Interfacial Bonding Mechanism And Numerical Simulation Of Cu/Cu Joint By Ultrasonic Spot Welding

With the world’s attention to energy conservation and emission reduction,new energy vehicles are gradually widely used,followed by higher and higher requirements for new energy vehicle batteries.Compared with other traditional welding technologies,ultrasonic welding has the advantages of short welding time,low energy consumption and suitable for sheet metal welding.Therefore,the research on ultrasonic welding has become a key technical issue to promote the development of new energy vehicles.At present,the research on Cu/Cu ultrasonic welding is mainly focused on the process parameters,and there is a lack of research on the joint connection mechanism.In addition,due to the limitations and errors of the test equipment,the inferences drawn during the test also need to be verified.In this paper,the joint mechanism of the joint is revealed by means of experiment and numerical simulation.Cu/Cu joints were fabricated with welding time of 0.5 s-0.9 s,and microstructure evolution and mechanical properties of the joints were analyzed.The results showed that the joint with welding time of 0.8 s has the highest temperature rise rate(607.5 ℃/s),and can withstand the maximum shear force(2370 ± 48.5 N),which indicates that under the conditions of ultrasonic frequency of 20 k Hz,amplitude of 60 % and clamping force of 0.4 MPa,0.8 s is the best welding time for preparing Cu/Cu joint.The microstructure and crystallographic information of Cu/Cu joints prepared under these technological parameters were systematically analyzed.The results show that the distribution of grain size,grain boundary,local orientation difference,Schmidt factor and grain orientation along weld thickness is uneven and asymmetrical,the grain size varies gradient from weld to base metal,the nano-sized equiaxed and refined elongated grains induced by dynamic recrystallization were formed at the welding interface.The nucleated fine grains between the elongated grains were beneficial to the grain boundary migration and consequent the achievement of the interfacial metallurgical bonding.The fracture was characterized,dimples and cleavage were observed,and there was an obvious necking phenomenon,indicating mixed fracture characteristics.A three-dimensional finite element model was established by ANSYS software to explain the dynamic process of ultrasonic welding,and the temperature distribution and plastic strain distribution of Cu/Cu joints were obtained by taking into account air convection,ultrasonic softening and other factors.Combined with the test results of grain size,hardness distribution and fracture morphology,the mechanical properties and bonding mechanism of the joint interface were revealed.Aiming at the problem that the welding rate could not be accurately quantified in the test result,a critical plastic strain value(0.11)was defined to quantify the welding rate by comparing the position where the joints formed good connections with the numerical simulation results.The welding rate of the joints with welding time of 0.8 s and 0.9 s was measured to be 76 %and 83 %,respectively.The experimental and simulation results show that the yield strength of the material decreases obviously under the action of ultrasonic vibration,which promotes the generation of violent plastic deformation,and forms a good metallurgical bond.