Research On The Investment Casting Technology For K4738 Alloy Exhaust Case

The exhaust case is generally made of superalloy by integral precision casting,which is the key turbine component of aircraft engine.Because of its large-scale thin-walled(the thinnest part of the wall is only 1.6 mm),requiring investment casting with complicated structure and great discrepancy of wall thickness,it is easy to introduce defects such as misrun,cold shut,shrinkage,and deformation during the pouring process.However,the part is required to have high dimensional accuracy,good comprehensive mechanical properties,excellent metallurgical quality,which enhance the difficulty in casting.Therefore,in this paper,taking K4738 exhaust case as the research object,the influence of casting process parameters on the microstructure and properties of K4738 alloy is studied.The gating system and process parameters are optimized by computer numerical simulation technology,and the exhaust case which meets the technical requirements is obtained.Studies have shown that as the pouring temperature and the shell temperature decrease,the grain size decreases,the dendrite structure refines,the secondary dendrite arm spacing decreases,and when the pouring temperature and the shell temperature are both low,the shrinkage tends to be serious.MC carbides are mainly precipitated between grain boundaries and dendrites,in the form of blocks,strips and short rods.As the pouring temperature and the shell temperature decrease,the size of the MC carbide decreases,the amount of precipitation increases,and the distribution is more diffuse.Under different processes,the interdendritic ?' phase did not show much difference in morphology,precipitation and size.When the shell temperature is 900? and 950?,the mechanical properties of the alloy are better.When the shell temperature is 900?,the pouring temperature is 1390? and the mold shell temperature is 1000?,the mechanical properties of the alloy are significantly reduced,and the selection of process parameters should be avoided as much as possible in this condition.Procast is used to simulate and analyze the filling and solidification process of the exhaust case,predicting the tendency of shrinkage.The research shows that the casting have no misrun and cold shut,shorter solidification time,fewer shrinkage by adopting the process scheme in which the shell temperature is 900? and the pouring temperature is 1450?.The research on the investment casting of the exhaust case shows that the the overall wax pattern used to manufacture exhaust case which conforms to the technical requirements can be produced by using ceramic core,secondary molding technology of cold wax block,wax orthopedic mold,wax separation combination technology et al.Adopting silica sol ceramic shell technology,the surface layer is coated with two layers,using silica sol and zircon powder,the back layer is coated with five layers,using silica sol and coal gangue powder,thus,the shell with excellent quality is obtained.The exhaust case is cast by using hot shell and gravity casting,double vacuum melting and pouring process.After inspection of size,fluorescence and X-ray,the result shows that K4738 alloy exhaust case haves high dimensional accuracy,excellent surface quality and less shrinkage defects,which is consistent with the simulation results.The mechanical properties of the bars are tested,and the results shows that mechanical properties of the bars meet the technical requirements.The microstructure of each section of the casting presents a difference,which is actually caused by the difference in the cooling rate of the section.