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.

Flux-cored Zn–2Al filler metals were subjected to stress-relief treatment at different temperatures. The salt-spray test was subsequently carried out on the specimens, and the corrosion behavior was analyzed. The findings indicated that Zn–2Al filler metal mainly consisted of α-Al and η-Zn phases. Before stress-relief annealing, α-Al showed a prominent continuous banded distribution. At a stress-relief annealing temperature of 100 °C, recovery and limited recrystallization occurred. This weakened the continuous banded morphology and refined the grain size. Under salt-spray exposure, electrochemical corrosion took place within the filler metal. α-Al acted as the cathode, while η-Zn served as the anode. Stress-relief annealing improved grain-boundary stability and thus enhanced corrosion resistance. In addition, the discontinuous distribution of α-Al impeded corrosion propagation. This further improved the corrosion resistance of the filler metal. After 15 days of salt-spray corrosion, the stress-relieved filler metal retained an elongation of 4.96%. In contrast, elevated stress-relief annealing temperatures promoted Al precipitation at grain boundaries and restored the continuous distribution of α-Al. This created preferential pathways for inward corrosion penetration, which degraded corrosion performance. Accordingly, after 15 days of corrosion, the elongation of filler metal annealed at 250 °C decreased to 2.65%.