| Thrust weight ratio is an important parameter of aeroengine.Ti-Al based alloy has low density and high specific strength,which can improve the thrust weight ratio.Nb-Si based alloy can be used at a higher service temperature and provide greater thrust.The research shows that refining grain can improve the strength and toughness at room temperature to meet the requirements of deformed components,while eliminating transverse grain boundaries can improve the high-temperature performance to meet the application environment of blades.In this paper,two representative Ti/Nb based alloys are selected:Ti-48Al-2Cr-2Nb-2.5C alloy and Nb-16Si-22Ti alloy.The microstructure of the as-cast Ti-48Al-2Cr-2Nb-2.5C alloy is coarse,with reinforcement phase segregation,the properties of which need to be improved;The shape of primary ductile phase in as-cast Nb-16Si-22Ti alloy is dendritic and metastable Nb3Si phase exists,which is unfavorable to the fracture toughness at room temperature.From the point of view of refining grains and eliminate transverse grain boundaries,ultrasound treatment and electromagnetic cold crucible directional solidification are used to control grain size and phase morphology to improve the comprehensive performance of materials.In this paper,numerical simulation and experiment are combined to systematically study the distribution of multiphysics in the process of ultrasound treatment and electromagnetic cold crucible directional solidification,further to reveal the distribution of physical fields in the process of ultrasound treatment and electromagnetic cold crucible directional solidification and gain the effects of ultrasound treatment and electromagnetic cold crucible directional solidification on the structure and properties.The calculation model of thermophysical parameters in Nb based alloy was established,calculated by the first principle molecular dynamics method,which makes up the blank of thermophysical parameters of Nb based alloy melt.At different temperatures,the equilibrium volume of Nb based melt was obtained by external pressure adjustment,and the physical parameters of Nb based melt were obtained.The results show that compared with pure Nb melt,the diffusion activation energy and viscosity activation energy of the melt doped with Si increase from 66.46 k J/mol to 76.38 k J/mol,and 39.809 k J/mol to 53.108 k J/mol,respectively,which makes the dynamic properties of Nb-16Si worse.Doping Ti reduces the diffusion activation energy and viscosity activation energy of the melt to 74.19 k J/mol and 50.779 k J/mol,respectively,which is beneficial to the diffusion of Nb-16Si.The physical parameters of Ti-48Al-2Cr-2Nb-2.5C alloy were obtained by JMat Pro software.The 3-D model of ultrasound coupled with arc remelting was established,and the variation law of multiphysics of ultrasound treatment in Ti-Al based alloy was obtained.The results show that:there is an obvious zoning in the melt,which is defined as weak ultrasound active zone,transition zone and strong ultrasound active zone.The boundary flow direction of strong ultrasound active zone is counterclockwise on the left side and clockwise on the right side.The influence of different parameters on each physical field is analyzed.It has been found that the sound pressure at the boundary between strong and weak ultrasound active zone of Ti-48Al-2Cr-2Nb-2.5C alloy is 10 MPa,and its cavitation threshold is 0.161 MPa.At20 k Hz,the maximum flow velocity of Ti-48Al-2Cr-2Nb-2.5C alloy is 0.49 m/s,and the acoustic streaming effect is significant.Combined with the distribution law of physical fields,the microstructure and mechanical properties of Ti-48Al-2Cr-2Nb-2.5C alloy treated by ultrasound were analyzed.The evolution of reinforced phase and lamellar colony and mechanical properties of were further obtained.It has been found that the size of the lamellar colony in the weak ultrasound active zone increases with the increase of superheat,and the size of the largest lamellar colony is about 57μm with the arc current of 300A～400 A.The aspect ratio of Ti2Al C reinforced phase decreased from 6.5 to 3.3,by49.2%;The minimum lamellar colony and the aspect ratio of Ti2Al C reinforced in the strong ultrasound active zone are 43.7μm and 2.5,respectively.The reinforced phase dispersed in the matrix,promoting heterogeneous nucleation.With arc current of 350A and ultrasound treatment time of 80 s can realize strong convection in the melt and promote the refinement of the structure.Strong acoustic streaming promotes the orientation ofγphase converges to[0 0 1]along the X0 direction（flow velocity direction）.At the strong ultrasound active zone,the maximum Vickers hardness of is227.7 HV.A 3-D numerical simulation model of ultrasound treatment of Nb-16Si-22Ti alloy was established to obtain the distribution law of multiphysics.The phase evolution and mechanical properties of Nb-16Si-22Ti alloy treated by ultrasound were analyzed combing with the distribution law of physical fields.The results show that the sound pressure at the boundary between strong and weak ultrasound active zone of Nb-16Si-22Ti alloy is 15 MPa,and its cavitation threshold is 0.218 MPa.After ultrasound treatment,Ti segregation is weakened in Nb-16Si-22Ti alloy.Primary Nbss dendrites are broken,the aspect ratio of primary Nbss phase gradually decreased from 2.07 to 1.51,and the volume fraction of primary Nbss phase increased from 26.3%to 35.81%,revealing that primary Nbss phase was spheroidized.After ultrasound treatment time of 80 s,eutectic transformation occurs,and the volume fraction of metastable Nb3Si phase decreases by 58.64%andγ-Nb5Si3 phase appears.Ultrasound treatment promotes inert eutectoid reaction.The influence of ultrasoud time on the fracture toughness at room temperature was analyzed.The results show that after ultrasound treatment time of 200 s,the plastic constraint of the ductile phase reduced,and the fracture toughness was improved by 69.16%.With arc current of 500A and the ultrasound treatment time of 200 s,the acoustic streaming effect in the melt is significant and the fluid shear force is strong.As a result,the initial ductile phase is spheroidized.The relationship between the interface curvature of Nbss phase and the ultrasound energy is expressed as:∫02πK?dα=σ4π（Qa+Qu-Qra-Qc-?Ge）Qa.A 3-D multiphysics model of electromagnetic cold crucible directional solidification of Nb-16Si-22Ti was established during starting up.By analyzing the numerical simulation results of the feeding and charage during starting up,the multiphysics distribution of solid-liquid interface was obtained.The results show that the corresponding relationship between frequency and power can be expressed as:P=49.75722-0.18483*f+8.31547×10-4*f2.Under the radiation boundary condition,the heat loss is higher,as a result,the height of the mushy increases.The solid-liquid interface drops,taking on a"W"shape.Under the condition of heat conduction,the solid-liquid interface is relatively straight.The upper eddy current and the lower eddy current converge in the middle,and the flow direction is opposite.A 2-D electromagnetic cold crucible directional solidification multiphysics model coupled with free surface and drawing velocity was established,the multiphysics distribution was obtained.The research shows that with the increase of power supply,the free surface of meniscus becomes"M"shape and gradually appears"hat edge".With the increase of the drawing velocity,the initial solid-liquid interface decreases,and the width of the mushy narrows,and then becomes stable.There are"S"and anti"S"circulations on the left and right sides of the meniscus,respectively.With the increase of the drawing velocity,the maximum velocity increasing from 1 m/s to 1.9 m/s.The power supply of 50 k Hz-48 k W,and the drawing velocity of 0.4 mm/min are conducive to the directional growth of microstructure.The maximum temperature gradients in the axial and radial directions are 61.46 K/mm and 63.84 K/mm,respectively. |
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