Effect of pinless friction stir processing on microstructure and properties of surface modification layer of 2024 aluminum alloy

Abstract Friction stir processing (FSP) is an advanced material surface modification technology that is both green and energy-efficient. This technology plays a crucial role in regulating the surface microstructure of alloys and improving alloys’ surface properties. It reaches this through the synergistic effect of non-equilibrium thermodynamic and surface mechanical deformation. In this work, the surface modification of an aluminum alloy was performed using pin-less FSP. Then, the modified surface was analyzed using stress–strain curves, optical microscopy (OM), x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical tests to investigate the impact of spindle velocity on the properties of the modified layer. Results of the study show that after undergoing pinless FSP modification treatment, the surface of the alloy appears bright and flat. The modified layer displays refined grains and numerous dispersed second-phase particles. Furthermore, the grains in the modified layer exhibit a gradient distribution from the surface to the matrix. Regarding the properties, compared to the base material (BM), the yield strength ( σ 0.2 ) and tensile strength ( σ b ) of the alloy-modified layer were increased by 34.8% and 29.4%, respectively. The maximum elongation ( δ ) of the modified coating reached 22.3%. The modified layer exhibits a tough-brittle mixed fracture pattern. Additionally, the modified layer’s corrosion resistance significantly improves. The performance of the modified coating shows the most significant improvement when the spindle speed reaches 1000 rpm.