累积反向挤压大变形诱导制备镁合金半固态坯料及组织性能调控

In the present work, accumulative back extrusion (ABE) based semi-solid processing is proposed as a novel method for semi-solid feedstock preparation. This intense plastic straining method has brought many advantages: no change in shape and dimension of preforms, simple procedure, easy to achieve accumulative induced strain, significant microstructural refinement, microstructure homogeneity and basal texture weakening. AZ61 and AZ91 magnesium alloys are specified as objects. According to the technical route of ‘homogenization and hot deformation behavior of as-cast magnesium alloys - simulation and experiment of ABE process - semi-solid isothermal heat treatment - semi-solid formability’, the scientific issues involved in the new approach will be systematically investigated by referring to theory and experimental research. Microstructure evolution laws of ABE processed alloys and the effect on the subsequent semi-solid microstructure will be clarified. The microscopic mechanism of semi-solid microstructural evolution affected by the dissolution of precipitate phases during ABE process will be revealed. The morphology evolution of solid and liquid components in semi-solid microstructure will be elaborated. A mathematical model for predicting the variations of thixotropic microstructural features versus the processing parameters will be developed. The intrinsic linkage among processing parameters, semi-solid microstructure and mechanical behavior in semi-solid state will be explicated. The success of this project is expected to develop an effective and practical technology of high-quality semi-solid feedstock preparation for magnesium alloys, rich semi-solid processing of magnesium alloys and the relevant scientific theory, and promote the industrial application of magnesium alloys.

本项目提出一种累积反向挤压(ABE)大变形诱导制备金属半固态坯料的新方法，该变形方法具有保持原始坯料形状尺寸不变、工艺简便、易实现累积诱导应变、组织细化效果显著、均匀性好及弱化基面织构等突出优点。以AZ61、AZ91镁合金为对象，围绕新方法涉及的科学问题，按照“铸态镁合金均匀化处理与热变形行为-ABE变形数值模拟与工艺-半固态等温热处理-半固态成形性能”的研究路线，通过系统的理论与实验研究，弄清ABE变形组织演变规律及其对后续半固态等温热处理组织演变的影响规律，揭示ABE诱导析出相回溶影响下的半固态组织演变微观机制，阐明固、液相组元的演变历程，建立预测半固态组织随工艺参数变化的数学模型，明确工艺参数-半固态组织特征-半固态力学行为的内在联系。本项目的研究成功，可望开拓一条制备高质量镁合金半固态坯料的实用化技术途径，丰富镁合金半固态成形工艺及相关领域的科学理论，推进镁合金的工业应用进程。

鉴于现有半固态坯料制备方法的不足，提出一种基于累积反向挤压制备金属半固态坯料的新SIMA法。该方法具有操作简便、单道次变形应变量大、保持坯料原有几何尺寸不变、材料利用率高等优点。基本过程是：利用累积反向挤压变形实现镁合金材料的大应变诱导变形和强烈剪切作用，使第二相充分碎化的同时，基体晶粒得到显著的细化；通过合理的等温退火处理，得到具有组织细小、球化程度好的半固态球晶化组织；利用半固态等温压缩模拟合金的半固态成形，证实基于累积反向挤压获得的半固态合金具有良好的触变成形性。.本项目围绕累积反向挤压制备金属半固态坯料的新SIMA法，以AZ91D镁合金为主要对象，系统地开展了镁合金半固态坯料制备与触变成形研究。借助等温热压缩模拟实验，构建了铸态镁合金的热变形本构模型和三维热成形加工图，建立了热形变过程的动态再结晶动力学方程；结合数值模拟与工艺实验研究，揭示基于累积反向挤压大变形镁合金的组织演变规律与微观机制；通过研究累积反向挤压镁合金等温退火处理时半固态组织的演变规律，揭示了累积反向挤压诱导析出相回溶影响下的半固态组织演变微观机制，阐明了固、液相组元的演变历程，阐明其半固态组织球晶化主要影响因素与微观机制，建立基于累积反向挤压与半固态等温处理工艺参数的协调匹配关系；通过半固态触变成形试验及材料半固态力学性能研究，明确工艺参数-半固态力学行为的内在联系，构建了半固态本构关系模型。.通过本项目的研究，确立了一条具有自主知识产权的制备高质量镁合金半固态坯料的实用化技术的途径。解决了相关的基础科学问题，突破关键技术难点，掌握了工艺规律，为该工艺的推广应用奠定基础，丰富了镁合金半固态成形工艺及相关领域的科学理论，推进镁合金的工业应用进程。