Study On Core Materials And Cleaning Technology Of Complex Magnesium Alloy Castings

With the rapid development of aerospace industry, the demand of complex thin-walled magnesium alloy castings is continuously increased, and the structures of magnesium alloy castings are developed toward to integration, hollow thin wall, precision and complication. Therefore, sand cores used to fabricate interior structure of castings are very complex and difficult to be formed, and also difficult to be clean after casting due to the low thermal melting amount of liquid magnesium alloy. In order to improve the formability and cleaning of complex magnesium alloy castings, the formability and collapsibility of complicated cores were systematically investigated, which was under the support of the National Nature Science Foudation of China, named "controlling mechanism of complicated sand cores formability and collapsibility for magnesium alloy aviation castings". Also, high-performance easy-collapsibility mould cores and effective cleaning technology were specifically studied. The results of this dissertation has great theoretical and practical significance.(1) The high-performance water-soluble sand core was cured by new twice microwave heating process using aqueous solution of magnesium sulfate as a main binder. The binder material composition and cure parameters fit for water-soluble cores of complex magnesium alloy castings were determined. The performance characteristic of magnesium sulfate bonded sand cores and its strengthening technology of moisture resistance were researched. The mechanisms of bonding, moisture absorption and strengthening technology of moisture resistance were analyzed.The results show that the water-soluble magnesium sulfate bonded sand core cured by microwave heating possesses many advantages including high room-temperature strength, fine surface stability, good high-temperature collapsibility, high water-solubility rate and low gas evolution, but its moisture resistance is poor. The moisture resistance of the water-soluble magnesium sulfate bonded sand core can be improved by modifying aqueous solution of magnesium sulfate and the modifiers are aluminum sulfate or kaolin. The moisture resistance can be greatly increased by surface coating strengthening methods including brushing paraffin or geoline. The water-soluble magnesium sulfate bonded sand core after casting magnesium alloy castings can be water-dissolved in a short time.(2) The soluble alumina-based core was fabricated by slip casting and low temperature sintering process using bauxite powder as refractory, silica sol as binder and light magnesia powder as curing agent for the first time. The effects of different process parameters on the properties of cores were systematically investigated. The relational expressions bewteen performances of core and its influencing factors were established. The mechanisms of slurry curing, strengthened sintering and solubility of core were discussed.The results show that the better formula of soluble alumina-based core is as follow: powder-liquid ratio 2.4-2.6 g/ml, curing agent content 1-2 wt.%, coarse powder content 0-20 wt.%. The better sintering process is as follow:sintering temperature 800?900?, holding time 2-3 h. The rise of sintering temperature can increase the content of 3Ak2O3·2SiO2 phase. Titania powder can improve strength, decrease soluble rate and apparent porosity of cores. Borate powder can greatly improve strength, decrease soluble rate and apparent porosity of cores. Quartz powder can improve soluble rate and apparent porosity, decrease strength of cores. Alkali liquor of sodium hydroxide can quickly dissolve alumina-based cores.(3) A new composite cleaning method of core was proposed, which synthesizes chemical dissolution, vibration crushing and ultrasonic cavitation. The effects of different process parameters on the cleaning efficiency of cores were tested by using a developed core cleaning device. The optimized process of core cleaning was confirmed and the mechanisms of chemical dissolution, vibration crushing and ultrasonic cavitation were analyzed.The results show that the new cleaning method and device can quickly clean residual cores in the inner chamber. The optimized process parameters are as follows:5 wt.% concentration of NaOH cleaning solution,2/18? eccentric block angle of vibration motor, 1200 W ultrasonic power. The main purpose of the alkaline cleaning solution is to dissolve and soften core, the purpose of vibration crushing is to quickly fall off the reactant from core and cleaning solution, and broken core utilizing inertia force, and the purpose of ultrasonic is to accelerate the cleaning efficiency using local high pressure and temperature due to ultrasonic cavitation effect.(4) The actual casting and cleaning of magnesium alloy component were investigated using self-developed core materials and cleaning device. The change of heating temperature of core during casting of complicated magnesium alloy component was simulated. The actual result of magnesium alloy component with complex inner cavity formed by different cores was compared.The results show that the heating temperature of core is low due to the low thermal melting amount of liquid magnesium alloy during casting, and the time of the heating temperature over 300? is short, and it makes residual cores in the inner chamber of magnesium alloy component difficult to be collapsed by low temperature heating. The complete casting and effective cleaning of magnesium alloy component with complex inner cavity were achieved using self-developed core materials and cleaning device, and it confirms that the research results can be applied for the actual casting of magnesium alloy component with complex inner cavity.