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

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-tension strain states is greater than that on the tension-compression strain states.