金属激光增材制造熔池传热流动行为与凝固组织研究

激光增材制造因其逐点逐层成型的特性，能够快速精确地制造具有复杂结构的零件和传统难加工材料的直接成型。但是将激光增材制造应用于航空航天等高附加值行业中仍存在诸多挑战，其中之一为合金凝固组织难以按需定制。熔池是激光增材制造中最基本的单元，其内部的传热流动行为直接决定了凝固条件和凝固组织特征。因此，揭示激光增材制造熔池传热流动行为、凝固条件与凝固组织的内在关联，探索有效的凝固组织调控手段，是实现凝固组织可控的高性能金属零件的激光增材制造所必须要突破的重点和难点。本文围绕金属激光增材制造中熔池传热流动行为与凝固组织进行研究，并从凝固行为调控和熔池流动行为调控两方面探索了可用于激光增材制造中的凝固组织定制方法。

激光粉床熔融（Laser powder bed fusion, L-PBF）和激光直接能量沉积（Laser direct energy deposition, L-DED）可视为两种具有不同能量输入水平的激光增材制造过程。通过建立L-PBF和L-DED两种工艺的熔池传热流动模型，对比研究了两种熔池的传热流动行为、凝固条件和凝固组织特征，揭示了激光增材制造熔池传热流动行为对凝固条件的影响机制。激光位置和熔体流动共同决定了熔池内部温度梯度的分布特征，熔体回流会导致凝固界面上的温度梯度等高线向熔池前方偏移。L-PBF熔池和L-DED熔池尾部熔体流动驱动力的差异使得其凝固界面上的热量传递机制分别为熔体流动主导和热传导主导。这种热量传递机制的差异导致了凝固界面的冷却速率分布特征在L-PBF熔池为底部高于顶部，而在L-DED熔池中则为顶部高于底部，进而导致了两种熔池内部的一次枝晶间距分布特征相反。

基于熔池模拟与凝固组织生长理论，研究了合金在激光增材制造中的柱状晶向等轴晶转变（Columnar-to-equiaxed transition, CET）行为。提出了一种新的过冷度与凝固速率的关联函数，成功将CET理论模型的适用范围拓展到涵盖L-PBF工艺的高凝固速率区间。结合熔池模拟，解释了在L-PBF工艺中，调整激光功率和扫描速度，晶粒仍以外延生长的原因。在L-PBF工艺的凝固速率区间，合金凝固过程中产生CET的临界温度梯度存在阈值（对镍基高温合金约为10⁷ K/m），而调整激光功率和扫描速度，熔池凝固界面的温度梯度仍高于CET阈值且未出现下降趋势。进一步分析材料参数对合金CET行为的影响，发现合金成分导致的CET临界凝固条件的变化比热物理属性导致的熔池凝固条件变化对合金CET行为的影响更显著。

基于激光增材制造熔池凝固条件特征与合金在其中的CET行为特征，提出了基于过冷度优化的晶粒形态定制方法。通过该方法预测了Al/Cu二元合金和Ti-Al-V系钛合金在激光增材制造中形成等轴晶的成分，并通过原位合金化技术制备试样，验证了该方法在激光增材制造工艺中应用的可行性。针对原位合金化中存在的成分不均缺陷，建立了耦合合金相图的熔池传热传质流动模型，研究了粉末熔化、熔体混合和成分不均区域形成的动态过程，揭示了提高能量输入促进元素均匀化的机制为对流搅拌时间和驱动力同步提高，提出了对流搅拌数来表征熔池流动特性，并建立了指导原位合金化工艺参数设计的工艺参数图。

基于熔池凝固条件受熔体流动影响的原理，采用仿真分析与实验相结合的方法，探究了可用于调控熔体流动的磁场技术在激光增材制造中应用的可行性。建立了耦合磁场的熔池传热流动模型，研究了磁场模式对熔池传热流动行为和凝固条件的影响。施加静磁场所激发的洛伦兹力有助于提高熔体流动的稳定性，而交变磁场和震荡磁场所激发的周期性变化的有效洛伦兹力会使熔体流速呈现周期性振荡，但施加静磁场和动磁场都未明显改变凝固界面的温度梯度和凝固速率。搭建了静磁场与交变磁场辅助激光增材制造实验装置。通过元素示踪实验和样品凝固组织表征分析，证实了通过改变磁场模式来调控熔池的熔体流动行为，能够用于激光增材制造合金的凝固组织定制。

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