ndeldin

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TechrovinH, A4

Received 10 May 2006; accepted 4 July 2006

restricted. Thus, the joining of aluminium to steel by use of an Al alloy filler metal containing Si. Murakami etal. [12] and Mathieu et al. [13] both point out that thetemperature probably determines the thickness of the

Materials Characterization 58 (1. Introduction

In order to reduce pollution and save energy, it isattractive to make car bodies lighter by introducing somealuminium parts as substitutes for the previous steelstructures [1,2]. Therefore, joining aluminium to steel hasbecome a major problem, requiring resolution. Directsolid-state joining can be used to make these dissimilarmetal joints by controlling the thickness of the interme-tallic compound layer that develops within a fewmicrometers of the joint interface [39]. However, theshape and size of such solid-state joints are extremely

fusion welding methods has been widely studied. As iswell known, the joining of aluminium to steel by fusionwelding is difficult because of the formation of brittleinterface phases which can deteriorate the mechanicalproperties of the joints. However, Kreimeyer and Sepold[10] have shown that if the layer is less than 10 m thick,the joint will be mechanically sound. In addition, theauthors also deem that the existence of a zinc coatingincreases the wettability of the Al to the steel substrate. Asanother approach, Achar et al. [11] reported that thethickness of the intermetallic compound layer formedduring TIG arc welding of Al to steel is decreased by theThe microstructure and properties of aluminiumzinc coated steel lap joints made by a modified metal inert gas CMTweldingbrazing process was investigated. It was found that the nature and the thickness of the high-hardness intermetallic compound layerwhich formed at the interface between the steel and the weld metal during the welding process varied with the heat inputs. From theresults of tensile tests, the welding process is shown to be capable of providing sound aluminiumzinc coated steel joints. 2006 Elsevier Inc. All rights reserved.

Keywords: Weldingbrazing; Heat input; Intermetallic compoundAbstractInterfacial microstructure aaluminiumzinc-coated ste

metal inert gas wel

H.T. Zhang a,, J.C. Fea State Key Laboratory of Advanced Welding Production

Heilongjiang Pb Fronius. Internation GMBCorresponding author. Tel.: +86 451 86412974; fax: +86 45186418146.

E-mail address: hitzht@yahoo.com.cn (H.T. Zhang).

1044-5803/$ - see front matter 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2006.07.008mechanical properties ofjoints made by a modifiedgbrazing process

a, P. He a, H. Hackl b

nology, Harbin Institute of Technology, Harbin 150001,ce, PR China600 Wels-Thalheim, Austria

2007) 588592intermetallic compound layer of the joint and recom-mended the use of lower heat input to obtain a sound joint.

The cold metal transfer process, identified here asCMT, is a modified metal inert gas welding processwhich invented by the Fronius Company. The principalinnovation of this method is that the motions of thewelding wire have been integrated into the weldingprocess and into the overall control of the process. Everytime the short circuit occurs, the digital process-controlboth interrupts the power supply and controls the re-traction of the wire. The wire retraction motion assistsdroplet detachment during the short circuit, thus greatlydecreasing the heat input during welding.

In this study, we selected the CMT process to joinTypical transverse sections of the samples were

observed using optical microscopy (OM) and scanningelectron microscopy (SEM). The composition of theintermetallic compound layer at the interface betweenthe steel and the weld metal was determined by energydispersive X-ray spectroscopy (EDX). Hardness valueswere obtained using a microindentation hardness testerwith a load of 10 g, and a load time of 10 s. In addition,the samples were cut in 10 mm widths, and transversetensile tests (perpendicular to the welding direction)were used to measure the joint tensile strength.

Table 1The welding parameters

Samplenumber

Meanweldingcurrent(A)

Meanweldingvoltage(V)

Wire feedrate(m/min)

Weldingspeed(mm/min)

Weldheatinput(J/cm)

Sample A 66 11.8 3.9 762 613.2Sample B 110 13.3 5.4 913 961.5

589H.T. Zhang et al. / Materials Characterization 58 (2007) 588592aluminium to zinc-coated steel using a lap geometry.The main purpose of this effort was to reveal the rela-tionship between heat input and the microstructure ofthe joint. Hardness testing was also used to characterizethe phases formed during the welding process. In ad-dition, the quality of the joints was assessed by tensiletesting.

2. Experimental

Deep drawn sheets of hot-dip galvanized steel andsheets of pure Al 1060 with thickness of 1 mmwere usedin the welding experiments. An Al sheet was lapped overa Zn-coated steel sheet on the special clamping fixture,and the ending of the weld wire was aimed at the edge ofthe aluminium sheet, as shown in Fig. 1. The MIGweldingbrazingwas carried out using the CMTweldingsource with an expert system and 1.2-mm-diameter AlSi filler metal wire. Argon was used as the shielding gasat a flow rate of 15 L/min. The surface of the samples wascleaned by acetone before welding. Two sets of weldingparameters of different heat inputs were selected, asshown in Table 1. The heat input, J, is calculated usingthe equation: J=(60UI)/v, where U is the meanwelding voltage, I is the mean welding current and v isthe welding speed.Fig. 1. Schematic plan of the welding process.

Fig. 2. Front (upper) and back (lower) appearances of typical jointswith different heat inputs: (a) Sample A; (b) Sample B.

3. Results and discussion

3.1. Macro- and microstructures

The appearance of the weld seams for different heatinputs are shown in Fig. 2. For all welding cases, asmooth weld seam was made. The molten metal wettedthe steel better when using lower heat input, i.e.,compare Sample A at lower heat input to Sample B.This may be related to the different degree of evapo-ration of the zinc coating at different heat inputs. Whileimproving the heat input, the greater evaporation ofzinc reduces the wettability of the molten metal on thesteel.

Fig. 3 shows a typical cross-section of the joints.Higher heat input (Sample B) resulted in a decrease inthe contact angle between the steel and the weld metal.Meanwhile, a special zone with lighter colour at the toe

Fig. 5. At lower heat input (Sample A), the inter-metallic compound presents a serrated shape orientedtoward the weld metal. When the heat input wasincreased (Sample B), the compound layer becamemuch thicker and grew into the weld metal with tongue-like penetrations. Anisotropic diffusion is a possibleexplanation for this irregularity. The intermetalliccompounds that form under these conditions generallyhave an orthorhombic structure (see below). Because ofthe high vacancy concentration along the c-axis of theorthorhombic structure, Al atoms can diffuse rapidlyin this direction and cause rapid growth of the inter-metallic compound.

EDX analysis was used to determine the phases ofthe intermetallic compound layer. The results show thatthe intermetallic compound layer of the joint made bylower heat input consists entirely of Fe2Al5. But whenthe heat input is increased, the intermetallic compoundlayer consists of two different phases, the FeAl2 phasenear the steel surface and a FeAl phase which

Fig. 4. Optical microstructures of interface between steel and weldmetal: (a) Sample A; (b) Sample B.

590 H.T. Zhang et al. / Materials Charaof the weldments can be found (designated by whitearrows in Fig. 3). Optical micrographs shows that avisible intermetallic compound layer has formed be-tween the steel and weld metal during the weldingprocess, Fig. 4. The thickness of the intermetallic com-pound layer changes not only with the location within agiven joint but also with the varying heat input betweendifferent joints. The thickness of the intermetalliccompound layer in the center is greater than at theedge of the seam within one joint. For Sample A, themaximum thickness of the compound layer is about10 m but is 4050 m for Sample B.

The microstructure of the intermetallic compound isshown in greater detail in the SEM micrographs in

Fig. 3. Cross-section image at limit of penetration in the joint, showingchange in contact angle with increased heat input. Arrows point to an

intermetallic compound at the tip of the weld metal: (a) Sample A; (b)Sample B.cterization 58 (2007) 5885923

penetrates toward the weld metal. Thus it is clear that

Fig. 7. Microindentation hardness test results of the joints made usingdifferent heat inputs.

591H.T. Zhang et al. / Materials Characterization 58 (2007) 588592the intermetallic compound layer that forms is closelyrelated to the heat input during the welding process.

With regard to the special zone designated by whitearrows in Fig. 3, dendritic-appearing structures can be

Fig. 6. Dendrite crystal structure at the toe of the weldment (Sample B).

Fig. 5. SEM micrograph of interface between steel and weld metal: (a)Sample A; (b) Sample B.distinguished on a high-magnification SEM micrograph(Fig. 6). EDX analysis results show that such dendrite-shaped crystals of an Al-rich -solid solution containingresidual zinc routinely formed at this location.

3.2. Hardness measurements

Hardness testing results also confirm the presence of ahard intermetallic compound layer. The hardness of theinterface layer is much higher than that of the base metaland the weld metal and is found to vary for thecorresponding intermetallic compound phases. For thehigh heat input weld (Sample B) the hardness is muchhigher, Fig. 7.Fig. 8. The location where the fracture occurred during tensile testing(designated by white arrows): (a) Sample A; (b) Sample B.

3.3. Tensile test results

The tensile tests were performed to provide aqualitative measure of the joint strength and behavior.These results show that the bond strength is excellent,with the fractures occurring in the HAZ of the Al evenwhen the thickness of the intermetallic compound layerwas greater than 40 m, Fig. 8. From a general view-point, the thickness of the intermetallic compound layer

Acknowledgements

The authors wish to acknowledge the financialsupport provided by the National Natural ScienceFoundation under Grant No. 50325517 for this work.

References

[1] Schubert E, Klassen M, Zerner I, Walz C, Sepold G. Light weightstructures produced by laser beam joining for future applicationsin automobile and aerospace industry. J Mater Process Technol

592 H.T. Zhang et al. / Materials Characterization 58 (2007) 588592obtain a sound joint. This implies that the joint madewith higher heat input should have a lower intrinsicstrength than the other because of the thicker brittleintermetallic compound layer. However, the intrinsicstrength of the joints cannot be determined when thefracture occurs in the HAZ of the pure Al. Nevertheless,according to the thickness of the compound layer, wecan presume that the intrinsic strength of the jointsshould be decreased when increasing the welding heatinput.

4. Conclusions

Based on the experimental results and discussions,conclusions are drawn as follows

1) Dissimilar metal joining of Al to zinc-coated steelsheet without cracking is possible by means of amodified metal inert gas (CMT) weldingbrazingprocess in a lap joint.

2) FeAl intermetallic compound phases were formedat the interface between the steel and the weld metal.The thickness and the composition of the interme-tallic compound layer varied with weld heat input.

3) Despite the formation of the intermetallic compoundphases, the interface between steel and weld metal isnot the weakest location of the joints. Tensile tests ofthe joints caused fractured in the Al HAZ, even whenthe intermetallic compound layer thickness exceeded40 m.2001;115:2.[2] Schubert E, Zernet I, Sepold G. Laser beam joining of material

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[5] Fukumoto S, Tsubakino H. Friction welding process of 5052aluminium alloy to 304 stainless steel. Mater Sci Technol1999;9:1080.

[6] Ochi H, Ogawa K, Suga Y, Iwamoto T, Yamamoto Y. Frictionwelding of aluminum alloy and steel using insert metals.Keikinzoku Yosetsu 1994;11:1.

[7] Shinoda T, Miyahara K, Ogawa M, Endo S. Friction welding ofaluminium and plain low carbon steel. Weld Int (UK)2001;6:438.

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[9] Adler L, Billy M, Quentin G. Evaluation of friction-weldedaluminum-steel bonds using dispersive guided modes of alayered substrate. J Appl Phys 2001;12:6072.

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[11] Achar DRG, Ruge J, Sundaresan S. Joining aluminum to steel,with particular reference to welding (III). Aluminum 1980;4:291.

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Interfacial microstructure and mechanical properties of aluminiumzinc-coated steel joints made.....IntroductionExperimentalResults and discussionMacro- and microstructuresHardness measurementsTensile test results

ConclusionsAcknowledgementsReferences