Softening behaviour of Al–Zn–Mg alloys due to welding

The softening behaviour of two Al–Zn–Mg alloys (7020 and 7022) due to welding was studied by means of simulation and real welding experiments. It was found that the heat-affected zone of the alloys can be divided in two parts: the dissolution zone and the overageing zone. The dissolution zone is characterised by dissolution of the strengthening precipitates, whereas the overageing zone is characterised by growth of the strengthening precipitates. It appears that the 7022 alloy has a stronger softening tendency than the 7020 alloy in both the dissolution zone and the overageing zone. It was also found that the degree of softening depends on the heat input: a higher input leads to more severe softening and a wider softening area, located at a larger distance from the fusion boundary. The hardness in the heat-affected zone can be recovered by post-weld heat treatment, especially in the dissolution zone. As far as the recovery of the hardness is concerned, artificial ageing is more effective than natural ageing.