WETTING BEHAVIOR IN ULTRASONIC VIBRATION-ASSISTED BRAZING OF ALUMINUM TO GRAPHITE USING Sn-Ag-Ti ACTIVE SOLDER

In this study, Sn-Ag-Ti ternary alloy has been used as the active solder to braze pure aluminum and graphite in atmospheric conditions using ultrasonic vibration as an aid. The authors studied the formation, composition and decomposition temperature of the surface oxides of the active solder under atmospheric conditions. In addition, the wettability of Sn-5Ag-8Ti active solder on the surface of pure aluminum and graphite has also been studied. The results showed that the major components presented in the surface oxides formed on the Sn-5Ag-8Ti active solder under ambient conditions are TiO, TiO2, Ti2O3, Ti3O5 and SnO2. Apart from AgO and Ag2O2, which can be decomposed at the brazing temperature (773 K), other oxides will not be decomposed. The oxide layer comprises composite oxides and it forms a compact layer with a certain thickness to enclose the melted solder, which will prevent the liquid solder from wetting the base metals at the brazing temperature. After ultrasonic vibration, the oxide layer was destroyed and the liquid solder was able to wet and spread out around the base materials. Furthermore, better wettability of the active solder was observed on the surface of graphite and pure aluminum at the brazing temperature of 773-823 K using ultrasonic waves. The ultrasonic wave acts as the dominant driving factor which promotes the wetting and spreading of the liquid solder on the surface of graphite and aluminum to achieve a stable and reliable brazed joint.