| With the rapid development of aerospace industry in our country, the demand forlarge-complicated and thin-walled castings is increasing greatly, and the light-weight ofaerospace-structural castings is becoming more important. Magnesium-alloy is one of themost potential materials in manufacturing aerospace-structural castings for it’s low-density,high-strength and good damping property.The formula of gypsum-mold of magnesium-alloy was studied in-depth in this paper.And the proportion of main material as following,-hemihydrate gypsum: magnesiumsulfate: quartz powder: bauxite powder=30:20:25:25. The effect of Bauxite powder andquartz powder could guarantee dimensional stability and prevent gypsum-mold fromcracking; and the magnesium sulfate could increase collapsibility of gypsum-mold inwater greatly. A variety of additives were added in order to improve the performance ofgypsum-mold, such as polycarboxylate-type superplasticizers, and it’s effect is reducingthe consumption of water and improving the fluidity of fresh gypsum plaster and strengthof gypsum-mold; The urea and barium chloride could reduce the linear shrinkage ofgypsum-mold; The trisodium phosphate could adjust the setting time of fresh gypsumplaster. In adition, The defoamers and high-temperature glass fiber could decrease bubbleof fresh gypsum plaster and crack of gypsum-mold. The gypsum-mold of this paper hashigh strength, low linear shrinkage and easy collapsibility in water.The migration of water and it’s impact to gypsum-mold were studied whenlarge-scale gypsum-mold (200mm200mm200mm) was drying. The process of dryingand roasting were combined with the loss of SLS (Selective Laser Sintering), The processof drying and roasting of large-scale gypsum-mold was determined, the process asfollowing,120℃×36h,200℃12h,300℃8h,500℃8h.The experimental result showed that the oxidation of magnesium-alloy casting wasserious when casting in gypsum-mold. The most important and difficult is preventingmagnesium-alloy casting from oxidation. The flake graphite at4%～6%which mixing ingypsum-mold could reduce the oxidation of magnesium-alloy casting obviously. The large-area black oxide on casting was eliminated, but there is still litter oxide on thesurface of casting. A small amount of sulfur which blowing into cavity of gypsum-moldcould eliminate oxidation successfully, and the magnesium-alloy casting was clean andbrightly. Nevertheless, it’s difficult to contorl the amount of sulfur. Therefore, One newvacuum forming process of gypsum-mold was invented in this paper. The process asfollowing, the vacuum equipment was connected with holes which on the upper of cavitythrough the stainless steel tube. The cavity is in a vacuum state before pouring and pouring.Not only could the process improve the filling capacity of liquid magnesium-alloy, butaslo reduce the oxidation of magnesium-alloy castings. The above-mentioned compositeprocesses could eliminate the oxidation of magnesium-alloy when casting ingypsum-mold completely.The process of filling and solidification of one typical complicated and thin-walledcastings were simulated using the numerical simulation software, and then the castingprocess was optimized. At last, some complicated and thin-walled magnesium-alloycastings were casted, and the superiority of vacuum-investment casting in complicated andthin-walled magnesium-alloy of gypsum-mold was proved. The defect characteristics andformation reason in vacuum-investment casting of complicated and thin-walledmagnesium-alloy of gypsum-mold were investigated, and the prevention measures basedon casting defects were proposed. |
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