Research On Microstructure Characteristics And The Change Of Performance Of Magnesium Alloy Solidified Under Pressure

Magnesium alloys will increasingly widely used in automotive, electronics, defense industry, aerospace, appliances and other fields, because of low density, high specific stiffness and strength, good damping properties and many other advantages. But in the practical application, strength, toughness and corrosion resistance, etc. of magnesium alloy are poor. This study strives to improve the structure of the magnesium alloy and increase the mechanical properties of magnesium alloys. During the experiment through pressurized solidification, by changing pressures of applying on magnesium alloy(AZ31 and AZ81) alloy liquid and solidification process pressure conditions(atmospheric pressure, static pressure, centrifugal pressure), solidification microstructures and mechanical properties of AZ31 and AZ81 magnesium alloy are researched in different pressure conditions, analysis impact behavior of pressure conditions on morphology and mechanical properties in the solidification process are analyzed, and phases evolution of the magnesium alloy in microstructure are explored at different pressures.The results show that:As the pressure gradually increases that from atmospheric pressure to additional pressure head pressurized of liquid metal(referred to static pressure) to centrifugation pressurized in magnesium alloy solidification process, microstructure of obtained samples had the following characteristics: two kinds solidification structure of AZ31, AZ81 have different degrees refinement, solidification precipitated nonequilibrium second phase ?-Mg17Al12 reduced accordingly. morphological distribution of the second phase of AZ31 along the grain boundaries change from net of normal pressure to discontinuous thin line of static pressure and then to dispersion small granules of centrifugal pressure. The average grain size under is 70.64 um in atmospheric pressure, when pressure increase to static pressure, centrifugal pressure, The average grain sizes reduced to 61.49 um, 47.59 um respectively. Volume fraction of second phase?-Mg17A112 are 25.89%(atmospheric pressure), 11.74%(static pressure), 4.37%( centrifugal pressure) respectively. Second phase of AZ81 atmospheric is coarse mesh, the second phase morphology of static pressure composed by thin line and the block small grain around boundaries. Finally, under the centrifugal pressure solidification, the second phase dispersed particles. The average grain size under atmospheric pressure is71.85 um, reduced to 63.05 um of static pressure, 52.81 um of centrifugal pressure. Volume fraction of second phase?-Mg17A112 are 31.28%(atmospheric pressure), 12.78%(static pressure), 6.96%( centrifugal pressure) respectively. Under the same pressure conditions, second phase precipitates of AZ81 more than of AZ31. Analysis pressure conditions on the effect of magnesium alloy microstructure evolution mechanism: First of all, with the increase of the solidification process pressure, melting point of alloy raises, super cooling degree and the nucleation rate are increase, so grains of AZ31 and AZ81 magnesium alloy refine with increasing pressure. Secondly, the pressure increases will reduce the diffusion rate of Al element, solid- liquid interface solidification speed is faster than the discharge rate of solute Al elements, a large number of Al elements dissolved in the primary phase, resulting in the amount of second phase precipitation in the microstructure reduce.As the solidification pressure increasing, strength of AZ31 and AZ81 alloy significantly improve, and tensile strength increased about twice compared with atmospheric samples. Tensile strength of AZ31 increases from 112.63 MPa of atmospheric pressure to 215.52 MPa of centrifugal pressure. Tensile strength of AZ81 increases from 106.51 MPa of atmospheric pressure to 197.45 MPa of centrifugal pressure. As Al element content of AZ81 than of AZ31, and at the same solidification pressure, the quantity of precipitated second phase of AZ81 solidification precipitation more than AZ31, resulting the tensile properties of AZ31 is better than AZ81. The results that Pressure conditions affect on the magnesium alloy performance show that: pressure results affect the main reason that mechanical properties have changes is microstructure changes due to the effects of pressure. After pressurized solidification magnesium alloy, grain refinement, solid solution strengthening and the small amount of second phase make the tensile properties of the magnesium alloy improved significantly.