Influence of air gap on the sapphire/Invar36 alloy joints by ultrafast laser direct micro-welding

The effect of interfacial air gap width on ultrafast laser micro-welding of sapphire to Invar36 alloy was systematically investigated for the first time. To precisely control the air gap width (optical contact, 0.36, 0.84, 1.32, 1.60, and 2.26 μm), ultrafast laser micromachining technology was applied. Small gaps (optical contact to 1.32 μm) promoted material intermixing and mechanical interlocking, while large gaps (1.60–2.26 μm) induced defects such as microcracks and voids. Therefore, shear strength gradually decreased from 242.7 to 13.6 MPa as the gap increased to 1.32 μm, and it suddenly dropped to 0 MPa when the gap was above 1.60 μm. Joint formation was attributed to nano-/micro-particle bridging from laser-induced Invar plasma. In summary, an acceptable gap tolerance between 0.36 and 0.84 μm is recommended for industrial applications, yielding 28–48.5 MPa strength which is compatible with the brazed joints.