technical trends
Developments in
composite powders
and sintered PM for
magnetic applications
In his follow-up coverage of papers presented at the
PowderMet 2012 show, Joe Capus examines the boundless
opportunities – as well as challenges – for PM for electronics.
A
s mentioned in the updated
PM Industry Roadmap discussed in the July/August
edition of Metal Powder
Report, electrical and electromagnetic applications represent both a
significant challenge and an opportunity for growth. As Sim Narasimhan
(Hoeganaes Corp) indicated in his
opening remarks at a session on magnetics at the PowderMet2012 conference in Nashville,Tennessee, USA, the
global market for soft magnetic materials exceeds US$12 billion. Laminated
steels make up the bulk of this at 86%,
while ferrites and amorphous materials
represent 12% and 2%, respectively.
Narasimhan suggested the PM industry
could aim for a target of 5% of the
market share, a sizeable business compared with the whole of the existing
PM ­market.
Conventional sintered materials,
although widely used in DC applications (discussed in a later section of
this article), are unsuitable for alternating current or rotating machine
applications. This is the sector for
which compacted composites of insulated iron powder have been under
development for the past few decades.
Insulated powder composites, however,
had some shortcomings; in a paper
with colleagues Marucci and Hanejko,
Narasimhan reviewed how some of
these could be overcome through recent
developments, expanding on what
had been presented at EURO PM2011
in Barcelona by co-author Michael
Marucci. Narasimhan’s presentation
focused on two main topics: the influence of particle size on magnetic characteristics of iron powder composites,
and the use of “double press – double
cure” processing (2P2C) to achieve density above 7.45 g/cm³ and the resulting
improved properties.
For the particle size study, highpurity water-atomized iron powder
(Ancorsteel 1000C) was sieved into
eleven separate size fractions with
mean particle sizes ranging from
224 µm (60 to 80 mesh) to 27 µm
(minus 400 mesh). Each powder fraction was coated with a proprietary
insulating material and then mixed with
0.75% zinc stearate before compacting into standard toroidal test pieces
at room temperature with a pressure
of 660 MPa. The toroids were cured
by heating at 450°C for one hour in
a nitrogen atmosphere before being
subjected to magnetic testing. DC hysteresis loops were generated at 800 to
3200 A/m, while AC hysteresis data
were generated at 500 Hz and peak
induction levels of 0.1- 0.5T. Tests of
initial permeability and core loss were
made at an induction level of 0.001T.
For the measurement of magnetic properties of AncorLam®* grade insulated
powder composites, tests were made at
a specified density. Toroid dimensions
were 36.1mm OD, 22.3 mm ID, and
5.76 mm thick.
Chemical analysis of the various
screen fractions showed very ­little
­variation: carbon content ranged
(coarse to fine) from 0.003% to
0.004%, while oxygen ranged from
0.056% to 0.065% and nitrogen from
0.002% to 0.001%. However, particle
size did have an effect on compressibility at 660 MPa, ranging from 7.2 g/
cm³ (finest) to 7.3 g/cm³ (coarsest), with
a parallel increase in maximum permeability from 160 to 260. On the other
hand, initial permeability was barely
influenced by compact density and
independent of frequency over a large
range. Core loss measured at an induction of 0.001T showed a significant
influence of particle size (Figure 1) in
addition to the trend with frequencies,
since the finer particles had lower eddy
current losses due to higher resistivity.
*AncorLam is a registered trademark of Hoeganaes Corp.
0026-0657/12 ©2012 Elsevier Ltd. All rights reserved
September/October 2012 MPR
25
10000
224
Core Loss (W/lb)
1000
195
164
138
117
102
83
72
60
51
37
100
224
195
164
138
117
102
83
72
60
51
37
10
1
0.1
0.01
0.1
1
10
100
1000
Frequency (kHz)
Frequency (KHz)
Figure 1: Core loss at an induction of 0.001T as a function of
­frequency.(After Narasimhan et al.)
Particle size also had a strong influence on Q ­factor, particularly at higher
frequencies between 20 and 300 kHz,
with finer particles resulting in higher
Q factor (Figure 2). To take advantage
of the variation in magnetic performance with particle size, two standardized particle size distributions were
­chosen, for moderate and higher-frequency ­applications, respectively. These
two grades, ­designated AncorLam 2
(140-224 µm) and AncorLam 2F (37140 µm) were tested for comparison
with the standard AncorLam grade
(37-224 µm).
Double pressing and
curing process (2P2C)
The desired microstructure of an insulated iron powder composite is a densified compact with minimal disruption
of the insulating layer. The preferred
standard process involves compaction
with a warm (80°C) die, to achieve high
density followed by curing at 450°C
for one hour in nitrogen to improve
strength and magnetic performance (by
partial stress relief). But to get to the
highest densities (>7.5 g/cm³) requires
very high pressures, e.g. >1100 MPa
and sophisticated presses and tooling.
To that end, Narasimhan et al recommend a 2-step process in which the
pressure can be limited to 830 MPa
and still reach densities over 7.5 g/cm³.
In this procedure, an extra curing step
at 400°C is inserted between the first
and second compactions, with the final
curing at 450°C, again in nitrogen. “In
general, the 2P2C process increases density by 0.07-0.09 g/cm³ in the standard,
26
MPR September/October 2012
Figure 2: Effect of particle size on Q factor at various frequencies.
(After Narasimhan et al.)
2 and 2F grades.” A density increase of
0.15-0.20 g/cm³ can be achieved with
an increased insulation coating thickness (HR grades). A substantial increase
in strength and reduction in core losses
can be obtained with the double compaction/curing process (See Table 1).
“The 2P2C process can be modified
to meet specific application needs such
as the highest possible induction, or to
lower core losses.”
The thermal conductivity of both
soft and hard (PM) magnetic materials
is important in the design of electrical devices such as motors. Because
of the large difference in thermal
conductivity compared with electrical steel, there is a significant impact
on heat flow and, hence, the design
of thermal insulation for the electrical windings. Barbara Slusarek (Tele
& Radio Research Institute, Warsaw)
reported on the development of a thermal conductivity testing apparatus and
its use in the measurement of thermal
conductivity for bonded Nd-Fe-B hard
magnetic material and several composite soft magnetic iron grades. Thermal
conductivity of MQP-B permanent
bonded magnets was affected by compacting pressure, rising from 1.0 ±0.1
to 1.2±0.1 W/mk as the pressure was
increased from 700 to 900 MPa, but
was hardly influenced by the percentage
of resin. Thermal conductivity of soft
magnetic materials based on AncorLam,
Somaloy 500 and Somaloy 700, respectively, fell in the range 7.3 to 7.7±0.6
W/mk and was unaffected by particle
size. Since electrical steel has a much
higher conductivity, in the range of 20
to 60 W/mk, designing electric motors
with soft magnetic iron powder composites and bonded magnets requires
an adjustment in the class of thermal
insulation for the wire windings. Lower
thermal conductivity raises the temperature of the windings with a significant
effect on the heating characteristics of
electric motors.
In another soft magnetic application sector, Ian Donaldson (GKN
Sinter Metals) and Peter Sokolowski
(Hoeganaes Corp) addressed the
improvement of mechanical strength in
sintered high-density components. This
Table 1: Effect of processing route on the core loss of iron powder composites.
(After Narasimhan et al.)
Core
loss
Strength
Core loss at 1 T (W/kg)
Material system
at 0.2T
(MPa)
(W/kg)
60 Hz 100 Hz 200 Hz 400 Hz 1 kHz 10 kHz
AncorLam – 1P1C
90
9
15
31
67
AncorLam HR – 1P1C
60
7.6
13
27
53
147
147
AncorLam 2 – 1P1C
40
9
15
31
67
245
172
AncorLam 2HR – 1P1C
60
6.7
12
25
54
163
AncorLam 2FHR – 1P1C
60
7.3
14
27
54
146
130
AncorLam 2HR – 2P2C
108
6.6
11
24
52
172
-
metal-powder.net
600
7500
7000
500
Mix 3
6500
Mix 4
6000
Permeability
Strength (Mpa)
400
300
200
100
5500
Mix 5
Mix 2
5000
Baseline
FY-4500
4500
Mix 3
1120 ºC
4000
0
Mix 1 (basline)
Mix 2
1120 C YS
1120 C UTS
Mix 3
Mix 4
1260 C YS
Mix 5
1260 C UTS
Figure 3: Comparison of YS and UTS for the various mixes at 1120°C
and 1260°C sintering temperatures (compacted at 760 MPa). (After
Donaldson & Sokolowski)
has become more significant in the light
of increased use of electrical systems
in auto body and chassis applications.
These authors reported on a study of
mechanical and magnetic properties of
iron-phosphorus alloys as influenced by
additions of prealloyed molybdenum
and/or warm compaction. The materials used in these tests were Fe-0.45%P
(FY-4500) based on Ancorsteel 1000B
atomized powder and Fe-0.85%Mo
(MPIF FL-4400) based on Ancorsteel
85HP with either 0.45%P or 0.80%P,
added as ferrro-phosphorus in the premix (see Table 2).
3000
300
350
400
450
500
550
UTS (MPa)
Figure 4: The relationship between maximum permeability and UTS.
(After Donaldson & Sokolowski)
Compaction with pressure of
760MPa was done at room temperature or with a heated die at 90°C.
Sintering in hydrogen was either at
1120°C or 1260°C. Mechanical and
magnetic properties were determined
by standard procedures. Tensile and
yield strength were increased by warm
compaction (which increased the green
density to 7.34/7.38 g/cm³) and also
by raising the phosphorus level from
0.45% to 0.80% and by the addition
of Mo (Figure 3). UTS values upwards
of 500 MPa were shown for 0.80%P
after sintering at both 1120°C and
Table 2: Composition of mixes used for the investigation with phosphorus added
as Fe3P.
Designation Description
Base iron
Phosphorus Lubricant
Mix 1
FY-4500
Baseline
Ancorsteel 1000B
0.45 w/o
0.75 w/o
Acrawax C
Mix 2
FL-4400 + 0.45 P
Ancorsteel 85 HP
0.45 w/o
0.75 w/o
Acrawax C
Mix 3
FL-4400 + 0.45 P
Warm-die
Ancorsteel 85 HP
0.45 w/o
0.6 w/o
AncorMax 200
Mix 4
FL-4400 + 0.80 P
Ancorsteel 85 HP
0.80 w/o
0.75 w/o
Acrawax C
Mix 5
FL-4400 + 0.80 P
Warm-die
Ancorsteel 85 HP
0.80 w/o
0.6 w/o
AncorMax 200
Table 3: Soft magnetic properties after 1260°C sintering. (After Donaldson &
Sokolowski)
Drive Frequency
Designation Sinter
µmax
Bmax (T)
HC
Br (T)
(Hz)
field
density
(A/m)
(g/cm³) (A/m)
Mix 1
7.29
7958
0.01
4463
1.57
95.5
1.21
Mix 2
7.31
7958
0.01
5052
1.56
79.6
1.19
Mix 3
7.5
7958
0.01
6767
1.64
71.6
1.29
Mix 4
7.24
7958
0.01
6235
1.53
71.6
1.21
Mix 5
7.39
7958
0.01
7024
1.6
71.6
1.21
metal-powder.net
1160 ºC
Mix 4
Mix 2
3500
1260°C. DC magnetic properties were
determined on toroid samples with an
applied field of 7958 A/m. As shown
in Figure 4, Maximum permeability
µmax fell slightly with the addition of
Mo after sintering at 1120°C, but
increased for sintering at 1260°C.
Increasing the phosphorus content
from 0.45% to 0.80% also increased
the permeability, but the biggest
increases were provided by the higher
densities reached by warm compaction.
Coercivity was lower in each of the
Mo-containing compositions compared
with the base-line Fe-0.45%P. Raising
the sintering temperature to 1260°C
improved all the soft magnetic properties. These results are summarized in
Table 3.
The authors concluded that the use
of a prealloyed 0.80% Mo iron powder
in conjunction with admixed ferrophosphorus enabled higher strengths to
be obtained with soft magnetic properties “comparable or better than” the
standard FY-4500 iron-based material.
“[With] proper selection of materials
and processing, performance targets
of specific soft magnetic applications
beyond typical MPIF materials can be
achieved.” Among specific achievements
listed were as follows:
• UTS above 500 MPa with 0.80%P
• Permeability increased by 13% (for
0.45%P) and 40% (0.80%P)
• Sintering at 1260°C and increasing
the green density increased permeability by 54% (0.45%P) and 57%
(0.8%P), respectively, over FY-4500
• Coercivity improved vs. FY-4500
with 0.85%Mo and 0.45% or
0.80%P.
September/October 2012 MPR
27