Effect of interlayer material on ultrasonic welding of dissimilar titanium and St12 steel sheets

This study examines the influence of metallic interlayers on ultrasonic welding of commercially pure Grade 2 titanium to St12 low-carbon steel. Three interlayers—copper, brass (70B) and zinc foils, each 0.05 mm thick—were evaluated. The welded joints were characterized by optical microscopy, SEM, EDS, tensile testing and microhardness profiling. Brass promoted the formation of mechanical interlocks at the interface, whereas copper facilitated extensive plastic deformation and localized diffusion without significant interlocking. Zinc, due to its limited deformability and poor adhesion to steel, failed to consolidate into effective joints. Mechanical testing showed that the brass interlayer achieved the highest yield load (∼42 kgf), nearly double that of copper (∼22 kgf), while zinc joints fractured during clamping. EDS analysis confirmed elemental interdiffusion in copper and brass joints but negligible interaction for zinc. Post-weld microhardness results indicated that copper induced strain hardening in both base metals, while brass caused softening on the titanium side due to grain coarsening. These findings highlight that interlayer selection critically governs bonding mechanisms, strength, and microstructural evolution in dissimilar titanium–steel ultrasonic welds. The comparative evaluation of copper, brass, and zinc interlayers under identical processing conditions provides practical insight for optimizing material combinations in advanced structural applications.