DC Applications/DC Magnetic Performance. DC magnetic components are produced by the conventional press and sintered P/M process. Magnetic properties of a P/M part can be varied by the raw material, the compaction, and the sintering conditions utilized. The range of alloys available varies from high-purity irons to alloys of nickel and iron; Table 2.6 summarizes the most common ferromagnetic powders used and the resulting magnetic properties. Note that the alloys of iron phosphorus and iron silicon are not prealloyed powders; rather, they are produced using a high-purity iron powder which is then premixed with either an iron-phosphorus inter-metallic or an intermetallic of iron silicon (either 31 percent or 50 percent silicon). This premixing approach preserves the high level of compressibility of the iron powder and allows for part-specific compositions utilizing various premix additives.
TABLE 2.6 Typical Properties of Sintered P/M Materials
| Density | Induction | ||||
| range, | Maximum | Hc, | at 15 Oe, | Resistivity | |
| Alloy system | g/cm3 | permeability | Oe | kG | iiiicm |
| Iron | 6.8-7.2 | 1800-4000 | 1.5-2.5 | 10-13 | 10 |
| Iron-phosphorus | 6.7-7.4 | 2500-6000 | 1.2-2.0 | 10-14 | 30 |
| Iron-silicon | 6.8-7.5 | 4000-10000 | 0.3-1.0 | 8-11 | 60 |
| 400 series | 5.9-7.2 | 500-2000 | 1.5-3.0 | 5-10 | 50 |
| 50Ni/50Fe | 7.2-7.6 | 5000-15000 | 0.2-0.5 | 9-14 | 45 |
FIGURE 2.70 Induction for Ancorsteel 45P and Ancorsteel 1000B.
Alloys of iron and phosphorus are used in applications requiring higher dc permeability without going to the expense of using a silicon-containing steel. Iron-phosphorus alloys can be sintered at 1120°C (2050°F) using either pure hydrogen or hydrogen-nitrogen blends as the sintering atmosphere. Alloys containing silicon require sintering temperatures of 1260°C (2300°F) to completely homogenize the silicon, thus giving the high level of magnetic properties. This elevated-temperature sintering, although becoming more commonplace, does increase the part cost. Iron-silicon alloys are used in applications where the low coercive force coupled with the high levels of permeability and resistivity are necessary.
The 400 series stainless steels are used in applications requiring magnetic performance coupled with corrosion resistance. Antilock brake wheel sensors represent the largest application for the 400 series stainless steels. The unique configuration of the wheel sensors combined with 100,000-mile durability make the 400 series stainless steels the logical choice. Prealloys of 50Ni/50Fe are the highest-cost P/M alloy and are used in the most demanding applications. Typically, this material is used in flux return paths for computer printing devices where the response time and low coercive forces are critical.
Processing Considerations for Sintered P/M Magnetic Materials. P/M processing is unique in that the magnetic properties can be tailored via part density and sintering conditions to meet the specific part requirements. As mentioned earlier, density has a significant effect on the part performance. Higher-density P/M parts exhibit increased permeability and saturation induction without any degradation of the coercive force. Techniques to increase the part density include double press/double sinter, warm compaction, or restriking a fully sintered part.
Increasing the density by either double press/double sinter or warm compaction processing results in sintered densities approaching 7.4 to 7.5 g/cm3. At these density levels, the permeability and saturation induction approach the values achieved for fully dense wrought steels. Table 2.7 shows a comparison of a low-carbon wrought steel with pure iron and phosphorus irons pressed to 7.3 to 7.35 g/cm3. The wrought steel was evaluated in the as-forged condition. Performance of the P/M materials is comparable in both permeability and saturation induction to the wrought AISI 1008 at 15 Oe. The P/M materials are superior in terms of lower coercive force values. Interestingly, the mechanical properties of the warm compacted Ancorsteel 45P are similar to the low-carbon steel forging. Thus, the P/M alternative produces a part that gives equivalent magnetic properties along with comparable mechanical properties.
TABLE 2.7 Comparison of Ancordense Processed Ancorsteel 1000B,Ancorsteel 45P, and AISI 1008
| Property | AISI 1008 | Ancorsteel 1000B 7.3 g/cm3 | Ancorsteel 45P 7.35 g/cm3 |
| Maximum permeability | 1900 | 2700 | 2700 |
| Induction 15 Oe, kG | 14.4 | 15.0 | 15.1 |
| Hc, Oe | 3.00 | 2.10 | 1.90 |
| Yield strength, | 42,000 (285) | 21,000 (145) | 42,000 (285) |
| lb/in2 (MPa) | |||
| Tensile strength, | 56,000 (385) | 32,800 (225) | 59,400 (405) |
| lb/in2 (MPa) | |||
| Elongation, % | 37 | 13.7 | 12 |
done at 982 to 1038°C (1800 to 1900°F) in a hydrogen-containing atmosphere, followed by slow cooling at a rate not to exceed 5.5°C/min (10°F/min).
The structure-sensitive properties of magnetic materials are the permeability, coercive force, and residual induction. With P/M magnetic parts, the permeability and residual induction are affected by the density of the component. However, it has been shown that the coercive force is not density sensitive. Permeability is affected by both the density and the microstructure of the final part. Several key parameters that influence the structure-sensitive properties in magnetic materials are grain size, pore size and morphology, and material purity (in particular, residual interstitial elements such as carbon and nitrogen). Figure 2.71 presents the permeability of
TABLE 2.8 Effect of Repressing on the Magnetic Properties of Ancorsteel 45P at 6.8 g/cm3
| Maximum | Hc at 15 Oe, | Induction at | |
| Condition | permeability | in Oe | 15 Oe, in kG |
| As sintered | 2260 | 1.98 | 11.0 |
| Sintered and sized | 1160 | 2.69 | 9.8 |
| Sintered—sized and | 2270 | 2.22 | 11.2 |
| annealed |
higher temperature results in a larger grain size, greater pore rounding, and, consequently, higher permeability.
Figure 2.72 illustrates the effect of both sintering temperature and elevated nitrogen levels on the coercive force and permeability. The data illustrate that sintering at an elevated temperature results in a significantly lower coercive force and greater permeability. However, the presence of increased nitrogen levels results in a degradation of the magnetic properties. Carbon has a similar effect to nitrogen on the magnetic performance; that is, the higher the carbon content, the lower the magnetic response. High nitrogen and carbon contents are the result of improper sintering and/or improper atmosphere selection. Care is necessary to make certain that proper lubricant burnout is effected and that a clean furnace is utilized to ensure a low sintered carbon content. Nitrogen content can be minimized by utilizing pure hydrogen in the sintering atmosphere.
Iron P/M alloys containing phosphorus and silicon are made by premixing a high-purity iron powder with an iron-phosphorus or iron-silicon intermetallic. The most common iron-phosphorus intermetallic is Fe3P, which contains approximately 16 percent by weight of phosphorus. This premix additive has a melting temperature of ~1066°C (1950°F); as such, sintering and complete homogenization of iron-phosphorus alloys can be accomplished at 1121°C (2050°F). Higher-temperature
FIGURE 2.71 Permeability of Ancorsteel 45P sintered at 1120°C (2050°F) and 1260°C (2300°F).
FIGURE 2.72 Effects of sintering temperature and elevated nitrogen levels on permeability and coercive force.
sintering will result in greater pore rounding and grain growth, thus enhancing the structure-sensitive properties of the iron-phosphorus material. Unlike the iron-phosphorus intermetallic, the iron-silicon intermetallic has a melting point in excess
of 1371°C (2500°F); sintering at 1121°C (2050°F) will not completely homogenize
the silicon within the iron matrix. To achieve complete homogeneity, sintering at 1260°C (2300°F) is necessary. Once properly sintered, the magnetic properties shown in Table 2.6 are achieved.
Sintering of the 400 series stainless steels and 50Ni/50Fe can be done at either
1121 or 1260°C (2050°F or 2300°F) with the higher temperature yielding a higher
level of magnetic performance through enhanced densification and grain growth. A word of caution on the 400 series stainless steels concerns the need to use a 100 percent hydrogen atmosphere. If a nitrogen-containing atmosphere is used, the affinity of the chromium for nitrogen will result in high nitrogen levels within the sintered part. These high nitrogen levels severely degrade the magnetic performance.
P/M Materials for AC Magnetic Applications. The rationale for using laminated steel assemblies for ac magnetic devices is well understood—specifically, the reduction of the eddy current losses resulting from the alternating magnetic field. A fully sintered P/M material is not suitable for ac applications because the inherent thickness of the P/M part will result in large eddy current losses even at low ac frequencies. Attempts at making thin laminations via the P/M process were not successful because the thinnest part practical via P/M is 0.060 in thick. Additionally, the cost of pressing and sintering laminations exceeds the cost of laminations made by the conventional stamping method.
Iron powder has been used in the manufacture of switch-mode power supplies, light dimmers, and loading coils for high-frequency applications. These applications utilize iron powder that has been treated with an electrically insulating surface coating and subsequently premixed with a polymeric binder and pressing lubricant. This so-called dust core technology utilizes the inherent fine particle size of the powder material to minimize the eddy current losses at higher frequencies. Limitations in compaction capacity and part densities focused this technology into generally smaller parts with generally higher operating frequencies.
Iron Powder-Polymer Composites. To extend the range of the dust core technology, experimental work was directed to develop new powder-coating technolo-
gies and new compaction techniques that enabled higher part densities with higher saturation induction levels and higher permeabilities. If a suitable coating technology could be developed, powder compacts potentially could displace laminations in low-frequency applications such as electric motors and transformers.
In the mid-1990s a new family of iron powder materials was introduced that was intended for medium- to high-frequency ac applications. These materials combined
polymer processing and iron powder metallurgy to produce an iron powder-polymer composite. The basic processing steps of these materials is illustrated in Fig. 2.73. 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. Once compacted, the part is suitable for use in ac applications; no sintering is required. The polymer coating, typically less than 1 percent by weight, both electrically insulates the iron particles and adds mechanical strength to the as-pressed component. An optional particle oxide coating can be applied for applications requiring greater interparticle resistivity.
Iron powder-polymer composites are designed to be used 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 strength of the insulated iron is approximately 15,000 lb/in2 (100 MPa) in the as-pressed condition. After an optional low-temperature heat treatment [316°C (600°F) for 1 h], the strength increases to nearly
35,000 lb/in2 (240 MPa). Comparable strength of
the standard P/M material in the green or as-
FIGURE 2.73 Processing steps for iron powder-polymer composites.
compacted condition is approximately 3000 lb/in2 (20 MPa).
Summary of DC Magnetic Performance of Iron Powder-Polymer Composites. Three distinct coated iron powders are currently available. Table 2.9 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; thus, powders can be customized to meet specific application requirements.
The effect of frequency on the permeability of these three materials is shown in Fig. 2.74. The Ancorsteel SC120 material provides the highest degree of permeabil-
TABLE 2.9 Magnetic Characteristics of Ancorsteel
| Polymer | Density | Initial | Maximum | Induction at | |||
| Ancorsteel | coating | Oxide | at 50 t/in2 | perme- | perme- | 40 Oe, | |
| material | (w/o) | coating | g/cm3 | ability | ability | Oe | kG |
| SC120 | 0.60 | No | 7.45 | 120 | 425 | 4.7 | 11.1 |
| SC100 | 0.75 | No | 7.40 | 100 | 400 | 4.8 | 10.9 |
| TC80 | 0.75 | Yes | 7.20 | 80 | 210 | 4.7 | 7.7 |
FIGURE 2.74 Effect of operating frequency on the 10-G permeability of the iron powder-polymer composites.
Figure 2.75 shows the effect of frequency on total core losses for the TC80 material compared with a lamination steel. The lamination steel is a nonoriented 3 w/o silicon iron rolled to a thickness of 0.2 mm (0.007 in). At low frequency levels, where the core losses are dominated by hysteresis losses, the laminated material shows lower levels of 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 those of laminated steels. However, the reduced eddy current loss inherent in the plastic-coated iron results in lower levels of losses and thus higher efficiency at the higher-frequency range where the total core losses are dominated by the eddy current losses.
Low-Core-Loss Insulated Powders. The widespread usage of iron powder-polymer composites remains limited because of the high hysteresis loss affecting high core losses at lower frequencies (<200 Hz). High hysteresis losses result from the cold-working imparted to the iron during compaction. Iron powder-polymer composites 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 G is approximately 2.0 Oe. Even minor
FIGURE 2.75 Frequency on total core losses for TC80 material compared with a lamination steel.
amounts of cold work raise the coercive force to approximately 4.5 Oe. This increase in coercive force raises the hysteresis losses, thus increasing the overall losses at low frequencies.
Thus, for the nonsintered iron powder composites to gain greater acceptance, this low-frequency deficiency must be overcome. To investigate the effects of powder compaction on the magnetic properties, a pure iron powder was surface-coated with 0.03 w/o phosphoric acid and then premixed with 0.75 w/o zinc stearate. Magnetic toroids were compacted over a range of pressures from 10 t/in2 (137 MPa) to 50 t/in2 (685 MPa). The as-compacted toroids were then cured at 177°C (350°F) and subsequently tested for their magnetic properties.
Table 2.10 presents the effects of compaction pressure on the dc permeability and coercive force of the pressed and cured iron toroid. Compaction pressures as low as 10 t/in2 (135 MPa) increase the coercive force to approximately 3.5 Oe, whereas compaction at 50 t/in2 (685 MPa) raises the coercive force to approximately 4.5 Oe. Magnetic data for pure iron compacts sintered at 1120°C (2050°F) gave a coercive force of approximately 2 Oe. Thus, even minor amounts of cold-working result in a significant increase in the coercive force, and, consequently, a significant increase in the hysteresis loss.
TABLE 2.10 Effects of Compaction Pressure on the Magnetic Properties of Iron Powder at 40 Oe
| Compacting | Density, | Hc, | Maximum | Induction, |
| pressure, t/in2 | g/cm3 | Oe | permeability | kG |
| 10 | 5.70 | 3.3 | 97 | 3.3 |
| 20 | 6.47 | 4.1 | 179 | 5.9 |
| 30 | 6.92 | 4.3 | 225 | 7.4 |
| 40 | 7.14 | 4.4 | 245 | 8.2 |
| 50 | 7.26 | 4.4 | 245 | 8.3 |
A proprietary compound was developed that met these criteria. This coating material is compatible with the iron powder and completely wets the powder surface. Once coated, the powder is compacted at 150°C (300°F) using lubricated dies. Annealing of the compacts is accomplished at 650°C (1200°F) in a nitrogen atmosphere for a minimum time of 1 h.
Figure 2.76 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 nontraditional look of the B-H curves of Fig. 2.76 is a result of the insulating material acting as 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 losses with minimal eddy current losses.
Comparisons of this new insulated material were made relative to cold-rolled motor laminations and an M-19 silicon steel, both at 60 Hz and 200 Hz. Figure 2.77 shows the dc hysteresis curves of the annealable iron powder composite and the M-50 steel lamination material. The wrought laminated materials exhibit significantly greater dc permeability and dc saturation. The reason for the reduced dc performance of the insulated powder is the presence of the powder coating.
Figure 2.78 presents the core loss as a function of the induction at 60 Hz for the annealable insulated powder material compared to M-50 and M-19 steel laminations. The annealable insulated powder material exhibits a total core loss at 60 Hz and 10,000 G of approximately 3 W/lb. This value is considerably lower than the value for the M-50 lamination steel and higher than that for the M-19 lamination steel. The total core loss at 200 Hz for the annealable insulated powder material is presented in Fig. 2.79. At this higher frequency, the annealable insulated powder has lower total core loss relative to the M-50 but is still higher than that of the M-19
FIGURE 2.76 DC and 60-Hz ac hysteresis curves.
FIGURE 2.77 DC hysteresis curves of annealing iron powder composite and M-50 lamination material.
material. These data indicate that the annealable insulated powder material can successfully replace components made from the M-50-type materials but is not a replacement for the M-19 at the two frequencies examined thus far.
Limitations of this annealable material are the low green strength of the anneal-able insulated powder (=10,000 lb/in2 TRS) and the low dc permeability. The low permeability is a direct consequence of the highly insulating coating. Efforts are under way to enhance the permeability of these materials without degrading the overall magnetic performance. However, this low-core-loss material is designed for powder applications where low total core losses are necessary and the consequence of the low permeability can be minimized by component design.