| Nickel-based single crystal superalloy,with excellent high temperature properties,good resistance to oxidation and corrosion resistance,was specially designed to fabricate blades.The single crystal superalloy which the service temperature is close to 90% of the melting point of the alloy,becomes irreplaceable structural material of the contemporary art aero engine hot section.When using conventional directional solidification process growth superalloy single crystal blades,there was prone to miscellaneous grain,grain free single crystal tissue,growth misaligned blade spindle,segregation,coarse dendrite and strengthening phase,tilt uneven solidification interface and other issues,which are leading to serious deterioration of performance of the blade.Currently,the searching for new technology to control directional solidification process to obtain essential quality blade is very importent.The casting process of superalloy single crystal blades and the influence of transverse static magnetic field on the directional solidification microstructures of Ni-based superalloy CMSX-6 have been studied.The specimens were prepared under different temperature gradient,different drawing speed and 0.5T magnetic intensitiy,respectively.The microstructures,primary dendrite arm spacing,interdendritic segregation ratio and dendrite orientation under different solidification conditions were investigated.The results are as follows:When the temperature gradient was 30°C/cm,the withdrawal rate was 3mm/min,it got the defect-free single crystal blades.It found that the deviation angle of crystal orientation of the single crystal blades at a temperature gradient of 30°C/cm,withdrawal rate of 3mm/min was controled in about 3?.However,the primary dendrite arm spacing was limited because the temperature gradient was 30K/cm and the withdrawal rate was 3mm/min.Transverse magnetic field can affect the solid-liquid interface morphology,tilted the solid-liquid interface.Applying a magnetic field generated a new convection-thermoelectric magnetic convection at the solid-liquid interface.The convection was perpendicular to solidification direction,flowed from one side of the sample to the other side.It would make a lot of solute enrichment on one side of the sample,so that this side appeared constitutional supercooling,therefore the dendrite growth rate of this side is less than the same level of dendrites at other locations.These resulted in solid-liquid interface tilt.Transverse magnetic field can decrease primary dendrite arm spacing about 50%,channels macro-segregation is induced in mushy zone,mushy zone length is shortened and the crystal orientation is more focused on the <001> direction.With thermoelectric magnetic convection generation,which led the movement of interdendritic solute,it makes the density of solution changing.Interdendritic Solution density becomes smaller,so that the density difference is aroused with lower density in the interdendritic fluid than that at top of the dendrite.The lower dense solute in the interdendritic fluid is advected out of the interdendritic region,and the downward flow brings higher temperature liquid from the bulk into the mushy zone.The higher temperature liquid from the bulk remelts the dendrite,changes the temperature gradient of the area of mushy zone,so the macro-segregation forms and the length of mushy zone decreased.Applying a magnetic field increases the temperature gradient in the mushy zone,which was direct impact on the growth orientation of grains,making the grain more focused on the <001> direction.Applying a magnetic field reduces the critical nucleation radius of the dendrite,increases the probability of crystal nucleation,and with the addition of a temperature gradient which is affected by magnetic field,which makes the primary dendritic spacing decreased.Transverse magnetic field allows the segregation of Al,Ti,Cr,Co,Mo and other alloying elements of directionally solidified single crystal superalloys to reduce and more uniform distribution;?? phase size of the single-crystal decreases abou 40%;monocrystalline eutectic volume fraction decreases about 70%,eutectic size is reduction.The magnetic field reduces the diffusion of the solute in the melt,increases the effective distribution coefficient of solute and increases the solidification rate,thereby reduces the segregation of elements.The magnetic field increases ?? nucleation rate,then increases the number of ??-phase precipitation,reduces ??-phase size.The magnetic field reduces the segregation of Al,Ti and other elements.Between interdendritic and dendritic shaft,Al,Ti element content distribution tends to be uniform,so the gap of ?? phase size between interdendritic and dendritic shaft decreases.Magnetic field reduces the segregation of solute elements,results in reduction of precipitates of eutectic,and the size becomes smaller.Based on the basic principle of the lattice Boltzmann method(LBM),a three dimensional LBM transport model describing the phenomena of melt convection and solute in dendritic growth is established.It can apply to simulate dendritic growth during forced convection and diffusion of solute transport by convection.This model simulated the dendrite growth with different degree of supercooling,under different magnetic field intensity.The results show that the thermoelectric magnetic convection changes the solute distribution of solid-liquid interface frontier;dendritic growth of solute-rich area was inhibited,and then affects the morphology of the solid-liquid interface.Thermoelectric magnetic convection enhances the solid fraction,indicating that solidification rate can be improved further by magnetic field.It can be corollaried magnetic field can make mushy zone length becomes shorter.Supercooling degree reduced,the magnetic field effect is increased.The effect of thermoelectric magnetic convection on solute distribution of solid-liquid interface and the solid fraction was influenced by the degree of supercooling. |
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