Research On Microstructure And Mechanical Properties Of Dissimlar Friction Stir Welded 6061 Aluminum Alloy/Pure Copper Joints

Aluminum and Copper are widely used in electrical,energy,transportation and other fields due to their excellent electrical and thermal conductivity.The Al/Cu composite structures are widely used in electronic industry.These structures can not only meet the service conditions,but also reduce the weight of the components and the production cost,which meet the requirements of modern industry for lightweight structure.However,because of the great difference in physical and chemical properties between Al and Cu,it is difficult to achieve effective or high-strength joints using traditional welding methods.Friction Stir Welding(FSW),as a solid-phase joining technology,has a series of advantages such as high welding efficiency,low energy consumption,low post-welding deformation and low residual stress.FSW is suitable for welding dissimilar materials,and the research of FSW on Al/Cu dissimilar metal materials joining has drawn many interets.Stationary Shoulder Friction Stir Welding(SSFSW)is a new type of solid phase joining technology developed based on traditional Friction Stir Welding.It has the advantages of smooth welding surface,almost no thinning of joint and uniform microstructure.However,the research of SSFSW on Al/Cu dissimilar metal joining is rarely reported until present.In this paper,the 4mm-thick 6061-T6 aluminum alloy and T2 industrial pure copper are butt-welded.The influences of the relative position of the base metal,the offset of the pin,the welding speed and the rotation speed on the welding surface and the internal forming of the joint are studied.The effects of welding speed and rotation speed on the microstructure,interface characteristics,intermetallic compounds and mechanical properties of the joint are analyzed.It is found by experiments that under inappropriate welding process parameters,Al/Cu dissimilar joints are particularly prone to defects.When Al is placed on the advancing side of the welding direction and the pin offset is 1mm towards the Al side,well-formed dissimilar joints without defects can be obtained both by FSW and SSFSW.Arc pattern can be seen on the surface of FSW joints.Due to the smoothing effect of the stationary shoulder,the welded surface of the SSFSW joints is relatively smooth and clean,and there is no arc pattern.In the dissimilar joints of FSW,a large number of broken Cu is stirred into the nugget zone and mixed with Al forming a complex area.Several types of intermetallic compounds are formed in nugget zone,including Al2Cu,Al4Cu9 and AlCu phases.As the rotation speed increases,the types of compounds detected in nugget zone increase and the thickness of the compounds layer at the interface becomes larger.The average microhardness value of nugget zone is higher than that of Al and Cu,and the hardness peak point appears in nugget zone.Hardness-softening zone appears on both sides of Al and Cu,and the area of softening zone become larger with the increase of rotation speed.The tensile strength and elongation both increase firstly and then decrease with the increase of rotation speed.Under the parameter of 1500 rpm and 30 mm/min,the tensile strength of the joint reaches the maximum(183 MPa),which is 71.8%of the copper base metal.The fracture occurs in the heat affected zone on the Al side,and the fracture mode is ductile fracture.In the process of SSFSW,the stationary shoulder does not rotate,and the friction heat generation and mechanical stirring of the stationary shoulder are inhibited.The internal microstructure of SSFSW joint is very uniform,and there is no region with complex structure where Al and Cu are mixed intensively.There are less type and content of intermetallic compounds are detected inside the joints.A transition layer composed mainly of Al2Cu is formed at the Al/Cu interface,and the formation of Al4Cu9 phase is detected near the interface.With the increase of rotation speed to welding speed ratio,the Al/Cu interface gradually changes from smooth and regular to curved and complex,and a laminated composite structure is formed at the interface,which increases the effective connection area between Al and Cu.The average hardness value of nugget zone is close to the average hardness value of base metal,and there is no peak hardness point in nugget zone.With the decrease of the rotation speed to welding speed ratio,the tensile strength and elongation of the SSFSW joints increase first and then decrease,and reach the maximum value of 211.4 MPa and 4.8%,respectively under the parameters of 1800 rpm and 30 mm/min The maximum tensile strength is 82.9%of Cu base metal,which is 15.5%higher than that of FSW dissimilar joints.