Micro-and meso-structures and their influence on mechanical properties of selectively laser melted Ti-6Al-4V

Abstract

The tensile properties, mode I fracture toughness (KIc), fatigue crack growth behavior, and unnotched fatigue strength of additively manufactured Ti-6Al-4V (Ti64) alloy using selective laser melting (SLM) technique were investigated. Four different combinations of layer thickness (t) - scan rotation between successive layers (ϕ), which resulted in mesostructures that range from through-thickness columnar prior β grains with square cross-sections, whose side lengths equal to the scan spacing, to near-equiaxed mesostructures in both build and transverse directions, were explored. Possible anisotropy in mechanical properties was investigated by conducting tests on samples whose loading axis is either parallel or perpendicular to the build directions. In all cases, the microstructure consisted of fine α/α′ lath structure, where α′ is the metastable martensitic Ti phase that is acicular in shape, within the prior β grains. Experimental results show that the process parameter combinations of t = 60 μm and ϕ = 67° results in an alloy that exhibits high yield strength (>1100 MPa) and ductility (>12%) simultaneously, KIc of 58 MPa m , and unnotched fatigue strength, which is similar to that of the same alloy but manufactured using conventional techniques. The anisotropy in properties, overall, was found to be not substantial, even in the case where columnar growth of prior β grains occurs in the build direction. The values of the Paris exponents for steady state fatigue crack growth (FCG) are much lower than those reported for conventionally manufactured Ti64, suggesting higher FCG resistance in SLM Ti64. Analysis of the effective microstructural length scale that controls the near-threshold FCG rate suggests that it is the colony size that dominates this behavior. Overall, the results of this study indicate directions for process parameter optimization that would lead to SLM Ti64 that is not only has high strength, but also is damage tolerant.