Influence of tool rotational speed on the microstructure and mechanical behavior of friction stir processed AA 7050 aluminum alloy

The present study investigated the effect of tool rotational speed on the microstructure and mechanical behavior of friction stir processed AA 7050 aluminum alloy using an optical micrograph, scanning electron microscope, tensile test, impact strength, and Vickers microhardness.The alloy was processed at 40 mm min -1 as a travel speed combined with three tool rotational speeds, 800, 1200, and 1600 rpm.The visual inspection of the surfaces of the samples revealed that the alloy could be successfully friction stir processed at any rotational speed.The results indicated that the mean grain size of the stir zone decreases with increasing the tool rotational speed (from 45 to 9 µm).The development of the precipitates θ, (η and η), (T and T), and S phases was significantly controlled by the tool rotational speed.Consequently, the microhardness, tensile strength, and impact strength of the AA7050 alloy were enhanced by friction stir processing due to the combination of grain refinement and precipitation strengthening.The optimum mechanical properties were obtained for the sample processed with a tool rotational speed of 1600 rpm.K e y w o r d s : friction stir processing