Composite Materials for Magnetic Field Control in EPM

Electromagnetic processing of materials requires generation of magnetic field in a very wide range of frequencies (from DC to several hundred kilohertz) and intensities (up to 12 T). AC magnetic systems are used for material stirring, casting, pouring control, transportation, forming etc. These systems typically may have magnetic circuits made of soft magnetic materials: laminations, ferrites and Soft Magnetic Composites (SMC) also known as Magnetodielectric Materials (MDM). Each type of materials has different electrical, magnetic and mechanical characteristics. This presentation gives an overview of characteristics of different soft magnetic materials and perspective of their use in the electromagnetic processing of materials. Main attention is paid to MDM, a relatively new type of materials. Presentation is based on experience of the authors, literature and discussions with experts in different industries. Introduction Magnetic fields are being widely used in electromagnetic processing of materials. It is natural, that magnetic field generation and control play a very important role. The goals of control are concentration of the field in some areas, shielding of the others and field distribution according to required pattern. Magnetic field control may be accomplished by variation of the coil turn shape and positioning, by insertion of non-magnetic shields and magnetic templates that may be called magnetic controllers. In different applications magnetic controllers have different names according to their role: concentrators, intensifiers, shields, cores, diverters. Non-magnetic shields, typically made in the form of copper rings or massive copper blocks, are often called “flux robbers”. Their use results in reduction of the induction system power factor and efficiency and they are not considered in this paper. Operating conditions of magnetic materials in EPM is very different than in traditional devises – motors, transformers, chokes, etc. In EPM the magnetic circuits are usually open or have a big gap. This circumstance dramatically changes requirements to material, first of all to its permeability. Special study made at Fluxtrol Inc. by computer simulation and confirmed by experiments, showed that in induction heating system the permeability of 40-50 is typically sufficient for good controller performance [1]. Moreover, in some cases, an excessive value of permeability can reduce the inductor efficiency. At the same time there are new requirements in EPM area compared to transformers etc., such as good performance in 3D fields, low losses at high frequency, good machinability, temperature resistance, etc. A course that contains the basics of magnetic flux control including the theory, methods of simulation and design and application technique guidelines may be found on the company website under the tab Training [2]. This information was developed mainly for induction heat treating but it may be effectively applied to other areas of EPM. The use of magnetic controllers on induction heat treating coils can provide accurate control of heat pattern, improve the coil efficiency and power factor, protect machine or the part components from unintended heating, better utilize power transferred to the part in local heating processes and reduce current demand to the coil thus improving performance of the whole system. In many applications controllers give more that one benefit. It is important to mention also that the controller design, selection of material and application technique can strongly influence lifetime of heavy loaded induction coils [3]. Materials Laminations are manufactured from silicon steel and are this material is traditionally used for magnetic field control in EPM. The main frequency range of their application is below 10 kHz though at times they are used at frequencies up to 50 kHz with reduced flux density and intensive cooling. Laminations have very high permeability with maximum values of several thousands and high saturation flux density (1.7-2.0 T). The main drawbacks of laminations are: frequency limits due to magnetic loss increase, laborious manufacturing of complex shaped controllers and poor performance in 3D magnetic fields, where they are being intensively heated by the field component perpendicular to the sheets. Laminations are manufactured in the form of sheets or rolls and may be used for very big cores by means of cutting or stamping and assembling. Ferrites may have very high permeability (in weak fields only!), low losses and can work in a very wide range of frequencies when properly selected and applied. However they have low saturation flux density and Curie temperature, are very sensitive to thermal and mechanical shocks and may not be machined except by grinding and cutting with diamond tools. For these reasons ferrites are used only at high frequencies in the form of simple shapes – cylinders, plates, rods or relatively small standard C or E parts. MDMs are made from magnetically soft particles and dielectric material which serves as a binder and electric insulator of the particles [2]. They are produced by pressing of different magnetic powders and binders with subsequent thermal treatment according to special technology. Fluxtrol Inc. is a primary manufacturer of SMC. Four grades of materials (Fluxtrol A, Fluxtrol 50, Fluxtrol 25 and Ferrotron 559H) are covering almost all the world’s induction heating market demands. Some properties of these materials are presented in Table 1 and in Fig.1. As it follows from Fig.1, magnetodielectric materials are quasi-linear, especially material Ferrotron 559. Its permeability is almost constant in very wide range of flux densities. More information about materials, their properties and application technique may be found in [2]. Properties Fluxtr ol A Fluxtrol 50 Fluxtrol 25 Ferr-n 559H Density, g/cm 6.6 6.1 5.6 5.9 Initial Permeability 63 36 22 18 Max Permeability 120 55 27 19 Saturation, T 1.6 1.5 1.4 1.0 Thermal cond-ty, W/cm K 0.20 0.06 0.05 0.05 Max frequency, kHz 50 50