Study On Microstructure And Mechanical Property Of ZM6 Heat-resistant Magnesium Alloy Prepared By Solid State Recycling

At present, magnesium alloy is the lightest metal structural materials. It has many advantages including low density, good specific strength and stiffness, steady dimension, good machinability and excellent damping capacity. Magnesium alloy is being used in aviation, spaceflight, automotive, computer, communications and electronic industries. Because mechanical and creep properties at elevated temperature of magnesium alloy are inferior, its applications at elevated temperature are limited. Mechanical and creep properties of magnesium alloy at elevated temperature are improved by adding lanthanon and heat-resistant magnesium alloy is made.In this paper, ZM6 heat-resistant magnesium alloy chips and scraps were prepared by solid state recycling. Microanalysis and test of mechanical properties were carried out by transmission electron microscope, scanning eletron microscope, optical microscope, inductively coupled plasma-atomic emission Spectrometry, electron universal strength testing machine and creep testing machine. Composition of the alloy, microstructural evolution, tensile properties, creep properties and fracture behavior were investigated. Strengthening mechanism and fracture mechanism of the alloy at different conditions were discussed.Effect of technologic parameters on micrcostructures and mechanical properties of ZM6 magnesium alloy prepared by solid state recycling were studied and plastic characteristics in solid state recycling were analyzed. At first, ZM6 magnesium alloy chips were cold-pressed to form a compact billet with the pressure of 350MPa. The density of the billet was 1.71g/cm³ and the surface of the chips exhibited some breaking and bonding. With extrusion ratios and temperatures increasing, grains of ZM6 magnesium alloy became refiner and tensile strength and elongation of the alloy improved. Recycled specimen that was prepared with extrusion temperature of 500℃and extrusion ratio of 25:1 showed higher mechanical property. Its ultimate tensile strength and elongation to failure were 280MPa and 29.7%, respectively. Ultimate tensile strength of ZM6 magnesium alloy prepared by solid state recycling was 171.9MPa at 250℃.Microstructural evolution, tensile properties and fracture mode of ZM6 magnesium alloy prepared by solid state recycling during ageing treatment were investigated. At the beginning of ageing, The discontinuous precipitation occurred near the grain boundaries. Precipitates became more and uneven distribution with aging time increasing. When aging time reached 16 hour, tensile strength of the alloy was maximal and main strengthening phases of the alloy wereβ' phase andβphase. Fracture mode of the alloy was mainly transgranular tearing fracture at the initial stage of ageing. Fracture mode of the alloy was a mix mechanism with transgranular dimple fracture and intergranular fracture in the peak-ageing. Bigger second phases had largely effected on fracture in the further-ageing.Mechanical properties of ZM6 heat-resistant magnesium alloy at different conditions were studied. Ultimate tensile strength and elongation of the cast alloy were 141.3MPa and 3.3%, respectively. Strength and ductility of the alloy were largely improved because microstructural state at the grain boundaries was destroyed through extrusion. Ultimate tensile strength of the alloy prepared by chip extrusion was equal to ultimate tensile strength of the alloy prepared by cast extrusion. Ductility of the alloy prepared by chip extrusion was lower than that of the alloy prepared by cast extrusion. Ultimate tensile strength of the alloy prepared by solid state recycling lightly increased and elongation to failure decreased after T5. Ultimate tensile strength of the alloy prepared by solid state recycling obviously decreased and elongation to failure increased after T4.Effect of Ce on microstructures and mechanical properties of ZM6 magnesium alloy prepared by solid state recycling was investigated. After the first extrusion, tensile strength and elongation to failure of the alloy were very low because Mg-Ce intermediate alloy chips have not been broken. Tensile strength and elongation to failure of the alloy were obviously improved because of well-proportioned distribution of Mg-Ce intermediate alloy chips after the fifth extrusion. Ultimate tensile strength and elongation to failure of the alloy increased with the extrusion times increasing. Increasing range decreased with the extrusion times increasing. The alloy was made ofα-Mg phase, Mg₁₂Ce phse, Mg₁₂Nd phase and MgO phase after the fifth extrusion. Ultimate tensile strength and elongation to failure of the alloy were 300MPa and 14.8%, respectively.Creep properties of ZM6 heat resistant magnesium alloy prepared by solid state recycling were studied. At 200℃temperature, creep properties of the alloy decreased and creep strain of Class I creep increased with the creep stress increasing. At 110MPa stress, creep properties of the alloy decreased and time of Class I creep reduced with the creep temperature increasing. The stress exponent and the activation energy of ZM6 magnesium alloy were 4.4 and 104kJ/mol. ZM6 magnesium alloy was controlled by the dislocations climbing mechanism.