Effects of surface roughness on the microstructure and mechanical properties of dissimilar sapphire/Invar36 alloy joints made by ultrashort pulsed laser micro-welding

The ultrashort pulsed (USP) laser microwelding of sapphire/lnvar36 alloy controlled by the surface roughness of metal was investigated for the first time. The surface roughness (S_a) of Invar alloys gradually decreased from 0.944 to 0.029 μm from the prime surface to grounded and polished surface. However, the joint shear strength first increased and then decreased with the lowered S_a, the maximum shear strength reached 107.87 MPa at the S_a ∼ 0.131 μm. Compared to other surfaces with low S_a, the relatively high surface roughness enhanced the interfacial thermal deposition both spatially and temporally which in turn promoted the diffusion of interface elements and the formation of jagged mechanical interlocking structures. Therefore, the appropriate rough metal surface was beneficial for the enhancement of sapphire/metal dissimilar joints. This report is of great significance in simplifying the surface preparation process in the USP laser microwelding of transparent hard and brittle materials with metals, therefore promoting this technique from lab to industry.