Fatigue behavior and modeling of additively manufactured IN718: The effect of surface treatments and surface measurement techniques

Abstract

• Effect of five surface treatments on surface topography of LB-PBF IN718 was studied. • Surface measurements using optical microscopy and X-ray CT (XCT) were compared. • Surface profiles were constructed from XCT images by curving-fitting edge detection. • The effect of surface roughness was more significant in high cycle fatigue regime. • A modified fatigue notch factor model correlated surface parameters with fatigue life. In this study, the effect of various surface treatments, including sand-blasting, drag-finishing, turning, grinding, and grinding + drag-finishing, on surface roughness and fatigue properties of laser beam powder bed fused Inconel 718 was examined. The surface roughness values obtained from two surface measurement techniques, i.e., optical microscopy and X-ray computed tomography, were compared. Both surface measurement techniques consistently indicated that all surface treatments led to improvements in surface roughness, although optical microscopy was influenced by surface glares and overestimated the surface roughness values of drag-finished specimens. Accordingly, all surface treatments also led to improvement in fatigue resistance with sand-blasting and drag-finishing yielding the least while grinding + drag-finishing causing the most. Notably, only the cracks of grinding + drag-finished specimens initiated from crystallographic facets while those in other conditions were surface initiated. Furthermore, by treating the surface valleys as micro notches, an effective fatigue notch factor model using a hybrid surface roughness metric that incorporates several standard surface roughness parameters was shown to correlate the fatigue lives of 94 % of specimens with various surface conditions within a scatter band of three.