Abstract
Stress corrosion cracking (SCC) resistance for weldment of Al–Zn–Mg alloys is enhanced with appropriate filler metal and post-welding heat treatment while maintaining the weld strength integrity. Tungsten insert gas (TIG) arc welding is used to join the AA7005-T4 plates with three high-Mg fillers, ER5183, ER5556 and ER5356. Four heat treatments, NA (natural ageing), T6, T73, and RRA, were applied to the welded plates. Micro-hardness test, tensile test and stress corrosion test were conducted on the heat-treated specimens. Differential scanning calorimetry (DSC) analyses and transmission electron microscopy (TEM) observations were also performed on the specimens cut from the fusion zone. For the three high-Mg filler metals, only the ER5356 filler with low Mn content showed improvement in the tensile property after different post-welding heat treatments. Weld strength improvement were ideal for the T7 post-welding heat treatment. No improvements were observed for the ER5183 and ER5556 filler wires. The ultimate tensile strength of ER5183, ER5556 and ER5356 weld were found to fall within the, respective, ranges of 250–280, 270–300, and 280–360 MPa corresponding to the different heat treatments. Post-welding heat treatment improves the SCC resistance of welds made with ER5356 and specimens with T7 and RRA heat treatments. DSC analyses showed that only the specimen fabricated from the weld made with ER5356 has an endothermic dissolution reaction peaked at 141 °C. The TEM images from the fusion zone (FZ) of T7 and RRA welds with ER5356 filler wire showed similar micro-structure and distribution of precipitates. They both have relatively large and non-continuous precipitates at the grain boundary. The size of precipitates differed in the matrix. The combination of ER5356 filler metal and post-welding T7 treatment can offer the AA7005 weld with better SCC resistance and high tensile strength.
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