Ni-based filler metal and Ni-Cu-based filler metal were used to obtain copper/stainless steel (Cu/SS) joints through wire-feeding laser welding. Along the SS/weld interface, there exist different grain sizes (from coarse columnar grains to fine equiaxed grains). The heat affected zone (HAZ) on the copper side consisted of two areas with different grain sizes and the size of the grain in the Cu-HAZ of the Ni-Cu-based filled joint was much smaller than that of the Ni-based filled joint. Our results showed that grain refinement at the copper/weld (Cu/weld) interface of the Ni-Cu-based filled joint was observed through high-resolution electron backscattered diffraction (EBSD). There was a hardness elevation at the Cu/weld interface of the Ni-Cu-based filled joint due to the grain refinement on the weld of the copper side. The maximum tensile strength of the Ni-Cu-based filled joint was obtained and reached 91.2% of the tensile strength of the copper base metal (Cu-BM). Joints in this study were observed to fracture in a ductile mode. Furthermore, the Ni-Cu-based filled joint exhibited a higher plastic deformation, which was primarily caused by the large deformation of the weld zone and the large deformation of the Cu-BM due to the high plasticity of the weld, which alleviated the stress concentration, as indicated by 2D-digital image correlation (DIC) test results.

Post-weld evaluation does not readily capture the dynamic evolution of nugget formation in resistance spot welding. In this study, an embedded ultrasonic probe was used to acquire A-scan signals during resistance spot welding of stainless steel. Consecutive signals were assembled into M-type ultrasonic images to analyze echo changes across the thermal cycle and under different heat inputs. Using 12 specimens with metallographic measurements of both nugget diameter D and thickness H, five- and three-dimensional acoustic features were extracted for D and H prediction, respectively. Random forest (RF) regression models were established and evaluated using leave-one-out cross-validation (LOOCV). The M-type images showed the streak evolution of interface echoes, back-wall echoes, and nugget-related echoes, reflecting differences in acoustic responses across welding stages and nugget states. The coefficient of determination (R²), mean absolute error (MAE), and root mean square error (RMSE) were 0.588, 0.674 mm, and 0.798 mm for D, respectively, with corresponding values of 0.570, 0.150 mm, and 0.198 mm for H. These preliminary results indicate quantitative correlations between the extracted low-dimensional acoustic features and nugget geometry, providing an experimental basis for nugget geometry evaluation using in-process ultrasound.