Brazing of Sn78Cu22 on aluminum substrates observed via in-situ experiments in a large-chamber SEM

In situ brazing is a powerful method for understanding dynamic material behavior during joining process, enabling real-time observation of melting, wetting, and interfacial reactions. The present study investigates the influence of aluminum alloy chemical composition on wetting behavior and interfacial phase formation during soldering with a Sn78Cu22 alloy. The experiments were performed inside a large-chamber scanning electron microscope (LC-SEM) to monitor heat-induced deformation, melting, melt propagation, elemental segregation, and solidification. Microstructural and compositional analyses were carried out using SEM/EDX and TEM, supported by thermodynamic calculations using Thermo-Calc. Despite the low melting point of Sn78Cu22, solder reactions could only be initiated at significantly higher temperatures, namely 466 °C for EN AC-42,100, 500 °C for EN AW-5083, and 600 °C for EN AW-3003, due to the presence of stable surface oxide films. The Si-rich EN AC-42,100 alloy exhibited early filler deformation and formation of Al–Cu and Cu–Sn intermetallic compounds, namely Cu6Sn5, Cu2Sn, while Sn wetting was limited by a Si diffusion barrier. In contrast, the EN AW-5083 and EN AW-3003 alloys showed delayed wetting caused by MgO and MgAl2O4 oxide layers, respectively. For EN AW-5083, Mg reacted with Sn to form Mg2Sn within the filler and at the interface, accompanied by Al–Cu formation. For EN AW-3003, Al–Cu interdiffusion led to the formation of Cu-rich Al4Cu9-type intermetallics with shallow Sn penetration. These results demonstrate that aluminum alloy chemistry critically governs wetting kinetics and interfacial reactions during soldering with Sn78Cu22.