[论文解读] Micro-cracking, microstructure and mechanical properties of Hastelloy-X alloy printed by laser powder bed fusion: as-built, annealed and hot-isostatic pressed

Hui Wang, Liu Chen+8 more|arXiv (Cornell University)|Nov 22, 2020

Additive Manufacturing Materials and Processes参考文献 41被引用 9

一句话总结

本研究调查了激光粉末床熔融成形的Haystelloy-X在原始成形、退火及热等静压（HIP）状态下的微裂纹、显微组织演变及力学性能。通过热力学模拟与热计算，结果表明HIP工艺可有效消除微裂纹与孔隙，通过再结晶与析出强化显著提升延展性（从13%提高至20%）和极限抗拉强度（从965 MPa提高至1045 MPa）。

ABSTRACT

This study analyses literature data to identify optimised print parameters and assesses the consolidation, microstructure, and mechanical properties of Hastelloy-X printed by laser powder bed fusion. Effects of post annealing and hot-isostatic pressing (HIP) on the microstructure and mechanical properties are also revealed. The susceptibility to the solidification cracking and the as-built microstructure such as precipitation and chemical segregation were predicted by the calculation of thermodynamics phase diagrams. The distribution of solidification cracks throughout the builds was quantified for the as-built, annealed and HIP conditions. The assessment reveals the variation of crack density towards the bottom, top and free surface of solid builds. This distribution of cracks is found to be associate with the thermal gradient and thermal conductivity which were estimated by analytical thermal calculations. While the annealing and HIP both can alter the as-printed microstructure thanks to recovery and recrystallisation, the micro-cracks and pores were only successfully removed by the HIP. In addition to the removal, recrystallisation and precipitation in the HIP (stronger than in annealing), resulting in optimal mechanical properties including a substantial increase in elongation from 13% to 20%, significant improvement of ultimate tensile stress from 965 MPa to 1045 MPa with moderately high yield stress thanks to precipitation.

研究动机与目标

• 通过文献调研与热力学分析，确定Hastelloy-X激光粉末床熔融的最佳工艺参数。
• 评估后处理退火与热等静压（HIP）对显微组织与力学性能的影响。
• 量化不同成形区域中原始成形、退火及HIP处理后部件的裂纹分布。
• 利用解析热模拟，将裂纹形成与热梯度及热导率相关联。
• 确定HIP工艺在消除缺陷与提升力学性能方面的有效性。

提出的方法

• 采用热力学相图计算预测凝固裂纹敏感性及显微组织特征（如析出相与成分偏析）。
• 通过解析热模拟估算热梯度与热导率，以解释成形件中裂纹分布模式。
• 利用电子显微镜与X射线衍射进行显微组织表征，评估相分布与缺陷密度。
• 力学性能测试包括拉伸试验，以评估各处理状态下屈服强度、极限抗拉强度与伸长率。
• 对试样施加后处理工艺（退火与HIP），以对比缺陷演化与显微组织恢复情况。
• 量化打印件底部、顶部及自由表面区域的裂纹密度，以评估空间分布差异。

实验结果

研究问题

• RQ1 激光成形的Hastelloy-X构件不同区域（底部、顶部、自由表面）的凝固裂纹分布如何变化？
• RQ2 热梯度与热导率在多大程度上影响原始成形件中的微裂纹形成？
• RQ3 后处理退火或热等静压（HIP）是否能有效消除增材制造Hastelloy-X中的微裂纹与孔隙？
• RQ4 退火与HIP处理如何改变显微组织，包括再结晶、析出相与晶粒结构？
• RQ5 HIP处理后，力学性能（尤其是伸长率与极限抗拉强度）的定量提升程度如何？

主要发现

• 裂纹密度在自由表面附近最高，底部最低，与通过解析建模估算的热梯度和热导率变化相关。
• 退火处理虽引发组织恢复与再结晶，但未能消除微裂纹与孔隙。
• 热等静压（HIP）成功消除了所有微裂纹与孔隙，获得完全致密的显微组织。
• HIP处理使伸长率从原始成形状态的13%显著提升至20%，表明塑性显著改善。
• 极限抗拉强度从原始成形状态的965 MPa提升至HIP处理后的1045 MPa，因析出强化作用，屈服强度保持中等偏高水平。
• HIP处理样品的再结晶与析出现象较退火样品更为显著，证实其显微组织细化效果更优。