The Reaserch Of Solidification Process Of TiAl Alloy And Influnce Of Interface Reaction On Lamellar Orientation In Vacuum Suction Casting

TiAl alloy is one of the most potential high-temperature structural materials for applications in aero-space due to its performance under high temperature and high elastic modulus. However, brittleness at room temperature, limiting its marketing. In order to improve the plasticity of TiAl alloy at room temperature, directional solidification technique was used to obtain castings with excellent performance, in which slice and grain boundary direction parallel to the loading axis.The possibility and method of controling slice orientation of TiAl alloy through sequential solidification method was studied, according to the basic principle of directional solidification. Two aspects of the work was done in this paper. The first, the rule of effects of mold heat capacity on the solidification structure and path were studied. The second, view the alpha phase as" seed"which was produced in the interection between mold material and TiAl alloy. And expected direction lamellar structure was got in Ti45Al alloy by controlling solidification conditionIt is founded that there is a geat influence of mold heat capacity on TiAl transition(β-α) point of alloy first precipitates phase. As the increase of mold heat capacity,the precipitate phase transition(β-α) point will be closer to low aluminum component. And the transition point is much deviation from the equilibrium phase diagram component49.4at.%in experimental cooling speed.Mold with diferent storage capacity lead to different levels of segregation in casting. The macro segregation becomes more and more serious with the increase of mold cooling capacity. It is analysed the reason of Formation mechanism of macrosegregation and the influence of mold cooling rate on macrosegregation is analysed. Grain of the surface growth is cellular growth, so the solute is pushed to the front of the solidification interface.with the temperature gradient decreasing,the grain growth gradually develop dendritic growth and the solute enrich in dendrite. Heat capacity of Mold affect not only the lamellar spacing,but also the the surface grain size, however,the size of surface grain also effacted by the heterogeneous nucleation and on the chemical composition of the alloysThe thickness of the interface reaction between mold and TiAl alloy is affected by types of surface coating of the mold.The reaction between TiAl alloy and mold with Zircon flour layer is the most intense,alumina is the second.Zirconium oxide reductionAt the same time,the mold marerial have a significant influence on surface grain lamellar orientation. Infiltration of O,Si and other elements will cause the metal surface inclined to a solidification,with the surface grain layer paralleling to the casting surface. For the same component alloy,,with the interface reaction enhancement,the number of surface grain will increase. Similar suction casting method was used in the experiment,to study the influence of interfacial reaction on the lamellae orientation of internal grains. It is found that it is feasible to control lamellar orientation of internal grains by interfacial reaction,however it is needed interaction of high active mould material and high temperature gradient.The influence of grain lamellar orientation on the mechanical properties is vevey important,castings with the fracture along the slice direction sften has a low strength.