| The rapid development of automobile and aerospace industies gives birth to the demand of alluminum castings with hollow configurations or internal channels, and results in resin sand core with high tensile strength being wildly used in manufacturing these castings, which causing serious enviromental pollution. Resin sand core also has bad colapsibility and high gas evolution. These can be settled by water soluble core, but its low strength and high hydroscopicity limits its application in practice. In the present dissertation, advantages and disadvantages of present water soluble core used in alluminum casting was summarized to develop inorganic salt refractory core. Afterward, methods and mechanisms of improving the strength and the humidity resistance of water soluble core were systematically researched. And then the process and the fomula of water soluble core with high strength and low hydroscopicity were obtained.The main contents of this paper were as follows:1) Influence of inorganic salts, base sands, heating temperature and heating time on tensile strength was investigated; the process and the basic fomula with the tensile strength 0.5MPa were obtained through orthogonal test optimization.2) The strengthening effect of the cores using different strengthening material was compared and the formula of the core with high strength was obtained. The results showed that:kaolin, magnesium borate whisker and diatomite could prominently improve the strength of the core and the tensile strength of the core could exceed 1.5MPa, which was two times higher than that of the basic fomula; the tensile strength of the core achieved 2.5MPa using composite strengthening method, which was 4 times higher than that of the basic fomula.3) Bonding mechanism and strengthening mechanism were analized. During the process of the core making, inorganic salt crystallized out to form bond briges from its solution which coated on the surface of sand and then the strength established; strengthening meterals could change the crystal morphology of the inorganic salt, turned flaky, massive and acicular structure to compact and smooth structure, which obviously improved the tensile strength.4) Hygroscopicity was established as the main index to evaluate the humidity resistance of the cores while surface hardness and tensle strength were the supplimentary, and the moisture absorption characteristic equation of the core was obtained. 5) Humidity resistance improved by precipitation method was investigated. The results showed that:the humidity resistance of the water soluble core treated by alcoholic solution dissolved with calcium chloride was improved prominently on the account of the porous surface coated by calcium chloride and the high humidity resistance products generated in the reaction between calcium chloride and potassium carbonate; under the condition of RH=75～85%, T=20～25℃, the strength loss of the water soluble core treated by preipitation method was 15%, which was 30% of that without any treated.6) The fomular of the water soluble core used in alluminum casting with high strength and low hygroscopicity was obtained and the mechanism of the colapsibility was investigated. The result showed that:the core made of the fomular with high strength and low hyproscopicity had high strength (2.5 Mpa), low gas evolution (6.2 mL/g), and good colapsibility and humidity resistance; potassium carbonate and silica sand could sinter at 600～800℃, but the sintering reaction was insufficiency and takes place at the solid-solid contact between potassium carbonate and silica, so cores without strengthening had good colapsibility on account of its porous structure and the property of potassium carbonate soluble in water easily; strengthening materials could not react with potassium carbonate, so there was no new sinter reaction in strengthening cores, which guaranteed that strengthening cores also had good colapsibilities. |
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