Engineered Approach to High Density Forming using Internal and External Lubricants.
Arthur Rawlings, Sydney Luk and Francis Hanejko
Hoeganaes Corporation
Cinnaminson, NJ 08077
Abstract
The drive to obtain higher densities, strength and dynamic properties in P/M has prompted the
development of methods, including Double Press / Double Sinter, Copper Infiltration, Warm
Compaction etc., to achieve these goals.
One of the contributing factors influencing higher densities is the addition of lubricant to the
premix. With advances in lubricant technology and binder-treated premixes it is possible to
achieve higher density parts, with reduction of internal lubricant and die wall lubrication
This study combines the effects of using a balance of internal and external lubrication to
improve the properties of P/M materials.
Introduction
Previous work, using die wall lubrication, has shown that by optimizing the lubrication
conditions, the internal lubricant can be reduced considerably, resulting in enhanced properties.
Studies using warm compaction techniques and die wall lubrication have been presented where
the objective was the reduction of the amount of internal lubricant, in order to maximize green
properties without degrading ejection characteristics. All the materials evaluated were binder
treated premixes and the die wall lubricant was applied using a tribostatic lubrication system.
Because of the elevated temperatures (290 °F / 145 °C) involved, traditional “room temperature”
die wall lubricants were found to be unsuitable. The lubricant exhibited melting when in contact
with the die surface. This melting lead to a reduction in green properties, due to pick up of the
lubricant in the green part, particularly when the spray time was increased. (1) Binder treated
premixes used in warm compaction contain a proprietary lubricant, which is specifically
designed to work efficiently at elevated temperatures. (2) Preliminary experimental work using a
modified ANCORDENSE lubricant, in the spray system, gave consistent spray coverage with
no melting. Optimizing the die spray parameters, a study was performed to evaluate the
ejection characteristics of a series of ANCORDENSE premixes. The results are presented in
Figure 1.
1
0.0% Lubricant
45.5
0.30% Lubricant
3.00
40.5
0.60% Lubricant
35.5
2.50
30.5
2.00
25.5
1.50
EJECTION PRESSURE (MPa)
EJECTION PRESSURE (tsi)
3.50
20.5
25
30
35
40
45
50
55
COMPACTION PRESSURE (tsi)
Figure 1:
Effect of Internal Lubricant and Die Wall Lubrication on Ejection Pressure
In Figure 1, the ejection characteristics for the 0.60% lube binder treated premix compacted
without any die wall lubrication is used as a reference. This data suggests that reduced internal
lubricant with optimized die wall lubrication can lead to reduced ejection forces over the range of
compaction pressures evaluated. Interestingly, the combination of die wall lube with 0.30%
internal lube gave the lowest ejection forces. This performance is attributed to the die wall lube
with the additional benefit of the internal lube.
GREEN DENSITY (g/cm³)
7.50
7.45
0.0% Lubricant
7.40
0.30% Lubricant
7.35
0.60% Lubricant
7.30
7.25
7.20
7.15
7.10
7.05
7.00
6.95
25
30
35
40
45
50
55
COMPACTION PRESSURE (tsi)
Figure 2:
Effect of Internal Lubricant and Die Wall Lubrication on Green Density
The effect of internal lubricant and die wall lubrication on the green density is shown in Figure 2.
Reducing the internal lubricant to the 0.30% level produced similar green densities to the
reference. With the total elimination of the internal lubricant, there was an increase in the green
density reaching a maximum of a 2% increase with compaction pressures of 50 tsi (690 MPa).
To understand the benefits of reducing the internal lube, it is worthwhile to measure the
increase in green density against the pore free density of the premix. Pore free density is
defined as the density of a green compact in which all interparticle porosity has been completely
eliminated. Obtaining a pore free compact is essentially impossible in conventional compaction.
The key factor being the elastic nature of the materials involved. The elasticity results in green
expansion after the compaction pressure is removed and the part is ejected. The pore free
2
density of a premix can be calculated by using a rule of mixtures calculation and the specific
density and percentage of each constituent in the premix.
DENSITY (g/cm³)
The pore free and measured densities were determined for the three lubricant levels, and the
results are shown in Figure 3. The compacted densities were determined after compaction at
50 tsi (690 MPa).
7.80
7.75
7.70
7.65
7.60
7.55
7.50
7.45
7.40
7.35
7.30
7.25
PORE FREE DENSITY
95.0%
96.5%
98.1%
MEASURED DENSITY
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
% LUBRICANT CONTENT
Figure 3:
Effect of Lubricant Content on Pore Free Density
The results show as the amount of internal lubricant is reduced, the percentage of pore free
achieved at 50 tsi and 290 °F is lower. At 0.60% lubricant level, the value is 98.1%, (less than
2% porosity) whereas at the 0.0% lubricant level the value is only 95.0%, (5% porosity). This
reduction experienced may be due the lack of internal lubrication, restricting particle
rearrangement during compaction.
In terms of green strength, reducing the internal lubricant increases the green strength for
equivalent green densities (Figure 4).
6000
0.0% Lubricant
46
0.30% Lubricant
42
0.60% Lubricant
38
5000
34
30
4000
26
22
3000
18
2000
6.90
7.00
7.10
7.20
7.30
7.40
GREEN STRENGTH (MPa)
GREEN STRENGTH (psi)
7000
14
7.50
GREEN DENSITY (g/cm³)
Figure 4:
Effect of Internal Lubricant Content and Die Wall Lubrication on Green Strength
The decrease in green strength, for the 0.60% lubricant level, at the higher green density may
result from the onset of laminations, because of over-pressing. Reducing the internal lubricant
shows the green strength continues to increase with density.
3
The reduction in green strength at the 0.60% lubricant level is also reflected in green expansion
(Figure 5).
0.40
% GREEN EXPANSION
0.35
0.30
0.0% Lubricant
0.30% Lubricant
0.60% Lubricant
0.25
0.20
0.15
0.10
0.05
0.00
6.90
7.00
7.10
7.20
7.30
7.40
7.50
GREEN DENSITY (g/cm³)
Figure 5:
Effect of Internal Lubricant Content and Die Wall Lubrication on Green Expansion
From the study, several questions were raised as to was this the optimum system for warm
compaction, or were there other options? This paper documents studies carried out on the
effects of balancing internal and external lubrication to improve on the properties of P/M
materials.
Procedure
Premixes were prepared using both the ANCORDENSE process and conventional methods.
The ANCORDENSE premixes contained ANCORDENSE lubricant, while the conventional
premixes utilized Acrawax as the lubricant. The base chemistry, for all the premixes, was
Ancorsteel 85HP (FL-4400) + 2 w/o nickel + 0.6 w/o graphite. The lubricant contents ranged
from 0.2 w/o to 0.75 w/o, depending upon the premix.
The die wall lubrication was applied using the Gasbarre Die Wall Lubrication System. Two die
wall lubricants were evaluated for warm compaction premixes and three lubricants were
evaluated for the conventional premixes (Table I).
Table I
Die Wall Lubricants
Application
Identity
Warm Compaction
Warm Compaction
Conventional
ADLI
ADLII
C-Lube
Chemtrend 101
H-Lube
E-Lube
Conventional
Conventional
The spray conditions were optimized, separately, for the warm compaction and conventional
premixes. The optimum conditions were determined by measuring the ejection pressure for
each condition.
4
Test specimens, measuring 1.25” (32mm) x 0.5” (13mm x 0.5” (13mm) were prepared at
compaction pressures ranging from 30 tsi (415 MPa) to 50 tsi (690 MPa), in order to evaluate
each premix / die wall lubricant. The warm compaction specimens were compacted at a die
temperature of 290 °F (145 °C) and a powder temperature of 285 °F (140 °C). The
conventional premix specimens were compacted at room temperature. Green density, strength,
expansion and ejection pressures were determined for all specimens compacted.
Results and Discussion
The green properties for the warm compacted premixes are summarized in Table II.
Table II - Green Properties – Warm Compacted Premixes
Premix
Compaction
Pressure
(tsi / MPa)
30 / 415
40 / 550
45 / 620
50 / 690
Green
Density
(g/cm³)
7.12
7.26
7.30
7.31
Green
Strength
(psi / MPa)
3149 / 22
3318 / 23
3640 / 25
3422 / 24
Green
Expansion
(%)
0.21
0.29
0.31
0.38
Ejection
Pressure
(tsi / MPa)
2.07 / 29
2.36 / 33
2.44 / 34
2.53 / 35
0.3% Lube
ADLI
Die Lube
30 / 415
40 / 550
45 / 620
50 / 690
7.10
7.26
7.30
7.33
3224 / 22
3842 / 26
4110 / 28
4142 / 29
0.19
0.25
0.29
0.32
2.03 / 28
2.15 / 30
2.18 / 30
2.22 / 31
0.3% Lube
ADLII
Die Lube
30 / 415
40 / 550
45 / 620
50 / 690
7.09
7.25
7.29
7.32
3186 / 22
3680 / 25
3875 / 27
3980 / 27
0.19
0.23
0.27
0.32
1.46 / 20
1.71 / 24
1.75 / 24
1.73 / 24
0.6% Lube
(Reference)
Three “conditions” were evaluated, 0.6% internal lube / no die wall lube (Reference), 0.3%
internal lube / ADLI die wall lube and 0.3% internal lube / ADLII die wall lube. The 0.3% internal
lube was chosen based on the original study using the modified ANCORDENSE lubricant.
For the three premixes evaluated, the green densities achieved at the each compaction
pressure were similar. The die wall lubricated samples showed higher green strengths,
particularly at the higher green densities. Unlike the reference material, the green strengths did
not decrease at 50 tsi (690 MPa).
The green expansion characteristics of the die wall lube materials were lower than the reference
mix. This difference in green expansion at the same density for the die wall versus. reference
mix can be explained by the amount of pore free density achieved. The reference mix at 7.31
g/cm³ is ~98% of the pore free density; whereas, the 7.32 g/cm³ for the 0.3% internal lube is
~96% of the pore free density. Thus the 96% of pore free density will have less green
expansion and a reduced tendency for green cracking. Although there was no improvement in
green density at the 0.30 % internal lube level, the green strength and reduced green expansion
are significant advantages.
The original study, using the modified ANCORDENSE lubricant, suggested that reduced internal
lube with die wall lubrication, could lead to reduced ejection pressures. (1) The current results
5
EJECTION PRESSURE (tsi)
4.00
0.6% Lube (Ref.)
3.50
50
0.3% Lube - ADLI
0.3% Lube - ADLII
3.00
40
2.50
30
2.00
1.50
20
1.00
10
0.50
0.00
EJECTION PRESSURE (MPa)
reaffirm that lower ejection pressures can be achieved. In fact, the ADLII lube produced even
lower ejection pressures than ADLI (Figure 6).
0
25
30
35
40
45
50
55
COMPACTION PRESSURE (tsi)
Figure 6
Effect of Die Wall Lubricants on Ejection Pressure
The reduction in ejection forces with the die wall lube at the warm compaction temperatures is
thought to result from improved distribution of the die wall lube to the surfaces of the die.
Relative to size effect, if larger lubricant particles are present, there is a possibility that they can
be removed by the top punch during the downward movement of the punch. The thought here
is that the clearance between the top punch and the die may be less than the largest particle
size. Thus, the lube particle can be removed prior to it facilitating part ejection or leaving a nonlubricated region of the die wall surface.
One of the advantages of the lower ejection forces can be seen in the ability to compact taller
parts, using the ADLII die wall lubrication (Table III).
Table III Effect of Die Wall Lubrication on Part Height
Part Height
(in / mm)
0.50 / 12.7
0.75 / 19.1
1.0 / 25.4
Reference
0.50” High
0.60% Lubricant
Green Density
(g/cm³)
7.32
7.30
7.29
Ejection Pressure
(tsi / MPa)
1.91 / 26
2.38 / 33
2.73 / 38
7.31
2.53 / 35
The results show, by utilizing die wall lubrication, similar ejection pressures were recorded for 1”
high parts (pressed at 50 tsi), as were recorded for 0.5” parts pressed using the conventional
ANCORDENSE premix.
The conventional premixes were evaluated in a manner similar to the warm compaction
materials. Green properties were determined at internal lube levels of 0.2%, 0.4% and 0.75%,
and utilizing three distinct die wall lubricants (Table IV).
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Table IV - Green Properties - Conventional Premixes
Premix
Internal
Lube
0.75% Lube
(Reference)
Compaction
Pressure
(tsi / MPa)
30 / 415
40 / 550
50 / 690
Green
Density
(g/cm³)
6.95
7.11
7.18
Green
Strength
(psi / MPa)
1410 / 10
1763 / 12
1855 / 13
Green
Expansion
(%)
0.19
0.24
0.30
Ejection
Pressure
(tsi / MPa)
2.02 / 28
2.49 / 34
2.71 / 37
0.4% Lube
C-Lube
30 / 415
40 / 550
50 / 690
6.98
7.16
7.29
1389 / 10
1878 / 13
2059 / 14
0.19
0.24
0.29
2.18 / 30
2.91 / 40
3.51 / 48
0.2% Lube
C-Lube
30 / 415
40 / 550
50 / 690
6.96
7.20
7.35
1481 / 10
2106 / 15
2529 / 17
0.19
0.24
0.29
2.47 / 34
3.51 / 48
4.66 / 64
0.4% Lube
H-Lube
30 / 415
40 / 550
50 / 690
6.99
7.19
7.29
1412 / 10
1827 / 13
2040 / 14
0.23
0.25
0.29
2.29 / 32
2.87 / 40
3.42 / 47
0.2% Lube
H-Lube
30 / 415
40 / 550
50 / 690
6.95
7.20
7.34
1472 / 10
1987 / 14
2647 / 18
0.20
0.24
0.29
2.27 / 31
3.44 / 47
4.40 / 61
0.4% Lube
E-Lube
30 / 415
40 / 550
50 / 690
6.97
7.17
7.29
1373 / 9
1909 / 13
2228 / 15
0.19
0.24
0.28
2.02 / 28
2.66 / 37
3.14 / 43
0.2% Lube
E-Lube
30 / 415
40 / 550
50 / 690
6.96
7.21
7.32
1541 / 11
2249 / 16
2566 / 18
0.19
0.24
0.27
2.11 / 29
3.14 / 43
3.60 / 50
The results presented in Table IV indicate that reducing the internal premixed lubricant, the
green density can be increased by about 0.11 g/cm³ for 0.4% internal lube and 0.17 g/cm³ for
0.2% internal lube (50 tsi compaction). Green strength is also increased, whereas green
expansion is similar to the reference. However, the ejection forces increased when the die wall
lubrication was used with either 0.4% or 0.2% internal lube. At the 0.4% internal lube level, the
increase in ejection pressure ranged from 20% to 40% (Figure 7). At the 0.2% internal lube
level, the increase in ejection pressure ranged from 30% to 70% (Figure 8).
7
EJECTION PRESSURE (tsi)
4.5
0.75% Lube (Ref.)
65.5
C-Lube
60.5
H-Lube
4.0
55.5
E-Lube
50.5
3.5
45.5
3.0
40.5
35.5
2.5
30.5
2.0
EJECTION PRESSURE (MPa)
5.0
25.5
1.5
20.5
25
30
35
40
45
50
55
COMPACTION PRESSURE (tsi)
Figure 7
Effect of Die Wall Lubricants on Ejection Pressure for 0.4% Lubricant Premix
5.0
4.0
C-Lube
60.5
H-Lube
55.5
E-Lube
50.5
3.5
45.5
3.0
40.5
2.5
35.5
30.5
2.0
EJECTION PRESSURE (MPa)
EJECTION PRESSURE (tsi)
65.5
0.75% Lube (Ref.)
4.5
25.5
1.5
20.5
25
30
35
40
45
50
55
COMPACTION PRESSURE (tsi)
Figure 8
Effect of Die Wall Lubricants on Ejection Pressure for 0.2% Lubricant Premix
The ejection behavior using ambient temperature die spray shows the importance of lubricant
chemistry and lube behavior. The E-Lube, which was uniquely formulated to produce low
ejection forces as an internal lube, showed a positive influence relative to the C-Lube or H-Lube.
However, relative to the ejection forces, the 0.75% internal lube was superior to the three die
wall lubes. The explanation for this behavior is again related to the adhesion of the spray lube
to the die wall surface. At room temperature the only force binding the lube to the die surface is
the tribostatic charge developed by the spray system. Powder filling and punch travel can wipe
away the deposited spray lube. Thus during ejection, the external lube effect is negated and the
ejection forces are reliant upon the internal lube of the premix.
To explain the total effectiveness of die spray lubrication, it is important to review three aspects
of the lubricant. One is the particle size effect, the size should be sufficiently fine to promote
uniform coverage and minimize possible wiping away of the lubricant during die fill and
compaction. The second key issue is the chemical make up of the lube. It was demonstrated in
the cold compaction that formulating the lube for the specific purpose of die wall lubrication
leads to improved performance. The third key is the temperature of die during spraying. It is
advantageous to maintain a die temperature that will not cause melting of the lubricant, such as
those used for room temperature compaction.
8
Additional experimental work is required to understand these effects to fully exploit the
advantages of reduced internal lube combined with die wall lubrication.
Conclusions
1) For warm compaction, better distribution and lubricity of the ADLII die wall lubricant
resulted in a 35% reduction in ejection pressure.
2) Utilizing the ADLII lubricant, allowed taller parts to be ejected with similar forces to
those required for conventional ANCORDENSE premixes.
3) At room temperature, higher green density and strength could be achieved in parts
pressed using lower internal lubricant content and die wall lubrication.
4) Room temperature die wall lubricants, evaluated in conjunction with lower internal
lubricant, resulted in higher ejection pressures.
5) The primary factors for optimum die wall lubrication are uniform lubricant
distribution, lubricant chemistry and die wall conditions.
References
1. Rawlings, A., Hanejko, F., “Die Wall Lubricant Utilizing Warm Compaction Methods”, Oral
Presentation, Special Interest Program, 2000 International Conference on Powder
Metallurgy and Particulate Materials.
2
Rutz, H.G., Hanejko, F.G., “High Density Processing of High Performance Ferrous
Materials”, Advances in Powder Metallurgy & Particulate Materials, Vol. 5, 1994, pp 117133 Metal Powder Industries Federation, Princeton, NJ.
9