Investigation Of The Performance Characteristics Of Typical Water-soluble Cores For Magnesium Alloy Casting

With the development of the aviation industry and the automobile industry, therehave been increasing demands for the complex thin-walled magnesium castings. Thestructure of casting parts are developing towards to integration, complication, andprecision, which exerts harsher requirements on the performance of the foundry coresand then makes the cores more complex and the cleaning of casting cavities moredifficult. As the most widely used cores in manufacturing these castings at present, theresin sand cores have a serious defect when used in the casting of complex magnesiumalloy parts due to their poor collapsibility. In comparison, water-soluble cores whichpossess outstanding collapsibility and thus make the castings cavities easy to clean, areexpected to overcome the difficulties in the casting of complex thin-walled magnesiumparts. In the present dissertation, the performance characteristics of three typicalwater-soluble cores were investigated, which laid a foundation for developing new typesof water-soluble cores and the corresponding forming method for the castings ofcomplex thin-walled magnesium parts. The research contents and results were asfollows:(1) The sodium silicate sand cores using different raw sand were hardened by twicemicrowave heating technology, and the performance characteristics of the cores wereresearched. The results showed that compared with the sodium silicate sand cores usingthe Dalin sand as raw sand, the sodium silicate sand cores using the ceramsite possessedhigher tensile strength and better humidity resistance. However, the ceramsite becamesintered with the sodium silicate when the temperature was higher than550℃, whichdeteriorated the collapsibility. The tensile strength and humidity resistance of thesodium silicate sand cores which used the mixed sand of the ceramsite and the Dalinsand as raw sand were poorer than those of the sodium silicate sand cores using theceramsite as raw sand, but were much better than those of the sodium silicate sand cores using the Dalin raw sand. Even though the amount of the binder for the mixed sand was1%, the tensile strength of the cores would be up to0.75MPa, and half of the strengthcould be kept after the cores were placed4hours in the air. The uneven expansion orcontraction of the mixed sand could counteract the sintering of the ceramsite and thesodium silicate, and led to a good collapsibility of the cores.(2) The effects of the salt binders, the raw sands, the microwave heatingparameters and the reinforcements on the strength of water-soluble sand cores werestudied. The results showed that the MgSO4water-soluble sand cores had high tensilestrength, and the strength could be remarkably improved when kaolin was added.Specifically, the strength could exceed1.05MPa when the MgSO4content was2%andthe addition of kaolin was2%. The Scanning Electron Microscope (SEM) analysisdemonstrated that with the addition of kaolin, there were less cracks and holes in thebinder bridge of the water soluble sand cores, and the mixed-mode fracture mechanismwas instead of the cohesive fracture mechanism. The other performance characteristicsof the MgSO4sand cores were also analyzed, which indicated that the MgSO4sandcores had excellent surface stability and good humidity resistance, but adding kaolincaused an increase in gas evolution. In addition, the MgSO4sand cores had superbcollapsibility in water and could be cleaned in several seconds.(3) The performance characteristics and process parameters of the calcia ceramiccores fabricated by gel-casting were investigated. The results indicated that the phenolicresin which was used as dispersant could significantly reduced the viscosity of slurry,and the minimum viscosity was obtained when the amount of the dispersant was6wt.%based on powder, in which way a low-viscosity slurry with48vol.%solid could befabricated. Epoxy resin was easy to operate when used as the gel former and the greenbodies made by it exhibited high strength and uniform microstructure. Moreover, as thesolid loading of slurry increased, the flexural strength of ceramic cores increased, whilethe linear shrinkage rate decreased. With the sintering temperature being increased, theflexural strength and the humidity resistance of ceramic cores were improved and the microstructure became more compact, but the sintered shrinkage rate increasedcorrespondingly. The calcia ceramic cores which were soluble in the water exhibitedexcellent collapsibility, and could be collapsed completely in the water in less20minutes.