By Dr. Kalathur Narasimhan | P2P Technologies
Insulated Iron or iron-silicon for soft magnetic powder composites (SMC) are a niche unique to powder metallurgy. This method uses pure iron powder with a highly insulating coating to provide a soft magnetic ferrous material that is suitable for a variety of electromagnetic applications. These coated powders offer 3D flux carrying capability compared to traditional lamination steels where flux is maximized in the rolling direction. This technology has been in commercial use since introduced in the early 1990s, but has only enjoyed limited applications. Recent improvements in the material systems and forming processes have enabled making this technology more attractive choice for some electronic and electric motor applications.
Although an attractive material, insulated composites have not displaced mainstream lamination steels in electric induction motors. This is because of insulating coating creates a distributed air gap in the material. This has the effect of lowering the saturation induction and permeability of the material. Mechanical strength, as measured by a three point bend method, is about 100 MPa, requiring additional means of strengthening, like resins.
In general, the higher the frequency the lower the core loss of insulated iron composites compared to lamination steels (see Figure 1). This figure shows the core losses as a function of frequency for AncorLam, an insulated iron powder for SMC applications made by Hoeganaes Corp., and typical lamination steel. At lower frequencies the lamination steel has lower losses than AncorLam. As the frequency increases the losses increase at a greater rate for the lamination steel. At lower frequencies the core loss is dominated by hysteresis losses, which are higher for insulated iron composites. At higher frequencies, the eddy current losses are lower for insulated iron composites as the eddy currents are localized to the particle surface compared to lamination steels where the eddy currents are generated at the larger surface of the steel. Data from past studies show that AncorLam type materials compete well in AC applications operating at frequencies >400 Hz with lamination steel thicknesses >0.35 mm5.
Within these limitations there is room for material and process optimization for specific applications. To this end a family of insulated composite material is available. Typically the particles size and coating type is modified to meet specific application types. In general, applications that need higher permeability have coarser particles and less insulation. Applications that need to operate at higher frequencies use finer particles (designation F) and or increased insulation (designated as HR). See table below on the effect of particle size on permability. A generic summary of materials and application types is presented in Table 1. The specific magnetic performance of each grade is summarized in Table 2.
The typical production process involves compacting the insulated iron particle and curing at a temperature that increases the mechanical strength without destroying the insulation. An insulated iron composite powder is compacted into the desired shape. The green part is then cured in N2 at 450 °C resulting in the final cured component. The curing has the benefits of increasing mechanical strength of the compact and it also partially stress relieves the iron improving magnetic response, specifically the hysteresis losses.
The current technology used globally to achieve density values >7.50 g/cm3 requires very high compaction pressures at least >1000 MPa. Compaction at these very high pressures limits the size and complexity of the final component. Recently Hoeganaes Corp., USA, developed a new technology called 2P2C to reduce the need for compaction at pressures greater than 800 MPa. The 2P2C process is outlined in Figure 2.
Major application for SMC is in ignition coils and fuel injectors for diesel engine common rail system, these utilize Powder metal complex shape capability and also tighter tolerances.
Transverse flux motors (TFM) using SMC is a recent development. YASA motors in UK and GKN Sintermetals in collaboration with Aachen University and a motor producer (Powder Metallurgy Review Fall 2013) are actively engaged in this. Permeability of SMC type is still low compared to lamination steel and saturation induction is also low limiting motors power density. Further research on improving SMC type materials continue. TFM offer size reduction and provide high torque at low speeds for direct drive systems.
Research continues on developing magnetic insulator coating to improve permeability (Narasimhan and Hanejko, US Patent publication 2014/0178376A1 ).
New applications such as inductors used in hybrid vehicles require permeabilities in the range of 100. Hence a pre-alloyed powder of iron with silicon was investigated. Gas atomized Fe-Si powders having low permeabilities have been reported in the literature for use at this permeability level.  .However, spherical powders obtained by gas atomization are difficult to process by conventional compaction process.
A new process, reactive atomization, utilizing water has been developed, which is able to produce powders that are coated with thin layer of oxide and are easily compacted. Fe-Si alloy powders were the first to be developed by this process. These reactive atomized magnetic (RAM) powders offer interesting properties useful in hybrid vehicles. Optical micrograph of Fe-3Si powder shows the particles are coated with a oxide coating as atomized and annealed as shown below:
Coreloss on Fe-3Si coated compacts cured at various temperatures is shown below. The dramatic drop in coreloss as the curing temperature is increased is related to relieving compaction stresses introduced when the parts are made.
The target market for these type of powders is in reactor core in hybrid vehicles. Further improvement in coreloss continues.
Summary: It has been nearly 18 years since the introduction of higher density insulated iron composite (SMC). The challenge continues to be in the permeability and induction of the material and cost to make a breakthrough in to higher volume applications such an as electric motors operating below 400 Hertz. Current state of the SMC is that it is attractive for 3D flux designs and ease of manufacturing. Newer type of powders like Fe-Si offer opportunities in inductors for hybrid vehicles. Research continues on improvement of the materials.
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About the Author
Dr. Kalathur Narasimhan has 40 years in Powder metallurgy related industry experience, 30 years at Hoeganaes Corp. as Vice President and Chief Technology Officer, 10 years at Crucible Materials Corporation as Technical Director, and 5 years as research assistant professor at University of Pittsburgh. His career focused on sustained development of technologies for the progress of Powder Metal processes starting from Research and Development to Commercialization. High Energy Permanent magnets (AncorMag grades, Crucore, CruMax) , High- density powder metal processes, high performance alloys, soft magnetic composites were developed to grow the Powder Metal and Permanent magnet Industries.