Study On Technology Optimization And Quality Control Of Low-pressure Expendable Pattern Casting Process For Magnesium (Aluminum) Alloy

The low-pressure expendable pattern casting (LP-EPC) process applies the principle of low-pressure casting to expendable pattern casting (EPC), in which the melt metal is filled under anti-gravity condition at low pressure. It obviously gains the advantages over die-casting, sand casting and conventional EPC process. In the present dissertation, the effect of main processing parameters on LP-EPC castings has been investigated systematically, which aims at the application of LP-EPC. Based on the modification of magnesium alloy, this study focuses on the methods to modify as-cast microstructure of magnesium alloy and improve its mechanical property.Based on analysis of theory in conventional EPC process, the technical parameters of LP-EPC were discussed. And the relations linking lifting pressure velocity (dP充)/(dτ) with gas flux Q , as well as the mechanics of solidification feeding in LP-EPC process were established. It was pointed that linear relation was existed between lifting pressure velocity (dP充)/(dτ) and gas fluxQ when the initial pressure of air supply was a certain value. The more gas fluxQ flowed in the crucible, the faster filling velocity. The grain number N of feeding melt metal flowing per unit of time at identical inter-granular location is mainly depended on holding pressure P_保when the alloy composition and pouring temperature have been selected. As the holding pressure increases, the grain number of feeding melt metal flowing would increase, the feeding capacity would become intensive and the microstructure would become more compact also.In this study, researches are focused on the effect of process parameters on the LP-EPC casting quality of AZ91D by using ladder samples. Those samples were tested for different combinations of the LP-EPC process parameters. Specifically, pouring temperature, vacuum level, filling velocity and coupling action of above factors were manipulated to observe their effect on the porosity and density distribution of casting. The optimal process parameters for the castings are pouring temperature 710～750℃, vacuum level 0.02-0.03MPa, gas flux 8～14m3/h. The unpressurized gating system and coupled modes of filling several seconds before suction also be selected. Complicated thin-walled castings require an upper limit magnitude(e.g. pouring temperature 750℃),and simple thick-walled casting should use lower limited magnitude。The physical feature and genesis of typical casting defects of LP-EPC process for magnesium castings were investigated. The result indicated that hole defects could be avoided through reducing pouring temperature, filling velocity and vacuum level, improving the permeability of coating as well as adjusting coupled modes of filling mold and vacuum level. Misrun and cold laps can be restricted by proper adjustments on pouring temperature and filling velocity, etc. While burning-on defect can be eliminated by increasing coating thickness,decreasing pouring temperature and vacuum,selecting smaller size sand to mold.Mechanical vibration, Ce-Sb alloying and semi-solid isothermal heat-treatment were used to improve the structure and performance of AZ91D magnesium alloy via EPC process. The result indicated that above three methods could refine its microstructure and improve tensile strength at room temperature:(1) The performance of AZ91D alloy via EPC process was improved when the casting was produced under mechanical vibration, and the microstructure evolved refiner and mechanical property became higher as the thickness of sample increase.(2) The optimal alloying composition was AZ91D with the addition of 0.4% Sb and1.0% Ce. Some new granulated CeSb and rod-shaped Al11Ce3 phases had been formed, which are present at the grain boundary and intracrystalline. Compared to AZ91D, the ultimate tensile strength and elongation of AZ91D-1.0%Ce-0.4%Sb alloy are enhanced by 35.5% and 36.7%, respectively. Ce-Sb alloying compound with mechanical vibration could further refine its microstructure and improve tensile strength. Its ultimate tensile strength and elongation were improved by11.2%,11%. The morphology of tensile fracture has more features of quasi-cleavage. It indicates that has had the bigger plastic deformation before failure.(3) When AZ91D magnesium is heat-treated at semi-solid temperature, the reticularβ-Mg17 Al12 phase almost solutionizes in matrix and primary grain gradually evolves into spheroidal shape. Its ultimate tensile strength and elongation were improved by32.4%,83.4%. The optimal process parameters of semi-solid isothermal heat-treatment are 560℃×60min, cooling in water.The trail manufacture of plate-fin cooler and compressor cylinder head indicated that LP-EPC possess has great mold filling ability which could largely reduce or overcome the defects like misrun and cold laps encountered in conventional EPC process. The complex thin-walled Mg (Al) alloy castings can be produced perfectly by LP-EPC, so the process is very suitable for casting Mg (Al) alloy parts with complex geometries and high dimensional tolerances.