Publications

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A particularly challenging problem associated with vacuum arc remelting occurs when trying to maintain accurate control of electrode melt rate as the melt zone passes through a transverse crack in the electrode. As the melt zone approaches the crack, poor heat conduction across the crack drives the local temperature in the electrode tip above its steady-state value, causing the controller to cut back on melting current in response to an increase in melting efficiency. The difficulty arises when the melt zone passes through the crack and encounters the relatively cold metal on the other side, giving rise to an abrupt drop in melt rate. This extremely dynamic melting situation is very difficult to handle using standard load-cell based melt rate control, resulting in large melt rate excursions. We have designed and tested a new generation melt rate controller that is capable of controlling melt rate through crack events. The controller is designed around an accurate dynamic melting model that uses four process variables: electrode tip thermal boundary layer, electrode gap, electrode mass and melting efficiency. Tests, jointly sponsored by the Specialty Metals Processing Consortium and Sandia National Laboratories, were performed at Carpenter Technology Corporation wherein two 0.43 m diameter Pyromet® 718 electrodes were melted into 0.51 m diameter ingots. Each electrode was cut approximately halfway through its diameter with an abrasive saw to simulate an electrode crack. Relatively accurate melt rate control through the cuts was demonstrated despite the observation of severe arc disturbances and loss of electrode gap control. Subsequent to remelting, one ingot was sectioned in the "as cast" condition, whereas the other was forged to 0.20 m diameter billet. Macrostructural characterization showed solidification white spots in regions affected by the cut in the electrode.

A new controller has been designed for vacuum arc remelting titanium alloys based on an accurate, low order, nonlinear, melting model. The controller adjusts melting current and electrode drive speed to match estimated gap and melt rate with operator supplied reference values. Estimates of gap and melt rate are obtained by optimally combining predictions from the model with measurements of voltage, current, and electrode position. Controller tests were carried out at Timet Corporation's Henderson Technical Laboratory in Henderson, Nevada. Previous test results were used to correlate measured gap to voltage and current. A controller test melt was performed wherein a 0.279 m diameter Ti-6Al-4V electrode was melted into 0.356 m diameter ingot. Commanded melt rate was varied from 20 to 90 g/s and commanded gap was held at 1.5 cm. Because no measure of electrode weight was available on the test furnace, electrode position data were analyzed and the results used to determine the actual melt rate. A gap-voltage-current factor space model was used to check estimated gap. The controller performed well, and both melt rate and electrode gap control were successfully demonstrated.

The dynamics of particle suspensions in heterodyned triaxial magnetic fields was discussed. Triaxial magnetic fields were used to create complex particle interactions. The interactions were observed after heterodyning of the field component was employed to produce slow oscillations. Analysis suggested the application of triaxial fields for producing improved materials.

Field-structured composites (FSCs) were produced by hosting micron-sized gold-coated nickel particles in a pre-polymer and allowing the mixture to cure in a magnetic field environment. The feasibility of controlling a composite's electrical conductivity using feedback control applied to the field coils was investigated. It was discovered that conductivity in FSCs is primarily determined by stresses in the polymer host matrix due to cure shrinkage. Thus, in cases where the structuring field was uniform and unidirectional so as to produce chainlike structures in the composite, no electrical conductivity was measured until well after the structuring field was turned off at the gel point. In situations where complex, rotating fields were used to generate complex, three-dimensional structures in a composite, very small, but measurable, conductivity was observed prior to the gel point. Responsive, sensitive prototype chemical sensors were developed based on this technology with initial tests showing very promising results.

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Electrode gap is a very important parameter for the safe and successful control of vacuum arc remelting (VAR), a process used extensively throughout the specialty metals industry for the production of nickel base alloys and aerospace titanium alloys. Optimal estimation theory has been applied to the problem of estimating electrode gap and a filter has been developed based on a model of the gap dynamics. Taking into account the uncertainty in the process inputs and noise in the measured process variables, the filter provides corrected estimates of electrode gap that have error variances two-to-three orders of magnitude less than estimates based solely on measurements for the sample times of interest. This is demonstrated through simulations and confined by tests on the VAR furnace at Sandia National Laboratories. Furthermore, the estimates are inherently stable against common process disturbances that affect electrode gap measurement and melting rate. This is not only important for preventing (or minimizing) the formation of solidification defects during VAR of nickel base alloys, but of importance for high current processing of titanium alloys where loss of gap control can lead to a catastrophic, explosive failure of the process.