Effects of build direction and microstructure on fatigue crack growth behavior of Ti6Al4V fabricated by laser powder bed fusion

This study compares the fatigue crack growth (FCG) behavior of Ti6Al4V parts fabricated by laser powder bed melting (LPBF) with four build directions (0 degrees, 30 degrees, 60 degrees, and 90 degrees) and two heat treatments and analyzed the effect of build direction and microstructure on FCG. The results show that the build direction has a significant effect on the FCG rate. Particularly, specimens with 90 degrees build direction have the best FCG resistance while the 0 degrees specimens have the worst. Furthermore, the fragile deposition layer boundary is the main cause of the crack deflection. The microstructure shows that the columnar beta grains have less effect on FCG. The increase in resistance to FCG after heat treatment is attributed to the lath-like alpha grains consuming the energy for FCG. As compared to needle-like alpha ' grains, the lath-like alpha grains are more likely to initiate intergranular fracture and secondary cracking, leading to rugged crack tip paths. The effect of build direction on FCG of LPBF-Ti6Al4V part is significant. The heat-treated lath-like alpha grains have better resistance to FCG. Cracks tend to expand in the direction of parallel deposition layers. Secondary cracking is related to build direction and microstructure.