材料设计

Effect of Al foils interlayer on microstructures and mechanical properties ofMg–Al butt joints welded by gas tungsten arc welding filling with Zn filler metalFei 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,Chinaa r t i c l e i n f o Article history:Received 24July 2012Accepted 6October 2012Available online 26October 2012Keywords:A.Non-ferrous metals and alloys D.Welding E.FractureF.Microstructurea b s t r a c tGas tungsten arc butt welding of Mg–Al filling with Zn filler metal without and with Al foils in different thicknesses was carried out.Additional Al element was introduced into the fusion zone to accurately modulate 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 solid solution can eliminate the stress concentration and hinder crack propagation,so the tensile strengths of the joints are improved.However,the immoderate quantity of Al element will lead to the formation of partially Al-rich zones and deteriorate the mechanical property of the joints.Ó2012Elsevier Ltd.All rights reserved.1.IntroductionMg alloys,the lightest structural metals,are in great potential for application to manufacturing industry due to their quite lower weight ratio,electromagnetic shielding capability and some other unique 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 alloys is required in order to achieve the application of Mg–Al combina-tion [3].Besides,gas tungsten arc (GTA)welding,as a traditional welding method,is commonly used in industry due to its series of advantages,such as high energy utilization [4]and easy opera-tion [5].In addition,butt welding is widely used in manufacturing structural components,such as 304L stainless steel [6]and 16MnCr5structural steel [7],so using GTA to butt weld Mg and Al alloys shows broad application prospects.In recently years,many welding and jointing technology are proposed to joint Mg/Al dissimilar metal.The Vacuum diffusion bonding was used and the result showed that the diffusion zone of Mg/Al diffusion bonded joint consists of intermetallic com-pounds MgAl,Mg 3Al 2and Mg 2Al 3[8].Laser welding was used and the results indicated that the intermetallic layer formed near interface between two metals significantly degraded the joining strength [9].Soldering was also proposed to joint Mg/Al dissimilar metal and the results showed that the bond strength increased obviously with the increasing of the holding pressure,and the bend strength as high as 22MPa was obtained [10].Yan YB and Zhang ZW proposed to use explosive welding and the results indicated that no intermetallics were formed on the interface,and the shear strength across the bonding interface of AZ31B/7075composite was ca.70MPa [11].Friction stir welding (FSW),as a new joining technology,could decrease the formation of Mg–Al intermetallic compounds (IMCs)and then improved the performance of the joints as reported by Chen and Nakata [12]and Kwon et al.[13].The major problem of Mg/Al welding is the formation of IMCs with a very high hardness and brittleness locating in the fusion zone as an interlayer.These IMCs preferentially act as the source of microcracks in mechanical property tests,and then deteriorate the mechanical property of the joint.Previous research indicated that the formation of Mg–Al IMCs could be avoided by filling Zn metal in the weld seam in GTA welding of Mg and Al alloys [14].By this method,Mg–Al–Zn ternary alloying weld seams were mainly composed of MgZn 2,Zn-based solid solution and Al-based solid solution.And the mixture of Zn-based solid solution and Al-based solid solution (MZAS)separated the continuous layer of MgZn 2IMCs locating in the alloying seams,so the tensile strength of 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 center of weld seam and near the Al base metal.Hence,the FZ near the Mg base 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 expected to 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,especially0261-3069/$-see front matter Ó2012Elsevier Ltd.All rights reserved./10.1016/j.matdes.2012.10.012Corresponding author.Tel./fax:+86041184707817.E-mail address:liulm@ (L.Liu).near the Mg base metal,were investigated.In addition,the mechanical properties and the fracture mechanisms of the joints were discussed.2.Experimental procedureThe base materials employed in the experiment were AZ31B Mg alloy with composition of Mg–3Al–1Zn–0.2Mn–0.1Si(wt.%)and 6061Al alloy with composition of Al–1Mg–0.6Si–0.15Cu–0.01Mn (wt.%).The dimensions of the two sheets were50mmÂ100mmÂ2mm.Zinc wire with a purity of99.9%and a diameter of3.5mm was used as thefiller metal.In order to modulate the quantity of Al element,four kinds of Al foils with a purity of 99.9%were used.The four Al foils remain the same size which is 50mmÂ4mm,and their thicknesses are0.1,0.2,0.3and 0.4mm respectively.Prior to welding,the sheets of Mg and Al alloy were machined into120°‘V’-type groove,and the oxide layers on the surface of the Mg and Al sheets were removed.All the speci-mens were ultrasonically cleaned in acetone to remove oil and other contaminants from the specimen surface.The schematic illustration of the GTA butt welding is shown in Fig.1.Al foil was preset on the groove of Mg base metal and Znfiller metal was placed in the groove.The optimized parameters in the experiment were125A GTA welding current,440mm/min welding speed and 15L/min argon shielding gasflow rate.After GTA welding,cross-sections of the specimens were cut and mounted in epoxy resin.The samples were then mechanically polished using120,600,1000and1200grades of SiC grinding pa-pers followed by polishing using a1l m diamond polishing paste. The polished specimens were etched by3%HNO3ethanol solution tension machine(Css-2205)under a travel speed of2mm/min at room temperature,and an average of at least three specimens was taken as the tensile strength.The joint tensile strength is cal-culated according to the equation r s=F/A,where F and r s are the load(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 discussion3.1.Analysis of the surface appearance and tensile strength of the welded seamFig.3shows the surface appearance of the butt welded seam formed between the Mg and Al alloys by GTA welding process.It can be seen that the welded seam width is uniform and no welding defects,such as spatter and undercut.The tensile strengths of the Mg/Al joints are shown in Fig.4.The average tensile strength of the GTA butt welding Mg–Al joints without additional Al element is93MPa.The average tensile strength of the joint increasesfirstly and then decrease as Al foil thickness increases,and reach the maximum value of104MPa with the addition of Al foil which is0.2mm thick.However,the tensile strength of the joint reduces to70MPa when the thickness of Al foil increases to0.4mm.Three typical joints(without additional Al foil,with0.2mm thick Al foil and with0.4mm thick Al foil)are selected to investi-gate the fracture.The joint with0.2mm thick Al foil possesses the maximum tensile strength,while the joint with0.4mm thick AlFig.1.The schematic illustration of the GTA butt welding. Fig.2.Sketch of butt welding tensile test specimens(mm).Fig.3.Surface appearances of the weld seam.Fig.4.Tensile strength of joints with different thicknesses of Al foils.420 F.Liu et al./Materials and Design46(2013)419–425metal.However the joint with0.4mm thick Al foil fractures at the TZ.Therefore the microstructures of the weak areas of the joints are investigated emphatically.Analysis of the microstructure of weak area in the joint microstructures of the weak areas of the typical joints observed,and the distributions of major elements(Al,Mg across the fracture location in the joints were detected investigate the fracture reason.Fig.6gives the back-scattered images and corresponding EPMA line analysis results present-the distributions of major elements(Al,Mg and Zn)in the cross-sections without and with different thickness of Al metal meltsfirst but solidifies last compared with that at the bot-tom of the joint.Consequently,the Mg atoms,at the top of Mg base metal,have more time to diffuse into the FZ.This is the reason why the TZ thickness becomes narrower from the top to the bottom for each joint.The TZ thickness decreases gradually with increasing the thick-ness of Al foil comparing Fig.6a–c,which is resulted from the fact that the Al foil hinders the diffusion of Mg atom to FZ.The line scans results show that the distributions of major elements in the TZ with0.2mm thick Al foil are more uniform than those in other joints,and the uniform composition is benefit for the prop-erty of the joints.To further investigate the failure mechanism of the joints,the microstructures of the weak areas in the joints were observed as shown in Fig.7a–c are SEM images of Mg/Al GTA butt welding joints without Al foil,with0.2and0.4mm thick Al foils,respec-tively.Figs.7d–f are the magnified images of the regions I,II and III in Fig.7a–c,respectively.It can be seen from Figs.7d and e that the grains near the Mg base metal are equiaxed crystals and the grain sizes are approxi-mately3–5l m.Some white phases(pointed by arrow1)are found in Fig.7d,and the white phases are less than those(indicated by arrow3)in Fig.7e.However,as shown in Fig.7f,it is quite difficult to distinguish the sizes of grains in the FZ of the joint with0.4mm thick Al foil and the microstructures of FZ are non-uniform,and some particles and cracks can be found.The reason may be that the molten of Al react with Mg and Zn,and form hard and brittle IMCs due to the addition of too much Al element in weak area of joint.These IMCs are very easy to be the origins of cracks.Mg alloy Mg alloy Mg alloy FZ FZ FZabcFracture location of the joints with different Al foil added:(a)withoutmm Al foil and(c)0.4mm Al foil.Line analyses of TZ and FZ near the Mg base metals:(a)without Al foil,(b)0.2mm Al foil and(c)0.4mmF.Liu et al./Materials and Design46(2013)419–425421from the testing positions denoted in Fig.7are summarized in Ta-ble1.When Mg and Al alloy are weldedfilling with Zn metal without Al foil,the compositions of white phases and gray phases,indicated by arrows1and2in Fig.7d,are shown in Table1.It can be inferred that the principal reactions in the order of their occurrence during the solidification process are[16]:L!MgZn2580 Cð1ÞL!ðAlÞþMgZn2480 Cð2ÞL!ðAlÞþMgZn2þAl6Mg11Zn11þðZnÞ337 Cð3Þ(Al)and(Zn)are representatives of Al-based solid solution and Zn-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)in term of Mg–Al–Zn ternary phase diagram presented in Fig.8. The gray phases are the IMCs of MgZn2and Al6Mg11Zn11according to their element compositions and the solidification process,which are indicated by arrow2in Fig.7d.The sketch of microstructures of the joint without Al foil is shown in Fig.9a.It can be seen that the FZ of the joint is composed of white and gray phases,and the gray phases(IMCs)are separated by the white phases.These hard and brittle IMCs are easy to be the crack origins when the specimens bear tensile stress during the tensile 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 can eliminate the stress concentration of the crack tip and hinder the crack 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 Zn filler metal[14].The microstructure of the joint near the Mg base metal with 0.2mm Al foil is shown in Fig.7e.It can be seen that the content of gray phase reduces significantly with the addition of Al foil. The welding pool near the Mg base metal precipitates MgZn2pri-marily in the welding process.Due to the addition of Al metal, the several reactions as listed below:L!MgZn2480 Cð4ÞL!ðAlÞþMgZn2380 Cð5ÞL!ðAlÞþMgZn2þðZnÞ280 Cð6Þa b c d e fzone near the Mg base metal:(a)without Al foil,(b)0.2mm Al foil,(c)0.4mm Al foil;(d),(e),(f)are the magnificationTable1The composition of welded joint detected by EPMA.Serial number Percentage composition(at.%)Inference compositionMg Al Zn113.443.142.5MZAS240.27.552.3MgZn2,Al6Mg11Zn113 3.020.776.3MZAS418.4 3.778.0Mg2Zn11,(Zn)529.648.621.8(Al),Al6Mg11Zn11639.414.146.5Al6Mg11Zn11,MgZn2422 F.Liu et al./Materials and Design46(2013)419–425Finally,the FZ near the Mg base metal is comprised of MZAS compounds.The sketch of microstructures of the joint mm Al foil is shown in Fig.9b.Hence,the tensile strengths are improved,compared with those of the jointsfilling metal but without the additional Al element.However,immoderate addition of mation of partial Al-rich zones in the Fig.7f.Due to the excessive content of near the Mg base metal precipitates marily in the solicitation process,andas listed below take place:L!MgZn2560 CL!ðAlÞþMgZn2530 CLþMgZn2!ðAlÞþAl6Mg11Zn11478 Therefore,the partial Al-rich zones areZn11,MgZn2)and Al-based solid solution nary phase diagram and the solidification microstructures of the joint with0.4 Fig.9c.The IMCs are very brittle and hard,Fig.8.Mg–Al–Zn ternary phase diagram.The sketch of microstructure of the joint with different Al foil added:Al foil,(b)0.2mm Al foil and(c)0.4mm Al foil.Comparison microhardness for GTA butt welding without andthickness of Al foil along the transverse Section1mm below topbears the tension stress,these partial Al-rich zones tend to produce cracks as shown in Fig.7f.Similar results are also reported by Mofid et al.[19].Fig.10shows the microhardness distribution of the transverse section of the typical joint along the line1mm below the top sur-face.The microhardness of the joint near the Mg base metal is much higher than other locations no matter without or with addi-tional Al element.In addition,the microhardness of the joint with 0.2mm Al foil is lower than that of without Al foil in the FZ.These results verify that the joint with0.2mm Al foil has more MZAS than that in the joint without Al foil,and the MZAS have excellent plasticity.In the area near the TZ,as the weakest area of the joint, the microhardness of the joint with0.4mm Al foil is much higher than that of the other two joints.This is caused by the formation of many brittle and hard IMCs in partial Al-rich zones[20].Fig.11a–c are the fracture surfaces of the typical joints without Al foil,with0.2mm and0.4mm thick Al foils,respectively.It can be seen that there are some torn ribs and polygonal particles (shown by the black rectangular in Fig.11a)in the fracture surface of the joint with no foil added.But the particles cannot be detected in the fracture surface of the joint added with0.2mm thick Al foil. When the thickness of Al foil is increased to0.4mm,the porosity, indicated by the black arrows as shown in Fig.11c,can be found in the fracture surface of the joint.The reason is that the excessive addition of Al element prevents the full mixture of element in the molten pool in welding process.From the above analysis,it can be concluded that the content of MZAS can be increased by the appropriate addition of Al element, and the properties of the joint are improved.However,the addition of excessive Al element leads to the formation of partial Al-rich zones which are composed of IMCs,such as Al6Mg11Zn11and MgZn2,resulting in reduction in the tensile strength of the joint.4.ConclusionsGTA butt welding of Mg/Alfilling with Znfiller metal without and with different thickness of Al foils are investigated.The major conclusions can be summarized as follows:(1)Magnesium alloy AZ31B and aluminum alloy6061werewelded by GTA weldingfilling with Znfiller metal without and with Al foils in different thickness.The composition and microstructure of the welding seam are accurately mod-ulated by introducing the additional Al into FZ.The TZ and FZ near the Mg base metal are the weak parts of the joints, and these areas are mainly composed of IMCs and a small quantity 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 can eliminate 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 to the formation of partially Al-rich zones,and subsequently the tensile strength of the joints is reduced.(3)The quantity of additional0.2mm thick Al foil is consideredas an appropriate option for the GTA butt welding of AZ31B Mg alloy to6061Al alloyfilling with Znfiller metal,and the tensile strength of the joint can reach104MPa. AcknowledgementsThe authors gratefully acknowledge the sponsorships from Pro-gram for Changjiang Scholars and Innovative Research Team in University(No.IRT1008)and National Natural Science Funds for Distinguished Young Scholar(51025520).References[1]Schubert E,Klassen M,Zerner I,Walz C,Sepold G.Light-weight structuresproduced by laser beam joining for future applications in automobile and aerospace industry.J Mater Process Technol2001;115:2–8.[2]Liu LM,Liu XJ,Liu SH.Microstructure of laser-TIG hybrid welds of dissimilarMg alloy and Al alloy with Ce as interlayer.Script Mater2006;55:383–6. [3]Somasekharan AC,Murr LE.Microstructures in friction-stir welded dissimilarmagnesium alloys and magnesium alloys to6061–T6aluminum alloy.Mater Charact2004;52:49–64.[4]Kumar Subodh,Shahi AS.Effect of heat input on the microstructure andmechanical properties of gas tungsten arc welded AISI304stainless steel joints.Mater Des 2011;32:3617–23.ab cFracture surface of the joints with different Al foil added:(a)without Al foil,(b)0.2mm Al foil and(c)0.4 424 F.Liu et al./Materials and Design46(2013)419–425[5]Kumar A,Sundarrajan S.Optimization of pulsed TIG welding processparameters on mechanical properties of AA5456aluminum alloy weldments.Mater Des2009;30:1288–97.[6]Bang Hee Seon,Bang Han Sur,Kim You Chul,Oh Ik Hyun.A study onmechanical and microstructure characteristics of the STS304L butt joints using hybrid CO2laser-gas metal arc welding.Mater Des2011;32: 2328–33.[7]Cemil C,Etinkaya,Arabaci U.Flash butt welding application on16MnCr5chainsteel and investigations of mechanical properties.Mater Des 2006;27:1187–95.[8]Liu P,Li YJ,Geng HR,Wang J.A study of phase constitution near the interface ofMg/Al vacuum diffusion bonding.Mater Lett2005;59:2001–5.[9]Borrisutthekul R,Miyashita Y,Mutoh Y.Dissimilar material laser weldingbetween magnesium alloy AZ31B and aluminum alloy A5052-O.Sci Technol Adv Mat2005;6:199–204.[10]Li XR,Liang W,Zhao XG,Zhang Y,Fu XP,Liu FC.Bonding of Mg and Al withMg–Al eutectic alloy and its application in aluminum coating on magnesium.J Alloys Compd2009;471:408–11.[11]Yana YB,Zhang ZW,Shen W,Wang JH,Zhang LK,Chin BA.Microstructure andproperties of magnesium AZ31B–aluminum7075explosively welded composite plate.Mat Sci Eng A–Struct2010;527:2241–5.[12]Chen YC,Nakata K.Friction stir lap joining aluminum and magnesium alloys.Scripta Mater2008;58:433–6.[13]Kwon YJ,Shigematsu I,Saito N.Dissimilar friction stir welding betweenmagnesium and aluminum alloys.Mater Lett2008;62:3827–9.[14]Liu F,Zhang ZD,Liu LM.Microstructure evolution of Al/Mg butt joints weldedby gas tungsten arc with Znfiller metal.Mater Charact2012;69:84–9. [15]Liu CT,Maziasz PJ.Microstructural control and mechanical properties of dual-phase TiAl alloys.Internet1998;6:653–61.[16]Villars P,Prince A,Okamoto H.Handbook of ternary alloy phase diagrams.ASMInternational;1995.[17]Zhao LM,Zhang ZD.Effect of Zn alloy interlayer on interface microstructureand strength of diffusion-bonded Mg–Al joints.Scripta Mater2008;58:283–6.[18]Straumal B,Valiev R,Kogtenkova O,Zieba P,Czeppe T,Bielanska E,et al.Thermal evolution and grain boundary phase transformations in severely deformed nanograined Al–Zn alloys.Acta Mater2008;56:6123–31.[19]Mofid MA,Abdollah-zadeh A,Malek Ghaini F.The effect of water coolingduring dissimilar friction stir welding of Al alloy to Mg alloy.Mater Des 2012;36:161–7.[20]Shang J,Wang KH,Zhou Q,Zhang DK,Huang J,Li GL.Microstructurecharacteristics and mechanical properties of cold metal transfer welding Mg/ Al dissimilar metals.Mater Des2012;34:559–66.F.Liu et al./Materials and Design46(2013)419–425425。