Manufacture of Magnesium Tubes with Gradient Hardness Distribution Using a Two-Stage Porthole Extrusion Die

Abstract

This paper aims to manufacture magnesium alloy tubes with gradient hardness using hot extrusion process. A two-stage porthole die together with a mandrel is designed to have a straight channel part combined with a conical part. Materials are pushed through this specially-designed die cavity to generate a non-uniform velocity distribution at cross sections between the mandrel and the die and result in different strain and strain rate distributions. Accordingly, a gradient microstructure or hardness product can be obtained. Using the finite element analysis, the effective strain distributions inside the die cavity and at the die exit are firstly discussed for various inclination angles in the conical part of the mandrel. Then, hot extrusion experiments with a two stage porthole die set are conducted to obtain magnesium alloy products with gradient microstructures and hardness. Using a die set of mandrel inclination angle of 10o and die inclination angle of 25o, gradient microstructures of grain sizes of 4.30μm, 5.92 μm and 3.67μm at the outer surface, center zone, and inner surface, respectively, are achieved.