The effect of shot peening time on mechanical properties and residual stress in Ti-6Al-4V alloy

Shot peening is a method that increases the surface compressive residual stress, controls the growth of surface micro-cracks, and improves surface properties such as increasing hardness of surface and wear resistance by fabricating a compact layer of nanostructure on the surface. In this study, the effect of shot peening process on compressive residual stress, microstructure, hardness of surface, wear, and surface roughness of Ti-6Al-4V alloy was investigated. Accordingly, specimens of Ti-6Al-4V alloy were shot peened with steel pellets with a diameter of 0.4 mm. Steel pellets were impinged on the surface of Ti-6Al-4V alloy with the nozzles air pressure of 1bar and the impinging angle of 90°. Then the samples were subjected to shot peening for 20, 40, and 60 min. Subsequently, the grain size, micro-strain and compressive residual stress of the surface were investigated by using X-ray diffraction (XRD) analysis. Also, microstructures formed on the surface were investigated with Scanning electron microscopy (SEM). In addition, mechanical properties of the surfaces were investigated by performing hardness and wear tests. The results showed that the grains were refined from the size of 150 nm in the raw specimen to 29.2, 28.5 and 28.3 nm over the shot peening times of 20, 40, and 60 min. Also, the shot peening operation led to the increase of 55%, 57%, and 63% hardness of surface, the increase of 32%, 37%, and 43% of surface wear resistance, and the increasing of surface roughness in comparison with raw specimen over the shot peening times of 20, 40, and 60 min. The reasons for improving the surface layer properties of titanium alloy include refining and nano-crystallization of the grains and creation of nanostructure on surface layer. On the other hand, as a compact layer of nanostructure is formed on the surface via shot peening, the amount of residual stress on the surface increased from 938 MPa during 20 min to 1232 MPa during 60 min of shot peening operation. Also, based on the surface wear investigation of titanium alloy the wear mechanisms included abrasive, adhesive, and tribo-chemical which decreased by increasing the hardness of surface in the shot peening process.