Study On Processign Technique And Properties Of Semisolid AZ91D Alloy

The rheo-forming technology of semi-solid metals has a wide application foreground. But by now, the study on the rheo-forming processing of the semi-solid magnesium alloy is still at the starting stage. In this thesis, the influences of the several melt treatment processinges (namely, modification treatment processing, electromagnetic stirring processing,"modification treatment and electromagnetic stirring"composite processing) on the microstructures, the mechanical properties and corrosion resistance of AZ91D alloy, have been investigated. And the processing to prepare the semi-solid slurry of AZ91D alloy has been explored, providing the basis for the further study on the rheo-forming technology of semi-solid AZ91D alloy.The influences of the melt treatment parameters (the addition of nano-sized carbon powder, cooling rate, electromagnetic stirring time, stirring manner, reversing time, frequency and temperature of beginning stirring) on the grain size and morphology of initial grain (α-Mg) and the second phase (β-Mg17Al12) of the AZ91D alloy, have been synthetically investigated. The experimental results indicate that the effect of the modification treatment on refining the AZ91D alloys grain is better when the addition of nano-sized carbon powder is more than 0.1%. The effects of the reversible electromagnetic stirring processing on the grain size and morphology of initialα-Mg are obvious. The grain size is about 70μm, the shape factor is about 0.85, and theβphase distributes continuously on the border of theα-Mg like network. The"modification treatment with carbon and reversible electromagnetic stirring"composite processing improves the quality of the semi-solid AZ91D alloy slurry. The grain size of initialα-Mg is approximately 55μm, the shape factor is about 0.85 and theβphase is more continuous and uniform with the network shape on the grain border.The experimental results of the further study indicate that the mechanical properties and corrosion resistance of AZ91D alloy are improved in different degree during the melt treatment processinges. The AZ91D alloy has the best mechanical properties under the condition of the"modification treatment with carbon and electromagnetic stirring"composite processing. The tensile strength increases from about 140MPa to 180MPa around, and the elongation increases from about 3% to 5% around. The tensile strength increases to 220MPa under the conditions of T4 and T6 treatment, and the T4 treatment improves the elongation while the effect of T6 treatment on the elongation is not significant. The T6 treatment improves the corrosion resistance of AZ91D alloy, but the T4 treatment reduces the corrosion resistance. The AZ91D alloy has the best corrosion resistance under the condition of the"modification treatment with carbon and electromagnetic stirring"composite processing, which there is a little influence of the T4 and T6 treatment on the corrosion resistance.The results show that the"modification treatment with carbon and electromagnetic stirring"composite processing is the effective processing to prepare the semi-solid slurry of AZ91D alloy, laying the foundation for the development of the rheo-forming technology of semi-solid magnesium alloys.