Effect of Processing and Material Selection on P/M Part Magnetic
Properties
Igor Gabrielov and Christopher Wilson
Borg-Warner Powdered Metals, Inc.
Livonia, Michigan 48150
Francis Hanejko
Hoeganaes Corporation
Cinnaminson, NJ 08077
Abstract:
The increased usage of electromagnetic components in the drive train of cars and SUV’s often
impose conflicting requirements of good magnetic properties with high strength and high
hardness. Traditionally, P/M soft magnetic materials utilizing iron-phosphorus alloys are
characterized by good magnetic performance but relatively low strength, hardness. Thus to
achieve the mechanical property and dimensional requirements it is often necessary to perform
secondary operations such as sizing, coining, or machining often in combination with a separate
heat treatment operation. This paper will focus on the effects of various secondary operations on
the mechanical and magnetic properties of soft magnetic materials in an actual component.
Additionally, data will be presented on a higher strength magnetic material and the potential for
this material to replace existing soft magnetic materials and possibly eliminate some of the
secondary operations intended to increase the strength of the actual component.
Introduction:
Four-wheel drive once a vehicle option designed primarily for the construction industry and the
off road enthusiast has proliferated into a vehicle option specified in nearly 30% of the domestic
auto sales. [1] Originally designed with manually locking hubs in which the driver had to exit the
vehicle to engage the system, the modern 4-WD system has shift on the fly capability frequently
coupled with wheel spin sensors that automatically engage the 4-WD system when the on-board
computer detects excessive wheel spin. The complexity of the modern systems utilizes a wide
variety of materials that transmit the torque to the front and rear axles but also
electromagnetically sense the need for plus engage the transfer system.
The primary function of the transfer case is to distribute torque between the front and rear wheels.
[2] The armature shown in Figure 1 is one component of the electromagnetic sub-system utilized
in the transfer case of light trucks and SUV’s. Combined with the armature is a rotor that
contains an electrical coil. When the driver shifts into 4-WD or if wheel spin is detected, an
electrical current is applied to the coil with the resulting induced magnetic induction causing an
attraction between the armature and rotor, thus activating the torque transfer. The degree of
torque transfer can be controlled electronically by the applied current resulting in various
amounts of slip/engagement between the armature and rotor. This duplex function of the
armature/rotor necessitates that both parts have good magnetic response along with good wear
and frictional characteristics. These two material characteristics often are mutually exclusive;
specifically high hardness required for wear resistance results in poor magnetic response and
good magnetic response often results in a part with relatively poor mechanical properties.
Figure 1:
Photograph of armature used in 4-WD transfer system
Despite the conflicting material requirements, the successful economic utilization of modern 4WD systems necessitates that a balance be achieved between these two disparate characteristics.
Figure 2 is a schematic of the processing route for the armature. The powder specified for the
armature is a MPIF FY-8000 (0.80 w/o phosphorus). This material has good magnetic
performance; thus minimizing the response time of the rotor / armature activation. The
ANCORBOND™ premix is compacted to a density of 7.1 g/cm³. Sintering is done in a belt
furnace with a set point of ~2080 °F (1140 °C) in a nitrogen / hydrogen atmosphere. After
sintering, the part is sized to insure flatness and dimensional precision. After sizing, the part is
then ground for parallelism, tumbled to remove any burrs, cross hatched to provide oil passages
on the surface, heat treated to increase the surface hardness, and finally phosphate coated for
corrosion protection and lubricity. These secondary operations are essential to meet the
functional mechanical requirements of the final component; however, each processing step
introduces the potential to degrade the magnetic response. Thus it is important to understand the
individual and cumulative effects that these processing steps have on the ultimate magnetic
properties of the actual component.
Processing Flow Diagram for Armature
Figure 2:
Compact to
7.1 g/cm³
Tumble
Sinter at
1140 °C
Cross Hatch
Size
Heat Treat
Grind
Phosphate
Schematic of process flow for armature
Previous experimental research performed by Frayman, Ryan, and Ryan demonstrated the effects
of secondary processing on the magnetic properties of magnetic toroids. [3] In their work, these
researchers utilized magnetic toroids to document the effects of various secondary processing. In
their work, the researchers focused on the effects of secondary processing as individual effects.
As a prelude to the present work, Gabrielov etal studied the magnetic response of these
components to understand the statistical variation in part production and magnetic property
response. [4] In the present experimental work, production components again were utilized and
tested because of the difficultly in processing magnetic toroids per the actual part processing
sequence. Additionally, the magnetic response was measured at each step in the processing to
monitor the potential degradation from the as-sintered magnetic performance. This procedure
had two advantages: one, there is no need to generate and process separate toroids and second,
testing of actual parts enabled real time statistical process control during part manufacture.
Experimental Procedure:
As previously noted, the armatures and rotors are made utilizing soft a magnetic material, MPIF
FY-8000 iron powder (high purity iron premixed via ANCORBOND technology with 0.80%
phosphorus). The armatures were compacted to a density of 7.1 g/cm and were sintered in
nitrogen/hydrogen atmosphere. Sizing reduced dimensional variations, improved flatness, and
increased the density by 0.05 g/m³. Once sized, the part manufacturing followed the process
outlined in Figure 2. Magnetic properties of the part were evaluated after each operation. To test
the magnetic properties, a specially designed test fixture was developed to eliminate manual
winding plus increase the stability of magnetic measurement (Figure 3).
Figure 3:
Photograph of test magnetic test fixture to automatically measure magnetic
properties
In addition to testing of actual armatures, additional experimental work was done evaluating both
the standard MPIF FY-8000 material and a blend of FL-4400 with 0.45% and 0.80 w/o
phosphorus. The objective of this additional testing was the potential to replace the standard FY8000 material with a material possessing higher tensile strengths and higher hardness without any
degradation of the magnetic performance. In this part of the study, magnetic toroids and MPIF
dog-bone type tensile specimens were compacted at 40 (555 MPa) and 50 tsi (690 MPa) and
sintered at 2050 °F (1120 °C) for 20 minutes at temperature in a nitrogen / hydrogen atmosphere.
Results:
The magnetic test results of the armatures are summarized in Figure 4 thru Figure 7. Each figure
shows the specific magnetic property at each step in the processing of production armatures. As
sintered, the armatures show the following magnetic properties
! Permeability
! Coercive Force →
! Maximum induction
→
2155
2.1 Oersted
→
11,470 Gauss
After sizing the magnetic properties are as follows:
! Permeability
! Coercive force
! Maximum induction
→
→
→
1170
3.2 Oersted
10,870 Gauss
Sizing of the part for dimensional precision and flatness severely degrades the permeability to
approximately 50% of the as sintered value while increasing the coercive force to a value 50%
higher than that of the as sintered value.
2500
Permeability
2000
1500
1000
500
0
Sintering
Figure 4:
Sizing
Grinding
Tumble
Cross
Hatching
Heat
Treating
Phosphate
Permeability of armatures after each processing step
Summarizing the data shown in Figures 4 through 7, the sizing operation produced the most
severe reduction in magnetic performance. The cold working associated with the sizing step
strained the iron lattice thus restricting the movement of the magnetic domains and hence
lowering the magnetic performance. [5] Operations subsequent to the sizing step produce only
minor losses in magnetic performance.
Interestingly, the heat-treating operation (nitriding) actually gave a small increase in the
permeability. Although the temperature of nitriding is below conventional annealing
temperatures, this relatively low temperature thermal treatment begins the initiation of annealing
specifically, the recovery stage. [6] These results were somewhat surprising because it was
originally speculated that each step would result in an additional measurable loss in magnetic
performance. However, the sizing operation was the most significant. Thus actual armature
performance is defined by the as sintered performance and the performance after the sizing. An
important implication of this data is that any additional loss in magnetic performance after the
sizing step indicates a deviation in processing from the specified processing conditions.
3.5
Coercive Force, Oe
3
2.5
2
1.5
1
0.5
0
Sintering
Figure 5:
Sizing
Grinding
Tumble
Cross
Hatching
Heat
Treating
Phosphate
Coercive Force of armatures after each processing step
The magnetic properties most affected by the secondary operations are the structure sensitive
properties that is, permeability, coercive force, and residual magnetism. Permeability of the
armature is important because it effects the response time of the armature, thus, effecting the
response time of the armature / rotor engagement. Unaccounted for reductions in the
permeability of the armature will potentially delay the initiation of the torque transfer leading to a
potential loss in control of the vehicle in emergency situations. Coercive force is a measure of the
current required to demagnetize the magnetic device. In a 4-WD system, a partially magnetized
armature could result in unintentional engagement of the transfer case leading to reduced fuel
mileage and excessive wear of the armature/rotor. Thus it is important to design for these values
and maintain them within the specified limits to ensure proper functioning and long-term
durability of the transfer case.
As anticipated, the magnetic induction results shown in Figure 6 do not show the level of
degradation as noted with the permeability or coercive force. The maximum reduction in
induction level is about 10%. For the case of induction, the sizing operation has the least effect,
whereas, the additional steps tend to further reduce the measured induction. Some of this
additional loss in the measured induction can be accounted for the reduction in mass of the part
with each additional process. Specifically, the grinding and cross hatching removes material thus
eliminating mass from the part.
11600
Induction at 40 Oersteds
11400
11200
11000
10800
10600
10400
10200
10000
9800
9600
Sintering
Figure 6:
Sizing
Grinding
Tumble
Cross
Hatching
Heat
Treating
Phosphate
Induction at ~40 Oersteds after each processing step.
The residual magnetism shows the same level of degradation as the permeability and coercive
force. Again, the sizing operation is the most significant. Additional secondary operations do not
result in any further decrease in residual magnetism.
Parallel to the study of the actual armature production, experimental work investigated the effects
of a potential change in the base iron to improve the mechanical properties without degrading the
magnetic performance. This work was a continuation of earlier work of Gabrielov, etal. [7] In
this parallel study, a MPIF FL-4400 base iron was investigated in addition to the standard pure
iron base material. In this part of the study, magnetic toroids and standard MPIF dog-bone tensile
bars were pressed and these test specimens were sintered in a laboratory belt furnace using the
same sintering conditions as used in the production of the actual armatures.
7000
Residual Magnetism, Gauss
6000
5000
4000
3000
2000
1000
0
Sintering
Figure 7:
Sizing
Grinding
Tumble
Cross
Hatching
Heat
Treating
Phosphate
Residual magnetism of the armatures after each processing step.
Shown in Table 1 is a summary of the magnetic data for the two experimental and the standard
MPIF FY-8000 materials. Table 2 presents the physical properties of the three materials. From
the data shown in Table 1, using an FL-4400 base iron in lieu of the pure iron, there is only a very
minor loss in the magnetic performance. The magnetic values presented are for the as sintered
condition and represent testing of magnetic toroids 2 inch OD x 1.8 inch ID and 0.25 in tall. The
magnitude of the data for the FY8000 material shown in Table 1 is significantly different from
the values obtained on testing of actual armatures. This difference can be accounted for by the
difference in geometries of the two test samples. Ideally, a magnetic toroid should have and
ID/OD ratio of ~0.8 to 0.9. [8] This prevents gross differences in the magnetic induction between
the core and the OD of the part. Unfortunately testing of the armature with its cored walls and
toothed outer diameter, the actual magnetic path length and magnetic cross sectional area could
only be approximated. This is not to imply that the data is invalid, rather to point out the
differences in the test procedures.
Table 1
Summary of Magnetic Properties of Test Materials
1.32
Induction at
15 Oe (kG)
13.1
Induction at
25 Oe (kG)
13.6
3750
1.47
12.8
13.4
3725
1.40
13.0
13.5
Material
Density
Max. Perm
Hc (Oe)
FY-8000
FL-4400 +
0.45% P
FL-4400 +
0.80% P
7.20
4500
7.12
7.21
The increase in mechanical properties by utilizing an FL-4400 base iron will not eliminate the
need for the secondary operations outlined in Figure 1. Unfortunately, the hardness is still
insufficient to resist long term wear at the rotor/armature interface. However, the equivalent
strength of the FL-4400 with 0.45% phosphorus offers higher elongation with reduced
dimensional change during sintering compared to the FY-8000 material. The FL-4400 with
0.80% phosphorus shows higher strength but also increased elongation. This material offers
increase flexibility in processing as the molybdenum prealloy eliminates phosphorus
embrittlement in steels.
Table 2
Summary of Mechanical Property Testing
7.20
YS, 1000 psi
(MPa)
49.6
UTS, 1000
psi (MPa)
53.5
Elongation,
%
1.4
Hardness,
HRB
71
7.14
44.5
62.4
7.2
68
7.27
52.2
65.4
8.0
75
Material
Density
FY-8000
FL-4400 +
0.45% P
FL-4400 +
0.80% P
From the data presented in Tables 1 and 2, the use of the FL-4400 base iron does not degrade the
magnetic properties relative to the FY-8000 material; however, the gain in mechanical properties
maybe significant for this high strength, high hardness application. Further production trials will
be necessary to evaluate this potential for the production of either armatures or rotors.
Summary:
Maintaining control of the magnetic property response of the armature and rotor is critical for the
proper functioning of modern 4-WD transfer cases. This investigation showed the effects of
various processing steps on the magnetic response of armatures from the sintered condition
through final phosphate coating for corrosion protection. The sizing operation produced the
largest degradation in magnetic response of the secondary operations. Subsequent operations
showed only minimal further reduction in magnetic response. The special test fixture developed
for testing of the armatures facilitated testing allowing the magnetic testing to be used as an in
process control.
As a secondary investigation, substituting a FL-4400 base iron for the pure iron did not degrade
the magnetic properties while giving increased tensile properties. Although the secondary
operations would still be required to give the surface hardness and surface detail, the FL-4400 is
less sensitive to phosphorus embrittlement. Additional testing of actual armature is necessary to
evaluate this possibility.
References:
1.) Ward Auto World, vehicle build statistics
2.) T.. R. Weilbaker, E. R. Lumpkins, “Creating Innovations in Torque Transfer Systems
Through Optimization of Powder Metallurgy Components”, SAE Paper # 2001-01-0350.
3.) Frayman, Ryan, & Ryan
4.) I. Gabrielov, P. Cook, E. Tews, C. Wilson, “Measurment of P/M Part Magnetic
Properties”, Advances in Powder Metallurgy and Particulate Materials – 2000, Compiled
by H. Ferguson, D. Whychell, Published by MPIF, pp. 7-57 to 7-65.
5.) C. Lall, Soft Magnetism, Fundamentals for Powder Metallurgy and Metal Injection
Molding,Metals Powders Industry Federation, Monographs in P/M Series No.2, 1992.
6.) Robert E.Reed-Hill, Physical Metallurgy Principles, Van Nostrand Reinhold Company,
New York, 1964.
7.) I.Gabrielov, F.Hanejko P/M Advances in Powder Metallurgy and Particulate Materials –
2001, Compiled by, Published by MPIF, pp. .
8.) Walker Scientific Inc. “Instruction Manual for AMH-20 Automatic Hysteresisgraph for
testing Soft Magnetic Materials, p. 2-2.