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 4-WD 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 FY-8000 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
Permeability of armatures after each processing step
Phosphate
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 Oersted 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 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
Material
FY-8000
FL-4400 +
0.45% P
FL-4400 +
0.80% P
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
Density
Max. Perm
Hc (Oe)
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
Material
FY-8000
FL-4400 +
0.45% P
FL-4400 +
0.80% P
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
Density
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.) % of vehicles with 4WD
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, C. Wilson, T. Wielbaker, A. Barrows, F.Hanejko, “P/M High Strength
Magnetic Alloys”, P/M Advances in Powder Metallurgy and Particulate Materials
– 2001, Part 7, pp. 10-19, Metal Powders Industries Federation, Princeton, NJ,
2001.
8.) Walker Scientific Inc. “Instruction Manual for AMH-20 Automatic Hysteresisgraph
for testing Soft Magnetic Materials, p. 2-2.