Modeling of and experiments on electromagnetic levitation for materials processing

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1998.Vita.Includes bibliographical references (p. 111-116).Electromagnetic levitation (EML) is an important experimental technique for research in materials processing. It has been applied for many years to a wide variety of research areas, including studies of nucleation and growth, phase selection, reaction kinetics, and thermophysical property measurements. However, the design of these systems has, for the most part, been empirical, and it will be shown that a more fundamental approach can provide benefits in a number of aspects, leading to a better design. The work presented here contributes to three aspects of levitation systems: modeling of electromagnetic effects, modeling of fluid flow characteristics, and experiments to measure surface tension and viscosity in microgravity. In this work, the interaction between the electromagnetic field and the sample were modeled, and experiments to measure the surface tension and viscosity of liquid metal droplets were performed. The models use a 2-D axisymmetric formulation, and use the method of mutual inductances to calculate the currents induced in the sample. The magnetic flux density was calculated from the Biot-Savart law, and the force distribution obtained. Parametric studies of the total force and induced heating on the sample were carried out, as well as a study of the influence of different parameters on the internal flows in a liquid droplet. The oscillating current frequency has an important effect on the feasible operating range of an EML system. Optimization of both heating and positioning are discussed, and the use of frequencies far from those in current use for levitation of small droplets provides improved results. The dependences of the force and induced power on current, frequency, sample conductivity, and sample size are given. A model coupling the magnetic force calculations to a commercial finite-element fluid dynamics program is used to characterize the flows in a liquid sample, including transitions in the flow pattern. The dependence of fluid flow velocity on positioning force, sample viscosity, and oscillating current frequency is presented. These models were applied to the design of thermophysical property measurements were performed in microgravity on the Space Shuttle. These experiments depend on careful control of the fluid flow in the sample, based on the MHD model presented. The measurements use the oscillating drop technique to provide very precise containerless measurement of surface tension, and the first containerless measurement of viscosity. Results are presented for surface tension and viscosity of a Pd-18Si alloy for a large range of temperature, including both the superheated and undercooled regimes, as an example of the many data taken on many materials, including zirconium, steels, and modern metallic glass forming alloys.by Robert W. Hyers.Ph.D

Numerical representations for flow velocity and shear rate inside electromagnetically levitated droplets in microgravity

Electromagnetic levitation techniques are used in a microgravity environment to allow materials research under containerless conditions while limiting the influence of gravity. The induced advective flow inside a levitated molten alloy droplet is a key factor affecting solidification phenomena while potentially influencing the measurement of thermophysical properties of metallic alloy. It is thus important to predict the flow velocity under various operation conditions during melt processing. In this work, a magnetohydrodynamic model is applied over the range of conditions under which electromagnetically levitated droplets are processed to represent the maximum flow velocity and shear rate as a polynomial function of heating voltage, density, viscosity, and electrical conductivity of molten materials. An example is given for the ternary steel alloy Fe-19Cr-21Ni (at%) to demonstrate how internal advection under different heater settings becomes a strong function of alloy temperature and is a determining factor in the transition from laminar to turbulent flow conditions. The results are directly applicable to a range of other materials with properties in the range considered, including Ni-based superalloys, Ti-6Al-4V, and many other commercially-important alloys

Impact of convection on the damping of an oscillating droplet during viscosity measurement using the ISS-EML facility

Oscillating droplet experiments are conducted using the Electromagnetic Levitation (EML) facility under microgravity conditions. The droplet of molten metal is internally stirred concurrently with the pulse excitation initiating shape oscillations, allowing viscosity measurement of the liquid melts based on the damping rate of the oscillating droplet. We experimentally investigate the impact of convection on the droplet’s damping behavior. The effective viscosity arises and increases as the internal convective flow becomes transitional or turbulent, up to 2–8 times higher than the intrinsic molecular viscosity. The enhanced effective viscosity decays when the stirring has stopped, and an overshoot decay pattern is identified at higher Reynolds numbers, which presents a faster decay rate as the constraint of flow domain size becomes influential. By discriminating the impact of convection on the viscosity results, the intrinsic viscosity can be evaluated with improved measurement accuracy

Electrostatic Levitation for Studies of Materials for Additive and In-Space Manufacturing

The electrostatic levitation (ESL) laboratory at NASA's Marshall Space Flight Center (MSFC) is a unique facility for investigators studying high-temperature materials. Electrostatic levitation minimizes gravitational effects and allows materials to be studied without contact with a container or instrumentation

Intelligent, Self-Diagnostic Thermal Protection System for Future Spacecraft

The goal of this project is to provide self-diagnostic capabilities to the thermal protection systems (TPS) of future spacecraft. Self-diagnosis is especially important in thermal protection systems (TPS), where large numbers of parts must survive extreme conditions after weeks or years in space. In-service inspections of these systems are difficult or impossible, yet their reliability must be ensured before atmospheric entry. In fact, TPS represents the greatest risk factor after propulsion for any transatmospheric mission. The concepts and much of the technology would be applicable not only to the Crew Exploration Vehicle (CEV), but also to ablative thermal protection for aerocapture and planetary exploration. Monitoring a thermal protection system on a Shuttle-sized vehicle is a daunting task: there are more than 26,000 components whose integrity must be verified with very low rates of both missed faults and false positives. The large number of monitored components precludes conventional approaches based on centralized data collection over separate wires; a distributed approach is necessary to limit the power, mass, and volume of the health monitoring system. Distributed intelligence with self-diagnosis further improves capability, scalability, robustness, and reliability of the monitoring subsystem. A distributed system of intelligent sensors can provide an assurance of the integrity of the system, diagnosis of faults, and condition-based maintenance, all with provable bounds on errors

Containerless Measurements of Density and Viscosity of Fe-Co Alloys

During the past years, extensive collaborative research has been done to understand phase selection in undercooled metals using novel containerless processing techniques such as electrostatic and electromagnetic levitation. Of major interest is controlling a two-step solidification process, double recalescence, in which the metastable phase forms first and then transforms to the stable phase after a certain delay time. The previous research has shown that the delay time is greatly influenced by the internal convection velocity. In the prediction of internal flow, the fidelity of the results depends on the accuracy of the material properties. This research focuses on the measurements of density and viscosity of Fe-Co alloys which will be used for the fluid simulations whose results will support upcoming International Space Station flight experiments

Advances in Non-contact Measurement of Creep Properties

Our team has developed a novel approach to measuring creep at extremely high temperatures using electrostatic levitation (ESL). This method has been demonstrated on niobium up to 2300 C, while ESL has melted tungsten (3400 C). High-precision machined spheres of the sample are levitated in the NASA MSFC ESL, a national user facility, and heated with a laser. The laser is aligned off-center so that the absorbed photons transfer their momentum to the sample, causing it to rotate at up to 250,000+ RPM. The rapid rotation loads the sample through centripetal acceleration, causing it to deform. The deformation of the sample is captured on high-speed video, which is analyzed by machine-vision software from the University of Massachusetts. The deformations are compared to finite element models to determine the constitutive constants in the creep relation. Furthermore, the noncontact method exploits stress gradients within the sample to determine the stress exponent in a single test. This method was validated in collaboration with the University of Tennessee for niobium at 1985 C, with agreement within the uncertainty of the conventional measurements. A similar method is being employed on Ultra-High-Temperature ZrB2- SiC composites, which may see application in rocket nozzles and sharp leading edges for hypersonic vehicles

Experimental Studies of Carbon Nanotube Materials for Space Radiators

Game ]changing propulsion systems are often enabled by novel designs using advanced materials. Radiator performance dictates power output for nuclear electric propulsion (NEP) systems. Carbon nanotubes (CNT) and carbon fiber materials have the potential to offer significant improvements in thermal conductivity and mass properties. A test apparatus was developed to test advanced radiator designs. This test apparatus uses a resistance heater inside a graphite tube. Metallic tubes can be slipped over the graphite tube to simulate a heat pipe. Several sub ]scale test articles were fabricated using CNT cloth and pitch ]based carbon fibers, which were bonded to a metallic tube using an active braze material. The test articles were heated up to 600 C and an infrared (IR) camera captured the results. The test apparatus and experimental results are presented here

Examining the influence of turbulence on viscosity measurements of molten germanium under reduced gravity

The thermophysical properties of liquid germanium were recently measured both in parabolic flight experiments and on the ISS in the ISS-EML facility. The viscosity measurements differed between the reduced gravity experiments and the literature values. Since the oscillating drop method has been widely used in EML, further exploration into this phenomenon was of interest. Models of the magnetohydrodynamic flow indicated that turbulence was present during the measurement in the ISS-EML facility, which accounts for the observed difference

Demonstration of the effect of stirring on nucleation from experiments on the International Space Station using the ISS-EML facility

The effect of fluid flow on crystal nucleation in supercooled liquids is not well understood. The variable density and temperature gradients in the liquid make it difficult to study this under terrestrial gravity conditions. Nucleation experiments were therefore made in a microgravity environment using the Electromagnetic Levitation Facility on the International Space Station on a bulk glass-forming Zr57Cu15.4Ni12.6Al10Nb5 (Vit106), as well as Cu50Zr50 and the quasicrystal-forming Ti39.5Zr39.5Ni21 liquids. The maximum supercooling temperatures for each alloy were measured as a function of controlled stirring by applying various combinations of radio-frequency positioner and heater voltages to the water-cooled copper coils. The flow patterns were simulated from the known parameters for the coil and the levitated samples. The maximum nucleation temperatures increased systematically with increased fluid flow in the liquids for Vit106, but stayed nearly unchanged for the other two. These results are consistent with the predictions from the Coupled-Flux model for nucleation