Effect of Multi-pass Friction Stir Processing on Microstructures and Mechanical Behaviors of As-Cast 2A14 Aluminum Alloy

Multi-pass friction stir processing with 100% overlap was applied to modify the as-cast 2A14 aluminum alloy plate through one to three passes. The effects of the number of FSP passes on microstructural evolution and mechanical behaviors of the stir zone (SZ) were comprehensively evaluated. The results indicated that obvious refined equiaxed grains with predominant high angle grain boundaries were formed in the SZ due to dynamic recrystallization (DRX). The grain size and tunnel defects decreased, while the second-phase particles are refined and distributed evenly as the number of passes increased. Moreover, the shear texture components of C and A1* are produced in the SZ through the DRX. As compared to as-cast alloy, the texture intensity of SZ is significantly reduced due to the formation of recrystallized grains with random orientation during FSP. Additionally, the effective improvement of strength and ductility was achieved in the two passes FSP (2P FSP) sample because of the grain refinement, fragmented particles and the elimination of casting defects. The ultimate tensile strength and elongation of 2P FSP sample reach up to 391.6 MPa and 10.8%, which are 120.9, 232.1% higher than that of the BM (177.23 MPa and 3.24%), respectively. Meanwhile, the hardness test results evidenced that 2P FSP not only enhances the microhardness, but also increases the width of SZ.