预孪晶-正挤压扭转剪切变形制备高强韧镁合金及组织演变机理研究

The main problem with the conventional plastic working method of Mg alloys lies with the difficulty to govern the evenness of fine crystals, texture weakening, and precipitation of strengthening phase, thereby lowering the strength and toughness of Mg alloys. These drawbacks restrain the mass development and wide application of processed Mg alloys. In this project, we apply a pre-twinning method by adding forward extrusion and torsion deformation, aiming at controlling the nano-micro structure and improving the strength and toughness of Mg alloys. We first use numerical simulation analysis to optimize the process parameters. The pretwinning-forward extrusion and torsion deformation are then carried out using the predicted parameters. The microstructure evolution is investigated to shed light on the effects of deformation process on grain refinement, texture softening, precipitates and microstrusture stability. In addition, we also provide the underlying origin of the pre-twinning, precipitates and dynamic recrystallization, and uncover the controling principles of grain refinement, texture softening, precipitates. A fitting model of strengthen-toughening theory for this new deformation technique will be established. Finally, by combining the numerical simulation with experiments, we further establish a strengthening and toughening theory based on the results. This study could add new knowledge to the understanding of the high-toughness magnesium alloys, and pave way for the establishment of the strengthen-toughening theory, thereby promoting the application of this deformation technique to Mg alloys.

传统的镁合金塑性加工方法存在细晶均匀性、织构弱化及强化相析出等难以控制问题，致使其强韧性能偏低，严重制约了变形镁合金的大规模发展与广泛应用。本项目拟采用预孪晶-正挤压扭转剪切变形方法调控纳微组织结构，提高镁合金的强韧性能。首先，采用数值模拟分析，初步优化工艺参数。其次，选择合理的参数进行预孪晶-正挤压扭转剪切变形实验及组织演变研究，阐明变形条件对晶粒细化、织构弱化、沉淀相析出和组织稳定性的影响及其机制；揭示预孪晶、沉淀相和动态再结晶之间的相互作用机理；确立晶粒细化、织构弱化与沉淀相析出的调控原理及方法；构建预孪晶-正挤压扭转剪切变形镁合金的强韧化机理模型。最后，结合模拟仿真分析与实验结果，完善预孪晶-正挤压扭转剪切变形工艺和强韧化理论，形成技术原型。本项目的开展，可高效低成本的获得高强韧变形镁合金，对丰富强韧化理论以及变形技术的推广应用具有重要的学术意义和工程价值。

针对变形镁合金的晶粒细化、织构弱化和析出相难以有效控制致使其强韧性能偏低的问题，本项目采用预孪晶处理与正挤压扭转剪切变形相结合的方法来调控镁合金的纳微组织结构，达到提高其强韧性能的目的。. 项目首先将铸态镁合金进行小变形量的室温预孪晶处理，并开展了不同温度和不同应变率条件下的热压缩实验，构建了预孪晶镁合金材料的本构方程和热加工图，创建了预孪晶镁合金的材料库；然后利用有限元模拟优化分析了预孪晶-正挤压扭转变形镁合金的变形行为等，如应变、应力、应变率、挤压力等，优化了预挤压温度、速度、挤压比、扭转结构和润滑等工艺参数，并结合塑性加工力学与有限元模拟计算出了较为精准的镁合金正挤压-扭转变形挤压力；最后进行了合理的预孪晶-正挤压扭转剪切变形实验。. 本项目重点研究了预孪晶镁合金在压缩减径变形、挤压比变形和扭转剪切变形各个阶段的变形机制；变形条件对晶粒细化、织构弱化以及沉淀相析出等的影响机理；定量性分析细晶组织均匀性及热稳定性；揭示正挤压-扭转剪切变形对镁合金组各向异性的影响机理；阐明了高速冲击与高速切削对挤压变形镁合金组织的影响机理；探明了晶粒大小、析出相、织构等纳微结构与强韧性能之间的关系；结合强韧性能分析优化预变形-正挤压扭转剪切变形工艺参数。此外，在本项目研究的基础上，初步开发了两种新型的镁合金新型特种塑性加工技术并开展了部分相关应用基础研究。. 本项目的开展，对于丰富强韧化理论和制备高强韧变形镁合金具有重要的理论支撑和工程意义，可高效低成本的获得优良强韧性能的变形镁合金，也为制备高性能其他高性能合金材料提供一条新的途径。