变位磁场控制电弧增材制造中的微观组织形成机理研究

Microstructure control is the key to improve performance of arc welding based additive manufacturing(AWAM) parts, magnetic field treatment is promising to solve this critical problem. This study utilizes deflection magnetic fields to treat the deposition region and solidification micro-region in GMAW-based AWAM process. The theoretical analysis of dendritic remelting, magnetic field energy and magnetic anisotropy, etc., multiscale coupling numerical simulation of electromagnetic-fluid-microstructure, and experimental test are combined together to investigate the problem of solidification crystallization and solid phase transformation in magnetically controlled on the GMAW process. The study will investigate the distribution of magnetic force in molten pool under the effect of deflection magnetic fields, the effect law of the magnetic force on molten pool heat and mass transfer, grain growth, and the effect law of magnetic fields on solid phase transformation. Especially, the disturbance of heat and solute distribution in frontier of growing dendrites under the magnetically forced convection, forced convection stress, and dendritic ripening resulting in dendritic root remelting, and the effect of magnetic field energy on the phase change free energy and phase distribution will be deeply studied. This study hopes to determine the mapping relationship between the deflection magnetic field parameters and microstructure, to establish the nucleation model of columnar-to-equiaxed transition(CET) and the rules of dendrite growth for magnetically controlled molten pool, and reveal the microstructure formation mechanism in deflection magnetic field controlled AWAM process, thereby providing theoretic basis and practice guidance for improving the mechanical properties of fabricated parts, and promoting the large-scale application of AWAM technology.

微观组织控制是提高电弧增材制造零件性能的关键，借助磁场作用技术有望解决此关键问题。本项目拟利用变位磁场作用于GMAW电弧增材制造中熔积和凝固微区域，采用枝晶重熔及磁场能量和磁各向异性等相关理论分析、电磁-流体-微观组织多尺度耦合数值模拟及实验测试结合的方法，对磁控焊接的熔池凝固结晶与凝固微区的固态相变问题展开研究。具体研究变位磁场作用下熔池电磁力分布，电磁力对熔池传热传质与晶粒生长及磁场对凝固微区固态相变的影响规律，特别深入研究电磁力强制对流对枝晶生长前沿热量和溶质分布扰动、强制对流应力和“熟化”导致的晶根重熔，以及磁场能量改变相变自由能和影响相分布。以期得到变位磁场参数与成形零件微观组织分布的映射关系，建立磁控焊接熔池柱状晶向等轴晶转变的形核模型和枝晶生长的规则，揭示变位磁场控制电弧增材制造中的微观组织形成机理，为提高成形零件性能提供理论依据和方向指导，推动电弧增材制造技术规模化应用。

本项目研究将外加磁场引入电弧增材制造过程，结合磁流体动力学理论、传热和传质仿真，磁控柱状晶向等轴晶转变的形核、枝晶生长与固态相变理论，揭示了磁场控制电弧增材制造凝固组织和相变的机理。主要研究结果包括：搭建了横向、纵向以及变位磁场辅助的电弧增材制造设备原型。建立了GMAW过程连续熔积电弧和熔池传热传质的弱耦合数值仿真模型，通过数值仿真和实验测试，揭示了焊枪偏转、焊枪-基板距离、特殊熔积位置、外加磁场对电弧形态，以及磁场对熔池流态、熔积层形貌、凝固组织和气孔的影响规律和机理；发现了纵向磁场通过周向电磁搅拌可以提高搭接精度、细化晶粒，通过磁场能量变化可以改变凝固过程中的相变能和相成分，通过电磁搅拌和磁场能量变化综合作用可以提升成形试样的静力学性能并减小其各向异性；发现了横向磁场作用可以使电弧向熔池后方偏转，使熔池发生单向强制对流并驱动熔融金属和热量向熔池后方运动，从而更加直接地冲刷熔池结晶面；变位磁场兼具纵向和平行磁场的功能，不仅可以搅拌熔池，还可以提高熔积层尺寸精度。通过建立基于有限元-元胞自动机的宏观传热与微观组织演变耦合的数值模型，模拟电弧增材制造过程中晶粒形核、生长过程，揭示了多层熔积温度梯度方向的变化对晶粒生长方向的影响规律和机理。将项目所开发的传热传质数值仿真模型应用于基于PAW的电弧增材熔积传热和流动的仿真，模拟了钛合金单道多层熔积的熔池流态和熔积层形貌，并揭示了熔池传热和流动的机理。初步开发了变位磁场与随焊辊压协同的电弧复合增材制造装置，并申请了相关专利。项目研究丰富了磁场辅助电弧增材制造技术的基本理论，为其研究提供了有价值的设备原型、计算模型和一些重要研究基础，实现了电弧增材凝固组织和力学性能的更优控制。