Modeling and Simulation for the Stress Relaxation Beha- vior of Ti-6Al-4V at Medium Temperature

摘要:

应力松弛是钛合金在升高温度和加载条件下的一个显著特性，也是热校形和热处理的理论基础。因此研究了一种Ti-6Al-4V钛板在923~1023 K温度范围内、几种应变水平下的拉伸应力松弛行为。结果表明，应力松弛速率随着温度的升高而增加，材料中的残余应力经过一段时间之后趋向应力松弛极限；另外，在相同温度下，不同应力水平的应力松弛极限相同。进而，建立了一种描述应力松弛行为的显式三次延迟函数，本构精度高达97%，可用于工艺设计及理论分析。最后，基于应力松弛和蠕变的关系，提出了一种隐式蠕变型本构方程描述应力松弛行为，并将识别的材料参数输入ABAQUS，数值模拟了Ti-6Al-4V的热应力松弛行为，发现模拟的应力变化规律符合应力松弛曲线，证明了蠕变型本构方程对应力松弛模拟的适用性。

Abstract:

Stress relaxation is a significant characteristic of titanium alloys at elevated temperature and load, which is the theoretical foundation of hot sizing and heat treatment. The tensile stress relaxation behaviors of a Ti-6Al-4V sheet over the medium temperature range from 923 K to 1023 K and at several strain levels were investigated. Comprehensive analysis results indicate that the stress relaxation rate increases with the increase of temperature. The residual stress in Ti-6Al-4V alloy reaches the relaxation limit gradually after a period of relaxation. In addition, the stress relaxation limits reach the same value for the different initial stresses at the same temperature. Furthermore, an explicit constitutive equation with a cubic delay function was established and its prediction precision is as high as 97% and this has laid the foundation of process design and theoretical analysis. Finally, the implicit creep-type constitutive equation was developed to describe the stress relaxation behavior of Ti-6Al-4V alloy, and identified material parameters were input into ABAQUS to simulate a stress relaxation process of Ti-6Al-4V alloy due to the creep forming. Comparison results show that the stress change simulated is in agreement with the stress relaxation curve. It is proved that the creep-type constitutive equation is valid for the simulation of stress relaxation.