Microstructures And Properties Of Vacuum Die Casting New Type Magnesium Alloys

Vacuum die casting magnesium alloys contain low porosity and lowcasting defects. The application of heat treatments on vacuum die castingmagnesium alloys achieves fine internal microstructures and improvesmechanical properties, corrosion resistance of the alloys. However,researches on the effect of heat treatments on vacuum die castingmagnesium alloys are quite few up to now. This thesis designs two vacuumdie casting magnesium alloys. Applying heat treatments on Mg-7Al-2Sn(wt.%) alloys and Mg-8Gd-3Y-0.4Zr (wt.%) alloys, it studied themicrostructures, corrosion resistance, porosity contents, mechanicalproperties and fracture behavior of these alloys.The results of vacuum die casting Mg-7Al-2Sn (wt.%) alloy (i.e AT72alloy) show that the microstructures of as-cast alloy consist of primaryα-Mg, Mg17Al12and Mg2Sn phases. Most of Mg17Al12phases dissolve afterT4treatment(410℃/16h), while Mg2Sn phases mostly still remain. Themicrostructure of T5-treated (150℃/16h) alloys contain coarse Mg17Al12phases and precipitated phases. The microstructures of T6-treatedalloysmainly contain small massive precipitated phases.The corrosion rates of T5-treated alloy (150℃/16h) are the lowestamong all the heat-treated alloys, which is related with the intensivemicrostructures of the alloy. However, the T4-treated and T6-treatedAT72alloys have serious corrosion condition due to the dispersedmicrostructures of Mg2Sn phases. The Mg2Sn phases form galvaniccorrosion with α-Mg matrix and cause severe corrosion. In this way, theoptimized heat treatment for improving corrosion resistance of vacuum die casting AT72magnesium alloys is T5treatment.The application of optical microscope is to calculate porosity contents ofMg-8Gd-3Y-0.4Zr (wt.%) alloys (i.e GW83K alloy), which turned out tobe0.268%. The microstructures of as-cast alloy mainly contain α-Mg andeutectic compound Mg24(Gd, Y)5which is around grain boundaries. Mostof the eutectic compounds dissolve into the matrix after solution treatment,while a cuboid-shaped phase which mainly contain Gd and Y elements getgrowth. The optimizedT4-treatment for fine grain size is475℃×2h.The elongation of T4-treated GW83K alloys improves greatly comparedwith as-cast alloy. The alloys T4-treated at500℃contain15%ofelongation and222MPa of ultimate tensile strength. The yield strength andultimate tensile strength of GW83K T6-treated alloys increase significantlywith the nano-scale metastable phases precipitated during ageing process.The fracture mode of as-cast alloy is ductile fracture+quasi-cleavagefracture. The fracture modes of T4-treated and T6-treated alloys arequasi-cleavage fracture. This shows that the as-cast alloy presents betterductility.