Fatigue crack growth in L-PBF Ti-6Al-4V: Influence of notch orientation, stress ratio, and volumetric defects

Abstract

This study investigates the fatigue crack growth (FCG) behavior of laser powder bed fused (L-PBF) Ti-6Al-4V parts with an emphasis on the effects of notch orientation, stress ratio, R, and process-induced volumetric defects. Process parameters were altered during fabrication to induce different defect types and populations. FCG tests were conducted using compact tension specimens at stress ratios of R = 0.1, 0.4, and 0.7 and in orientations of vertical, horizontal, and diagonal. Detailed fractography as well as post-mortem microstructure characterization were performed. R was found to significantly influence both the threshold and stable fatigue crack growth behavior. An increase in FCG rate with increasing R was observed due to lower degrees of plasticity induced and roughness induced crack closure. Specimens with a vertical notch orientation exhibited the highest threshold stress intensity factor range at all R values due to higher levels of crack closure resulting from greater crack surface roughness caused by grain orientation. Crack path tortuosity for different orientations was observed which was driven by the crystallographic orientation of prior β grains and α-laths. Interestingly, specimens containing more defects had slightly higher ΔK_th however, the stable crack growth behavior was unaffected.