Materials and Design 46 (2013) 419–425

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Materials and Design

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Effect of Al foils interlayer on microstructures and mechanical properties ofMg–Al butt joints welded by gas tungsten arc welding filling with Zn filler metal

Fei Liu, Daxin Ren, 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 i n f o

Article history:Received 24 July 2012Accepted 6 October 2012Available online 26 October 2012

Keywords:A. Non-ferrous metals and alloysD. WeldingE. FractureF. Microstructure

0261-3069/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.matdes.2012.10.012

⇑ Corresponding author. Tel./fax: +86 0411 847078E-mail address: liulm@dlut.edu.cn (L. Liu).

a b s t r a c t

Gas tungsten arc butt welding of Mg–Al filling with Zn filler metal without and with Al foils in differentthicknesses was carried out. Additional Al element was introduced into the fusion zone to accuratelymodulate microstructure and composition of the welding seam. Microstructures and mechanical proper-ties of the welded joints were examined. Results show that the addition of appropriate quantity of Al ele-ment increases the content of Al-based solid solution in the fusion zone near the Mg base metal. The solidsolution can eliminate the stress concentration and hinder crack propagation, so the tensile strengths ofthe joints are improved. However, the immoderate quantity of Al element will lead to the formation ofpartially Al-rich zones and deteriorate the mechanical property of the joints.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Mg alloys, the lightest structural metals, are in great potentialfor application to manufacturing industry due to their quite lowerweight ratio, electromagnetic shielding capability and some otherunique properties [1]. Al alloys are also widely used in many appli-cations such as automobiles, aerospace and electronic industries,because of their attractive mechanical and metallurgical proper-ties, including high specific strength and excellent corrosion resis-tance [2]. Thus, welding reliability between Al alloys and Mg alloysis required in order to achieve the application of Mg–Al combina-tion [3]. Besides, gas tungsten arc (GTA) welding, as a traditionalwelding method, is commonly used in industry due to its seriesof advantages, such as high energy utilization [4] and easy opera-tion [5]. In addition, butt welding is widely used in manufacturingstructural components, such as 304 L stainless steel [6] and16MnCr5 structural steel [7], so using GTA to butt weld Mg andAl alloys shows broad application prospects.

In recently years, many welding and jointing technology areproposed to joint Mg/Al dissimilar metal. The Vacuum diffusionbonding was used and the result showed that the diffusion zoneof Mg/Al diffusion bonded joint consists of intermetallic com-pounds MgAl, Mg3Al2 and Mg2Al3 [8]. Laser welding was usedand the results indicated that the intermetallic layer formed nearinterface between two metals significantly degraded the joiningstrength [9]. Soldering was also proposed to joint Mg/Al dissimilarmetal and the results showed that the bond strength increased

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17.

obviously with the increasing of the holding pressure, and the bendstrength as high as 22 MPa was obtained [10]. Yan YB and ZhangZW proposed to use explosive welding and the results indicatedthat no intermetallics were formed on the interface, and the shearstrength across the bonding interface of AZ31B/7075 compositewas ca. 70 MPa [11]. Friction stir welding (FSW), as a new joiningtechnology, could decrease the formation of Mg–Al intermetalliccompounds (IMCs) and then improved the performance of thejoints as reported by Chen and Nakata [12] and Kwon et al. [13].

The major problem of Mg/Al welding is the formation of IMCswith a very high hardness and brittleness locating in the fusionzone as an interlayer. These IMCs preferentially act as the sourceof microcracks in mechanical property tests, and then deterioratethe mechanical property of the joint. Previous research indicatedthat the formation of Mg–Al IMCs could be avoided by filling Znmetal in the weld seam in GTA welding of Mg and Al alloys [14].By this method, Mg–Al–Zn ternary alloying weld seams weremainly composed of MgZn2, Zn-based solid solution and Al-basedsolid solution. And the mixture of Zn-based solid solution and Al-based solid solution (MZAS) separated the continuous layer ofMgZn2 IMCs locating in the alloying seams, so the tensile strengthof the joint was improved. However, the content of MZAS in the fu-sion zone (FZ) near the Mg base metal is less than that in the centerof weld seam and near the Al base metal. Hence, the FZ near the Mgbase metal becomes the weakest area of the joint.

In the recent work, the additional Al element was added be-tween the Zn filler metal and Mg base metal, which is expectedto increase the amount of MZAS near the Mg base metal and sub-sequently improve the tensile strengths of the joints. The influ-ences of Al foils on the microstructure of fusion zone, especially

420 F. Liu et al. / Materials and Design 46 (2013) 419–425

near the Mg base metal, were investigated. In addition, themechanical properties and the fracture mechanisms of the jointswere discussed.

2. Experimental procedure

The base materials employed in the experiment were AZ31B Mgalloy with composition of Mg–3Al–1Zn–0.2Mn–0.1Si (wt.%) and6061 Al alloy with composition of Al–1Mg–0.6Si–0.15Cu–0.01Mn(wt.%). The dimensions of the two sheets were 50 mm �100 mm � 2 mm. Zinc wire with a purity of 99.9% and a diameterof 3.5 mm was used as the filler metal. In order to modulate thequantity of Al element, four kinds of Al foils with a purity of99.9% were used. The four Al foils remain the same size which is50 mm � 4 mm, and their thicknesses are 0.1, 0.2, 0.3 and0.4 mm respectively. Prior to welding, the sheets of Mg and Al alloywere machined into 120� ‘V’-type groove, and the oxide layers onthe surface of the Mg and Al sheets were removed. All the speci-mens were ultrasonically cleaned in acetone to remove oil andother contaminants from the specimen surface. The schematicillustration of the GTA butt welding is shown in Fig. 1. Al foil waspreset on the groove of Mg base metal and Zn filler metal wasplaced in the groove. The optimized parameters in the experimentwere 125 A GTA welding current, 440 mm/min welding speed and15 L/min argon shielding gas flow rate.

After GTA welding, cross-sections of the specimens were cutand mounted in epoxy resin. The samples were then mechanicallypolished using 120, 600, 1000 and 1200 grades of SiC grinding pa-pers followed by polishing using a 1 lm diamond polishing paste.The polished specimens were etched by 3% HNO3 ethanol solutionreagent for 3s to reveal the microstructures of the joint. Macro-structures of the joints were observed using an optical metallo-graphic microscope. The microstructures and compositions ofdifferent zones in the joints were determined using a scanningelectron microscope (SEM) with energy-dispersive X-ray spec-trometer (EDS). Element distribution in the welding seam wasexamined with electron probe micro-analyzer (EPMA).

The tensile specimens were machined as per ASTM: E-8/E8M-11 sub-size specifications perpendicular to the welded seam withgauge lengths of 25 mm and width of 6 mm as shown in Fig. 2.The specimens underwent tensile strength test in an electronic

Fig. 1. The schematic illustration of the GTA butt welding.

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

tension machine (Css-2205) under a travel speed of 2 mm/min atroom temperature, and an average of at least three specimenswas taken as the tensile strength. The joint tensile strength is cal-culated according to the equation rs = F/A, where F and rs are theload (N) and the ultimate tensile strength (MPa), respectively; A,calculated by multiplying the width by the thickness of the speci-men, is the cross sectional area (mm2) of the tensile test specimen.

3. Results and discussion

3.1. Analysis of the surface appearance and tensile strength of thewelded seam

Fig. 3 shows the surface appearance of the butt welded seamformed between the Mg and Al alloys by GTA welding process. Itcan be seen that the welded seam width is uniform and no weldingdefects, such as spatter and undercut.

The tensile strengths of the Mg/Al joints are shown in Fig. 4. Theaverage tensile strength of the GTA butt welding Mg–Al jointswithout additional Al element is 93 MPa. The average tensilestrength of the joint increases firstly and then decrease as Al foilthickness increases, and reach the maximum value of 104 MPawith the addition of Al foil which is 0.2 mm thick. However, thetensile strength of the joint reduces to 70 MPa when the thicknessof Al foil increases to 0.4 mm.

Three typical joints (without additional Al foil, with 0.2 mmthick Al foil and with 0.4 mm thick Al foil) are selected to investi-gate the fracture. The joint with 0.2 mm thick Al foil possesses themaximum tensile strength, while the joint with 0.4 mm thick Alfoil gives the lowest strength. The fracture locations of these typi-cal joints were observed. Fig. 5 shows the cross-sections of thefractured specimens without and with different thicknesses of Alfoils. It can be seen that there is a transitional zone (TZ) betweenthe Mg alloy substrate and FZ. The fracture of the joint withoutand with 0.2 mm thick Al foil occurs at the FZ near the Mg base

Fig. 3. Surface appearances of the weld seam.

Fig. 4. Tensile strength of joints with different thicknesses of Al foils.

Mg alloy

Mg alloy

Mg alloy

FZ

FZ

FZ

a

b

c

Fig. 5. Fracture location of the joints with different Al foil added: (a) without Al foil,(b) 0.2 mm Al foil and (c) 0.4 mm Al foil.

F. Liu et al. / Materials and Design 46 (2013) 419–425 421

metal. However the joint with 0.4 mm thick Al foil fractures at theTZ. Therefore the microstructures of the weak areas of the jointsare investigated emphatically.

3.2. Analysis of the microstructure of weak area in the joint

The microstructures of the weak areas of the typical joints areobserved, and the distributions of major elements (Al, Mg andZn) across the fracture location in the joints were detected toinvestigate the fracture reason. Fig. 6 gives the back-scattered elec-tron images and corresponding EPMA line analysis results present-ing the distributions of major elements (Al, Mg and Zn) in the jointcross-sections without and with different thickness of Al foils.From Fig. 6a–c, it can be seen that the width of TZs is not constantfor each joint. At the top, close to the heat source, the Zn filler

Fig. 6. Line analyses of TZ and FZ near the Mg base metals: (a

metal melts first but solidifies last compared with that at the bot-tom of the joint. Consequently, the Mg atoms, at the top of Mg basemetal, have more time to diffuse into the FZ. This is the reason whythe TZ thickness becomes narrower from the top to the bottom foreach joint.

The TZ thickness decreases gradually with increasing the thick-ness of Al foil comparing Fig. 6a–c, which is resulted from the factthat the Al foil hinders the diffusion of Mg atom to FZ. The linescans results show that the distributions of major elements inthe TZ with 0.2 mm thick Al foil are more uniform than those inother joints, and the uniform composition is benefit for the prop-erty of the joints.

To further investigate the failure mechanism of the joints, themicrostructures of the weak areas in the joints were observed asshown in Fig. 7a–c are SEM images of Mg/Al GTA butt weldingjoints without Al foil, with 0.2 and 0.4 mm thick Al foils, respec-tively. Figs. 7d–f are the magnified images of the regions I, II andIII in Fig. 7a–c, respectively.

It can be seen from Figs. 7d and e that the grains near the Mgbase metal are equiaxed crystals and the grain sizes are approxi-mately 3–5 lm. Some white phases (pointed by arrow 1) are foundin Fig. 7d, and the white phases are less than those (indicated byarrow 3) in Fig. 7e. However, as shown in Fig. 7f, it is quite difficultto distinguish the sizes of grains in the FZ of the joint with 0.4 mmthick Al foil and the microstructures of FZ are non-uniform, andsome particles and cracks can be found. The reason may be thatthe molten of Al react with Mg and Zn, and form hard and brittleIMCs due to the addition of too much Al element in weak area ofjoint. These IMCs are very easy to be the origins of cracks.

3.3. Analysis of the tensile strength and microstructure evolution ofweak area in the joint

The tensile strengths of materials are determined by theirmicrostructures and compositions [15]. For this reason, the micro-structure and composition of the weakest area of the joints that arewithout Al foil, with 0.2 mm thick Al and with 0.4 mm thick Al foil,are investigated emphatically. The distributions of primary ele-ments (Mg, Al and Zn) were detected by EPMA to investigate thecompositions of white phases, gray phases, particle phases andnon-uniform microstructure in Fig. 7d–f. The EPMA results taken

) without Al foil, (b) 0.2 mm Al foil and (c) 0.4 mm Al foil.

a

b

c

d

e

f

Fig. 7. SEM images of fracture zone near the Mg base metal: (a) without Al foil, (b) 0.2 mm Al foil, (c) 0.4 mm Al foil; (d), (e), (f) are the magnification of I, II, and III from (a), (b)and (c), respectively.

422 F. Liu et al. / Materials and Design 46 (2013) 419–425

from the testing positions denoted in Fig. 7 are summarized in Ta-ble 1.

When Mg and Al alloy are welded filling with Zn metal withoutAl foil, the compositions of white phases and gray phases, indicatedby arrows 1 and 2 in Fig. 7d, are shown in Table 1. It can be inferredthat the principal reactions in the order of their occurrence duringthe solidification process are [16]:

L!MgZn2 580 �C ð1Þ

L! ðAlÞ þMgZn2 480 �C ð2Þ

L! ðAlÞ þMgZn2 þ Al6Mg11Zn11 þ ðZnÞ 337 �C ð3Þ

(Al) and (Zn) are representatives of Al-based solid solution andZn-based solid solution for similarly hereinafter.

Therefore, it can be concluded that the white phases are mix-ture of Zn-based solid solution and Al-based solid solution (MZAS)

Table 1The composition of welded joint detected by EPMA.

Serial number Percentage composition (at.%) Inference composition

Mg Al Zn

1 13.4 43.1 42.5 MZAS2 40.2 7.5 52.3 MgZn2, Al6Mg11Zn11

3 3.0 20.7 76.3 MZAS4 18.4 3.7 78.0 Mg2Zn11, (Zn)5 29.6 48.6 21.8 (Al), Al6Mg11Zn11

6 39.4 14.1 46.5 Al6Mg11Zn11, MgZn2

in term of Mg–Al–Zn ternary phase diagram presented in Fig. 8.The gray phases are the IMCs of MgZn2 and Al6Mg11Zn11 accordingto their element compositions and the solidification process, whichare indicated by arrow 2 in Fig. 7d.

The sketch of microstructures of the joint without Al foil isshown in Fig. 9a. It can be seen that the FZ of the joint is composedof white and gray phases, and the gray phases (IMCs) are separatedby the white phases. These hard and brittle IMCs are easy to be thecrack origins when the specimens bear tensile stress during thetensile strength test [17]. On the contrary, the alloys whose com-positions are approximately to those of MZAS have excellent plas-ticity [18]. When the sample bears the tensile stress, the MZAS caneliminate the stress concentration of the crack tip and hinder thecrack propagation. For this reason, tensile strength improved sig-nificantly by ameliorating the microstructures in the joint, com-pared with the directly welding of Mg and Al alloy without Znfiller metal [14].

The microstructure of the joint near the Mg base metal with0.2 mm Al foil is shown in Fig. 7e. It can be seen that the contentof gray phase reduces significantly with the addition of Al foil.The welding pool near the Mg base metal precipitates MgZn2 pri-marily in the welding process. Due to the addition of Al metal,the several reactions as listed below:

L!MgZn2 480 �C ð4Þ

L! ðAlÞ þMgZn2 380�C ð5Þ

L! ðAlÞ þMgZn2 þ ðZnÞ 280 �C ð6Þ

Fig. 8. Mg–Al–Zn ternary phase diagram.

Fig. 9. The sketch of microstructure of the joint with different Al foil added: (a)without Al foil, (b) 0.2 mm Al foil and (c) 0.4 mm Al foil.

Fig. 10. Comparison microhardness for GTA butt welding without and withdifferent thickness of Al foil along the transverse Section 1 mm below top surface.

F. Liu et al. / Materials and Design 46 (2013) 419–425 423

Finally, the FZ near the Mg base metal is comprised of MZAS andMgZn2 compounds. The sketch of microstructures of the joint with0.2 mm Al foil is shown in Fig. 9b. Hence, the tensile strengths of thejoints are improved, compared with those of the joints filling withZn metal but without the additional Al element.

However, immoderate addition of Al element leads to the for-mation of partial Al-rich zones in the FZ as zone 5 shown inFig. 7f. Due to the excessive content of Al element, the liquid metalnear the Mg base metal precipitates Al-based solid solution pri-marily in the solicitation process, and then the several reactionsas listed below take place:

L!MgZn2 560 �C ð7Þ

L! ðAlÞ þMgZn2 530 �C ð8Þ

LþMgZn2 ! ðAlÞ þ Al6Mg11Zn11 478 �C ð9Þ

Therefore, the partial Al-rich zones are composed of IMCs (Al6Mg11-

Zn11, MgZn2) and Al-based solid solution in terms of Mg–Al–Zn ter-nary phase diagram and the solidification process. The sketch ofmicrostructures of the joint with 0.4 mm Al foil is shown inFig. 9c. The IMCs are very brittle and hard, therefore when the joint

a

b c

Fig. 11. Fracture surface of the joints with different Al foil added: (a) without Al foil, (b) 0.2 mm Al foil and (c) 0.4 mm Al foil.

424 F. Liu et al. / Materials and Design 46 (2013) 419–425

bears the tension stress, these partial Al-rich zones tend to producecracks as shown in Fig. 7f. Similar results are also reported by Mofidet al. [19].

Fig. 10 shows the microhardness distribution of the transversesection of the typical joint along the line 1 mm below the top sur-face. The microhardness of the joint near the Mg base metal ismuch higher than other locations no matter without or with addi-tional Al element. In addition, the microhardness of the joint with0.2 mm Al foil is lower than that of without Al foil in the FZ. Theseresults verify that the joint with 0.2 mm Al foil has more MZASthan that in the joint without Al foil, and the MZAS have excellentplasticity. In the area near the TZ, as the weakest area of the joint,the microhardness of the joint with 0.4 mm Al foil is much higherthan that of the other two joints. This is caused by the formation ofmany brittle and hard IMCs in partial Al-rich zones [20].

Fig. 11a–c are the fracture surfaces of the typical joints withoutAl foil, with 0.2 mm and 0.4 mm thick Al foils, respectively. It canbe seen that there are some torn ribs and polygonal particles(shown by the black rectangular in Fig. 11a) in the fracture surfaceof the joint with no foil added. But the particles cannot be detectedin the fracture surface of the joint added with 0.2 mm thick Al foil.When the thickness of Al foil is increased to 0.4 mm, the porosity,indicated by the black arrows as shown in Fig. 11c, can be found inthe fracture surface of the joint. The reason is that the excessiveaddition of Al element prevents the full mixture of element inthe molten pool in welding process.

From the above analysis, it can be concluded that the content ofMZAS can be increased by the appropriate addition of Al element,and the properties of the joint are improved. However, the additionof excessive Al element leads to the formation of partial Al-richzones which are composed of IMCs, such as Al6Mg11Zn11 andMgZn2, resulting in reduction in the tensile strength of the joint.

4. Conclusions

GTA butt welding of Mg/Al filling with Zn filler metal withoutand with different thickness of Al foils are investigated. The majorconclusions can be summarized as follows:

(1) Magnesium alloy AZ31B and aluminum alloy 6061 werewelded by GTA welding filling with Zn filler metal withoutand with Al foils in different thickness. The compositionand microstructure of the welding seam are accurately mod-ulated by introducing the additional Al into FZ. The TZ andFZ near the Mg base metal are the weak parts of the joints,and these areas are mainly composed of IMCs and a smallquantity of MZAS.

(2) The content of MZAS near the Mg base metal is increased bythe addition of a critical level of Al element. The MZAS caneliminate the stress concentration and hinder crack propa-gation, and thus improves the tensile strength of the joint.However, the immoderate quantity of Al element leads tothe formation of partially Al-rich zones, and subsequentlythe tensile strength of the joints is reduced.

(3) The quantity of additional 0.2 mm thick Al foil is consideredas an appropriate option for the GTA butt welding of AZ31BMg alloy to 6061 Al alloy filling with Zn filler metal, and thetensile strength of the joint can reach 104 MPa.

Acknowledgements

The authors gratefully acknowledge the sponsorships from Pro-gram for Changjiang Scholars and Innovative Research Team inUniversity (No. IRT1008) and National Natural Science Funds forDistinguished Young Scholar (51025520).

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