Fatigue performance of laser powder bed fusion hydride-dehydride Ti-6Al-4V powder
Fatigue performance of laser powder bed fusion hydride-dehydride Ti-6Al-4V powder
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DOI：
10.1016/j.addma.2022.103117
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发表时间：
2022-08
影响因子：
11
通讯作者：
Mohammadreza Asherloo;Ziheng Wu;M. Heim;Dave Nelson;M. Paliwal;A. Rollett;A. Mostafaei
中科院分区：
文献类型：
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作者：
Mohammadreza Asherloo;Ziheng Wu;M. Heim;Dave Nelson;M. Paliwal;A. Rollett;A. Mostafaei
采用优化的工艺参数对粒度分布为50-120µm的氢化物-脱氢钛铝4V合金进行了激光粉床熔化处理，获得了密度为99.9%的近全致密组织。显微组织观察和物相分析表明，在铸态下形成了柱状β相和针状α/α‘相。制得的样品表面粗糙度显著，平均粗糙度Ofra=115.71±103.96µm，均方根粗糙度Rrms=108.4±24.9µm，而机械研磨后，这两个值分别减小了TORA=90.19±100.04µm和Rrms=14.9±100.6µm。力学测试是在制造的试件上进行的，然后进行应力消除处理。所有试件均在R=−1的全反拉压条件下进行疲劳失效试验。预制试件表面裂纹萌生主要发生在微观缺口处，而机械磨削后裂纹萌生转变为气孔等亚表面缺陷。通过机械磨削将表面粗糙度降至最低，消除了表面微缺口，从而提高了高周疲劳区域的疲劳强度。疲劳缺口因子计算表明，与标准球形粉末相比，使用HDH粉末时表面粗糙度的影响明显降低。X射线衍射分析表明，机械研磨样品表面存在面内压应力、微观应变和晶粒细化现象。断口观察(宏观尺度)显示，在裂纹扩展的第一阶段为完全脆性断裂，在裂纹扩展的第三阶段转变为以韧性为主的断裂。另一方面，在微观尺度上，即使是脆性断裂区也显示出α马氏体板条内的延性断裂。
Hydride-dehydride (HDH) Ti-6Al-4V alloy with particle size distribution of 50–120 µm is laser powder bed fusion (L-PBF) processed using optimum processing parameters and a near-fully dense structure with a density of 99.9 % is achieved. Microstructural observations and phase analyses indicate formation of columnar β grains with acicular α/α′ phases in as-built condition. The roughness of the as-fabricated samples is significant with an average roughness ofRa= 15.71 ± 3.96 µm and a root mean square roughness ofRrms= 108.4 ± 24.9 µm, however, both values are reduced toRa= 0.19 ± 0.04 µm andRrms= 4.9 ± 0.6 µm after mechanical grinding. Mechanical tests are carried out on as-fabricated specimens followed by stress relief treatment. All samples are tested to failure in fatigue, under fully-reversed tension-compression conditions of R = −1. The as-built samples failed from the surface with crack initiation mainly at micro-notches, whereas after mechanically grinding, crack initiation changed to subsurface defects such as pores. Minimizing surface roughness by mechanically grinding eliminates surface micro-notches which improves fatigue strength in the high cycle fatigue region. Fatigue notch factor calculations showed that the effect of surface roughness was significantly lower when HDH powder is used compared to standard spherical powder. X-ray diffraction analysis revealed an in-plane compressive stress, micro-strain and grain refinement on the surface of the mechanically ground samples. Fractography observations (macroscale) revealed a fully brittle fracture in the first stage of crack growth with a transition to a dominantly ductile fracture in the third stage of crack growth. On the other hand, at the micro scale, even the brittle fracture regions showed evidence of ductile fracture within the α′ martensite laths.