Microtopographic, mechanical and microstructural evaluation of ultrasonic torsional welds for Al-Cu electrical connections

• Developed a surface-quality-based framework for assessing ultrasonic torsional welds of Al–Cu joints. • Integrated 3D surface topography, microhardness mapping, and microstructural analysis for weld evaluation. • Employed torque-controlled ultrasonic welding to precisely regulate energy input and bond formation. • Established direct correlation between surface morphology, interfacial integrity, and mechanical strength (2481 N) • Introduced a predictive, non-destructive method enabling early detection of weak zones and reliable process optimization. Reliable joining of aluminum and copper is a major challenge for creating efficient and lightweight electrical systems in cars and other new energy vehicles. This study investigates a high-tech solution using an ultrasonic torsional welding machine with precise torque control to carefully manage the joining energy. This study presents an integrated framework for assessing and predicting the quality of aluminum-copper (Al-Cu) dissimilar joints produced by an advanced torque-controlled ultrasonic torsional welding (USTW) system. We introduce the combined use of high-resolution 3D surface topography and microstructural analysis as a powerful, non-destructive method for evaluating weld integrity. The precision torque-control mechanism enabled the detailed study of how controlled energy input manifests at welded interface. Our results successfully establish a direct correlation amid specific surface topography signatures such as defined roughness parameters (Ra, Rz, Rp, Rv, Rq) and 3D volumetric uniformity. Microhardness mapping revealed a consistent and intense work-hardened bond line (peak ∼ 155 HV), confirming robust solid-state bonding. Energy-dispersive spectroscopy (EDS) verified the process efficacy in creating a clean and contaminant-free interface. Crucially, we demonstrate that by quantifying surface heterogeneity, potential weak zones can be identified before mechanical failure. Good welds showed fine-grained metal grains but poor welds showed cracks, spattered metal dots and trapped oxides during SEM analysis. During the tensile test, the strongest sample withstood 2481 N when we pulled the joints apart but the most flexible one stretched longer.