This research investigates the effect of the formation of an oxygen-stabilised titanium alpha layer – called alpha-case at the surface – on the fatigue properties of Ti-6Al-4V (Ti64) alloy components produced by Laser Powder Bed Fusion (L-PBF). Three post processing heat treatments with different controlled atmospheres were carried out on samples with as-built surfaces to evaluate how differences in alpha-case layer thickness and hardness affect the material’s susceptibility to surface embrittlement and its overall fatigue performance. The investigation includes bulk and subsurface microstructural analysis, surface characterisation by X-ray computed tomography (XCT), and fatigue testing. Key findings show that alpha-case layers can reduce the fatigue resistance of L-PBF fabricated Ti64. The presence of a 70±3 µm thick alpha-case layer was found to promote crack initiation. This is emphasised by a higher density of initiated cracks, thus leading to a reduction in fatigue life. Conversely, thinner alpha-case layers were found to have a reduced impact on the fatigue performance, highlighting the critical role of post processing heat treatments in modulating the fatigue resistance of the material. The use of XCT to characterise the surfaces of the specimens in 3D confirms that fatigue cracks primarily initiate at surface notches, highlighting the predominance of as-built surfaces over microstructure in determining the fatigue resistance of L-PBF Ti64 components.

中文翻译：

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Alpha-case 促进激光粉末床熔融热处理 Ti-6Al-4V 表面疲劳裂纹的萌生


这项研究调查了氧稳定钛 α 层（称为表面 α 层）的形成对激光粉末床熔融 (L-PBF) 生产的 Ti-6Al-4V (Ti64) 合金部件疲劳性能的影响）。对具有竣工表面的样品进行了三种不同受控气氛的后处理热处理，以评估 α 壳层厚度和硬度的差异如何影响材料对表面脆化的敏感性及其整体疲劳性能。研究包括整体和次表面微观结构分析、X 射线计算机断层扫描 (XCT) 表面表征以及疲劳测试。主要发现表明，α 壳层可以降低 L-PBF 制造的 Ti64 的抗疲劳性。发现 70±3 µm 厚的 α 壳层的存在会促进裂纹萌生。更高密度的引发裂纹更加强调了这一点，从而导致疲劳寿命缩短。相反，较薄的 α 壳层对疲劳性能的影响较小，凸显了后处理热处理在调节材料抗疲劳性方面的关键作用。使用 XCT 对样品表面进行 3D 表征，证实疲劳裂纹主要始于表面凹口，这凸显了在确定 L-PBF Ti64 部件的抗疲劳性时，竣工表面相对于微观结构的优势。