激光选区熔化增材制造过程中的元素蒸发机理及抑制方法

Element evaporation during selective laser melting (SLM) additive manufacturing process results in the deviation of alloy composition of the as-built parts, bringing about serious defects such as composition segregation, element dilution and abnormal microstructure. In this project, the mechanical model of migrating and diffusing during element evaporating was established combined with the thermodynamics of evaporation, and then the critical escaping condition as element penetrating the liquid-gas interface was determined. Based on the above, the evaporation mechanism was revealed accordingly. Optical emission spectrum (OES) of the vapour was collected and spectrum characters of evaporation intensity were extracted, and the quantitative characterization relationship between process parameters, spectrum characters and element loss was discussed, then the prediction model of element evaporation intensity was established. The dominant influence mechanism of evaporation intensity under multi-factor interaction was explored, which contributed to the determination of critical process conditions as element reaching the equilibrium during migrating and diffusing process, and parameter optimization can be conducted accordingly. Thus the objective of element evaporation inhibition in SLM additive manufacturing can be achieved. This project is conducive to improving the uniformity and stability of component, microstructure and performance of SLMed parts, there are great scientific significance and practical application value for improving the application of SLM technique in the fields such as aeronautics & astronautics and biological medical, etc.

激光选区熔化（SLM）增材制造过程中的元素蒸发导致成型材料的成分发生偏离，造成合金成分偏析、元素贫化和微观组织异常等缺陷。因此，本项目结合蒸发的热力学特性，建立蒸发过程中合金元素迁移扩散的力学模型，确定合金元素挣脱液/气相界面的临界逸出条件，揭示元素蒸发的机理；采集金属蒸汽的光发射信号并提取反映蒸发强度的光谱特征，探讨工艺参数、光谱特征和元素损失量之间的定量表征关系，建立元素蒸发强度预测模型；探索多因素交互作用下蒸发强度的主导影响机制，确定元素分子达到迁移扩散平衡时的临界工艺条件并进行参数优化设计，实现SLM增材制造中元素蒸发的抑制。本项目有助于提高SLM增材制造零件的成分、组织和性能的均匀性和稳定性，对促进SLM增材制造技术在航空航天、生物医疗等领域的应用具有重要的科学意义和工程应用价值。

在本项目的资助下，项目组针对激光选区熔化（SLM）的增材制造过程、微观组织和相应的力学性能展开研究，主要研究发现如下：.1.通过高温合金熔池温度的有限元模拟建立了微裂纹密度与熔池温度的数学关系；分析了飞溅和元素蒸发导致孔洞缺陷以及不同熔池温度下的元素分布和微观组织的不均匀性，揭示了原始粉末化学成分和成型熔池温度对合金元素中Al元素蒸发的影响规律，建立了铝元素的蒸发强度与激光能量密度的数学关系。.2.探明了冲击载荷下激光增材制造金属材料动态响应规律。研究了SLM成形钛合金在不同温度、应变率下的动态力学性能，动态压缩强度随着温度的提升而急剧降低，增材制造钛合金具有较强的温度敏感性，而应变率敏感性没那么强烈。基于实验结果标定了J-C本构参数，建立了SLM成型钛合金的率相关本构关系。.3.揭示了高温高应变率条件下激光增材制造钛合金的微观组织演变规律。高温冲击下，增材制造钛合金局部发生绝热温升，产生绝热剪切带，剪切带内存在大量纳米等轴晶；剪切带附近由于剪切应力导致马氏体发生弯曲，产生弯曲针状马氏体；远离剪切带的区域晶粒仍然能够保持较为完整的针状马氏体。增材制造钛合金的针状马氏体在高温冲击下容易发生横向断裂，从而引起晶粒细化。.4.开展了基于固溶处理的激光增材制造TC4钛合金的动态力学性能调控。采用不同的固溶处理调控增材制造钛合金的α和β相，获得较多α/β界面相。结果显示，高速拉伸后的α+β固溶钛合金的颈缩区域的α/β界面相附近形成致密的变形孪晶，说明界面相能够通过形成变形孪晶增加材料的变形协调性，从而提升了材料的塑性。