Thermo-Mechanically Activated Transient Plasticity and Elevated Temperature Forming Limit Diagram

材料塑性变形净成形是一种高效的制造工艺。 在净成形中，期望高塑性应变和高速度以及低成形能量。 超塑性成形可以实现极高的应变，但是应变速率对于大规模生产来说太低，因此对于汽车工业来说是不可行的。 在高应变速率（~ 1/s）下，由于缺乏抵抗局部颈缩的硬化机制，热成形中的成形性通常低于室温下的成形性。 本计画的目的是开发一种新的成形技术，以改善成形极限，降低成形能量与成本，并改善机械性能。 该方法是使用一个新的概念，热机械激活瞬态塑性的基础上，以实现在低应力下的高速率变形。 在该过程中，工件在热机械激活变形条件下在不稳定的微观结构状态下变形。 通过连续的后热处理，可以显著提高成形部件的机械性能。 建立了考虑应变速率和组织演变的高温成形极限图，为工艺设计提供了依据。 该项目预计将产生重要的知识基础热机械激活瞬态塑性。 这些知识将是重要的净成形技术的进步，特别是对铝和高强度钢的可成形性的改善，以及制造业教育的重要意义。

Netshape material forming through plastic deformation is an efficient manufacturing process. A high plastic strain and a high speed with a low forming energy is desired in netshape forming. Superplastic forming can achieve extremely high strains, but the strain rate is too low for mass production and thus is not feasible for the automobile industry. At a high strain rate (-1/s) the formability in hot forming is usually lower than that at room temperature, due to the lack of a hardening mechanism to resist localized necking. The objective of this project is to develop a new forming technique with improved formability limit, reduced forming energy and cost, and improved mechanical property. The approach is to use a new concept based on thermomechanically activated transient plasticity to achieve high rates of deformation at low stresses. In this process the workpiece is deformed under a nonstable microstructure state at a thermomechanically activated deformation condition. With a sequential post heat treatment, the mechanical properties of the formed parts can be significantly improved. The elevated temperature forming limit diagram with the consideration of strain rate and microstructure evolution will be established to serve as a foundation for manufacturing process design. This project is expected to generate important knowledge fundamental to thermo-mechanically activated transient plasticity. This knowledge will be important to the advancement of netshape forming technology, especially on formability improvement for aluminum and high-strength steels, as well as significant implication for manufacturing education.