Interfacial Characteristics and Metallurgical Mechanism of Sapphire/TC4 Alloy Joints Produced by Femtosecond Laser Welding

Femtosecond laser welding of sapphire to TC4 (Ti–6Al–4V) alloy was conducted under atmospheric conditions, and the interfacial characteristics, metallurgical reaction mechanism, and the effect of laser power on joint strengthening were systematically investigated. The results revealed that the femtosecond laser induced transient high temperatures, elemental intermixing, and interfacial metallurgical reactions at the sapphire/TC4 alloy interface, producing an interfacial compositional transition zone with a maximum measured width of 5.72 μm after rapid cooling. Reaction products including TiO, TiO2, and Al3Ti were identified by XRD, while EDS revealed interfacial redistribution of Ti, Al, and O. At a laser power of 8.4 W, the joint exhibited a relatively flat interfacial morphology and a lower apparent shear strength than that at 12.4 W. The highest mean apparent shear strength of 51.55 MPa was obtained at 12.4 W, which was associated with the characteristic wavy interfacial morphology that promoted crack deflection and energy dissipation. At 16.6 W, interconnected microcracks formed near the interface and within the sapphire substrate due to excessive heat input and thermal mismatch, and the apparent shear strength decreased to a level statistically comparable to that at 8.4 W.