Evolution of microstructure and nanohardness in CuCr1Zr alloy under various electric current sources: Insights into electromigration-assisted recrystallization and precipitation

The degradation of CuCr1Zr electrodes during resistance spot welding (RSW) is a critical issue in the automotive industry, primarily driven by microstructural softening at the electrode tip. While electrode thermal cycles during RSW are well-documented, the role of electromigration—induced by high intensity electric currents—on the evolution of precipitation, grain structure, and mechanical properties remains poorly understood. This study investigates the microstructural and nanomechanical responses of a precipitation-hardened CuCr1Zr alloy subjected to sub-critical annealing treatments at 500 °C, 600 °C, and 700 °C, using for the first time three distinct heating modes: conventional radiative heating, direct current (DC), and pulsed current (PC) Joule heating. Using CuCr1Zr/steel/CuCr1Zr sandwich-type samples to simulate electrode aging in RSW conditions, we employ nanoindentation, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) to characterize the evolution of nanohardness, precipitate distributions, and grain boundary structures. Our results reveal a first-order temperature dependence of alloy softening, consistent with previous studies on thermal aging. However, electromigration induced by DC and PC significantly enhances the precipitation of fine (100–200 nm) particles at 500 °C, a phenomenon not observed under conventional heating. At 700 °C, twinning-assisted recrystallization occurs across all annealing modes, but electromigration—particularly under pulsed current—promotes a higher fraction of Σ 3 twin boundaries, suggesting an acceleration of grain boundary mobility. The hardness reduction at 700 °C is most pronounced under pulsed current (31% decrease relative to the as-received state), compared to 25% and 23% for DC and radiative heating, respectively. These findings highlight the role of current-induced atomic diffusion in microstructural evolution, with implications for electrode lifespan and RSW process optimization.