增材制造铝合金复杂热流传输与微观组织演化多尺度研究

Additive manufacturing of metallic materials is flourishing in recent years and developing rapidly into a key technology in the field of advanced manufacturing. In this project the research of process and microstructure of selective laser melting of high strength 2024 aluminum alloy through numerical modeling and experiments is proposed. A multi-scale numerical model will be developed to simulate the complex thermo-fluid transport processes on the level of powder particle, molten pool, and solid component during the selective laser melting process. The interaction between the laser beam and the alloy powders will be explored. The motion of the gas/liquid interface of the molten pool and the temperature and velocity fields within the molten pool will be revealed. The multiple thermal cycles experienced by the build during the additive manufacturing process will be examined as well. Moreover, a grain structure evolution model will be further developed to investigate the grain growth process of aluminum alloy during selective laser melting. The data for the shape and size of the molten pool and the critical solidification parameters such as the temperature gradient and the crystal growth rate at the solidification front of the molten pool obtained from the process model will be used in the grain growth model. The evolution of grain structure under the constricting solidification conditions of the moving molten pool and the influence of multiple thermal cycles experienced by the component will be studied. The temporal evolution process and the three-dimensional spatial distribution of the grains will be revealed with high resolution. This project is committed to realizing the comprehension of the physical processes and the microstructure evolution characteristics of high-strength aluminum alloy by selective laser melting. The achievements can serve as the critical basis for the scientific regulation and control of mechanical properties of the components.

金属材料增材制造近年来蓬勃发展并且正在迅速成为高端制造业的关键技术。本项目拟针对激光选区熔化高强2024铝合金的工艺过程与微观组织开展数值模拟与实验分析综合研究。项目拟开发多尺度数值仿真模型以在颗粒、熔池与构件层面分别对激光选区熔化的复杂热流传输过程进行研究，探索激光与粉末的相互作用，揭示熔池金属的气/液界面运动规律以及温度场与流场分布，阐明构件在三维空间内经历的多重热循环过程。项目拟进一步协同开发激光选区熔化铝合金晶粒组织演化模型，并基于工艺过程模型获得的熔池形貌与尺寸、凝固前沿的温度梯度与晶体生长速度以及构件多重加热冷却过程等关键数据，综合考虑移动熔池约束凝固条件与构件多重热循环对晶粒组织演化的共同影响，以高时间和空间分辨率揭示晶粒组织的演化过程以及晶粒组织的空间分布特征。项目致力于实现对激光选区熔化高强铝合金工艺过程与微观组织演化规律的综合把握，为构件力学性能的科学调控提供关键基础。

本项目针对激光选区熔化多维渐进过程特征，结合2024铝合金的材料特性，采用多尺度模拟仿真与实验手段相结合的新方法，综合研究了激光选区熔化2024铝合金复杂热流传输过程与晶粒组织演化的关键科学问题。开发了多物理场耦合模拟仿真模型，通过求解质量、动量、能量守恒以及流体体积偏微分方程，探索并揭示了激光选区熔化2024铝合金过程中激光与铝合金粉末的相互作用机理，获得了液态金属熔池的三维温度场与流场分布特征，阐明了熔池气/液自由界面的高速非稳态运动过程。获得了移动熔池凝固前沿的温度梯度与晶体生长速度等关键凝固参数以及构件三维空间多重热循环温度-时间-空间数据，发现激光选区熔化极端热循环条件下熔池冷却速率可达一百万K/s。进一步阐明了移动熔池约束的非平衡凝固条件与构件多重热循环交互作用影响下的晶粒组织时变演化过程，揭示了晶粒组织在三维空间内的分布规律。测试表征结果结果表明激光选区熔化2024铝合金微观组织以柱状晶为主且容易产生伴生凝固裂纹。裂纹尺寸、位置和扩展路径与激光能量输入、扫描策略及道间距离等因素影响下的熔池局部凝固条件密切相关。总之，关于工艺过程与微观组织的模拟仿真与实验分析综合研究为阐明激光选区熔化“过程-组织-性能”关系提供了关键基础依据，对于建立和发展激光选区熔化2024铝合金科学体系具有重要意义。本研究全部完成了项目合同书中的主要研究内容，满足了所有项目研究目标，并进一步超额拓展和深化了项目研究成果在更广范围的增材制造方法与材料方面的应用。