定向洛伦兹力调控激光熔注增强颗粒分布机理研究

The distribution of the reinforcement particles is one of the most important factors which decides the performance of the laser melt injection layer. However, it is difficult to regulate the distribution of the reinforcement particles by traditional laser melt injection technologies at present. The Lorentz force induced by both external steady electric fields and steady magnetic fields, as a volume force, was applied to control the melt flow and regulate the distribution of reinforcement particles in the molten pool. In this project, multi-energy fields numerical model, concerning electric field, magnetic field, flow field and thermal field, was employed to study the heat and mass transfer in the molten pool of laser melt injection under the steady Lorentz force. Meanwhile, the particles kinematic model was coupled with Lorentz force and fluid drag force to reveal the movement trajectories and trapping mechanism of the reinforcement particles in the molten pool. Under the control of Lorentz force, the distribution of the reinforcement particles in the laser melt injection layer can be regulated as required. By using this method, both the particles content in the key zone of the laser melt injection layer and the surface wear resistance of the parts can be increased, the cracks risk could be reduced and the post-machining property could be improved as well. The theory and the technology of this project could be further applied in the laser additive manufacturing and laser welding for reducing the defects such as the pore.

增强颗粒在激光熔注层中的分布状态是决定熔注层性能的关键因素，但是现有的激光熔注技术难以对熔注层中增强颗粒的分布进行有效调控。本项目提出利用外加稳态磁场和稳态电场的协同作用，在熔注层熔池内形成定向恒稳洛伦兹力，利用这一外加体积力，作用于熔池流体和增强颗粒，实现对增强颗粒分布趋势的灵活调控。本项目拟建立激光熔注熔池中关于电场、磁场、流场和热场的多能量场耦合模型，研究洛伦兹力作用下熔注层熔池内的传热传质变化规律。同时，根据洛伦兹力和流体拖曳力共同驱动下的颗粒运动模型，揭示增强颗粒在熔池中的精确运动轨迹和固化规律。利用外加定向洛伦兹力可实现对熔注层增强颗粒分布梯度的按需调控，可显著提高关键层域中的有效颗粒含量，大幅提高材料表面耐磨性能、降低熔注层的开裂风险并优化熔注层后续机加工性能，有望大幅拓宽激光熔注技术的应用范围。同时，本项目研究成果还可实现激光焊接和增材制造过程中气孔的排出与功能梯度的调控。

激光熔注技术是一种直接将硬质颗粒注入金属基体的方法，虽然可用于获得具有超高性能的材料表面，但在实际应用场合，受到熔注层后续机加工困难、熔注层易开裂、增强颗粒有效含量存在瓶颈等问题，限制了激光熔注技术的应用范围。本项目提出在激光熔注过程同时耦合稳态电场和稳态磁场，利用电磁复合场的协同作用，在熔注层熔池区域产生感应洛伦兹力和定向洛伦兹力，作为一类外加体积力，影响熔池内部的对流速度，同时改变增强颗粒在熔池中所受到的等效浮力，实现对增强颗粒运动及固化位置的精准调控。通过建立耦合电磁场、流场、热场、颗粒运动和固液相变等多物理场的仿真模型，揭示了电磁复合场对熔池传热传质过程和颗粒分布之间的影响机理。讨论了由增强颗粒分布梯度变化所引起的熔注层组织、成分、硬度和耐磨性的梯度变化差异。.1）电磁复合场可在熔注区域同时引入感应洛伦兹力和定向洛伦兹力，前者为时刻与熔池对流方向相反的阻力，可对熔池流速产生明显的抑制作用，后者为一类与重力类似的体积力，可改变颗粒所受的等效浮力。.2）在电磁复合场协同作用时，通过定向洛伦兹力的方向变化，可实现增强颗粒分布梯度的反转。.3）当未附加磁场和电场时，熔注层上部和下部的颗粒含量接近，分别占比50%左右，当洛伦兹力与重力同向时，熔注层上部的颗粒含量占到所有颗粒的80%以上，而熔注层下部的颗粒含量不到所有颗粒的20%。.4）当表面颗粒含量较高时，熔注层熔凝区的最高硬度接近900 HV，其磨损机理主要为磨粒磨损，且局部微区还同时存在粘着磨损和疲劳磨损现象。.本项目利用电磁复合场协同作用，在熔注区域外加方向可控的洛伦兹力体积力，获得了熔注层中WC颗粒分布梯度可按需调整的制备工艺，突破了传统只能通过激光工艺参数进行调节所存在的瓶颈，拓宽了激光熔注技术的使用场合和范围。