On the severe shot peening effect to generate nanocrystalline surface towards enhancing fatigue life of injection-moulded Ti-6Al-4V alloy

Severe shot peening (SSP) is one of the nanocrystalisation surface treatments, which has received much interest because of its capability to enhance cyclic loading. Metal injection moulding (MIM) is a cost-effective mass-production process that can fabricate complex net-shape Ti-6Al-4 V alloy with excellent physical and mechanical properties. However, the MIM parts usually have surface characteristic and microstructure that are detrimental to fatigue properties, which are notches at the surface and large lamellar grains generated by pressureless sintering. In the present study, the SSP treatment was applied to the Ti-6Al-4 V alloy manufactured by MIM. After SSP treatment, notches at the surface were eliminated with a decrease in arithmetical mean roughness (~ 50%). The near-surface hardness significantly increased (~ 40%) corresponding with the generation of compressive residual stress (800 MPa). In addition, the fatigue strength also improved (~ 14%) due to the surface quality improvement, the formation of compressive residual stress and surface nanocrystalline with the average grain size of 92.5 nm, which simultaneously changed the crack origination site at the surface to sub-surface and delayed the crack propagation rate when compared to those without SSP treatment. Graphical abstract