Study On The Performance And Hardened Process Of The Water-soluble Composite Sulfate Sand Core

The rapid development of aerospace and automotive industries has driven the increasing demand in the market for complex thin-walled light alloy castings. This kind of castings often possess complex intracavity structures, including elongated holes and thin-walled structures, which not only have high performance requirements for foundry sand cores, but also cause a problem that the casting parts are difficult to be clean after casting. In this paper, in order to overcome the poor leachability of the sand cores for complicated thin-walled magnesium/aluminum alloy casting, a water-soluble sand core using sulfate aqueous solution as a binder was investigated systematically, including material composition, properties and hardening process of the sand core. The water-soluble sand core possessed an excellent performance and outstanding collapsibility, which satisfied the production needs of the complex thin-walled light alloy castings.The performance characteristics of the water-soluble sulfate sand core prepared by different sulfate binders and different hardening processes were analyzed and compared.The results show that the water-soluble sand core prepared by simple substance sulfate binder(magnesium sulfate or aluminum sulfate) possesses a poor comprehensive properties, and the composite sulfate binder(Mg SO4+Al2(SO4)3 or Na2SO4) can increase the strength of the sand core. The twice microwave heating technology is suitable to harden the water-soluble sulfate sand core, which can exert adequately the bonding efficiency of the sulfate binder, and the sand core possesses high strength, low addition amount of the binder and high hardening efficiency, which accords with the development trend of green foundry.The performance characteristics of the water-soluble composite sulfate sand core hardened by twice microwave heating technology were tested and investigated. Two material composition formulas of water-soluble sand cores possessing excellent performance are obtained:(1) the raw sand is Dalin scrubbed silica sand, and thecomposite sulfate binder consists of Mg SO4 solution mass fraction of 30%, Mg SO4 solution addition amount of 5%(by weight of the raw sand, the same as below) and Al2(SO4)3�18H2O crystals addition amount of 0.5%.(2) the raw sand is Dalin scrubbed silica sand, and the composite sulfate binder consists of Mg SO4 solution mass fraction of 30%, Mg SO4 solution addition amount of 5% and Na2SO4�10H2O crystals addition amount of 0.5%. The two kinds of water-soluble composite sulfate sand cores both possess a better comprehensive performance with room temperature strength of 1.2MPa,4h storage strength above 1MPa in a moist environment(strength decrease rate is less than 20%). In addition, the two sand cores possess a lower water absorption rate and gas evolution with a high surface stability, which can be used for practical production.The practical casting test of magnesium alloy castings was carried out with water-soluble composite sulfate sand cores. The results show that the magnesium alloy castings in still water can be cleaned easily, and its internal water-soluble composite sulfate sand core can rapidly dissolve within 15 s. Besides the intracavity surface of castings is smooth, and there is no obvious surface defects as well.The micro-morphology of water-soluble composite sulfate sand cores was analyzed and studied. The microscopic analysis indicates that the composite sulfate binder can form bonding bridges which possess high bonding strength among sand grains, and so the sand core represents high strength in macroscopic view. The composite sulfate binder absorbs water in a moist environment and then causes that bonding bridges appear micro-holes or cracks, which is the reason that the bonding strength of bonding bridges decreases, then the strength of the sand core decreases in macroscopic view.