On the origin of microstructural banding in Ti-6Al4V wire-arc based high deposition rate additive manufacturing
On the origin of microstructural banding in Ti-6Al4V wire-arc based high deposition rate additive manufacturing
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DOI：
10.1016/j.actamat.2018.12.038
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发表时间：
2019-03-01
期刊：
影响因子：
9.4
通讯作者：
Prangnell, Philip B.
中科院分区：
文献类型：
--
作者：
Ho, Alistair;Zhao, Hao;Prangnell, Philip B.
定向能高沉积速率增材制造工艺涉及比粉末床技术更大的熔池直径（类似于5-10 mm）和层高度（1-2 mm），这通常导致更大的微观结构不均匀性和更严重的热影响区（HAZ）条带。虽然HAZ条带在AM中已被广泛报道，但在这项研究中，线弧增材制造（WAAM）生产的样品中看到的条带特征比以前更严格地量化，使用统计可靠的成分和目的开发的微观结构分析绘图工具，这为它们的性质和形成机制提供了新的见解。除了HAZ条带，在每个熔化轨迹的熔合边界处也发现了偏析层。这是第一次在AM沉积物中看到的瞬时偏析层和弱成核，可归因于Fe在钛中的分配系数较低，以及有限的V和Al偏析。详细的微观结构演变发生在热影响区带已被重新审视，新的证据的基础上，并涉及黑暗和白色蚀刻带。较低温度的暗蚀刻区域不仅是由于粗化导致的α片层间距的增加，而且还由于温度升高导致的更大的化学分配。此外，它是由热模拟表明，薄的白色带发生在重新加热到略低于β transsus温度，这是向上移动，由于在AM的高加热速率。该白色条带与精细α层状菌落形态的形态变化相关，其表现出较少的溶质分配。讨论了相关机制。在β接近曲线的范围内发生的快速粗化归因于从β再生长的界面迁移，而不是传统的表面张力驱动效应，而提出的精细菌落微观结构是由菌落成核引起的，在随后的冷却中，在低体积分数的残余α上。(C)2019作者由Elsevier Ltd代表Acta Materialia Inc.发布这是一个在CC BY许可证下的开放获取文章（http：//creativecommons.org/licenses/by/4.0/）。
Directed energy high deposition-rate additive manufacturing processes involve a larger melt pool diameter (similar to 5-10 mm) and layer height (1-2 mm) than powder bed technologies, which generally leads to greater microstructural heterogeneity and more severe Heat Affected Zone (HAZ) banding. While HAZ banding has been widely reported in AM, in this study the banding features seen in samples produced by Wire-Arc Additive Manufacturing (WAAM) have been more rigorously quantified than previously possible, using statistically reliable compositional and, purpose developed, microstructure analysis mapping tools, which has provided new insight into their nature and mechanisms of formation. In addition to HAZ banding, a segregation layer has also been discovered at the fusion boundary from each melt track. This transient segregation layer and the weak coring seen, for the first time in the AM deposits, can be attributed to the lower partition coefficient of Fe in titanium, as well as limited V and Al segregation. The detailed microstructure evolution occurring in the HAZ bands has been revisited, based on new evidence, and is shown to involve both dark and white etching bands. The lower temperature dark etching region is caused not just by an increase in the alpha lamellar spacing due to coarsening, but also by greater chemical partitioning with temperature rise. In addition, it is shown by thermal simulation that the thin white band occurs on re-heating to just below the beta transus temperature, which is shifted upwards owing to the high heating rate in AM. This white band is associated with a morphological change to a fine alpha lamellar colony morphology, which exhibits less solute partitioning. The mechanisms involved are discussed. The rapid coarsening that occurs in the range of the beta approach curve is attributed to interface migration from beta re-growth, rather than conventional surface tension driven effects, whereas the fine colony microstructure is proposed to be caused by colony nucleation, in subsequent cooling, on a low volume fraction of residual alpha. (C) 2019 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).