Microcrackless weld formation under unilateral melting in laser transmission welding of sapphire and stainless steel

Abstract

Sapphire-steel interfacial bonding determines the reliability and safety of aerospace, marine, medical devices, and nuclear systems. However, conventional laser transmission welding induces microcracks in sapphire, compromising joint integrity. In this work, a controlled unilateral melting (UM) strategy using a nanosecond fiber laser was proposed to suppress microcrack formation. By controlling the welding temperature (1672–2323 K) at the interface, only stainless steel 304 (SS304) was selectively melted while maintaining sapphire in a solid state. A simulation model based on Finite Element Method (FEM) was established to describe the welding process. This method effectively minimized interfacial thermal stress and preserved the single-crystal integrity of sapphire. Compared to conventional bilateral melting (BM), UM welding increased the cracking load by 703 % and enhanced shear strength by 70 %. Elemental and microstructural analyses confirmed the formation of a stable (Cr, Al)₂O₃ transition layer in UM weld. Failure mechanisms changed from brittle fracture within sapphire to ductile failure in SS304, ensuring superior mechanical performance. The proposed UM method demonstrates significant potential for high-reliability sapphire-metal welding in industries and provides a novel inspiration for laser welding mechanisms.