Microstructure And Mechanical Properties Of Ultrasonic And Electrical Current Assisted Ultrasonic Welded Cu And Al Alloys

Copper and aluminum are widely used in battery electric vehicles, aerospace and electronics industries. However, joining of copper and aluminum by conventional fusion welding methods is difficult due to the high electrical and thermal conductivity. Ultrasonic welding(USW) is currently one of the most widely used joining techniques for battery pack assembly, such as tab to tab and tab to bus-bar joining. Microstructure evolution and mechanical properties of Cu–Cu and Cu–Al joints fabricated high power USW and electrical current assisted USW(EUSW) techniques were studied to improve the joint performance.A high power USW system and an EUSW system were constructed. With the welding system, the joining process was realized by applying ultrasonic, heat and force on the welding interface, and the welding process information, such as the driving signals of PZT, displacement of the sonotrode tip and temperature of the interface during welding process, was acquired. The relationship between the microstructure evolution of the interface and the process information was studied.Copper to copper joints were produced by high power USW. The evolution of plastic deformation at the contact interface as a function of welding energy was analyzed in detail. The influence of the microstructure development on the mechanical properties was also studied. With increasing welding energy, the material flow at the welded interface resembled those of vortex, crest, ridge and the Kelvin-Helmholtz instability waves. The morphology of the weld interface evolved from microbondings to discontinuous linear joining to linear joining to band joining to wave-like joining.The lap shear load of the joints increases with increasing weld energy. The failure mode transitioned from interfacial debonding to nugget pullout as the weld energy increased. Evidence of dynamic recrystallization was observed at the welded interface. With increasing the weld energy, the grain size of the recrystallized grains increases. The hardness values in the weld zone were lower than those in the parent material. Based on the results of the tensile lap shear tests and the dynamic interfacial friction model, the influence of clamping force on the joining process was also studied.Dissimilar joints of copper to aluminum were produced by high power USW. The interfacial reaction between copper and aluminum alloy as a function of welding time was characterized in detail. The influence of the microstructure development on the mechanical properties was also studied. There is an increase trend in the peak temperature with increasing the welding time. The maximum interfacial temperature reached at the welding time of 0.7 s is about 510 ?C, which is slightly lower than the melting point(548 ?C) of the Al–Cu eutectic alloy. A discontinuous IMC layer formed at local points along the interface was clearly visible for a short welding time(0.3 s). With further increasing the welding duration, the IMC layer grew along the interface and became thickening until a continuous IMC layer was formed. The IMC layer could be composed mainly of Cu Al2 by EDS and XRD analysis. It was found that the growth rate of the reaction layer in the RUSW is much more rapid than that for static growth. When the welding time reached 0.7 s, a solidified microstructure seems to be observed in the irregular shaped region along the weld zone. This means that the eutectic reaction α-Al+θ→L has occurred in local area at the interface. The lap shear load of the joints first increases and then decreases with increasing welding time, and the failure of the joints occurred dominantly at the interface.However, the USW technique is also difficult to apply to the joining of miniature work pieces. This challenge results from the area of the weld tip, which limits the ultrasonic power delivered in the weld zone. A new hybrid welding technique, the electrical current assisted USW(EUSW) technique, is proposed to further contribute to the improvement of USW quality. The overall design plan of ultrasonic power supply was made, with completion of design and calculations the main circuit, control circuit, and matching circuit to achieve the stable ultrasonic vibration output. Based on the designed ultrasonic power supply, an EUSW system was constructed, which was composed of a low power USW power source and a DC inverter resistance spot welding power source. The DC could be adjusted from 0 to 2 k A continuously during the joining process. A comparison is conducted between two dissimilar Cu–Al joints, one produced by USW and the other by EUSW. In EUSW, the peak power of ultrasonic vibration and the peak temperature of the interface increase significantly. The interfacial reaction between aluminum and copper is studied as a function of the current. An IMC layer is clearly visible at the faying surface of the EUSW. The thickness of the intermetallic compound layer increases with the current. At a relatively high current(1500 A), evidence of solidified microstructure was observed at the interface. The influence of current on the mechanical properties of the joints is also discussed.