Vacuum Electromagnetic Levitation Melting and Undercooling of Peritectic Terfenol-D

For the first time, the undercooling of a magnetostrictive material a near peritectic Tb 0.27 Dy 0.73 Fe 1.90 alloy was realized by vacuum electromagnetic levitation melting and 60 K undercooling was obtained. There is one recalescence behavior during solidification of the undercooled melt,which can attribute to the priority precipitation of REFe 2 phase instead of REFe 3 phase, due to preferential nucleation and higher crystal growth rate of REFe 2 phase and the suppression of peritectic reaction. According to the crystal structural characteristics of REFe 2 and REFe 3, REFe 2 is a Laves phase intermetallics with MgCu 2 type structure, which has similar polytetrahedral structure with short range ordered structure in undercooled melt and has lower potential barrier for nucleation than that of REFe 3,which lead to the preferential nucleation of REFe 2 phase directly from the undercooled melt. Also, the similarity of structures between REFe 2 phase and undercooled melt leads to higher crystal growth rate of REFe 2 phase than that of REFe 3.

Effect of sample radius on stability of electromagnetic levitation melting

Based on the power dissipating model of spherical sample in free convection gas medium and the expression of input power, the model of temperature calculation for electromagnetic levitation melting sample was established. Considering the limitation of levitation force and levitation sample temperature,the principle of stability levitation zone computation was determined. A spherical sample （ThDy）Fe2 under the protection of argon gas was examined, and the effect of radius of levitation sample and perturbation on the stable levitation zone was investigated. The results show that longitudinal perturbation and transverse perturbation can shorten the length of stable levitation zone and the range of levitation sample radius. By increasing the sample radius and weakening the perturbation the electromagnetic levitation melting stability of sample can be improved.

THE ELECTROMAGNETIC FIELD CALCULATION AND ANALYSIS OF LEVITATION MELTING WITH COLD CRUCIBLE

In this paper, using the quasi-3D coupled current method, the influences of structure of cold crucible, the power frequency, the electricity property of melt, the coil position and current on the electromagnetic field (EMF) and the levitation characteristics in the melting processes are analyzed. It is shown that in the processes of levitation melting with cold crucible, the power frequency and cold crucible structure are the decisive factors for the ability of magnetic flux penetrating into cold crucible. The magnetic flux density in cold crucible is reduced as the increasing of power frequency, and this tendency becomes stronger when the power frequency is higher than 100kHz. The segmented structure of cold crucible can reduce the induction eddy in itself effectively, and the higher the power frequency is, the better the result is. So, a cold crucible can be segmented into 16-20 sectors for high frequency electromagnetic field and/or 8-12 sectors for lower frequency one. It is also shown that the levitation force of melting charge is related to coil current as a parabolic function.

New Two-Frequency Method and Devices for Electromagnetic Levitation Drip- and Leakage-Free Melting of Metals

A new method and devices for the electromagnetic levitation melting of metallic samples with greater weights and axisymmetrically stabilized positions is presented in this paper.The new method is applying two homogeneous magnetic fields of different frequencies,whose field lines are in absence of a charge horizontally and reciprocally normal in order to exert electromagnetic lift forces also on the axis of the levitated sample.Therefore the weight of the charge can be increased and the charge can be drip- and leakage-free melted.The method can be used in a melting furnace with or without a cold protective wall,as well as for the coreless induction valves applied for flow rate control,e.g.in the continuous casting of molten metals.The applicability of the method was experimentally examined and proved by tests conducted with three types of laboratory setups.

Rapid solidification of Cu_(60)Co_(30)Cr_(10) alloy under different conditions

Metastable liquid phase separation and rapid solidification in a metastable miscibility gap were investigated on the Cu60Co30Cr10 alloy by using the electromagnetic levitation and splat-quenching.It is found that the alloy generally has a microstructure consisting of a（Co,Cr）-rich phase embedded in a Cu-rich matrix,and the morphology and size of the（Co,Cr）-rich phase vary drastically with cooling rate.During the electromagnetic levitation solidification processing the cooling rate is lower,resulting in an obvious coalescence tendency of the（Co,Cr）-rich spheroids.The（Co,Cr）-rich phase shows dendrites and coarse spheroids at lower cooling rates.In the splat quenched samples the（Co,Cr）-rich phase spheres were refined significantly and no dendrites were observed.This is probably due to the higher cooling rate,undercooling and interface tension.

H_∞ control design for unstable plants

For the control of unstable plants,it is required that the Nyquist locus of the system should encircle anticlockwise the point-1+j0,and the performance weighting function must be selected based on the specific requirement of the mid-frequency gain. Besides,the performance limitation imposed by the Bode's integral also must be considered. Therefore the H∞ design for unstable plants is different from that in the regular case. It is pointed out that the sensitivity function for unstable plants must remain constant over the mid-frequency range to ensure a robust design. The selection of weighting functions to meet these requirements in the H∞ design is discussed in detail in the paper. A mixed sensitivity H∞ design example is also given to demonstrate the special considerations of the design.

Numerical Simulation of Fluid Flow Inside a Molten Liquid Metal Droplet Located in an Electromagnetic Induction Generated by Two Different Inducting Currents

Electromagnetic levitation of electrically conductive droplets by alternating magnetic fields is Sa technique used to measure the physical properties of liquid metallic alloys such as surface tension,viscosity,heat capacity.Experiments can be conducted in microgravity,to reduce electromagnetic stirring and shaping of the droplet.The inductor of the EML is composed of a single inductor which two types of voltage are imposed.This type of electrical installation generates a field to center the metallic droplet and another field to heat and excite it.We use a commercial code join to a homemade code to compute flows generated by inducting voltages recorded in the sounding rocket TEXUS-EML-2(Feb2008).In this flight,two samples(Cu75Co25 at%and A168,5Ni31,5 at%)of 8 mm diameter have been processed.

Single-electromagnet levitation for density measurement and defect detection

This paper presents a single-electromagnet levitation device to measure the densities and detect the internal defects of antimagnetic materials.The experimental device has an electromagnet in its lower part and a pure iron core in the upper part.When the electromagnet is activated,samples can be levitated stably in a paramagnetic solution.Compared with traditional magnetic levitation devices,the single-electromagnet levitation device is adjustable.Different currents,electromagnet shapes,and distances between the electromagnet and iron core are used in the experiment depending on the type of samples.The magnetic field formed by the electromagnet is strong.When the concentration of the MnCl aqueous solution is 3 mol/L,the measuring range of the single-electromagnet levitation device ranges from 1.301 to 2.308 g/cm.However,with the same concentration of MnCl aqueous solution(3 mol/L),the measuring range of a magnetic levitation device built with permanent magnets is only from 1.15 to 1.50 g/cm.The single-electromagnet levitation device has a large measuring range and can realize accurate density measurement and defect detection of high-density materials,such as glass and aluminum alloy.