Innovations in Soft Magnetic Composites and their Applications in Induction Systems

R. Ruffini, N. Vyshinskaya, V. Nemkov, R. Goldstein, C.J. Yakey

Fluxtrol Inc., Auburn Hills, MI USA fluxtrol@fluxtrol.com, 248-393-2000, www.fluxtrol.com

Abstract

New soft magnetic composites have been added to the current family of materials produced by Fluxtrol Inc. which allows users to increase their range of magnetic flux controller applications and improve their overall inductor performance. A new material (Fluxtrol 100) is a substitution for a current well known material (Fluxtrol A). This new material has better mechanical properties, machinability and low anisotropy. The formable materials of Alphaform are effective on I.D. induction coils and various small coils of complex geometries. These materials may be used at any frequency, up to several megahertz. Along with the description of new materials, this presentation contains information about recent improvements in application of Fluxtrol materials including preparation, forming and gluing. One of new methods is impregnation of magnetic concentrators. This advanced technology consists in vacuum treatment of magnetic controllers or whole induction coils with subsequent placing them into a bath of a special resin. Resin penetrates into the material pores and gaps between the concentrators and copper turns and polymerizes inside of them. This treatment increases mechanical strength of the material and total assembly and improved corrosion resistance. Induction coils for axle and crankshaft hardening as well as small ID coils are selected for illustration.

Introduction

Magnetic Controllers on Heat Treating Inductors Modification of magnetic field distribution and control of its intensity on the surface of the parts to be heated may be accomplished by different methods: by variation of the coil turn shape and positioning, by insertion of non-magnetic shields and magnetic templates that may be called magnetic controllers. Non-magnetic shields, typically made in the form of copper rings or massive copper blocks, are often called “flux robber rings” [1]. Their use leads to reduction of the coil power factor and efficiency and they are not considered in this paper. Magnetic flux controllers are made of soft magnetic materials: steel laminations, ferrites and magnetic composites. Magnetic controllers can concentrate field in required areas (field concentration), change field distribution, shield certain areas from unintended heating and strongly reduce the magnetic field outside the treatment area. The team of Fluxtrol Inc. has developed the basics of magnetic flux control including the theory, methods of simulation and design, application technique guidelines. A course that contains these topics as well as the basics of induction heating may be found on the company website under the tab Training [2]. A role of magnetic flux control and methods of computer design of induction coils with magnetic controllers are presented also in multiple papers, e.g. in [3-5]. The use of magnetic controllers on heat treating induction coils can provide accurate control heat pattern, improvement of the coil efficiency and power factor, better utilization of power transferred to the part in local heating processes. It can also result in reduction of the coil current demand thus improving performance of the whole induction system and protect machine or the part components from unintended heating Technical and economic effects of magnetic flux controllers are the following: better part quality, higher production rate or energy savings, reduction of required power of the heating equipment. The most effective design method of the induction coil with magnetic controllers is to use computer simulation [3]. In this way both the coil copper and concentrator may be optimized for the best performance in a particular application. It is important to state that the controller design, selection of material and application technique can strongly influence performance and lifetime of heavy loaded induction coils. The goal of this paper is to inform the induction community about the latest improvement in development and application of magnetic controllers.

Materials for Magnetic Controllers on Heat Treating Inductors There  are  three  groups  of  materials  that  can  be  used  for  magnetic  flux  controllers:  laminations,  ferrites  and  Soft  Magnetic  Composites  (SMC),  aka  MagnetoDielectrics  Materials  (MDM).          Laminations  are  thin  sheets  of  electrical  steel  with  thin  electrical  insulation  on  their  surface.  They  are  working  well  in  plane-­‐parallel  (2D)  magnetic  fields  at  frequencies  up  to  20  kHz,  sometimes  even  at  30  kHz.  Advantages  of  laminations  are:  very  high  permeability,  high  temperature  resistance,  high  thermal  conductivity  in  the  plane  of  sheets,  low  magnetic  losses  at  low  frequencies.  Lamination  drawbacks  are:  overheating  in  3D  magnetic  fields,  limited  frequency  range,  difficulty  in  machining  and  installation,  resulting  in  high  labor  costs  in  the  case  of  complex  coil  geometry.        Ferrites  are  glass-­‐like  materials  made  of  oxides  of  iron,  manganese,  zinc  and  other  elements.  In  spite  of  high  permeability  (in  weak  magnetic  fields  only!)  and  relatively  low  losses,  they  are  used  in  rare  cases  of  high  frequency  coils  of  small  sizes  due  to  the  following  drawbacks:

- They  are  very  hard  and  brittle  and  practically  non-­‐machinable - Saturation  flux  density  is  low  (up  to  0.3-­‐0.4  T) - Low  service  temperature  for  majority  of  types  due  to  low  Curie  point - Low  thermal  conductivity

SMCs are made from ferrous particles (iron and its alloys), covered with very thin insulation layer, mixed with organic or inorganic binder, pressed at high pressure (up to 720 MPa and even higher) and cured according to a special technology. Majority of SMC used in induction industry have organic binders, which provides good machinability. All pressed materials have certain anisotropy (up to 1.5-2 times in permeability depending upon structure) but all of them work well in 3D fields. High frequency materials have low anisotropy. Possibility to work in 3D fields and good machinability are highly valued by the coil manufacturers. Different types of SMC cover the whole range of frequencies used in induction heating (50 Hz – 13.56 MHz). Losses at low frequency are comparable to laminations and at high frequencies – to ferrites. Temperature resistance is lower than for laminations but usually sufficient for induction applications. High thermal conductivity (up to 0.23 W/cmK, i.e. 35% higher than solid stainless steel material) and possibility of effective thermal management using external or internal cooling can keep controllers safe in heavy loaded cases. The main drawbacks of SMC are limited dimensions (up to 220 mm long plates at present time and higher price of material. However with account for labour cost and possible improvement in coil life time, use of SMC in many cases occurs cheaper that laminations. It is especially correct when using net-shape manufactured or machined controllers, fig.1. Technical and economic analyses show that in some cases a combination of different materials give excellent results. For example, laminations may be used for the regular part of controllers and SMC for areas with complex shape and 3D field, such as the end zones of seam annealing coils.

Figure 1: SMC blocks painted for identification (left) and different machined magnetic controllers (right)

SMC is a class of materials that was significantly improved during the last decade. There are newer materials with improved properties such as Fluxtrol 100, Fluxtrol LF designed for low frequency applications (shielding of melting furnaces, low frequency heat treating, etc.) and formable materials of Alphaform type, which can be applied to inductors of irregular shape manufactured with low tolerance. Several studies have been performed in order to improve technique for application of magnetic controllers to the coils and to develop corresponding guidelines for users.

New SMC Materials

Properties of new materials are presented in Table 1 in comparison with traditional material Fluxtrol A. Table 1: Properties of Fluxtrol 100, Fluxtrol LF and Alphaform materials in comparison with Fluxtrol A

Fluxtrol 100 Fluxtrol 100 is a new material with different insulation and binder complex than traditional Fluxtrol materials (Fluxtrol A, 50 and Ferrotron 559 and 119). It is designed for use in a wide range of frequencies up to 50 kHz instead of Fluxtrol A. Material has lower anisotropy than Fluxtrol A and better mechanical properties, which allows the users to machine parts with sharp corners and thin walls. Magnetic properties of Fluxtrol 100 and A are very similar in favorable direction perpendicular to direction of pressing, fig.2. Permeability of Fluxtrol 100 in direction of pressing is much higher than of Fluxtrol A and it does not require the user to care about material orientation when designing the controllers. Thermal conductivity of Fluxtrol 100 is also higher and it allows us reduce the rated temperature of material to 200 C. However material can work for a long time at temperature 250 C with the same magnetic properties and reduced electric resistivity.

Figure 2: Magnetic permeabilities of Fluxtrol A and 100 in two directions

Alphaform materials

These materials are manufactured from magnetic particles of different dimensions for “lower” (LF), middle (MF) and high frequencies (HF) mixed with a special epoxy compound. Alphaform material is supplied in tins, which is advised to keep refrigerated for longer life time, fig. 3, left. Materials may be manually formed/shaped when warm. After that the coil must be heated for curing. During heating the material passes through the transient stage when it becomes relatively thin to flow out and special coating or wrapping is necessary.

Figure 3: Tins with Alphaform materials (left) and ID induction coil with magnetic core (right)

Alphaform materials may be effectively used on ID induction coils and wrapped tubing coils of complex or irregular geometries due to its flexibility during application. Material sticks to copper tubing resulting in good mechanical integrity and very good thermal contact even for non-machined coils with significant tolerances, fig.3, right. Due to the ease of installation (and removal when needed) this SMC are also great for lab and development projects where immediate results are needed.

SMC Controllers on Crankshaft Hardening Coils

Over the last 5 years a big progress has been made in use of Soft Magnetic Composites in the Elotherm (rotational) style and clamshell (non-rotational) style crankshaft induction hardening coils. In rotational style inductors laminations have been the norm for decades, but SMC have proven to be more cost effective in coil assembly techniques and overall coil performance, including inductor lifetime. The ease of installation and modification make for easier adjustments at setup. More complex coil designs can be achieved to deal with more challenging aspects of crankshaft hardening such as fillets and undercuts due to the flexibility/machinability of SMC materials, fig.4.

Figure 4: Hardness pattern (left) and induction coil with Fluxtrol 100 concentrators, right

In clamshell or non-rotational inductors the application of SMC controllers provides excellent heat pattern control in journal circumference and width while reducing the required amount of power needed to achieve pattern specifications. Along with improved heat pattern uniformity we now have industry feedback confirming increased coil life due to the application of side shields in these types of inductors. As these types of applications grow, more and more data is being gathered for analysis and comparisons to older styles of crankshaft magnetic controllers.

Recommended Application Techniques for Soft Magnetic Composites

Fluxtrol Inc. is constantly pursuing the best ways to not only adhere our material to inductors, but to make it easier for our customers to access this technology and apply it themselves with ease and confidence. This way insuring our material is performing at its peak, is structurally sound and being cooled to the best of its applications ability. All of which leads to the best performing induction systems available. Many factors come into play when attaching Fluxtrol material to an inductor. First and foremost is the use of the proper grade of Fluxtrol for your application

Conclusions

Fluxtrol, Inc. continues to improve existing and introduce new composite materials to meet industry demands. Magnetic flux controllers can improve heat pattern, prevent unintended heating of the part or hardening machine, improve induction coil parameters and performance of the whole induction installation. As a result, proper application of magnetic flux controllers can strongly improve heat pattern control, increase production rates, save energy and cut manufacturing costs. Soft Magnetic Composites manufactured by Fluxtrol Inc. are the primary choice for magnetic controllers. They cover the whole range of frequencies used for induction heat treating (from line frequency up to several megahertz), may be easily machined to any desirable shape and used as constructive elements of the coil. Magnetic permeability of these SMC reaches 120, which is sufficient for almost all induction heating applications. The most effective way to design induction coils with magnetic controllers is to use computer simulation, which can predict the coil performance prior to its manufacturing.

References

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[2] Website www.fluxtrol.com [3] Goldstein, R. et al., “Virtual Prototyping of Induction Heat Treating”, Proc. of the 25th Conf. ASM Heat

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Metallurgical Process Design. Chapter 15. Marcel Dekker; New York, NY-USA. 2004; pp. 591–640 [5] Nemkov V., Goldstein R., Ruffini R., “Optimal Design of Induction Coils with Magnetic Flux Controllers,” ”

Proc. of Int. Symposium HES-07 “Heating by Electromagnetic Sources”, Padua, Italy, 2007 [6] Ruffini, R., Nemkov, V., Vyshinskaya, N., “New Magnetodielectric Materials for Magnetic Flux Control.

”Proc. of Int. Symposium HES-04, “Heating by Electromagnetic Sources”, Padua, Italy, June 2004 [7] Nemkov, V., Goldstein, R., “Optimal Design of Internal Induction Coils,” Proc. of Int. Symposium HES-04

“Heating by Electromagnetic Sources”, Padua, Italy, 2004 [8] Myers, C. et al., “Optimizing Performance of Crankshaft Hardening Inductors,” Industrial Heating, December,

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