Effect of traverse and rotational speeds on microstructure, texture, and mechanical properties of friction stir processed AZ91 alloy

In the current work, the influences of both traverse and rotational speeds of friction stir processing (FSP) on the microstructural evolution, crystallographic texture, and mechanical properties of AZ91 alloy were investigated. The tool traverse and rotational speeds used in the present study were 20, 40, 60 mm/min, and 400, 800, 1200 rpm, respectively. The average grain size significantly decreased from 61.6 mu m in the as-cast condition to below 10 mu m in the FSP-processed samples owing to the occurrence of dynamic recrystallization (DRX). By increasing the tool traverse speed and decreasing the rotational speed, the grain size reduced. The grain boundaries pinned by fine beta-Mg17Al12 particles, retarding the grain growth. The fraction of the beta-Mg17Al12 remarkably decreased after the FSP owing to fragmentation and dissolution. The high strain rate of the 800-40 and 1200-40 samples coupled with their higher peak temperatures increased the dissolution of beta-Mg17Al12 during FSP. It was found that a texture transition from (10.1) pyramidal to (00.2) basal occurred by decreasing the traverse speed and increasing the rotational speed. The highest microhardness, which was equal to 104.4 HV, belongs to the 800-20 sample due to its very fine grains. When the rotational speed increased from 400 to 1200 rpm, the ultimate tensile strength (UTS) improved from 236.8 to 267.1 MPa, the total elongation (TE) decreased from 7.5% to 6.6%, and the energy absorption (EA) improved from 13.1 to 14.2 J/cm3. These results were due to the higher accumulated strain, the larger solid solution strengthening, the lower aspect ratio of beta-Mg17Al12, stronger {0002} texture, and the lower content of beta-Mg17Al12 of the 1200-400 sample. The UTS and TE of the 1200-40 sample showed an enhancement of 96.4% in UTS, 26.9% in TE, and 132.8% in EA as compared to the as-cast AZ91. Up to the true strain of 0.05, with increasing the rotational speed, the strain hardening rate increased owing to decreasing the fraction of grain boundaries. After the true strain of 0.05, the strain hardening rate reduced by increasing the rotational speed due to strengthening the {0002} texture. The fractured surface of the 800-20 sample had the largest number and smallest size of dimples as compared to the other FSP-processed samples owing to its lowest grain size. Finally, the strong {0002} basal texture in the 800-40, 1200-40, and 800-20 samples led to tear ridges aligning with the normal direction of fractured surfaces.