Research On Process And Mechanism Of Friction Stir Spot Welding For Al/Cu

In order to use the scare metal copper resource reasonably and save the costs, Al/Cu composite structure has developed rapidly in recent years, and the requirement for processing technology is increasing. Friction stir spot welding, a new type of solid-phase welding technology based on friction stir welding, has the advantages of simple process and high quality, is competitive in joining of Al/Cu dissimilar metals. In this paper, friction stir spot welding of 2-mm-thick 1060 aluminum and T2 copper was studied. The influence of rotation speed, dwell time, plunge depth and tool shape on microstructure and properties of joint were analyzed and the mechanical properties are optimized. The mechanism of friction stir spot welding for Al/Cu was proposed.Al/Cu friction stir spot welded joint was symmetric about the rotation axis of pin tool. There was an exit hole at the center of the weld and the stir zone was formed nearby. Copper was brought into aluminum with the formation of Hook. The joint was formed by mechanical intermixing and metallurgical bonding between aluminum and copper. With the increase of the heat input, Hook Height/Full Bonded Region(HH/FBR) of the Hook geometric parameters increased first and then decreased. Microstructure analysis results showed that Cu Al2-Cu Al-Al4Cu9 laminated structure was formed in the interface of Hook near exit hole, while Cu Al2 layer and discontinuous Cu Al phase were found in the interface of Hook away from exit hole. With increasing heat input, the thickness of interfacial compounds increased.The mechanical properties test results showed that the microhardness distribution in friction stir spot welded Al/Cu was symmetrical about the center line of the joint. The hardness in the SZ was the highest and that in the HAZ was the lowest. With increasing heat input, the hardness decreased slightly and the tensile shear failure load increased firstly then decreased. When cylindrical screwed pin tool was used, the maximum tensile shear failure load of 4.304 k N was obtained at the rotation speed of 2250 r/min, dwell time of 5 s and plunge depth of 0.1 mm. The tensile properties of joint were dependent on weld defects, the shape of Hook and the thickness of intermetallic compounds, while the type of intermetallic compounds was the decisive factor of the failure path during tensile shear test. The optimized tensile shear failure load of 4.398 k N was obtained at the rotation speed of 2371.27 r/min, dwell time of 4.96 s and plunge depth of 0.1 mm by the response surface methodology.The process parameters during the welding process were measured. With increasing interaction between the pin tool and material, the peak temperature in the joint increased, while the plunge force and the peak torque decreased. The formation thermodynamics and kinetics of the compounds at the Al/Cu interface were analyzed. It was indicated that Cu Al2 phase was first formed, and then Cu Al and Al4Cu9 nucleated and grown to form the Cu Al2-Cu Al-Al4Cu9 laminated structure. The dispersed composites distributed in the SZ and part of them degenerated to Cu Al2 particles eventually. The type and thickness of the interfacial intermetallic compounds were disparate under different welding heat input, while the flow behavior of material was different caused by the difference of plasticizing degree. Accordingly, the mechanism of the friction stir spot welding for Al/Cu was proposed.