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

网状材料的塑性成形是一种高效的制造工艺。网状成形要求塑性应变大、成形速度快、成形能低。超塑性成形可以获得极高的应变，但应变速率太低，不适合大批量生产，因此不适用于汽车工业。在高应变速率(-1/S)下，由于缺乏抵抗局部颈缩的硬化机制，热成形的成形性通常低于室温。本项目的目标是开发一种新的成形工艺，提高成形极限，降低成形能量和成本，并改善力学性能。该方法是使用基于热机械激活的瞬时塑性的新概念来实现低应力下的高变形率。在此过程中，在热机械激活的变形条件下，在非稳定的组织状态下变形工件。采用顺序后热处理工艺，可显著提高成形件的力学性能。建立了考虑应变速率和组织演化的高温成形极限图，为制造工艺设计提供了依据。该项目有望产生热机械激活瞬时塑性的重要基础知识。这一认识将对发展网形成形技术，特别是提高铝和高强度钢的成形性具有重要意义，并对制造业教育具有重要意义。

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.