镁合金转模挤压成形机理与变形协调控制

The proposed project is aiming to investigate the coordinated deformation control and extrusion forming mechanism of magnesium alloy extrusion with rotating container. In this study, theoretical analysis, finite element simulation, and technical tests were used to investigate the mechanism of coordinated deformation, as well as the characterization methods of torque and the torsion deformation degree in the characteristic region during container rotation extrusion; Then finding out the effects of rotating container on the shearing stress, eddy current, forming load, and temperature rise of magnesium alloy, and thus to build the theoretical model basing on the results of quantitative analysis and optimization of the forming parameters; Revealing the evolution laws of the microstructure and mechanical properties, as well as the mechanism of grain refining strengthening of magnesium alloy during the rotating container forming process. Sequentially, provide the theoretical basis for the design of magnesium alloy rotating container forming process. The container rotation extrusion method can be utilized to form products with complex cross-sections under the same loading condition. Compared with the other methods, this method effectively decreases the forming load and the tonnage of the forming equipment when achieving the products with the same cross-sections. In addition, it facilitates the realization of the aim to form large products with small equipments. And last, it also provides the technical reserve and scientific guidance for the complex shaped, large integral, high performance extrusion components used in the aviation, astronautics and automotive industries.

本项目旨在研究凹模扭转作用下的镁合金挤压成形方法及机理。采用理论分析与有限元模拟及工艺实验相结合的方法，探索镁合金转模挤压成形中特征区之间的变形协调机制、扭矩及扭转变形量的表征方法等基础理论问题；研究凹模扭转和挤压的双向同时加载对成形过程中受力状态及变形模式的精确控制；明晰凹模扭转对镁合金挤压成形中剪切应力、涡流行为、成形载荷和温升变化的影响及交互作用关系，使工艺理论模型建立在对成形参数的定量分析和优化基础上；揭示镁合金转模挤压成形中微观组织及性能的演变规律和细晶强化机理，形成镁合金转模挤压成形理论基础。该方法可在相同载荷情况下成形复杂截面挤压制品；在相同截面形状情况下能有效地降低载荷及设备吨位，易于实现"以小干大"的目的。对进一步扩大形状复杂、大型整体、高性能轻量化挤压材在航空航天及汽车等领域的工程化应用提供技术储备和科学指导。

传统镁合金成型多采用铸造工艺，但铸件材质不够致密且制品组织性能等存在不足。挤压成形时金属处于强烈的三向压应力状态，可充分发挥其塑性，易获得较优异的组织性能，因此，特别适于低塑性镁合金的加工成形。但成形温度区间窄、所需能耗大及材料利用率低等一直以来是制约镁合金挤压成形技术快速发展的瓶颈，更难于直接制造大尺寸、复杂截面的高性能制品。本项目在扭转-体积复合成形机理深入剖析基础上，提出了对凹模施加转动的成形新技术—转模挤压法。与芯模转挤压仅适于圆截面制品相比，对凹模施加转动可有效地避免异形截面制品挤出成形时引起垂直模口截面上轴向流线的切断及附加应力分布复杂等难题。与普通正挤压相比，在同样载荷情况下，凹模施加转动可成形复杂截面制品；在相同截面形状时可降低成形载荷，进而易于实现“以小干大”地进行复杂截面、低塑性材料的加工及成形。.本项目重点开展了转模挤压成形过程力学分析，转模挤压成形中塑性变形规律及影响因素分析，材料扭转变形行为及协调变形控制和微观组织演变等方面的研究工作。.将转模挤压成形中坯料变形部位定量划分成三个特征区，给出了各特征区内表面转速的分布规律。建立了扭矩和扭转变形长度的理论计算公式。研究结果表明，当凹模转速在一定范围内(0~1rad/s)，随着摩擦因子的增加或凹模转速的变大，定凹模内材料发生扭转变形的长度逐渐减小。.引入了滑动参量对凹模旋转使坯料发生打滑的现象进行表征。研究了扭转方式对塑性区形成及演变的影响，分析了各特征区的受力状态和流动模式对变形协调控制的影响机制。阐明了工艺条件对AZ31镁合金转模挤压中涡流行为、温升变化、成形载荷及缺陷的影响规律，基于此构建了合理的工艺理论模型。.建立了坯料外表面周向扭转角的理论模型，研究结果表明：当其它条件不变时，随着摩擦因子或凹模转速的增加，坯料外表面周向扭转角均呈增大趋势变化；当凹模转速一定时，随着冲头压下量的增加，径向扭转角则呈增大趋势变化；而在其它不变条件下径向扭转角的增幅随着转速增加而减小。.揭示了AZ31镁合金转模挤压中微观组织演变规律，研究结果表明，凹模转速的增加促使变形区内坯料所受剪切变形量的持续增大，动态再结晶进行的也更加充分，相应条件下动态再结晶体积分数也明显增多。可知在一定范围内适当增大凹模转速可达到平均晶粒尺寸减小且圆整度和分布均匀性都提高的效果。