Effect of pre-strain on the brazing characteristics of a high-strength aluminum brazing sheet with a barrier layer

Several interactions can occur between the liquid clad and solid core alloys during brazing of multi-layered Al brazing sheets used in automotive heat exchanger fabrication. The objective of the current study is to evaluate the use of a physical barrier layer between the clad and core alloys to prevent the occurrence of certain solid–liquid interactions during brazing. A novel brazing sheet comprised an AA4045 clad, AA1100 barrier layer, and Al–1.95 Mg–1.5Mn–0.5Cu core alloy (NS) was investigated and compared with a commercially available AA4045/X901 brazing sheet (CS). Interrupted tensile tests were performed to impart strains between 0 and 10% in both sheet conditions prior to brazing. Differential scanning calorimetry (DSC) was used to measure the brazing characteristics through the ratio of solidification peak energy to initial melting peak energy after a 3-min hold at 600 °C (i.e., ΔH_sol/ΔH_melt). The post-braze microstructures of DSC samples and samples subjected to furnace brazing were investigated. The ΔH_sol/ΔH_melt ratio for both CS and NS conditions exhibited a non-linear relationship with the level of applied strain prior to brazing, but the nature of the relationship was different for the two sheets. Liquid film migration (LFM) was generally observed in CS conditions exhibiting low values of ΔH_sol/ΔH_melt. The NS barrier layer was found to effectively prevent LFM in strained brazing sheet, but diffusional solidification of the clad layer still resulted in relatively low ΔH_sol/ΔH_melt values at low levels of pre-strain. It was also found that the post-braze microstructures of samples heat treated in the DSC and furnace were different, with furnace-treated samples containing a greater volume of Al–Si eutectic due to the shorter time at elevated temperatures. It is concluded that the barrier layer can be an effective method to prevent liquid film migration in strained brazing sheet.