Microstructure and properties of graphite plate joints soldered with Sn–Zn–Ti solder via ultrasonic-assisted soldering

Poor wettability and reliable bonding of carbon materials pose significant challenges in the thermal management materials. In this study, Sn–9Zn–3Ti active solder was applied to the side surfaces of graphite plates via ultrasonic coating. The bonding of graphite plates was achieved by applying ultrasonic treatment after butt-jointing their coated surfaces. The influence of ultrasonic coating duration on the microstructure and mechanical properties of brazed joints was analyzed. The results revealed that at 300 °C, the Sn–Zn eutectic matrix of solder existed in a molten state, with solid Ti₂Sn₃ phases distributed within the solder. At ultrasonic coating times of 5 s and 10 s, ultrasonic energy transferred to the boundaries of solid Ti₂Sn₃ phases, inducing their dissolution and transformation into TiSn₃Zn₅ phases, which further lowered the melting point of Ti-bearing compounds. At 20 s, the Ti₂Sn₃ and TiSn₃Zn₅ phases melted and refined. Ultrasonic waves propagated to the interface between the graphite plate and Sn–Zn–Ti solder, impacting the solder to penetrate into the interlayers of the graphite plate and form an interlocking structure. Part of the graphite film achieved interfacial bonding with the solder, and the fracture location shifted from the interface to the solder, achieving a tensile strength of 11.78 MPa. At 40 s, the Ti₂Sn₃ and TiSn₃Zn₅ phases aggregated at the graphite plate/solder interface and reacted with C to form TiC, enhancing the wettability of the solder on the graphite plate surface and contributing to increasing the strength to 12.32 MPa. This study provides a theoretical foundation for the application of Sn–Zn–Ti solder and the bonding between carbon materials.

Graphical abstract