Research On Interface Characteristics And Diffusion Behavior Of Cu/Al Ultrasonic And Electric-ultrasonic Hybrid Welding

Aluminum and copper are widely used in power electronics,instrumentation,aerospace,national defense industry and new energy vehicles owing to their good electrical conductivity,thermal conductivity and corrosion resistance.However,due to the marked discrepancy in the thermal and mechanical properties and the rapid development of intermetallic compounds,the traditional fusion welding methods are confronted with a difficult situation during the dissimilar joining of aluminum alloy and copper.Ultrasonic welding(USW)can overcome the harmful effect of the differences in thermophysical properties to realize the joining between copper and aluminum alloy,but the joint properties are still restricted by the intermetallic compounds and interface microstructure.In the paper,the intermetallic compounds growth and interface microstructure were investigated during ultrasonic welding of pure copper to 5652 aluminum alloy.Besides,the microscopic characteristics and mechanical properties were compared between ultrasonic and electric-ultrasonic hybrid welding.They were provided theoretical reference for improving the properties of copper-aluminum welding joints.The growth model of intermetallic compound was established during ultrasonic welding of pure copper to aluminum alloy.The static growth model of the intermetallic compound was built based on the results of isothermal aging treatment of Cu-Al ultrasonic pre-welded samples.The dynamic growth model of intermetallic compound in ultrasonic welding process was established by combining the dynamic change process of vacancy concentration,vacancy diffusion mechanism and material constitutive equation.The research results show that the predicted value of the static model is far less than the measured value of the intermetallic compound,while the predicted values of the dynamic model are consistent with the measured values at different welding amplitudes.The diffusion of the copper atoms into aluminum alloy was accelerated by the supersaturated vacancy concentration in aluminum alloy,which is responsible for the different types of intermetallic compounds between under ultrasonic welding and under the isothermal aging based on the effective heat of formation model and the change of gibbs free energy.Changes of the interfacial characteristics(plastic flow,intermetallic compound and grain morphology)and mechanical properties of Cu-Al welded joint with welding time were systematically studied during ultrasonic welding.The results show that the plastic flow degree increases with welding time at the welding interface.The morphology and size of the copper grains adjacent to the welding interface were basically unchanged,However,the grain morphology and size of aluminum alloy have significantly changed owing to the occurrence of grain fragmentation,dynamic recrystallization and grain growth.The texture strength of both copper and aluminum alloy at the welding interface decreased first and then increased with welding time.The shear texture and recrystallization texture in aluminum alloy were formed,while the texture type of copper did not change with the welding process.The tensile strength and fracture energy of copper-aluminum welded joints increase first and then decrease with the increasing welding time.The tensile strength of the welded joint reaches its maximum when the large depth of local mechanical self-locking,thin intermetallic compound layer and fine equiaxed grain were distributed on the welding interface.In addition,the formation mechanism of cracks at the prepery of welding region was analyzed during ultrasonic welding of copper to aluminum alloy.The relationship between the edge cracks and ultrasonic power variation curve is discussed.The influence of the auxiliary current on interface characteristics and mechanical properties of Cu-Al welded joint was studied.All of the ultrasonic power,the plastic flow degree and the intermetallic thickness increase with the increase of the auxiliary current.Compared to the single ultrasonic welding,the rising rate of interface temperature is increased and the deeper embedded depth between copper and aluminum is generated at a shorter period of time during electric-ultrasonic hybrid welding.Besides,the same shear deformation degree,thinner intermetallic compound and more fine equiaxed grains of aluminum alloy can be formed at the welding interface,which are contributed to improve the tensile strength and fracture energy of Cu-Al joints.The tensile strength and fracture energy of the copper-aluminum welded joint first increased and then decreased with the increase of auxiliary current.The maximum tensile strength is obviously greater than that of single ultrasonic welding.