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Page 1: HLW Report Shahnawaz Lodhi

iii

University of Waterloo Faculty of Engineering

Department of Mechanical and Mechatronics Engineering

Hybrid Laser Welding

Prepared by

Shahnawaz Lodhi

ID 20194907

20 December 2014

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Abstract

Hybrid laser welding (HLW) is a technology that utilizes both a laser heat source and an

additional (secondary) arc welding source in welding applications in a synergistic

manner. This combined approach results in a welding process that can deliver faster

processing speeds, deeper penetration, and greater control of the weld pool. This

technology is of particular importance in the automotive industry, railway industry,

aircraft industry, shipbuilding industry where the focus is weight reduction and use of

aluminum alloys allows this due to its nature of having low density and high-specific

strength. The technology utilizes a very high energy intensity from the laser to rapidly

reach the vaporization temperature of the work piece forming a vapor cavity. The

resulting cavity acts as a radiation blanket leading to the buildup of radiative energy and

acting directly (in combination with the filler wire) onto the work piece. In this paper, we

will discuss the several categories of hybrid welding including laser-gas tungsten arc

welding and laser-gas metal arc welding. We will also study the primary laser heating

sources along with relevant operational parameters.

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Table of Contents

Abstract ........................................................................................................................................... iv List of Figures ................................................................................................................................. vi 1 Introduction .............................................................................................................................. 1 2 Types of lasers ......................................................................................................................... 3 3 Types of laser arc hybrid welding processes ........................................................................... 6 4 Shielding gases......................................................................................................................... 7 5 Gap tolerance and depth penetration ...................................................................................... 11 6 Welding Direction .................................................................................................................. 14 7 Power and Beam Spot Diameter ............................................................................................ 17 References ...................................................................................................................................... 20

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List of Figures

Figure 1 Comparison of welding processes (B. Ribic, 2009) .......................................................... 2 Figure 2 Comparison of arc, laser and hybrid welding processes (B. Ribic, 2009) ......................... 2 Figure 3 Quality of a laser beam: Beam Parameter Product (Paleocrassas, 2010) ......................... 4 Figure 4 illustrating feature comparison for typical materials processing laser sources (Olsen, 2009)

......................................................................................................................................................... 5 Figure 5 Overview of the HLW Process (Kah, 2011) ...................................................................... 7 Figure 9 A sectional view of hybrid welds (P Kah, 2011) ............................................................... 8 Figure 10 Function and effect of different shielding gases used in laser and arc welding (P Kah,

2011) ................................................................................................................................................ 9 Figure 6 Welding parameters for this experiment (Ming Gao, 2007) .............................................. 9 Figure 7 the arrangement parameters of gas nozzle (Ming Gao, 2007) ......................................... 10 Figure 8 Plasma shapes of laser-TIG under different shielding gas parameters (Ming Gao, 2007)

....................................................................................................................................................... 10 Figure 11 Comparisons of cross-sections of YAG-MIG hybrid welds produced at various laser-

focused point and MIG wire target (FUJII, 2007) ......................................................................... 11 Figure 12 X-ray inspection results of YAG-MIG .......................................................................... 12 Figure 13 Hybrid welding conditions (Jing-bo Wanga, 2009) ...................................................... 13 Figure 14 Effect of laser-MIG distance on arc current at (Jing-bo Wanga, 2009) ........................ 14 Figure 15 the welding parameters (WangZhang, 2014) ................................................................ 15 Figure 16 The weld pool profile (WangZhang, 2014) ................................................................... 16 Figure 17 the detail information of weld pool (WangZhang, 2014) .............................................. 16 Figure 18 Effect of laser beam diameter on bead shape (T U E Y A M A, 2004) ......................... 17 Figure 19 Depth of penetration as a function of welding speed (Hilton, 2005) ............................. 18

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1 Introduction

Laser welding is a popular choice that has developed over the course of the last two

decades for use in many industries. The valuable features of hybrid fusion welding is the

combination of two heat sources with different energy densities which makes it possible

to utilize efficiently the welding and technological special features of each method and, at

the same time, eliminate their shortcomings (Bernadskiy, 2014). In hybrid welding, the

laser and arc are arranged in a manner such that a common interaction zone is achieved.

This interaction zone yields high welding speed, deep penetration, and improved weld

quality with reduced susceptibility to pores and cracks and excellent gap bridging ability.

HLW can be categorized into three main categories: (1) laser-gas tungsten arc (GTA)

welding; (2) laser-gas metal arc (GMA) welding; and (3) laser-plasma welding (H.L.,

2012).

Prior to commencing the study of hybrid laser welding (HLW), it is important to

understand what is laser welding and arc welding. Laser welding is a technology that

utilizes a laser as a power source (Nd:YAG or CO2 amongst others) to apply heat to the

work pieces such that the melting temperature is exceeded resulting in a small heat

affected zone (Welding Procedure Services, n.d.). The result is a weld with narrow, high

depth of penetration and high welding rates. Arc welding technology most applicable in

context of HLW are the TIG-Tungsten Inert Gas and MIG-Metal Inert Gas. TIG process

uses a non-consumable tungsten electrode that delivers the current to the welding arc.

The tungsten and weld puddle are protected and cooled with an inert gas, typically argon

(Gomez, 2013). The MIG process uses a wire connected to a source of direct current acts

as an electrode to join two pieces of metal as it is continuously passed through a welding

gun. A flow of an inert gas, typically argon, is also passed through the welding gun at the

same time as the wire electrode. This inert gas acts as a shield, keeping airborne

contaminants away from the weld zone (Gomez, MIG Welding: The Basics for Mild

Steel, 2014).

Figure 1 below illustrates very accurately the weld pools of arc, laser and the HLW

processes. Figure 1c (arc welding weld pool) is shallower than both the laser (a) and

hybrid weld pools (b). This shows the advantages of HLW (keyhole penetration and

controlled gap coverage) over both traditional processes.

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Below is a summary showing the important characteristic of welding. It is apparent that

HLW process is the optimal method that allows the process to achieve benefits of both

arc and laser welding without their negative effects.

Figure 2 Comparison of arc, laser and hybrid welding processes (B. Ribic, 2009)

The use of aluminum as a structural component is widely increasing in numerous

industries. However, by itself as an element it has low mechanical strength and corrosion

resistance. But to improve these properties, different concentrations of differing elements

are mixed with aluminum. The following are the alloying elements used to reinforce

aluminum (Modern technologies of welding aluminium and its alloys, 2012):

1. Copper

Figure 1 Comparison of welding processes (B. Ribic,

2009)

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2. Silicon

3. Manganese

4. Zinc

5. Magnesium

The alloying elements mentioned above are crucial to the acceptance of aluminum for use

in industry. Zinc and magnesium have a tendency to increase the corrosion resistance and

improve overall mechanical strength. Elements such as copper, chromium, iron,

zirconium, silicon, vanadium, bismuth, nickel and titanium are also added. They have the

benefit of improving the susceptibility to heat treatment, as well as strength and corrosion

resistance. Small amounts of these elements are usually not taken into account when

assessing the weld ability of aluminum alloys. But often it is these elements that have a

significant impact on their weld ability.

Another very important consideration when HLW of aluminum is the very high reflection

of the laser radiation and a tendency for porosity and cracks to occur in the welds. It is

especially difficult to start the process of laser welding due to this high reflectivity and a

large power input is required to penetrate into the substrate. The power density must be

high enough above such that “a gas-dynamic capillary tube is created, filled with gases

and metal vapor, surrounded by a layer of liquid metal” (Modern technologies of welding

aluminium and its alloys, 2012). This will ensure that the laser radiation is absorbed by

the metal vapor contained in the capillary through multiple reflections of the laser beam

from the capillary walls covered with liquid. For aluminum and its alloys, due to their

much higher thermal conductivity and high reflection coefficient of laser radiation, the

threshold power density is approximately 1.5 x 10^6W/cm2 and not less than 2x10^6 to

have a stable process. Comparatively for iron alloys the power density is typically 0.15

x10^6, and for a stable process (Modern technologies of welding aluminium and its

alloys, 2012).

2 Types of lasers

The selection of a laser is an extremely important topic to understand prior to

commencing the study of HLW. An important parameter is beam quality which is of

major relevance in materials processing with high power laser. The beam quality is

defined by ISO 11146:1999 by the beam parameter product (BPP) or the M2 factor, this

assess the ability to focus a laser beam. The beam quality of a solid state laser is often

specified by the beam parameter product (BPP) defined as follows:

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Low values of BPP express good quality beams or the ability to focus a beam to a small

spot. As mentioned previously aluminum is one of the best reflectors of light. In

addition, many aluminum alloys contain magnesium or zinc, which are easily vaporized

forming plasma that blocks the incident beam. Additionally, aluminum also by its nature

absorbs very little energy and the surface must be properly prepared. Typically the laser

must have a minimum power density of 3kW to overcome the reflectivity of aluminum

(L. Quintino, 2012).

The major types of lasers used in HLW are the: carbon dioxide (CO2) and Nd:YAG

(solid state).

The CO2 laser is a gas laser emitting at a wavelength of 10.6 microns with higher output

powers (near 50kW) and efficiencies near 20% with a good beam quality but the light

cannot be transferred via fiber optics. However, a major drawback is that the wavelength

is not readily absorbed by the aluminum as the majority is absorbed/reflected by the

plasma created during the arc/laser interaction (Eboo, 1978). This results in reduced

energy reaching the substrate and a reduction in important characteristics such as

penetration depth occurs.

Nd: YAG lasers are widely used in the HLW of aluminum because their wavelength of

1.06 microns is easily absorbed by aluminum even though their overall power output is

much lower than CO2 (<10kW). However, they can still deliver significant power to the

substrate as the plasma does not absorb much of the laser energy. For this reason the Nd:

YAG can deliver same penetration with a lower power output.

Figure 4 below illustrates the types of lasers and their important parameters as discussed

above.

Figure 3 Quality of a laser beam: Beam Parameter Product (Paleocrassas, 2010)

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Figure 4 illustrating feature comparison for typical materials processing laser sources (Olsen, 2009)

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3 Types of laser arc hybrid welding processes

There are numerous types of laser-arc welding processes that exist and are summarized

from the research article (Kah, 2011).

Laser-MIG/MAG welding process: This process combines the use of a laser as a primary

source and the MIG arc as secondary source. This process allows for larger gaps to be

covered via the filling of the gap with a filler metal. The main benefit of this method

benefit is that the microstructure and mechanical properties of the weld metal can be

improved by controlling its chemical composition when using an appropriate filler

material. This approach can be used with either a pulse or continuous laser.

Laser-TIG welding process: This process combines the use of a laser as a primary source

and the TIG arc as a secondary source. This process is very fast and can be operated

either with or without filler metal addition. This process is mainly used in thin sheet

applications (0.4-0.8mm). However, since the electrode is non-consumable heat is

consumed from the arc-laser zone to reach the melting point of the filler wire and deposit

into gap.

Laser-PAW process: This process combines the use of a laser and a high frequency

microwave based source which induces a plasma jet. The plasma creates (upon

application of a current between substrate and nozzle) sufficient heat to ionize the air gap.

In this process, the laser beam is surrounded by a plasma arc. As with the TIG and MIG

process, a gas is used to prevent exposure of the substrate to the environment’s damaging

effects.

The figure below illustrates the experimental setups of the above mentioned processes.

As mentioned, for the purpose of this report we will only be investigating the one electric

arc and laser source setup (similar to a) below.

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4 Shielding gases

Shielding gas selection and its distribution into the heat source–material interaction zone

is one of the most important parameters in a welding process and can deeply affect the

quality and reliability of the welded joints. This consideration is particularly true if hybrid

laser-GMAW processes are considered, because the synergist effect between the two heat

sources may be achieved only if a trade-off condition in gas composition is obtained.

Aluminum for example readily reacts with the environment in its molten state which may

lead to the formation of porosities, oxides or nitrides inclusions and thus to performance

reduction of the welded joints. Shielding gases need to fulfill different requirements: (a)

composition: the gases have to be inert with respect to the working materials (in this case

aluminum) also at high temperatures, but some oxygen content is useful for improving

arc stability; (b) flow: the gas must be capable of moving the laser induced plasma plume

away from the process zone but it must be low enough not to blow the liquid phase away

from the bead; (c) ionization potential: high ionization potentials help to reduce the

amount of plasma during welding and thus lead to deep penetration but they often lead to

arc instability (G. Campana, 2008).

In laser–arc hybrid welding, the shielding gas is supplied first to (as mentioned

previously) to isolate the molten metal from the ambient air. But it also has a very

important secondary function which is to suppress the laser-induced plasma, remove the

plume of metal vapor out of the keyhole, and to stabilize the metal transfer. Typically,

when welding with a carbon dioxide (CO2) laser, a phenomenon called plasma shielding

may cause problems in welding, when the evaporated metal plume prevents (shielding)

the laser beam to reach the work piece surface by reflection and absorption, causing an

unstable keyhole and thus lowering the penetration depth. In the case of autogenously

Figure 5 Overview of the HLW Process

(Kah, 2011)

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laser welding, the velocity of the shielding gas is sufficient to displace the induced plume

but with arc welding this can change the direction of the filler wire deposition and

negatively affect the appearance. To overcome this plasma shielding effect, selection of a

type of laser to be used in the process and shielding gas is very important. As discussed

since CO2 lasers which have a very high wavelength (10.6 microns), it tends to reflect

energy back from the aluminum lowering the effectiveness of its plasma shielding effect.

helium and argon are used extensively as shielding gases. Helium because it has a high

ionization potential. (P Kah, 2011). However, helium is not conducive to promoting arc

stability due to its high thermal conductivity. In order to reduce its high cost, helium is

often combined with argon to enhance the overall performance of the blend while

minimizing its cost as well as achieving maximum efficiency. Also it is important that

when using helium, the gas flow has to be three times greater compared to argon in other

achieve an efficient shielding due to its lighter weight (Tusek, 1999) .

In Nd: YAG lasers however, a mixture of argon and helium can be used due it’s the

smaller wavelength and thus does not have the negative effect as a CO2 laser (Tusek,

1999).

In addition to argon and helium, numerous studies have found that adding up to 12%

CO2 gas which is reactive to a mixture of argon and helium can improve metal transfer.

However, above 16% leads to spatter during welding. Studies have also found that adding

between 5-10% of CO2 improves the appearance (shape of the weld). The figure 9 below

very well illustrates the interaction of the mixture of helium and argon through a study

performed with a 12kW CO2 laser. The effectiveness of the helium gas is obvious due to

its ability to prevent laser induced plasma formation. Furthermore, a small amount of

argon can be added to the mixture without significant reduction in the weld geometry (P

Kah, 2011).

Figure 6 A sectional view of hybrid welds (P Kah,

2011)

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The summary below shows the different shielding gases and their relevant parameters.

As mentioned previously, plasma shielding is a phenomenon that occurs when using a

CO2 laser. In a study performed (Ming Gao, 2007), the effect of shielding gas parameters

were studied experimentally using a 5 kW Rofin-sinar TR050 CO2 laser together with a

Miller 300A conventional DCEN TIG welder. The shielding gases were the standard

argon and helium. The following were the weld parameters:

The experiment utilized the argon gas for the paraxial and coaxial nozzles and helium in

the tig torch nozzle offset as shown in figure 7 to study impact of the velocity and angle

of dispersion on the effectiveness of the shielding gas.

Figure 7 Function and effect of different shielding gases used in laser and arc welding (P

Kah, 2011)

Figure 8 Welding parameters for this experiment (Ming

Gao, 2007)

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The results obtained below show the effect of the angle and velocity of the shielding gas

dispersion on plasma height (see figure 8 below). The higher the interacting plasma

height with the laser, the smaller the laser energy absorbed by work piece and the

shallower the weld penetration.

This shows that the shielding gas angle and velocity can be effectively used to change the

plasma height and thus increase weld penetration. This is quantified by the following eqn:

I(h) ≈ I0e(−βh)

Where I (h) is the laser energy absorbed by work piece, I0 the laser incident energy, β the

plasma absorption coefficient for laser energy and h is the plasma height interacting with

incident laser.

This resulting interaction between laser and arc plasma leads to further impact on

interacting plasma height and weld penetration depth. “When the laser induced plasma

Figure 9 the arrangement parameters of gas nozzle

(Ming Gao, 2007)

Figure 10 Plasma shapes of laser-TIG under

different shielding gas parameters (Ming Gao,

2007)

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has high temperature and density of charged particle, which is usually far higher than that

of arc plasma. When the two plasmas encounter spatially, an electric channel is formed.

Through the channel, the charged particle of laser induced plasma will enter into arc

plasma, resulting in the dilution and reduction of charged particle density of laser induced

plasma. This will reduce the plasma defocusing effect for laser and enhances the laser

energy effectively absorbed by work piece. On the other hand, the increase of charged

particle in arc plasma will improve the ionizability of arc to decrease the resistance of

arc.” (Ming Gao, 2007)

5 Gap tolerance and depth penetration

In HLW, weld depth penetration and gap coverage is extremely important. The very

purpose of welding is to attach two similar or dissimilar pieces of metal together such

that the depth penetration and gap coverage between them is maximized. We will look at

various studies where parameters were modified to maximize the above mentioned.

In a literature study (FUJII, 2007), an experiment was performed using an A5052

aluminum alloy (plate thicknesses: 2, 3 and 4 mm) and A5356 MIG wire (1.2 mm

diameter). An Nd:YAG laser (maximum power: 4 kW) and a DC pulsed MIG welding

machine (maximum current 350 A) were employed as hybrid welding power sources to

study impact of process distance penetration depth and width. The process distance “d” is

the distance between the laser beam irradiation (spot) position and the wire target

position as shown in figure 11 below:

Figure 11 Comparisons of cross-sections of YAG-MIG

hybrid welds produced at various laser-focused point and

MIG wire target (FUJII, 2007)

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The experiment wanted to study what impact if any the distance has on the resulting weld

formation.

The results showed that the deepest penetration occurred with a 0-2mm process distance.

As the process distance was increased, the penetration depth decreased and resembled

conduction style welding. The further the distance between the laser beam irradiation

(spot) position and the wire target position, the larger the process zone and greater energy

was dissipated. The experiment also looked at what happens when the welding speed is

increased. The hypothesis was that as the speed is increased not sufficient enough energy

would penetrate the substrate. The effect on penetration depth was predictably similar

also when increasing welding speed. As the welding speed was increased between 30-

90mm/s, partial penetration welds were apparent. Only between 30-40mm/s did the weld

resemble keyhole welding -see figure 12 below.

Figure 12 X-ray inspection results of YAG-MIG

hybrid welds produced (FUJII, 2007)

In another similar experiment performed (Jing-bo Wanga, 2009) using a Nd:YAG laser

and fiber laser with MIG welding, the effect of laser beam parameters on depth

penetration and gap tolerance were studied on an aluminum alloy. The figure 13 below

shows the parameters of the laser and MIG energy sources. The lasers used were a

continuous wave (CW) Nd:YAG laser (wavelength 1064 nm) and fiber laser (wavelength

1070 nm).

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The experiments were performed where the lasers were irradiated directly on the wire

surface varying distance between -6 to +2 mm. The study found similar results as the

above experiment regardless of the laser power source and arc current. It was also found

(predictably) that if the distance was zero there was a reduction in the molten pool size

(due to pressure reduction from arc), making a reduction in the bead width and

penetration depth. However, the study was performed using a 2.7kW power laser. But

deviating from that (increasing power) it is unknown the effect on the process. It would

be speculated that wire melting rate tends to increase when the laser output power is

increased and it can be anticipated that the laser beam energy absorbed by the droplets

formed at the tip of the wire will also increase. If a droplet absorbs too much laser beam

energy, its temperature rise becomes too rapid, resulting in violent vaporization. The wire

feed rate was also studied as it was varied from 115 to 230mm/s for its effect on arc

current and laser-mig distance. It was found that if the wire feed rate was increased then a

large arc current was required otherwise the penetration depth would be negatively

affected as shown in figure 14 below.

Figure 13 Hybrid welding conditions (Jing-bo

Wanga, 2009)

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This leads us into the discussion of the impact of welding direction on a range of

resulting weld characteristics.

6 Welding Direction

The HLW process can be oriented in two directions: arc leading or laser leading. This

means that the laser or arc is the power source that leads in the welding direction. The

difference in welding direction can produce different weld surface geometries and

penetration. To optimize the welding process and obtain the desired welding quality, a

fundamental understanding of the complex phenomena between the two process

orientations is essential. In a study performed (WangZhang, 2014), the effect of various

welding directions on the behavior of CO2 laser GMAW-P hybrid welding processes was

investigated. The laser was a TLF15, 000 CO2 laser (TLF 15000, TRUMPF) was used as

the laser source with a maximum output power of 15Kw. The filler wire was a steel

CHW-50C6, and is 1.2 mm in diameter. It was fed at a velocity of 4 m/min. A 15 mm

thick aluminum work piece was employed. Shielding gas was argon–helium mixtures

(50% argon and 50% helium), flowed at 40 L/min.

Below were the welding parameters shown in figure 15:

Figure 14 Effect of laser-MIG distance on arc current

at (Jing-bo Wanga, 2009)

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The results of the experiment showed that in the arc leading welding process, the work

piece is firstly heated by the arc, and the arc has a lower peak power density distribution

than the laser. As Figures 16 and 17 show, “the molten pool can be divided into three

regions, and the outlines of these regions are shown as dashed lines. The outer region is

the preheating zone, in this area the work piece is at a semi-solid state, so only an outline

can be seen. The second part is the arc melting zone, which is melted by the arc power

density peak. Most of the molten metal is mainly from the filling droplets, in addition a

relatively shallow weld pool is formed. The third part is laser melting zone. Due to the

sharp and highly intense power density distribution of laser, a keyhole is produced, the

metal flows out from the keyhole, resulting in a deep weld pool.” (WangZhang, 2014)

It's interesting to note in this study that the penetration depth is the higher in an arc

leading process then in a laser leading process. A deeper penetration reached with a

leading torch can be explained by the fact that the arc is already melting the work piece

surface, and when the laser beam reaches the location of the molten material at an

elevated temperature, it is able to start penetrating the metal on an already warm surface.

The leading torch also ensures better weld quality.

Figure 15 the welding parameters (WangZhang,

2014)

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Figure 16 The weld pool profile (WangZhang, 2014)

Revisiting the experiment we discussed in the section regarding maximizing depth

penetration and gap coverage. The experiment was performed (FUJII, 2007) using an

A5052 aluminum alloy (plate thicknesses: 2, 3 and 4 mm) and A5356 MIG wire (1.2 mm

diameter). A YAG laser equipment (maximum power: 4 kW) and a DC pulsed MIG

welding machine (maximum current 350 A) were employed as hybrid welding power

sources to study impact of welding direction on penetration characteristics and porosity.

The study was performed both as Nd:YAG leading and MIG leading. The results showed

that both kinds of welding exhibited similar porosity levels. However, as was found in the

previous experiment, the laser leading penetration was shallow as compared to the arc

leading. Similarly, the width coverage was the inverse relationship as should be expected

as the filler metal is able to spread out over the gap when not penetrating deeply.

However, there was an additional factor here of weld surface quality. The x-ray

Figure 17 the detail information of weld pool

(WangZhang, 2014)

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inspection showed that in the case of the arc leading the formation of AlMgO particles

which were most likely blown onto the bead surface from MIG torch gas resulting in the

unclean appearance. Typically, laser leading HLW provides a cleaner bead appearance

as the arc following the laser beam the process gas would blow the oxide particles away

(Jing-bo Wanga, 2009).

7 Power and Beam Spot Diameter

The spot diameter of the laser beam is an important factor in determining the

characteristics of the laser beam. In Nd:YAG laser and CO2 laser, energy density is

increased by making the beam spot diameter small so that keyhole welding can be

realized. An experiment was performed to study the effect of beam spot diameter on bead

shape and penetration geometry and the laser beam diameter of the 2kW Nd:YAG laser

was varied from 1.09, 3.34, 5.14, 6.92mm. The analysis of the results showed that as the

beam diameter increases to 6.92mm, the penetration depth decreases and the width of the

beam decreases. Further analysis explains that the method of welding was conduction and

not keyhole (T U E Y A M A, 2004). This is the result of the defocussing effect of the

laser as shown in figure 18.

There is also a direct relationship between spot size and power density and welding

speed. In studies performed using 4 and 6mm thick 5083 aluminum with spot sizes of

0.14 and 0.40mm, the welding speed was approximately 60% and 21% higher with a

0.14mm spot size, compared with a 0.4mm spot size. Figure 19 below also shows, on the

other hand, that welding at fixed speeds of 5 and 2m/min, means that 5.0 and 6.8mm of

Figure 18 Effect of laser beam diameter

on bead shape (T U E Y A M A, 2004)

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aluminum can be penetrated with the smaller spot, compared with only 4.3 and 6.4mm,

using the larger spot size. This is an increase in depth of penetration of 16 and 6%

respectively (Hilton, 2005).

8 Conclusion

An extensive literature study was undertaken to develop this report on hybrid laser

welding. The two most common laser sources are the CO2 and Nd:YAG. The wavelength

of the CO2 is higher than the Nd:YAG and as such there is reduced absorption of energy

into the substrate. To maximize the energy delivered and achieve excellent weld

appearance, a shielding gas is necessary. A shielding gas is important because of

its strong plasma-defocusing effect. For a constant combination of the laser beam and arc

sources, there should be an optimal combination, and composition of types of shielding

gases. The shielding gases must be inert with respect to the working materials, must

change the laser-induced plasma plume and must have high ionization potential. Argon

and helium are two most commonly used shielding gases in the welding processes due to

their cost and effectiveness. The process distance was shown to be an important

parameter. The results derived from the experiments showed that as the process distance

was increased, the penetration depth decreased and resembled conduction style welding.

The further the distance between the laser beam irradiation (spot) position and the wire

target position, the larger the process zone and greater energy was wasted. The effect on

penetration depth was predictably similar also when increasing welding speed. As the

welding speed was increased, partial penetration welds were apparent. Welding direction

it was found was very important in controlling depth penetration. It was found that

penetration depth is higher in an arc leading process then in a laser leading process. A

deeper penetration reached with a leading torch can be explained by the fact that the arc

is already melting the work piece surface, and when the laser beam reaches the location

of the molten material at an elevated temperature, it is able to start penetrating the metal

on an already warm surface. Furthermore, in the case of the arc leading the formation of

AlMgO particles which were most likely blown onto the bead surface from MIG torch

gas resulting in the unclean appearance. Lastly, it was found that there exists a direct

Figure 19 Depth of penetration as a function of welding speed

(Hilton, 2005)

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relationship between spot size and power density and welding speed. In studies

performed, it was shown that increasing the spot size the energy density and speed are

reduced.

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References

(n.d.). Retrieved 11 05, 2014, from Welding Procedure Services:

http://www.wpsamerica.com/library/Laser%20Welding%20Basics.pdf

B. Ribic, T. A. (2009). Problems and issues in laser-arc hybrid. Institute of Materials, Minerals

and Mining and ASM International, 54(4). doi:10.1179/174328009X411163

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