Evolution of grain structure across joints in friction stir welded EN AW 5083 H111 plates during thermal exposure

The mechanisms involved in the coarsening of the grain structure across FSW joints produced in EN AW 5083 H111 plates with different pin profiles were investigated. The grain coarsening across the FSW joints invariably starts inside the shoulder flow zones and involves both abnormal growth of the grains inside the dynamically recrystallised regions as well as recrystallisation of the deformed grains outside these regions. The elliptical nugget zones produced with the threaded circular pins appear to be more stable with respect to the basin shaped counterparts obtained with the triangular pin. It takes an annealing temperature of 550 degrees C for the former to become fully coarse grained across the joint while the joint produced with the triangular pin is largely covered with coarse grains starting at 500 degrees C. The higher resistance of the nugget zone obtained with a threaded circular pin to grain coarsening appears to be critical in the selection of the pin profiles when the manufacturing cycle of the welded EN AW 5083 plates involves post-welding heat treatments.