SOME STUDIES ON ULTRASONIC JOINING OF THIN COPPER, ALUMINIUM AND PHOSPHOR BRONZE SHEETS

Abstract

The quest to produce cost-effective, efficient, and ergonomically designed products demands the use of assemblies fabricated with assorted materials. Because of the differences in their physical, chemical, and metallurgical properties, joining dissimilar metals has been a difficult task for the researchers. Ultrasonic metal welding has overcome some of these limitations due to its unique characteristics. A number of diversified applications, ranging from small components used in the electronics industry to aerospace and solar, are being fabricated by Ultrasonic Spot Metal Welding (USMW). USMW uses vibrational energy to produce heat at the interface of the faying surfaces. The sheets are subjected to combined normal and shear loading with the help of sonotrode assembly. These combined loads disperse the oxides and contaminants as well as remove the surface asperities so as to form pure metallic bonds in cold conditions without filler metal, flux, or shielding gas. It’s an efficient, green process that takes very little processing time. Since USMW is designed to join dissimilar metals/materials, this study used both similar and dissimilar combinations of phosphor bronze (UNS C51100), copper (UNS C10300), and aluminum (Al 3003). It was observed through the available literature that there is a need for the optimization of the process parameters along with the characterization of the weld joint in the case of USMW. With these goals in mind, experiments were carried out in both 'Time Control Mode' and 'Energy Control Mode' using different experimental designs. The Analysis of Variance (ANOVA) is utilized on the response parameters-tensile shear load and the weld area. Weld pressure is observed as the most significant parameter, followed by weld time, and vibration amplitude, in affecting the weld strength. A reasonably good correlation is observed between the tensile shear load and the weld area between all the combinations of the weld metals. The process parameters are optimized by coupling the regression model as a fitness function with vii

the simulated annealing optimization algorithm. Finally, the confirmatory experimental results substantiated the predicted results and validated the proposed methodology. The modeling and simulation of the USMW process is carried out using FEM. The model is utilized for the study and prediction of the thermal profiles at the weld interface. The heat fluxes generated due to deformation and friction are calculated and assigned as boundary conditions during thermal simulation. The forecast of temperature is done under various welding conditions. The maximum temperature obtained by transient simulation at the weld interface is 368.8℃, 369.4℃ and 296.1℃ for PB-PB, PB-Cu, and PB-Al, respectively. The continuous reduction in the temperature is observed towards the extremes of the weld metal. The sonotrode and the anvil achieve a lower temperature in comparison to the weld interface. The effect of clamping force and bonding ratio on the interface temperature is observed to be positive. The weld interface is distinguished as the weld zone, TMAZ and HAZ. The model is validated with the maximum absolute errors within 5% for PB-PB joints, 6.26% for PB-Cu joints, and 5.68% for PB-Al joints between the observed and predicted temperature results. A correlation coefficient of 0.96, 0.87, and 0.86 is established between the simulated temperature result and the weld strength for PB-PB, PB-Cu, and PB-Al, respectively. Thus, it is clear that the interface temperature has a strong linear relationship with joint strength and is a major deciding factor for achieving strong joints. The effect of the weld energy on interface temperature and weld strength is also explored. It is observed that the values of peak interface temperature and tensile-shear strength increase with the welding energy. The failure mode changes from interfacial to nugget pull-out at a considerably high energy level during the tensile-shear load test. There is a significant rise in the tensile-shear load initially, but a negligible change is observed in the last stage. The scanning electron microscopy (SEM) revealed that the joining line appears almost straight at a low energy level but fades away at a higher energy level. The bonding region ultimately viii

acquires the shape of a wavy, convoluted interface. Micro-bonding accompanied by interlocking is observed as the primary joining mechanism at high energy level. Hence, it can be concluded that joint strength in USMW was the combined result of the formation of micro-bonds and mechanical interlocking due to the swirling of metal at the interface. The observations and the results of the current study reflect that different combinations of PB, Cu, and Al give very good responses to the ultrasonic spot metal welding in the given ranges of the parameters. Hence, this joining technique can be effectively used for the fabrication of thin components made of these metals. This study can provide useful inputs for the industries involved in the manufacturing of battery electrical vehicles, solar panels, small electrical and electronic products like relays, contacts, and heat sinks etc.