Effects of pretreatment and HVOF sprayed cermet coating on fatigue properties of TC21 titanium alloy

The effects of grit blasting (GB), shot peening (SP) pretreatment and high velocity oxygen fuel(HVOF) sprayed WC-17Co cermet coating on the fatigue properties of the novel ultra-high strength TC21 titanium alloy were investigated with a rotating bending fatigue test machine. The basic properties and surface integrity of the coating were investigated by using X-ray diffraction (XRD), surface roughness meter, microscopic hardness tester, scanning electron microscopy (SEM) and X-ray stress test instruments. The results showed that the residual compressive stress could be introduced into the surface of TC21 alloy by GB and SP pretreatment, and that HVOF sprayed WC-17Co coating was compactly bonded with TC21 alloy substrate and it significantly improved the surface hardness of the substrate. However, there was a certain residual tensile stress in the sublayer of the coating. SP could significantly increase fatigue resistance of TC21 alloy due to the surface residual compressive stress. There was no significant effect on fatigue resistance by GB treatment due to the offset between the influence of surface residual compressive stress and the surface notch effect. The fatigue resistance of TC21 alloy was significantly reduced by HVOF WC-17Co coating prepared with GB pretreatment. This could be attributed to the relaxation of surface residual compressive stress during the HVOF heating process, pore defects and residual tensile stress distribution in the WC-17Co coating with low toughness, and the surface notch induced by GB. The fatigue resistance of TC21 alloy was slightly reduced by HVOF WC-17Co coating prepared with SP pretreatment. This was attributed to the offset between the advantage effect by SP and disadvantageous effect of WC-17Co coating and high temperature factor during the HVOF process.