Research On The Ceramic Shell Fabrication And Application Of The Expendable Pattern-Shell Compound Casting

The ceramic shell casting process based on expandable pattern possesses some advantages both of lost foam casting and lost wax casting. It can be used to eliminate carburization of low-carbon steel castings in LFC process and reduce size over-tolerance of large-scale castings in lost wax casting process. It has advantages in the production of low-carbon steel castings and large precise castings. The papre attempts to introduce this new shell casting process to the production of aluminum and magnesium alloys, and concentrates on fabrication process of ceramic shells. The effect of process parameters and materials selection on the quality of ceramic shells has been investegated through many experiments, and characteristics and properties of aluminum(magnesium) alloys poured under gravity or antigravity condition are compared and analyzed. This new shell casting process shows broad prospects in the field of military and aerospace industry.Rapid fabrication of high-strength and low-roughness ceramic shell base on EPS pattern, and mechanism of shell's cracking when removing foam pattern and roasting are studied systematically. The results show that ceramic shells produced with silica sol as binder and bauxite as refractoty filler and stucco produce much higher wet-strength and high-temperature strength, 1.60MPa,2.92MPa respectively; adding of commonly used organic binders into silica sol, slurries or refractory stucco was investigated in the paper. The results show that adding of organic binder into refractory stucco can increase the wet-strength of shells. Among the investigated binders, PVA produces the largest increase rate, up to 20%, and increase rates of dextrin, CMC,α-starch are 11.3%, 9.4% and 5.0%, respectively. A series of experiments were conducted to determine the cracking mechanism of shells when removing foam pattern and roasting shells. The shell was heated immediately after been put into the roasting furnace and then shrinked sharply. Because of the heat resistance of the shell, the temperature of the foam pattern rises slowly, and its dimention-size keeps stable. The thrinkage of the shell was hindered by the foam pattern, as a result, the shell cracked.The surface-smoothing coatings for EPS patterns were prepared to improve the surface roughness of castings, and the results showed that: the uniform and compact smoothing film outside the foam pattern formed by the nitrocellulose coating and molten wax can not only fill in grooves, but also prevent facecoat slurry from infiltrating inner void. Therefore, striated network structure on the surface of shells and burrs are eliminated, which greatly improves the surface quality of shells. The results show that, The nitrocellulose coating would decrease the surface roughness of the foam pattern and the shell fabricated by it from 3.0μm, 4.5μm to 0.87μm, 1.7μm respectively, and correspondingly 1.1μm, 1.9μm for the wax film. Furthermore, the surface roughness of shells decreases with the increasing mesh number of filler. Compared to sodium silicate, silica sol and hydrolysates of ethyl silicate employed to prepare the facecoat slurry would produce a 4~5um lower surface roughness of castings. This is due to reason that Na2O in the sodium silicate could react with SiO2 and generate low melting point compounds.The casting simulation software was used to contrast and analyse the characteristics of mold-filling and solidification in the lost foam casting and this ceramic shell casting process. The results show that, because the foam prototypes are removed before pouring, the filling capacity of the molten metal can be improved for the ceramic shell casting process. Low pressure pouring process can brings stable mold filling of molten metal, and the thrinkage of moten metal and its solidification can also be filled fully. In our experiments, low pressure shell casting process produce highest tensile strength, elongation, hardness of ZL101 alloy, up to 278.27MPa,8.10%,93.1HBS respectively, while, which correspondingly are 20.5%, 166.4%, 17.6% greater than those of castings produced by common lost foam casting.