Microstructure And Mechanical Properties Of Pulse MIG Welded 6061/A356 Aluminum Alloy Dissimilar Joints

With the rapid development of economy in modern society,the global energy consumption is becoming more and more serious,meanwhile,the environmental pollution caused by energy consumption becomes urgent problems which need to be solved.Energy-saving and emission-reduction can reduce environmental pollution effectively,and lightweight of automotive can achieve energy-saving and emission-reduction through reducing vehicle exhaust emission,so this proposition attracts much attention of scientific researcher.With the wide application of aluminium alloy structural components and castings in automotive industry,the welding of dissimilar aluminium alloys is inevitable.It combines the advantages of high strength,corrosion resistance and excellent casting performance of different kinds of aluminium alloys,so that the structure and components design of vehicles can be more reasonable and efficient.Therefore,the high quality and high efficiency welding of wrought/cast aluminium alloys have great application prospect and research value.In this paper,the butt,lap and T joining between 6061-T6 and A356-T6 aluminium alloys was conducted by pulsed MIG(P-MIG)welding with ER4043 filler wire,and the thickness of aluminium alloy plates is 4mm.The weld forming mechanism of 6061-T6/A356-T6 dissimilar aluminium alloy joints is studied,and the effect of welding speed,welding current and joint type on weld forming,the defect formation and distribution in weld,mechanical properties and microstructure are investigated.The results are as follows:The 6061-T6/A356-T6 dissimilar aluminum alloy butt joints were obtained by pulsed MIG welding.When welding current is 165 A and the welding speed is 12mm/s,the tensile strength of butt joints reaches 235 MPa,which is 83% of that of 6061-T6 aluminum alloy and can meet strength requirements in engineering application,and the weld formation is good and the weld reinforcement and melting width are moderate.The phenonmenon of grain boundary liquation(GBL)existed in partial melting zone(PMZ)on 6061 aluminum alloy side,affecting the strength of joints.Decrease of welding heat input,resulting in decrease of PMZ width,can weaken the GBL extent,and prevent cracks formation in PMZ.Needle-like ?-Al5 FeSi phase and granular ?-Al9FeMg3Si5 phase appeared in PMZ on A356 aluminum alloy side.The needle-like ? phase could deteriorate the strength of the joints,it is necessary to limit its size and quantity.The 6061-T6/A356-T6 dissimilar aluminium alloy lap joining by pulsed MIG welding was conducted.When welding current is 165 A,welding speed is 10mm/s and joint type is type?(A356 aluminum alloy is the upper plate,6061 aluminum alloy is the lower plate),the maximum tensile strength of lap joints is 96 MPa,which is 30% of that of 6061-T6 aluminum alloy,and its line intensity is 380N/mm.The lap joints are mainly fractured in weld zone,and the fracture originates from the root of joints.The weld formation is better when using joint type?,and the weld width and penetration are moderate.The extent of GBL decreases greatly when the lap joint type is changed into type ?.The root of lap joints could gather large pores easily,making the root become the weakest region.The 6061-T6/A356-T6 dissimilar aluminium alloy T joint was obtained by pulsed MIG welding.When welding current is 165 A,the welding speed is 12mm/s and joint type is type ?(6061 aluminium alloy is the web,A356 aluminum alloy is wing plate),welding heat input is appropriate,forming convex weld;the shape of the weld is bowl-shaped,thus the continuity is better;the ventral weld depth is relatively small,and the wing plate weld depth is large,so the weld strength is relatively high.The tensile performance of T joints is well,thus the maximum tensile load reaches 9488 N.When joint type is changed from type?to type?,the weld is good.The microhardness in weld decreased seriously compared with base material,the average microhardness was 77~82HV.