Magnetic Flux Controllers in Induction Heating and Melting
MAGNETIC FLUX CONTROLLERS are materials other than the copper coil that are used in induction systems to alter the flow of the magnetic field. Magnetic flux controllers used in power supplying components are not considered in this article. Magnetic flux controllers have been in existence since the development of the induction technique. Michael Faraday used two coils of wire wrapped around an iron core in his experiments that led to Faraday’s law of electromagnetic induction, which states that the electromotive force (emf) induced in a circuit is directly proportional to the time rate of change of the magnetic flux through the circuit. After the development of the induction principle, magnetic flux controllers, in the formof stacks of laminated steel, found widespread use in the development of transformers for more efficient transmission of energy (Ref 1, 2). Magnetic cores gained widespread use in the transformer industry because they increased the amount of magnetic flux produced with the same alternating current. The higher the magnetic flux, the higher the emf, which results in an increase in energy transfer efficiency from the primary winding to the secondary winding. Similar to transformers, magnetic cores were used on early furnaces for induction melting (Ref 1, 2). The benefits of magnetic flux controllers vary depending on the application. For induction heating, magnetic flux controllers can provide favorable and unfavorable paths for magnetic flux to flow, resulting in increased heating in desired areas and reduced the heating in undesirable areas, respectively. Magnetic flux controllers are not used in every induction heating application, but their use has increased (Ref 3, 4).
Optimizing Axle-Scan Hardening Inductors
hile being asked to produce better product, automotive parts suppliers have been faced with increased competition and relentless demands to lower costs. A fundamental study using computer simulation revealed that there was signifi cant potential for cost savings on induction-scanning coils by incorporating precisionmachined inductors with selective use of magnetic fl ux controllers. The cost savings come in the form of improved part quality, reduced sensitivity, increased repeatability, increased production rates and energy savings. These results have been validated in several production facilities on both semi-fl oat and full-fl oat axles. A case story will be presented to demonstrate typical results. Introduction In the early 1930s, the fi rst industrial induction heat-treating processes were developed. Vehicle axles were some of the fi rst parts induction hardened. There are two main processes for induction heat treating of axles – scanning and single shot. This paper focuses on induction-scanning processes. Induction axle-scanning installations typically utilize single or two-turn profi led inductors made from formed tubing. Two-turn profi led inductors are used wherever possible because they offer energy savings and higher possible production rates. The drawbacks of two-turn inductors are the ability to properly treat the fi llet area and run-out at the end of the shaft. Single-turn profi led inductors are used when it is not possible to meet the heat-treatment specifi cations using a two-turn inductor. A picture of typical single and two-turn scanning inductors along with a brass quench ring is shown in Fig. 1. The brass quench ring is mechanically fastened to, but electrically isolated from the coil interconnect with the buss bars. In some cases, a nonmetallic quench ring is used instead of brass. A picture during the axlescan hardening process is shown in Fig. 2. These traditional inductor designs were developed empirically during the rapidtechnology-adoption time for induction heat treating (1930–1970), and they offered signifi cant cost savings compared to other competitive heat-treating technologies. Through process settings and coil diameter adjustment, it was possible to treat a wide variety of axles using the same equipment. To succeed in today’s more competitive W
EXPERIMENTAL AND NUMERICAL STUDY OF THE COLD CRUCIBLE MELTING PROCESS
The cold crucible, or induction skull melting process as is otherwise known, has the potential to produce high purity melts of a range of difficult to melt materials, including Ti–Al and Ti6Al4V alloys for Aerospace, Ti–Ta and other biocompatible materials for surgical implants, silicon for photovoltaic and electronic applications, etc. A water cooled AC coil surrounds the crucible causing induction currents to melt the alloy and partially suspend it against gravity away from water-cooled surfaces. 
 
Strong stirring takes place in the melt due to the induced electromagnetic Lorentz forces and very high temperatures are attainable under the right conditions (i.e., provided contact with water cooled walls is minimised). In a joint numerical and experimental research programme, various aspects of the design and operation of this process are investigated to increase our understanding of the physical mechanisms involved and to maximise process efficiency. A combination of FV and Spectral CFD techniques are used at Greenwich to tackle this problem numerically, with the experimental work taking place at Birmingham University. Results of this study, presented here, highlight the influence of turbulence and free surface behaviour on attained superheat and also discuss coil design variations and dual frequency options that may lead to winning crucible designs.