Laser beam welding and post weld heat treatment of DP1000 ...

UNIVERSITY OF MISKOLC

FACULTY OF MECHANICAL ENGINEERING & INFORMATICS

INSTITUTE OF MATERIALS SCIENCE AND TECHNOLOGY

Laser beam welding and post weld heat treatment of

DP1000 automotive dual-phase steel

2020

By

ADLE S. A. MOSLEH

MSc student

Neptun-IBBKB1

UNDER THE GUIDANCE OF

Supervisor:

Dr. Marcell Gáspár Associate Professor

Consultant:

Raghawendra Pratap Singh Sisodia

PhD Student and Assistant Research Fellow

University of Miskolc

Miskolc, Hungary, 2020

Declaration of Authorship

I, ADLE S. A. MOSLEH; Neptune-code: IBBKB1, MSc student of the Faculty of

Mechanical Engineering and Informatics, University of Miskolc, being acutely aware of

my legal liability, hereby confirm, declare and certify with my signature, that the

assignment, entitled “Laser beam welding and post weld heat treatment of DP1000

automotive dual-phase steel”.

–except where indicated by referencing –, is my own work, is not copied from any

other person’s work, and is not previously submitted for assessment at University of

Miskolc or elsewhere, and all sources (both the electronic and printed literature, or any

kind) referred to in it, have been used in accordance with the rules of copyright.

I understand that a thesis work may be considered to be plagiarized if it consists of

• Quoting word by word or referring to literature either with no quotation marks or

no proper citation;

• Referring to content without indicating the source of references;

• Representing previously published ideas as one’s own.

I, hereby declare that I have been informed of the term of plagiarism, and I understand

that in the case of plagiarism my thesis work is rejected.

Miskolc 30/04/2020. Signature:

ACKNOWLEDGMENT

First and foremost, my sincere thanks and gratitude to the god, Allah, the

exalted, the majestic, who gave me the strength and ability to successfully achieve

my thesis and inspired me to be patient during my stay away from my family.

I would like to express my special thanks to my supervisor Dr. Marcell Gáspár

and Mr. Raghawendra P. S Sisodia for their cooperation and advising throughout

my research work. I enjoyed being part of your team. Thanks for giving me the

opportunity of being one of your students.

I would like to express my deep appreciation to the Budai Benefit Ltd.,

Budapest, Hungary for their generous cooperation in the production of the laser-

welded joints used in this research.

To those who always inhabits in my heart and mind in all steps in my life, my

father: Salem Mosleh and my mother: Kifah Ismail, without you I would not have

succeed in anything in my life.

I cannot forget any advice and sentence that I heard from my grandfather and

grandmother. You are invaluable in my life.

To my friends, thanks for all the time we have been together. I enjoyed being

with you. Thanks for your supporting.

I would like to thank Tempus Public Foundation for nominating me to get

Stipendium Hungaricum scholarship. Your help has allowed me to pursue my

master’s degree at the University of Miskolc. Thank you for your investment in

my life.

To her as well, my heart still standing on your hands, as always.

CONTENT

ACKNOWLEDGMENT

INTRODUCTION ............................................................................................... 1

1. Literature review ............................................................................................. 3

1.1. Advanced high strength steels (AHSS) ............................................................ 3

1.1.1. Advanced high strength steels background ........................................................ 3

1.1.2. Dual-Phase steels (DP) ....................................................................................... 4

1.2. Laser welding overview ................................................................................... 5

1.2.1. Introduction to laser welding .............................................................................. 5

1.2.2. Types of laser welding........................................................................................ 7

1.2.3. Characteristics of diode lasers compared to other lasers .................................... 7

1.2.4. Laser welding parameters ................................................................................... 8

1.3. Diode laser system ............................................................................................ 9

1.3.1. Principle structure of a diode laser ................................................................... 10

1.3.2. Diode laser models ........................................................................................... 11

1.4. Advantages and limitations ............................................................................. 11

1.5. Solid state transformation in weldments ......................................................... 12

1.6. Introduction to weldability .............................................................................. 13

1.6.1. Weldability definition ....................................................................................... 13

1.6.2. Weldability factors ........................................................................................... 14

1.6.3. Weldments cracks ............................................................................................. 15

1.6.4. Weldability evaluation...................................................................................... 15

1.6.4.1. Carbon content and alloying elements ..............................................................16

1.6.4.2. Weldability tests ................................................................................................17

1.7. Weldability analysis of dual-phase high strength steels ................................. 18

1.7.1. Laser welding process and related issues ......................................................... 18

1.7.2. Laser Welding of AHSSs in Automotive industry and research work ............. 19

1.7.3. Weld geometry and defects .............................................................................. 20

1.7.4. AHSS with galvanization and coating .............................................................. 22

1.7.5. Oil and contaminations on surfaces .................................................................. 22

1.7.6. Shielding gases ................................................................................................. 23

1.7.7. Laser welding of thick high strength steels plates ............................................ 23

1.8. Metallurgy of laser welded joints ................................................................... 24

1.8.1. Microstructural analysis and main parts of laser welded joint ......................... 24

1.8.2. Dimensions of FZ and HAZ ............................................................................. 28

1.9. Hardness of LBW joints and formation of soft zone ...................................... 29

1.10. Effect of post-weld heat treatment ................................................................ 39

1.11. Some studies on mechanical properties of laser welded joints ..................... 42

1.12. Quality control .............................................................................................. 43

1.12.1. Welding procedure specifications (WPS) ...................................................... 43

1.12.2. WPS for laser beam welding .......................................................................... 44

1.12.3. Welding procedure test ................................................................................... 44

2. Materials and experimental procedure ....................................................... 46

2.1. Material and composition ............................................................................... 46

2.2. Welding and Post-Weld Heat Treatment ........................................................ 47

2.2.1. Preparation for welding .................................................................................... 47

2.2.2. Laser welding set up ......................................................................................... 48

2.2.3. Post weld heat treatment (PWHT) .................................................................... 49

2.3. Weld characteristics and metallurgical examination ...................................... 51

2.3.1. Sample preparation ........................................................................................... 51

2.3.2. Optical microscopy ........................................................................................... 51

2.3.3. Micro hardness tests ......................................................................................... 52

2.4. Tensile testing ................................................................................................. 53

2.5. Bending test ..................................................................................................... 55

3. Results and Discussion .......................................................................... 56

3.1. Microhardness profile ..................................................................................... 56

3.2. Microstructural analysis .................................................................................. 58

3.2.1. Base Material .................................................................................................... 58

3.2.2. Laser Welded Joint ........................................................................................... 59

3.2.2.1. Fusion zone (FZ) ...............................................................................................59

3.2.2.2. Heat affected zone (HAZ) .................................................................................60

3.2.2.3. Coarse-grained HAZ (CGHAZ) ........................................................................60

3.2.2.4. Fine-grained HAZ (FGHAZ) ............................................................................61

3.2.2.5. Intercritical HAZ (ICHAZ) ...............................................................................62

3.2.3. Effect of PWHT on LBW joints ....................................................................... 62

3.3. Tensile testing ................................................................................................. 64

3.3.1. Tensile properties of LBW DP1000 steel ......................................................... 64

3.3.2. Fracture location of LBW joints ....................................................................... 65

3.3.3. Effect of post weld heat treatment on tensile properties .................................. 66

3.4. Bending tests ................................................................................................... 67

4. Conclusions ............................................................................................. 70

5. References ............................................................................................... 72

ANNEXURE ................................................................................. 81

List of Figures.

Figure 1. Illustration of automotive sheet steel grades based on strength and ductility [1]. .............. 4

Figure 2. Fundamental Modes of laser welding. The keyhole formation on the right-hand and the

conduction welding on the left [18]. ....................................................................................................... 6

Figure 3. Laser outputs: pulsed, CW and modulated [19]. ................................................................ 7

Figure 4. Absorptions for different materials as a function of wavelength [20]. ............................... 8

Figure 5. Wavelength and polarisation coupling of a diode laser [21]. ............................................10

Figure 6. Structure of a direct diode laser [21]. ................................................................................11

Figure 7. Schematic illustration of typical microstructure history of the HAZ [23]. ........................12

Figure 8. Schematic illustration of the microstructural variation in the heat-affected zone of a carbon

steel containing 0.15 wt% [23]. ..............................................................................................................13

Figure 9. OM and SEM micrographs of different microstructures in different subzones of the LBW-

welded joint of DP800 steel. a Base material. b ICHAZ. c FGHAZ. d CGHAZ. e WZ [64]. ...............24

Figure 10. Microstructure of DP600 base metal [65]. .......................................................................25

Figure 11. . Microstructure of DP1000 base metal [65]. ...................................................................25

Figure 12. Micrographs of martensite and tempered martensite in a 0.12 wt% C steel [66]. ...........30

Figure 13. Summary of Fe-C martensite tempering processes as a function of temperature during a

1-hour heat treatment [66]. .....................................................................................................................30

Figure 14. Hardness profiles of DP980 and HSLA at welding speed of 1.0 m/min [69]..................31

Figure 15. Hardness profiles of DP980 and HSLA welds with different welding speeds [69]. .......32

Figure 16. Variation of FZ hardness with carbon content and carbon equivalent in laser welds with

both similar and dissimilar combination [70]. ........................................................................................33

Figure 17. Variation of FZ hardness as function of carbon content [70]. .........................................33

Figure 18. Hardness profiles across welds with diode laser welding at speed of 1.3 m/min [71]. ...34

Figure 19. HAZ softening of DP welds made with different heat inputs resulting in different time

constants at the Ac1 [71]. ........................................................................................................................34

Figure 20. Maximum amount of HAZ softening at Ac1 versus martensite volume fraction [71]. ....35

Figure 21. Martensite softening of studied DP alloys with respect to heat input [71]. .....................35

Figure 22. TEM images of cementite particles found in the subcritical HAZ of the lean chemistry

DP780 welded with low and high heat input [68]. .................................................................................36

Figure 23. Minimum HAZ hardness vs time constant for DP780 materials [68]. ............................37

Figure 24. Effects of chemistry on softening of the BM and nonisothermally tempered DP steels [72].

................................................................................................................................................................38

Figure 25. Effect of PWHT on hardness profile of a. LBW DP800 b. LBW DP1200 [64]. .............38

Figure 26. PWHT thermal cycles. .....................................................................................................40

Figure 27. Effect of PWHT on hardness profile of diode laser D980 welds. ...................................41

Figure 28. Effect of LBW and PWHT on Stress vs Strain curve of DP980 base metal. ...................41

Figure 29. Effect of PWHT on hardness profile of a. LBW DP800 b. LBW DP1200. ....................42

Figure 30: test piece for linear butt joint [79]. ..................................................................................44

Figure 31: Location of test specimens for a linear butt weld [79]. ...................................................45

Figure 32. Dimensions of specimens to be welded. ..........................................................................47

Figure 33. a Clamping Device on Worktable. b Spot welding against misalignment. .....................48

Figure 34. a Laser head and robotic arm. b LBW experimental set up. ............................................49

Figure 35. LBW butt-welded joint. a Face side. b Root side. ...........................................................49

Figure 36. A welded and treated specimen before cut. b PWHT face side. c PWHT root side. .......50

Figure 37. a Control display. b PWHT experimental set up. ............................................................50

Figure 38. Location for thin sections used for microstructural analysis. a. LBW joint. b. PWHT joint.

................................................................................................................................................................51

Figure 39. Microscopic experimental set-up. ....................................................................................52

Figure 40: Microhardness tester (MVK-H1 Mitutoyo). ....................................................................52

Figure 41: LBW specimen. ...............................................................................................................53

Figure 42: PWHT specimen. .............................................................................................................53

Figure 43. Tensile test specimen. ......................................................................................................54

Figure 44: MTS 810.23-250 KN electric hydraulic testing equipment. ............................................54

Figure 45. Schematic diagram of three point bend test set up. .........................................................55

Figure 46: Hardness profile on LBW. ...............................................................................................57

Figure 47: Hardness profile on PWHT. ............................................................................................57

Figure 48: Comparison of LBW and PWHT. ...................................................................................58

Figure 49. OM micrograph of DP1000 Base material. .....................................................................59

Figure 50. OM micrograph of FZ in LBW DP100 at 200X Magnification. .....................................59

Figure 51. LBW sup-regions for microstructural analysis where A: FZ, B: CGHAZ, C: FGHAZ, D:

IVHAZ and E: BM. ................................................................................................................................60

Figure 52. OM Micrographs of a. Boundary FZ to HAZ. b. CGHAZ. At 200X magnification. ......61

Figure 53. OM Micrographs of a. FGHAZ. b. CG to FGHAZ boundary of LBW joint. At 200x

magnification. .........................................................................................................................................61

Figure 54. OM Micrographs of a. ICHAZ. b. ICHAZ to Base material boundary of LBW joint. At

200x magnification. ................................................................................................................................62

Figure 55. . PWHT specimen's sup-regions for microstructural analysis. ........................................63

Figure 56. OM micrograph of a. FZ, b. CGHAZ, c. FGHAZ, d. ICHAZ in LBW DP100 at 200x

magnification. .........................................................................................................................................63

Figure 57. OM micrograph of boundary between a. FZ & CGHAZ, b. CGHAZ & FGHAZ, c. FGHAZ

& Base material, d. Base material in LBW DP100 at 200x magnification. ...........................................64

Figure 58. OM micrographs of fracture location of DP1000 LBW joints. .......................................65

Figure 59. Tensile tested LBW joints. ..............................................................................................66

Figure 60. OM micrographs of fracture location of DP1000 PWHT joints. .....................................67

Figure 61. Tensile tested LBW Specimens. ......................................................................................67

Figure 62. Bend tested samples with no cracks. ...............................................................................68

Figure 63. Bend tested specimen with Bending angles .....................................................................68

Figure 64. OM micrographs of Bend location of DP1000 LBW and PWHT joints. ........................69

List of tables.

Table 1. The effect of CE ranges on weldability...............................................................................17

Table 2. CE and CET alloying additions weight percentages limits. ................................................17

Table 3. Comparison between the sizes of weld zone based on different laser welding processes [10].

................................................................................................................................................................28

Table 4. Chemical compositions of the investigated base material (DP1000). .................................46

Table 5. Mechanical properties of DP100. ........................................................................................47

Table 6. Laser beam welding experimental set up. ...........................................................................49

Table 7. PWHT parameters. ..............................................................................................................50

Table 8. Dimensions in mm. .............................................................................................................54

Table 9: average HV. ........................................................................................................................56

Table 10. Tensile properties of LBW DP1000. .................................................................................65

Table 11. Tensile properties of PWHT DP1000. ..............................................................................66

Table 12. Bending properties of LBW and PWHT DP1000 Joints. .................................................68

1

INTRODUCTION

Increasing attention on safety, durability, economy and environmental requirements has

made steel producers and automotive companies looking for new steel grades that meet

these requirements [1]. Engineers have been utilizing the variety of desired properties

combination via careful and precis control of chemistry and production methods [2].

Developing new steels with higher strength properties in combination with improved

formability properties allows more decrease in the required steel weight to get the

desired properties [3]. This will lead to reduction in total structure weight and in turn

reduce the fuel consumption and CO2 emissions [1] [4].

Advanced high strength steels (AHSS) can be the material of choice when

considering the mentioned requirements by automotive industry. AHSS holds the

combination of high strength properties and high formability. As a result, AHSSs allow

the use of lower parts thicknesses, which in turn lead to car weight reduction and achieve

the automotive industry requirements [4] [5]. Although mild steels still used in

automotive steel, this unique combination in AHSSs makes the superior to conventional

steels in many aspects [2].

The trend of using AHSSs in automotive industry appeared in many projects For

example but not limited to, ULSAB-VAC project suggested up to 83% use of AHSS in

medium size car [6]. In ULSAB-VAC, DP steels represent the highest used steel by 73%

of car body. World steel association suggested in a case study, mass saving up to 25%

of car weight by using AHSS instead of conventional steel, which contribute to about

9% reduction in the total car weight [7].

Dual phase steels (DP) are one of the most used AHSS in automotive industry. Its

rise was in 1975 [8]. It is two-phase microstructure consists of ferrite which gives the

ductility and martensite islands which responsible for the strength properties [2], [9].

2

Intercritical annealing followed by quenching to get the desired combination of ferrite

and martensite and possibly other phases, which results in the desired properties, is the

process used in DP steels production [2].

DP steels hold unique properties including excellent combination of strength and

ductility, high strain hardenability and the bake hardening behaviour [2]. In addition,

DP steels, in terms of car safety, are the material of choice when maximizing the crash

energy absorbing is required [1]. Furthermore, it is used in anti-intrusion application. It

also gives the required fatigue life since the offered tensile strength [1].

When comparing DP steels to other AHSS, it is lower in production cost than TWIP

steels and it can reach higher strength than micro-alloyed steels [10], [11].

The increasing use of AHSSs in automotive industry makes weldability assessment

of it a critical consideration since any car typically has thousands of weld [5]. Before

the rise of AHSS in automotive industry, mild steels were extensively used and normally

fabricated by resistance spot welding (RSW) [7], [12]. With the trend of using AHSSs,

other welding methods are being used. Laser welding technology is currently used in

joining AHSSs for automotive industry applications since it holds two main features, it

is highly flexible and can be robotized, and the considerable lower heat input than other

conventional arc welding technologies [13], [14].

In this work, the effect of laser welding that carried by 4KW diode laser on the

microstructure and mechanical properties of DP1000 was analysed. In addition, the

enhancement of post-weld heat treatment carried by diode laser on LBW DP1000 butt

joints was examined. The microstructural analysis was carried out by optical microscope

Axio observer D1m Zeiss. Transverse tensile tests and three-point bending tests were

performed by MTS 810.23-250 KN electric hydraulic testing equipment and ZD20

hydraulic testing equipment, respectively. In the literature review, detailed study on

weldabilty aspects of DP steels by laser welding, introduction to laser welding and DP

steels and the previous studies on the effect of laser welding on DP steels are provided.

In addition, some of the previous related studies on the effect of PWHT on lase welded

joints are mentioned. Chapter 2 discusses the experiments done during the work.

Chapter 3 contains the results and discussion of the tests and then the conclusion.

3

1. LITERATURE REVIEW

1.1 Advanced high strength steels (AHSS)

1.1.1 Advanced high strength steels background

Through over 100 years of automotive industry applications development, steel has been

a characteristic and effective solution for vehicle body structure. The safety, durability,

economy and environment requirements have made steel producers and automotive

engineers developing new grades of steel in order to meet these requirements of different

vehicle structures [1].

From a metallurgical point of view, automotive steels can be classified into different

groups including the low strength steels, conventional high strength steels (HSS) and

advanced high strength steel (AHSS). The microstructure is the main feature to

distinguish between these groups of steels. Conventional HSS (such as carbon-

manganese, bake hardenables and high strength low alloy steels) are mainly a single-

phase ferritic or pearlitic steel. On the other hand, the microstructure of AHSS contains

a second phases such as martensite, bainite, austinite and sometimes retained austenite

in sufficient amounts that affect and produce the desired combination of mechanical

properties. Fig. 1 shows the different steel grades and the relation between the strength

and ductility which specifies the proper application of each steel [3], [15].

Via careful and precise control of chemistry and processing, engineers have been

utilizing the combination of steel mechanical properties, especially the strength and

ductility. Some types have a higher balance of strength and ductility due to higher strain

hardening rates than conventional steels and some types have ultrahigh yield strength

4

and bake hardening. Producing process of AHSS are mainly by controlling the cooling

rates and stages from austenite or austenite plus ferrite phases [2], [3].

The dual phase (DP) steels, transformation induced plasticity (TRIP) steels, complex

phase (CP) steels, martensitic (MS) steels, ferritic bainitic (FB) steels, and twinning

induced plasticity (TWIP) steels are the common types of AHSS [15]. The different

grades of steel can be seen in Fig. 1.

Figure 1. Illustration of automotive sheet steel grades based on strength and ductility [1].

1.1.2 Dual-Phase steels (DP)

DP steels are one of the most used steels in the automotive industry. The rise of DP

steels was in 1975 [8].

DP steels are normally a two-phase microstructure, the first one is hard martensite

particles sprinkled in a ferrite matrix which is the second phase characterized by softness

and ductility. Featuring excellent combination of strength and ductility. The tensile

strength of DP steels ranges from 450 to 1400 MPa [2], [9].

DP steels are normally produced by intercritical annealing followed by rapid cooling

[2]. During the intercritical annealing small spheres are formed inside the ferrite matrix,

which then transformed into martensite due to the rapid cooling. Martensite portion

which is hard and deformation resistance phase is subsequently introduced in the

microstructure during the austenite to martensite transformation which cause a volume

expansion and subsequently a mobile dislocation in the surroundings of the ferrite

5

matrix are formed. These mobile dislocations are interacting with each other and with

the grain boundaries, which is the reason for high initial work hardening rate and

continuous deformation behaviour [3]. It is also important to emphasize that

transformation induced plasticity steels (TRIP) and medium Mn steels are produced by

intercritical annealing [4].

What makes the DP steel a part of the automotive industry, is the unique mechanical

properties such as the excellent combination of strength and drawability due to the

outstanding combination of strength and ductility, their high strain hardenability and the

bake hardening behaviour [2].

In terms of safety, DP steels have been the material of choice when maximum

absorbed crash energy is required, although mild steels is still used, the application of

DP steels are expanding rapidly when maximum ability to absorb energy is needed [1].

Moreover, DP steels are used in anti-intrusion applications in the passenger safety cell.

The high tensile strength allows using them in parts requiring resistance to fatigue,

intrusion, denting, impact and so on [1].

In terms of chemical composition, the carbon content of DP steels is normally less

than 0.2%. This provides a good weldability for all welding methods. Other materials

are present in different percentages depending on the desirable properties. These

additives may be Manganese in amounts of 1 to 1.5% to provide sufficient hardenability.

Chromium and molybdenum have also been inserted into the microstructure in amounts

about 0.6% maximum. Other elements might be added such as niobium, titanium,

vanadium to control the grain size or to provide more precipitation hardening. Nitrogen

is sometimes added to active the effects of vanadium intensely [9].

1.2 Laser welding overview

1.2.1 Introduction to laser welding

Laser beam welding is a process by which heat is generated to join metals by using a

moving high power-density heat source called as laser. Laser is an abbreviation for light

amplification by stimulated emission of radiation. The laser energy in the form of

electromagnetic energy interacts with the material to generate enough heat to melt and

6

penetrate the surface of metals to be joint. The steps of melting and penetrating are

multidisciplinary and need a wide knowledge of several physics’ areas. The nature of

laser beam interactions with materials and heat delivery is presented in [14], [16].

During the laser welding, a liquid pool is formed and remains stable under

solidification through a very delicate balance between heating and cooling within a

localized volume overlapping the interface between two or more solids. The liquid melt

pool is created by absorption of incident radiation. The liquid melt pool will grow to the

desired size and then propagate through the solid interface between materials to be

joined [17].

Laser welding can use shielding gases, pressure or both during welding procedure.

Conduction welding and keyhole or penetration welding are the two basic modes of laser

beam welding. During heat conduction mode the material is heated above its melting

point by the absorbed laser heat energy, but only to the point without building up enough

vapour pressure to break the weld pool surface. The lower energy input into the

workpiece, limited melt volume and the nature of the process gives better results

compared to conventional welding. On the other hand, the absorbed laser beam energy

in deep penetration mode heats the material above its evaporation point and

consequently the weld pool opens to allow laser beam to penetrate and form a vapour

capillary called the keyhole inside the weld pool [18]. Heat conduction welding and

keyhole welding mode are schematically illustrated in Fig. 2. It is worth to mention the

transition keyhole mode or the existence of transition regime between the two basic

modes [19].

Figure 2. Fundamental Modes of laser welding. The keyhole formation on the right-hand and the

conduction welding on the left [18].

7

1.2.2 Types of laser welding

In general, there are two basic types of laser used in metal joining based on the material

inside the tube. The first type is solid state laser which uses a solid medium. YAG laser,

disk laser, fiber laser and diode laser are solid state lasers. The second type is gaseous

laser which uses a mixture of gases in the tube such as helium, nitrogen and 𝐶𝑂2 [3],

[7].

In terms of wave operation, lasers can be divided into pulsed and continuous wave

operation. A pulsed laser produces a series of discrete energy packets with laser power

exceeds the average power. On the other hand, continuous wave laser with power does

not exceed the average rated power. It is worth to mention that continuous wave (CW)

can produce a pulsed laser known as modulated laser. Fig. 3 shows the three different

wave operations. Diode lasers operate in continuous wave mode while an Nd:YAG

operates only in pulsed wave mode and fiber laser can operates in both wave operations

[19].

Figure 3. Laser outputs: pulsed, CW and modulated [19].

1.2.3 Characteristics of diode lasers compared to other lasers

High power diode lasers which consists of small light sources not a single high-power

light source. The diode laser combined a several small lasers each with few watts and

consequently the power densities achieved are much lower than other high-power lasers.

In most cases, laser welding with diode laser uses heat conduction welding mode, which

is stable, and results into lower penetration and wider welds. The diode lasers provide

8

more possibilities since its higher absorption for metallic materials [20]. Fig. 4 shows

the relation between each laser type and materials absorptivity as a function of

wavelength.

Diode lasers offer a power range from 10W to 8KW, but suitable power range for

welding from 1 to 4 KW. The area of laser beam focal point is rectangular and range in

millimetres while the beam shape ration describes the ratio between longer and shorter

side of the rectangular area [20].

𝐶𝑂2 and Nd:YAG offers a fast and accurate welding and provide deep and narrow

welds. In these types, the focal point is relatively small, and it requires highly accurate

positioning and well joint preparation and fixing. These types are usually performed in

keyhole welding mode due to using higher power densities and single high-power

source. The drawback of the process, in comparison to conduction welding, is the

decreased heat input and because of that, a lower level of distortion in case of diode laser

[20].

Figure 4. Absorptions for different materials as a function of wavelength [20].

1.2.4 Laser welding parameters

Laser parameters determine the weld quality extensively [3]. By changing these laser

parameters, the manufacture can improve the quality of the welded joint. While welding

parameters in pulsed wave (PW), CW and modulated lasers have some differences in

the nature since the nature of the output power [19].

9

In general, the laser parameters in PW and modulated lasers are similar. Theses

parameters are peak power, pulse width, pulse energy, pulse repetition rate, average

power and optical spot size. Peak power is the maximum power of each pulse measured

in watts while average power is a term used when more than one pulse is used, and it

represents the average of the power over the pulse period. Pulse width is measured in

milliseconds and describes the duration of the pulse. On the other hand, in CW lasers,

the laser parameters of concern are optical spot size, beam quality and rated power [3]

[19].

Two more important laser parameters are the optical spot size which defines the

diameter of the focused laser on the work piece, and the beam quality which describes

the ability of laser to be focused to a small spot size. Increasing the intensity or power

per unit area in case of CW or energy per unit area for pulsed beams to a high value

sufficient to do useful work with small beam spot size. The quality of a beam also known

as 𝑀2 number. It is worth to mention that CW does not have the high peak powers of

the PL and consequently the optical spot size is smaller to ensure enough power density

for welding [3] [19].

Welding speed is a basic and important parameter. This parameter defines the

distance of the material to be welded that laser beam will cover per unit time. Laser

welding can use filler materials, but usually does not uses filler material which means it

is an autogenous welding procedure. Shielding gases can be used to prevent oxidation

and contamination during welding [19].

In general, the highest beam quality does not mean necessarily a better weld, which

means there is always the option to decrease the beam quality if needed to match the

application requirements. As a result, there must be a balance between welding

parameters, welding performance, application requirements and the process window in

any manufacturing process.

1.3 Diode laser system

A standard diode laser system consists of three main parts, a diode laser head that also

called the beam source, supply unit and the control unit [21].

10

1.3.1 Principle structure of a diode laser

The diode laser head consists of group of individual diode laser components with

lenses for shaping and projecting the beam. Fig. 5 shows schematic illustration of a

diode laser head.

Figure 5. Wavelength and polarisation coupling of a diode laser [21].

Where the components of wavelength and polarisation coupling are the following:

(1) Polarisation coupling.

(2) Wavelength coupling.

(3) Laser diode stack.

(4) Laser spot.

(5) Focussing lens.

(6) Collimation lens.

Many individual emitters with a size of 0.5×3 μm2 are arranged in a row and placed

on cooling element. The power output of such a diode laser bar equal to 30 watts. These

components are arranged in way allows to produce laser beam with intensity over 100

W [21].

Electrical power is the source of energy while cooling is upon water. Laser is

produced by exciting the semiconductor material by DC voltage source. The waste heat

during converting electrical output into optical output reaches 70 % and it is dissipated

to the ambient air via pumping water to an external heat exchanger, which avoids heat

accumulation in the diode laser stack [21].

Focusing procedure consists of three steps, at first the beam is collimated vertical to

emitter level by a microbar lens, then horizontally by a cylindrical lens. This two-fold

is bundled and projected onto a minute area by the laser’s focusing optics [21].

11

In order to increase the output intensity of laser system by about of factor 4, two

procedures are used. Wavelength coupling by combining two different laser wavelength

that doubles the emitted laser performance. In addition, linear polarization is used to

increase the output performance [21].

The big disadvantage of such a diode laser is the extreme angle of divergence with

which the beam is emitted [21].

1.3.2 Diode laser models

Direct diode laser (LDL), fiber-coupled diode laser (LDF), DioScan, multi diode laser

are some of diode laser models. In this study, the laser model used is LDL 160-3000. A

schematic illustration of this head is shown in Fig. 6.

Figure 6. Structure of a direct diode laser [21].

(1) Safety label.

(2) Housing.

(3) Supply lines connections.

(4) Optics.

(5) Protective glass.

(6) Optics cooling hose.

1.4 Advantages and limitations

Industrial applications of lasers have found that welding with lasers offers several

benefits to conventional welding processes [22]. These advantages include [14]:

1. Laser welding provides high processing speeds with very rapid starting and

stopping become possible as the energy source is light which is inertia-less.

2. Multiple optical paths can be used. In other words, the laser beam can be

shared in energy and time.

12

3. Filler materials and electrode are not necessarily required.

4. Laser welding is compatible with atmospheric conditions and gas mixtures.

5. Laser welding provides high energy density which comes from focused laser

light.

6. Titanium, quart and other difficult to weld materials can be joined using laser

welding.

7. Very precise welds can be achieved in terms of penetration, positions and

diameter.

8. Level of contamination can be very low.

9. Very narrow heat affected zone can be obtained.

10. Considerable distance between laser source and workpiece.

11. It can be automatically controlled.

Limitations of laser welding includes the following [14]:

1. The typical focal spot for laser beam range is small which makes parts fit-up

and alignment much more critical.

2. Laser welding is expensive.

3. Penetration obtained in case of electron beam welding is more due to the

kinetic energy of electrons with inertia is greater than photons.

1.5 Solid state transformation in weldments

Solid-state transformation occurring in welds are non-equilibrium in nature. The

metallurgy of the welded joints can be divided into two main portions: the fusion zone

(FZ) and the heat-affected zone (HAZ) [23]. For example, the transformations occur

during welding of low carbon steels are illustrated in Fig. 7.

Figure 7. Schematic illustration of typical microstructure history of the HAZ [23].

ΔT8/5 describes the time interval during cooling from α+γ region at 800 ℃ to α region

at 500 ℃ where the solid-state transformations occur in most steels. In the heat affected

13

zone the material is not melted but the mechanical properties and the microstructure are

modified by the heat of welding process. Fig. 8 shows an illustration of the

microstructure variation of the heat-affected zone for carbon steel containing 0.15 % wt

% carbon.

Figure 8. Schematic illustration of the microstructural variation in the heat-affected zone of a

carbon steel containing 0.15 wt % [23].

1.6 Introduction to weldability

1.6.1 Weldability definition

All materials can be considered suitable for use in structural applications such as

automotive industry, only if, it can be fabricated into components. One of the main

fabricability factors is the weldability that is also known as joinability. However,

welding is the main fabrication technique for joining materials from same or different

types and grades including steels [24].

A wide variety of material characteristics plays role in describing the weldability of

any material when it is subjected to welding process. Since that, it is hard to define the

14

weldability term as a quantity term but a quality term [25], [26]. However, many

standards and welding institutes defines the weldability. According to International

Institute of Welding (IIW document no. IIW/IIS-22-59), the weldability is defined as ‘A

metallic material is considered to be weldable to a certain degree by a given process and

for a given purpose when a continuous metallic connection can be obtained by welding

using a suitable procedure so that the joints comply with the requirements specified both

in regard to their local properties and their influence on the construction of which they

form a part’ [24], [26]. The American Welding Society describe it as ‘Capacity of metal

to be welded under fabrication conditions into suitably designed structure to perform

intended service’ [27]. According to the International Organization for Standardization

ISO (ISO/TR 581:2005) , weldabililty as reference: ‘A component consisting of metallic

material is considered to be weldable by a given process when metallic continuity can

be obtained by welding using a suitable welding procedure. At the same time, the welds

shall comply with the requirements specified in regard to both their metallurgical and

mechanical properties and their influence on the construction of which they form a part’

[28].

1.6.2 Weldability factors

Weldability in all definitions are dependent on three main factors, which affects the

ability of the material to be welded; they are the material properties (chemical

composition, metallurgical properties, physical properties), design properties (welding

safety and structural standpoint), and production properties [28], [29].

In addition to the ability of the materials to be welded, the resulting welded structure

has to perform as needed in its service life [30]. This is the welded joint integrity, which

combine the mechanical performance of the joint such as the toughness, strength,

ductility, hardness, impact etc.) [31].

The welded joint has to be in good quality and free from weld defects and failures

such as cracks, and flaws such as incomplete penetration and rollover. All these factors,

individually or collectively, can cause different types of cracking in the weldments due

to increasing stress concentrations [26].

15

1.6.3 Weldments cracks

Several types of cracking can cause the material to be poor weldable including [32]:

Cold cracks.

Hot cracks.

Lamellar tearing.

Cracking during PWHT.

Cracks that develop under the solidus temperature are called cold cracks or subsolidus

cracks. It is also sometimes called the hydrogen-induced cold crack since the major role

of hydrogen in these types of cracking after welding procedure [33]. It is delayed cracks

which means it can appears after hours or more before or during service [34]. During

cooling in welds, hydrogen which may come from the base metal or filler material or

the environment during welding, diffuse out of the area and the remaining gathers

around the imperfections and martensite or other places causing an increase in pressure

and residual stress. This in turn cause failure of the welded joint [34].

Cracks that develop above the solidus temperature are called hot cracks or

supersolidus cracks. While other cracks can appear during cooling or heat treatments of

the joint [26], [33] .

Not only the cracks during and after welding play major role in weldability but also

the welded joint performance during service [35].

1.6.4 Weldability evaluation

It is necessary to study the weldability. This means it is needed to study the material to

be welded and its design configuration, the welding environment, and the welding

procedure [36]. However, optimizing the best weldability by manipulating all these

factors is impossible or expensive to do and long try and error process [36].

International organizations for standardization and welding institutes has come out

with standards for testing, evaluating and indicating weldability factors in qualitative

and quantitative ratings or by limiting curves [37].

16

1.6.4.1 Carbon content and alloying elements

Increasing the carbon content in steels normally increase the tendency to form hard and

brittle areas such as martensite and other carbides in the FZ and HAZ during welding

[27], [36], [38]. The carbon content defines the maximum possible hardness and it is

inversely proportional to the weldability of the steel [24]. Thus, the combination of high

tendency to form brittle and hard areas within the FZ and HAZ during welding with the

presence of other factors that enhance susceptibility to form weld defects and cracks

make the material to be poor weldable.

In addition to the effect of carbon content on hardenability and weldability, other

alloying elements effects on maximum possible hardness and weldability need a special

attention and the effect of thermal cycles [24].

In general, low carbon steels with carbon level less than 0.15 % by weight are

normally very weldable, mild steels with carbon level between 0.15 % and 0.3 % are

good weldable steels. However, with increasing carbon content reaching 0.3 % by

weight, special actions such as preheating, controlling heat input and travel speed, and

post weld heat treatment are normally required [26].

IIW has come out with a term called carbon equivalent (CE) and generalize equations

to evaluate it. CE considers the carbon content and other alloying elements to indicate

the weldabiltiy of the materials [32].

The first use of carbon equivalent were initially to characterize the hydrogen cold

cracking as the increase in carbon content and alloying levels increase the probability of

cold cracks occurrence [36]. In other words, thick steels with low ductility and high

carbon equivalent are more likely to shrink after welding with increasing in stress levels

causing cracks such as cold cracking [34]. However, steels in general with good strength

and toughness in its service life is considered as a weldable material [35], while the

hardenability of steel is inversely proportional to hydrogen cracking tendency. Thus, the

carbon equivalent is used to indicate the weldability and hardenability of steels based

on their alloying elements [34].

IIW defines standard equations to quantify the effect of carbon and other alloying

elements on weldability and hardenability of steel. IIW has come out with CE value. In

17

japan, similar approach is also used by Ito-Bessyo composition characterizing parameter

Pcm. Table 1 shows the effect of CE ranges on weldability [36].

Table 1. The effect of CE ranges on weldability.

According to EN 1011:2001, two empirical formulas to calculate carbon equivalent

value, CE and CET depending on the percentages of alloying elements present. The

limits for alloying elements are shown in Table 2 for the two approaches. These limits

are used to avoid hydrogen cracking for non-alloyed, fine-grained and low-alloyed steels

[39]. CE and CET are calculated based on Eq. 1 and Eq. 2.

𝐶𝐸(𝐶𝐸𝑉) = 𝐶 +𝑀𝑛

6+

𝐶𝑟+𝑀𝑜+𝑉

5+

𝐶𝑢+𝑁𝑖

15 ….. (1)

𝐶𝐸𝑇 = 𝐶 +𝑀𝑛+𝑀𝑜

10+

𝐶𝑟+𝐶𝑢

20+

𝑁𝑖

40 ……… (2)

Table 2. CE and CET alloying additions weight percentages limits.

Element C Si

(Max) Mn

Cr

(Max)

Cu

(Max)

Mo

(Max)

Vi

(Max)

Va

(Max)

Nb

(Max)

CE 0.05-0.25 0.8 1.7

Max 0.9 1.0 0.75 2.5 0.2 ----

CET 0.05-0.32 0.8 0.5-1.7 1.5 0.7 0.75 ---- ---- 0.6

1.6.4.2 Weldability tests

Weldability tests can be divided into five main categories. These categories are

theoretical tests, simulated test, visual examination, component sampling and actual

welding test [26].

CE Weldability

< 0.4 Excellent

0.41-0.45 Good

0.46-0.52 Fair

> 0.52 Poor

18

Some tests are designed to determine the cracking tendency of welds to crack such as

cold cracking and cracking associated with the specific structural fabrication. Those

tests are the fabrication weldability tests [26].

During the service life of the fabricated component, it is necessary to ensure that the

welded joint has the needed mechanical properties in service. Those tests, including

tensile tests, formability tests, fatigue tests, corrosion tests, creep tests, hardness tests

impact tests etc., are called the service weldability tests [40]. However, not only the

mechanical properties need to be tested but also metallurgical changes or microstructural

changes [27].

In order to get the reasonable degree of confidence in the resulting welded joint, some

precautions such as preheating, appropriate selection, controlling heat input, post weld

heat treatment etc. are needed since the weldability test can provide useful clues on the

weldability limitations [26].

1.7 Weldability analysis of dual-phase high strength steels

1.7.1 Laser welding process and related issues

Despite the high investment cost of laser welding technologies [41], it holds two main

features that make the use of laser welding wider and more suitable for different

applications such as automotive industry. These two factors are that the laser welding

is highly flexible process and the low heat input introduced to the material to create the

weld [41] [13].

As to maintain the metallurgical heterogeneity as low as possible, it is necessary to

keep the heat input as low as possible [42]. Laser welding carries this advantage of high

power intensity of the order up to 108 𝑊

𝐶𝑚2 which is almost 4 times higher in order

compared to conventional arc welding methods [43].

Thus, the high power intensity minimizes the thermal energy needed to be put into

material to melt and solidify to form the weld by laser welding [44]. As a result, a

combination of high welding speed can be achieved with as low heat input as possible.

19

This in turn leads to good quality welds that are narrow with lower level of deformation,

good weld mechanical properties integrity and low heat distortion [41], [43], [10].

It is not possible to consider the thermal efficiency of welding process to be constant

percentage since several parameters related to the process, material to be welded with

or without the addition of filler materials and the environment and circumstances of the

welding procedure. However, the general thermal efficiency of LBW is K= 0.5 to 0.75.

This value can get as low as 0.005 when welding against high reflectivity surfaces while

it may reach 0.75 in keyhole welding mode [45].

Laser welding offers a great combination of speed and flexibility [46]. One of the

main reasons as to why laser welding is flexible process is that LBW is free contact

process. Secondly, it is possible to create a full penetration when access is possible from

only one side. In addition, LBW can be robotized [47]. Furthermore, wide variety of

material can be welded with wide variety of geometries [48].

The flexibility in combination with low heat input and high welding speed in laser

welding caused LBW to be common in welding tailor blanks and other application. As

a result, cost minimization, material usage decrease, weight reduction, enhance the

rigidity and other advantages can be obtained [49].

1.7.2 Laser Welding of AHSSs in Automotive industry and research work

AHSSs allow more decrease in the thickness of steels used in different industries

especially the automotive industry, which in turn lead to reduction in the weight of parts

made from this group of steel and reduce the fuel consumption [10]. ULSAB-VAC

project clearly pointed out the trend of using AHSSs in the auto-body. The project

suggested that it is possible to use AHSSs up to 83 % of a median size car body in which

73 % are dual phase steels. In comparison between AHSSs, DP and TRIP are lower in

cost than TWIP and have more formability than martensitic steels [10]. In addition, DP

steels can reach higher tensile strength than micro-alloyed steels and this in combination

with good formability and weldability [10], [11].

20

The high use of AHSSs in automotive industry makes understanding the weldability

of these steels very essential since any car typically has thousands of welds [5]. Since

DP steels are widely used, its weldability present a critical issue.

Automotive industry generally uses the keyhole-welding mode but much research

work has been done in the conduction-welding mode to study the metallurgical and

mechanical changes [10]. The reason that automotive industry normally depends on

keyhole welding mode is to get narrow welds and deep penetration with highest possible

welding speed. However, in keyhole mode, the rapid fluid velocity as high as 3000mm/s

combined with the instability of keyhole often results in less acceptable weld with rough

and ropy bead surface [50]. By controlling the welding parameters smooth, smooth weld

bead surfaces with improved level of acceptance can be obtained since the surface

appearance can be an issue in automotive industry [10].

In general, the weldability of Docol DP steels is very good since it contains low

alloying elements content and its high strength [41]. Docol DP is a dual phase steels

group manufactured by SSAB Company [11]. In welding DP steels, all the common

welding processes can be used but mainly spot welding and laser welding are the most

common technologies involved in automotive industry [12]. However, AHSS are

sensitive to heat input, which makes keeping the heat input as low as possible essential.

This can be seen from the short cooling rates recommended [42].

LBW can be used to weld all Docol AHSS/UHSS since the risk of defects is low in

these steels and the FZ and HAZ are narrow in combination with high load bearing

capacity. LBW of DP steels is similar to LBW of mild steels since normally the same

parameters are used for both. However, the strength of DP welded joint is higher than

the conventional high strength steels [41].

1.7.3 Weld geometry and defects

Laser welding can be used to weld similar and dissimilar materials with different joint

types, but laser welding is highly accurate process since the accurate heat input and the

small spot size of laser beam compared to other fusion welding [51]. Thus, due to the

21

narrow beam size of laser, the process requires precise fit up and alignment with the

right joint type choosing where sometimes fixturing of parts or sheets are needed [52].

In case of welding tailor blanks, laser welding is the preferred method of joining since

continuous weld line can be obtained while in joining AHSS [53], butt welding is the

preferred method, and in welding tailor blanks, it is the only process that can be used

[10]. The main reason as to why butt joint configuration is preferred in joining tailor

blanks made from AHSSs is to eliminate the issues related to forming and die design

[10].

In general, a concave shaped beam formation on the top of weld zone can sometimes

results when laser welding is applied [54]. Thus, any gap between the steel sheets during

laser butt welding should be avoided since it may results in significant weld concavity

and undercut, which results in lower weld performance [10]. Furthermore, edges of work

sheets must be machined before welding while any misalignment will cause a notch and

degrades the fatigue performance [10]. However, good quality edge and precis fit up are

necessary to not only obtain an acceptable welding result, but also avoid undesirable

underfill on the top side or sometimes a lack of fusion can occur [55].

Yang and Galter [54] observed concavity formation on the top of weld pool in laser

welded EDDS and DP590 and other combinations. Due to effects of gravity, a concavity

is rarely appears on the bottom of welds. A tear drop with concave up shape formed in

the bottom even if samples are partly penetrated. Yan and Galter return this to the shape

of laser beam which is generally a Gaussian shape. However, the bottom of fusion zone

was concave up.

Yan and Galter [54] studied the effect of concavity on tensile properties and observed

that if the concavity is lower than 20 % of the sheet thickness, a little effect will appear

in tensile properties. However, other weld defects will enhance the effect of concavity

and it will be considerably affecting the tensile strength. For example, a concavity in

combination with HAZ softening in DP980 will cause lower tensile results. This can be

due to the high strength of FZ an HAZ laser welded.

Porosity appears during butt laser welding of 800 MPa TRIP steel. Han et al stated

that porosity would decrease with increasing welding speeds [56].

22

1.7.4 AHSS with galvanization and coating

In case of galvanized and coated AHSSs, vaporization of zinc will occur [12]. It is

essential to allow zinc vapour to escape from the weld pool. When butt-welding of

coated AHSSs, vaporization of zinc would not be harmful. Very small gap can be used

while too closed gap are more important in case of autogenously welding [41], [55] [57].

This issue can be resolved by adding filler material with very small gap. Liu et al suggest

to keep a small gap at 0.04 mm between sheets edges [57].

In case of lap welding of alloyed hot-doped galvanized steel (GA) sheets, zinc

vaporization will occur at the molten area of contact surfaces between sheets. As a result,

a large quantity of spatter will appear especially if overlap without gap is used [12].

Keeping gap between sheets is one of the methods to avoid pores, spatter and other

defects in lap welding. 0.1 - 0.2 mm gap is recommend by SSAB the steel producer [41].

Filler materials can help solving this issue [58]. In addition, shims or spacers are used

to keep the same gap along the weld line [55].

Another way, to improve weld quality and reduce weld defects related to galvanizing

such as pores, is to use a laser spot welding method. By splitting the laser beam into two

laser, which create a larger keyhole, allowing more time for zinc vapour to escape from

the molten pool. 0.8 to 1.2 s or 0.3 to 0.5 s is recommended [41].

Fixturing is more critical in case of lap welds but it is needed for butt joints too [55].

1.7.5 Oil and contaminations on surfaces

Oil on sheet surface represent an issue since too much oil results in hydrogen pick up in

the weld, which creates weld defects and degrades the weld performance. For example,

in case of roll forming line, preheating can effectively get out the oil from the area near

edges to be welded. This can be achieved by high frequency induction device prior to

welding equipment [41] .

Contaminations and oxide layers on the surfaces of materials to be welded are

harmful since it reduce the weld properties and cause defects such as porosities. This is

as to why it is essential to get rid of these oxides before start welding [59].

23

1.7.6 Shielding gases

In order to improve weld quality and avoid hydrogen embrittlement, it is necessary to

protect the weld metal by shielding gas. Different types with different flowrates can be

used depending on the welding application. However, Nitrogen is not preferred since it

would get into the material and react with the molten pool [59]. The shielding gas of

choice seems to be argon (Ar). In some cases, a combination of argon and CO2 can be

used. For example, Hartly and Ono [60]choose corgon 18 (28 % Ar, 18 % CO2) and

successfully welding DP steels with it.

1.7.7 Laser welding of thick high strength steels plates

With the development of laser sources providing higher power intensities, laser welding

of thick steel plates is becoming wider and a good level of stable single pass laser

welding can be achieved even though for medium and high thick materials [13].

There are some disadvantages when welding thick plates. The limited available laser

welding power source typically up to 20 kW commercially lasers put some difficulties

on welding thick plates. In addition, as the thickness of material increased, the

possibility of weld defects increased. For example, molten material droplet, cracking,

mis-tracking and porosities and tear up or porosities can appear [13].

In a single pass laser welding, Vollersten et al. [61] observed a successfully full

penetration of carbon steel up to 20 mm maximum thickness with 19 kW laser source.

However, the typical laser welding penetration ratio in single pass welding process is 1-

2 mm/kW. Even though with high power laser power sources, there is limited

penetration that can be achieved. For example, the maximum penetration achieved with

high power fiber lasers reached up to 30 [62].

Filler material carries some advantages, which makes the use of laser welding wider

in many cases by getting better weld qualities while the main features of laser welding

still exist. Relaxing the tight joint fit up can be achieved when using filler materials. In

some cases, by modifying the FZ composition, it is possible to get better homogeneity

and weld properties. Multi-pass with filler materials can be used [63].

24

In lap welding of thick steel plates, it is not always possible to achieve a full

penetration. In this case, up to 50 % penetration in the lower plate is recommended [41].

1.8 Metallurgy of laser welded joints

1.8.1 Microstructural analysis and main parts of laser welded joint

Similar to other welding methods, base metal (BM), heat-affected zone (HAZ) and

fusion zone (FZ) are typically the main three parts of that form the microstructure of

laser welds [10]. A typical example of the microstructure of welded DP800 steel by 4

kW diode laser equipment is illustrated in Fig. 9. It shows the main parts of welded joint.

Figure 9. OM and SEM micrographs of different microstructures in different subzones of the

LBW-welded joint of DP800 steel. a Base material. b ICHAZ. c FGHAZ. d CGHAZ. e WZ [64].

In DP steels, the microstructure of the base metal normally consists of ferrite grains

with martensite islands that elongated in the rolling direction during the production

process. As an example, the base metal of DP600 and DP1000 are shown in Fig. 10 and

Fig. 11, respectively. It is clear that the higher grades of DP steels have higher

martensitic fraction.

25

Figure 10. Microstructure of DP600 base metal [65].

Figure 11. . Microstructure of DP1000 base metal [65].

The HAZ can be divided into three subparts. The peak temperatures experienced

during welding by each part, and the critical transformation temperature, at when, the

phase changing can occur or initiated, determine the boundaries of these three subparts.

Tempered region or subcritical HAZ, intercritical HAZ and supercritical HAZ are the

three parts that form the HAZ, while the supercritical HAZ consists of two main

subzones: coarse-grained HAZ (CGHAZ) and fine-grained HAZ (FGHAZ) [27].

If the peak temperature experienced by the part is below the Ac1 temperature of the

material, then normally tempering of martensite will occur. This part of HAZ of the

welded joint called tempered region or subcritical HAZ. This subpart is in touch with

the unaffected base metal [13], [10].

26

The intercritical HAZ can be defined as the part of HAZ that experienced peak

temperatures above Ac1 but below Ac3 temperature of steel. In other words, the

temperature boundaries at which intercritical HAZ formed are Ac1 and Ac3 [27], [10].

Supercritical area where no melting occurs but experienced peak temperatures much

above Ac3. This region is in between the intercritical HAZ and FZ. Coarsing normally

occurs in this subpart [10].

If the peak temperature exceeds the melting temperature of steel then this part called

the fusion zone (FZ) [13], [10].

HAZ metallurgical changes primarily depends on solid-state transformation since no

melting occur in this region. On the other hand, FZ experienced two stages of

transformation or metallurgical changes since melting occurs. Thus, the microstructure

depends not only on solid state transformation during cooling down from melting

temperature, but also on the solidification mechanism as the material state converts from

liquid to solid [27].

FZ experienced non-equilibrium solidification mechanism during material state

change from liquid to solid. This is due to the very high cooling rates that resulted from

the high welding speeds and focused heat input. The solidification mechanism of FZ can

be expressed as epitaxial mechanism. Solidification starts at the FZ-HAZ boundaries

and gradually growing toward the weld centreline, while the growth direction of solid

grains depends upon the initial orientation of grains at the boundaries of FZ before

solidification [27] , [10], [45].

In general, FZ microstructure and its constituents in AHSS welded joints depends

mainly on two factors: the cooling rates experienced and the chemistry (carbon content

and other alloying elements) of AHSS [5].

Normally with increasing the welding speed, the time available to complete the

solidification and solid-state transformation decreased. In other words, the cooling rates

is higher with higher welding speeds [27].

AHSS are very sensitive to heat input and it processes high hardenability since the

high alloying content. Thus, in turn results in that the critical cooling rates to form

27

martensite in AHSSs is lower than cooling rates experienced during laser welding. This

is the reason as to why the most common observed microstructure in laser-welded joints

is martensite [4], [42], [10].

Carbon content directly affect the microstructure of FZ. Carbon is directly

proportional to hardenability and it causes shifting in continuous cooling transformation

curves of steel to the right. However, not only the carbon content but also other alloying

additions could affect the microstructure of FZ of welded joints [10].

Carbon content enhance the formation kinetics of martensite while other alloying

additions initiated the retarding kinetics of ferrite and bainite formation. Thus, since the

effect of alloying elements and carbon content present in AHSSs in combination with

high welding speeds the FZ normally is mostly or even fully martensitic [10].

HAZ experienced different peak temperatures from FZ, but also experienced rapid

cooling. This results in microstructural changes within the HAZ as it heated up and

cooled down during welding thermal cycles [5]. Different subparts, normally three

subparts, results with microstructural differences since different boundaries in term of

peak temperatures experienced within the HAZ.

Next to the unaffected base metal, the subcritical HAZ that experienced the lowest

peak temperatures. No austenizing occurs in this subpart since peak temperature is

below Ac1 of the steel being welded [5]. As a result, in this region, lower martensitic

content is present due to the tempering occurs results in new phases. In DP steels with

tensile strength of 800 MPa or more, softening normally occurs in which lower hardness

values than the unaffected base metal could result [10].

Austenite start forming in the intercritical HAZ as the area experienced peak

temperatures between Ac1 and Ac3 and so partially, austenizing occurs. Austenite starts

nucleating at the grain boundaries. However, during cooling down, austenite typically

transform into martensite since the high cooling rates in combination with relatively

high hardenability of AHSS. No changes on ferritic portion normally during cooling

while the volume of fraction of martensite gradually increases with increasing

temperature from Ac1 to Ac3 [27], [10].

28

In the supercritical area, fully austenitic phase formed upon heating above Ac3. In this

subpart, grains growth normally occurs depending on what are the peak temperatures

reached above the Ac3. This results normally in coarser grains of mostly or fully

martensite due to the high cooling rates experienced in combination with high

hardenability [10], [5].

1.8.2 Dimensions of FZ and HAZ

Welding parameters and the type of laser power source used in welding AHSS are the

main factors that define the dimensions and geometries of HAZ and FZ. As shown in

Table 3, a comparison between HAZ and FZ dimensions results from different laser

technologies with different welding parameters used in welding DP980 steel sheets in

butt configuration [10].

Table 3. Comparison between the sizes of weld zone based on different laser welding processes

[10].

Welding

Type

Power

(kW)

Welding

speed

(m/min)

Spot size

(mm)

Thickness of

the

workpiece

(mm)

Average

width of

the

HAZ (μm)

Average

width of the

fusion zone

(μm)

Diode 4 1.6 12×0.5 1.2 4000 3000

Nd-YAG 3 3 0.6 1.17 1000 750

Fiber 6 16 0.6 1.2 250 450

CO2 6 6 -- 1.8 1000 1000

In case of 4 kW diode laser welding, the HAZ and FZ results is relatively wider than

other welding types. This is because of the considerably large spot size of laser beam

produced by diodes, which results in conduction welding mode with comparatively high

heat losses leading to higher heat input and wider affected area. On the other hand, in

case of 6 kW fiber laser welding with comparatively smaller spot size as 0.6 mm

compared to 12×5 mm, which in turn, lead to higher power intensity resulting in keyhole

welding mode. This higher intensity gives a possibility of welding at higher speeds with

lower heat input, which in turn lead to narrower FZ and HAZ [10].

29

1.9 Hardness of LBW joints and formation of soft zone

Hardness profiles of laser-welded joints are basically determined by the local

corresponding microstructure [10]. The parameters that affect the microstructure of

welded joints will in turn affect the corresponding hardness profiles.

In general, DP steels consists of martensite, ferrite and possibly other phases such as

bainite or retained austenite [2]. Martensite especially represents an important issue

when welding DP steels due to the formation process and kinetics of martensite.

Martensite typically formed via diffusionless transformation when cooling rapidly from

austenitic phase [66]. As a result, the so-called martensite is thermal unstable

microstructure and supersaturated with carbon [66]. Carbon content in martensite

exceeds the solubility limits and equals to carbon content in the austenite. Rapid cooling

prevents or minimizes the fractions of austenite that transform into stable phases such

as ferrite at higher temperatures than martensitic start temperature. However, martensite

presents a very hard microstructure. The transformation process can be fully or partially

martensitic depending on the cooling rates with the corresponding carbon content [67].

During welding, microstructure will be modified through the thermal cycle into new

microstructure. Heating martensite represents a critical issue since it is thermal unstable

supersaturated diffusionless microstructure. Martensite will tent to decompose and the

excess carbon content will form stable phase such as carbides and ferrite as predicted

from the phase diagram [67]. This process is well known as martensite tempering. Fig.

12 shows an example of lathe martensite and the corresponding tempered

microstructure. However, martensite tempering depends basically on peak temperatures

and time. Fig. 13 shows typical Fe-C martensite decomposition summary. It is clear that

tempering is function of temperature and time.

30

Figure 12. Micrographs of martensite and tempered martensite in a 0.12 wt% C steel [66].

Figure 13. Summary of Fe-C martensite tempering processes as a function of temperature during a

1-hour heat treatment [66].

Tempering of HAZ in DP steels represents a critical issue since DP steels

microstructure contain sufficient fraction of martensite. As a result, tempering will

modify the microstructure during welding and in turn modify the hardness profile of

welded joints especially in the outer HAZ or the so-called subcritical HAZ [10]. This

phenomenon has been long recognized as HAZ softening. In the 1970𝑠 was the first

failures explained by the HAZ tempering of DP steels when DP steels for wheel rim

applications were proposed to flash welding and later failures in the outer HAZ where

the softening normally occurs and the process was named HAZ softening [66].

It is clear now that HAZ softening is unavoidable in DP steels and normally occurs

in the outer HAZ where the peak temperatures are at Ac1 temperature or below where it

is only sufficient for tempering [42], [68]. However, martensite decomposition in the

softened area will not only modify the microstructure, but other consequences will

appear. For example, yield and tensile strength will decrease, and ductility may

31

increases. Strain localization also may results causing failures at lower stress levels than

the base material [68].

Since HAZ can affect the weld integrity and adversely affect the weld performance,

much work has been done to characterize this phenomenon. Softening is hot topic in

welding DP steels and has been long recognized in research work and practical

applications.

Sreenivasan et al [69] studied the effect of diode laser welding on DP980 and HSLA

hardness profiles. DP980 butt laser welded joints showed a combination of high levels

of hardening and local softening. DP980 showed a relatively flat profile in the FZ with

peak hardness values. They return the high hardness values in FZ to the formation of

fully or mostly martensitic microstructure as the hardness values were very close to the

expected martensite hardness. However, hardness was progressively decreasing when

moving outward from the FZ due to progressive decrease in peak temperatures, which

results in microstructural changes. Further, formation of hardness valleys in the outer

portion of HAZ or the so-called subcritical HAZ appears. They return this to the

decomposition of local martensite and formation of carbide, in other words, due to

tempering of martensite. On the other hand, HSLA showed lower hardness values

without formation of soft zone, but smoothly decrease in hardness from flat FZ toward

the un-affected base material. Fig. 14 shows the hardness profiles of DP980 and HSLA

at the same welding parameters.

Figure 14. Hardness profiles of DP980 and HSLA at welding speed of 1.0 m/min [69].

32

In addition, Sreenivasan et al [69] compared the effect of welding speed on hardness

profiles DP980 steels. There was no significant effect on FZ while the width of soft zone

and its location has been significantly modified by changing welding speed. They

concluded that the higher welding speed results in smaller degree of softening and return

this to the smaller heat input to the material and shorter thermal cycle. Fig. 15 shows the

effect of welding speed on hardness profiles of HSLA and DP980 steels.

Figure 15. Hardness profiles of DP980 and HSLA welds with different welding speeds [69].

Santaline et al [70]. studied the effect of carbon content in diode laser welded joints

of similar and dissimilar combinations of DP600-980, TRIP780 and HSLA steels on FZ

hardness. They observed a nearly linear relation between the carbon content and

hardness values in FZ while equivalent carbon content that represents alloying additions

fits more linearly to FZ hardness as illustrated in Fig. 16. The decrease in calculated

hardness (expected fully martensitic hardness) in the FZ was explained by decrease in

carbon content and alloying additions in corresponded steel as shown in Fig. 17.

33

Figure 16. Variation of FZ hardness with carbon content and carbon equivalent in laser welds with

both similar and dissimilar combination [70].

Figure 17. Variation of FZ hardness as function of carbon content [70].

Xia et al [71] studied the effect of heat input, welding speed, and martensite content

on softening occurs in DP laser welded joints. Several DP450, DP600 and DP980 steels

has been welded with two laser sources: Nd YAG and diode laser.

Xia et al [71] indicates that laser source type and material characteristics both affect

the weld hardness profile. They observed that Nd: YAG produces harder welds with no

softening in DP450 and DP600, but only in DP980. On the other hand, softening

occurred in all diode laser welds. They return this to that diode laser produces higher

linear heat input. Fig. 18 shows the hardness profiles for diode laser welds. Further, it

was clear that DP steels with higher strength have higher hardness in welds and base

34

metal. They return this to the high alloying additions in higher strength DP steels, which

increases the hardenability and ability to form martensite when welding under same

conditions.

Figure 18. Hardness profiles across welds with diode laser welding at speed of 1.3 m/min [71].

Xia et al [71] noted that the maximum HAZ softening occurs at Ac1 temperature or

just below it. This conclusion helped them in studying the relation between heat input

and degree of softening by calculating time constant that represents the time required to

reach Ac1 by the base metal and then modelling the relation graphically by Rosenthal

formulation. However, Ashby and Easterling as well as Ion et al have successfully used

Rosenthal formulation to model grain growth and martensite development. Fig. 19

shows the results of modelling time constant vs the degree of softening.

Figure 19. HAZ softening of DP welds made with different heat inputs resulting in different time

constants at the Ac1 [71].

35

Xia et al [71] concluded that reducing heat input would shorten the thermal cycle in

which time constant is lower and so there is no sufficient time to fully tempering the

martensite. Thus, increasing the welding speed will reduce the heat input and further

reduce the degree of softening. In order to understand the role of martensite in material

softening, they made several diode welds for steels with different M fraction. It was

clear that with increasing martensite content, the degree of softening increases. In

addition, in order to isolate the martensite effect, they divided the degree of softening

over the martensitic fracture. They observed that, in all steels, the maximum degree of

martensite hardness change was the same.

Figure 20. Maximum amount of HAZ softening at Ac1 versus martensite volume fraction [71].

Figure 21. Martensite softening of studied DP alloys with respect to heat input [71].

36

Biro et al [68] studied the effect of heat input and steel chemistry on the softening

kinetics of DP steels during laser welding process. They used three different laser-

welding sources with different welding heat power: 4 kW Diode, 8 kW CO2, 1 kW Yb

fiber laser to weld four different steels: two DP600s and two DP780s that are different

in chemistry (alloying additions and carbon content) and martensitic fractions. It was

possible to ignore extra parameters effects (beam diameter, material absorptivity, and

welder efficiency) by using Rosenthal’s two-dimensional. Heat input was given a

physical meaning by normalizing it using time constant. Xia et al [71] previously used

Rosenthal’s equation while the time constant is similar to Ion et al study.

Biro et al [68] found that HAZ minimum hardness is directly proportional to heat

input until an extent when no further decrease in HAZ hardness with increasing heat

input. However, HAZ softening was found to be function martensite tempering by

cementite forming. With increasing the heat input, the cementite particles became larger

with higher percentage as shown in Fig. 22. In terms of martensite C content, they found

that in steels with the same amount of carbide alloying forming elements, HAZ softening

requires lower heat input with increasing C content in martensite.

Figure 22. TEM images of cementite particles found in the subcritical HAZ of the lean chemistry

DP780 welded with low and high heat input [68].

37

In DP780 without Cr and Mo added, softening level is higher at lower heat input

compared to DP780 with alloying additions. This is due to the absence of carbide

forming alloying elements, which stabilized the martensite, and slowing tempering

reaction. Fig. 23 shows how Cr and Mo affect the softening-heat input relation in DP780.

Note that weld heat input increases with increasing time constant.

Figure 23. Minimum HAZ hardness vs time constant for DP780 materials [68].

Nayak et al [72] studied the alloying additions effect on severity of softening degree

in Dual phase steels. They found that with decreasing alloying additions, the degree of

softening decreases. They attributed this to the higher level of martensite decomposition

in DP steels with lower alloying additions, in other words, the stability of martensite is

lower since the lower equivalent carbon content. Fig. 24 compared the degree of

softening of different DP alloying levels. In Fig. 24, L represents the lean chemistry with

low alloying additions (e.g. manganese, chromium and silicon), M medium alloying

content, and R rich DP steel with carbon content. In M steel, the concentrations of

alloying additions are between that of lean and rich steel

38

Figure 24. Effects of chemistry on softening of the BM and nonisothermally tempered DP steels

[72].

In a recent study, Sisodia et al [64] compared the hardness profiles of DP800 and

DP1200 welded with 4KW diode laser. They observed a nearly flat FZ hardness in both

steels with higher values in DP800 than the base metal with 100 to 40 HV differences

while FZ of DP1200 showed lower hardness values than the base metal with average

between 20 to 50 HV. In both steels, the maximum hardness obtained in the inner part

of HAZ with smoothly decrease in DP800 toward the base metal without significant

softening while in DP1200 softening occurred in the outer HAZ or subcritical HAZ.

Sisodia et al [64] also studied the effect of laser post-weld heat treatment (PWHT)

on same welded joints. PWHT significantly reduced the hardness peaks in HAZ for both

steels. In DP800, FZ showed nearly the same hardness of base metal without notable

softening in HAZ. On the other hand, the degree of softening increased in DP1200 in

both the FZ and the outer HAZ. Fig. 25 shows the corresponding hardness profiles.

Figure 25. Effect of PWHT on hardness profile of a. LBW DP800 b. LBW DP1200 [64].

39

1.10 Effect of post-weld heat treatment

DP steels chemical composition has been designed in such a way that martensite will be

present in significant fraction in combination with ferrite [73]. As a result, DP steels

welds are likely to be martensitic. In addition, especially in laser welding, cooling rates

experienced are very high which led to mostly or fully martensitic. Increasing cooling

rates will fundamentally decrease the diffusion rates whilst driving forces for martensitic

transformation increases. Martensite formation does not require carbon to diffuse in

combination with high equivalent carbon content will lead to no deformation of

diffusional phase such as ferrite or even pearlite. As a result, especially in FZ and inner

HAZ, sever hardening will occur [73].

In weldability assessment, chemistry and carbon levels in combination with cooling

rates define the weldability of steel. Thus, affecting the resultant weld performance [24].

This is the core of the problem in laser welding of DP steels for automotive steel

industry. Carbon concentrations is increased in DP steels to get higher strength

properties in combination with high cooling rate experienced during laser welding, will

results in hard welds. As there is no enough time for carbon to diffuse into stable phases

(ferrite or even bainite) [73]. Further, the DP steels laser welds will tend to be very hard

welds with martensitic microstructure. As a result, the hardness values will be

unacceptable and the welds will tend to brittle behaviour. In general, 450HV is the

acceptable hardness level (martensitic hardness) which is not easy to achieve without

any post process or modification on welding process itself [73].

Post weld properties, mainly, can be modified by two methods. The first option is

decreasing the cooling rates in order to reduce martensite formation and produce as soft

welds as possible. This process affects not only the hardness but also may lead to wider

welds and wider HAZ softened area. In addition, it affects the productivity since it

requires more time until welding completion. The second option is by applying post

weld processes [73].

Post weld heat treatment (PWHT) is the preferred option. This process can be applied

to welded joint after completing welding process [74]. This process can reduce the weld

40

tendency to brittle fracture (get lower hardness values) and improve microstructure

homogenously within the weld zone. In addition, PWHT can enhance the mechanical

properties of joints (fatigue properties, toughness, etc.). However, PWHT needs more

time to complete fabrication process [73].

Farabi et al [7] observed sever hardening in the FZ and inner part of HAZ of DP980

diode laser welds. In order to get rid of this hardness peaks, they tried four heat treatment

processes with peak temperatures (750, 780, 810, 840 ℃) in atmospheric controlled

furnace for 15 minutes and then water quenching as shown in Fig. 26. However, this

procedure is more like intercritical annealing followed by quenching in producing DP

steels. They studied the enhancement of this four-peak temperatures PWHT

enhancement on DP980 welded joints. They studied the effect on hardness profile,

tensile properties, fatigue life and fracture mechanism.

Figure 26. PWHT thermal cycles.

Farabi et al [7] observed significant improvement in hardness values that were mostly

flat within the weld zone. The flatness of weld hardness was attributed to the improved

homogeneity in the microstructure. This was supported with heat-treated weld

microstructure observation. It was clear that the hardness increased with increasing peak

temperature during heat treatment. The optimum values in terms of hardness was 750 to

780 ℃ where the hardness were totally recovered to nearly the base metal hardness. Fig.

27 shows the hardness profile of treated joints compared to the simple laser welded joint.

41

Figure 27. Effect of PWHT on hardness profile of diode laser D980 welds.

Farabi et al [7]. observed smooth and continuous stress-strain curves for PWHT at

750 ℃, PWHT at 810 ℃, DP980 base metal and DP980 welded joint. As shown in Fig.

28, DP980 PWHT at 810 ℃ has high tensile properties but ductility has been sufficiently

decreased. In terms of fatigue performance, PWHT improve the fatigue life in case of

810 ℃, while no significant effect in case of 750 ℃.

Figure 28. Effect of LBW and PWHT on Stress vs Strain curve of DP980 base metal.

42

In a recent study, Sisodia et al [64] used laserline LDF 5000-40 diode laser in

applying post heat treatment on DP800 and DP1200 in order to study the change in their

microstructure and mechanical performance (tensile, hardness, bending properties). The

PWHT was done at 275 W output power at 4 mm/s welding speed. There was no

significant effect in terms of tensile properties since the tensile test result showed a very

slight difference compared to as weld joints. In addition, all tested samples fractures at

the same fracture location (weld zone). Further, there was no visible fractures after

bending tests, which can be an indication for good formability with low effect of weld

imperfection on the joint crack tendency. The significant change was in hardness profile.

In both DP800 and DP1200, PWHT reduced the hardness peaks. However, welds tend

to be soft welds after applying PWHT as shown in Fig. 29.

Figure 29. Effect of PWHT on hardness profile of a. LBW DP800 b. LBW DP1200.

1.11 Some studies on mechanical properties of laser welded joints

Hartley and Ono [60] conducted a research work on the laser weldability of different

of different Dual-Phase steel grades from 270 to 980 MPa ultimate tensile strength.

thicknesses range from 1 to 1.6 mm. The results showed that DP-steel laser welded

joints has a good soundness and weld geometry, and can achieve good levels in both

formability and fatigue performance near the base metal values.

Uchihein & Fukui [75] compared the effect of different welding technologies on the

formability and fatigue properties of different HSS grades that range from (270 to 780

MPa) and thicknesses were 1, 1.4 & 1.6 mm. The results showed that the laser welding

is the most appropriate welding procedure in terms of both the fatigue and formability.

43

Galtier & Dutch [76] studied the fatigue behaviour of different high strength steel

including DP-600, DP-450, Trip 800 etc., using different joining techniques including

spot welding, laser welding, and clinching. They studied a linear laser butt welded joints

of thin sheets with (0.7 to 1.2 mm) thicknesses. The comparison between the fatigue

limits for those welded materials showed no difference compared to the base material,

and the lower obtained values referred to the effect of size and shape. The research

showed a significant effect of the misalignment on the fatigue limit.

Farabi et al [3] studied the fatigue properties of diode laser welded dual-phase steel

joints. The study was made on DP-600 and DP-980 with 1.2 mm and showed that the

degree of softening in DP-980 was higher than in DP-600. The higher degree of

softening causes a significant decrease in the fatigue limit for welded DP-980 but still

higher than both DP-600 base metal and welded joint.

1.12 Quality control

1.12.1 Welding procedure specifications (WPS)

WPS have two main purposes, the first is providing the basis for planning of the welding

operations and the second one is to ensure quality control during welding [77].

In terms of quality control, welding is a special process which must be carried out in

accordance with written procedure specification in order to fulfil the quality requirement

during and after welding. However, following the welding procedure specification does

not mean necessarily that the welds fulfil the quality required. This is due to the

metallurgical deviations and imperfections or distortion which represents a special

problem resulted in the development of qualification of welding procedure prior to the

final specifications to actual procedure. general rules can be found in EN ISO

15607:2003.

WPS are considered as preliminary welding procedure specification until being

qualified by one of the standardized methods of qualifications. These methods include

the welding procedure test. More methods are mentioned in EN ISO 15607:2003.

44

The format for WPS for electron beam welding and laser beam welding can be found

in prEN ISO 15609-3 and prEN ISO 15609-4 respectively.

1.12.2 WPS for laser beam welding

WPS covers all technical parameters and information needed for complete laser

welding. WPS includes different sections starts with the manufacture, identification of

any equipment used and its relevant parameters such as beam delivery and focusing

system, gas system and the welding equipment itself, base materials details, filler

material or any additional material, joint design and surface preparation, the tools,

welding position, backing, welding technique sketch, welding parameters such as the

beam parameters, mechanical parameters such as travel speed and other details. More

details and format example can be obtained from EN ISO 15609-4:2009 [78].

1.12.3 Welding procedure test

The shape and dimensions, components and geometry, rolling direction shall be

specified and done according to EN ISO 15614-11:2002. Fig. 30 and Fig. 31 show the

test piece for linear butt joint and location of test specimens for a linear butt weld

respectively [79].

Figure 30. Test piece for linear butt joint [79].

Key

1 Edge preparation and fit-up as detailed in the preliminary Welding Procedure

Specification (pWPS)

45

a = 3 x t; minimum value 150 mm.

b = 6 x t; minimum value 300 mm.

t = thickness of the thinner material in a dissimilar thickness joint.

Figure 31. Location of test specimens for a linear butt-weld [79].

Key

(1) Area 1 for 1 tensile specimen; 1 root and 1 face bend specimen or 2 side bend

specimens.

(2) Area 2 for impact and additional test specimens if required.

(3) Area 3 for 1 tensile specimen; 1 root and 1 face bend specimen or 2 side bend

specimens.

(4) Area 4 for 1 metallographic specimen/hardness (if required).

(5) Discard: if t ≤ 25 mm: discard 25 mm, if t > 25 mm: discard 50 mm minimum.

(6) Welding direction.

46

2. MATERIALS AND EXPERIMENTAL PROCEDURE

This chapter includes the materials used in this thesis work, welding related information,

sample preparation, and describes the experimental procedure for all tests including

destructive test in order to get mechanical properties evaluation and non-destructive to

evaluate metallurgical changes and properties.

2.1 Material and composition

The selected material in this study is DP1000 commercial uncoated cold-rolled

continuous annealed steel sheet supplied by Swedish steel company (SSAB). The steel

sheet thickness is 1mm and the steel sheet was cut with rolling direction.

Material supplier (SSAB) provides the chemical composition and mechanical

properties of the selected steel and data are shown in Table 4 and Table 5, respectively.

The equivalent carbon content can be calculated using the formula provided by SSAB

in the material inspection certificate in the following equation. Material specification is

attached in Appendix A.

𝐶𝑒𝑞 = 𝐶 +𝑀𝑛

6+

𝑁𝑖 + 𝐶𝑢

15+

𝐶𝑟 + 𝑀𝑜 + 𝑉

5= 0.39

Table 4. Chemical compositions of the investigated base material (DP1000).

Element C Si Mn P S N Cr Ni Cu V B Nb

Weight

percentage.

(wt%)

0.123 0.19 1.5 0.01 0.003 0.004 0.03 0.03 0.01 0.01 0.0002 0.014

47

Table 5. Mechanical properties of DP100.

𝐑𝐏𝟎.𝟐, MPa 𝐑𝐦, MPa 𝐀𝟖𝟎%

821 1074 9.5

2.2 Welding and Post-Weld Heat Treatment

2.2.1 Preparation for welding

Two pieces of 300 mm × 150 mm (Fig. 32) were cut from DP1000 steel sheet of 1250

mm × 2000 mm according to EN 15614-11:2002. The edges of the specimens were

cleaned with emery paper and acetone to remove surface oxides before welding.

Clamping devices was used to fix the work pieces before welding to protect it from

misalignment and distortions, which affects the weld properties as shown in Fig. 33a.

However, spot welding by laser is used to fix the two pieces together against

misalignment as shown in Fig. 33b. pWPS is prepared according to EN ISO 15609: 4:

2009 and attached in Appendix B.

Figure 32. Dimensions of specimens to be welded.

48

Figure 33. a Clamping Device on Worktable. b Spot welding against misalignment.

2.2.2 Laser welding set up

The laser welding was done using a direct diode laser head LDL 160-3000 Mounted on

Reis SRV 40 Robotic Fig. 34b. The working stretch of the robotic arm was 3800 mm ×

3800 mm. The laser beam was characterized by rectangular spot size of 2 mm × 2 mm.

the laser was emitted in a continuous wave mode at 940-980 nm wavelength with a

maximum output power of 5 kW.

All welds achieved full penetration to ensure uniformity and stability of the HAZ

properties through the thickness of the welded joint. In addition, the welding was a

conduction-welding mode with no keyhole formation.

The specimens were placed on working table and fixed by clamping device in order

to protect the specimens to be welded against distortion. The laser welding experimental

set up is shown in Fig. 34a.

During autogenous laser welding of butt-welded joints (Fig. 35) , ultra-high purity

argon gas (Argon 4.6, 99.996%) was used as a shielding gas with a flow rate of 7 l/min.

welding was conducted at a welding speed of 8 mm/s in full penetration bead on plate

for uniform thickness butt joints. The welding parameters used in this study are

summarized in Table 6.

49

Table 6. Laser beam welding experimental set up.

Laser system Laser source Laser Power

(KW) Welding speed (mm/s) Beam dimensions (mm)

LDL 160-3000 Diode 1 KW 8 2*2

Figure 34. a Laser head and robotic arm. b LBW experimental set up.

Figure 35. LBW butt-welded joint. a Face side. b Root side.

2.2.3 Post weld heat treatment (PWHT)

The welded joints had cooled down to room temperature and then half of it had been

subjected to PWHT (Fig. 36) which was carried out using LDF 5000-40 laser head

mounted on a robotic arm Reis SRV 40 (Fig. 37). The maximum power of such a diode

50

laser is 5 kW. The beam has a rectangular shape with dimensions of 15 mm × 6.5 mm.

PWHT was conducted at half of welding speed with 4 mm/s. Ultra-high purity argon

was used as a shielding gas with 7 l/min. PWHT parameters are summarized in table 7.

Figure 36. A welded and treated specimen before cut. b PWHT face side. c PWHT root side.

Table 7. PWHT parameters.

Laser system Laser source Laser Power

(KW) Welding speed (mm/s)

Beam dimensions

(mm)

LDF 5000-40 Diode 0.275 KW 4 15×6.5

Figure 37. a Control display. b PWHT experimental set up.

51

2.3 Weld characteristics and metallurgical examination

2.3.1 Sample preparation

Metallographic examination and hardness measurements were used to analyse the weld

microstructure and profile characterization. Five thin sections were cut from laser

welded and laser weld post treated specimens as shown in Fig. 38. The sections were

cut from the specimens, mounted, polished and etched using Nital (HNO3). Different

silicon carbide paper was used in grounding as per metallographic standard.

Figure 38. Location for thin sections used for microstructural analysis. a. LBW joint. b. PWHT

joint.

2.3.2 Optical microscopy

The microstructure of laser welded and post weld heat treated specimens was observed

by optical microscope. The optical microscope Axio observer D1m Zeiss, connected to

computer with image analysis software, as shown in Fig. 39 was used for microstructural

analysis at 200x magnification.

The macro samples can be divided into three main regions: base metal, weld zone

and heat affected zone that can be divided into three sub-regions: coarse grained HAZ,

fine-grained HAZ and intercritical HAZ as illustrated in Fig. 38 above.

52

Figure 39. Microscopic experimental set-up.

2.3.3 Micro hardness tests

Vickers micro hardness tests were achieved using MVK-H1 Mitutoyo hardness tester

(Fig. 40). In order to study the effect of laser welding and the effect of post heat

treatment on the properties of the joint, tests were done on unetched two samples:

welded specimen and post welded specimen. The dwell time and load were 15 s and 200

gm, respectively. The centre point was accurately determined by observing weld

geometry under microscopic. 0.1 mm was the distance between each two indentation to

avoid any potential effect of strain field by precious indentation (Fig. 41, 42).

Figure 40. Microhardness tester (MVK-H1 Mitutoyo).

53

Figure 41. LBW specimen.

Figure 42. PWHT specimen.

2.4 Tensile testing

Tensile tests were carried out at room temperature using MTS 810.23-250 kN electric

hydraulic testing equipment. Tensile specimens were milled from butt-welded sheet

according to EN ISO 15614-11: 2002. The geometry and dimensions of test specimens

are shown in Fig. 43 and table 8. Tensile test were performed on three laser welded

samples and three post weld heated samples in order to study the effect of post weld heat

treatment using laser on the enhancing the mechanical properties. Tensile test machine

set up is shown in Fig. 44.

54

Figure 43. Tensile test specimen.

Table 8. Dimensions in mm.

Symbol Value

A 100

B 50

C 24

W 12

R 25

T 1

L 232.5

Figure 44. MTS 810.23-250 KN electric hydraulic testing equipment.

55

2.5 Bending test

The presence of weld imperfections in the area of face and root side affect the tendency

of laser welded and laser weld heat-treated joints to crack during deformation. The

ability to withstand the deformation against crack initiation was studied and tested using

three-point bend test according to EN 5137: 2010. The bend test was carried out by

ZD20 hydraulic testing equipment. Schematic diagram of three point bend test set up is

shown in Fig. 45. The indenter diameter was 10 mm, fixed support roller diameter is 50

mm and the distance between the rollers (span distance) is 70 mm.

Figure 45. Schematic diagram of three point bend test set up.

56

3. RESULTS AND DISCUSSION

3.1 Microhardness profile

The micro-indentation hardness profiles of laser welded DP1000 and PWHT DP1000

specimens are shown in Fig. 46 and Fig. 47, while Fig. 48 shows a comparison between

both. The average values of hardness for each zone are calculated in Table 9.

Significantly higher hardness values approximately 1.5 times higher than the base

metal observed in the FZ due to formation of mostly martensite microstructure with

maximum hardness of 421 HV. As seen in Fig. 46 a continuous decrease of hardness

outward from FZ until the BM. The lowest values were present in the outer portion of

HAZ or the subcritical area of HAZ, which were called the soft zone. The existence of

such a soft zone was mainly due to decomposition and tempering of pre-existed

martensite during welding.

The severity of soft zone decreases the tensile strength and mechanical properties

compared to the BM. To eliminate and reduce the effect of soft zone and enhance the

toughness of welded joint, PWHT is a possible way. PWHT enhanced the hardness

profile and reduced the effect of soft zone. Fig. 48 compares the effect of PWHT and

laser welded joint.

Table 9. Average HV.

LBW BM HAZ FZ HAZ BM

average hardness 253 319 367 339 239

PWHT BM HAZ FZ HAZ BM

average hardness 226 253 271 269 228

57

Figure 46. Hardness profile on LBW.

Figure 47. Hardness profile on PWHT.

200

230

260

290

320

350

380

410

440

470

-3.6 -3.1 -2.6 -2.1 -1.6 -1.1 -0.6 -0.1 0.4 0.9 1.4 1.9 2.4 2.9 3.4

Har

dn

ess

HV

0.2

Distance from weld centerline, mm

Hardness Profile

LBW

BMBM HAZ HAZFZ

soft zone

soft zone

180

200

220

240

260

280

300

320

-3.6 -3.1 -2.6 -2.1 -1.6 -1.1 -0.6 -0.1 0.4 0.9 1.4 1.9 2.4 2.9 3.4

Hardness Profile

PWHT

BM HAZ FZ HAZ BM

58

Figure 48. Comparison of LBW and PWHT.

3.2 Microstructural analysis

In this section, microstructural analysis for the base material DP1000, effect of diode

laser welding on microstructure of DP1000 and the effect of post weld heat treatment

(PWHT) are discussed. Optical microscope at magnification level of 200x was used in

this Microstructural analysis.

3.2.1 Base Material

The base metal DP1000 clearly consists of two micro-constituents. In the base

material, Martensite islands appears in black while ferrite phase appears in white. Fig.

49 shows the morphology and micro-constituents of DP1000.

The microstructure can be described as a network like microstructure since the high

Martensitic to ferritic ratio (45 % to 55 %) which means that almost ferrite is connected

with martensite forming. However, there is a possibility for bainite and other micro-

constituents to appear depend upon cooling rates, but at this level of magnification (200

x), we can distinguish between two main micro-constituents as shown in Fig. 49.

100

150

200

250

300

350

400

450

-4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

Hard

nes

s H

V0.2

Distance from weld centerline, mm

Hardness Profile

PWHT

LBW

59

Figure 49. OM micrograph of DP1000 Base material.

3.2.2 Laser Welded Joint

3.2.2.1 Fusion zone (FZ)

In this region and at this level of magnification (200x), the clearest micro-constituent is

martensite upon high cooling rate which is a good explanation of high hardness values

at this region. The morphology of FZ in LBW joint is shown in Fig. 50. Martensite

grains are lined up due to quick solidification in the growth direction. Martensite lathe

seem to be very thin in this region.

Figure 50. OM micrograph of FZ in LBW DP100 at 200X Magnification.

60

3.2.2.2 Heat affected zone (HAZ)

The HAZ can be divided into three main sub-regions in order to study the gradual

variation in the microstructure from FZ forward the unaffected base material, which

depends on peak temperatures and the response of ferrite and martensite in these critical

regions. The sub-regions as mentioned before are coarse-grained HAZ (CGHAZ), fine-

grained HAZ (FGHAZ) and intercritical HAZ (ICHAZ) as shown in Fig. 51.

Figure 51. LBW sup-regions for microstructural analysis where A: FZ, B: CGHAZ, C: FGHAZ,

D: IVHAZ and E: BM.

3.2.2.3 Coarse-grained HAZ (CGHAZ)

In CGHAZ martensite is present in this sub-region but it is distinguished from the

martensite in FZ since the martensite in CGHAZ is longer and coarser than that in FZ.

This explains the high hardness values. However, other micro-constituents can be

distinguished at this level of magnification such as retained austenite. However, there

could be some ferrite or bainite caused by cooling rates and response of microstructure.

The boundary between CGHAZ and FZ, and the CGHAZ morphology and that shows

the difference between the two regions are shown in Fig. 52a and Fig. 52b, respectively.

61

Figure 52. OM Micrographs of a. Boundary FZ to HAZ. b. CGHAZ. At 200X magnification.

3.2.2.4 Fine-grained HAZ (FGHAZ)

Fine and homogenous martensite grains are present in this sub-region since phase

changes and recrystallization takes place at this stage. However, in this zone, during

cooling austenite starts to transform into fine martensite, bainite or ferrite. This explains

the gradual and sharp decrease in hardness values further from the CGHAZ as the

tempering increased toward the base material. Fig. 53a, 53b show the morphology of

FGHAZ microstructure and boundary between FGHAZ and CGHAZ, respectively.

Figure 53. OM Micrographs of a. FGHAZ. b. CG to FGHAZ boundary of LBW joint. At 200x

magnification.

62

3.2.2.5 Intercritical HAZ (ICHAZ)

Regarding the ICHAZ, there is a change in ferritic to martensitic ratio from the base

material. The martensite content is lower than the base material and this could be due to

tempering of the original martensite, which caused the softening in this sub-region.

Ferrite, bainite and other carbides could be formed in this sub-region due to tempering.

Fig. 54a and 54b show the morphology of ICHAZ and the boundary between ICHAZ

and the base material.

Figure 54. OM Micrographs of a. ICHAZ. b. ICHAZ to Base material boundary of LBW joint. At

200x magnification.

3.2.3 Effect of PWHT on LBW joints

Applying heat treatment by laser beam caused more existence of tempered martensite,

which caused the martensitic to ferritic ratio to decrease. This happens especially in the

CGHAZ and ICHAZ that seems to be finer. However, this change is in good agreement

with the gradual decrease in the hardness values. The microstructure of FZ, HAZ are

more homogenous which is good explanation for the more steady values of hardness

values.

The sub-regions of PWHT specimens are shown in Fig. 55, while the morphology of

FZ and HAZ of PWHT joint and the boundary between Sub-regions are shown in Fig.

56 and Fig. 57, respectively.

63

Figure 55. PWHT specimen's sup-regions for microstructural analysis.

Figure 56. OM micrograph of a. FZ, b. CGHAZ, c. FGHAZ, d. ICHAZ in LBW DP100 at 200x

magnification.

64

Figure 57. OM micrograph of boundary between a. FZ & CGHAZ, b. CGHAZ & FGHAZ, c.

FGHAZ & Base material, d. Base material in LBW DP100 at 200x magnification.

3.3 Tensile testing

In order to study the effect of both laser welding and laser heat treatment on the tensile

properties, Transverse tensile tests (weld perpendicular to load direction) were done on

six specimens cut from the same welded steel sheet, three are laser welded and three

with laser post weld heat treatment. In this section, the tensile properties and fracture

location related to softening degree, and the effect of post weld heat treatment are

discussed.

3.3.1 Tensile properties of LBW DP1000 steel

The ultimate tensile strength (UTS) of DP1000 welded joints was observed to be

significantly lower than that for base metal. The average UTS of the three specimens

65

was around 800 MPa, which is 20 % lower than that for base metal. Experiment results

of the tensile properties of LBW DP1000 joins are present in Table 10.

Table 10. Tensile properties of LBW DP1000.

The hardness values and tensile properties are usually highly connected in steels.

Within the given welding parameters, which resulted in 30HV, degree of softening

caused a significant decrease in tensile strength.

3.3.2 Fracture location of LBW joints

All LBW specimens were fractured in the region between the weld centreline and the

inner part of heat-affected zone with marginally differences in the fracture location in

the tested samples as shown in Fig. 58 and Fig. 59.

The changes in hardness profile along the weld zone and HAZ along with the degree

of softening affects the location of fracture. Since the majority of the plastic deformation

was accumulated in the weld zone until final fracture, there is a reduction in total

elongation compared to the base metal along with the reduction in tensile strength.

Figure 58. OM micrographs of fracture location of DP1000 LBW joints.

Sample No

Laser Power

(W)

Speed (mm/s)

Force Tensile

Strength Fracture Location Delta t (s)

Strain rate 1/s

1F1 1000 8 9962.631 819.969621 Weld Metal 0.25 0.002

1F2 1000 8 9729.995 824.575856 Weld Metal 0.25 0.002

1F3 1000 8 9161.328 763.444008 Weld Metal 0.25 0.002

66

Figure 59. Tensile tested LBW joints.

3.3.3 Effect of post weld heat treatment on tensile properties

The average UTS of the tested PWHT joints was 715 MPa, which is 11 % less than

average UTS of the tested LBW joints and 30 % lower than that for the base metal. The

tensile properties and welding parameters are summarized in Table 11.

Table 11. Tensile properties of PWHT DP1000.

Sample

No

Laser

Power

(W)

Speed

(mm/s) Force (N)

Tensile

Strength

Fracture

Location

Delta t

(s)

Strain

rate 1/s

1FP1 275 4 8817 705 Weld Metal 0.25 0.002

1FP2 275 4 8677 714 Weld Metal 0.25 0.002

1FP3 275 4 8808 728 Weld Metal 0.25 0.002

As the tensile properties are usually highly related to hardness in steel, the decrease

in tensile strength is in good agreement with the decrease in hardness values. PWHT

cause a decrease in UTS and HV.

In terms of fraction location, PWHT has marginally effect on the location of fracture.

In case of PWHT joint, the fracture is closer to weld centreline. The fracture location,

most of plastic deformation and crack initiated is concentrated within the weld centreline

and the fusion zone as shown in Fig. 60 and Fig. 61. In addition, the fracture mode is

ductile fracture as well.

67

We can conclude that the PWHT has no significant enhancement on the welded joints,

which is slightly different in fracture location and tensile properties as well.

Figure 60. OM micrographs of fracture location of DP1000 PWHT joints.

Figure 61. Tensile tested LBW Specimens.

3.4 Bending tests

The results of bending tests showed good plastic properties without any cracks for both

LBW and PWHT joints of DP1000 steel. The four tested specimens showed no cracks

during the test and after it, which indicates good plastic properties.

The test results indicates that the presence of weld imperfections has no significant

effect on the plastic properties of welded and heat-treated joints.

The bending test results are summarized in Table 12 while the tested specimens of

LBW and PWHT joints of DP1000 are shown in Fig. 62-64.

68

Table 12. Bending properties of LBW and PWHT DP1000 Joints.

Sample No Process Bending angle Laser Power

(W) Speed Final result

1F1 Face LBW 143 1000 8 No cracks

1F2 Root LBW 143 1000 8 No cracks

1FP1 Face PWHT 137 275 4 No cracks

1FP2 Root PWHT 136 275 4 No cracks

Figure 62. Bend tested samples with no cracks.

Figure 63. Bend tested specimen with Bending angles

69

Figure 64. OM micrographs of Bend location of DP1000 LBW and PWHT joints.

70

4. CONCLUSIONS

The following conclusions could be drawn based on the discussion on the LBW and

PWHT joints:

1. The laser welding of DP1000 steel results in significant hardening in FZ and

inner part of HAZ while sever softening occurred in the outer HAZ.

2. The sever hardening in FZ resulted from formation of mostly martensitic FZ

due to the rapid cooling experienced during laser welding in combination with

the chemistry of DP1000 steels which has been designed in such a way to

produce high fraction of martensite up to 50%.

3. HAZ softening occurs in the subcritical HAZ can be attributed to the

tempering of original martensite as the peak temperatures experienced were

below the Ac1 of the steel and only sufficient to produce tempered martensite.

As a result, lower martensite content caused lower hardness values.

4. The average ultimate tensile strength (UTS) was of LBW joints was up to 20

% lower than the DP1000 base metal with 800 MPa average UTS while the

PWHT joints showed 30 % lower than the base metal with 720 average UTS.

5. There were no visible cracks after bending test completion for both LBW and

PWHT joint, which can be an indication for the good plastic properties.

6. All tensile tested specimens were fractured in the weld zone with marginally

differences in the location of fracture. The problem of weld softening can be

solved by the application of an alloyed filler metal (wire). PWHT had no

significant effect on tensile properties.

7. PWHT had significant effect on hardness profiles of welded joints. Hardness

peak has been significantly reduced while the degree of softening increased in

71

the HAZ. Hardness decrease with an average of 25 % within the weld zone

has observed.

8. Laser welding can be successfully applied to join DP1000 uncoated steel.

9. PWHT successfully reduced the hardness peaks to acceptable level as the weld

can be considered as soft weld.

72

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81

ANNEXURE

1. Appendix A (Material Specification)

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2. Appendix B (pWPS)

Welding Procedure Specification

(WPS) identification: EN ISO 15609: 4:2009.

Manufacture: Laserline GmbH.

Equipment identification: direct diode laser head LDL 160-3000

Mounted on Reis SRV 40 Robotic

laser source and beam implant

system: Direct diode laser.

Shielding gas system: Ultra-high purity argon gas (Argon 4.6, 99.996

%)

Shielding gas flow rate: 7 l/min

Filler material(s) feeding system: No filler material.

Parent material specification:

1. DOCOL 1000 DP

2. DOCOL 1000 DP

Material thickness: 1 mm

filler or other additional material: No additions.

Joint type:

Sheet metal closed square butt joint (no gap).

Joint Design: closed square Butt joint. 0 mm gap.

Welding sequence: one single run.

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One run (single pass welding).

Full penetration.

jig, fixture and tooling Yes

Type Mechanically fixed.

Tool Clamping device.

Tack weld Yes

Tack welding process Nd: YAG laser welding, code: 521

EN ISO 4063: 2009

Back support No

Backing gas No

Preparation emery paper and acetone to remove oxides

Welding position PA

EN ISO 6947: 2011

welding process 523

EN ISO 4063: 2009

Beam Power at work piece 1 kW

— Welding mode Continues wave mode (CW)

— Peak power (kW) 4 kW

— Shape Rectangular spot.

— Dimensions 2mm × 2mm

Travel speed (mm/s) 8 mm/s

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Shielding gas nozzle orientation Perpendicular to welding direction.

Preheating No

Post-weld heat treatment Yes

Operations after welding Post-weld heat treatment

Post weld heat treatment process Laser post-weld heat treatment.

Equipment LDF 5000-40 laser head mounted on a robotic

arm Reis SRV 40.

Beam Power at work piece 0.275 kW

— Peak power (kW) 5 kW

— Shape Rectangular spot.

— Dimensions 15 mm × 6.5 mm

Travel speed (mm/s) 4 mm/s

Date: 30-4-2020