Soft Magnetic Materials Utilizing via Conventional And Warm Compaction Technology

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1 Soft Magnetic Materials Utilizing via Conventional And Warm Compaction Technology Francis G. Hanejko & Michael Marucci Hoeganaes Corporation Abstract: Sintered P/M soft magnetic materials are used extensively in automobiles. Their applications are limited to DC applications. Using a sintered material in an AC application results in high heat losses and a loss in magnetic performance. To overcome this obstacle, a new class of soft magnetic materials was developed with the objective of displacing laminated steels in AC applications. Early experimental work demonstrated that the warm compacted materials offer optimal levels of strength and magnetic performance. The strength of these materials may permit the use of these warm compacted materials in low stress applications. However, many applications do not require the strength of the warm compacted materials and can utilize a lower strength material provided there is no compromise in the magnetic performance. This paper will review the magnetic and physical properties of both the warm compacted and conventionally compacted materials. Introduction: Opportunities for magnetic materials in automobiles continue to grow. There are numerous sensors for the engine, transmission, ABS wheel systems, etc. These parts are DC magnetic applications requiring ease of processing and good magnetic properties with good dimensional stability. [1] Their complex shape makes them ideal candidates for the P/M process. Presently, most of the DC applications within the automobile have been designed as a P/M component. However, the growing trend is still greater usage of electrical devices for motors and the like. Many of the new devices are using brushless DC (BLDC) or other types of high efficiency AC electric motors. In these motor designs, sintered P/M magnetic materials are not well suited. Although not a motor applications, a good example for the innovative use of materials is the ignition system used in many cars. The advent of coil at plug and coil near plug has 1

2 given rise to the development and use of P/M non-sintered AC magnetic materials. These materials are coated with an organic or inorganic coating that effectively insults each powder particle from its nearest neighbors thus preventing excessive eddy current losses from developing within the device. Advantages of these materials includes! Minimal material waste! Excellent high frequency properties! Low eddy current losses! Ability to customize the magnetic response to the demands of the particular application! 3-D flux carrying capability Despite the many advantages of insulated powder technology, there are specific drawbacks that preclude their broad acceptance. The limitations are:! Low strength with no ductility! Reduced permeability because of the distributed air gap! Reduced magnetic saturation because of the intrinsic porosity and non-flux carrying insulation. Shown in Figure 1 is a representation of the classification for non-sintered powder materials for AC applications. The earliest materials were the fluid bed coated materials; these materials were specifically designed for the ignition coils and utilized a polymeric material that could resist the temperatures experienced in under-hood type applications. These materials were introduced in the early 1990 s. The following discussion will focus on each of these materials with their respective advantages and disadvantages. Fluid Bed Coated Insulated Iron Powder Pressed & Cured Materials Conventionally Mixed Figure 1: Classification of AC magnetic materials derived from P/M technology. Iron Powder Polymer Composites (Fluid Bed Coated) Annealed Materials Annealed at 650 C The initial concept for these materials was to extend the range of the so-called dust core materials in applications requiring higher part densities with corresponding higher saturation induction levels and higher permeability. [2] A suitable polymer coating technology was developed with the objective of displacing laminations in medium to high frequency applications such as specialty electric motors, transformers, etc. As stated previously, these materials were commercialized in the mid 1990 s. [3] These materials combined polymer processing and iron powder metallurgy to produce an iron powder polymer composite. The basic processing steps of these materials are illustrated in Figure 2. The process starts with a high purity iron powder that is coated with a polymer via fluid bed processing. The suitable polymer is dissolved in a solvent and is subsequently sprayed onto the iron powder. After the coating step the coated iron powder is compacted in heated tooling, no sintering is required. The polymer coating, typically less than 1% by weight, both electrically insulates the iron particles and adds mechanical strength to the as pressed component. 2

3 An optional particle oxide coating can be applied for applications requiring greater interparticle resistivity. Iron powder polymer composites are designed for use in the as pressed condition. As such, the strength of the green part must be sufficient to withstand the stresses associated with the winding or final assembly of the component. The transverse rupture strength of the insulated iron is approximately 100 MPa (15,000 psi) in the as pressed condition. After compaction an optional low temperature heat treatment (315 C or 600 F for one hour), the strength increases to nearly 240 MPa (35,000 psi). Comparable strength of the standard P/M material in the green or as compacted condition is approximately 20 MPa (3000 psi). Pure Iron Powders Optional Oxide Coating Fluid Bed Coating with High Temperature Polymer W arm Compaction Powder Temperature C (350 F) Die Temperature 275 C (525 F) Optional Thermal Treatment--315 C (600 F) In Air Finished Component Figure 2: Processing steps for Iron Powder Polymer Composite materials Table 1 Summary of DC Magnetic Performance of Iron Powder Polymer Composites Material 690 MPa Perm 40 OE Hc, Oe SC SC TC Three distinct coated iron powders are currently available. Table 1 shows the composition and the DC magnetic properties of these three materials. Although three grades are presented, the manufacturing process for these powders is flexible and thus powders can be customized to meet specific application requirements. The effect of frequency on the permeability of these three materials is shown in Figure 3. The Ancorsteel SC120 material provides the highest permeability at low frequencies while the TC80 material gives the best high frequency performance. The increasing eddy currents as the frequency increases results in a decrease in effective permeability and the roll-off observed. The SC120 material is designed to provide the highest performance at lower frequency levels by controlling the particle size and utilizing just a single polymer coating. The TC80 material uses a finer particle size distribution, the oxide coating and a higher level of polymer coating minimizing the eddy current losses. The performance of the SC100 material lies between the other two grades. 3

4 SC-120 SC-100 Core Loss (µw/cm³) mm (0.007in) thick, 3% Si-Iron TC-80 Permeability at 10G TC E+02 1.E+03 1.E+04 1.E+05 1.E+06 Frequency (Hz) E+02 1.E+03 1.E+04 1.E+05 1.E+06 Frequency (Hz) Figure 3: Effect of operating frequency on the 10 Gauss permeability of the Iron Powder Polymer Composites Figure 4 shows the effect of frequency on total core losses for the TC80 material compared with lamination steel. The lamination steel is a non-oriented 3 w/o silicon iron rolled to a thickness of 0.2 mm (0.007 inches). At low frequency levels, where the core losses are dominated by hysteresis losses, the laminated material shows lower losses than the coated iron powder material. This limits the low frequency performance of plastic coated iron compacts, as the hysteresis losses are greater than laminated steels. However, the reduced eddy current loss inherent in the plastic coated iron results in lower losses and thus higher efficiency at the higher frequency range where the total core losses are dominated by the eddy current losses. Figure 4: Effect of operating frequency on the total core loss of Ancorsteel TC80 and a 3% silicon lamination steel. Conventionally Mixed Insulted Iron Powder Although the fluid bed coated material offers the optimal strength and magnetic performance, these strength levels are often unnecessary in many nonstressed applications. Additionally, the complication of using heated tooling and heated powder makes the manufacturing process for components unnecessarily complex. To compliment these high strength materials, a lubricant insulation was developed that offers the ease of room temperature compaction with the magnetic performance of the fluid bed coated material. These products are referred to as Ancorperm 100 and Ancorperm 500. They represent products that are equivalent to the Ancorsteel TC80 and Ancorsteel SC120, respectively. 4

5 Grade AncorPerm 100 AncorPerm 500 HS Ancorperm 500 LS Ancorperm 500 HP Table 2 Ancorperm Material Properties Den at 690 MPa Max Perm Hc Oe Bmax at 40 Oe, kg Low Core Loss Insulated Powders One disadvantage of both the fluid bed coated materials and the room temperature compacted materials is the high hysteresis losses resulting from the cold working of the iron during compaction. Because these materials are used in the as compacted condition; the cold working during compaction affects the structure sensitive magnetic properties (in particular the permeability and coercive force). For pure iron, the coercive force of a fully annealed material toroid at induction level of 12,000 Gauss is approximately 2.0 Oersteds. Even minor amounts of cold work raise the coercive force to approximately 4.5 Oersteds. This increase in coercive force raises the hysteresis losses, thus increasing the overall losses at low frequencies. Table 3 presents the effects of compaction pressure on the DC permeability and coercive force of the pressed and cured iron toroids. Compaction pressures as low as 135 MPa (10 tsi) increase the coercive force to approximately 3.5 Oersteds while compaction at 685 MPa (50 tsi) raises the coercive force to ~ 4.5 Oersteds. Magnetic data for pure iron compacts sintered at 1120 C (2050 F) gave a coercive force of approximately 2 Oersteds. Thus even minor amounts of cold working result in a significant increase in the coercive force; consequently, a significant increase in the hysteresis loss. Table 3 Effects of Compaction Pressure on the Magnetic Properties of Iron 40 Oersteds Comp Press, MPa Den, g/cm³ Hc Oe Max Perm Bmax, kg Testing was done to determine the minimum annealing temperature to reduce the coercive force. Experimental testing determined that a 650 C (1200 F) annealing cycle is adequate to raise the DC permeability and lower the DC coercive force while also giving reduced AC core losses. However, conventional iron powder polymer composite materials cannot withstand this temperature without degradation of the polymeric material. This effort suggest that a new type of insulating material is needed that is compatible with conventional P/M techniques and allows for a modified magnetic annealing at a minimum temperature of 650 C (1200 F). A proprietary compound was developed that met the above criteria. This coating material is compatible with the iron powder and completely wets the powder surface. Annealing of the compacts is accomplished at 650 C (1200 F) in a nitrogen atmosphere for a minimum time of one (1) hour. 5

6 Induction, (kgauss) 60 Hz DC Oersteds Induction, (kgauss) 15 M50 Steel Laminations Annealable IP Oersteds Figure 5: DC and AC (60 Hz) B-H curves for annealed insulated powder Figure 5 presents the DC and 60 Hz AC hysteresis curves of this new material. Both the AC and DC curves exhibit low coercive force and a low hysteresis loop area. The non-traditional look of the B- H curves of Figure 6 is a result of the insulating material acting a distributed air gap within the part thus shearing the B-H curve. Note the similarity of the DC and AC curves; indicating that the losses are primarily hysteresis loses with minimal eddy current losses. Comparisons the as pressed insulated material were made relative to cold rolled motor laminations and an M19 silicon steel both at 60 Hz and 200 Hz. Shown, as Figure 9 is the DC hysteresis curves of the annealable iron powder composite and the M50 steel lamination material. The wrought laminated materials exhibit significantly higher DC permeability and DC saturation. The reason for the reduced DC performance of the insulated powder is the presence of the powder coating. Figure 6: DC B-H curve of annealed insulated powder vs. cold rolled motor laminations Core Loss, (w/lb) Annealable IP M50 M Induction, (kgauss) Figure 7: Core loss of annealed insulated powder and lamination steels tested at 60 Hz. Figure 7 presents the core loss as a function of the induction at 60 Hz for the annealable insulated powder material compared to M50 and M19 steel laminations. The annealable insulated powder material exhibits a total core loss at 60 Hz and 10,000 Gauss of approximately 3 watts/pound. This value is considerably below the value of the M50 lamination steel and higher than the M19 lamination steel. The total core loss at 200 Hz for the annealable insulated powder material is 6

7 presented in Figure 11. At this higher frequency, the annealable insulated powder has lower total core loss relative to the M50 but is still higher than the M19 material. This data indicates that the annealable insulated powder material can successfully replace components made from the M50 type materials but is not a replacement for the M19 at the two frequencies examined thus far. Limitations of this annealable material are the low green strength of the annealable insulated powder 67 MPa (~ 10,000 psi) TRS and the low DC permeability. The low permeability is a direct consequence of the highly insulating coating. SUMMARY Powder metal processing is mass production metal forming process capable of producing a wide range of magnetic components for both DC and AC applications. The specifics concerning the processing for DC applications are well established and the interested reader should consult MPIF Standard 35. choice between the iron powder polymer composites or a powder that can be given a low temperature annealing is a function of the part function and ultimate part requirements. REFERENCES: 1.) D. G. White, Powder Metallurgy in 1995, Advanced Materials and Processes, August 1995, pp ) Micrometals Iron Powder Cores, Catalog 3, Issue E, RF Applications, ) C. Oliver, H. Rutz, The Manufacture of Electronic Components by the Powder Metallurgy Process, Advances in Powder Metallurgy and Particulate Materials , Vol. 3 Part 11, pp , Metal Powder Industries Federation, Princeton, NJ. For AC applications, there are three distinct types of insulated iron powder that can be used. The fluid bed coated iron optimizes strength and magnetic performance for those applications requiring moderate stress carrying capability. The room temperature compacted materials offers ease of manufacturing plus magnetic properties equivalent to the warm compacted fluid bed materials. The drawback of the room temperature materials is low strength. Lastly, if the minimization of losses is a key concern, the annealed insulated powder can be considered. Annealing eliminates the increase in hysteresis losses resulting from cold work. However, these materials have low permeability and low strength. The 7