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July/August 2004
Technology pushes green strength up in single press and
sinter cycle
Imagination and innovation are among the energy sources the
PM industry needs to maintain growth rates. Research in New
Jersey shows that binders and lubricants have a role to play
in pushing green densities higher…
The powder metal industry has experienced an average growth
rate approaching 10 per cent a year for the last decade, according
to the PM Industry Vision and Technology Roadmap published
by the US Department of Energy. To maintain this growth in
the PM industry requires the continuous development of new
products, processes and technologies.
A team from Hoeganaes Corporation has demonstrated that improvements
in binder and lubricant technology make it possible to reach
green densities approaching 7.4g/cm3, continuing a story that
began several years ago with the warm compaction of iron powder
to achieve higher densities. The Hoeganaes ANCORDENSE process
(ANCORDENSE, AncorMax D, and Ancorsteel are registered trademarks
of Hoeganaes Corporation), combined with highly compressible
powders, provides the means to reach green densities in the
range of 7.3g/cm3 to 7.4g/cm3. However this technology requires
the heating of both the iron powder and the die to a temperature
in the 270ºF to 290°F (132°C to 143°C) range.
To eliminate the need to heat the iron powder but still reach
a green density similar to ANCORDENSE, a new proprietary binder/lubricant
system, AncorMax D was introduced, that yielded higher densities
at die temperatures of 140°F to 160°F (60°C to
71°C) [2,3]. Although this technology does not require
the preheating of the iron powder, one major limitation to
this system is a restriction of the overall length of the
parts that can be processed. In general, the AncorMax D system
is limited to parts with overall lengths less than or equal
to 0.75 in (19 mm).
To overcome this limitation a more refined lubricant system
has been developed to improve the length capability of this
high-density system. This paper discusses the physical and
mechanical properties of the refined system that makes it
possible to reach larger part lengths. This new AncorMaxD
Mod system will be discussed in comparison to the standard
AncorMax D.
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Staff from press builder Cincinnati were
among those who assisted in the project.
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The testing was carried out on FLN2-4405 premixes made with
Ancorsteel 85HP, a base iron prealloyed with 0.85 w/o Mo,
0.6 w/o graphite (Asbury 3203) and 2.0 w/o nickel (Inco 123).
Two versions were prepared using both proprietary AncorMaxD
and AncorMaxD Mod binder/lubricant systems. The mixes were
made as 500 lb (227 kg) mixes with compositions listed in
Table I.
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A Tinius-Olsen hydraulic press was used for pressing the
TRS bars and the tensile bars with a heated die that maintained
temperature at +/-5°F (+/-2.8°C). Compaction of test
samples was performed between 40 tsi and 60 tsi (550 MPa and
830 MPa). After compaction the test specimens were sintered
in a belt furnace at 2050°F (1120°C) for 30 minutes
in an atmosphere of 90 v/o N2 - 10 v/o H2. The parts were
subsequently tempered at 400°F (204°C) for 1 hour
in 100 v/o N2. A ToniTechnik hydraulic press was used to press
cylinders with a diameter of 0.563 in (14.3 mm) at various
lengths to compare the ejection properties of the two mixes.
The die was preheated to 145°F (63°C) and the cylinders
were compacted to 60 tsi (830 MPa). The ejection force was
collected as a function of time with an ejection rate of 0.04
in/s (1.0 mm/s). The resulting force data was then normalised
by dividing it by the surface area of the cylinder wall adjacent
to the die surface (different for each part length). This
results in pressure data that is largely dependent on the
performance of the lubricant.
Ejection measurements
Green properties and static and dynamic ejection pressures
were determined on test bars measuring 1.25 in (32 mm) x 0.5
in (13 mm) x 0.5 in (13 mm). Sintered properties were determined
on TRS bars measuring 1.25 in (32 mm) x 0.5 in (13 mm) x 0.25
in (6 mm). Tensile properties were determined from flat, unmachined
"dog bone" tensile bars according to ASTM E8 and
MPIF Standard 10. Ejection pressure, measured continuously
as a function of time, was determined on test cylinders 0.563
in (14.3 mm) in diameter, with heights of 0.3 in (8 mm), 0.5
in (13 mm), 0.8 in (20 mm) and 1.1 in (28 mm), respectively.
As the height changes, the surface area of the part/die interface
increases. This increases the ratio of the interface area
to punch area. As this number goes up the demand on the lubricant
increases.
The ejection testing was done at several part heights to
examine the lubricant system's ability to maintain high density
in taller parts while maintaining a good surface finish. Samples
were pressed at a high compaction tonnage [60 tsi (830 MPa)]
to gauge the ejection characteristics under an extreme condition.
Figure 1 shows a comparison between the two binder/ lubricant
systems of how density changes with part length and the absolute
density difference between each material. The Mod version
achieves a slightly lower density than the standard, however,
the drop off in density for both materials is 0.01g/cm3 -
0.02g/cm3. This is a very tight range considering the large
change in fill length within the die from 0.3-1.1 in (8-28
mm).
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Figure 1: Effect of part length on the green
density. Cylinders compacted to 60 tsi (830 MPa),
die preheated to 145°F (63°C).
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Figure 2 shows the ejection curves for slugs pressed to 60
tsi (830 MPa). Note that as the part length increases the
ejection forces increase. However, the AncorMax D Mod exhibits
a large improvement in ejection pressure over the standard
version. This lowers the strip (break free pressure, see arrow
in Figure 2a) and the sliding pressure (pressure as the part
exits the die, see arrow in Figure 2b) by 30-40 per cent.
Also noteworthy is the shape of the ejection curves, as this
indicates how the lubricant is behaving. For instance, the
lubricant is breaking down if the ejection pressure sharply
rises as the sliding pressure is approached. A flat or decreasing
curve is preferred, indicating good lubrication. A part free
of scoring and laminations will have a flat ejection curve.
Examining the curve shapes in Figure 2 shows that the ejection
curves are fairly flat with only a slight rise as the sliding
pressure is approached. This shape is consistent for both
versions of the binder/lubricant system. The only indication
that the lubricant is beginning to be strained is that the
overall pressures rise as the part length increases (see Figure
2c).
Figure 3 shows a photograph of the green slugs. The AncorMax
D Mod slugs are slightly more reflective than the standard
material. This indicates that the level of fine score marks
is lower due to the improved lubricity. This improved surface
finish is beneficial and can be translated into taller part
lengths with high densities.
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Figure 3: Photograph of FLN2-4405 green slugs.
The slightly more reflective surface of the Ancormax
D Mod slugs indicates that the level of fine score
marks is lower due to improved lubricity. The improved
surface finish is beneficial and can be translated
into taller part lengths with high densities.
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Figure 4a shows a compressibility curve for FLN2-4405 pressed
at 145°F (63°C). At the higher compaction pressures
the Mod version results in a slightly lower green density.
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Figure 4: a.) Compressibility of FLN2-4405
pressed at 145°F (63°C) and b.) effect of
die temperature on the density of FLN2-4405 pressed
at 60 tsi (830 MPa).
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Compact Properties
This indicates that some high density performance is sacrificed
for taller components and the binder/lubricant system should
be chosen for the particular application. The impact of compaction
temperature on the density is shown in Figure 4b. This shows
that the die temperature must be held in a tight range to
ensure the maximum density benefit. Also note the density
decreases as the range is exceeded. The shaded area is the
recommended temperature range for both versions of this binder/lubricant
system.
Another important aspect to consider when adjusting the lubricant
system is the part performance. It is important to ensure
that the high density lubricant does not degrade the properties.
Table II and Table III summarise the green and sintered properties
for both systems. Aside from the differences in ejection pressures
discussed above the only other notable difference is the
reduction of green strength for the Mod version. All other
parameters including TRS and hardness are consistent with
the standard version. The dimensional change is also consistent
for both
systems.
Over the range of densities tested, the Mod version exhibits
strip and sliding pressures that are consistent as compaction
pressure increases. This is beneficial for producing higher
density parts without breaking down the lubricant.
Conclusions
1. The improved binder/lubricant system provides 30 per cent
to 50 per cent lower ejection pressure than the standard AncorMax
D, thus making it possible to press taller parts to high
density in a single press and single sinter process.
2. The ejection pressures were constant as compaction pressure
increased when the Mod version was used, especially near the
recommended processing temperature of 145°F (63°C).
3. Increasing the die temperature from room temperature to
145°F (63°C) at the same compaction pressure resulted
in an increase in green density. However there was no further
increase in density when the die temperature was raised to
180°F (82°C).
4. The as-sintered mechanical properties were consistent for
both systems.
References
1. PM2 Industry Vision and Technology Roadmap, U.S. Department
of Energy, Office of Industrial Technologies, September 2001,
p.iii.
2. Donaldson, I W, Luk, S H, Poszmik, G, Narasimhan, K S,
Processing of
Hybrid Alloys to High Densities, Advances in Powder Metallurgy
& Particulate Materials - 2002, Part 8, pp 170-185, Metal
Powder Industries Federation, Princeton, NJ.
3. Poszmik, G, Luk, S H, Binder Treated Products for Higher
Densities and Better Precision, Advances in Powder Metallurgy
& Particulate Materials - 2003, Part 3, pp 33-44, Metal
Powder Industries Federation, Princeton, NJ.
4. Standard Test Methods for Metal Powders and Powder Metallurgy
Products - Metal Powder Industries Federation, Princeton,
NJ.
The authors
George Poszmik and Michael Marucci of Hoeganaes Corporation
are co-authors of Higher Density and Higher Performance by
Single Pressing and Sintering, a paper given at PM²TEC
2004 in Chicago under the auspices of the Metal Powder Industries
Federation, from which this article is abstracted.
Acknowledgements
The authors wish to thank Ken Cradler from Cincinnati Inc
and Barry Diamond from Hoeganaes Corporation for their help
in this work.
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