Precipitate-free zone

Watch out with precipitation-hardening alloys: When welding, the heat-affected zone gets so hot that the fine particles that make the material hard dissolve. On rapid cooling, they don't form back properly; instead, the alloying elements migrate to the grain boundaries and form coarse chunks there. This creates a soft seam right next to the weld, the so-called precipitate-free zone. That's the weak point of the whole joint. If you load the weld, this soft zone deforms first, and it can tear easily. Especially with high-strength aluminum alloys, this is a classic failure pattern, and corrosion resistance also suffers there. To get this under control, you have to keep the heat input as low as possible. Use processes with concentrated energy, like laser or electron beam, and don't preheat the part unnecessarily. If possible, you can do a complete heat treatment after welding – re-solution annealing, quenching, and aging – then the zone is gone again. You can check it with a hardness test or a micrograph.