Influence of repetition rate on the microstructure and mechanical properties of dissimilar sapphire/Invar36 alloy joints by ultrafast laser micro-welding

This study systematically investigated the effects of thermal accumulation and thermal diffusion on the microstructure and mechanical properties of sapphire/Invar36 alloy joints fabricated by ultrafast laser micro-welding at different repetition rates. The results indicated that as the repetition rate decreased, the shear strength of the joints progressively improved, accompanied by a significant increase in interfacial mixing and the mixture layer attained its maximum thickness. A high-strength joint with a shear strength of 269.94 MPa was successfully produced at a repetition rate of 200 kHz, providing crucial technical support for the practical application of this material system in industrial fields. Fractographic analysis on the Invar36 alloy side revealed a typical dimpled morphology, characteristic of ductile fracture, which confirmed that sufficient interfacial mixing had significantly enhanced the mechanical properties. Furthermore, this study revealed, for the first time through numerical simulation, the evolution behavior of the temperature field at the joint interface under different repetition rates, elucidated the energy relaxation process from electrons to the lattice and the thermal accumulation and thermal diffusion behavior of the joint, and proposed the joint formation mechanism.