Diffusional solidification phenomena in clad aluminum automotive braze sheet

Differential scanning calorimetry (DSC) was used to investigate clad/core interactions and the evolving microstructure during simulated brazing of aluminum brazing sheet alloy No. 12, which consists of an AA3003 core with AA4343 cladding layers on either side. During the brazing operation the clad layers can alloy with the core as a result of Si diffusion before and after clad melting (i.e. in the solid-state and the solid/liquid state). This interaction affects the amount of liquid formed and its duration, which will impact on joint formation and mechanical performance. Metallographic data and DSC liquid fraction measurements show that solid-state interdiffusion prior to melting is responsible for some initial liquid suppression, but the amount of liquid present during the brazing cycle decreases rapidly as a result of isothermal, or diffusional, solidification. The liquid phase can be completely removed via diffusional solidification at the brazing temperature prior to cooling, leaving behind a significantly coarsened clad structure. Using this novel and practical DSC technique, experimental insight regarding Si diffusion into the core can be obtained as well as the evolving clad/core solidification structure.