With the trend towards more widespread use of automotive electric systems such as electric power steering, new opportunities exist for P/M soft magnetic alloys. These applications require high density for magnetic properties and precision. To meet density, precision and geometry complexity requirements, secondary operations are usually employed, which degrade magnetic properties. Annealing can be utilized for recovery of the magnetic properties, but with the potential for dimensional changes. Through the use of an advanced binder system, higher densities with subsequent increases in magnetic properties can be achieved in a single compaction step. The influence of secondary operations, processing methods such as the use of an advanced binder system and annealing are presented for Fe, Fe-P and Fe-Ni materials.
The powder metallurgy process offers near net shape for magnetic components. This, coupled with the ability to modify and control the chemical composition along with the resultant magnetic properties comparing to wrought materials, have led to growth opportunities. The trend in automotive applications has been with more complex geometries and tighter tolerances, which has allowed for the replacement of low carbon steels.
The proper selection of the P/M materials along with the appropriate processing conditions will result in the magnetic properties required for the specific application.
Processing and post processing effects on density and microstructure, which in turn affect the soft magnetic response and physical properties, must be understood and controlled. The effects of various processes and microstructures on these properties have been written about earlier [1,2,3,4]. This paper further explores these effects for development of tactics in manufacturing prototypes and production parts.
Test specimens were processed and evaluated as described in each section. For warm compaction, AncorMax D processing was with a heated die at 60°C and with
ANCORDENSE processing, the powder and die were heated to 135°C. Tensile properties were developed from flat, un-machined “dogbone” tensile bars according to
ASTM E8 and MPIF Standard 10 [5]. DC hysteresis loops were generated per ASTM
A773/A773M-01 on either standard toroid shapes (3.60 cm OD x 2.23 cm ID x 0.62 cm high) or on other samples as described with an OS Walker AMH 20 Hysteresisgraph.
After processing, the samples tested for magnetic response were wound with primary and secondary turns of #28 AWG wire.
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RESULTS AND DISCUSSION
Primary and post processing considerations can be more important for soft magnetic components than structural components due to the significant impact these can have on the soft magnetic response. Figure 1 shows various processing routes that can be utilized in producing magnetic components.
Pure Iron Powders
Premix Operation Lubricants, Alloys (P, Si)
Compaction
Pre-Sinter
ANCORDENSE or
AncorMax D
Process
Warm Compaction
Sinter
Repress
DPDS Process
Secondary
Operations
Machining, Coining, Sizing,
Annealing, etc.
Finished Part
Figure 1: Processing Routes for P/M Soft Magnetic Components
Prototype Processing
At a prototype stage, machining of blanks is a common method for testing the feasibility of design. But this can lead to a difference in properties as compared to the production process that may be utilized. A test was performed comparing machined blanks and production parts made with Ancorsteel 45P. The production process was compaction to
7.15 g/cm 3 , sintered at 1120°C, then coined to a 7.2 g/cm 3 dimensions. The blanks were pressed to a 7.2 g/cm
density as a means to qualify
3 density, sintered at 1120°C, and then machined to the final dimensions. No annealing was performed on either of the sample groups. The samples were wound with 58 primary and secondary #28 AWG wire and tested with a drive field of 1195 A/m. Both samples were measured at <0.01% total carbon. It was found that the machined samples had a 34% lower permeability
(1290 versus 1950) and 25% higher coercivity (236.4 A/m versus 179.1 A/m) than the coined parts.
Compaction
Magnetic performance improves with increasing density provided the post compaction processing is the same. Figure 2 shows maximum permeability of different materials
(Ancorsteel 1000, 1000B, 1000C, 45P and 80P) as a function of density and purity level.
Impurities levels are lower for Ancorsteel 1000C as compared to Ancorsteel 1000B, which is lower than Ancorsteel 1000. All samples were sintered at 1120°C in 25-75 v/o
N
2
-H
2
atmosphere. As noted, the permeability increases with increasing density level and with increasing purity level. Additions of Fe
3
P improve both the maximum permeability and material resistivity because the phosphorus promotes liquid phase sintering and stabilization of the BCC phase.
2

5000
4500
4000
3500
3000
2500
2000
Ancorsteel 80P
Ancorsteel 45P
Ancorsteel 1000C
Ancorsteel 1000B
Ancorsteel 1000
1500
6.6
6.7
6.8
6.9
7 7.1
7.2
7.3
7.4
7.5
Density (g/cm 3 )
Figure 2: Maximum Permeability as a Function of Density and Purity Level
Increasing density at compaction results in an increase in sintered densities with subsequent increase in soft magnetic properties. Figure 3 shows the comparison of
Ancorsteel 45P compacted via conventional and warm compaction (AncorMax D and
ANCORDENSE) methods. As shown, the soft magnetic properties show an increase with increasing density. Linear regression of the properties for these processing conditions revealed a strong linear relationship with density regardless of compaction method, with R 2 values > 0.96.
8000
7000
6000
5000
4000
3000
2000
1000
0
Conventional
Conventional
AncorMax D
AncorMax D
ANCORDENSE
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
6.73
6.90
7.07
Density (g/cm
3
)
7.23
7.42
Max Perm Bmax
Figure 3: Permeability and Bmax for Ancorsteel 45P Sintered at 1120°C in 75 v/o
Hydrogen and 25 v/o Nitrogen as a Function of Density and Compaction Method
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Sintering
The sintering process can have an affect on the magnetic performance. Increasing temperature will provide for an improvement. Interstitials such as carbon, nitrogen and oxygen will also have an influence, such as forming magnetic domain-pinning precipitates or oxides, which degrade the properties. For example, with increasing nitrogen content, a decrease in permeability and increase in coercivity occurs (see Figure 4), with a more pronounced effect occurring at a higher sintering temperature. Control of the sintering process to minimize the effect is of importance.
The effect of sintering temperature on 50/50 Fe-Ni with a 0.4% Si content is detailed in
Table 1 . As expected, the soft magnetic properties improve with increasing temperature since grain growth and pore coalescence is enhanced at the higher temperature. With this material, the Si content can affect the sintering kinetics and resultant permeability. A comparison at 1180°C is shown, with a greater difference seen at the higher compaction pressure. Permeability was improved up to 46% at the higher density.
5500
5000
1120°C - Hc
4500
1260°C -µmax
1260°C - Hc
4000
3500
3000
1120°C -µmax
2500
0.000
0.002
0.004
0.006
0.008
0.010
Sintered Nitrogen Content (w/o)
Figure 4: Effect of Nitrogen and Sintering Temperature on 45P Compacted at 7.3 g/cm 3 via ANCORDENSE and Sintered in a 25-75 N
2
-H
2
Atmosphere
Si Content Compaction
Pressure,
MPa
0.4%
415
Sintering
Temperature
°C
Sintered
Density g/cm
3
Permeability Coercive
Force
A/m
Maximum
Induction
T
1120 6.60 5000 49.35 0.81
1180 6.65 7900 45.37 0.84
690
1120 7.15 8000 54.9 1.01
1180 7.22 9500 51.74 1.09
0.3%
Table 1: Effect of Sintering Temperature and Si Content for 50/50 Fe-Ni Sintered in a 25-
75 N
2
-H
2
Atmosphere Tested at a 1195 A/m Drive Field
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Secondary Operations
Various secondary operations can be utilized to achieve the performance requirements of the application. Quite often, sizing or coining is used to qualify the dimensions. The effects of this process will vary based on the amount of cold work that is induced. The plastic deformation associated with sizing strains the iron lattice, which restricts the magnetic domain movement under a magnetic field resulting in a decrease in maximum permeability and coercivity. Table 2 shows a comparison of magnetic properties for 45P undergoing full feature sizing at 6.8 g/cm 3 . As shown, a significant drop in magnetic properties occurs with sizing. With the addition of annealing, the maximum permeability can be recovered.
Condition
Sintered, Sized and
Annealed
(815°C for 60 Minutes)
Max. Permeability HC (A/m) Induction (T)
As-Sintered 2260 157.6 1.10
Sintered and Sized 1160 214.1 0.98
2270 176.7 1.12
Table 2: Effect of Sizing on 45P at 6.8 g/cm 3 Measured at 1195 T Drive Field
With the addition of an annealing operation, the amount of time at temperature can affect the overall cost of the component, so optimization of the process with respect to the performance objectives of the application needs to be realized. Sample parts were compacted at 7.15 g/cm 3 with Ancorsteel 45P, sintered at 1120°C, then sized to 7.20 g/cm 3 . These were then tested as a function of time at temperature. The results are shown in Table 3. As can be seen, only a 2% and 9% improvement was seen in permeability at 30 and 60 minutes, respectively, as compared to 15 minutes. Coercivity decreased 2% and 5% at 30 and 60 minutes, respectively, as compared to 15 minutes.
Another aspect of the coining process that needs to be considered is the reduction of elongation. For example, this is important when the magnetic component is press fit over a shaft. The effect can be significant. For example, an Fe ring compacted to a 7.0 g/cm 3 density, sintered at 1180°C in an 80-20 N
2
-H
6%. Coining the ring to a density of 7.3 g/cm
2
3 atmosphere had an elongation greater than
resulted in a reduction of the elongation to
1.6%.
Drive Field @1195 A/m Drive Field @1990 A/m
Condition Max Perm
As Coined 1100
Hc
A/m
Bmax
T
230.8 1.03
Br
T
0.583
Max
Perm
1100
Hc
A/m
Bmax
T
242.0 1.215
Br
T
0.616
15 Min 3640 134.5 1.314
1.172
3417 140.1 1.371
1.186
30 Min 3700 132.1 1.328
1.185
3550 136.9 1.382
1.211
60 Min 3950 128.2 1.371
1.234
3712 132.1 1.428
1.248
Table 3: Effect of Annealing Time at 815°C in 25-75 N
2
-H
2
for 45P at 7.2 g/cm 3
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Another secondary operation that has been utilized on magnetic components as a method to seal porosity prior to a surface coating is steam treatment. The effect of steam treatment on Ancorsteel 45P at 7.2 g/cm 3 density sintered at 1120°C in 90-10 N
2
-H
2 atmosphere is shown in Table 4. An 18% decrease in maximum permeability and an 8% increase in coercivity were measured.
Condition Max. Permeability HC (A/m) Induction (T)
As-Sintered 3250 125.0 1.306
Steam Treatment 2660 136.1 1.212
Table 4: Steam Treatment Effect on 45P at 7.2 g/cm 3 Measured at 1195 A/m Drive Field
SUMMARY
The P/M route provides a flexible and cost effective method for manufacturing parts for soft magnetic applications. Various processing operations that can be utilized for achieving desired part performance objectives, but the proper selection coupled with the appropriate processing method is essential to meet the required performance targets of the specific application. Increasing densities, either by conventional compaction or warm compaction methods, and increasing sintering temperatures provide for improved magnetic properties. The choice of secondary operations needs to be evaluated with respect to the impact it will have on the magnetic properties. The addition of annealing may be required to restore the properties to a level that is necessary for the performance of the component. Each of the process steps employed in the manufacture of the P/M component must be understood and controlled.
REFERENCE
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Containing Mixes in Low Hydrogen Atmospheres”, Advances in Powder Metallurgy & Particulate
Materials-2002, Vol. 14, pp. 132-137
2. Guo, R., Cheng, C., Lee, J., “Developing a Soft Magnetic P/M Component used in Wireless
Communication Devices with High Green Strength Lubricants”, ”, Advances in Powder Metallurgy &
Particulate Materials-2002, Vol. 14, pp. 73-78.
3. Hanejko, F.G., Rutz, H.G., Oliver, C.G., “Effects of Processing and Materials on Soft Magnetic
Performance of Powder Metallurgy Parts”, Advances in Powder Metallurgy & Particulate Materials-
1992, Vol. 6, pp. 375-401
4. Lall,
P/M Materials”, Advances in Powder Metallurgy & Particulate Materials-1992, Vol. 3, pp. 129-138
5. “Standard Test Methods for Metal Powders and Powder Metallurgy Products”, Metal Powder
Industries Federation, Princeton, NJ, 2003
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