基于M-K理论的6016铝合金成形极限曲线预测

Prediction of forming limit curve of 6016 aluminum alloy based on M-K theory

近年来,为实现汽车车身轻量化,大量的铝合金材料被用于汽车车身制造,由于6016铝合金具有良好的烘烤性能,被大量使用.但是传统的冷成形技术并不能成形复杂零件,因此热冲压-冷模具淬火成形技术被用到铝合金的成形过程中,板材成形领域中一个重要的性能指标是成形极限.本论文使用理论预测和试验两种方法对6016铝合金成形极限曲线进行了研究.首先,建立了考虑应变强化和应变速率强化的Fields-Bachofen本构方程,并将此本构方程引入到成形极限理论推导过程中;然后,基于M-K凹槽理论,对6016铝合金成形极限曲线进行了理论预测,并且采用Nakazima试验方法对预测结果进行了验证.结果显示,随着初始厚度不均度的增加,预测曲线向纵坐标的正方向移动;通过实验值和预测值的对比发现M-K凹槽理论对成形极限曲线的预测是可行的、准确的.

In recent years,due to the increasing demand for lightweight products in automotive industries to save energy and decrease CO2 gas emissions,many aluminum alloy materials are being used in cars. Due to its good baking performance,6016 aluminum alloy is popular. However,traditional forming technology cannot produce complex parts. Furthermore,recent studies have focused on the hot stamping of aluminum alloy sheets and,in particular,of 6016 aluminum alloy sheets. It is well-known that sheet-metal formability is enhanced when the blanks are formed in hot temperatures. When this is done,the forming limit curve will rise. The forming limit of the material during the hot forming process is an important index and studying the forming limit of aluminum alloys at high temperature is of direct significance to production practices. In this paper,the forming limit curve of 6016 aluminum alloy was studied by theoretical prediction and experimentation. First,to evaluate the flow stress of a 6016 aluminum alloy sheet,the uniaxial hot tensile tests were conducted over a strain rate range of 0. 01-1 s-1 and a temperature range of 400-500 ℃. Then,the Fields-Bachofen constitutive equation was established with considering strain hardening and strain rate enhancement,which matched well with the experimental measurements. Then this constitutive equation was introduced into the forming limit theory. Finally,based on the M-K groove theory,the forming limit curve of 6016 aluminum alloy was theoretically predicted,and the prediction results were validated by using the Nakazima test method. The comparison of the experimental and predicted values shows that the M-K groove theory is reasonable and accurate in predicting the forming limit curve. The effect of the initial inhomogeneity factor was analyzed on the forming limit curve.The results show that the prediction curve moves in the positive direction of the vertical coordinate,with an increase in the initial inhomogeneity factor. Also,the effect of the initial inhomogeneity factor on different strain paths differs,and the impact on the tension-ten-sion strain states is greater than that on the tension-compression strain states.