Effects of heat treatment and Nd: YAG laser repair welding parameters on the microstructure and properties of a Cu–Ni–Si–Cr mold alloy

The novel Cu–Ni–Si–Cr alloy (Cu-6.7Ni-1.6Si-0.3Cr) has good prospects for substitution with toxic Be–Cu alloy. This study used a Nd: YAG laser process with a variety of heat treatment processes and repair welding parameters to investigate the effects of heat treatment and repair welding parameters on the microstructure and properties of the Cu–Ni–Si–Cr mold alloy. To eliminate the disadvantages of the laser welding process for the Cu–Ni–Si–Cr mold alloy, such as high thermal conductivity and high thermal expansion coefficients, many improved approaches have been used, including pre-weld heat treatments (expected to reduce thermal conductivity), control of the laser shape, and preheating (to slow down the cooling rate and avoid excessive expansion and contraction). The results showed that after the pre-weld heat treatment and preheating, deeper penetration and defect-free welds were obtained. However, continuous Ni/Si phase precipitates were observed along the grain boundaries in the welding fusion zone (WFZ), which may have harmful effects on the mechanical properties of the welds. This problem was improved by post-weld heat treatment (PWHT). The repair welds (including WFZ and HAZ) after PWHT showed a good combination of hardness, tensile strength, and thermal properties. The high hardness or tensile strength in the weld was attributed to the formation of a nano-sized Ni/Si phase, which was identified via a transmission electron microscope.