Investigation On Transfer Behaviors During Directional Solidification Of TiAl-based Alloys By Electromagnetic Cold Crucible

Due to the low density,high specific strength,high temperature strength,excellent oxidation and creep resistance of TiAl-based alloys,they have the superiority of light weight for application in aeroengine blade.Based on the previous investigations,the directionally solidified TiAl-based alloys with fully lamellar structure paralleled to growth direction could promote the creep and fatigue resistance observably.Cold crucible directional solidification?CCDS?technique overcomes the problems of contamination and size limitation during traditional directional solidification of TiAl-based alloys,and exhibits promising foreground for directionally solidifying high temperature and active materials.However,there are complex heat and mass transfer behaviors during CCDS,which affects the directionally solidified microstructure.In this study,the characteristics of heat and mass transfer during CCDS and their effects on the microstructures were investigated,the mechanisms of solidification interface and solute controlling were revealed and the process of CCDS were optimized.A 3-D model of the cold crucible for induction heating was established,and the model was verified by magnetic field and temperature field measurement.The magnetic field and temperature field of the materials under different process parameters were calculated,and the induction heating efficiency was optimized.Results indicate that the maximum magnetic flux density could be reached when the relationship of h_m<h₁+h₂<h_m+? is satisfied.Under these conditions,the materials could be melted with less time consumption and have uniform temperature field.With increasing current intensity and frequency,the time consumption for melting decreases while the temperature difference increases.The TiAl alloys could be melted with lower power by optimized process.The influences of structure and electric parameters on Lorenz force in melt were analyzed,and four dimensionless parameters that controlling fluid flow in cold crucible were defined,these being the Hartman?Ha?,magnetic Reynolds?R_??,coils-melt position?h?and the ratio of the melt height to length?H/L?numbers.A 3-D model for flow field calculation was established and verified by tracer method.It is found that there is a vigorous three-dimensional melt flow in the cold crucible.A meridional flow with two eddies in the half meridian plane,the maximum velocity locates within the skin layer.The coupled effects of alternating magnetic fields and the configuration of cold crucible induce an azimuthal melt flow.The flow fields in the melt were calculated under various dimensionless parameters.Results indicate that the electromagnetic coupling increases with increasing Ha and H/L,and thus the velocity and turbulent kinetic energy increase.R_?significantly influences the distribution of eddies and flow velocity,the core of eddies move to the surface of melt with increasing R_?and the maximum velocity could be obtained when R_?=50.The relative scale of upper and lower eddies is affected by the value of h,the flow velocity decreases while the turbulent kinetic energy increases with increasing h.Multi-physics coupling model of magnetic field,shaping field,flow field and temperature filed during CCDS was established,the variation of melt temperature was calculated and verified by experimental measurement.The variation of melt temperature increment with powers fits the Dose-response curve,and the rising rate of temperature with powers reaches the maximum within the range of 47.9 kW and 55.1kW.Moreover,the temperature increment reaches the maximum when h_c ranges from36 mm to 38 mm.The model of heat transfer in the liquid zone,the mushy zone and the solid zone were established,the characteristics of convective heat transfer between the liquid zone and the mushy zone,the heat transfer behaviors between the mushy zone and the solid zone were obtained.Further,the influence mechanisms of CCDS processes on the shape of solidification front were revealed.Based on the analysis of convective heat transfer between liquid zone and mushy zone,the axial-radial heat flux density and the distribution of temperature field in the mushy zone were deduced under the condition of convective heat transfer,and its influences on the shape of solidification front were analyzed with considering the meniscus shape.Based on the analysis of heat transfer between mushy zone and solid zone,the distribution of temperature in axial in the mushy zone and solid zone was deduced.It revealed that the height of solidification front decreases with increasing convective heat transfer,induction heat and solidification velocity.According to the analysis above,the planar solidification front can be obtained by optimizing the processing parameters.Based on the flow field in the melt,the convective mass transfer in the meniscus and solute distribution in the melt was analyzed.It found that larger value of Ha,R_?and H/L,as well as smaller h are beneficial to promote the uniformity of solute in the melt and the solidification front.Moreover,the intensive three-dimensional flow in the cold crucible contributes to more efficient melt stirring.The convective mass transfer model in the solute boundary layer with planar solidification front during CCDS was established,influence mechanisms of electromagnetic stirring on the solute distribution in the directionally solidified ingot were revealed.Based on the convection-diffusion parameter,functions of solute distribution in the melt and the ingot with solidification distance were deduced;the expression of constitutional supercooling at the solidification front was derived considering the convective heat transfer.It reveals that the solute concentration in the melt and constitutional supercolling at the solidification front decrease with increasing flow intensity.TiAl-based alloys were directionally solidified by CCDS,and it revealed that the segregation in the melt decreases with increasing power,meanwhile,the uniformly distributed solute in the vicinity of solidification front leads to homogeneous solidification microstructure.The effective solute distribution coefficient approaches to 1 with lower cold crucible cross section size and powers,as well as higher pulling velocity.Under these conditions,the uniformity of solute distribution increases.The directionally solidified TiAl-based alloys ingot with large columnar crystal,uniform composition and microstructure was obtained by optimized CCDS.