Femtosecond laser welding of sapphire to Invar36 alloy with enhanced strength by Ti film coating

High-strength femtosecond laser welding of metals to ceramics remains a critical challenge for the practical application of dissimilar joints in aerospace, marine, and medical devices. In this work, Ti films with different thicknesses are proposed to enhance the bonding strength of sapphire/Invar36 joints, and the ultrafast laser-induced dynamics on Ti-coated Invar36 surfaces are further investigated. The results show that Ti films markedly enhance mechanical interlocking at the sapphire/Invar36 interface, significantly increasing both shear and tensile strengths. Under the same laser fluence, the Ti-coated surface exhibited a larger focal region and a faster decrease in reflectivity, whereas the bare surface showed a slower decrease in reflectivity and pronounced Newton's rings within the focal region. In addition, greater melt depth and increased plasma intensity and lifetime were observed on Ti-coated surfaces, suggesting that Ti films promote interfacial melting and bonding. However, increasing the Ti thickness also caused more severe cracking in the sapphire. An optimal Ti thickness of 150 nm yielded peak joint strengths of 212.96 MPa (shear) and 64.40 MPa (tensile). These optimized joints retained relatively high strength after thermal aging and thermal cycling, indicating improved thermal stability compared with joints without an interlayer.