Surface Integrity Study On Shot Peened Ti-6Al-4V Titanium Alloy

Due to the high specific intensity and excellent medium temperature stability, Ti-6A1-4V alloy has been the first kind of titanium alloys used to make compressor blades and turbine disks of aircraft engines. However, the poor anti-fatigue properties limited its future application in the aerospace industry. Shot peening technique is one of effective cold working processes widely used to improve the fatigue life and resistance to stress corrosion of metallic components. In this paper, dry shot peening using cast steel shot and wet shot peening using ceramic beads were used to strength the surface of Ti-6Al-4V alloy. The surface integrity and fatigue properties of shot peened Ti-6A1-4V were investigated by optical microscopy, scanning electron microscopy, X-Ray diffraction and fatigue testing machine etc.Ti-6A1-4V alloy was treated by dry shot peening using cast steel beads. The results showed that highly compressive residual stress and work hardening states on the surface layer were induced. With improving the shot peening intensity, surface micro-hardness, surface residual stress, maximum compressive residual stress and the depth of compressive residual stress layer increased. The maximum compressive residual stress located about subsurface 30 μm. The depth of work hardening layer is substantially consistent with the depth of the residual stress field; surface roughness increased greatly with the shot peening intensity. Surface structure changed from equiaxed structure to fibrous structure. Under 0.3 mmA shot peening intensity, the depth of the fibrous structure was about 100 μm.Ti-6A1-4V alloy was also treated by wet shot peening using ceramic beads and water. The effect of wet shot-peening intensity and peening coverage on surface integrity and fatigue property of Ti-6A1-4V alloy were investigated. The results demonstrated that wet shot peening effectively induced highly compressive residual stress and work hardening states on the surface layer. With improving the shot peening intensity, maximum compressive residual stress and the depth of compressive residual stress layer also increased. The maximum compressive residual stress located subsurface 25 μm. By wet shot peening of 0.1-0.2 mmN intensity, the surface machining scratches of specimen disappeared and Ra is lower than base metal. With high wet shot peening intensity, Ra increased obviously Because of the cyclic plastic deformation, surface hardness and the dislocation density were enhanced. Meanwhile the microstructure was refined and the diffraction peaks broadened in the surface layer after wet shot peening. Increasing peening coverage can reduce the surface roughness, but with too high peening coverage, the depth of compressive residual stresses reduced and obvious softening occurred. Fatigue experiment results show that wet shot peening can improve the fatigue life of Ti-6A1-4V alloy. But there is no monotonous relation between the increase of fatigue life and shot peening parameters. The optimized shot peening intensity is 0.3 mmN and the optimized peening coverage is 400%. The cracks of wet shot peened Ti-6A1-4V specimens occurred on the subsurface while the cracks of un-peened specimens occurred on the surface. Wet shot peening mainly affects crack initiation and the first stage of crack propagation.