Improvement of Forgeability of a Commercial AZ31B Magnesium Alloy in Cold Backward Extrusion with Counter Pressure

Abstract

In order to improve the forgeability of a commercial wrought AZ31B magnesium alloy (Mg-3%Al-1%Zn) at room temperature, backward extrusion is carried out with applying counter pressure. A counter pressure is applied from a die exit of backward extrusion. By applying counter pressures of 100–200 MPa during backward extrusion, the critical punch stroke for fracturing is improved by 20% because the ductility increases under a high pressure. To predict the occurrence of fracturing of magnesium alloys, the distributions of the stress, strain and temperature during forging are calculated by the finite element simulation because the existing fracture criteria are not adequate to predict the occurrence of fracturing of magnesium alloys in forging. The mechanism of fracturing is discussed on the basis of plastic deformation, and a fracture criterion of magnesium alloy in cold forging is suggested. The newly proposed criterion provides much better results than the existing criteria.