| Preparation of nickel based superalloy single crystal by directional solidification is the key technology in the manufacture of aero engine and industrial gas turbine.Directional solidification shell demand reliability and safety which will withstand even the most extreme working conditions, the performance of the shell is directly related to the quality and yield of the blade. Many problems exist in the widely used nickel based superalloy directional solidification shell, such as high heat resistance,low intensity, anti deformation capacity, long production cycle etc. So it is difficult to meet the production requirements of directional solidification of nickel based superalloy.In this study, the different length and content of short carbon fiber were used to modify the corundum shell. The effect of short carbon fiber on the viscosity of the slurry, the shell thickness and the drying property of the shell were studied, and the preparation technology of the modified shell was determined. Then the modified shell was prepared, and the effects of short carbon fiber on the properties of the normal temperature strength, sintering strength, anti deformation ability, and dimensional change of the shell were studied. Finally directional solidification experiments were carried out to test the solidification temperature gradient, and the macro morphology and microstructure of the single crystal casting were compared.Study on the preparation technology of shell show that, by adding the short carbon fiber to the silica sol in advance, the carbon fiber is evenly distributed in the slurry. Short carbon fiber increase the viscosity of the silica sol, and the content of fiber is lower than 7g/L, which can guarantee the normal coating operation. The shell thickness increases with the increase of the viscosity, peaked at 101%, which reduces the number of shell coating and shorten the production cycle.By extending the 1 hour drying time to ensure the modified shell drying thoroughly.By vacuum roasting process to prevent the burning loss of carbon fiber.the ultimate success of the prepared short cut carbon fiber shell. The modified shell of short carbon fiber modified shell was successfully prepared.Strength research shows that, compared to the strength of specimens without carbon fiber, the green strength and sinter strength of modified shell on the traight parts are obviously increased, peaked at 6.31 Mpa and 10.73 MPa,which respectively increased by 116% and 92%.the green strength and sinter strength on the corner parts reach 3.32 Mpa and 10.98 Mpa, which respectively increased by 97% and 148%.With the increase of fiber content and fiber length, the strength of modified shell are the increasing trend, but not without limit. When the fibers are added to a certain extent,the strength of the shell increase began to slow down, and even decreased, fiber length also the same trend.Anti-deformation ability research shows that, with the increase of carbon fiber content, the moderated-temperature shrinkage and high-temperature shrinkage decrease to 0.04% and 0.74%,and the former is greater than the latter. Short carbon fiber improve the ability of shell to resist high-temperature gravitational distortion,with the increase of carbon fiber content and length, the high-temperature gravitational distortion decreases, and the minimum is 0.72%.The experimental results of single crystal directional solidification show that,short carbon fiber modified shell can provide steeper temperature gradient than the unmodified shell, the highest solidification temperature gradient was 66℃.Under the same coagulation parameters the modified shell can eliminate the miscellaneous grains caused by the heat dissipation, and improve the structure of the single crystal.At the pulling rate of 3 mm/s, the primary dendrite arm spacing was 296.2μm, and the two dendrite arm spacing was 65.6 μ m. Therefore, the single crystal casting microstructure has improved. |
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