IMPROVED POWDER PERFORMANCE THROUGH
BINDER TREATMENT OF PREMIXES
Chris Schade, Mike Marucci, & Fran Hanejko
Hoeganaes Corporation
Cinnaminson, NJ 08077
ABSTRACT
Powder mixes used in the PM industry contain ingredients of substantially different
particle sizes and specific gravities that have a strong tendency to segregate during
handling. Reducing or eliminating this segregation is essential for the part producer to
achieve consistent precision and optimum performance. Treating the premixes with
various types of binders improves the performance of the premix as a whole. A
laboratory study was conducted with various binder/lubricant systems and results relating
to segregation, flow and press performance are reviewed. Developments that provide
improved flow rates combined with increased green densities and green strengths as well
as their impact on overall productivity and performance are outlined.
INTRODUCTION
Binder treatment or bonding has been well established in the PM industry and is currently
used for a variety of applications. Binder treatment has progressed from simply reducing
segregation and improving flowability to a process that can be engineered to improve
green strength, green density and dimensional change as well as improving the pressing
and ejection of parts. There are several different commercial binder systems available
from simple oils to engineered polymers. To study the advantages and disadvantages of
these systems a laboratory study was conducted. The two main objectives of the study
were to study the bonding efficiency and compaction characteristics of the various
bonding systems.
Bonding Mechanisms
The mechanisms which occur during bonding are mainly a result of the physical and
chemical properties of the binder system. Bonding is the formation of a solid interface
between the particles and a bonding agent such as an oil or resin after drying. The
resultant properties of the bonded mix are heavily dependent on both the processing and
the type of bonding agent used. Some binders have excellent bonding capability but have
poor lubricating qualities. This leads to a mix with poor compressibility and ejection
properties. Other binders have good lubricity but poor bonding performance. This can
lead to problems such as segregation, poor die fill and weight control and dusting.
Obviously, the optimum binder would maximize both bonding capability and lubricity. In
order to measure the bonding capability of a binder system a simple test is used as
illustrated in Figure 1.
Figure 1: Device used to measure dust resistance or bonding capability of a binder
system.
In this test 25 grams of the binder treated mix are introduced into a graduated cylinder
with a 400 mesh screen placed at the bottom. These two components are sealed to a
flask. Low pressures nitrogen is blown through the premix causing any particles that are
not bonded to the iron powder to be carried from the mix. Chemical analysis of the
powder mix before and after give a percentage of the amount of additive that was bonded
called the dust resistance. If the additives were fully bonded, this would lead to 100%
dust resistance.
Pore Free Density
Although bonding was initially the primary purpose of the binder system, in many
applications the binder also is now used to achieve improved lubrication leading to
improved part densities. However, in order to understand the effect of the binder system
on achievable densities, it is necessary to understand the concept of pore-free
density(PFD). PFD is defined as the density of any material when all pores within that
material are eliminated. For wrought steels, PFD is assumed to be ~7.85 g/cm³, but even
this density is dependent upon alloy composition, microstructural features, thermomechanical processing, etc. For PM materials, because they begin as a heterogeneous
mixture of iron, plus alloy additions, plus lubricants; the PFD of the premix is quite
different. Equation 1 calculates PFD as a function of premix additions. Typical densities
of various alloying elements are shown in Table 1. Both graphite and admixed lubricants
are low-density constituents and have a significant negative effect on PFD.
PFD = (1 / (∑ (% element in premix / specific gravity of additive)
eq. 1
Table 1: Specific Density of Various Elemental Materials
Material
Ancorsteel 1000B
Ancorsteel 4600V
Distaloy 4800A
Atomized Copper
Inco Nickel 123
Graphite
Lubricants
Specific Density, g/cm³
7.841
7.844
7.896
8.047
8.846
2.295
0.90 to 1.15
Because graphite and lubricant (or binder) have the lowest specific densities, the key to
obtaining high densities is minimizing the amount of these two constituents within the
premix. An additional consideration of rigid die compaction is elastic spring-back after
ejection. This elastic spring-back can result in internal micro-cracking if excessive
compaction pressures are utilized. Prior work has shown that achieving 98% of pore free
density after ejection from the die is a practical upper limit that can be achieved without
initiating micro-laminations within the part. Elastic spring-back is also influenced by the
premix composition; higher lubricant amounts causing larger green expansion. Figure 2
highlights the effect due to different amounts of lubricant on PFD in the material system
in this study.
7.70
Pore-Free Density
98% Pore-Free Density
7.60
Density (g/cm3)
7.50
7.40
7.30
7.20
7.10
7.00
6.90
6.80
0.20
0.40
0.60
0.80
Weight Percent Lubricant Added to Premix
1.00
Figure 2: Effect of w/o lubricant on the achievable density in an iron + 2 w/o Ni + 0.6
w/o graphite mix (FN-0205).
Figure 2 highlights the fact that if a binder system is going to improve density and still
bond effectively, it needs to do so by utilizing low levels of binder. In this study, the
performance of several binders systems are studied in relation to compaction
performance and bonding capability.
Experimental Procedure
Laboratory procedures were performed in accordance with appropriate MPIF standards.
In order for direct comparison of the performance of the various binder systems, a FN0205 mix composition was utilized in all testing. ANCORBOND is a patented premix
treatment system developed by Hoeganaes Corporation [1]. All other ingredients are
commercially available. All the premixes were made from the same lot of iron powder
(Ancorsteel 1000B). The nickel was T123 from INCO, Acrawax-C was from Lonza Inc.,
and the graphite was 3203HS from Asbury Graphite Mills. Conventional premixes were
made in a double cone blender and binder treated mixes were made in a high shear mixer.
Samples for transverse rupture (TR) and green strength were compacted uniaxially at 690
MPa and 65 oC. The test pieces were sintered in a high temperature Abbott continuousbelt furnace at a temperature of 1120 °C for 30 min in a mixed atmosphere of 90 v/o
nitrogen and 10 v/o hydrogen.
Prior to mechanical testing, green and sintered density, dimensional change (DC), and
apparent hardness, were determined on the TR samples. Five TR specimens were
evaluated for each composition. The densities of the green and sintered steels were
determined in accordance with MPIF Standard 42. Green strength measurements were in
accordance with MPIF standard 15. Apparent hardness measurements were conducted on
tensile and TR specimens, in accordance with MPIF Standard 43.
To verify ejection characteristics, rectangular green strength specimens 32 mm x 12.7 mm
x 12.7 were compacted in an instrumented hydraulic compaction press. The characteristics
measured are shown schematically in Figure 3. Ejection was evaluated by comparing two
values: the pressure required to initiate ejection (strip pressure) and the pressure as the compact
is exiting the die (slide pressure).
Figure 3: Schematic of ejection measurements. [2].
Bonding with Oil
Utilizing oil as a binder is perhaps the simplest method and requires the least amount of
capital equipment. Processing is simple and does not require heating or cooling of the
powder. Some of the disadvantages of this process are that the finished product is not dry
and the flow rate and apparent density of the powder can change with time. In addition,
because the oil thoroughly coats the iron powder the apparent density is generally lower
than a simple premix. Table 2 shows the green and sintered properties of a standard
premix and two mixes bonded with 0.10 w/o of Oil 1and 0.20 w/o of Oil 2. The bonded
mixes were made blended in the high shear mixer for 30 minutes at ambient temperature.
Table 2: Properties of Mixes Bonded with Oil.
Apparent
Green Green
Green
Flow
Strip
Density
Density Strength Expansion
Material
Premix
0.10 w/o Oil 1
0.20 w/oOil 2
(g/cm3)
3.06
2.92
2.97
(secs) (g/cm3)
35.8 7.21
NF
7.24
NF
7.20
(MPa)
15.4
15.4
15.6
(%)
0.15
0.12
0.13
(MPa)
34.2
30.5
27.0
Slide
Sintered
DC
Density
(MPa) (g/cm3)
18.9
7.23
16.2
7.24
15.4
7.21
TRS
(%) (MPa)
0.06 961
0.04 1072
0.01 1058
Apparent
Hardness
(HRA)
44
46
45
Because the oil effectively coats the powder and remains on the surface the apparent
density of the powders is lower than the standard premix. Another disadvantage of the
bonding oils is the poor flow. Since the oil itself remains on the individual particles there
are cohesive forces between powder particles and flow is inhibited. These mixes were
dried for several more days at room temperature and the apparent density was found to
increase and the flow rate also improved with time. The green strength of the oil bonded
mixes is also less then the standard premix. In this case the oil tends to decrease the
interlocking between powder particles and lowers the green strength. The ejection
properties, as measured by the strip and slide, improve with the use of oil bonding
indicating that these oils have good lubricity. The higher TR strength and hardness of the
oil bonded mixes in comparison to the standard premix, may be related to their better
retention of carbon. The bonded carbon, as measured by the dust resistance test (Figure
1), of Oil 1 and Oil 2 were 95% and 88% respectively versus the premix which only had
43% bonded carbon.
Bonding with Epoxies
Generally epoxies have been used in PM for making composites (magnetic materials) and
for rapid prototyping. However epoxies have potential advantages in bonding metal
powders. First, the bonds developed with epoxies are very strong and tend to hold with
aggressive handling of the mix. Secondly, epoxies tend to wet a wide range of metals,
which would be an advantage for PM where a wide variety of additives are used. In
general epoxies are stable and are not influenced by moisture. Epoxies are more difficult
to process because they are generally multi-component systems that require temperatures
of approximately 70 oC to cure or react. The components of epoxies have a high
viscosity and require a high shear mixer to coat the powder and in general they are more
expensive then other bonding systems.
Table 3: Properties of Epoxy Bonded Mixes.
Apparent
Green Green
Green
Flow
Strip
Density
Density Strength Expansion
Material
Premix
Epoxy 1
Epoxy 2
(g/cm3)
3.06
3.23
3.21
(secs) (g/cm3)
35.8 7.21
25.3 7.22
25.0 7.17
(MPa)
15.4
11.0
11.8
(%)
0.15
0.11
0.12
(MPa)
34.2
27.2
29.0
Slide
Sintered
DC
Density
TRS
(MPa) (g/cm3) (%) (MPa)
18.9
7.23
0.06 961
21.2
7.23
-0.03 996
19.3
7.19
-0.05 954
Apparent
Hardness
(HRA)
44
45
46
Table 3 compares the green and sintered properties of two epoxy systems to a
conventional premix. The hard-smooth coating produced by the epoxy systems lead to a
mix that has higher apparent density and enhanced flow than the conventional premix.
However these same attributes of the epoxies lead to a lower green strength and the
sliding pressure is slightly higher for the epoxy systems than for the premix due to the
hard nature of the binder. In general, there is no significant improvement in either
sintered density or sintered properties, in fact Epoxy 2 has slightly lower properties than
the premix. In addition to having very high carbon bonding efficiency (91 and 97 %
respectively), Epoxy 1 and Epoxy 2 mixes had excellent nickel bonding of over 60%.
Bonding with Polymers
Various polymers can be used to bond additive particles to iron. There are several key
properties of the polymer that must be taken into consideration. First, the melting point
of the polymer is crtical. Obviously the polymer should have a low enough melting point
that it can be softened enough to provide bonding at the processing temperature.
However the melting point must be high enough to withstand the frictional heat generated
during compaction. If the polymers melt or soften significantly during compaction,
powder can stick to the parts leading to poor surface finish. Second the particle size of
the polymer binder must be fine enough so that agglomerates do not form in the mix and
lead to large pores or surface imperfections. Lastly, the polymer must have adequate
lubricity to allow for press performance while still being able to bind the additives. In
many cases, the polymer will excel in one manner but be deficient in another. Table 4
shows the performance of two polymer systems that show improvements in several areas.
The bonding efficiency for carbon of the two polymer systems were 88 and 90%
respectively.
Table 4: Properties of Polymer Bonded Mixes.
Apparent
Green Green
Green
Flow
Strip
Density
Density Strength Expansion
Material
Premix
Melt Bond 1
Melt Bond 2
(g/cm3)
3.06
3.33
3.32
(secs) (g/cm3)
35.8 7.21
30.2 7.28
30.5 7.26
(MPa)
15.4
14.1
13.6
(%)
0.15
0.19
0.11
(MPa)
34.2
25.4
29.9
Slide
Sintered
DC
Density
(MPa) (g/cm3)
18.9
7.23
13.7
7.29
16.0
7.26
TRS
(%) (MPa)
0.06 961
0.08 1023
0.07 1010
Apparent
Hardness
(HRA)
44
46
46
The apparent density and flow characteristics are improved over the conventional premix.
The polymers in both melt bond systems coat the iron powder rounding the particles
leading to the a higher apparent density and flow. The polymers binders were chosen
because of their increased lubricity and this manifests in improvement of the green
density and the ejection charateristics (strip and slide). The better performance in
compaction leads to an increase in sintered density and hence mechanical properties.
ANCORBOND® Technology
ANCORBOND premixing technology encompasses a family of engineered materials that
satisfy the demands of the PM industry for enhanced flow, enhanced densification,
reduced segregation, greater green strength, and reduced part to part variability. When
introduced in 1988, ANCORBOND was designed primarily to produce enhanced flow
and die fill while simultaneously giving reduced dusting and reduced part to part
variability. This original binder treated premix concept utilized a binder in addition to
the added premix lubricant. That is, the lubricant level remained constant with the binder
as an additional additive. The binder added was an additional low specific density
additive that reduced the pore-free density (PFD) of the premix. Consequently, the first
generation ANCORBOND material was limited in the densities it could achieve.
However, it is significant that this initial material satisfied the market need for improved
flow, increased press rates (up to 25%), and reduced part to part variability [3]. The first
generation ANCORBOND premixes possessed higher apparent density relative to that of
the regular premix.
Table 5: Properties of ANCORBOND Mixes.
Apparent
Green Green
Green
Flow
Strip
Density
Density Strength Expansion
Material
Premix
ANCORBOND
ANCORMAX 200
ANCORMAX 450
3
(g/cm )
3.06
3.21
3.33
3.28
(secs) (g/cm3)
35.8
7.21
27.6
7.20
25.5
7.34
26.2
7.47
(MPa)
15.4
20.0
26.3
33.9
(%)
0.15
0.15
0.11
0.22
(MPa)
34.2
28.8
29.2
22.8
Slide
Sintered
DC
Density
(MPa) (g/cm3)
18.9
7.23
15.1
7.16
17.7
7.35
22.8
7.49
TRS
(%) (MPa)
0.06 961
0.06 961
0.08 1023
0.11 1100
Apparent
Hardness
(HRA)
44
44
46
41
Note: Ancormax 450 mix has 0.40 w/o graphite to lower PFD
With the introduction of the first generation of ANCORBOND, market trends in the PM
industry indicated that PM part densities were increasing and the future for PM was in
parts that displaced steel forgings or high strength castings. Higher mechanical property
requirements can only be satisfied by PM parts with higher densities.
To address the need for high part densities, it became necessary to lower the total amount
of lubricant and binder added to the premix so that the PFD could be increased. The
optimal situation was to utilize a polymeric binder that had both bonding and lubricating
characteristics. Developmental work at Hoeganaes Corporation in the late 1980’s
produced a lubricant-binder system that reduced the total organic content of the premix
enabling part densities up to 7.1 g/cm³. The benefits of this second generation
ANCORBOND are a parity of green density with conventional mixes with improved
green strengths and lower ejection forces. Additionally, this second generation material
exhibited equivalent compacted densities at low to moderate pressures but showed
increased green densities of up to 0.1 g/cm³ at compacting pressures in the range of 550
to 690 MPa [4]. Results are cited in Table 5. The second generation binder treated
premixes exhibited the same higher apparent density compared with a non-bonded
material but continuous improvement in the processing enabled retrofit premixes to be
formulated that can be utilized in existing fixed fill dies. The benefits of increased press
rates, enhanced powder flow, and increased material consistency that were inherent in the
initial ANCORBOND are present in this second generation material, with the additional
benefit of increased green densities of up to 7.1 g/cm³.
More recently, a methodology to achieve higher green densities via single press/single
sinter, employing heated dies without any powder preheating, identified as AncorMax
200™ has been developed [5]. AncorMax 200™ processing requires die heating (60-95
°C), without any preheating of the powder. Analogous to warm compaction processing,
this technique combines binder and lubricant technology to achieve an increase of 0.05 to
0.15 g/cm3 in density relative to a conventional premix (Table 5). Similar to warm
compaction along with this increase in green and sintered densities, the AncorMax 200
process offers reduced alloy segregation, reduced dusting, superior flow, and enhanced
die fill. These factors give the opportunity for greater consistency and higher quality PM
parts.
Heated powder / heated die processing commonly referred to as warm compaction
utilizes iron or low alloy powders premixed with a unique binders and lubricants to
facilitate compaction at temperatures ranging from 120 °C to 150 °C [3]. Both the
premixed powder and compaction tooling are heated to this temperature range. Higher
green densities are obtained via a temperature-induced reduction in compressive yield
strength coupled with a softening of the lubricant, resulting in a pseudo-isostatic
compaction condition within the die as densities approach PFD [4,5]. The amount of
lubricant used in warm compaction is typically 0.6%, thus promoting higher PFD relative
to conventional compaction. Warm powder pressing has demonstrated greater uniformity
of density through out the part, minimizing the neutral zone typically associated with PM.
[6,7] This reduced neutral zone is desirable because greater uniformity of density implies
more consistent properties within the part thus less concern about low-density zones and
their impact on final part performance. Table 5 shows results for an ANCORDENSE
450™ mix (warm compaction). This bonding system shows improvements in green
strength, green and sintered density and ejection characteristics (strip and slide).
It should be noted that all the Ancorbond systems have carbon bonding (dust resistance)
of all of the Ancorbond products was > 90%, therefore the systems can be chosen based
on the compaction conditions and density requirements.
Summary
Binder treatment technology is now common in the PM industry. What was once a
technology primarily utilized to reduce dusting and segregation is now used to improve
lubricity and density. The properties of bonded mixes are dependent on the processing
and bonding agents used. The binder used can range from simple oils to complex
polymer systems; each which has advantages and disadvantages. Most binders, are now
engineered to enhance specific performance criteria depending on customer needs. As
parts producers, seek higher densities binder technology will be key in order to reduce the
total lubricant required and hence increase the PFD. A necessary goal is to engineer
binder systems that can provide lubricity with minimal levels of organics.
REFERENCES
1. F.G. Hanejko, S.H. Luk, A.J. Rawlings, and T.J. Hale, “Applications of High
Performance Binder-Treated Materials,” Advances in Powder Metallurgy and
Particulate Materials – 1998, Metal Powder Industries Federation, Princeton, NJ,
1998, part 11 pp.121-133.
2. M. Kondohi, H. Okajima, “High Density Powder Compaction Using Die Wall
Lubrication:, Advances in Powder Metallurgy and Particulate Materials-2002”, Part
3, pp. 3-47 thru 3-55, Metal Powder Industry Federation, Princeton, NJ 2002.
3. M.J. McDermott, “P/M Parts Fabrication Experience with ANCORBOND (Binder
Treated) Premixes, Advances in Powder Metallurgy & Particulate Materials - 1990,
Vol. 1, Metal Powder Industries Federation, Princeton, NJ, pp 209-237.
4. F.J. Semel, S.H. Luk, “Continuing Improvements in Binder Treatment Technology”,
Advances in Powder Metallurgy & Particulate Materials - 1996, Part 13, Metal
Powder Industries Federation, Princeton, NJ, pp 353-363.
4. G. Poszmik, S. Luk, “Binder Treated Products for Higher Density and Better
Precision”, Advances in Powder Metallurgy and Particulate Materials – 2003, Metal
Powder Industries Federation, 1993, p. 3-33 to 3-44.
5. H. G. Rutz, F. G. Hanejko, “High Density Processing of High Performance Ferrous
Materials”, Advances in Powder Metallurgy and Particulate Materials – 1994, Vol. 5,
Metal Powders Industries Federation, 1994, pp 117-133.
6. F. Chagnon, Y. Trudel, “Effect of Compaction Temperature on the Sintered
Properties of High Density PM Materials”, Advances in Powder Metallurgy and
Particulate Materials – 1995, Vol. 2 Part 5, Metal Powder Industries Federation,
1995, P. 106-126.
7. O. Mars, U. Engstrom, “Warm Compaction and its Influence on the Dynamic
Properties of Sintered Steels”, Powder Metallurgy in Automotive Applications, P.
Ramarkrishnan, Ed. Science Publishers Inc., 1998, p. 147-155.
8. T. Miller, F. Hanejko, “Development of a Warm Compacted Automatic Transmission
Torque Converter Hub”, Paper 970428, Society of Automotive Engineers.
9. J. Chemlar, B. Nelson, H. Rutz, M. Lutz, J. Porter, “An evaluation of
ANCORDENSE Singel Compaction Process and HPP Processing Technique on Fine
Pitched Spur and Helical Gears”, Advances in Powder Metallurgy and Particulate
Materials, Vol. 5, Metal Powder Industries Federation, 1994, pp 73-89.