Aluminum-Magnesium-Silicon Aging
Formation of magnesium-silicon precipitates and cluster formation during artificial aging of Al-Mg-Si wrought alloys. This is the standard mechanism for medium-strength, easily extrudable structural profiles.
AlMgSi alloys are hardenable. When welding, you lose the heat treatment in the heat-affected zone, the material becomes softer there. Hardening happens via solution annealing and subsequent quenching, then the material ages at room temperature or artificially warm. When welding, you have to be careful not to put too much heat in, otherwise the structure overages or you get hot cracks. If the weld is to have strength again afterwards, you have to solution anneal, quench, and age the whole area again – that is often difficult in practice.