A comparative analysis of residual stresses from friction stir processing of aluminum cast 380 and wrought 7075 alloy sheets: Experimental characterization and modeling

Residual stresses are often overlooked in friction stir processing (FSP), but their significant impact on performance necessitates their consideration in optimizing processing parameters. The first step in this effort is to understand the effect of process conditions on residual stress distributions, especially in the context of different alloys. This study uses hole-drilling electronic speckle pattern interferometry (ESPI) and the thermal pseudo-mechanical (TPM) model within finite element analysis to study the 3D residual stress distributions in friction stir processed thin aluminum sheets of two alloys under various conditions. Using this approach, three new aspects of residual stresses from FSP were experimentally revealed, and simulations explained the underlying mechanisms. First, notable stress variation through the thickness was observed. In the A7075 alloy, stress gradually reduced from the tool shoulder side to the bottom surface, while in the AA380 alloy, stresses sharply increased near the surfaces. This is because the cast A380 alloy's surface material is significantly harder than its interior, i.e., due to the “skin effect.”. This study also clarifies the opposite effects of process temperature on residual stress: increased temperature reduced stress in AA380 but increased it in AA7075. This is because AA7075, with its higher yield stress, undergoes plastic deformation only during heating, unlike AA380, which deforms during both the heating and cooling phases. Lastly, residual stresses in AA380 increased with an additional pass due to material hardening from the initial pass. All these findings highlight the crucial role of yield stress and its variation with alloy type, location within the material, and the number of passes in determining residual stresses.