Fracture of Magnesium Alloy in Cold Forging

In order to clarify the forming limit of magnesium alloy in cold forging, the workability of magnesium alloy AZ31B (Mg-3%Al-1%Zn) is examined by upsetting and backward extrusion. In the upsettability test, shear type fracture occurs at a small equivalent strain of about 0.15 at temperatures lower than 170 °C. At a higher temperature, the flow stress curve has a peak at an average equivalent strain of 0.2-0.3, showing a clear tendency to work soften. In cold backward extrusion with a conical punch, the local strain can reach a very large value, 3.0 or greater, without fracture. To explain the experimental results, the mechanism of fracturing is discussed on the basis of strain localization, and a fracture criterion of magnesium alloy in cold forging is suggested. The newly proposed criterion provides much better results than the existing criteria.