Improved Die Fill Performance Through Binder Treatment
Kylan McQuaig, Sunil Patel, Peter Sokolowski
Hoeganaes Corporation
Cinnaminson, NJ 08077
Suresh Shah, Gilbert Schluterman, Jerry Falleur
Cloyes Gear & Products, Inc.
Subiaco, AR 72865
Abstract
Powder premixes used in the PM industry contain ingredients of substantially different particle sizes and
specific gravities that have a strong tendency to segregate during handling. Binder treatment or bonding
has been well established in the PM industry to combat these problems and is currently used for a variety
of applications. The objective of this study is to gain an understanding of how binder treatments can
improve the flow of powder from the feed system into the die cavity. The part chosen is an ABS wheel
sensor component that has fine gear teeth on the OD requiring consistent fill to ensure uniform part to part
densities and dimensional changes upon sintering. The die fill capability of binder-treated premixes with
different apparent density and flow characteristics was reviewed relative to a conventionally prepared
powder premix in actual part production. Testing included evaluation of the regular premixed and bonded
powders coupled with press productivity and key characteristics of the as-compacted and sintered
components.
Introduction
The use of binders in the PM industry has been a common practice for many years. Bonding is used to
prevent segregation of powder premixes by attaching smaller additives such as graphite, metallic alloying
elements, and lubricant to the base iron [1-3]. While the addition of smaller additives is beneficial due to
increased diffusion rates during sintering, it also contributes to segregation, a natural phenomenon that
occurs in premixes containing particles of varying size [4]. Bonding helps to maintain a homogeneous
premix composition, which prevents variation in structure and mechanical properties from part to part, or
within a single pressed part. Figure 1 shows scanning electron microscopy (SEM) images of the particles
in a regular premix compared with a premix bonded using the ANCORBOND system patented by
Hoeganaes Corporation. Figure 2 shows a high magnification image of one of the particles in the
ANCORBOND premix. In this image, a high percentage of the small additives in the mix such as the Ni
and graphite have been successfully bonded to the larger base alloy particles.
Figure 1: SEM images of a regular premix (A) and an ANCORBOND premix (B).
Figure 2: High magnification SEM image of an ANCORBOND premix particle.
In addition to maintaining premix homogeneity, bonding can be used to help tailor the properties of the
powder premix depending on the desired press settings and part application. Various types of binders and
bonding procedures are available to improve powder flowability, apparent density, green density, green
strength, and dimensional tolerance [3, 5]. Bonded powder premixes are also far less prone to dusting
than regular premixed alternatives, improving part consistency, reducing waste material, and resulting in
cleaner working conditions.
More recently, European Union regulation REACH (Registration, Evaluation, Authorization and
Restriction of Chemicals) has required that PM producers assess the environmental and health effects of
all materials manufactured or imported in Europe [6]. Additive particles that are prone to dusting, then,
pose a potential threat to press operators as respirable dust, depending on the particle size. Nickel, in
particular, is of great concern in the PM industry due to its widespread use, small particle size, and
potential health implications. By using a bonding system such as ANCORBOND to essentially glue the
premix additives to the base iron, the respirable dust can be reduced to considerably safer levels, as shown
in Figure 3.
Figure 3: Respirable dust levels observed at various locations using a regular premix and a bonded
premix.
In this study, two new experimental binder systems developed by Hoeganaes Corporation were examined
and compared with the ANCORBOND system and a regular powder premix. The relative ability of the
powder premixes to bond efficiently, as well as improve powder properties, pressing characteristics, and
mechanical properties of the finished parts was observed. The increase in powder flowability and
apparent density achieved through bonding can result directly in increased productivity and part
consistency.
Experimental Procedure
A regular FC-0205 premix and three bonded premixes of similar composition were prepared for this study
using commercially available Ancorsteel 1000C iron powder from Hoeganaes Corporation. All four
premixes contained 2.1 mass % ACuPowder International 8081 Cu, 0.6 mass % Asbury type 3203H
graphite, and 0.75 mass % total lubricant content. The nominal compositions and identifications for each
premix are shown in Table 1. One bonded premix was prepared using the ANCORBOND system and
two were made using experimental variations of ANCORBOND labeled EXP1 and EXP2.
Table 1: Nominal compositions and identifications of premixes studied (mass %).
Regular Premix
ANCORBOND
EXP1
Iron
Copper
Graphite
Lubricant
EXP2
Balance
Balance
Balance
Balance
2.1
2.1
2.1
2.1
0.6
0.6
0.6
0.6
0.75
0.75
0.75
0.75
FLN
All laboratory procedures were carried out in accordance with the appropriate MPIF standards [7]. The
apparent density and flowability of each powder premix was tested using a Hall Apparatus according to
MPIF Standards 3 and 4. The green density and ejection characteristics were observed on rectangular
bars with 32 x 12.7 x 12.7 mm dimensions according to MPIF Standard 15. Using a hydraulic
compaction press, the initial ejection pressure (strip) and pressure applied as the bar is exiting the die
(slide) was measured over time. Figure 4 shows a schematic of a standard pressure vs. time graph,
including the points at which strip and slide pressures are recorded. Green density and ejection data were
recorded at compaction pressures of 415, 550, 690, and 830 MPa (30, 40, 50, 60 tsi) at both room
temperature and with a die heated to 75° C.
Figure 4: Schematic of ejection measurements [5].
Each powder premix was also tested for dust resistance using an elutriation test. Using the set-up shown
in the schematic in Figure 5, nitrogen was forced through a small characteristic sample of each premix. A
chemical analysis was performed on powder before and after the elutriation in order to determine any
change in the percentage of bonded additions. In this study, the carbon content was measured before and
after to determine dust resistance and bonding capacity of the graphite for the four tested premixes. The
percent bonded graphite for each bond type is also a good indication of the dust resistance of other
elements in the premix as well.
Figure 5: Schematic of the elutriation set-up for testing dust resistance [5].
Mechanical properties were measured using transverse rupture (TR), dogbone tensile, and unnotched
Charpy impact bars compacted to green densities of 6.80 and 7.00 g/cm3. These samples were sintered at
1120° C for 25 minutes in a mixed atmosphere of 95 v/o nitrogen and 5 v/o hydrogen. Prior to
mechanical testing, green and sintered density, apparent hardness, and dimensional change were
determined on the TR samples using MPIF Standards 42, 43, and 44, respectively. The mechanical
properties were evaluated on sets of five bars for each premix and density combination. TR strength was
determined using MPIF Standard 41, tensile properties were found in accordance with MPIF Standard 10,
and impact energy was measured using MPIF Standard 40. Sintered carbon values were also measured
using a Leco 200 carbon-sulfur combustion gas analyzer with reference standards run before and after test
samples.
A compaction study was also performed in order to determine consistency of each premix in terms of
dimensional control between parts, overall mass variation, and die fill. A round ABS wheel sensor
component with small teeth, displayed in Figure 6, with an approximate height of 2 cm and mass of 130 g
was chosen for this study. Using each of the four premixes, 300 total parts were pressed to a green
density of 6.80 and 7.00 g/cm3. For 50 consecutive parts during each run, mass and thickness were
measured to determine the consistency of the die fill and powder homogeneity. The parts were also
measured for inner and outer diameter length, as well as roundness. Part temperature was observed over
time, as well as overall part-to-part density and density of the various quadrants within a given part. For
each premix and density combination, five parts were tested for crush strength and hardness to determine
the relative mechanical properties of the parts following sintering. Finally, the surface finishes of the
green and sintered parts were photographed, and photomicrographs of each premix and density were
captured.
Figure 6: Photo of one of the pressed parts used in the study.
Results
Powder Properties
Before pressing samples and observing mechanical properties, the properties of the powder premixes
themselves were tested. The results, shown in Table 2, highlight the flexibility available through the use
of different bonding techniques. The regular premix has an apparent density of 3.04 g/cm3, which can be
matched closely using the ANCORBOND system. The two experimental bonds resulted in apparent
density values of 3.28 and 3.27 g/cm3, respectively, but these numbers could be lowered through
adjustments in the lubricant.
Differences can be seen in the flow and dust resistance characteristics, as well, though they are not as
pronounced as is possible with other lubrication systems. All three bonding systems were found to
increase powder flowability slightly and drastically improve dust resistance of the premix. While the
regularly premixed material lost nearly 50% of the carbon present during the elutriation test, all three
bonded premixes maintained approximately 90% carbon. This bonding effectiveness results in less
segregation and better powder homogeneity, lower health risks for press operators, and improved
cleanliness of work areas. Powder properties such as apparent density, flow, and dust resistance can be
adjusted to meet user specifications through control of bond type and lubricant selection when designing a
premix.
Table 2: Powder characteristics of the four tested premixes.
Apparent
Density
Mix
(g/cm3 )
Flow
(s/50g)
Dust
Resistance
(% carbon)
Premix
3.04
31
55
ANCORBOND
3.03
31
90
EXP1
3.28
30
92
EXP2
3.27
28
90
Green Properties
The green properties for the four premixes are shown in Table 3 at room temperature and after pressing
with a die heated to 75° C. Because the premixes used in this study are of the same composition and
lubricant content, many of the green properties were found to be within experimental error. When used at
room temperature, EXP1 was found to have a slightly lower green density, as seen in Figure 7. Because
this bond is designed for use at slightly elevated temperature, the premixes were again observed after
heating the die to 75° C, and EXP1 was found to have compressibility similar to the other bonded
premixes, as shown in Figure 8.
Table 3: Green properties of the four premixes at room temperature and with a heated die (75° C).
Mix
Regular Premix
ANCORBOND
EXP1
EXP2
Compaction
Pressure
(MPa)
415
550
690
830
415
550
690
830
415
550
690
830
415
550
690
830
GD
3
(g/cm )
6.93
7.13
7.23
7.27
6.92
7.13
7.23
7.28
6.86
7.07
7.19
7.25
6.93
7.15
7.26
7.32
Room Temp.
GS
Strip
(MPa)
(MPa)
15.6
20.9
23.7
25.6
15.1
19.7
21.8
23.2
13.7
19.4
22.5
25.6
16.2
22.4
26.3
27.2
15.0
18.0
19.7
20.5
14.7
17.7
19.8
20.5
14.3
17.9
20.5
22.1
15.2
18.9
21.1
22.4
75° C
Slide
(MPa)
10.0
13.3
15.3
15.8
10.9
13.9
16.6
17.4
11.2
15.0
18.3
20.9
12.3
16.1
20.2
22.5
GD
3
(g/cm )
7.00
7.20
7.28
7.30
7.01
7.22
7.29
7.31
7.05
7.23
7.27
7.28
7.04
7.22
7.27
7.29
GS
(MPa)
Strip
(MPa)
Slide
(MPa)
13.4
17.1
18.8
21.3
13.2
14.8
17.1
19.2
14.3
17.7
18.1
18.2
14.3
17.0
18.1
20.6
24.1
25.0
25.6
25.5
21.4
22.6
22.0
21.6
12.7
16.8
17.3
18.2
13.5
16.9
18.1
18.1
11.5
12.0
11.0
10.1
12.0
11.5
8.6
6.7
7.6
11.7
12.3
13.7
7.4
10.6
12.5
13.4
Figure 7: Compressibility of the four premixes at room temperature.
Figure 8: Compressibility of the four premixes with die heated to 75° C.
There was little variation observed in green strength at elevated die temperature, but EXP2 showed
improved green strength over the other premixes when used at room temperature. This additional green
strength can be beneficial for applications in which parts are handled to some extent before sintering or
must be machined in the green state. Both experimental bonds also showed a reduction in the necessary
ejection pressures, especially at elevated die temperature. Figure 9 shows the effect of the different bond
types on the stripping pressure at a die temperature of 75° C. Decreasing the stripping pressure can help
maintain an excellent surface finish on pressed parts and extend the life of the tooling.
Figure 9: Strip pressure vs. compaction pressure at a die temperature of 75° C.
Mechanical Properties
The mechanical properties found using TR, unnotched Charpy impact, and dogbone tensile bars are
displayed in Table 4. The bars were pressed for each of the four premixes to a green density of 6.80 and
7.00 g/cm3 for testing. As shown in the table, there was little difference in properties between the
regularly premixed material and the bonded material, or between the various bond types. All four
premixes were found to have consistent mechanical properties in the as-sintered condition, which were
dependent on overall premix chemical composition and density. None of the three bonding techniques
affected these properties in a negative manner.
Table 4: Mechanical properties of the four premixes pressed to green densities of 6.80 and 7.00 g/cm3.
Mix
Regular Premix
ANCORBOND
EXP 1
EXP 2
Green
Density
Sintered
Density
(g/cm3 )
6.80
7.00
6.80
7.00
6.80
7.00
6.80
7.00
(g/cm3 )
6.73
6.94
6.73
6.92
6.73
6.93
6.72
6.93
Dimensional
Change
(%)
TRS
(MPa)
HRA
Charpy
Impact
(Joules)
UTS
(MPa)
+ 0.49
+ 0.53
+ 0.51
+ 0.53
+ 0.51
+ 0.55
+ 0.50
+ 0.53
903
1063
869
1049
897
1031
921
1067
44
48
44
48
44
47
45
48
9
14
8
11
8
11
9
12
391
458
367
426
401
443
356
431
0.2% Offset
Elongation
YS
(%)
(MPa)
306
351
303
340
316
334
300
323
1.5
1.9
1.3
1.6
1.4
1.8
1.5
1.9
Sintered
Carbon
(%)
0.55
0.57
0.58
0.56
Compaction Study
A compaction study was performed at Cloyes Gear & Products, Inc. to determine the performance of each
premix in a production environment. Round ABS wheel sensor parts with an approximate height of 2 cm
and mass of 130 g were pressed in a Cincinnati Rigid Reflex 200-C-6 mechanical compacting press to a
green density of 6.80 and 7.00 g/cm3 in runs of 300 parts. The part consistency in terms of density, mass,
height, outer diameter, inner diameter, and roundness was measured for 50 consecutive parts in the
middle of each run.
The consistency of the mass and height for 6.80 g/cm3 density green parts is shown in Figures 10 and 11
for all four premixes. The distributions shown in the graphs were used to determine overall consistency
and standard deviation from the mean value. EXP1 showed excellent consistency in both mass and height
due to the bonding methods used. All three bonded premixes showed an increase in uniformity of the part
height at both densities. As shown in the figures, the two experimental bonds resulted in exceptional
repeatability of part height and mass in the green condition at this density. Similar trends were observed
for the parts pressed to 7.00 g/cm3.
Figure 10: Mass variation of green parts pressed from each of four premixes at 6.80 g/cm3 density.
Figure 11: Height variation of green parts pressed from each of four premixes at 6.80 g/cm3 density.
Table 5 outlines the data obtained during the pressing study at Cloyes Gear & Products, Inc. for each of
the four premixes. The dimensional consistency is reported using the standard deviation value calculated
for height, inner diameter, outer diameter, and roundness for parts pressed to a 6.80 and 7.00 g/cm3 green
density. As seen in the table, there is good dimensional control for each of the premixes in the sintered
condition. Overall standard deviation in height, inner diameter, and outer diameter was only 10 µm or
less for the majority of conditions. The values for the bonded premixes were in agreement with those
calculated for the regular premix. Slightly better dimensional consistency was found for the bonded
premixes pressed to 6.80 g/cm3, while the inverse was true for the 7.00 g/cm3 samples.
There was a slightly higher standard deviation for all four premixes at both density values. The values for
the ANCORBOND premix were consistent with those found for the regular premix. In terms of
roundness for both the inner and outer diameter, the experimental premixes were both found to have
higher standard deviation values. The crush strength values were similar for all four premixes, with the
bonded premixes showing some benefit over the premix at the higher density.
Table 5: Statistical data outlining the sintered dimensional consistency and crush strength of the four
premixes at two densities.
Mix
Regular Premix
ANCORBOND
EXP 1
EXP 2
Green
Density
(g/cm3 )
6.80
7.00
6.80
7.00
6.80
7.00
6.80
7.00
Height
Inner Diameter ID Roundness Outer Diameter OD Roundness
St. Deviation St. Deviation St. Deviation St. Deviation St. Deviation
(µm)
(µm)
(µm)
(µm)
(µm)
7.94
7.56
9.42
12.05
6.27
9.74
5.62
13.87
4.49
3.67
4.05
3.73
4.13
4.36
3.89
4.71
14.47
12.46
16.62
11.28
23.36
20.06
15.60
21.34
11.23
7.76
10.37
12.64
5.45
9.24
10.00
9.44
15.34
15.33
17.71
11.91
21.46
19.47
13.25
19.06
Crush
Strength
(MPa)
5.26
5.64
5.21
5.81
5.07
5.83
4.83
5.76
Surface Cleanliness
Following the press study, sintered parts were photographed to determine the presence of any sooting that
may have occurred during sintering. Figures 12 and 13 show the cleanliness of the surfaces of a part
pressed using each premix at 6.80 and 7.00 g/cm3. The figures show that with the current lubricant
system employed, sooting was present on parts pressed from the standard premix and the ANCORBOND
premix at both densities. Using EXP1 and EXP2 bonding techniques reduced the sooting significantly,
even at higher density.
Figure 12: Sooting observed following sintering for all four premixes pressed to 6.80 g/cm3.
Figure 13: Sooting observed following sintering for all four premixes pressed to 7.00 g/cm3.
Conclusions
Bonding can be utilized in most instances to immediately improve powder flow, apparent density, dust
resistance, surface finish and ejection characteristics without any degradation of mechanical properties in
the sintered product. This study showed the overall flexibility of bonding and how unique bonding
techniques can be employed to meet specific goals in terms of powder, mechanical, and pressing
properties. In addition, elutriation tests showed a substantial reduction in respirable dust for all three
variations of the ANCORBOND system studied. A number of binder and lubricant combinations exist
that allow parts producers to improve powder properties over a regular premixed material.
Acknowledgements
The authors would like to thank Chris Schade for his contributions to this work. Sample pressing and
characterization done by Chen Wing Hong, Jerry Golin, Barry Diamond, and Tom Murphy was also vital
to the completion of this paper. The work at Cloyes was performed John Combs, Sam Schluterman and
Gerry Wewers, who deserve acknowledgement as well.
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