Investigation on the Effect of Sub-Zero Treatment on Micro-Hardness and Microstructure of GTAW Welded Al–Si–Mg–Mn Alloy

In this work, the effect of sub-zero treatment on the mechanical properties of an Al–Si–Mg–Mn alloy welded by GTAW (gas tungsten arc welding) leads to significant softening in the welded region. The latter is due to melting and resolidification in the welded region, which have resulted in decomposition of the strengthening precipitates. The experiments were performed on GTAW welded plates of 6 mm thickness by varying the heat inputs, namely, of 370, 317.1, 277.5, 246.4, and 222 J/mm, and sub-zero treatment time periods. The Sub-Zero treatment was performed at–45°C using dry ice; hardness and microstructure investigations were performed in the welded region of the Al‒Si–Mg–Mn alloy that was studied in two different conditions, namely, as-welded and in that formed after post weld sub-zero treatment with artificial aging. It was found that the post weld Sub-Zero treatment followed by artificial aging had led to realization of significantly higher hardness values in the welded region due to the recurrence of the precipitation sequence.