Journal of Iron and Steel Research, International Volume 18 Issue 9 2011 [Doi 10.1016_s1006-706x(12)60037-5] Fatih Hayat -- Comparing Properties of Adhesive Bonding, Resistance Spot

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JOURNAL OF IRON AND STEEI. RESEARCH, INTERNATIONAL. 2011, 18(9): 70-78

Comparing Properties of Adhesive Bonding Resistance Spot Welding and Adhesive Weld Bonding of Coated and Uncoated DP 600 Steel

Fatih Hayat (Metallurgy and Materials Engineering Department, Engineering Faculty, Karabuk University Baliklarkayasi, Karabuk 78050, Turkey)

Abstract: Zinc coated dual phase 600 steel (DP 600 grade) was investigated, utilisation of which has gradually increased with each passing day in the automotive industry. The adhesive bonding ( AU) , resistance spot welding (RSW) , and adhesive weld bonding (AWE) joints of the zinc coated DP 600 steel were investigated. Additionally, the zinc coating was removed using HCL acid in order to investigate the effect of the coating. The microstructure, tensile shear strengths, and fracture properties of adhesive bonding (AB) , resistance spot welding (RSW) and adhesive weld bonding (AWB) joints of the coated and uncoated DP 600 steel were compared. In addition, a mechanical-electrical-thermal coupled model in a finite element analysis environment was utilised. The thermal profile phe- nomenon was calculated by simulating this process. The results of the tensile shear test indicated that the tensile load bearing capacity (TLBC) values of the coated specimens among the three welding methods were higher than those of the uncoated specimens. Additionally, the tensile strength of the AWE joints of the coated and uncoated specimens was higher than that of the AB and RSW joints. It was determined that the fracture behaviours and the deformation caused were different for the three welding methods. Key words: advanced high strength steel; DP600 ; adhesive weld bonding; microstructure; deformation; fracture

In recent years, a great amount of attention has been focused on reducing the weight of vehicles to reduce fuel consumption and exhaust gas emission. Therefore, using high strength materials, which provide higher strength for automobile body parts, has rapidly increased in the sheet metal industr$'-41. The dual phase DP 600 steel, which is among the steels developed within the scope of measures to provide fuel saving, has become a product widely used in automotive industryC5-". The DP 600 steel enables both a reduction in vehicle weight, an increase in strength for those carrying greater loads, and provides thinner application thickness.

The DP 600 steel was reported as being used in Arcelor MittalC7] automobile applications, in parts such as seat flange, wheel, wheel webs, light- weighted longitudinal rails, shock towers, and fas- teners. In the ULSAB-AVC-PESCB1 report, it was also stated as being used in different parts of an automobile such as ancillary parts, body structure, clo- sures, and suspension/chassis. Additionally, SSABCgl ,

Biography: Fatih Hayat(l980-), Male, Doctor, Assistant Professor;

another report, revealed that DP 600 steel is used in several parts like critical transverse and longitudinal cross members. Fig. 1 illustrates automobile parts in which the DP 600 is used in accordance with the SSAB reportCg3.

According to the service information from Daimler Chrysle&lo3 , the impact performance increases by using DP 600 steel in frame rails and A-pillars parts of the 2006 Jeep Grand Cherokee and In re-

Roof bow

rrinforccmcnt

Fig. 1 The automobile parts in which DP 600 steel is used

E-mail: fhayatBkarabuk. edu. tr; Received Date: June 18. 2010

Issue 9 Comparing Properties of Coated and Uncoated DP 600 Steel 71

cent years, DP 600 applications are widely used in different automobile models such as Porsche Cay- enne["], Land Rover LR2C'z3, and Jaguar XFC13]. While the reported weight saving is 50% in the SAB report due to DP 600, the reported weight reduction in the ULSAC reportc"] was 46%. The SAB reported that a weight saving of 17. 9% was due to reducing the material thickness in the rear side rails from 6.7 to 5. 5 mm thickness by using DP 600 steel. In 2002, Corus conducted a research that intended to reduce the weight of the vehicles without reducing the impact strength. This research reported that DP 600 steel is the most axial crash and bending crash resistant steel after TRIP 600 steel in durable steel rankingClS3.

In recent years, car manufacturers have started using coated sheets in automobile Galvanizing, which is important for corrosion pro- tection, is carried out prior to forming any component, while finished components are paint bakedCl7]. Galva- nized DP steel sheets are widely used in construction with corrosion resistance and especially in the automotive industry. Resistance spot welding, one of the oldest electric welding processes, is the most- frequently-used joining technique, particularly in the automotive industry for sheet materials. Today, the body-in-white is assembled using spot welds in the industryC'8-203.

Adhesive bonding is the process in which two different materials are joined using an adhesive[21-251 . Weld bonding is a modern and promising metal joining technology. The structural adhesive must have good wetting and flow characteristics in order to obtain a good quality bond of the faying metal surfaces, and premature curing, before or during spot welding, must be avoided since it can significantly increase the electrical contact resist; ance. High electrical contact resistance may result in excessive heat generation in the vicinity of the inter- face followed by subsequent metal expulsion, or it may simply impede the current from running throughCz1].

Although in literature there are studies on the joints of the steels used in the automotive industry, there is no study about the comparative analysis of the effects of galvanized coating on adhesive bonding, resistance spot welding, and adhesive weld bonding joints. Moreover, the study provides important data for the automotive industry regarding the deformation and shock absorption that occurs at welded joints in the tensile direction during a crash. The study investigated the microstructures, me-

chanical properties, and fracture behaviours of the coated and uncoated AB, RSW, and AWB joints of DP 600 steel, one of the commonly-used steel types in automobile manufacturing.

1 Materials and Experimental Methods

The study used Zinc coated DP 600 dual phase automotive steel. Zinc coated material has a 1. 2 mm thick ferrite+martensite microstructure. The chemical composition (mass percent, %) of DP 600 steel is C 0.11, Mn 1 . 6 , Si 0.182, Cr 0.34, Ni 0.027, Nb 0.0037, Ti 0.002, V 0.0035, and Mo 0.098.

The 30 mmX30 mm areas were removed under laboratory conditions by submerging them into HCl (hydrochloric acid, hold time: 30 s) acid as sche- matically shown in Fig. 2 from the coats of galvanized specimens before spot welding was applied. Two different materials were used in the removal process of galvanized coating during this experimental study.

, MS930

Adhesive Spot welding Adhesive weld bonding bonding

Fig. 2 Dimensions of welded specimens and pre-joining removal of zinc coated

Specimens were coded as DP6OOZ and DP6OOU in the experimental study ( 2 : zinc coated and U: uncoated). A clean and dry surface is a necessary prerequisite for AB, RSW, and AWB. Therefore, the mechanical and chemical treatment of the surface was performed just before the bonding process. Loc- tite, Terostat MS 930 (white colour, code: W ) , and Terostat 9220 Polymer Power Adhesive (black colour, code: B) grade adhesives were applied onto the bonding surface. Since the adhesives act as good electrical insulators that impede the current from running through, they were removed from the electrode contact surface. The remaining layer of adhesive was about 1 .2 mm. The other adhesives had a pot life of 60 min and were cured at room temperature. As for the adhesive weld bonding procedure, the above-mentioned welding parameters were used.

The welding procedure was carried out using water cooled by a conical Cu-Cr alloy electrode,

72 Journal of Iron and Steel Research, International Vol. 18

whose contact surface diameter was 8.0 mm. As a result of the preliminary work, because of the non- conducting adhesive between the AWB joints, the passing current was not the same as in the spot welding, so its weld nugget and nucleus values were lower. Therefore, parameters with the same heat input and nugget geometry value in both joints were selected. T h e parameter with the same nugget geometry was determined as 10.5 kA in the RSW joint, and 12 kA in the AWB joint. RSW and AWB were applied while all weld parameters, such as electrode pressure ( 6 X lo5 P a ) and holding time of electrode (20 cycles), were kept constant. Specimens were coded as 2-AB-W, U-AB-W, Z-RSW, U-RSW, 2- AWB-W, U-AWB-W, 2-AWB-B, and U-AWB-B. ( Specimen codes: 2-zinc coated, U-uncoated; AB-adhesive bonding, RSW-resistance spot welding, AWB-adhesive weld bonding; W-white, B- black. For example, the zinc coated specimen to which adhesive bonding was applied with the Z-AB- W = W adhesive).

In this study, the resistance spot welding process is treated a s an electrical-thermal-mechanical problem solved using the finite element method. Commercial finite element code ANSYS was used to model the coupling between electrical and thermal phenomena and between the thermal and mechanical phenomena. Temperature-dependent physical and mechanical properties of materials, including thermal conductivity, coefficient of thermal expansion, electrical resistance, specific heat, density, enthalpy, elasticity of yield stress, and Poisson's ratio, were used for both electro-thermal and thermal-mechanical anaIysisCz61.

In the study, a Nikon DIC microscope was used to carry out the optical examination of samples. T h e Photoshop computer program was used to obtain the microstructure profile of welded materials. T h e fracture surfaces were analysed using scanning elec- tron microscopy (SEMI.

A tensile shear test was applied to the AB, RSW, and AWB samples using a Shimadzu UH 5 000 kN type testing machine under laboratory conditions. During the tests, the crosshead speed was kept constant a t 2 mm min-I. Tensile shear tests were performed for flat test pieces in accordance with the DIN E N 10002 Standard.

2 Results and Discussion

2 .1 Microstructure Since the DP 600 steel used in the study has

high hardenability, it can harden even at lower cooling rates. In addition, due to i ts lower C content, it has high formability and good weldability. There- fore, martensitic transformation may occur in the HA2 that is exposed to heat and in the weld metal that is exposed to melting and solidification for a short time. Fig. 3 ( a ) , ( b ) , (c) , and ( d ) illustrate the microstructure images of the welded specimens.

Fig. 3 ( a ) illustrates how dual-phase steel has primarily equiaxed grains. It is established that it consisted of light brown (martensite) and white ( ferrite ) phases. There are typically two phases present in the microstructure of dual phase steels, body centred cubic ( bee)-ferrite and body centred- tetragonal ( bct ) m a r t e n ~ i t e ~ ~ " ~ ~ . Fig. 3 ( a ) illustrates that the microstructure in the base metal basi- cally consists of evenly distributed martensite within the somewhat elongated ferrite phaseCz6].

T h e martensite phase in the base metal is small in size along the ferrite grain boundaries and has a low volume ratio. T h e measured martensite volume ratio of the main material is approximately 18% f 5 % . T h e HA2 region, which is between the base metal and the weld metal and affected by heat, undergoes phase transformations when exposed to critical heat ( HAZ-1) between ACl-A, and austenitising heat (HAZ-2) between Ae3 and the melting temperature. Due to these thermal factors, the HA2 contains fine and coarse textured regions. T h e formula developed by Andrews [Eqn. ( 1 ) and Eqn. (Z)] was used to determine these temperatures.

Ac1=723-10.7~Mn-l6. 9w~i+29.1wsi+ 1 6 . 9 ~ c , + 2 9 0 ~ A ~ + 6 . 3 8 ~ w (1)

AC3=910-Z03 &-15.2wNi+44.7wsif 1 0 4 ~ ~ + 3 1 . 5 ~ M , , + 1 3 . lww (2)

After calculating the temperatures using these equations the temperature values started from A,] =

717 'C and Ae3 = 854 "C , while the austenitisation temperature started from 854 "C and continued until the melting temperature. T h e heat distribution sys- tem of resistance spot welding on DP 600 steel was conducted according to 10.5 kA 20 cycles welding parameter using the finite element method. Fig. 3 illustrates the temperature distribution of this parameter.

Fig. 4 illustrates the temperature distribution, which ultimately shows the increase in imposed heat according to distance. As seen in Fig. 3 and Fig. 4 the phase transformation starts in the region where the base metal is affected by heat, i. e. , from 717 "C. T h e increase in the martensite grain size and the volume ratio starts in the HA2 region of the weld metal


(a) Uase metal; ( b ) . (c) HAZ; (d ) Weld metal.

Fig. 3 Microstructure profile of 10.5 kA 20 cycles welding

Fig. 4 Temperature distributions at the end of welding process

(fusion zone) because of the increase in exposed temperature. T h e study proved that an increase in temperature, from 717 to 854 "C , occurred in a 3 5 0 h

10 pm area. T h e martensite volume fraction in the HAZ-1 region increased from 18% + 5 % to approximately 9 0 % + 5 % in a 3 5 0 h 1 0 pm area from the region that was exposed to the initial temperature. O n the other hand, in the HAZ-2 region, there was a coarse textured region, which was exposed to temperature between 854 and 1535 'C in a 5 1 0 h 10 pm area, because austenitising was incomplete and the growth of the austenite grains that did form were re- stricted due to the formation of martensite and thermal cycIes[261.

T h e temperature in the RSW joint, increased to approximately 2 000 'C , depends on the properties of material and thickness of the part. In a study conducted by Eisazadeh et a1L261, they reported that the


temperatures at the spot weld of a 1. 52 mm thick steel exposed to a 14. 2 kA welding current at different periods (8 and 1 4 cycles) was between 1900 and 2150 "C. Fig. 3 ( d ) illustrates that the martensitic structure forming on the microstructure images of the weld metal is lath martensite, whose formation is associated with the low carbon content.

The results of the study showed that there was no difference between the micro-structural photos of the joints belonging to the coated and uncoated specimens. Similarly, there was no microstructural difference between the coated and uncoated AWB and RSW joints, which was thought to be due to the fact that the formed heat inputs were the same for both the AWB and RSW joints.

2 .2 Microhardness The graphic illustrated in Fig. 5 shows the

changing microhardness profiles of RSW and AWB

AU-RSW X Z-AWB-B +U-AWB-B 8 ; l o o I . I I * 0 0 0 0 a 3 8 1

7 (5 5 4 3 2 1 2 3 4 5 6 7 Microhardness point

Fig. 5 Typical microhardness profiles of DP 600 steel after RSW and AWB joints

welded zinc coated and uncoated DP 600 steels, from the weld centre towards the base metal.

In conclusion to assessing the graphics, the microhardness values of both coated, uncoated RSW and AWB specimens were similar for all heat inputs. In the base metal, the microhardness values vary by approximately 190 f 10 HV. Microhardness values showed a significant increase compared to the base metal as the weld metal got closer in the HAZ. Mi- crohardness values rose to approximately 440 f 10 HV in the weld metal. The high values of hardness may be associated with the martensitic formations of alloy ele- ments (Mn, Si, Cr, etc. that increase the hardenability in the chemical compositions of steels in states of rapid heating and cooling during the welding process. When the microhardness values of RSW and AWB joints in the HA2 were compared, it be- came evident that coated specimens had higher values than that of the uncoated specimens; the reason is that Zinc coating speeds up the cooling process in the HAZ. In the study conducted by M Goodarzi et alCz7' using 1. 1 mm thick galvanized low carbon steel sheet, they emphasized on the importance of cooling rate of samples welded with resistance spot welding. Moreover, it was stated that cooling rate had an effect on martensite formation. The study proved that the microhardness of the weld metal was 2.4 times harder than the base metal. Furthermore, H A 2 displayed-a linear increase between base metal and weld metal.

2 .3 Tensile shear test Fig. 6 (a) and Fig. 6 (b) illustrate the results of

0 1200 1600 I

j 2 800

400

0 1 2 3

I" 000

10 000

8 000

6 OOC

4 ooc

2 00c

0 6 12 18 24 3 Strain/%

Fig. 6 Tensile shear curves of AB (a ) , RSW, and AWB samples (b)


the tensile shear tests carried out on the AB, RSW, and AWB joints of the coated and uncoated specimens.

As seen in Fig. 6 ( a ) , joints formed using adhesive B in the AB joint have a higher ultimate tensile shear load bearing capacity (TLBC) than the joints formed using adhesive W. Additionally, the TLBC value of the adhesive bonding joint of the zinc coated steels is higher than the joint values of the uncoated specimens. While the TLBC value of the uncoated U-AB-W specimen was approximately 750 MPa, the TLBC value of the coated 2-AB-B specimen reached 1200 MPa.

When the tensile graphics of the RSW and AWB joints are compared according to Fig. 6 (b> , the lowest tensile values are in the spot welded specimens. The TLBC values of the zinc coated DP 600 steel are 9800 MPa in the RSW joint, 10406 MPa in the AWB joint conducted using adhesive W , and 10 800 MPa in the AWB joint conducted using adhesive B. This result proves that AWB joints can resist higher loads than the RSW joints.

When we compare the AWB joints of the coated and uncoated specimens, it can be seen that the tensile and elongation values of the AWB joints of the coated specimens are higher. For example, while the TLBC value of the U-AWB-B specimen is approximately 10262 MPa, the TLBC value of the 2-AWB-B specimen is approximately 10 800 MPa.

The experimental results show that the TLBC value for the joints of the coated specimens is higher than the TLBC value for the joints of the uncoated specimens among. the three welding methods. The reason why the TLBC values of RSW and AWB joints of the coated specimens are high in all three of the welding processes formed is because hardenability of DP600 steel is better due to its high Mn content. In steel that hardens well, the change in the cooling speeds affects its hardness and ~ t r e n g t h ~ ~ * - ~ ~ ~ . As the increase in TLBC of joints is connected to the mechanical properties of the HAZ, the cooling speed

of the tear that occurs as a result of the HA2 at the end of the tensile shear tests in RSW and AWB joints carries grave importance. The RSW and AWB joints in coated specimens have a higher TLBC due to the increase in the cooling speed in the HA2 because the thermal conductivity of Zinc is better than DP 600 steel; therefore, enables rapid cooling. The cooling rate in the H A 2 of zinc-coated specimens is higher than that in the HA2 of uncoated specimens. The increased rate of cooling enables the hardness and the resistance values of the HA2 to be higher (Fig. 5 ) .

When physical analysis is carried out on the HAZ, it can be seen that the coating does not melt, but simply changes colour. Due to the cooling rate being higher in coated specimens, the area of the HA2 is narrower in coated specimens when compared to uncoated specimens (Fig. 2 ) . The results obtained prove that the reason behind high TLBC values of joints in coated specimens is related to the rate of cooling; therefore, forming an area that is narrow and highly resistant.

One of the characteristics of the adhesives used in AB joints is that they are also effective in coated materials. The reason behind the fact that the AB joints in coated specimens have a higher TLBC values is associated with the roughness difference in surface structure between Zinc (Zn) and steel; the adhesive sinks in deeper in zinc-coated surfaces and affects a broader area due to the fact that the surface is rougher.

2.4 Fracture mode Among the three types of the joints, only the

AB joint did not have any deformation in DP 600 steel. The fracture occurred along the adhesive surfaces. Fig. 7 illustrates the types of disjunction that occurs in RSW and the AWB joints after the tensile and shear test.

As seen in Fig. 7 (a) , (b), (c), and (d), knob and

Fig. 7 Failure modes of RSW and AWB samples

- 76 Vol. 18 Journal of Iron and Steel Rescarch, International -

tear shaped fractures occurred in all of the welded specimens, during all ol the weld times. The physical analysis, carried out on the fracture regions of the welded joints, indicated that the deformation that occurred during the fracture displayed different characteristics in the AWE and RSW joints. T h e RSW specimens deformed considerably during the tensile shear test and they curved back at approximately 70" to 80". This indicates that DP 600 steel, which is widely used in the automotive industry, may endan- ger the safety of people in an automobile during an accident that occurs towards the tensile direction.

On the other hand, the deformation during the tensile shear test was more limited (approximately 5" to l o " ) , and the corners of the specimens curved back less in the AWB joints. This means that the behaviour of the welded regions, especially for DP 600 specimens, will appear during a , crash ( acci-

dent). T h e combined joints will be able to carry more loads and stress in the tensile direction and ab- sorb shocks during any accident. Moreover, it will not allow the welded regions to be exposed to the damaging curves against any load and it will allow the other parts to cover them. In the spot welding joint, both the tensile and shear strength are lower, causing an extreme physical deformation, which en- dangers those in the vehicle. This result proves that the steel in question provides better results in the AWB joint during an accident.

Fig. 8 illustrates the SEM images of fractured surfaces formed at the end of the tensile shear exper- iment.

Fig. 8 illustrates that the ductile fractures, formed due to the failure of specimens, consist of dimples and large craters. Cone-cup type ductile fractures, which form due to ductile holes, occur in

Fig. 8 Fracture surface morphology of joints


the HAZ region of the said material. Additionally, round and small-sized cavities, an indicator of the ductile fracture, are seen on the fractured surfaces of the material.

T h e SEM images illustrated in Fig. 8 ( f ) indi- cate inclusions. T h e cavities occur on the second phase (inclusions) during tension and the fracture occurs on the second phase regions. I t is thought that the regions where deep tears occur are the ones where martensite or other phases pile up. It is thought that the hard martensite phase separates during the fracture and possibly forms cavities in this region.

3 Conclusions

1) After comparing the TLBC and displacement values obtained for the three separate types of joints, the AWB joints were higher than the AB and RSW joints.

2) Among the specimens, the AWB joint, conducted using the black coated adhesive, had the

3) After investigating the fracture types of the AB, RSW, and AWB specimens, no deformation occurred in the AB specimen. T h e RSW specimen underwent maximum deformation and curved back during the fracture. T h e AWB specimen partially curved over the other specimen and ruptured during the fracture.

4) T h e SEM analysis of the ruptured regions indicated that a dimple structure and a ductile fracture occurred in the specimens.

' highest TLBC value.

The authors wish to acknowledge the material support of the TOFA$ A $, M I S T A S A $ and the help of lecturer M YASAR f o r this study.

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Journal of Iron and Steel Research, International Volume 18 Issue 9 2011 [Doi 10.1016_s1006-706x(12)60037-5] Fatih Hayat -- Comparing Properties of Adhesive Bonding, Resistance Spot

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