Industrial Performance of a New Lubricant for Manufacturing PM Gears
Peter Sokolowski1, Andreas Milbrath2, Davide Vitti3, and Stefano Fontana3
1
Hoeganaes Corporation, 08077 Cinnaminson, NJ, USA
Hoeganaes Corporation Europe GmbH, 40667 Meerbusch, Germany
3
Metalsinter srl, 9, via Messina, 20038 Seregno (MI) Italy
2
Abstract
Admixed lubricants used in Powder Metallurgy (PM) have been effective at meeting the needs
of the industry to manufacture traditional PM parts. With increasing demand to improve upon
part tolerances, surface finish, density distribution, weight consistency, and reduced
environmental impact; parts makers desire to replace traditional lubricants such as Acrawax and
Kenolube. In an effort to do so, Ancorlube has been developed as an engineered lubricant
system to enable robust part manufacturing. This work compares the production characteristics,
based on lubricant type, for a water pump gear pressed to a density of 6.80 g/cm3. The weight
consistency was evaluated during serial production of more than 5,000 gears, each weighing
approximately 191 grams. A standard deviation of 0.6 grams was measured, indicating that a
stable process was achieved with a premix containing Ancorlube. Metallographic inspection of
surface quality and density distribution was evaluated and compared against parts made with
Kenolube. In addition, laboratory studies show improved ejection characteristics are obtained at
typical press operating temperatures approaching 60 °C (140 °F).
Introduction
Traditional PM lubricants typically consist of metallic stearates, synthetic amide waxes, and
composite hybrids of stearates and waxes admixed into premixes at 0.5 – 1.5 mass percent [14]. Lubricants have a strong influence on powder behaviors such as apparent density and flow
and are critical for providing an even distribution of forces for uniform green density during
compaction. Furthermore, lubricants are necessary to reduce ejection forces to improve tool life
and permit production of high precision components with good surface finish. Ideally, a single
lubricant would provide these advantages while maintaining high compressibility of the powder
premix, providing excellent green strength of the powder compact, and burning out cleanly
during sintering. Currently, there is no single lubricant that can meet all these demands in one
package, whereby selection of a lubricant over others could possibly lead to a sacrifice in one
property in order to improve another for a specific processing requirement or application need.
Consequently, a need exists to develop a lubricant that can better support all the demands
needed to enable robust part manufacturing and reduce environmental impact.
This paper will focus on comparing traditional admixed lubricants with a recently developed
lubricant system, Ancorlube, in a manufacturing setting. Additionally, laboratory test samples
were compacted at room temperature and using a warm die at a temperature of 60 °C. When
using lubricants designed for warm die compaction, this approach can effectively lower the
necessary ejection forces and increase the achievable green density, often resulting in
improved properties overall [5].
Experimental Procedure
Laboratory premixes of similar composition were prepared using commercially available
Ancorsteel 1000 base iron powder from Hoeganaes Corporation. All premixes contained 2.0
mass % Royal Cu 155 powder, 0.6 mass % Asbury type 3203H graphite, and 0.8 mass %
lubricant content. The nominal compositions for each premix are shown in Table 1. Commonly
used lubricants, Acrawax C and Kenolube, were used as a reference for evaluation against
Ancorlube from Hoeganaes.
Table 1: Nominal compositions and identifications of lubricants studied (mass %).
Mix
Acrawax
Kenolube
Ancorlube
A1000
Balance
Balance
Balance
Cu
Graphite
Lubricant
2.0
0.6
0.8
2.0
0.6
0.8
2.0
0.6
0.8
Laboratory procedures were carried out in accordance with the appropriate MPIF standards [6].
The green density was determined using rectangular bars with dimensions of 32 x 12.7 x 12.7
mm according to MPIF Standard 15. Bars were compacted at 414, 552, and 690 MPa (30, 40,
and 50 Tsi) at room temperature and with a heated die at 60 °C. Cylindrical slugs with
dimensions of 14 x 25 mm (diameter x height) were compacted at 600 MPa (43.5 Tsi) using an
instrumented hydraulic compaction press to study ejection characteristics. Effect of lubricant
type was evaluated by comparing the initial ejection pressure (strip) and pressure applied as the
sample exits the die (slide) over time.
Results & Discussion
Laboratory Evaluation of Lubricants
The green properties of samples compacted at room temperature are listed in Table 2. The
green density of Ancorlube premixes were found to be similar to Acrawax, though slightly less
than Kenolube. The green strength, however, of the Ancorlube premix was found to be more
comparable with Kenolube containing premixes under room temperature compaction conditions,
with a range of 13 – 17 MPa. The springback behavior of the Ancorlube premixes was found to
be slightly higher than both Acrawax and Kenolube at most compaction pressures.
Table 2: Green properties from compaction at room temperature
Information
Compaction Die Temp
Tsi MPa
°C
30
414
Acrawax 40
552
50
690
30
414
Room
Kenolube 40
552
(~22)
50
690
30
414
Ancorlube 40
552
50
690
Mix
Density
g/cm3
6.71
6.98
7.11
6.75
7.00
7.12
6.72
6.96
7.09
Green
Springback
%
0.18
0.22
0.28
0.16
0.21
0.25
0.19
0.24
0.28
Strength
psi
MPa
1,637
11
2,084
14
2,195
15
1,978
14
2,387
16
2,470
17
1,852
13
2,241
15
2,425
17
The green properties of samples compacted at 60 °C are listed in Table 3. The green density of
Ancorlube premixes were found to be similar to both Acrawax and Kenolube containing
premixes, though the green springback was higher with Ancorlube. With the elevated die
temperature, green densities were increased by 0.05 – 0.10 g/cm3 for all premixes as compared
with the densities achieved using a room temperature die. The green strength of the Ancorlube
premix was equivalent to Kenolube (19 – 22 MPa). Both Ancorlube and Kenolube are superior
to the Acrawax premix (14 – 18 MPa).
Table 3: Green properties from compaction at 60 °C
Information
Mix
Compaction Die Temp
Tsi MPa
°C
30
414
Acrawax 40
552
50
689
30
414
60
Kenolube 40
552
50
689
30
414
Ancorlube 40
552
50
689
Density
g/cm3
6.82
7.06
7.17
6.83
7.10
7.17
6.83
7.05
7.16
Green
Springback
%
0.12
0.18
0.24
0.14
0.20
0.26
0.17
0.23
0.29
Strength
psi
MPa
2,006
14
2,525
17
2,621
18
2,553
18
3,160
22
3,070
21
2,696
19
3,172
22
3,223
22
To further assess the influence of the different lubricants, the ejection characteristics are
compared in Figure 1. The initial peak of each curve, representing the stripping force required to
start ejection, can be compared at both room temperature, Figure 1 - A, and 60 °C die
temperature, Figure 1 - B. It was found that the stripping force for Ancorlube was equivalent to
Kenolube and better than Acrawax under room temperature conditions. Moreover, in the
Ancorlube sample, the sliding force to eject the bar completely from the die was similar to
Kenolube, with both being lower than Acrawax. At increased die temperature, Figure 1 - B, the
stripping force for all lubricants was higher than found at room temperature. The sliding force
was better (lower) for both Ancorlube and Kenolube at elevated temperature, indicating these
two lubricants are more effective at increased temperatures. During the sliding action, both
Acrawax and Kenolube containing samples exhibited a stick and slip behavior which resulted in
an erratic force measurement.
A
B
Strip Slide Figure 1: Ejection characteristics at (A) room temperature and (B) 60 °C.
Manufacturing Compaction Trial
Water pump gears, pictured in Figure 2, were compacted to 6.80 g/cm3 on a Dorst TPA
160/3HP hydraulic press using a gravity powder feed system to evaluate the lubricant in a
manufacturing setting. The composition of the premix used was A1000 base iron with 1.0
mass% copper, 0.6 mass% UF4 graphite, and 0.85 mass% Ancorlube. For comparison, a
similar premix with Kenolube was used to manufacture the same gears. The part temperature
was consistently measured to be between 55 - 60 °C upon ejection from a water cooled die. The
weight consistencies of the trial runs are shown in Figure 3. Both trends have similar average
weights with low standard deviations of 0.4 g for Kenolube and 0.6 g for Ancorlube indicating
that a stable process was achieved for both.
Location of
cross-section
presented in
Figure 5
Figure 2: Sintered water pump gear
Average = 191.4 g
STDEV = 0.4 g
A
Average = 191.2 g
STDEV = 0.6 g
B
Figure 3: Weight of gears compacted with (A) Kenolube; (B) Ancorlube.
Parts from both the Kenolube containing premix and Ancorlube trial were sintered at 1120 °C in
an endothermic atmosphere. The surface quality from each premix was evaluated after
compaction of more than 5,000 parts, Figure 4. No visible scratching or lack of lubricity was
evident during this trial. Further, samples of the gears were sectioned to visually evaluate
density distribution and pore structure as a result of the lubricant. Figure 5 is a montage crosssection of the water pump gear from both the Kenolube and Ancorlube premixes. From this
evaluation, no substantial difference in density distribution or pore structure based on lubricant
type was observed. In addition to these observations, no sooting was found on the gears
sintered with Ancorlube. The sintered carbon content of the gears was measured using a Leco
CS200 where the Ancorlube mix was measured to be 0.57% and the Kenolube containing mix
was recorded to have 0.60% by mass. The clean burning behavior of the Ancorlube is
consistent with a prior evaluation [7] with a focus on providing a lubricant which burns out
cleanly for aesthetic, productivity, and environmental reasons.
A
B
Figure 4: Sintered gear surface compacted with (A) Kenolube; (B) Ancorlube.
A
B
Compaction direction
Sectioned to fit in
mount
OD
ID
OD
ID
Figure 5: Sintered gear cross-section compacted with (A) Kenolube; (B) Ancorlube.
Conclusion
A new lubricant, Ancorlube, has been introduced to the market with a focus on good lubricity,
green strength, and clean burning to support the PM industry needs. In lab trials, Ancorlube
showed improved lubricity and green strength over Acrawax, being more similar to Kenolube at
both room temperature and at 60 °C die temperature compaction. This performance was
similarly achieved in an industrial setting during the manufacturing of a water pump gear. With
over 5,000 gears compacted using an Ancorlube containing premix, a consistent weight was
maintained with a low standard deviation of 0.6 g, indicating a stable process was reached.
Acknowledgements
The authors would like to thank Metalsinter for permitting the manufacturing trial and collection
of associated data at their facility. Additionally, Christian Schneider and Eric Alesczyk, from
Hoeganaes, are recognized for the laboratory evaluation and metallographic inspection
presented.
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