M A T E R I A L S C H A R A C T E R I Z A T I O N 6 9 ( 2 0 1 2 ) 8 4 – 8 9

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Microstructure evolution of Al/Mg butt joints welded by gastungsten arc with Zn filler metal

Fei Liu, Zhaodong Zhang, Liming Liu⁎

Key Laboratory of Liaoning Advanced Welding and Joining Technology, School of Materials Science and Engineering,Dalian University of Technology, Dalian, 116024, China

A R T I C L E D A T A

⁎ Corresponding author. Tel./fax: +86 411 8470E-mail address: liulm@dlut.edu.cn (Limin

1044-5803/$ – see front matter © 2012 Elseviedoi:10.1016/j.matchar.2012.04.012

A B S T R A C T

Article history:Received 21 November 2011Received in revised form2 March 2012Accepted 20 April 2012

Based on the idea of alloying welding seam, Gas tungsten arc welding method with pure Znfiller metal was chosen to join Mg alloy and Al alloy. The microstructures, phases, elementdistribution and fracture morphology of welding seams were examined. The resultsindicate that there was a transitional zone in the width of 80–100 μm between the Mg alloysubstrate and fusion zone. The fusion zone was mainly composed of MgZn2, Zn-based solidsolution and Al-based solid solution. The welding seam presented distinct morphology indifferent location owning to the quite high cooling rate of the molten pool. The addition ofZn metal could prevent the formation of Mg–Al intermetallics and form the alloyed weldingseam during welding. Therefore, the tensile strengths of joints have been significantlyimproved compared with those of gas tungsten arc welded joints without Zn metal added.

© 2012 Elsevier Inc. All rights reserved.

Keywords:WeldingIntermetallicsMicrostructureTensile strength

1. Introduction

Al alloys have a lot of attractive mechanical and metallurgicalproperties, including high specific strength and excellent corro-sion resistance; this is why they are widely used for structuralcomponents in many applications such as automobiles, aero-space, electronic industries [1,2]. Due to their unique propertiessuch as lower weight ratio and electromagnetic shieldingcapability, Mg alloys also have great potential in manufacturingindustry [3,4]. Besides, with the growing economical andenvironmental needs, the Mg alloy has been a favorite choicein automobile field. Thus, welding Mg to Al alloys to formcompound structure is necessary, which can not only improvethe flexibility and availability of components substantially butalso reduce the weight and the cost of component [5–7].

Vacuum diffusion bonding, laser welding, soldering, ex-plosive welding and friction stir welding (FSW) are sometechniques proposed to dissimilarly weld Mg alloys and Alalloys [8–14]. The researches indicate that brittle and hardintermetallics formed by fusion Mg–Al welding can deteriorate

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r Inc. All rights reserved.

the performances of welded joints seriously. As a solid-phasebonding process, FSW can yield joints to a better level thanfusion welding technology; however, there are still the Mg–Alintermetallics in the welding seam. In another word, if thereis a direct contact between Mg and Al, it is very difficult toeliminate the formation of intermetallic compounds (IMCs) byfusion welding or solid-state joining.

Alloying is commonly used to increase the strength ofmaterial, which means adding other elements may be aneffective method to enhance the strength of welded joint.Commonly, Zn acts an alloying element in producing Mgalloys and Al alloys to improving the mechanical properties.Therefore, if Mg–Al–Zn ternary alloy can form in the weldingseam of Mg/Al instead of Mg–Al intermetallic phases, the jointstrength will be increased. Based on the alloying weldingseam idea, it will be promising that to add pure Zn in weldingof Mg/Al can improve the tensile strength of the joints byforming Mg–Al–Zn ternary alloying welding seams.

In this present paper, Mg alloy AZ31B and Al alloy 6061were welded by Gas tungsten arc (GTA) butt welding, which is

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widely used in industrial production [15,16]. The feasibilityof GTA welding of Mg–Al with a Zn filler metal has beeninvestigated. By analyzing the microstructure evolution of thejoints, the effect of the additional Zn metal on interfacemicrostructure and strength of the joint was studied, and theformation mechanism of the alloyed welding seams has beenalso presented. So far, no reports have been published onusing GTA welding arc as the welding heat source to realizedissimilar material butt welding between Mg and Al alloysfilling with pure Zn metal.

Fig. 2 – Sketch of butt welding tensile test specimens (mm).

2. Experiments

The base materials employed in this investigation were magne-sium alloy AZ31B sheet with nominal composition of Mg–3Al–1Zn–0.2Mn–0.1Si (wt. %) and aluminum alloy 6061 sheet withnominal composition of Al–1Mg–0.6Si–0.15Cu–0.01Mn (wt. %).Thedimensions of the two sheetswere 2mm×50mm×100mm.Zn metal with a purity of 99.9 wt. % and a diameter of 3.5 mmwas used. Before welding, the butt sheets were machined into“V”-type groove of 120 degrees, degreased and ground byacetone and abrasive paper, respectively. Zn metal was placedin the groove. The schematic illustration of welding process isshown in Fig. 1. The optimized parameters in the experimentwere GTA welding current of 120A and welding speed of440mm/min.

The cross-sections of the welded joint, perpendicularly tothe welding direction, were prepared in accordance with themetallographic method and were etched by 3% HNO3 alcoholsolution. The cross-sections and the fracture surface of thewelded joint were observed with metalloscope and scanningelectron microscope (SEM). Element distribution in the weld-ing seam was examined with electron probe micro-analyzer(EPMA). Phases in the welding seamwere analyzed with X-raydiffraction (XRD).

The tensile specimens have undergone tensile strength testson an electronic tensionmachine (Css-2205) under a travel speedof 2mm/min at room temperature. The dimension of the tensilespecimens is shown in Fig. 2. The joint tensile strength iscalculated according to the equation σs=F/A, where F and σs arethe load (N) and the ultimate tensile strength (MPa), respectively;A, calculated by multiplying the width by the thickness of thespecimen, is the cross sectional area (mm2) of the tensile testspecimen.

Fig. 1 – Schematic illustrations of TIG butt welding.

3. Results and Discussion

3.1. Analysis of the Microstructure of the Joint

Fig. 3 shows the surface appearance of the butt welded jointformed between the Mg and Al alloys by GTA welding process.Welding defects, such as spatter and undercut are not found.

The cross-section perpendicularly to the welding directionis shown in Fig. 4(a), which presents themacrostructure of thewelded joint. It can be seen that nomacroscopic cracks appearin the joint, but some pores exist in the fusion zone (FZ). Thismay be resulted from that the fully vaporized Zn could notoverflow the weld pool during the welding process.

Fig. 4(b), (c) and (d) presents the microstructures of thewelding seam near the Mg alloy substrate, in the fusion zoneand near the Al alloy substrate, which are the magnifiedimages of the regions 1, 2 and 3 in Fig. 4(a), respectively.

It can be seen from Fig. 4(b) that there is a transitional zone(TZ) with 80–100 μm in width between the Mg alloy substrateand FZ. However, no apparent TZ between the Al alloysubstrate and FZ can be observed in Fig. 4(d). In order toinvestigate the element distributions of the welding seam,EPMA detection was carried out and the analysis results areshown in Table 1. The results show that the compositionranges of TZ are 71.70–66.11 at.% Mg, 10.05–12.59 at.% Al and18.25–21.30 at.% Zn, indicating that the TZ is mainly com-posed of the solid solution of Mg and Al6Mg11Zn11 in terms ofMg–Al–Zn ternary phase diagram [17] as shown in Fig. 5.

Fig. 3 – Surface appearances of the weld seam.

Fig. 4 –Microstructures of welded joint. (a) is the macrostructure of the joint; (b), (c) and (d) are the magnification of 1, 2 and 3 in(a); (e), (f) and (g) are the magnifications of I, II, and III from (b), (c) and (d), respectively.

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Fig. 4(e), (f), and (g) is the magnified images of the regions I,II, and III from Fig. 4(b), (c), and (d), respectively. Fig. 4(e) showsthat the grains are equiaxed crystals in the FZ near the Mgalloy substrate and the grain size is approximately 3–8 μm.Fig. 4(f) and (g) shows that most of the grains which locate inthe center of the welding seam and near the Al alloy substrateare dendrite crystals with the grain sizes of 8–20 μm. The XRDanalysis of FZ shown in Fig. 6 reveals that the welding seamconsists of the compound MgZn2, Zn-based solid solution andAl-based solid solution.

The composition of the gray grains (area 3, 5 and 7) inFig. 4(e), (f) and (g) is 31–40 at.% Mg, 1–8 at.% Al and 68–52 at.%Zn, suggesting that the gray grains are composed of MgZn2

Table 1 – Chemical compositions of different areas.

Serialnumber

Percentagecomposition (at. %)

Inferencecomposition

Mg Al Zn

1 71.7 10.1 18.3 (Mg), Al6Mg11Zn11

2 66.1 12.6 21.3 (Mg), Al6Mg11Zn11

3 40.2 7.5 52.3 (Zn), MgZn2

4 13.4 43.1 42.5 MZAS5 34.1 4.8 61.1 (Zn), MgZn2

6 6.2 7.0 86.8 (Zn)7 31.6 1.2 67.2 (Zn), MgZn2

8 7.3 26.2 66.5 MZAS

Note: (Mg) and (Zn) in the table represent Mg-based and Zn-basedsolutions, respectively.

and a little Zn-based solid solutionwith reference to the ternaryphase diagram and the XRD results. In addition, the composi-tions of the gray grains do not show gradient changes in thedifferent locations of the FZ from the near theMgalloy substrateto near the Al alloy substrate.

The composition percentage of the white phases (see thearea 4 in Fig. 4(e)) is 13.4 at.% Mg, 43.1 at.% Al and 42.5 at.% Zn.It can be deduced that those phases are a mixture of Zn-basedsolid solution and Al-based solid solution (MZAS). The molarratio of Zn-based solid solution and Al-based solid solution isclose to 1:1. The composition percentage of white phase (area6 in Fig. 4(f)) is 6.2 at.% Mg, 7.0 at.% Al and 86.8 at.% Zn,suggesting that most of those white phases are Zn-based solidsolution. The white phases, located near the Al alloy substrate,are shown in the area 8 in Fig. 4(g), whose percentage com-positions are 7.3 at.%Mg, 26.2 at.%Al and 66.5 at.%Zn, suggestingthat thosewhite phases are alsoMZAS, and themolar ratio of Zn-based solid solution and Al-based solid solution is approximately2.5:1.

Taking Fig. 4(e), (f) and (g) for comparison, it can be seen thatthe amount of white phases from the side of Mg alloy substrateto the side of Al alloy substrate is gradually increasing. Bycalculating the area of the white phases in Fig. 4(e), (f) and (g), itcan be obtained that the percentages of white phases areapproximate to 15%, 52% and 66%, respectively.

In thewelding process, Zn fillermetal, below the electric arc,melts firstly to form the welding pool. Then as the base metalsmelt, Mg andAl elements go intomolten pool from the oppositesides. The cooling rate of the molten weld pool is quite high,so the Mg, Al and Zn elements can not mix soundly during the

Fig. 5 – Mg–Al–Zn ternary phase diagram.

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welding process, resulting in that different microstructuresform in the different locations of the FZ.

According to the Mg–Al–Zn ternary phase diagram shownin Fig. 5, in welding pool near the Mg alloy substrate, theMgZn2 precipitates primarily, and then the composition ofliquid alloy reaches the e2E2 line and the reaction occurs:liquid→ (Al)+MgZn2 ((Al) representative of Al-based solidsolution for similarly hereinafter). Meanwhile, in the weldingpool near the Al alloy substrate MgZn2 precipitates primarily,and then takes the reaction of liquid→ (Al)+MgZn2; subse-quently the last remaining liquid alloy experiences the reaction:liquid→ (Al)+MgZn2+(Zn). In the center of welding pool Zn-

Fig. 6 – XRD analysis of FZ.

based solid solution precipitates primarily and subsequentlythe eutectoid reaction, liquid→ (Al)+MgZn2+(Zn), takes place.

When Mg and Al alloy are welded directly by GTA weldingprocess, the major problem is the formation of many Mg–AlIMCs with high hardness and brittleness, and the IMCs locatebetween Mg and Al alloy as an interlayer.

These hard intermetallic compounds act preferentially asthe source ofmicrocracks in themechanical property tests, andcan make joint performances deteriorate seriously. Similarresults are also reported by Liu etc. [18]. However, by filling Znmetal the direct contact between Mg and Al alloys is preventedand hence the Mg–Al IMCs are avoided. Consequently, Mg–Al–Zn ternary alloyedwelding seams are formed. AlthoughMg andZn also produced MgZn2 IMCs, these IMCs are separated by theMZAS, which can not form a continuous layer.

3.2. Analysis of the Tensile Strength and Fracture of theWelded Joint

Tensile tests were carried out to measure the tensile strengthsof GTA welded Mg/Al joint with and without Zn filler metal.The results show that the average tensile strength of jointswith Zn filler metal can reach to 93 MPa, which is improvedsignificantly compared with that of the Mg/Al direct fusionjoint (28 MPa). In the tensile test, the Mg/Al direct fusion jointfractured in the interlayer of IMCs in the joint, while allthe welded joints with Zn filler metal fractured in the FZ nearthe Mg alloy substrate, as shown by the white dotted line inFig. 4(a).

Fig. 7 – SEM images of fracture surface. (a) Mg/Al direct joint. (b) Mg/Al joint filled with Zn metal.

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From the above analysis, it can be known that the weldingseam consists of MgZn2 compound, Zn-based solid solutionand Al-based solid solution, and the alloyed welding seamform by adding pure Zn metal. The alloys whose composi-tions are approximate to those of MZAS have excellentplasticity [19]. When the sample bears the tension stress,the MZAS can eliminate the stress concentration of cracktip and hence hinder crack propagation. Therefore, tensilestrength would be improved significantly by amelioratingthe microstructures in the joint. However, the alloys of thewelding seam near the Mg alloy substrate have less MZASthan those in the center and near the Al alloy substrate.This is the reason for all the welded joints fractured in theFZ near the Mg alloy substrate.

The fracture surfaces of Mg/Al joints with and without Znfiller metal are shown in Fig. 7. As shown in Fig. 7(a), thefracture surface of welded joint without Zn filler metaldisplays brittle feature and implies no plastic deformationoccurred before fracture. However, several tearing ribs can beseen on the fracture surface of Mg/Al joint with Zn filler metalas shown in Fig. 7(b), suggesting that plastic deformationoccurred before fracturing.

According to their shape and the XRD results given inFig. 8, the integrated grains pointed by the arrows in Fig. 7(b)are deduced to be MgZn2 grains, implying that intercrystalline

Fig. 8 – XRD analysis of fracture surface.

fracture occurred in the tensile test. The fractographies of thejoints in Fig. 7 also present that the additional Zn makes themicrostructure of the fracture zone finer.

Asmentioned above, forming an alloyed welding seam canimprove significantly the tensile strength of the welded joints,so the composition design of the alloyed welding seam fordissimilar welding between Mg alloy and Al alloy is essentialand would need to be further investigated.

4. Conclusions

(1) A novel method, based on the idea of alloying weldingseam, was applied to dissimilar welding between Mgalloy and Al alloy, and the plates of AZ31B alloy and6061 alloy were successfully welded with Zn filler metalby GTA butt welding.

(2) The alloyed welded joint was obtained, and the FZ wasmainly composed of MgZn2, Zn-based solid solutionand Al-based solid solution. A TZ of 80–100 μm in widthwas formed between the Mg alloy substrate and FZ,while no obvious TZ existed between the FZ and Al alloysubstrate. The welding seam presented distinct mor-phology in different location owning to the quite highcooling rate of the molten pool.

(3) The tensile strength of the Mg/Al joint formed with Znfiller metal could reach to 93 MPa, which is improvedsignificantly compared with that of the GTA weldedjoint without Zn filler metal. During tensile test, thejoints fractured in the FZ near the Mg alloy substrate,and integrated MgZn2 grains could be seen in thefractography, indicating that the intercrystalline frac-ture mode occurred.

(4) Direct contact between Mg and Al alloys was preventedand the Mg–Al IMCs were avoided by filling Zn metalduring GTA welding; thus the Mg–Al–Zn ternary alloyedwelding seams could be obtained and exhibited goodmechanical proprieties in the tensile test.

Acknowledgments

We really appreciate the supports from National NaturalScience Funds for Distinguished Young Scholar (51025520).

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