Microstructure evolution of warm deformed multilayered Al alloy sheet during brazing

Warm forming can improve the formability of Al alloy sheets that are used in heat exchanger production, but the effect of warm forming on the brazing performance of the sheet is not well understood. This investigation evaluates the brazing performance and corresponding microstructure evolution of a warm formed sheet comprised of an Al-Mn core alloy and an Al-Si clad alloy. Understanding the conditions that lead to the detrimental process of liquid film migration (LFM) during brazing is of particular interest. Warm forming was simulated by tensile tests performed between room temperature and 250 degrees C, and at strain rates between 7E-4 s(-1) to 7E-2 s(-1). A brazing treatment consisting of a 3 min hold at 600 degrees C was simulated using differential scanning calorimetry. Brazing performance was evaluated by the ratio of the clad alloy solidification energy to the initial melting energy (i.e. Delta H-sol/Delta H-melt). The post-braze microstructure and Delta H-sol/Delta H-melt depended on strain, strain rate, and forming temperature. Delta H-sol/Delta H-melt was 0.5 or less when LFM occurred during brazing, but improved to 0.7 when the core alloy recrystallized. The dislocation density in the sheet was calculated using the Bergstrom hardening model; dislocation densities over which LFM and full core recrystallization occurred were distinct from one another. When LFM did not occur, isothermal solidification reduced the amount of clad alloy available for brazing by 23 %. It is concluded that the post-braze microstructure strongly depends on the sheet forming conditions, and that the driving force for LFM is the reduction of stored deformation energy.