Effects of post-processing and loading orientation on high-cycle fatigue of selective laser melted Ti-6Al-4V

Recent research on selective laser melted (SLM) Ti-6Al-4V showed that fatigue fracture differs depending on the post-processing and loading orientation. To better understand their influences, tensile and fatigue tests on samples with three post-processing conditions, namely, stress relief (SR), Sub-transus and hot-isostatic-pressing (HIP), were conducted in both the vertical and horizontal loading orientations. The elongation-to-failure of horizontal specimens are consistently higher than the vertical ones for the SR and Sub-transus state, but the opposite trend is observed for the HIP state. This discrepancy is owing to the reduced defect in combination with the {1120} alpha preferred orientation. Comparison of the SR and Sub-transus states suggests that coarser alpha-lath contributes to a longer initiation phase of microcrack from the defect, which improves overall fatigue performance. Fatigue strength of the HIP state is significantly higher than the other two states, exhibiting little anisotropy, and the fatigue-cracking source shifts from the defect to the microstructural facet. Subsequently, a model is proposed to evaluate the high-cycle fatigue of SLM-built Ti-6Al-4V, by incorporating yield strength and elongation-to-failure as additional variables, achieving the life prediction error band within two to five times with respect to the experimental data.