Effect Of Temperature Gradient Investment Casting Shell On Microsturcture And Properties Of K418 Alloy Thin-wall Castings

K418 alloy is a nickel based superalloy with excellent properties,it is widely used in aviation field.At present,lightweight has become a development trend in the aviation field,castings are mostly designed as thin-walled structure.Thin-walled castings often suffer from shrinkage and cold-separation defects because of difficulty in filling and insufficient solidification feeding,which seriously affected the yield of this type of casting.At present,the uniform temperature investment casting shell is used in the research and production process,which is not conducive to controlling the solidification sequence and solidification structure of the alloy.In order to solve the problem of defects in thin-wall castings and meet the needs of current aviation production,we propose a new process,temperature gradient investment casting shell process.The combination of numerical simulation and experiment is used to research this process' s influences to the microstructure and properties of thin-walled K418 alloy castings.According to the structural characteristics of the castings,the partition heat preservation scheme of the melting model shell is designed.Different heat preservation density is applied in different regions.By controlling the cooling process,the investment casting shell has a certain temperature gradient before pouring.The temperature gradients of the three investment casting shell schemes are 0.5?/mm,1?/mm,and 1.5?/mm,respectively.The numerical simulation results show that the castings are solidified in a sequential solidification mode when using a gradient temperature shell temperature field for plates of different thicknesses,liquid metal can effectively feed,the amount of defects is less than one that obtained in the traditional heat preservation process.The defects of 3mm and 4mm plates decrease with the increase of temperature gradient.When the temperature gradient is 1.5?/mm,the minimum defects are 1.426cm3 and 1.093 cm3 respectively.Firstly,2mm plate's defect volume decreases and then increases with the increase of temperature gradient.When the temperature gradient is 1?/mm,the defect volume is at least,2.088 cm3.This results indicate that the temperature investment casting shell process can effectively reduce casting defects volume.At the condition of 900? shell,the casting tends to solidify at the same time as the thickness of the castings increase,and the ratio of the defect to the volume increases.With a temperature gradient shell,with the increase of the thickness,the temperature gradient favors the solidification of the solidification feeding,the ratio of defects to volume decreases.The results of a comparison experiment show that,through the gradient heat preservation method,before casting,the temperature gradient of investment casting shell is about 0.85?/mm.The grain of the casting is small and the defect is less.The secondary dendrite arm spacing in different positions is obviously smaller than that of the traditional heat preservation process.The ?? precipitates is more tiny,the shape is mostly round and square.Most of the size of ?? precipitates is between 323 nm and 812 nm.The harmful(?+??)eutectic is smaller,and the volume fraction is less.The tensile strengths and elongations are significantly improved at-cast.The average tensile strength of the panel increases by 10.07% and the elongation rate increases by 31.11%.