Extrusion Limit Diagram of Mg-5bi-3al-1zn Alloy and Effects of Extrusion Parameters on its Microstructures and Mechanical Properties

Abstract

A Mg-5Bi-3Al-1Zn (wt.%) alloy, in this work, with a great potential on excellent extrudability and high mechanical properties was carried out hot-compressive tests and finite element simulations at different extrusion conditions to construct an extrusion limit diagram. Based on the pressure limit and cracking limit, a safe extrusion processing zone was established. With ~ 310 °C as the dividing line, when the extrusion temperature was below this value, the safe extrusion zone was narrow because the stress required to induce the plastic flow of alloy was higher than the pressure limit. Once this critical temperature was exceeded, high incipient melting temperature caused a wide safe extrusion zone controlled by the cracking limit. The theoretical maximum ram extrusion speed was calculated as ~ 48 mm/s at ~ 310 °C. As this alloy was extruded at 30 and 44 mm/s, no hot cracks occurred on the alloy surface; while at 56 mm/s, there existed a large amount of orange peel defects. Through further microstructural observation of 44 mm/s extruded (E44) alloy, it exhibited a complete dynamic recrystallization structure with an average grain size of ~ 21.9 μm, along with a relatively weak basal fiber texture (maximum pole intensity of ~ 5.8) and plentiful nano-Mg3Bi2 phases. These microstructure features co-dominated the E44 alloy showing an excellent tensile data (tensile yield strength of ~ 270 MPa, ultimate tensile strength of ~ 375 MPa and elongation of ~ 15%) at high-speed extrusion.

Keywords: Mg-5Bi-3Al-1Zn alloy, Extrusion limit diagram, Extrusion parameter, microstructure, mechanical properties