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.

中文翻译：

---

激光选择性熔化Ti-6Al-4V微细结构及其对力学性能的影响

摘要 研究了使用选择性激光熔化 (SLM) 技术增材制造的 Ti-6Al-4V (Ti64) 合金的拉伸性能、I 型断裂韧性 (KIc)、疲劳裂纹扩展行为和无缺口疲劳强度。层厚度 (t) 的四种不同组合 - 连续层 (ϕ) 之间的扫描旋转，这导致细观结构的范围从具有方形横截面的全厚度柱状先验 β 晶粒，其边长等于扫描间距，到近探索了构建和横向上的等轴细观结构。通过对加载轴平行或垂直于构建方向的样品进行测试，研究了机械性能的可能各向异性。在所有情况下，微观结构由精细的 α/α' 板条结构组成，其中 α' 是亚稳马氏体 Ti 相，呈针状，位于先前的 β 晶粒内。实验结果表明，t = 60 μm 和 ϕ = 67° 的工艺参数组合导致合金同时表现出高屈服强度 (>1100 MPa) 和延展性 (>12%)、58 MPa m 的 KIc 和无缺口疲劳强度，这与相同的合金相似，但使用常规技术制造。总体而言，发现性能的各向异性并不显着，即使在先前 β 晶粒在构建方向上发生柱状生长的情况下也是如此。稳态疲劳裂纹扩展 (FCG) 的巴黎指数值远低于传统制造的 Ti64 所报告的值，表明 SLM Ti64 具有更高的 FCG 抗性。对控制近阈值 FCG 率的有效微观结构长度尺度的分析表明，主导这种行为的是菌落大小。总的来说，这项研究的结果表明了工艺参数优化的方向，这将导致 SLM Ti64 不仅具有高强度，而且具有耐损伤性。