Effects of surface integrity of shot peening, grit blasting and HOVF sprayed WC-17Co coating on fatigue properties of TC18 titanium alloy

In order to investigate the effect of surface integrity of shot peening(SP), grit blasting(GB) and HVOF sprayed WC-17Co coating on the fatigue behavior of Ti5Al5Mo5V1Cr1Fe(TC18)alloy, the surface integrity of the surface treatment was evaluated by using X-ray diffraction (XRD), surface roughness meter, microscopic hardness tester, scanning electron microscopy (SEM) and X-ray stress test instrument. The fatigue property of TC18alloy influenced by above surface treatment was studied by using rotating bending fatigue test machine. The result indicates that the surface integrity after SP treatment is better than that after GB process. Therefore, the fatigue life of TC18titanium alloy increases dramatically by SP, but there is no significant change of fatigue life by GB treated TC18alloy. The fatigue life of HVOF WC-17Co coated TC18alloy with SP pretreatment is increased six times larger than that of the alloy substrate. This is attributed to the retained large residual compressive stress in the SP layer. The crack initiation and earlier propagation are delayed by the residual compressive stress. However, the fatigue life of HVOF WC-17Co coated TC18alloy with GB pretreatment is obviously decreased. This is due to the low toughness, large surface roughness, and pore defects of WC-17Co coating. The relaxation of residual compressive stress influenced by the HVOF high temperature process and the notch effect in GB layer are also the reasons.