Effects Of Alloying And Heat Treatment On The Microstructure And Property Of Az91d Magnesium Alloy Prepared By The Lfc Process

As"the 21st Century Green Engineering Material", Magnesium alloy has been widely used in modern industrial products. As "the 21st century casting technology", Lost Foam Casting(LFC) has broad development prospects. However, in LFC, dry sand mold is used and the solidification range of common magnesium alloy is broad, which leads grain of castings to be coarse and hence has confined the applications of the magnesium alloy in LFC. Alloying and heat treatment are the primary methods to strengthen magnesium alloy at present. In this paper, microstructures of the LFC AZ91D magnesium alloy were improved through three methods: adding rare-earth element Y singly, adding rare-earth element Y and Gd together, and heat treatment, respectively. Furthermore, test measures like OM, XRD, SEM, EDS, DTA, tensile test and hardness test are utilized to study changing law for microstructure, solidification characteristic and mechanical property of the LFC AZ91D magnesium alloy. This research has obtained rational alloying scheme and heat-treatment scheme, and has explored the mechanisms of improving microstructure and enhancing mechanical property.Adding Y element singly to LFC AZ91D magnesium alloy could make the morphology ofβ-Mg17Al12 phase turn into intermittent and granular forms and make grains refined. When the content of Y was up to 1.0wt%, the effect of refinement was the best. As Y increased gradually, the primary nucleation temperature ofα-Mg was heightened gradually and the eutectic reaction temperature was dropped, but when the content of Y was increased to a certain extent, eutectic reaction temperature would be re-heightened due to the accumulation and growth of Al2Y phase. Mechanical property of LFC AZ91D magnesium alloy was improved through adding Y. When the content of Y is 1.0wt%, the maximum values of the tensile strength, elongation rate and hardness were all up to their maximum values which are 153.54MPa, 2.62%, 58.1HB, increasing 12.8%, 44.8%, 7.0% respectively compared with common AZ91D magnesium alloy.Keeping the content of Y at 0.6wt% stably and increasing the content of Gd, grain size ofα-Mg matrix shrunk, meanwhile, the shape ofβ-Mg17Al12 phase turned into intermittent and granular forms and grains were refined. When the content of Gd was 0.9wt%, the effect of refinement was the best, and which was better than the effect when adding 1.0%wt Y singly. As Gd increased gradually, the primary nucleation temperature ofα-Mg heightened gradually, and when the content of Gd was less than 0.9wt%, the eutectic reaction temperature dropped gradually. Mechanical property of LFC AZ91D magnesium alloy was improved remarkably through adding Y and Gd together. When the contents of Y and Gd are 0.6wt% and 0.9wt%, the tensile strength, elongation rate and hardness were up to their maximum values which are 161.68MPa, 2.80%, 64.7HB respectively, increasing 18.8%, 54.7%, 19.2% respectively compared with AZ91D magnesium alloy.In the LFC AZ91D magnesium alloy and AZ91D+Y magnesium alloy,β-phase dissolved intoα-Mg matrix after solution treatment. For the AZ91D+Y magnesium alloy, Al2Y phase had still existed inα-Mg matrix steadily after solution treatment, because it is heat-stable phase. After solution treatment, the values of tensile strength and elongation of the two alloys were increased, but hardness were decreased. In the process of subsequent aging treatment, theβ-phase precipitated from supersaturatedα-Mg matrix. For the AZ91D+Y magnesium alloy, the rate ofβ-phase precipitating became slow, because nucleation and growth ofβ-phase was hold back by Al2Y phase. After T6 treatment, the values of tensile strength and hardness of the two alloys were increased, but elongation rate were decreased, furthermore, Y played a role of delaying the age-hardening on the AZ91D magnesium alloy. For LFC AZ91D magnesium alloy, the best scheme to gain comprehensive mechanical properties is solution treated at a temperature of 420℃for 20 h and then ageing treated at a temperature of 250℃for 20 h while for LFC AZ91D +1.0% Y magnesium alloy is solution treated at a temperature of 420℃for 20 h and then ageing treated at a temperature of 250℃for 25 h.