Bottom Pouring Vacuum Suction Casting Technology Of Small Thin Walled TiAl Based Alloy Component

TiAl based alloys are considered as promising structural materials in aerospace, energy and automotive industries. Their advantages are mainly low density, high specific strength, high specific stiffness, good oxidation resistance, and good creep property up to high temperatures. TiAl based alloys have low ductility, low deformation limit, and bad cold-forming property, so these bad properties limit the cold processing technology of TiAl based alloys.Casting is the most economical way to produce TiAl based alloy components. Different casting methods such as low pressure casting, centrifugal casting, shell mould casting etc. are used. However, there are some problems in casting small thin walled component of TiAl based alloy. Therefore, the TiAl based alloy precise forging thin blades were generally used in small engine. However, this thin blade has deformation problem, and precision allowance was removed by hand polishing, and the air-actuated shape of the blade was assured by cross section pattern.In this article, a new net-shape casting method for TiAl based alloy was developed - bottom pouring vacuum suction casting using permanent mold. Through the numerical simulation and experimental verification, the detailed study on filling and solidification process of TiAl based alloy melt during bottom pouring vacuum suction casting has been carried on. On the basis of the numerial simulation an experimental results, the defects and structure characteristics of small thin sheet Ti-47Al alloy components prepared by bottom pouring vacuum suction casting were studied also. Bottom pouring vacuum suction casting technique is to melting TiAl based alloy in a water-cooled crucible, and molten alloy will fill into metal mould through the suction hole at the bottom of the crucible,and then solidify. In the process of bottom pouring vacuum suction casting, the melt fills and solidifies under vacuum so as to overcome the oxide formation, on the other hand, melt the mould cavity in the coupled effect of the pressure difference and gravity, and the smaller superheat can also form small thin walled component, at the same time,use of metal mold can refine TiAl based alloy structure and weaken interface reaction.The suction hole diameter will affect filling behavior of TiAl based melt. With the increase of suction hole diameter, the max jet width increases. In this article, the effects of suction hole diameter, pouring temperature, pouring velocity, mold temperature, alloy element and coefficient of heat transfer on filling ratio have been studied. The suction hole diameter and filling ratio have linear relationship. With the increase of pouring temperature, the filling ratio increases. When pouring velocity is less than 2m/s, there has a linear relationship between the pouring velocity and the filling ratio. With the increase of the mold temperature, the filling ratio increases. The addition of alloy element, which will increase melting point and viscosity, will decrease the filling ratio. The proper current density and electrode height are benefit to decrease gas hole of casting.The effect of technology parameter on solidification time and solidification fraction has been studied. With the increase of pouring temperature, the solidification time of front melt is extended, which will benefit to feeding; The increase of suction hole diameter and mold temperature are benefit to solidification of blade hawk after the front melt solidification, and the solidification fraction is reduced, which will benefit to improve feeding; With the increase of pouring velocity, the solidification closed area decreases first and then increases, and the critical value is 1m/s, when pouring velocity is less than 1m/s, the increase of pouring velocity will benefit to improve feeding. The local solidification shrinkage ratio 1.55% of TiAl based alloy is used for shrinkage criterion, the experimental results and numerical simulation results agree well. The suction hole diameter has the greatest effect on shrinkage cavity of TiAl based alloy component, followed by the pouring temperature, with the increase of suction hole diameter and pouring temperature, the shrinkage cavity is reduced. With the increase of pouring temperature, the stress is reduced; the increase of suction hole diameter will lead to decrease blade leaf stress; the increase of pouring velocity and mold temperature will benefit to decrease blade stressIn this article, the qualified TiAl based alloy thin walled components have been obtained. The structure character of the thin sheet casting and blade casting of TiAl based alloy has been studied. The grain size of Ti-47Al alloy thin sheet casting increases with increasing distance from the bottom, and the entire cross section has equiaxed grain. The inner grain size of Ti-47Al alloy blade increases with increasing distance from bottom. The inner grain shape of Ti-47Al alloy blade is changes from equiaxed to columnar crystal at the distance from the bottom 20mm, and the columnar crystal fraction increases with increasing distance from bottom. The lamellar spaces of blade and thin sheet casting increase with the distance increase from bottom. Compared with centrifugal casting, the bottom pouring vacuum suction casting has several advantages, that is, less segregation, no interface reaction, and smaller grain size. The effect of alloying element on Ti-47Al alloy structure has been studied also in this article. The first secondary dendrite arm was refined, because of the W element addition; Because of B and Si element additions, second phase will form and result in refinement of Ti-47Al alloy structure.