Research On The Expendable Pattern-Shell Compound Casting Of AZ91D Magnesium Alloy

:With rapid development of the aerospace, automotive,3c industry, magnesium alloy castings are widely used. Nowadays, the forming technology of magnesium alloy include die casting, investment casting, sand casting, lost foam casting(LFC). However, these methods are difficult to meet requirements of magnesium alloy castings.In this thesis, foam pattern preparation of LFC and shell precision fabrication of investment casting are combined to form magnesium alloy expendable pattern-shell compound casting technique. The technique possesses some advantages:low-cost, small shrinkage, dimensional stability of the foam pattern, structural design flexibility; high accuracy of the ceramic shell. This thesis focused on the basic research of magnesium alloy expendable pattern-shell compound casting:mainly including ceramic shell fabrication based on foam pattern; optimization of expendable pattern-shell compound casting process parameters for magnesium alloy. Improve the mechanical properties of AZ91D alloy, the effect of mechanical vibration on the microstructure and mechanical properties of AZ91D alloy under expendable pattern-shell compound casting were investigated.Using foam cutting machine cut foam pattern, the sol binder and corundum refractory to prepare a thin-walled shell of magnesium alloy, the relationship and law of the coating powder-liquid ratio, the coating viscosity, sintering temperature and other parameters was investigated on the shell bending strength, fracture morphology. The results show that: with coating powder-liquid ratio, the coating viscosity is increasing, but the shell strength first increase and then decreases. When powder-liquid ratio is3.2, the shell bending strength reaches maximum4.0MPa. With sintering temperature from800℃to1200℃, the shell bending strength is slowly increased, when the sintering temperature is1200℃, the shell bending strength reaches maximum4.3MPa. Sol-corundum shell has high strength, and can meet the need of magnesium alloy expendable pattern-shell compound casting. Considering shell strength for the casting needed, the appropriate baking temperature is800℃. By change of pouring temperature and shell preheating temperature of expendable pattern-shell compound casting AZ91D shows:AZ91D magnesium alloy microstructures consisted of the matrix a-Mg and the second phase β-Mg17Al12, in addition to the small amount of Al8Mn5phase. With improvement of pouring temperature and shell preheating temperature, grain coarsening and the distribution of the second phases are turned from refining and scattering to meshing. The mechanical properties of the alloy are decreased, because the casting solidification conditions have a significant effect on AZ91D alloy microstructures and properties, higher pouring temperature or shell preheating temperature will decrease the liquated magnesium alloy cooling rate, which result in coarse casting structure and poor mechanical properties.The effect of vibration frequency on microstructures and properties of AZ91D magnesium alloy of expendable pattern-shell compound casting have investigated. Mechanical vibration forces, the matrix α-Mg from coarse dendrites to bulk, while the grain size decreases with increase of vibration frequency. When the vibration frequency is30HZ, the average grain size of AZ91D alloy is141μm less21%than no vibration. Alloy’s tensile strength, elongation and hardness all increase with increase of vibration frequency. Through vibration frequency of30HZ, the tensile strength, elongation and hardness are205MPa,6.88%and65.93HV, increased by14.5%,75%and5.3%, respectively, than no vibration.