Modelling microstructural evolution during multiple pass friction stir welding

The effect of multiple weld passes on the microstructure and properties of friction stir welds in aerospace aluminium alloy AA 7050 has been investigated with the aid of a microstructure model. Each weld pass leads to a reduction in minimum strength in the heat affected zone, the decrease being greatest after the first pass. The model predicts that this is due to partial dissolution and coarsening of the strengthening precipitatates, which up to weld pass 3 is dominated by dissolution, and beyond weld pass 3 by coarsening. Microstructural evolution in the nugget is predicted to involve complete precipitate dissolution during each pass, followed by reprecipitation on the refined grain boundaries. A post-weld aging treatment after multiple passes is predicted not to be effective in improving minimum strength since precipitation occurs via growth and coarsening of existing particles, not nucleation of new precipitates.