基于特征点及物理软化参数法的金属热成形本构关系建模研究

Constitutive model is essential in material plastic processing field, but there are some problems in existing models, such as strain is not included in the semiempirical creep equations, multitudinous parameters appear in the model based on strain, the parameters of some models have few definite physical meanings and correlative variables exsit. This project aims at establishing a more physically appropriate constitutive model of metal based on characteristic points and physical softening parameter method during hot forming. The mechanism of microstructure evolution will be reflected by the model. The flow stress could be obtained by the following way. The flow stress before DRX will be got based on the relationship between the flow stress and the rate of work hardening. The characteristic point variables and material parameter are included. The flow stress of dynamic recrystallization(DRX) grains will be used in the modelling of the flow stress after DRX. A new more remarkable physical significances and practically useful fractional softening model is presented for the simulation of high temperature austenite flow curves. General equations describing the flow stress after DRX are derived from the formula that represents the dependence of the dislocation density on plastic strain and stress. It contains the stress, strain values on characteristic points and the rate of dynamic recovery. Compression experiments will be carried out to obatain the flow cueves.The key characteristic points of flow curve will be evaluated by the flow curves. The model parameters are identified by inverse ananlysis of the experimental stress-strain curves obtained from the hot compressive deformation on Gleeble-3500 thermo-simulation machine, and then the variation of model parameters with deformation condition Z will be investigated. The Avrami formalism is used here to describe the kinetics of DRX. Flow curve modeling requires that the experimental dependences on Z of the initial stress, critical stress, the saturation value of the work hardening flow curve, the rate of dynamic recovery, the Avrami constant and the Avrami time exponent must first be established. They shall be displayed as functions of Z. That will complete the establishing of the constitutive model. The effects of grain size on the values of the characteristic points will be studied.And the model will be applied to the given steel. The research and analysis of this project will enhance the control model for production process during meatl materials and high-performance precision manufacturing forming of metal materials. It will provide theoretical reference for actual production of hot forming for metal materials.

本构模型是进行材料塑性加工的基础，但现有模型中存在如半经验蠕变方程不含应变，基于应变的模型参数众多、物理意义不明确，有非独立参数等问题。 项目将研究建立基于特征点和物理软化参数法、有明确物理意义反映材料内部微观组织演变机理、参数明晰的金属高温热成形本构模型：(1)考虑金属热成形流变应力曲线上特征点出现的微观机理，由应变硬化率与应力的关系出发，建立动态再结晶(DRX)前流变应力关于特征点变量和材料参数的模型；(2)提出已发生DRX晶粒流变应力的概念，构造更具物理意义的DRX软化分数计算式，以位错密度为内部变量，借助其与应力、应变的关系求解DRX后流变应力关于关键特征点变量以及动态回复参数的模型；(3)利用实验曲线，求解特征点变量、动态回复参数、Avrami方程待定参数与Z参数的关系，并进行实验验证。 项目的研究将丰富金属材料热成形及高性能精确制造成形生产控制工艺，为金属热成形生产提供参考。

高性能精确成形制造能通过成形过程使零构件宏观性能在坯料性能基础上得到提高，是先进制造发展的重要方向，支撑国民经济发展与国防建设的主要技术之一，为真正使成形过程不但能“成形”，也能“成性”，应重点研究微观组织演化与表征、宏微观本构建模支撑成形过程的多尺度模拟方法等，高性能精确成形过程的工艺制定、建模仿真与优化，需要更深入的物理模拟实验对塑性成形理论、反映塑性成形过程中的材料宏微观本构模型、材料参数的确定等方面进行支持。这样就需要建立较为精确的材料高温下的热成形本构模型，来描述材料变形的基本信息，阐明在热加工变形条件下流变应力与温度、应变速率以及真应变之间的依赖关系。热成形本构模型在制定、优化成形工艺参数及有限元数值模拟预测中发挥着十分重要的作用。由于工业生产实验成本非常高，结合数值模拟可以经济且更好地对生产工艺进行研究。.随着自动化、工程实际要求越来越严格、新产品不断研发，迫切需求针对我国实际情况以及具体钢种建立更精确、更具明确物理意义的金属材料高温变形本构模型。本项目考虑到动态回复和再结晶造成的材料热加工过程中软化和位错密度的变化都影响着流变应力曲线的形状，根据高温流变应力行为主导软化机制的不同，建立金属材料热塑性成形过程的动态回复和动态再结晶两阶段本构模型，采用内部变量位错密度经过推导描述加工硬化和动态回复对流变应力的影响，引入新的物理软化参数法计算DRX软化分数，据实验流变应力曲线确定本构模型中的关键特征点，并分析模型参数随变形条件的变化规律。对于控制金属材料热变形过程中的组织演变、工艺参数优化和提高塑性成形产品的质量具有重要意义，本项目的研究丰富了金属材料高性能精确制造成形生产工艺控制模型，为生产提供精准在线控制本构模型，提高其在线控制能力，在很大程度上有助于新产品开发以及准确的模拟热成形过程中金属内部微观组织演变和力学性能预报，有利于制造业向绿色、低碳、环保生产方向发展。