Wetting kinetics and spreading phenomena of the precursor film and bulk liquid in the AgCuTi/TC4 system

The reactive wetting of AgCuTi filler metal over a TC4 substrate was conducted using real-time in situ hot stage microscopy. In addition to the commonly observed bulk liquid, a rapidly spreading precursor film was observed. Both the precursor film and bulk liquid were studied and characterized. The wetting kinetics of the bulk liquid within the main spreading stage was predominantly controlled by chemical reactions, in which Ag-based solid solutions (Ag(s,s)), Ti-based solid solutions (Ti(s,s)) and intermetallic compounds (Ti2Cu) were formed within the central domain. The high temperature phase transition of α-Ti to β-Ti induced by the diffusion of Ag into the TC4 substrate was also observed at the edge domain. In the mid-to late-stage spreading process, a thin precursor film, with an extremely rapid spreading speed, appeared at the front of the triple line. According to compositional analysis, the precursor film was predominantly composed of Ag and exhibited similar spreading characteristics to pure Ag on TC4. The formation mechanism of the precursor film can be explained by the mechanism of rapid adsorption followed by the film overflow; based on this mechanism, a physical model was proposed.