Optimization Design And Solidification Control Of Electromagnetic Levitation Process For Bulk Metallic Materials

Electromagnetic levitation（EML）technique has the advantages of strong levitation force and high heating efficiency,which is widely used in the fields of rapid solidification,thermophysical properties measurement of liquid alloys and preparation of advanced materials.Due to the fact that the current EML system is mainly used for millimeter-level metallic materials,this paper aims at enhancing the levitation performance and investigating the heating feature in EML system with bulk metallic materials.So the EML manufacture instrument for bulk metallic materials was designed.The EML experiments of bulk metallic materials and finite element analysis（FEA）with multi-physics were employed to study the associative process of EML manufacture,including the levitation performance of various coil configurations,the levitation characteristics of bulk metallic materials,the migration dynamics of solid/liquid interface,the levitation behaviors of liquid metal and the solidification characteristics in the levitation manufacture process.The main conclusions are summarized as follows.1.Equipment design for electromagnetic levitation manufacture of bulk metallic materialsA three-dimensional heat transfer model of electrode flange under long-time and high-amplitude current conditions was established.The characteristics of the absorbed power and temperature distribution of the flange surface were analyzed,and three types of water cooling optimization schemes were proposed.It was found that the S-type water cooling scheme can make the maximum temperature and average temperature of the flange reach the lowest.A scientific equipment,which is capable to implement electromagnetic levitation and levitation manufacture,has been designed and manufactured.The equipment provides 800 to 2500 K heating temperature on 0.01 to2 kg sample while creates 5×10^-5 Pa vacuum.The velocity of levitation manufacture moulds reaches 2 to 6 m/s,produces maximum cooling rate of 100 K/s and temperature gradient from 100 to 5000 K/m.Levitation heating,precise control of levitation position and quick response of the mould were realized through the combination of pressure solidification forming technology,the high frequency induction heating technology and the control system.The levitation heating,melting and levitation solidification process of Al,Cu and Ti were realized.To achieve precise control of the response time,the associative process of electromagnetic levitation manufacture was established.2.Optimization design of electromagnetic fieldThe electromagnetic field calculation and the optimized solution were established based on finite element analysis.The distribution of electromagnetic field in the levitation space,Lorentz force and the eddy current in the levitated sample were analyzed systematically.Meanwhile,the correlations among levitation force,levitation position,sample size and electrical conductivity were discussed.Three types of optimized structure,such as single-layer coil,double-layer coil and coneshaped coil were obtained based on the optimized solution.The levitation capability of double-layer coil and coneshaped coil were improved by 110%and 21%,respectively.Furthermore,the influence of electromagnetic field distribution on the levitation force was experimentally verified by the above coil structures and various metallic spheres with the mass range from 10 to 100 g.All of the optimized coils contribute to the improvement of magnetic field in the levitation area and the reductions of minimum levitation current reach up to 8.1 and 33.1 pct,respectively.This shows a good agreement with the computational results and the double-layer coil is an excellent configuration to increase the levitation force.Electromagnetic levitation of 1033 g of aluminum,1179 g of copper,424 g of titanium and 282 g of nickel have been successfully achieved by greatly enhancing the levitation force through the optimized geometric structure of the coil.3.The solid-liquid phase transition of levitated metals driven by induction heatA thermo-electromagnetic-hydrodynamic model was established to explore the phase transition including the migration of liquid/solid interface,the evolution of melt flow and the temperature distribution under various EML conditions.To quantitatively describe the temperature uniformity of levitated solid sample,a defined number related to sample properties D²ρσ^-1k^-1was proposed.The space and time evolution of liquid/solid interface migration features are strongly correlated with the electromagnetic field of levitation space.In heating process,the sample reached the lowest equilibrium temperature and the minimum melting rate when it was levitated in the middle of the levitation zone.An oval molten pool formed near the lower sample surface firstly,then,the liquid/solid interface moved up.The migration behaviors under extreme conditions were predicted by simulation that molten pools formed simultaneously on the upper and lower surfaces when the sample was balanced near the levitation ceiling.4.Levitation characteristics of bulk liquid metal in electromagnetic fieldBased on the two types of optimized electromagnetic design,the levitation behaviors of bulk melts were investigated using computational fluid dynamics（CFD）modeling coupled with high-frequency electromagnetic field analysis and the arbitrary Lagrangian–Eulerian（ALE）method.The motion of the mass center,the deformation of melt shape,flow pattern,and Lorentz force in the levitated melt were determined within a wide excitation current range and various coil structures.The difference on the oscillation behavior between the solid and molten samples was explained by the damping of the electromagnetic induction and liquid viscosity.With the increase of the applied current,the melt’s centroid position rose sharply in the area with low current but slightly in the area with high current.The internal flow pattern transformes from a simple double-loop structure to a complex configuration with three or four loops.Meanwhile,the bulk melt cross section exhibites an evolutionary pattern that can be described as‘‘long taper→short taper→rhombus.’’The strong correlations among the deformation information,melt properties,and applied current were investigated using the Bond number.A series of melting experiments that used aluminum samples under the protective inert gas condition were executed to analyze the shape control and the swing process.5.Electromagnetic levitation manufacture of Al-Si eutectic alloysIn order to systematically explore the EML manufacture process,3 types of Al-Si alloys were electromagnetically levitated,and containerlessly melted and then solidified in the EML instrument.The maximum undercoolings of hypoeutectic Al-5at.%Si alloy,eutectic Al-12.2at.%Si alloy and hypereutectic eutectic Al-20 at.%Si alloy are 22 K,44 K and 70 K,respectively.Primary Al phase and eutectics are evenly distributed in hypoeutectic Al-5at.%Si alloy.Al-12.2at.%Si eutectic alloy has a uniform needle-like eutectic structure.Silicon phase is rich in the surface layer of the Al-20at.%Si hypereutectic alloy sample,and the inside is uniform eutectic structure.The taper and wedge-shaped components were made by 3 types of Al-Si alloys.A heat transfer model for the rapid solidification of spherical and taper-shaped components was established to explore the migration of liquid/solid interface,temperature change,heat flow direction and cooling rate.The spherical Al-Si alloy with a radius of 10 mm takes 0.5 s to solidify.In this solidification process,the cooling rate decreases gradually from 10⁵ K/s to 7×10² K/s.The temperature distribution in sphere center is higher and the surface layer is lower.The heat flow is emitted from the center to the surface layer,and the liquid-solid interface moves from the outer shell gradually towards the center.The taper-shaped Al-Si alloy takes 0.8 s to solidify.