Formation And Microstructure Of Zn-added Friction Stir Spot Welded Joints Of Aluminum And Copper Dissimilar Alloys

The aluminum-copper composite structures have the advantages of low cost,lightweight and excellent comprehensive properties,which can satisfy the requirements of high-quality components in the high-tech field.However,the aluminum-copper dissimilar materials have a big difference in melting point,and the aluminum-copper dissimilar joints are prone to form large amount of brittle Al-Cu intermetallic compounds,which makes it difficult to improve the joint quality.This restricts the application of aluminum-copper composite structures in the aerospace field.Friction stir spot welding（FSSW）,as a solid-phase joining technology,has great technical advantage in joining the aluminum and copper.Zn as the interlayer can prevent the brittle Al-Cu intermetallic compounds from generating.The FSSW technology with assisted Zn interlayer was put forward in this study to improve the distribution of intermetallic compounds in the joint,thereby promoting the joint quality.The 1060 aluminum and T2 copper alloys were used as the base materials,and the tool pin was not inserted into the lower plate.The thickness of the Zn interlayer is 0.02,0.05 and 0.1 mm.The results stated that the aluminum-copper dissimilar materials had a higher weldability when the copper alloy was placed as the upper plate.Copper alloy as the upper plate could obtained the higher welding heat input,which was more conducive for the joint formation.The addition of 0.02 mm thick Zn interlayer significantly improved the joint weldability.The addition of Zn interlayer directly affected the microstructures evolution in the joint,which made intermetallic compounds in the lap interface change from Al2Cu andαAl+Al2Cu eutectic to Al-Zn-Cu and αAl+Al2Cu eutectic.The process parameters can affect the thermal cycle,pressure and material flow at the lap interface,thereby affecting the atom diffusion behavior.The pin length,rotational velocity and the thickness of Zn interlayer can significantly affect the atomic diffusion behavior at the lap interface.The joint quality can be promoted by optimizing the above-mentioned process parameters.The regulation between the tensile shear strength and pin length under the conventional FSSW was opposite to that under the FSSW process with Zn interlayer.The microhardness distribution of the Cu/Al joints all showed an inverted V-shaped.The joints all fractured at the lap interface,which belonged to the shear fracture mode.Taguchi method was used to obtain the optimal parameters combination of 1.3 mm-1750 rpm-0.02 mm,and the tensile shear load in the optimal parameters combination could reach 7.03 kN.