The Effect of Processing and Density on P/M Soft Magnetic Properties
Ian W. Donaldson, GKN Sinter Metals, Worcester, MA, USA
Fran Hanejko, Hoeganaes Corporation, Cinnaminson, NJ, USA
Abstract:
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.
Introduction:
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 Methods:
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.
1
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
ANCORDENSE or
AncorMax D
Process
Pre-Sinter
Warm Compaction
DPDS Process
Repress
Sinter
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/cm3, sintered at 1120°C, then coined to a 7.2 g/cm3 density as a means to qualify
dimensions. The blanks were pressed to a 7.2 g/cm3 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
N2-H2 atmosphere. As noted, the permeability increases with increasing density level and
with increasing purity level. Additions of Fe3P improve both the maximum permeability
and material resistivity because the phosphorus promotes liquid phase sintering and
stabilization of the BCC phase.
2
5000
Maximum Permeability
Ancorsteel 80P
4500
4000
Ancorsteel 45P
3500
Ancorsteel 1000C
3000
Ancorsteel 1000B
2500
Ancorsteel 1000
2000
1500
6.6
6.7
6.8
6.9
7
7.1
7.2
7.3
7.4
7.5
3
Density (g/cm )
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 R2 values > 0.96.
8000
1.6
ANCORDENSE
AncorMax D
6000
1.4
1.2
AncorMax D
5000
4000
1
Conventional
0.8
Conventional
3000
0.6
2000
0.4
1000
0.2
0
Bmax (T)
Maximum Permeability
7000
0
6.73
6.90
7.07
7.23
7.42
Density (g/cm3)
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
3
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.
5000
4500
2.0
160
1120°C - Hc
1.5
120
1260°C -µmax
4000
1260°C - Hc
80
1.0
3500
3000
40
0.5
1120°C -µmax
2500
0.000
0.002
0.004
0.006
0.008
Coercive Force
@1195 T (A/m)
Maximum Perm
5500
0
0.0
0.010
Sintered Nitrogen Content (w/o)
Figure 4: Effect of Nitrogen and Sintering Temperature on 45P Compacted at 7.3 g/cm3
via ANCORDENSE and Sintered in a 25-75 N2-H2 Atmosphere
Si Content
Compaction
Pressure,
MPa
415
0.4%
690
0.3%
415
690
Sintering
Temperature
°C
Sintered
Density
g/cm3
Permeability
Coercive
Force
A/m
Maximum
Induction
T
1120
1180
1260
1120
1180
1260
1180
1180
6.60
6.65
6.74
7.15
7.22
7.27
6.72
7.24
5000
7900
11000
8000
9500
12000
8500
13900
49.35
45.37
41.39
54.9
51.74
43.78
50.15
50.94
0.81
0.84
0.95
1.01
1.09
1.14
0.85
1.07
Table 1: Effect of Sintering Temperature and Si Content for 50/50 Fe-Ni Sintered in a 2575 N2-H2 Atmosphere Tested at a 1195 A/m Drive Field
4
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/cm3. As shown, a significant drop in magnetic
properties occurs with sizing. With the addition of annealing, the maximum permeability
can be recovered.
Condition
As-Sintered
Sintered and Sized
Sintered, Sized and
Annealed
Max. Permeability
2260
1160
2270
HC (A/m)
157.6
214.1
176.7
Induction (T)
1.10
0.98
1.12
(815°C for 60 Minutes)
Table 2: Effect of Sizing on 45P at 6.8 g/cm3 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/cm3 with Ancorsteel 45P, sintered at 1120°C, then sized to 7.20
g/cm3. 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/cm3
density, sintered at 1180°C in an 80-20 N2-H2 atmosphere had an elongation greater than
6%. Coining the ring to a density of 7.3 g/cm3 resulted in a reduction of the elongation to
1.6%.
Drive Field @1195 A/m
Drive Field @1990 A/m
Condition
Max Perm
Hc
A/m
Bmax
T
Br
T
Max
Perm
Hc
A/m
Bmax
T
Br
T
As Coined
1100
230.8
1.03
0.583
1100
242.0
1.215
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 N2-H2 for 45P at 7.2 g/cm3
5
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/cm3 density sintered at 1120°C in 90-10 N2-H2
atmosphere is shown in Table 4. An 18% decrease in maximum permeability and an 8%
increase in coercivity were measured.
Condition
As-Sintered
Steam Treatment
Max. Permeability
3250
2660
HC (A/m)
125.0
136.1
Induction (T)
1.306
1.212
Table 4: Steam Treatment Effect on 45P at 7.2 g/cm3 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.
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