2003-01-0448
Soft Magnetic Materials Utilizing via Conventional
And Warm Compaction Technology
Francis G. Hanejko & Michael Marucci
Hoeganaes Corporation
Abstract:
Introduction:
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.
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
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
Insulated Iron Powder
Pressed & Cured
Materials
Fluid Bed
Coated
! 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
Conventionally
Mixed
Annealed
Materials
Annealed at
650° C
Figure 1: Classification of AC magnetic
materials derived from P/M technology.
Iron Powder Polymer Composites
(Fluid Bed Coated)
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.
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.
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.
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.
2
Table 1
Summary of DC Magnetic
Performance of Iron Powder Polymer
Composites
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).
Material
SC120
SC100
TC80
Den
@
690
MPa
7.45
7.40
7.20
Perm
425
400
210
B@
40
OE
11.1
10.9
7.7
Hc,
Oe
4.7
4.8
4.7
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.
P u re Iro n P ow ders
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.
O p tio n al O x id e C oatin g
Fluid Bed C oa ting w ith
H igh T em perature P olym er
W arm C om paction
P ow der T em perature-175°C (350°F )
D ie T em perature
275°C (525°F )
O ptional T herm al
T reatm en t--315°C (600°F )
In Air
Finishe d C om pone nt
Figure 2:
Processing steps for Iron
Powder Polymer Composite materials
3
100000
Core Loss (µW/cm³)
10000
140
S C-120
Permeability at 10G
120
S C-100
100
0.2mm (0.007in) thick,
3% Si-Iron
1000
100
TC-80
10
1
TC-80
0.1
80
0.01
60
1.E+02
40
1.E+03
1.E+04
1.E+05
1.E+06
Frequency (Hz)
20
Figure 4:
Effect
of
operating
frequency on the total core loss of
Ancorsteel TC80 and a 3% silicon
lamination steel.
0
1.E + 02
1.E + 03
1.E + 04
1.E + 05
1.E + 06
Frequency (H z)
Figure 3:
Effect
of
operating
frequency on the 10 Gauss permeability
of the Iron
Powder
Polymer
Composites
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.
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.
4
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 2
Ancorperm Material Properties
Grade
AncorPerm
100
AncorPerm
500 HS
Ancorperm
500 LS
Ancorperm
500 HP
Den
at 690
MPa
6.58
Max
Perm
Hc
Oe
75
3.1
Bmax
at 40
Oe, kG
3.0
7.21
235
4.6
8.1
7.22
426
3.3
10.6
7.18
524
3.9
11.2
Table 3
Effects of Compaction Pressure on
the Magnetic Properties
of Iron Powder @ 40 Oersteds
Comp
Press,
MPa
155
310
415
550
690
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.
Den,
g/cm³
Hc
Oe
Max
Perm
Bmax,
kG
5.7
6.5
6.9
7.1
7.3
3.3
4.1
4.3
4.4
4.4
97
179
225
245
245
3.3
5.9
7.4
8.2
8.3
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).
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
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
-80
Annealable IP
10
4
2
0
-60
15
M50 Steel Laminations
Induction, (kGauss)
Induction, (kGauss)
60 Hz
DC
-40
-20
0
20
40
60
80
-2
5
0
-60
-40
-20
0
20
40
60
-5
-10
-4
-15
Oersteds
-6
Oersteds
Figure 5:
DC and AC (60 Hz) B-H
curves for annealed insulated powder
Figure 6:
DC
B-H
curve
of
annealed insulated powder vs. cold
rolled motor laminations
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 BH 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.
10.00
Core Loss, (w/lb)
Annealable IP
M50
M19
1.00
5000
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.
7000
9000
11000
Induction, (kGauss)
13000
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
15000
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.
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 49 - 51
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.
2.) Micrometals Iron Powder Cores,
Catalog 3, Issue E, RF Applications,
1994.
3.) C. Oliver, H. Rutz, “The Manufacture
of Electronic Components by the
Powder Metallurgy Process”, Advances
in Powder Metallurgy and Particulate
Materials - 1995, Vol. 3 Part 11, pp 87 102,
Metal
Powder
Industries
Federation, Princeton, NJ.
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.
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