On the microstructure and mechanical properties of similar and dissimilar AA7075 and AA2024 friction stir welding joints: Effect of rotational speed

The similar and dissimilar aluminum alloys (5 mm thick AA7075-T651 and AA2024-T351) joints were performed by friction stir welding to assess the influence of rotational speed (600, 950, 1300 and 1650 rpm) on the microstructure features, materials flow and mechanical properties of the joints. The results indicated that the width of the thermos-mechanically affected zone (TMAZ) on the retreating side (RS) is greater than that of the advancing side (AS). Increasing the rotational speed results in widening the TMAZ on the AS and RS. The mixing degree in the joints is remarkably affected by the rotational speed. Low rotational speed leads to a limited materials mixing, while the typical onion ring of mixing pattern can be acquired at the high rotational speed. Compared to the base materials, significant grain refinement (average grain size: 1.7 mu m) occurs in all the joints at a rotational speed of 600 rpm. Increasing the rotational speed is prone to result in grain coarsening. The crystallographic texture in the nugget zone of all the joints is dominated by a simple shear texture and varies with the rotational speed. The hardness values of all the joints almost increase and then decrease from the top to the bottom along the welding center thickness direction. The tensile fracture locations coincide with that of minimum hardness values in all the joints at various rotational speeds.