镁合金薄壁件热振联合渐进成形机理及形性协同控制研究

Magnesium alloy sheet parts enable lightweight, high performance solutions, which are vital for the development of advanced manufacturing technology. The major problem with magnesium alloy is related to its poor formability at room temperature which leads to the difficulty during various forming processes and limits the industry applications. The purpose of this project is to increase the formability of magnesium alloy by using an innovative incremental forming technology assisted with heat and vibration, namely combined heat-vibration incremental sheet forming process. The interrelationship among forming process, material property and deformation behaviour for magnesium alloy are investigated by means of theoretical, experimental and numerical methods. First, the experimental set-up integrated with industrial robot, heating system and vibrating tool is developed. Second, the dynamic numerical model that couples thermo-mechanical multi-physics is established. A viscoelastic-plastic constitutive model describing the material behaviours of magnesium alloy under complex deformation paths is developed as well as the implementation of the corresponding user subroutines. Third, to explore the source of twist springback of the formed part, the residual thermal/internal stress fields are analyzed based on the numerical model and experimental observation. Meanwhile, the microstructural evolution of magnesium alloy under combined heat-vibration incremental forming process is also motivated by means of microstructural characterization and crystal plasticity model. The coupled effects of heat and vibration on the microstructural property and formability of the material are assessed. Fourth, the co-control strategy of shape and property for magnesium alloy sheet parts based on combined heat-vibration incremental forming process is performed. Finally, the goal of this project is to identify ways to improve the forming quality of the magnesium alloy parts, and to establish a flexible and robust process condition for stable and precise manufacturing of lightweight materials.

镁合金薄壁件的成形工艺是轻量化、高性能装备技术研究、发展及应用迫切需要解决的关键基础科学问题。本项目从镁合金常温下塑性差，难变形的特点出发，提出采用热振联合渐进成形工艺，结合粘弹塑性变形理论和多场耦合数值模拟技术，揭示成形工艺—材料特性—变形行为三者间的内在联系。首先，构建石英管阵热辐照系统和振动辅助渐进成形装置相集成的物理实验平台；其次，开发考虑材料各向异性、温度、应变速率相关的粘弹塑性本构模型及其子程序，建立动态热-力多场耦合数值模型，分析热振联合渐进成形产生的残余热/内应力和板材扭曲回弹的内在关联，提出改进温度场、振动场、成形轨迹等工艺优化策略；再次，结合微观测试表征和晶体塑性模型模拟分析，阐明热振耦合作用下镁合金板材微观组织演变及成形性能增强机理；最后，提出改进热振联合渐进成形工艺的形性协同控制策略。本项目研究结果对发展高性能轻金属合金复杂构件柔性化成形先进理论和技术具有重要意义。

汽车、航空航天等领域对轻质、高性能、复杂形状镁合金薄壁件的制造和性能要求不断提高，急需解决现有成形工艺的瓶颈问题。本项目从镁合金常温下塑性差，难变形的特点出发，提出一种热振联合渐进成形新工艺，结合渐进成形工艺实验、多场耦合数值模拟技术和材料微观表征分析，探索了热振联合对镁合金板材渐进成形中塑性变形的影响规律及作用机理。首先搭建了热空气/碳纤维电热管加热工作台及振动辅助渐进成形物理实验平台；其次，设计组建了热振联合拉伸实验力学平台，并以AZ31B镁合金为研究对象，分析了热振联合对材料拉伸力学行为的影响；基于热激活机制和位错密度演化理论构建了热振联合辅助下材料本构模型；采用完全热力耦合法建立了热振联合渐进成形有限元数值模型，解决了动态加载过程中的接触边界问题描述、温度场的演化、超声场的施加以及材料参数设定等建模关键技术；结合热振联合渐进成形工艺实验，分析了热振联合作用对镁合金零件成形过程中成形力、成形极限、几何精度和表面质量的影响。最后结合微观表征测试技术研究了热振联合对镁合金显微组织、断口形貌、动态再结晶行为、位错结构等微观组织的作用机制，阐明了镁合金材料在热振联合作用下潜在的变形机理。主要研究结果表明：热振联合作用下镁合金的屈服强度和抗拉强度下降，延伸率和塑性提高，动态再结晶延迟，连续动态再结晶占比增加。随着振幅的增加，屈服强度和抗拉强度降低的幅度增加，而延伸率先增大后减小；超声振动通过影响位错的运动和分布，进而影响动态再结晶行为，最终导致材料性能发生变化；结合热振联合渐进成形仿真和实验结果，在适当温度下施加超声振动，镁合金渐进成形工件的成形力降低，成形极限、几何精度和表面质量等得到显著提高。本项目研究成果为热振联合渐进成形工艺优化和性能调控提供有效的理论依据，扩展了金属多场耦合材料形变基础理论，对推动镁合金等难变形轻金属材料塑性加工技术的发展具有重要意义。