The wetting and spreading of AgCu filler metal over Ti-6Al-4V (TC4) substrates in a high-purity argon atmosphere were investigated. The influences of substrate surface roughness and temperature on the wetting and spreading mechanisms were studied. The substrate surface roughness had a limited impact on the wetting and spreading process, while the temperature dictated the spreading process and mechanisms. At 860 A degrees C, the entire spreading process involved four distinct stages: (I) an initial stage (II) a rapid spreading stage (III) a limited spreading stage, and (IV) an asymptotic stage. On the other hand, at 940 A degrees C, owing to the more drastic chemical reaction, the entire spreading process exhibited only three stages: (I) an initial stage (II) a rapid spreading stage, and (III) an asymptotic stage. At 860 A degrees C, the wetting kinetics of the rapid spreading stage were determined by the chemical reaction rate, where the interdiffusion between Ti (from the substrate) and Cu (from the AgCu filler) was the dominant rate-limiting factor during the limited spreading stage. However, at 940 A degrees C, the wetting kinetics was dominated by the mixed effects of the chemical reactions, dissolution, and diffusion into the solid. These differences indicate that the wetting mechanism was changed from the reaction-limited (in stage II) and diffusion-limited (in stage III) modes at lower temperatures to a different mode at higher temperatures that were controlled by chemical reactions, dissolution, and diffusion into the solid.