几何和晶粒尺度效应对金属薄板屈服强化特性及屈服准则影响规律的研究

With the ubiquitous trend of product miniaturization and rapid development of microforming technology, a demand for the precision description of multi-scale deformation behavior of sheet metal increases urgently. Due to the size effect with miniaturization, the material yielding behavior and hardening law may be different under multi-scale, meanwhile, the knowledge of macroscopic plastic deformation can not be completely applicable to micro-scaled plastic forming process. In addition, the multi-scale yielding and hardening behaviors and the validity of existing yield criteria and hardening models have not yet been systematically explored and studied, which would cripple the wide applications of microforming technology in production of microparts. In this project, the yielding behavior and hardening law of sheet metal in multi-scale plastic deformation process will be systematically explored and how different geometry and grain size effects, material microstructure evolution affect them will be also extensively explored via multiaxial loading experiment and finite element method. The validity of yield criteria and hardening models, which are widely used in macro-scale forming, will be evaluated under different workpiece geometry and grain size scenarios. By employing the outcome of size affected yielding and hardening characteristics, the yield criterion and hardening model, which are suitable for describing micro-scaled plastic deformation of sheet metal will be established, Through the micro hydrodynamic deep drawing (MHDD) of a semicircle micropart, the developed knowledge of yielding and hardening behaviors and the determined criteria will be verified and validated. The project will thus focus on (a) Geometry and grain size effects on yielding behavior and hardening law of sheet metal; (b) Validity of existing yield criteria and hardening models under micro-scale plastic deformation; (c) Case study and verification of the developed knowledge.

医疗植入器械等微型产品的广泛应用和微成形技术的快速发展对金属板料在不同尺度下塑性变形行为更为精确的描述提出了新的要求。由于尺度效应的存在，不同尺度下材料屈服行为和强化规律存在差异，宏观尺度下的塑性变形知识无法完全适用于微塑性成形过程。同时，由于缺乏对多尺度下材料屈服强化特性的全面认知以及现有屈服准则和强化模型在微尺度下有效性的研究，使得微成形技术离批量化和精确化生产还有一定的差距。本项目将通过十字形试件双向加载实验和有限元方法系统研究不锈钢薄板在不同尺度下的屈服行为和强化规律，探讨几何尺寸、晶粒大小和材料微观组织对薄板屈服轨迹和强化特性的影响，并验证和评估现有屈服准则和强化模型在微塑性变形过程中的适用性及有效性。基于尺度效应和材料微观组织演变对屈服强化特性的影响规律，提出适合描述金属薄板微塑性变形的屈服准则和强化模型，并通过微充液拉深工艺实验验证所建立的微尺度屈服准则和强化模型的有效性。

在产品微型化和功能集成化的背景下，金属箔材及其微成形技术广泛应用于电子通信、精密仪器、新能源汽车、生物医疗等领域。与传统板材塑性加工技术相比，微成形过程中金属薄板几何和晶粒尺寸所引起的尺度效应会改变其屈服强化特性和失效机制等。因此，为揭示金属箔材微成形过程尺度效应机制，提高成形缺陷的预测精度，需要对微尺度下金属薄板的屈服强化行为及成形极限等进行系统研究。. 本项目基于自主开发的微尺度双轴双向加载实验平台和微尺度十字形试样双向拉伸测试方法，开展了不同材料状态下SUS304箔材屈服强化行为研究。通过对比理论与实验屈服轨迹，发现Yld2000-2d屈服准则相对于其它准则可以较为准确描述金属箔材的屈服行为。然而，随着箔材厚度减小、晶粒尺寸增大以及塑性应变增加，理论计算与实验结果偏差愈发明显。基于Taylor-Bishop-Hill多晶体塑性模型，揭示了织构演化-屈服轨迹-成形性能之间的关联关系，为微成形过程形性协同调控提供了理论基础。通过将尺寸因子和塑性应变引入到Yld2000-2d屈服准则的最优指数和各向异性系数中，建立了耦合尺寸效应和塑性变形的微尺度Yld2000-2d-μ准则，并通过了实验验证。. 此外，通过对比不同尺度下SUS304箔材初始屈服轨迹和预拉伸后继屈服轨迹的形状和尺寸，发现背应力与尺寸效应表现出明显的正相关关系，并且等向-随动混合强化模型对金属箔材回弹的预测精度最高。基于所开发的微尺度成形极限实验平台，研究了金属箔材的破裂机制和成形极限演变规律，阐明了表面粗化和尺度效应对成形极限的耦合影响机制，建立了综合考虑表面粗化和失效机制转变的分段型失稳准则，并通过了实验验证。. 本项目通过对不锈钢箔材微尺度下屈服强化行为与成形极限的研究，系统评估了现有屈服准则、强化模型和失稳模型在微成形中的有效性和适用性，阐明了尺寸效应和微观组织演变对屈服行为、强化特征和成形极限的影响规律，提出了适合金属箔材的微尺度屈服准则和分段失稳模型，对完善微尺度金属箔材本构理论，推进微成形工艺发展具有重要意义。研制的多功能微型双轴双向拉伸试验机已应用于中铝材料应用研究院，支撑了汽车铝板的产业升级，提出的跨尺度分段失稳准则，应用于锂电极片（厚度10μm）裁切过程毛刺的预测与改善。