Feedback-Based Control Over the Spatio-Temporal Distribution of Arcs During Vacuum Arc Remelting via Externally Applied
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Feedback-Based Control Over the Spatio-Temporal Distribution of Arcs During Vacuum Arc Remelting via Externally Applied Magnetic Fields MATT CIBULA, PAUL KING, and JOSH MOTLEY Ampere Scientific’s VARmetricTM measurement system for Vacuum Arc Remelting (VAR) furnaces passively monitors the distribution of arcs over time during VAR in real time. The arc behavior is known to impact both product yield and quality and can pose potentially catastrophic operating conditions. Arc position sensing with VARmetricTM enables a new approach to control the heat input to the melt pool. Transverse external magnetic fields were applied to push the arcs via the Lorentz force while measuring the arc location to control the arc distribution over time. This has been tested on Ampere Scientific’s small-scale laboratory arc furnace with electromagnets used for control for up to 60 seconds while monitoring the arc location with VARmetricTM. The arc distributions were shown to be significantly different from the uncontrolled distributions with distinct thermal profiles at the melt pool. Alternatively, this type of control can be periodically applied to react to undesirable arc conditions. https://doi.org/10.1007/s11663-020-01959-w Ó The Minerals, Metals & Materials Society and ASM International 2020
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INTRODUCTION
VACUUM Arc Remelting (VAR) furnaces are the workhorse for the manufacture of high value metals and alloys (Ni, Ti, Nb, Zr, Hf, etc.). During VAR, the input material (electrode) is lowered into a water-cooled copper crucible and heated under vacuum by an electric arc (50kW-5MW), the liquid metal drips into the crucible, and the molten pool solidifies into a homogeneous ingot.[1] The controllable parameters of the solidification process, including the input heat (current and voltage), crucible dimensions, materials, and cooling rate, are critical to the production of defect-free homogeneous materials.[2] These input parameters and associated boundary conditions control the properties of the molten pool, such as the pool depth, solidification angle, Rayleigh number, liquid velocity, and circulation time—all of which affect the persistence of defects, such as inclusions, freckles, white spots, etc., into the final ingot.[3] Despite the importance of producing defect-free ingots for safety-critical applications, including jet engines and medical implants, the VAR process has remained relatively unchanged since its introduction in the 1940s. Notable improvements in control include the
addition of cameras to visually monitor the melt around the annulus between the electrode and crucible wall, which improved safety during the melting process (by separating the operators from the furnace) while helping to better understand the arc dynamics via visual inspection[4,5] and the introduction of drip-short control over the vertical position of the electrode.[6,7] While drip-short control may provide more consistent melting and heating over time, it does not provide a mapping of the spatial distribution of heat at the surface of the melt pool, nor doe
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