Ultrafast Laser Direct Welding of Dissimilar Materials: ZnS Crystals and Titanium Alloys

Direct welding of ZnS crystals to titanium alloys is critical for the integration of optical window and structural components in advanced optoelectronic and aerospace systems. In this study, we report the first systematic investigation of ultrafast laser welding between ZnS and Ti-6Al-4V alloys using picosecond laser pulses. A record-high shear strength of 67.7 MPa is achieved—85% higher than that of conventional brazing methods—while maintaining a low residual stress of 0.286 GPa, representing only 13% of the typical value found in brazed joints. Detailed SEM images, Raman spectra, and mechanical analyses reveal a flat interface formed via localized melting of ZnS and bonding mechanisms dominated by chemical bonding. A novel joint reinforcement mechanism is proposed, where microvoids—formed during rapid solidification—enhance the joint strength by impeding crack propagation. This work not only demonstrates a robust, clean, and stable joining strategy for dissimilar materials but also lays the foundation for future applications in optoelectronic integration.