Effects of the stress concentration factor and residual stress on the improvement in the fatigue properties of powder-bed-fused Ti6Al4V via cavitation abrasive surface finishing

Abstract

Additively manufactured metals, such as powder-bed-fused titanium alloy (PBF/Ti6Al4V), are promising materials; however, their fatigue strength is only half that of wrought metals, which limits their practical applications. In the as-built condition, their low fatigue strength is caused by the high surface roughness due to the partially melted particles remaining on the material surface after PBF, surface defects and internal or near-surface defects. In this study, PBF/Ti6Al4V was treated by cavitation abrasive surface finishing (CASF) post-processing to improve the fatigue strength. Results were compared with those of grinding. During CASF, abrasive impacts removed the surface defects and promoted smoothing, and cavitation impacts introduced compressive residual stress. The fatigue properties were evaluated using a plane bending fatigue test. The fatigue strength of the as-built PBF/Ti6Al4V at 10⁷ cycles was improved via CASF by up to 65 %. To quantitatively determine the effects of surface smoothing on the improvement in the fatigue properties via post-processing, the effective stress concentration factor was estimated using the surface roughness and profile valley radii. Additionally, the residual stress was determined using X-ray diffraction. The results revealed that the stress concentration factor and compressive residual stress are the key parameters to improve the fatigue strength of as-built PBF/Ti6Al4V.