Precipitation hardening

Solution-annealed aluminum alloys get their strength from finely distributed precipitates that form during aging. When welding, you heat the heat-affected zone so much that these particles dissolve or coarsen – the zone becomes soft and stays that way even after cooling. If the design allows, you must re-solution anneal, quench, and age after welding. Otherwise, you have to design the weld or choose cross-sections so that the soft zone still carries the load. Keep heat input as low as possible, i.e., narrow weld beads and don't get too hot.