Materials and Design 45 (2013) 24–30

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Materials and Design

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Short Communication

Research on microstructure and mechanical properties of laser keyholewelding–brazing of automotive galvanized steel to aluminum alloy

M.J. Zhang, G.Y. Chen ⇑, Y. Zhang, K.R. WuThe State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, PR China

a r t i c l e i n f o

Article history:Received 5 June 2012Accepted 12 September 2012Available online 23 September 2012

0261-3069/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.matdes.2012.09.023

⇑ Corresponding author. Tel./fax: +86 731 8882389E-mail addresses: chengy@hnu.edu.cn, hdgychen@

a b s t r a c t

Laser keyhole welding–brazing (LKWB) technique with fiber laser was used for butt joining of automotivegalvanized steel to aluminum alloy using a filler wire of 4043 aluminum alloy (Al–5Si) without groove.The dissimilar joint was obtained both by welding the aluminum alloy with the filler wire in keyholewelding mode, and by brazing the molten aluminum alloy and filler wire to the solid galvanized steel.The results show that the galvanized steel was brazed by the molten aluminum alloys without melting.There was a thin layer of the intermetallic compositions (IMCs) on the interface between galvanized steeland welded seam. The thickness of IMCs layer was varied with joint zone ranged from 1.5 lm to 13 lm.The IMCs layer was composed of three different phases: a(s5)-Al8Fe2Si, h-Al13Fe4 and f-Al2Fe withmicrohardness ranged from �811 HV to �1060 HV, compared with �53.5 HV in brazed seam and�480 HV in galvanized steel substrate. Transverse tensile test results indicate that the maximum tensilestrength of the butt joint reached 162 MPa and ductile fracture occurred at heat affected zone (HAZ) ofaluminum alloy with appearance of necking in tension.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

As a result of new policies related to global warming, avoidingunnecessary energy waste and reducing environmental pollutionlevels are becoming a major issue in the automotive industry. Theamount of steel used in car body manufacturing has decreased con-tinuously. As a consequence, light metals and plastics have replacedmany parts previously manufactured from steel [1]. For this pur-pose, technologies have been developed in recent years to producefusion welded hybrid blanks of aluminum and steel [2]. However, inorder to achieve great steel–aluminum joints, these novel technol-ogies must overcome challenges arising mainly from differences inphysical and mechanical properties of the parent materials such asmelting point, thermal expansion coefficient, thermal conductivity,corrosion potential, as well as the excessive formation of brittle Al-rich intermetallic compositions (IMCs) owing to the low solubilityof Fe in Al [3]. So some researchers had proposed the welding meth-ods without liquid phases, such as diffusion welding, explosionwelding, friction welding, and friction stir welding to produce alu-minum/steel joints [4–7]. However, since the shape and size of suchsolid state joints are extremely restricted, those methods are notsuitable for the auto-body assembly.

Nowadays, welding–brazing process offers a great potential fordissimilar metal joining, such as aluminum to steel, aluminum totitanium. In this process, metal with low melting point and the

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filler wire are melted and welded by heating source, and metalwith high melting point is brazed by the molten metal in solidstate. The resulted joint has dual characteristics: it is a welded jointin low melting point metal side and a brazed joint in high meltingpoint metal side [8]. Compared to the pure fusion welding pro-cesses, this process is effective to limit the mixture of the dissimi-lar molten metal, and inhibit formation of the brittle IMCs at theinterfacial layer [9].

Recently, considerable work has been reported on welding–brazing process for dissimilar metal joining. Dharmendra et al.[8] studied lap joining of zinc coated steel with aluminum alloyby laser welding–brazing process with 85% Zn and 15% Al fillerwire. The resulted hybrid joints failed on the base metal of alumi-num alloy. Song et al. [10,11] butt-joined aluminum alloy to stain-less steel by tungsten inert gas (TIG) arc welding–brazing processwith V-shaped groove, and investigated spreading behavior of li-quid filler metal on the front and back surfaces of the groove. Chenet al. [12,13] investigated laser welding–brazing dissimilar alloysof titanium alloy to aluminum alloy in butt configuration, andfound that it is essential to fabricate grooves on parent materialsto obtain a well brazed joint with appropriate wetting length onthe top and bottom surfaces of the titanium alloy sheet. Jacomeet al. [14] used cold metal transfer (CMT) arc to butt-join alumi-num alloy and zinc coated steel in voestalpine geometry with awedged steel plate end, and found that the presence of silicon inthe filler material significantly inhibited the formation of IMCswhich has been already generated in the early growth stage. Asthe authors presented, the weld pool was large with the special

Table 2Typical mechanical properties of base materials.

Material Yield strength,ry (MPa)

Ultimate tensile strength,rUTS (MPa)

Elongation (%)

H220YD 275 410 326016 125 230 24

M.J. Zhang et al. / Materials and Design 45 (2013) 24–30 25

shaped groove which allows the interfacial reaction of Al–Fe tokeep at a high temperature for a long time, resulting in a thickerIMCs layer. Furthermore, the sharp steel tip at the end of the dis-similar Al-filler/steel interface acted as a stress concentrator dete-riorating the mechanical performance of joint.

Laser welding–brazing of galvanized steel to aluminum alloywithout any additional shaped groove has practical significancein the industrial application. In this paper, laser keyhole weld-ing–brazing (LKWB) technique for joining automotive galvanizedsteel to aluminum alloy was developed. In the process, laser weld-ing operates in keyhole welding mode. This study focuses on inves-tigating the feasibility and characteristics of laser keyholewelding–brazing of automotive galvanized steel to aluminum alloyin butt configuration without groove.

2. Materials and methods

The experimental materials consisted of 1.2 mm thick H220YD+ ZF automotive galvanized steel and 1.15 mm thick 6016 alumi-num alloy. The Zn layer of the galvanized steel, approximately10 lm thick, was coated on both sides of the steel sheet. The fillermetal was eutectic 4043 aluminum alloy (Al–5Si) wire of 1.2 mmdiameter with melt point of 650 �C. Chemical compositions andmechanical properties of base materials and filler wire are tabu-lated in Tables 1 and 2, respectively.

An IPG YLR-4000 fiber laser with wavelength of 1070 nm, awelding head, a welding robot and a filler wire feeder were appliedin the experiment. The laser beam was transmitted through a pro-cessing fiber with diameter of 300 lm, collimated by a lens with150 mm focal length, and then focused on the workpiece surfaceby a focusing lens with 200 mm focal length. Accordingly, the spotsize of focused laser beam is approximately 0.4 mm. The schematicof experimental setup is shown in Fig. 1. Argon gas was suppliedco-axially as shielding gas to avoid the oxidation of melt pool.The laser beam irradiated on the workpiece with an inclination an-gle of 10� relative to the norm of workpiece surface. The angle be-tween filler wire and workpiece surface was 35�. The laser keyholewelding–brazing parameters were adopted as follows: laser power,2300–2600 W; welding speed, 1 m/min; filler wire feeding speed,2.22 m/min; defocusing value, +5 mm; (The plus sign means focalplane is above of the workpiece surface.) shielding gas flow rate,16 l/min.

Before welding, the oxide layers on the surface of aluminum al-loy were removed using abrasive paper, and then all the workpiec-es were cleaned using acetone and dried. The brazing flux,composed of KAlF4, was dissolved in acetone and smeared homog-enously on the workpiece surface near the butting edge with thick-ness of 0.1–0.3 mm.

After welding, typical cross-sections of the samples were cut byelectro-discharge machining (EDM). Then the samples were polishedwith abrasive paper and diamond slurry and etched with Keller’s re-agent and Nital acid for aluminum alloy and steel, respectively.

Macrostructure of joint was observed using an optical micro-scope (OM) and microstructure and composition of joint wereidentified using scanning electron microscopy (SEM) equippedwith an energy-dispersive X-ray spectrometer (EDS). Vickers

Table 1Chemical compositions of base materials and filler wire.

Material Elements (wt.%)

Mg Si Cu Mn Zn

H220YD – 0.01 0.3 0.70 –6016 0.25–0.6 1–1.5 0.2 0.2 –4043 0.05 4.50–5.5 0.30 0.05 0.10

microhardness was measured using HXD-10007 digital and intelli-gent micro-hardness tester with a load of 500 gf and a durationtime of 15 s with reference to standard ASTM: E384-11e1. Accord-ing to the standard of GB/T 228-2002 [15] which is equal to thestandard of ISO 6892:1998 [16], all transverse tensile tests werecarried out at room temperature using WDW-100 universal tensiletesting machine operating with a crosshead speed of 1 mm/min.The dimensions of a tensile test sample are illustrated in Fig. 2.In each case three samples were tested and average value for trans-verse tensile strength of joint was obtained. After transverse ten-sile test, the failed sample was examined using SEM and X-raydiffraction (XRD) to identify the fracture pattern and phase compo-sition in the IMCs layer.

3. Results and discussion

3.1. Joint geometry

The geometric characteristics of galvanized steel–aluminum al-loy butt joint produced by laser keyhole welding–brazing tech-nique is presented in Fig. 3 with laser power, 2600 W; weldingspeed, 1 m/min; filler wire feeding speed, 2.22 m/min; defocusingvalue, +5 mm; shielding gas flow rate, 16 l/min. Obviously, thejoint exhibited dual characteristics: a typical keyhole welded seamformed on the aluminum alloy side, while a brazed seam generatedon the solid galvanized steel side. During laser keyhole welding,the laser beam propagates to bottom of the slender keyhole result-ing in uniform energy distribution along the whole penetrationwith Fresnel absorption and multiple reflections on the keyhole[17–19]. For this reason, the molten filler metal spread consistentlyon the top and bottom surfaces of galvanized steel. Therefore, thebrazed joint with well double-face appearance was obtained with-out any shaped grooves. The wetting lengths of brazed joint on topand bottom surfaces of galvanized steel were almost similar. How-ever, the wetting angle on top surface was a little smaller than thaton bottom surface. It maybe attribute to intense evaporation ontop surface.

3.2. Microstructure of interfacial reaction layer

Fig. 4 illustrates the metallographic microstructure of laser key-hole welding–brazed joint of galvanized steel to aluminum alloy.At the center of welded seam, the dendritic eutectic structurewas formed due to smaller temperature gradient and larger degreeof supercooling during crystallization. At the same time, a greatquantity of equiaxed crystal was formed due to envelopment ofhigh-temperature molten liquid, as shown in Fig. 4a. On the other

Ti P S C Fe Al

0.12 0.06 0.025 0.01 Bal. 0.02– – – – 0.5 Bal.0.20 – – – 0.80 Bal.

Fig. 1. Schematic view of the laser keyhole welding–brazing process from different perspectives: (a) general view, (b) lateral view, and (c) view along welding direction.

Fig. 2. Schematic illustration of transverse tensile testing sample.

Fig. 3. Macroscopic cross sectional view of galvanized steel–aluminum alloy buttjoint with LKWB technique.

26 M.J. Zhang et al. / Materials and Design 45 (2013) 24–30

hand, the columnar crystal was generated nearly vertical to theinterface between galvanized steel and brazed seam, which was in-duced by higher cooling rate near the galvanized steel matrix andpreferential conducting direction of thermal, as shown in Fig. 4b.

Backscatter electron (BSE) analysis was performed to exhibitthe microstructure of interfacial reaction layer with regions A–Dmarked by rectangles in Fig. 3, as shown in Figs. 5 and 6. Therewas a thin and even IMCs layer between brazed seam and galva-nized steel, and the thickness was varied with joint zone. At theinterface between brazed seam and top surface of galvanized steel(zone A in Fig. 3), the interfacial reaction layer had continuousmorphology with thickness of �2.5 lm, as shown in Figs. 5a and6a. Correspondingly, at the interface between brazed seam andbottom surface of galvanized steel (zone D in Fig. 3), the interfacialreaction layer had thinner continuous morphology with thicknessof �1.5 lm, as shown in Figs. 5d and 6d. The IMCs layer at thetop surface was thicker than that at the bottom surface mayberesulted from the heating effect of plasma/plume above the weld-ing pool. At the upper part of the interface between brazed seamand butting edge of galvanized steel (zone B in Fig. 3), the thickness

of the interfacial layer decreased from up to down, with maximumthickness of �13 lm. There also exhibited needle-shaped mor-phology with average thickness of �5 lm, as shown in Fig. 5b. Thismay be attributed to both effects of Fresnel absorption with multi-ple reflections on the keyhole wall and inverse bremsstrahlungabsorption of the keyhole plasma at the upper part of welding poolwith laser beam irradiating directly [19], which leaded to largerheat input and longer reaction time between molten aluminumand solid galvanized steel. At the lower part of the interface be-tween brazed seam and butting edge of galvanized steel (zone Cin Fig. 3), there was a thin and even IMCs layer with thickness of�5 lm, as shown in Figs. 5c and 6c. It should be noted that forma-tion of the keyhole during laser keyhole welding–brazing processensures more energy was transmitted into deeper welding pool.Consequently, there was obvious reaction layer generated at the

Fig. 4. Microstructure of the laser keyhole welding–brazed joint: (a) welded joint and (b) brazed joint.

Fig. 5. Backscatter electron (BSE) images of galvanized steel/brazed seam interfacial reaction layer in Fig. 3: (a) A zone (c) B zone (e) C zone and (g) D zone.

M.J. Zhang et al. / Materials and Design 45 (2013) 24–30 27

lower part of joint, compared to the usual laser welding–brazingprocess as reported in Ref. [20].

In order to identify the phase compositions of the IMCs layer be-tween galvanized steel and brazed seam, EDS analyses of the IMCslayer were performed at the points marked by crosses in Fig. 6, andthe results were plotted in Table 3. According to the Al–Fe–Si ter-nary phase diagram [21] (Fig. 7), and the characterization of theternary solid phases in the Al–Fe–Si system [22–25], the EDS anal-ysis indicates that the IMCs layer near brazed seam, maybe consistof a(s5)-Al8Fe2Si phase. Since filler metal contained an amount ofSi (5 mass%) and growth energy of Al–Fe–Si ternary phase was low-er than that of Al–Fe binary phases [21], a(s5)-Al8Fe2Si phase firstlyformed on the interfacial layer. The primary layer inhibited thedirect reaction between liquid aluminum alloy and solid galva-nized steel. During primary layer’s growing, the high temperatureliquid filler eroded and dissolved the layer, and Al atoms coulddiffuse through the layer and react with melted Fe matrix to form

Fe–Al phases. The IMCs in the internal part of the reaction layernear galvanized steel were varied with joint zone. In details, h-Al13-

Fe4 or Al3Fe phase was thought to be generated at the whole inter-face between welded seam and galvanized steel. f-Al2Fe or g-Al5Fe2 phase may be generated at the interface between brazedseam and butting edge of galvanized steel.

3.3. Mechanical properties of joints

Vickers microhardness of the interfacial layer in the laser key-hole welding–brazed joint was measured using a digital and intel-ligent microhardness tester with 500 gf loading force and 15 sholding time. The averages of three measurements are shown inFig. 8. The average hardness of brazed seam was �53.5 HV withAl–5Si filler metal and the value of galvanized steel substratewas �480 HV. However, the hardness increases quickly in theinterface of the joint. At the interfaces between brazed seam and

Fig. 6. BSE microstructures of galvanized steel/brazed seam interfacial layer in Fig. 5: IMCs layer in (a) E zone, (b) F zone, (c) G zone and (d) H zone.

Table 3EDS analyses results of IMCs layer pointed in Fig. 6.

Points Main elements

Al Fe Si

wt.% at.% wt.% at.% wt.% at.%

1 65.68 78.08 30.42 17.47 3.9 4.452 55.59 71.07 41.94 25.91 2.46 3.023 38.11 55.47 60.42 42.48 1.47 2.054 64.05 78.23 34.99 20.65 0.96 1.125 61.9 76.48 36.8 21.97 1.31 1.556 58.38 73.67 40.04 24.41 1.58 1.927 55.33 71.23 43.06 26.78 1.6 1.988 52.63 68.88 45.49 28.76 1.88 2.369 41.27 58.36 56.5 38.61 2.23 3.03

10 71.77 83.22 26.5 14.85 1.73 1.9311 60.01 74.65 37.79 22.71 2.2 2.6312 52.69 68.85 45.28 28.59 2.04 2.5613 29.8 46.19 68.51 51.3 1.69 2.5114 73.96 84.53 24.04 13.27 2 2.215 61.62 75.34 34.96 20.65 3.41 4.0116 46.02 62.82 51.55 33.99 2.43 3.19

Fig. 7. Isothermal section of the Al–Fe–Si system at 600 �C [21].

28 M.J. Zhang et al. / Materials and Design 45 (2013) 24–30

top/bottom surfaces of galvanized steel, the average hardness val-ues were�1032 HV in the layer of a(s5)-Al8Fe2Si, h-Al13Fe4 or Al3Feand �811 HV in the layer of f-Al2Fe or g-Al5Fe2, as shown withlines I–IV in Fig. 8. At the interface between brazed seam and butt-ing edge of galvanized steel, the average hardness values were�1060 HV in the layer of a(s5)-Al8Fe2Si, h-Al13Fe4 or Al3Fe, and�829 HV in the layer of f-Al2Fe or g-Al5Fe2, as shown with linesII and III in Fig. 8 [26].

Transverse tensile test was carried out to evaluate the tensilestrength of the laser keyhole welding–brazed steel–aluminum hy-brid joint. Fig. 9 shows that three failure modes of joints were ob-served with different laser powers of 2300 W (Fig. 9a), 2500 W(Fig. 9b) and 2600 W (Fig. 9c) and keeping other parameters con-stant. The best was fractured at heat affected zone (HAZ) of alumi-num alloy with appearance of necking in tension, as shown in

Fig. 9a. The average tensile strength was up to 162 MPa. Following,the specimens were fractured through HAZ of aluminum alloy andwelded seam simultaneously with average tensile strength of140.5 MPa, as shown in Fig. 9b. The worst was detached betweengalvanized steel and brazed seam with average tensile strengthof 124 MPa, as shown in Fig. 9c. This implied that the joint formedin higher heat input should have a lower tensile strength than inlower heat input because the brittle IMCs layer was thicker. There-fore, in order to improve the strength of joint, it should limit theheat input.

In order to have knowledge of fracture behavior of the tensilespecimens, the fractured surfaces were observed by SEM, as shownin Fig. 10. The morphology of tensile specimen failed at HAZ of alu-minum alloy, exhibited ductile dimple fracture with characteristic

-2 0 2 4 6 8 100

200

400

600

800

1000

1200

1400 I II III IV

Distance from interface in steel side (μm)

Vic

kers

mic

roha

rdne

ss (

HV

)

Steel

Seam Seam

IIIIIIIV

Fig. 8. Microhardness distribution across the interface between galvanized steeland brazed seam.

Fig. 9. Macroscopic view of failure specimens: (a) fractured at HAZ of aluminumalloy, (b) mix-failed through HAZ of aluminum alloy and brazed seam, and (c)detached between galvanized steel and brazed seam.

35 40 45 50 55 60 65 700

20

40

60

80

100

120

140

160

180

Inte

nsit

y (a

.u.)

2ϑ (degrees)

Fig. 11. X-ray diffraction profile of the fracture plane in steel side.

M.J. Zhang et al. / Materials and Design 45 (2013) 24–30 29

of dimples, as illustrated in Fig. 10a. Fig. 10b shows the morphol-ogy of the tensile specimen failed through HAZ of aluminum alloyand brazed seam simultaneously. In the center of fractured blade,

Fig. 10. SEM fractography of the ruptured specimens: (a) fractured at HAZ of aluminudetached between galvanized steel and brazed seam.

the fractography exhibited ductile dimples like failure at HAZ ofaluminum alloy. However, the morphology of the broke seam zonerevealed brittle fracture pattern. The morphology of the tensilespecimen detached along the IMCs layer exhibited brittle fracturepattern, as shown in Fig. 10c.

XRD analysis was carried out on the fracture plane of specimendetached between galvanized steel and brazed seam, as shown inFig. 11. It can be confirmed that the fracture layer containeda(s5)-Al8Fe2Si, h-Al13Fe4 and f-Al2Fe phases [10–11,27–28]. Thebrittle h-Al13Fe4 and f-Al2Fe phases in the IMCs layer reduced themechanical resistance of the joint and could be failed by a smalltensile force, especially when its thickness exceeded permissiblevalue (about 10 lm) [26].

4. Conclusion

The butt joining of automotive galvanized steel to aluminum alloywas carried out by fiber laser keyhole welding–brazing technique.The microstructure characteristics and mechanical properties ofthe butt joint were investigated. Major conclusions could be summa-rized as follows:

(1) A sound butt joint of galvanized steel to aluminum alloy,exhibited dual characteristics: a typical keyhole weldedseam formed on the aluminum alloy side, and a brazed seamgenerated on the solid galvanized steel side, was obtained byfiber laser keyhole welding–brazing technique without anyshaped grooves.

(2) The dendritic eutectic structure and a great quantity of equi-axed crystal were formed at the center of welded seam. Thecolumnar crystal was generated nearly vertical to the inter-facial layer between galvanized steel and brazed seam.

(3) There was a thin and even interfacial reaction layer betweenbrazed seam and galvanized steel. The thickness of the inter-facial reaction layer was varied with joint zone ranged from1.5 lm to 13 lm.

m alloy, (b) mix-failed through HAZ of aluminum alloy and brazed seam, and (c)

30 M.J. Zhang et al. / Materials and Design 45 (2013) 24–30

(4) The interfacial reaction layer was composed of threedifferent phases: a(s5)-Al8Fe2Si, h-Al13Fe4 and f-Al2Fe withmicrohardness ranged from �811 HV to �1060 HV.

(5) The maximum transverse tensile strength of the butt jointreached 162 MPa. The corresponding joint was fractured atHAZ of aluminum alloy with typical ductile dimple fracturepattern.

Acknowledgements

The authors would like to appreciate the financial support fromthe National Natural Science Foundation of China (No. 51175165).And the discussion about MS with Dr. Qun Wang and Dr. YingzheZhang from College of Materials & Engineering of Hunan Universityis highly appreciated.

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