Use of Binder-Treated Ferrous PM Premixes for Improved
PM Part Production and Part Density
Francis Hanejko & William Tambussi
Hoeganaes Corporation
Cinnaminson, NJ 08077
USA
Abstract:
The benefits of utilizing phosphorous as an alloying element in pure iron are well
documented. These include improved DC magnetic performance, higher sintered densities,
and good mechanical properties. A major disadvantage of alloying phosphorus in iron is the
greater tool wear experienced with using ferro-phosphorous additives. Utilizing an
experimental binder treatment process, a reduction in the strip and slide was observed during
laboratory testing of both traditional binder treated premixes at sintered densities less than 7.0
g/cm³ and with warm die compaction materials at sintered densities of >7.2 g/cm³. This
reduction in strip and slide has the potential to minimize die wear. Both the mechanical and
magnetic properties of the new material will be presented.
Introduction:
Advantages of using phosphorus in PM steels include good magnetic properties, higher
magnetic resistivity, and good mechanical properties with excellent elongation and impact
toughness. [1,2] Typical applications for phosphorous containing materials (FY-4500) include
automotive speed sensors, flux rings for DC electric motors, and electromagnetic actuators.
[3] Additionally, phosphorous is added in FC-0205 type steels utilized in automotive main
bearing caps because of the unique combination of strength, ductility, and good sintering
response. [4]
The major disadvantage of phosphorous containing materials is that phosphorous is added as
a ferro-phosphorous inter-metallic. Whether Fe2P or Fe3P, these inter-metallic compounds
possess high hardness, which then results in a powder premix consisting of a relatively soft
iron matrix incorporating a high hardness second phase particle that can cause significant die
wear. The hardness values of Fe2P and Fe3P are ~1050 HV and 1000 HV, respectively; this is
compared to an iron particle hardness of ~100 BHN (no conversion to HV is realistic at this
low hardness). [5] These high hardness particles result in abrasive wear of the punches and
often the actual tool material. Frayman has reported that the hard particle additions abrade the
metallic binder of carbide inserts. [6] The consequence of these hard particles is excessive
tool wear with greater frequency of tool repair and replacement. Utilizing advanced die
coatings has lessened tool wear but not eliminated it. [7] Thus, users of FY-4500 simply “live
with” the inherent high tool wear and frequent die repair associated with ferro-phosphorus
containing materials.
Hoeganaes Corporation produces FY-4500 materials as a binder treated premix
(ANCORBOND™). Although this binder treatment does not reduce the abrasive nature of
ferro-phosphorus, it minimizes segregation thus preventing localized tool wear because of
potential segregation. The experimental work described in this report focuses on an
experimental enhanced binder treatment system designed to counteract the abrasive
characteristic of ferro-phosphorus particles. Although the exact nature of this new binder
system is proprietary, it does function with both conventional PM lubricants designed for both
conventional compaction and with modifications can be utilized with warm die compaction
processes. The work discussed in this report will document the mechanical properties,
laboratory measured ejection characteristics of this experimental binder treatment system.
Experimental Procedure:
In this study, four laboratory premixes were prepared and evaluated; these premixes are listed
in Table 1. All premixes were prepared by adding 2.9% Fe3P plus the appropriate powder
lubricant; thus producing a final sintered phosphorus content of ~0.45%.
Table 1
Premixes Evaluated in this Investigation
Premix ID
1
(ANCORBOND
FY-4500)
2
3
(AncorMax 200
FY-4500)
4
Base Iron Type
High
Compressibility
Iron Powder
High
Compressibility
Iron Powder
High
Compressibility
Iron Powder
High
Compressibility
Iron Powder
Premix Type
Conventional
Binder treated,
cold compaction
Enhanced binder
treated, cold
compaction
Warm die
compaction,
conventional
Enhanced binder
treated, warm
die compaction
Lubricant
added
% Lubricant
Added
Acrawax
0.75%
Proprietary
0.75%
Proprietary
0.40%
Proprietary
0.40%
The premixes were evaluated for compressibility, transverse rupture strength, green strength,
tensile and impact properties. Compaction conditions for the “cold” compaction conditions
were a die temperature of ~25 °C (75 °F) and compaction pressures of 415, 550, and 690
MPa. For the warm die premixes, the die temperature was ~93 °C (200 °F) with compaction
pressures of 550, 690, and 830 MPa. In addition to the mechanical property specimens
magnetic toroids were pressed from mixes 1 and 2 to a density of 6.9 g/cm³ and from mixes 3
and 4 to a density of 7.25 g/cm³.
After compaction, all samples were sintered in a laboratory belt furnace on ceramic trays.
Sintering conditions were 1120 °C (2050 °F) in a 90% nitrogen / 10% hydrogen atmosphere,
with a cooling rate after sintering of ~0.6 °C per second. The time above 1095 °C (2000 °F)
was approximately 20 minutes.
After sintering, the samples were tested via the appropriate MPIF test standard. [8] In addition
to the mechanical property test samples, ejection characteristics were quantified. The
methodology employed was to compact a green strength bar at 690 MPa, then measure two
key characteristics, strip and slide. Strip is defined as the initial break free load required to
initiate movement of the test specimen and slide is defined as the load immediately before the
specimen exits the die. Strip and slides pressures are then calculated by dividing the
measured force by the area of the bar in contact with the die surface.
Results:
Presented in Table 2 are the powder properties of the four premixes evaluated in this study.
The advanced binder treatment utilized in combination with the high compressibility iron
does not affect the apparent density (AD) or flow characteristics relative to ‘conventional’
processing. The data presented in Table 2 was collected on 250 kg laboratory prepared
premixes.
Table 2
Powder Properties of Materials Evaluated
Premix ID
Base Iron Type
1
FY-4500
2
FY-4500
3
FY-4500
4
FY-4500
Premix Type
Conventional,
cold compaction
Enhanced, cold
compaction
Warm die,
conventional
Enhanced, warm
die
AD, g/cm³
Flow, sec / 50 g
3.15
29.2
3.16
29.0
3.09
25.9
3.10
26.3
Figure 1:
Compressibility data for four materials evaluated, all premixes with 2.9%
added ferro phosphorus
Compressibility information is presented in Figure 1. As expected, the warm die compaction
premixes show ~0.10 g/cm³ to 0.20 g/cm³ improvement in compressibility compared to the
conventional premixes. Compressibility of FY-4500 for the standard and experimental
enhanced binder systems is identical for both cold and warm die compaction grades. Thus,
the experimental binder system had no detrimental effects on the compressibility of the FY4500 alloy system.
Figure 2 presents the green strength of the FY-4500 materials compacted at both room
temperature and at 93 °C (200 °F). The experimental binder system has no detrimental effect
on the green strength of FY-4500 regardless of compaction conditions. It is noteworthy;
warm die compaction gives an approximate 75% increase in green strength over the range of
green densities evaluated. This higher green strength can result in a potential reduction in
green part damage during part handling prior to sintering.
Figure 2:
Green Strength of Tested Materials
Table 3
Sintered Mechanical Properties of the Six Premixes Evaluated
Mix ID
Std FY4500 cold
compaction
Sintered
Density,
g/cm³
6.70
6.95
7.09
Exp FY4500 cold
compaction
6.71
6.94
7.1
Warm die
compaction
FY-4500
7.12
7.34
7.40
Exp warm
die
compaction
FY-4500
7.12
7.31
7.42
Yield
Strength,
MPa (psi)
218
(31,700)
245
(35,700)
267
(38,800)
216
(31,400)
241
(35,000)
258
(37,500)
232
(33,700)
258
(37,500)
269
(39,100)
229
(33,300)
261
(37,900)
273
(39,700)
UTS,
MPa (psi)
280
(40.600)
324
(47,100)
356
(51,700)
273
(39,600)
322
(46,800)
353
(51,300)
327
(47,500)
368
(53,400)
382
(55,600)
324
(47,100)
371
(53,900)
390
(56,700)
Elongation,
%
Impact,
Joules
(ft.lbf)
Hardness,
HRA
5.6
10 (7)
28
6.8
24 (17)
34
7.8
31 (22)
38
5.0
14 (10)
28
7.8
20 (14)
34
8.9
28 (20)
37
9.1
17 (13)
34
10.6
34 (25)
38
10.6
52 (39)
41
9.8
15 (11)
34
10.5
32 (22)
38
10.9
57 (42)
40
Table 3 presents the mechanical properties developed for the four alloys investigated.
Transverse ruptures test samples were prepared for each of the materials; however, the
ductility of the six materials invalidated these data. The TRS samples were utilized to
quantify the DC after sintering. Referring to Table 3, premixes 1 and 3 are commercially
available materials containing ferro phosphorus. Premixes 2 and 4 represent the experimental
binder system materials. Comparing the commercial premix to the experimentally prepared
premix, there is no difference in either the measured tensile or impact properties.
Figure 3 presents the dimensional change data for the four materials after sintering at 1120 °C
(2050 °F) in a 90% nitrogen / 10% hydrogen atmosphere. The experimental binder system
for the “cold” die compaction showed slightly higher shrinkage by approximately 0.02% at
the 6.8 g/cm³ sintered density compared to the commercial material (within experimental
measuring error). The sintered dimensional change for the warm die compaction FL-4500
was identical for both the conventional material and the experimental binder system.
Figure 3:
Dimensional change of the premixes, premixes 3 and 4 are excluded because
of carbon pick up during sintering.
Table 4 presents the DC magnetic data developed for each premix. From the data two trends
are apparent. First, the experimental binder system does not affect the magnetic performance
of the either FY-4500 at either 6.9 g/cm³ or 7.25 g/cm³ sintered densities. The data
presented in Table 4 appears to suggest that higher density does not improve the magnetic
properties of PM materials. This is contradictory to data presented in MPIF standard 35,
which indicates that higher part density produces higher magnetic performance. Sintered
carbon and sintered oxygen results for 4 materials were identical at <0.01% and ~0.06%
respectively. It is expected that the permeability at this ~7.3 g/cm³ sintered density should be
~3500.
Ejection characteristics as measured by strip and slide are presented in Table 5. At ambient
compaction conditions, the experimental binder system shows a 10% lower stripping pressure
and ~5% lower sliding pressure compared to the standard ANCORBOND treated material.
At warm die compaction conditions, the standard material has a significantly higher strip and
slide compared to the lower density green strength bars. However, the modified experimental
binder produces a dramatic decrease in both the strip and slide.
Table 4
Sintered DC Magnetic Data
Mix ID
Std FY4500 cold
compaction
Sintered
Density,
g/cm³
Applied
Field,
Oe
6.90
15
Max DC
Perm
25
Exp FY4500 cold
compaction
6.90
Warm die
compaction
FY-4500
7.25
Exp warm
die
compaction
FY-4500
7.25
15
25
15
25
2200
2700
2600
15
25
2600
Max
Induction,
kGauss
Hc,
Oersteds
Br,
kGauss
11.3
2.0
11.3
11.9
2.0
11.9
11.8
1.8
11.7
12.2
1.9
12.2
13.1
2.2
11.8
13.7
2.2
11.9
13.2
2.2
11.9
13.8
2.3
12.1
Table 5
Ejection Characteristics of FY-4500
Premix ID
Std FY-4500 cold
compaction
Exp FY-4500 cold
compaction
Warm die
compaction FY-4500
Exp warm die
compaction FY-4500
Green Density,
g/cm³
Stripping Pressure,
MPa (psi)
Sliding Pressure,
MPa (psi)
6.90
38.8 (5630)
19.8 (2870)
6.90
35.4 (5150)
19.1 (2780)
7.20
59.9 (8700)
48.2 (7000)
7.20
33.0 (4800)
29.6 (4300)
Discussion:
The experimental binder treatment described showed reduced ejection forces for FY-4500
materials as measured by strip and slide stress. In addition to the reduced ejection forces,
mechanical property testing demonstrated no deleterious effect on the compressibility or
sintered mechanical properties of the FY-4500 material system. To verify the usefulness of
the laboratory data, preliminary testing was performed on production PM components. Beta
site testing showed reduced ejection pressures, reduced die wear, and an overall improvement
in the compaction characteristics of FY-4500 material at green densities ranging from 6.8 to
7.0 g/cm³. A qualitative observation was reduced press adjustments during extended
production runs. The fewer number of press adjustments was attributed to reduced
entrapment of powder between the punches and die thus enabling the press to return to the fill
position with less powder drag between the punches and die assembly. In terms of die wear,
measurement of the top punch showed no measurable wear on the top punch after a
production run of ~50,000 parts. This represented a considerable improvement from the
commercially available material. It was also noted that no differences were observed in either
the powder compressibility or sintered dimensional change response.
This experimental binder system for the FY-4500 material system has demonstrated that
equivalent powder compressibility with identical sintered mechanical properties can be
achieved. The implication of this data is reduced die wear and potentially lower die repair
and replacement cost. This new development can potentially lead to further utilization of
ferro-phosphorus containing premixes in new magnetic applications.
Conclusions:
From the results presented in this paper, the enhanced binder treatment of ferro-phosphorus
containing materials has demonstrated the following:
 Warm die compaction of FY-4500 type materials gives a 0.1 g/cm³ to 0.2 g/cm³
increase in green density at equivalent compaction pressures.
 Warm die compaction results in an increase in green strength of ~ 75% at equivalent
green densities.
 The experimental binder system reduces the stripping and sliding stresses of both cold
compacted and warm compacted FY-4500.
 This experimental binder system has identical compressibility to the ANCORBOND
and AncorMax 200 materials.
 No difference in DC was observed with the either “cold” die or warm die compacted
materials.
 Magnetic performance was unaffected; as anticipated, higher sintered densities should
produce better magnetic performance.
 Mechanical properties are identical.
The experimental binder system described in this report is still undergoing laboratory
development and is not currently commercially available.
References:
1. Material Standard for PM Structural Parts (MPIF Standard 35), 2009 Edition,
Published by Metal Powders Industries Federation, Princeton, NJ, 2009.
2. H. Rutz, F. Hanejko, C. Oliver, “Effects of Processing and Materials on Soft
Magnetic Performance of Powder Metallurgy Parts”, Advances in Powder Metallurgy
and Particulate Materials – 1992, Vol. 6, pp. 375 – 405, Metal Powder Industries
Federation, Princeton, NJ, 1992.
3. H. Rutz, F. Hanejko, G. Ellis, “The Manufacture of Electromagnetic Components by
the Powder Metallurgy Process”, International Conference on Powder Metallurgy and
Particulate Materials, Metal Powders Industry Federation, Princeton NJ, 1997.
4. Donald White, “Auto Industry Boosts PM Parts”, Heat Treat Magazine, January
1993, p. 19-21.
5. J. Nowacki, “Phosphorus in iron alloys surface engineering”, Journal of
Achievements in Materials and Manufacturing Engineering, Vol. 24, Issue 1,
September 2007, pp. 1-57 to 1-67.
6. Leonid Freyman, Advanced Cemented Carbides for PM Tooling Applications,
Presented at 2009 PM Parts Compacting/Tooling Seminar, Cleveland OH, Oct 27 –
28, 2009, Sponsored in Cooperation with the Powder Metallurgy Equipment
Association.
7. Robert Jacoby, Coating Technology for Metal Forming Operations, Presented at 2009
PM Parts Compacting/Tooling Seminar, Cleveland OH, Oct 27 –28, 2009, Sponsored
in Cooperation with the Powder Metallurgy Equipment Association.
8. Standard Test Methods for Metal Powders and Powder Metallurgy Products, 2008
Edition, Published by Metal Powders Industries Federation, Princeton, NJ, 2008.