Precipitate stability in the heat-affected zone of a welded Al–Mg–Si–Cu alloy

Aluminium alloys are used in component manufacturing due to their favourable properties like high strength-to-weight ratio, good formability, and corrosion resistance. However, since welding processes are often an integral part in multi-component assemblies, the thermal stability is an important criterion when selecting Al alloys. Among possible candidates, Al-Mg-Si-Cu alloys have demonstrated a remarkable good thermal stability. This is primarily attributed to the formation of ordered and partly disordered precipitates like the L phase during precipitation hardening. However, despite these alloys’ promising properties, their performance during rapid heating and cooling remains relatively unexplored. This study systematically investigates the microstructure evolution within the heat-affected zone (HAZ) of a welded peak-age Al-Mg-Si-Cu alloy on the advancing side of the joint, and assesses how these changes relate to measured hardness and tensile properties using electron microscopy and mechanical testing. Scanning precession electron diffraction is used to identify and spatially map the precipitates. The results reveal that almost all precipitate types dissolve in the HAZ where the peak temperature reaches approximately 285-375 °C. At higher peak temperature, approaching 450 25 °C, only the L phase and precipitates typically observed in the alloy’s over-aged condition remain. The potential of exploiting the L phase’s thermal stability to narrow the HAZ width through thermomechanical processing is briefly discussed toward the end of the paper.