Non-isothermal thermal cycle process model for predicting post-weld hardness in friction stir welding of dissimilar age-hardenable aluminum alloys

A challenge in welding age hardenable aluminum alloys is the strength loss in the heat affected zone (HAZ) due to transformation of precipitates upon non-isothermal heating cycles. The present work focuses on understanding the evolution of the HAZ microstructure of age hardened Al-Mg-Zn and Al-Mg-Si alloys joined together by dissimilar friction stir welding (FSW). An analytical model was applied to describe the thermal history of the dissimilar alloy FSW joints processed at 125, 250 or 500 mm min(-1) and measured by embedded thermocouples. The Shercliff-Ashby age hardening model was fit to the isothermal ageing kinetics obtained from Vickers microhardness measurements of each individual alloy. The FSW process cycles were modelled in terms of an equivalent isothermal treatment and coupled to the Shercliff-Ashby model to predict the hardness values, and compared to Vickers microhardness profiles and microstructures observed after FSW. The model predicts softening in the HAZ, which correlated to the precipitate evolution observed by transmission electron microscopy.