| The solidification process is an important step in the metallic material processing.The control of the solidification process and solidification structure is critical to the subsequent material processing and the final work process.Defects such as non-metallic inclusions,looseness,shrinkage cavities,macrosegregation,and coarse structure formed during solidification process are so difficult to eliminate even with the plastic deformation process with large deformation,which will induce huge defects or potential safety hazards,or greatly reduce the service performance of materials.Therefore,how to effectively control the heat,mass and momentum transfer process during solidification,and then control the solidification structure,solute distribution,and eliminate solidification defects has always been a key research goal in the field of solidification.During the solidification process,the heat transfer process is disturbed by many factors,resulting in the inhomogeneity temperature in the melt,which will induce the buoyancy flow;during dendritic or solidification interface advancement,the rejection or segregation of solute will cause the change of the melt density,which will also induce the buoyancy flow;the surface tension gradient in the melt caused by the temperature gradient will induce the Marangoni convection;the application of the ultrasonic,electromagnetic field,mechanical agitation,etc.will induce the forced flow.These flows will further affect the melt temperature gradient,solute distribution,interface growth process,and ultimately affect the solidification quality.Therefore,recent years,how to control the flow during the solidification process or to introduce a specific forced flow to regulate the solidification process has been widely concerned by the solidification workers.The application of an electromagnetic field during the solidification process can significantly affect the heat transfer,mass transfer and momentum transfer in the melt,which is beneficial not only for controlling the solidification structure,improving the solidification defects,eliminating component segregation,but also ultimately improving the properties of the solidification structure.However,due to the complexity of solidification conditions in actual industrial production,the diversity of forced flow induced by the electromagnetic field,and the coupling with the flow in the melt,the research on the influence mechanism how the forced flow induced by the electromagnetic field effect the solidification structure is far from deep.Directional solidification technology is beneficial to simplify the solidification process and is an important means to study the solidification theory.In order to simplify the solidification process in industrial production,in this paper,directional solidification technology is used as the research means and the simple binary alloys such as Sn-Bi,Sn-Pb and Al-Sn alloys are used as the research objects.Through the solidification experiment combined with the numerical simulation,the influence mechanism of different types and different magnitudes of electromagnetic field-induced forced flows on the directional solidification structure of binary alloy was quantitatively studied.The results obtained in this paper may contribute to the theoretical and experimental references for the development of solidification electromagnetic control technology.The main research content is as follows:1.The magnitude and direction of the thermoelectric magnetic force and the thermoelectric magnetic convection during directional solidification under a transverse static magnetic field have been studied.Through the determinations of the absolute thermoelectric power of Sn-10 wt.% Bi alloy and the actual cooling curve in the solidification experiments,the internal thermoelectric current was calculated,and thus the magnitude and direction of the thermoelectric magnetic force were deduced in the directionally solidified Sn-10 wt.% Bi alloy.A three-dimensional numerical model for the directional solidification of binary alloys was performed to study the magnitude and distribution of the thermoelectric magnetic convection in the mushy zone at solidifying front during directional solidification.The results show that as the magnetic field intensity increases from 0 – 0.5 T,the thermoelectric magnetic convection gradually increases.The effect of the thermoelectric magnetic convection on the solute distribution at solidifying front during the directional solidification of binary alloys was simulated.The results show that the rejected solute will be unidrectionally transported to one side of the sample by the thermoelectric magnetic convection.The directionally solidified structure was compared to confirm the correctness of the simulation result.2.The effect of a transverse static magnetic field-induced thermoelectric mangetic convection on the microstructure,dendrite spacing,composition distribution was studied in the directionally solidified Sn-10 wt.% Bi alloys.Through the determination of the average temperature gradient in the mushy zone under different magnetic field intensities,the influence mechanism of the transverse static magnetic field on the directional solidification structures of binary alloys was proposed from the perspective of solute redistribution and verified by the corresponding numerical simulation.In order to further prove this mechanism,the transverse static magnetic field is also applied to the directional solidification of pure Sn,Sn-Bi and Sn-Pb alloys by Cz method.It can be found that magnetic field does not change the shape of pure Sn sample,but it can make Sn-Bi and Sn-Pb alloy samples bend and induce the “dendrite” like surface.When the direction of the magnetic field is reversed,the bending direction of the alloy sample is also reversed.3.The thermoelectric magnetic convection produced by the interaction of the internal thermal current and the external magnetic field can induce the macrosegregation in the directional solidification structure of Sn-10 wt.% Bi alloy.Thus,an electromagnetic compounding method was proposed to eliminate the macrosegregation,i.e.an external DC current was applied to interact with the external magnetic field to induce an electromagnetic convection which is opposite to the direction of thermoelectric magnetic convection in the mushy zone during directional solidification under a transverse static magnetic field.The feasibility of this method was investigated by corresponding solidification experiments from the directional solidification microstructure,dendritic spacing and composition distribution and further verified by numerical simulation.Furthermore,the effect of different DC current densities on the directional solidification structure was investigated.The corresponding influence mechanism of the electromagnetic convection on the directional solidification structures was revealed.The modification of the directional solidification structures under the combined action of the electromagnetic convection and thermal electromagnetic convection must be attributed to the change of the solute redistribution at solidifying front,which is consistent with that under the thermoelectric magnetic convection.4.In order to further investigate the effect of forced flow on directional solidification structure,a pulse-electromagnetic convection was induced by a pulse current and a transverse static magnetic field.This convection was regulated by changing the pulse current frequency(1,10 and 100 Hz).This can be considered as the influence of the electromagnetic vibration frequency on the directional solidification structure of Sn-10 wt.% Bi alloy.The experimental results show that the solidification structures under a 10 or 100 Hz pulse current and a 0.5 T transverse magnetic field are similar to that only treated by the 0.5 T transverse magnetic field,but the solidification structure at 1 Hz changes obviously.The main reason is that the forced flow velocities at different electromagnetic vibration frequency are different,which leads to the difference in the transportation of the rejected solute at solidifying front,thereby yielding the difference of solidification structure.In order to prove this influence mechanism,the effects of different vibration frequencies on the forced flow velocity and particle transportation were studied by numerical simulation.The results showed that the lower the frequency,the faster the forced flow,and the easier the particles are transported.Then,the electromagnetic convection velocities and solute transportations under different electromagnetic vibration frequencies were numerical simulated during directional solidification.The results show that the lower the frequency,the easier the rejected solutes are transported out of the solute boundary layer.Comparing the thickness of the solute boundary layer and the moving distance of the solute in the half cycle,the result shows that the solute can be moved out of the solute boundary layer at 1 Hz only,thereby yielding the change of solidification microstructure,which is consistent with the experimental results.5.Further,the influence of electromagnetic vibration frequency on the macrostructure of semicontinuous casting Al-15 wt.% Sn alloy was investigated from the perspective of flow-induced solute redistribution at solidifying front.It can be found that the lower the frequency,the higher the equiaxed grain rate of the macrostructure of semicontinuous casting Al-15 wt.% Sn alloy.The flow fields in the semicontinuous casting process under different electromagnetic vibration frequencies were simulated.The result shows that the lower the frequency,the lager the forced flow veloctiy.Moreover,the moving distance of the equiaxed grain in the half cycle at different vibration frequencies was calculated,which shows that the lower the frequency,the lager the moving distance of the equiaxed grain.Comparing the thickness of the solute boundary layer and the moving distance,the result shows that the equiaxed grain may move out of the solute boundary layer at less than 50 Hz.Moreover,when the equiaxed grain moves out of the solute boundary layer,i.e.the undercooled and growing equiaxed grain moves into the non-undercooled melt,the growth of equiaxed grain will be inhibited,thereby yielding refined grains.This is in agreement with the experimental results.In summary,the effect of forced flow of different types and different magnitudes induced by the applied electromagnetic field on the solidification structure,interface shape,and macrosegregation during directional solidification and semi-continuous casting process has been deeply investigated by solidification experiment combined with the numerical simulation method.This will have certain reference significance and important reference for the solidification of large castings and the organization control of continuous casting process!... |
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