The effect of heat generated on mechanical properties of friction stir welded aluminum alloys

The aim of this work is to analyze the effect of the heat generated within the joints during the friction stir welding process. Four different aluminum alloys were considered, i.e., three precipitation-hardening alloys (AA2024-T3, AA6082-T6, and AA7075-T6) and a work-hardening alloy (AA5754-H111). In order to study the cooling effect, the welds were performed with and without a water-cooling system. The temperatures reached during the FSW process were measured by three type-K thermocouples positioned near the nugget, in the thermo-mechanically affected zone and in the heat-affected zone. The results suggest a dependence of the thermal gradient on both the process parameters used and the alloy considered. From the mechanical point of view, the Rockwell B hardness tests and tensile tests were performed. It was possible to evidence a relationship between the hardness distribution and the cooling systems. In fact, the water-cooled joints showed higher values of hardness reached in the thermo-mechanically affected zone and in the heat-affected zone for all the precipitation-hardening-tested alloys. The tensile tests executed orthogonally to the welding direction showed, for all the alloys, a reduction of the real elongation percentage in water-cooled joints; this can be related to the hardness increase that implies a decrease in the elongation of these welds.