Effects of double-pass welding and extrusion on properties of fiber laser welded 1.5-mm thick T2 copper joints

The elongation of the laser beam welded joint is much lower than that of base metal due to the existence of softened zone and coarse columnar grain structures in the fusion zone. Mechanical extrusion can enhance the micro hardness of the softened zone in copper joint, whereas double-pass laser beam welding will disrupt the growth direction of the columnar grain structures in the fusion zone and decrease the size of columnar grains. Four types of copper joints were prepared, which included the single-pass laser beam welded (LBW) joint, single-pass laser beam welded + extrusion (LBW & E) joint, double-pass laser beam welded (DLBW) joint, and the double-pass laser beam welded + extrusion (DLBW & E) joint. The microstructure and mechanical properties of these four types of joints were characterized and compared. The results show that the mechanical properties of the DLBW & E joint exhibited the most pronounced improvement, where its tensile strength and elongation increased by 8.5% and 20.1% compared with those of the LBW joint, respectively. The extrusion process would increase the number of dislocations in the FZ of copper joints and consequently lead to a slight degradation of both thermal conductivity and electrical conductivity.