Guinier-Preston zones

Guinier-Preston zones are tiny clusters of alloy atoms that form during aging of hardenable alloys. They are the first step of precipitation hardening and cause a significant increase in strength, but also make the material more susceptible to stress corrosion cracking. When welding hardened aluminum alloys, you need to be aware that the heat input dissolves these fine structures again. The microstructure in the heat-affected zone becomes soft and loses strength – this is called overaging. If you want to regain strength afterwards, you have to completely re-solutionize and artificially age the component after welding. This is often not feasible for large or already assembled structures. Then you have to live with the strength loss in the heat-affected zone and design the weld accordingly. In practice, this means: for welded structures, it is better to use non-hardenable alloys or plan the heat treatment from the start. The exact times and temperatures for solutionizing and aging are in the manufacturer's data sheet – stick to that, it's not rocket science, but it requires a feel for it.