Study On Investment Casting Shell Deformability Of TiAl Alloy

Titanium aluminides alloys has low density,good oxidation resistance,high temperature strength and excellent fatigue performance.Because of its intermetallic compounds,so that they are both metal and ceramic properties,is often used as aerospace and automotive engine heat-resistant structural parts of the material.The defect of TiAl alloy is also clear.Its plasticity and toughness at room temperature are low and machinability is poor.The problem of poor machinability can be effectively solved by investment casting,but because of the considerable solidification shrinkage,the castings are not difficult to produce cracks and the primary forming rate is low.Some researchers have improved the shrinkage of castings by reducing stress and the crack defects.Because the raw materials and production cost of tial alloy are very expensive,it is not good to use the traditional trial and error method.Using the computer numerical simulation method to optimize the precision casting process can improve the casting quality,reduce the cost and shorten the product trial cycle.In this thesis,ProCAST was used to simulate the investment casting process of TiAl alloy.The numerical simulation results were reported for the experimental results.It is shown that the numerical simulation can predict the solidification defects and the formation of the microstructure of the TiAl alloy.It provides a theoretical basis and strong support for the optimization design of the casting process.In order to facilitate calculation,stress type of the mold is usually set for rigidity when the ProCAST is used for numerical simulation.However,in reality,the mould cannot be rigid,and it will deform with the deformation of the casting.This deformation of the mold will affect the stress distribution within the casting and the quality of the casting.In this experiment,ProCAST software was used to set the mold for linear elasticity.By changing the preheating temperature of the shell,the stress,strain,and microstructure of the casting under different shell preheating temperatures were simulated,and the linear elastic type was assessed by comparison and theory.Simultaneously,the simulation of the different Young's modulus of the shells was also carried out,so as to make some reference for the follow-up study of shell deformability by ProCAST.Due to the bending strength is generally used to characterize the deformability of the mold,the method of adding fiber in the ceramic shell of TiAl alloy for precision casting is adopted inthis thesis.The effect of different content of carbon fiber and nylon fiber on the bending strength of the shell in different states is studied.The results show that adding a small amount of carbon fiber can increase the wet strength of the shell,reduce the shell at room temperature and high temperature dry strength,but when the amount of carbon fiber added reaches 10%,it will also increase the wet strength of the shell,room temperature and high temperature dry strength.When nylon fiber content of less than 10% is added to the shell,the wet strength,room temperature,and high-temperature dry strength of the shell will decrease as the amount of nylon fiber added increases.Considering the requirements for wet strength and room temperature dry strength during the process of shell-type transportation,assembly,and post-cast cleaning,and the relatively low high-temperature dry strength required for improving deformability,the proper amount of added carbon fiber can improve the deformability of the shell while satisfying the use requirements.