基于同步辐射X射线成像的选区激光熔化Ti-6Al-4V合金缺陷致疲劳行为

Defect Induced Fatigue Behaviors of Selective Laser Melted Ti-6Al-4V via Synchrotron Radiation X-Ray Tomography

基于自主研制的原位疲劳试验机和高分辨同步辐射X射线三维成像技术,采用Feret直径和极值统计方法定量表征选区激光熔化Ti-6Al-4V合金的缺陷特征尺寸、数量、位置及形貌,原位观测疲劳裂纹的萌生与扩展行为,通过辨识疲劳断口源区的缺陷特征,开展缺陷诱导的疲劳损伤评价研究,从而建立缺陷特征与疲劳寿命之间的关系。分析表明,缺陷主要为未熔合和气孔,等效直径小于50μm的频率为90%,球度分布于0.4~0.65之间;在不考虑表面粗糙度的情况下,疲劳裂纹优先在试样表面或近表面缺陷处萌生,呈现出典型的半椭圆形貌;同时缺陷特征尺寸越大,疲劳寿命越低。研究结果为增材高性能部件的疲劳性能及寿命评估提供了重要的理论参考。

As a very promising additive manufacturing(AM) technique, selective laser melting(SLM)has gained considerable attentions due to the feasibility of producing light-weight metallic components directly from virtual design data. On the other hand, high strength, low density and high corrosion resistance Ti-6 Al-4 V alloy has been a preferred AM used material for the aviation and military industries. However, the fatigue damage behaviors of SLMed or AMed components usually suffer from interior defects such as incomplete fusion and gas pores due to unstable process or unsuitable processing parameters.Therefore, thorough investigations on process-induced and metallurgical defects and its influence on the fatigue behavior is required for robust designs and engineering applications of high performance SLM components. As an advanced characterization approach, synchrotron radiation micro computed X-ray tomography(SR-μCT) has been recently to investigate the fatigue damage behaviors of critical components with defects. Based on self-developed in situ fatigue testing rig fully compatible with the BL13 W1 at Shanghai Synchrotron Radiation Facility(SSRF), several AMed specimens were prepared for in situ fatigue SR-μCT. The Feret diameter and extreme values statistics were then adopted to characterize the defect size, morphology, population, location and the influence on fatigue life. Fatigue fractography was also examined to further identify the defect to really initiate a fatigue crack. Results show that two types of defects including gas pores and the lack of fusion can be clearly distinguished inside SLM Ti-6 Al-4 V alloys. Fatigue crack with a typical semi-ellipse usually initiates from the defects at the surface and near the surface. Besides, the defects less than 50 μm and sphericity of 0.4~0.65 dominate for the SLM Ti-6 Al-4 V alloys. It is also found that the larger the characteristic size of the defect, the lower the fatigue life. Current results can provide a theoretical basis and support to predict the fatigue performance of SLM Ti-6 Al-4 V alloys. Further investigations should be performed on the relationship between the critical defect and fatigue strength by introducing the Kitagawa-Takahashi diagram.