Effect of defects on the constant-amplitude fatigue behavior of as-built Ti-6Al-4V alloy produced by laser powder bed fusion process: Assessing performance with metallographic analysis and micromechanical simulations

Abstract

The influence of various process parameters on the mechanical behavior (including tensile strength, strain-to-failure and constant-amplitude fatigue performance) of as-built Ti-6Al-4V alloy, produced via laser powder bed fusion (L-PBF) process, is demonstrated using complementary information obtained from mechanical (uniaxial tensile and fatigue) tests, fractography analysis, metallographic analysis and micromechanical simulations. Four different build conditions were considered such that the laser power and velocity were varied for each build while holding the build orientation, hatch spacing, layer thickness, laser spot size and hatch rotation constant. The volumetric energy densities (E) of the four unique builds were: 33 J/mm3, 66 J/mm3, 130 J/mm3 and 180 J/mm3, respectively. Two coupons per build were subjected to constant-amplitude fatigue loading while one coupon per build was subject to uniaxial tensile loading at room temperature. Coupons belonging to Builds 1 and 2 (corresponding to E values of 33 J/mm3 and 66 J/mm3, respectively) had considerably longer fatigue lives compared to coupons from Builds 3 and 4 (corresponding to E values of 130 J/mm3 and 180 J/mm3, respectively).To reconcile the relative lifing capability of the four builds, fractography, two-dimensional (2D) pore analysis, and multiscale finite-element simulation campaigns are detailed. Collectively, these exercises underscore the centrality of pore clustering on the fatigue performance of the builds. Moreover, they highlight the role that microstructural character (viz. grain orientation with respect to loading) plays on the accumulation of plastic strain in the vicinity of pores. Specifically, it is demonstrated that a pore embedded in a grain favorably oriented for slip accumulates a significant amount of plastic strain compared to one embedded in a grain not favorably oriented for slip. Comparing the relative performance and defect character of the four builds, this study suggests that both pore spacing and relative grain hardness act in concert with other fatigue-limiting characteristics including pore size, shape and distance to free surface.