Study on the structural transformation of 6xxx composite four-layer aluminum alloy sheet during brazing and artificial aging and its influence on strengthening and corrosion behavior

The microstructural transformation, strengthening and stress corrosion cracking (SCC) mechanism upon brazing and subsequent artificial aging in the novel multi-layered sheets with adequate strength and corrosion resistance were clarified using comprehensive characterization methods, electrochemical measurements and slow strain tensile tests. The microstructure and overall strength of the brazed sheets was improved significantly due to the formation of various precipitates in the AA6A02 core after post-weld aging. The top layers can provide sacrificial protection to the core due to its more negative potential and higher localized corrosion reactivity caused by the large number of micro-galvanic couples formed between intermetallic particles and the substrate, thus increasing the corrosion resistance of the sheets. The galvanic effect and cathodic protection to the core diminished with increased aging time. The peak-aging and over-aging treatments led to reduced corrosion resistance and increased SCC susceptibility, attributing to the reduced potential difference and inefficient cathodic protection effect between the core and clad caused by full deposition of precipitates and severe intergranular corrosion of the core.