Alpha-case promotes fatigue cracks initiation from the surface in heat treated Ti-6Al-4V fabricated by Laser Powder Bed Fusion

IF 5.7 2区材料科学 Q1 ENGINEERING, MECHANICAL

International Journal of Fatigue Pub Date : 2024-09-26 DOI:10.1016/j.ijfatigue.2024.108621

Quentin Gaillard , Florian Steinhilber , Amélie Larguier , Xavier Boulnat , Jean-Yves Buffiere , Guilhem Martin , Sylvain Dancette , Sophie Cazottes , Rémy Dendievel , Christophe Desrayaud

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Abstract

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

\pm

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.

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通过激光粉末床熔融技术制造的热处理 Ti-6Al-4V 中，α-壳促使疲劳裂纹从表面萌生

本研究调查了氧稳定钛α层（表面称为α-壳）的形成对通过激光粉末床融合（L-PBF）生产的 Ti-6Al-4V (Ti64) 合金部件疲劳性能的影响。我们对带有坯料表面的样品进行了三种不同控制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的表面脆化敏感性及其整体疲劳性能。调查包括块体和次表层微结构分析、X 射线计算机断层扫描（XCT）表面表征和疲劳测试。主要研究结果表明，α-壳层会降低 L-PBF 制成的 Ti64 的抗疲劳性能。研究发现，70±3 µm 厚的α-外壳层会促进裂纹的产生。这突出表现在萌生裂纹的密度更高，从而导致疲劳寿命缩短。相反，较薄的α-表壳层对疲劳性能的影响较小，这凸显了后处理热处理在调节材料抗疲劳性能方面的关键作用。使用 XCT 对试样表面进行三维表征证实，疲劳裂纹主要从表面凹口处开始，这突出表明在决定 L-PBF Ti64 部件的抗疲劳性时，竣工表面比微观结构更重要。

本文章由计算机程序翻译，如有差异，请以英文原文为准。

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来源期刊

International Journal of Fatigue 工程技术-材料科学：综合

CiteScore

10.70

自引率

21.70%

发文量

619

审稿时长

58 days

期刊介绍： Typical subjects discussed in International Journal of Fatigue address: Novel fatigue testing and characterization methods (new kinds of fatigue tests, critical evaluation of existing methods, in situ measurement of fatigue degradation, non-contact field measurements) Multiaxial fatigue and complex loading effects of materials and structures, exploring state-of-the-art concepts in degradation under cyclic loading Fatigue in the very high cycle regime, including failure mode transitions from surface to subsurface, effects of surface treatment, processing, and loading conditions Modeling (including degradation processes and related driving forces, multiscale/multi-resolution methods, computational hierarchical and concurrent methods for coupled component and material responses, novel methods for notch root analysis, fracture mechanics, damage mechanics, crack growth kinetics, life prediction and durability, and prediction of stochastic fatigue behavior reflecting microstructure and service conditions) Models for early stages of fatigue crack formation and growth that explicitly consider microstructure and relevant materials science aspects Understanding the influence or manufacturing and processing route on fatigue degradation, and embedding this understanding in more predictive schemes for mitigation and design against fatigue Prognosis and damage state awareness (including sensors, monitoring, methodology, interactive control, accelerated methods, data interpretation) Applications of technologies associated with fatigue and their implications for structural integrity and reliability. This includes issues related to design, operation and maintenance, i.e., life cycle engineering Smart materials and structures that can sense and mitigate fatigue degradation Fatigue of devices and structures at small scales, including effects of process route and surfaces/interfaces.