Bottom Pouring Vacuum Suction Casting Technology Of TiAl Based Alloy Small Casting

TiAl based alloys have a series of merits, e.g., low density and oxidation resistance, which possess good application prospect as high-temperature material. However, due to low plasticity and poor machinability, casting is principal method to produce TiAl based alloys. Bottom pouring vacuum suction casting which is a new net-shape method has been put forward by our research group. This method use gas pressure to drive filling of TiAl based melt, which makes TiAl based melt possess good filling ability. It can be used to produce small complex casting. In this article, effects of gas pressure on flow process of TiAl based alloy melt and the formation of pore have been studied in detail. On that basis, TiAl based alloy formation technical technology in metal mould and ceramic shell mould has been studied.Filling process of TiAl based melt alloy under pressure has been simulated using Fluent software. Influencing factors of TiAl based alloy flow are presented and possible defects are predicted during suction casting. Under gravity, the alloy melt flow more steadily into the mold, but the liquid flow is not continuous; while under the gas pressure, spurting liquid is injected into the mold, leading to unstable melt alloy surface. After the first stock of melt flows into the mold, gas with melt also gets into the mold, which has a strong impact on the filled liquid metal. With the increase of the gas pressure, the size of pits of melt metal surface increases; When the diameter of the suction hole increases, the gas may earlier enter to the mould, and the mass of first flow melt is smaller. When gas accesses earlier to the casting mould, remain melt metal in the crucible increases. Because of the impact of gas, the top fluid is turbulent flow conditions, while the bottom fluid is laminar flow conditions which can flow free.The experiment of vacuum suction casting exhaust valve was carried out to verify the law obtained by simulation. Pores inside the exhaust valve can be divided into two cases by the causes:one is involved pores. In the melting process, the melt alloy is rapidly rotating, which draw gas into the liquid. After melt alloy fill mould, the gas stay inside and form pores. The other pores are formed due to uncontinuous liquid flow. The pores are closed in casting mould by subsequent liquid. The increase of pressure will cause more gas into the mold, and increase the size of the pores in the castings. But if there is no pressure, it also will form gas trapping in the mold. It was found that the optimal gas pressure is0.03MPa. With increasing the mass of the metal in the crucible and reducing the size of the suction hole diameter, the amount and size of pores gradually decrease. In this article, complete and compact exhaust valve is prepared by a reasonable combination of metal mass and the suction diameter.Vacuum suction casting was combined with investment casting technology to prepare small complex shape casting, i.e., connecting rods and blades. It was found that increase of preheat temperature of the shell and enhancement of melting current are beneficial to fill. The optimal preheat temperature is900℃, and the optimal melting current is1000A. The vent in casting mould also has a very important effect on the formation of casting. Complete casting is prepared by choosing reasonable vent in casting mould.The composition of TiAl based casting was detected by energy spectrum analysis. It was found that the casting composition is uniform, and no obvious macrosegregation exists. The microstructure of casting is investigated by metalographic microscope, scanning electron microscopy. The macrostructure of exhaust valve prepared in metal mould is mainly equiaxed grains. Due to the surface chilling role, fine crystal layer is formed in the casting surface, and the grain size is30～40μm. The size of grain in the center of exhaust valve is200～300μm. In the blade prepared in ceramic shell mould, the distribution of grain has the similar rule, but the whole grain size is larger than the exhaust valve. The hardness in casting gradually decreases from the surface to the internal casting. The hardness of castings prepared in metal mould is higher than that of the ceramic shell casting. The maximum tensile strength of the TiAl based alloys casting prepared in ceramic shell moud is354MPa, and the elongation is0.28%.