COLD-HARDENING ALUMINOPHOSPHATE COMPOSITION FOR MAKING OF CERAMIC SHELL MOLDS FOR INVESTMENT CASTING
The purpose is to develop a cold-hardening mixture with an inorganic and environmentally friendly binder for the manufacture of ceramic shell molds for investment casting.
Methods. The determination of the qualitative and quantitative phase composition of the molding compositions was carried out on the Rigaku Ultima IV; the dynamics of transformations of compositions at heating is established by means of the differential thermal analyzer of the STA 449 C Jupiter. Bending strength control of ceramic shells-molds was performed on samples of 40 x 20 x 3 mm at normal temperature, as well as in the hot state, which used the original method based on the installation of US-700. One of the methods of experiment planning and experimental data processing was used to ensure the reliability of the results and reduce the amount of experiments.
Results. The dynamics of cold hardening of refractory suspensions based on powdered quartz, which contain orthophosphoric acid, aluminum hydroxide and the addition of kaolin clay, has been studied. The phase composition of the mixture after cold hardening and after calcination at 800 °C and at 900 °C was determined and it was shown that its changes are minimal. The hardening products are aluminum orthophosphates of various crystal structures, which act as a binder.
The properties of samples of ceramic shell molds from the developed aluminophosphate mixture were determined and it was shown that the bending strength at room temperature, in the hot state at a temperature of 600 to 1000 °C, after calcination and cooling, is not less than 4.0 MPa. The strength fully satisfies the conditions of the investment casting method.
Originality. For the first time, the kinetics of the formation of aluminum phosphates upon contact of an aqueous solution of orthophosphoric acid (2...10 mass parts) with an excess of aluminum hydroxide (10...30 mass parts), where the rest is a refractory filler, at room temperature has been studied. This composition of the mixture has not been previously considered in research. All examples of the synthesis of aluminophosphate and complex binders involved a significant excess of orthophosphoric acid, as a result of which the aluminum phosphates formed in solution did not provide cold hardening of the compositions.
Practical implications. Ceramic shell molds have passed production tests and have shown their suitability for the manufacture of castings from carbon and chromium alloyed steel. The developed aluminophosphate compositions and refractory suspensions based on them can be considered as an alternative to ethyl silicate and liquid glass suspensions for investment casting.
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