ASM Handbook,Volume 7: Powder Metal Technologies and Applications
P.W. Lee, Y. Trudel, R. Iacocca, R.M. German, B.L. Ferguson, W.B. Eisen, K. Moyer,
D. Madan, and H. Sanderow, editors, p 376-381
Copyright © 1998 ASM International®
All rights reserved.
www.asminternational.org
Warm Compaction
Francis G. Hanejko,
Hoeganaes
Corporation
THE FERROUS P/M INDUSTRY continues
to grow because of developments in raw materials and part production processes enabling the
manufacture of components with greater complexity and higher levels of performance. Advances in raw materials include higher compressibility iron powders, molybdenum prealloyed
steels, diffusion-alloyed powders, and the use of
binder-treated iron powders. These new powders
and premix technologies offer P/M users greater
flexibility in mechanical properties at traditional
part densities, typically less than 7.1 g/cm 3.
However, end users of P/M parts are demanding
still higher levels of mechanical properties available solely through higher part densities. Traditional methods used to achieve higher densities
include the use of copper infiltration, doublepressing/double-sintering (DP/DS), and powder
forging. Because these techniques involve the
use of secondary processing, significant cost
penalties are encountered, often negating the potential cost savings realized by powder metallurgy. Warm-compaction process is a technique
to achieve DP/DS densities and mechanical
properties utilizing a single compaction process.
The process incorporates the use of heated powder and heated tooling in standard compacting
presses to achieve higher green and sintered densities.
Temperatures above room temperature and below the hot-forging range are used extensively in
wrought steels to lower forging loads and minimize distortion. Work at MeriSinter AG in the
mid-1980s found certain advantages to be present when ferrous, bulk lubricated powders were
heated in the vicinity of 100 °C. It was of particular interest that compressibility improved
with heating as compared to the some powders
in an unheated condition (Ref 1). The practical
Powder l
prem,x
I
1 -I
Press ready
binder-treated
premix
Fig. 1 Warm-compaction process
Powder I
he=or
EITemp
Abbott heater
Slotheater
application of warm compaction of powders was
realized in 1994 with the introduction of
Hoeganaes Inc. Ancordense and Densemix powders (Ref 2). Figure 1 schematically illustrates
the warm-compaction process. The powder and
die temperatures used vary from 75 to 150 °C,
with every 100 °C rise in compaction temperature
resulting in a 0.08 g/crn3 increase in green density
(Ref 3). Experimental work at H6gantis AB
showed a 30% decrease in the compressive yield
strength of iron powder when the powder was
heated to 150 °C (Fig. 2) (Ref 4).
The maximum green density achieved via P/M
techniques depends on the amount and type of
premix additives used with iron powder. To calculate the maximum green and sintered densities
attainable, it is useful to review the concept of
pore-free density (PFD). Pore-free density is defined as the density of a green compact in which
all the interparticle porosity is eliminated (Ref
2). This PFD can he calculated from the specific
density and percentage of each additive in the
premix. The calculation for PFD is given as:
Table I Specific density as measured by
pycnometry
PFD =
l/[E (% element in premb/specific gravity of additive)]
(Eq 1)
in which the percentage of each element is the
weight percentage used and is expressed as a
decimal.
Once the PFD is calculated, a practical upper
limit of green density is 98% of the calculated
value. The specific density of several common
ferrous powders and typical premix additives are
listed in Table 1. Additions of materials with
specific densities higher than the base iron increase the PFD, while additions of materials
~
I
I- I
with lower specific densities (lubricants and
graphite) will lower the PFD.
Because of the reduced compressive yield
strength at 150 °C, attaining 98% of the PFD is
achieved with lower compaction pressures. In addition, compaction at 150 °C reduces the amount
of lubricant between the particles while simultaneously increasing the amount of added lubricant reaching the die-part interface (Ref 5). This
redistribution of the lubricant not only increases
the green density, but reduces the ejection forces
by 25 to 33% (Ref 2, 5). This enhanced lubricity
implies that lower amounts of lubricant are necessary (typically 0.60% lubricant for warm compaction compared to 0.75% in conventional
compaction), which again contributes to the attainment of higher green and sintered densities.
Advantages of the warm-compaction process
include higher green densities, enhanced green
strength, improved mechanical and soft magnetic properties, and greater uniformity of den-
Heated
tooling
I
I1
Powder and die
heated to
130-150 °C, +2.5 °C
(265-310 °F, _+5°F)
=
Single'pressed
high-density
P/M part
0.10 to 0.25 g/cm 3
improvement in
green density
high green strength
improved ejection
Specific density,
g]cm3
Material
Ancorstee11000B
Ancorstee14600V
Distaloy4800A
Atomizedcopper
Inconickelpowder123
Graphite
Lubricants
7.841
7.844
7.896
8.047
8.846
2.295
0.90-1.15
300
to 260
g 220
.¢
180
~ 140
lOO
Fig. 2
o
50
1oo
150
Temperature, °C
200
Effectof temperature on the yield strength of pure
iron powder. Source: Ref4
Warm Compaction / 377
sity throughout the as-sintered part. The balance
of this article details the process and the improved properties resulting from warm compaction.
Effects on Green
and Sintered Properties
Warm compaction results in a 0.10 to 0.25
g/cm3 increase in the green and sintered densities of P/M parts (Ref 2). Figure 3 shows the
improved green and sintered densities achieved
with a diffusion-alloyed powder premixed with
0.6% graphite. At lower compacting pressures,
the beneficial effect of warm compaction is
greater than the improvement observed at higher
compaction pressures. Figure 4 summarizes the
transverse rupture strength results of the diffusionbonded material compacted by both conventional and warm-compaction techniques under
compaction conditions of 410 to 690 MPa.
Table 2 summarizes the as-sintered mechanical properties of various warm-compacted premix compositions (Ref 6). This processing is
applicable to all iron and low-alloy powder compositions. The magnitude of the increase in sintered density depends on the material system and
subsequent part processing. Premixes containing
copper additions exhibit growth during the sintering process; this growth negates the beneficial
effects of the warm-compaction processing. Con-
415
7.5
I
Compaction pressure, MPa
550
690
I
I
I
.~i I
P'~
p,¢~,_
.(9
c~ 7 . 1
,~,,~ -
6.9
~
r
Table 2 As-sintered tensile properties of warm-compacted P/M materials, sintered at
1120 °C (2050 °F)
Sinlered
density,
g/cm3
Composition(a)
FL-4405 with0.6% graphite
FLN 2-4405 with 2% Ni and 0.6%
graphite
FL-4205 with 0.6% graphite
Ancorstee1150Mo (b) with 2% Ni
and 0.6% graphite
FDr0405 with 0.6% graphite
Iron plus 0.45% phosphorus
FN 0250 with 0.6% graphite
0.2 % offset
yield strength
MPa
ksi
Ten.~le strength
MPa
ksi
7.37
7.44
273
444
40
65
471
628
69
92
3.5
2.8
77
87
7.24
7.40
417
533
61
78
506
718
74
105
1.7
1.3
81
93
7.25
7.39
7.37
425
267
267
62
39
39
117
422
452
800
62
66
2.6
25.2
3.52
97
67
79
Sintered
Green [
Conventional
Material
Simering
tempermure
*C
OF
Heat
treat
Dendty,
g/tin a
7.23
7.40
7.33
7.50
7.07
7.17
7.19
7.32
7.26
7.37
7.20
7.32
7.18
7.34
7.21
7.37
7.31
7.35
7.20
7.16
7.27
7.16
7.28
7.23
7.17
7.30
7.19
7.29
2050
No
Fe-0.45 wt% P
1260
2300
No
Fig. 3 Compressibility of diffusion-bonded 4% Ni, 1£%
FC-0208
1120
2050
No
lubricant
FD-4805
1120
2050
No
FD-4805
1260
2300
No
FD-4805
1120
2050
Yes
A150HP, 2% Ni
and0.6 Gr
A 150HP, 2% Ni
and 0.6 (Jr
FLN2-4405
FLN2-4405
FLN2-4405
A41AB
1120
2050
No
1260
2300
No
1120
1260
1290
1290
2050
2300
2350
2350
No
No
No
No
A41AB
1290
2350
Yes
FN0250
FL-4405
1120
1120
2050
2050
Yes
Yes
FD-0205
1120
2050
Yes
(9
35
40
45
50
Compaction pressure, tsi
55
60
Cu, and 0.5% Mo with 0.6% graphite and 0.6%
•~ 260
.¢=
30
240
r
r
t
I
Warm compacted at 150 °C
~'o
t , ~ °~
1800 ~.
1700 ~
16oo
220
1500 rn
200
1400
(9
~nventional
o 180
~- 1613
6.8
Fig. 4
1 3 0 0 (9
12oo >=
oo
6.9
~rn~t
laardaes~ BRB
Table 3 Fatigue data of warm-compacted ferrous materials
1120
25
Elongation,
%
"~;"" ~Green
Fe-0.45 wt% P
6.7
formed by Donaldson and others, warm-compacted P/M parts were presintered at 870 °C
(1600 °F) and subsequently re-pressed at up to
690 MPa (50 tsi) at room temperature (Ref 1012). Following re-pressing, the part was then sintered at either 1120 °C or 1260 °C, resulting in
sintered densities ranging from 7.5 to 7.6 g/cm3.
Figures 5 and 6 present data on transverse rupture strength and impact energy from this study.
These higher densities of the doublepressed/double-sintered (DP/DS) warm compacted materials produced an approximately
15% improvement in the transverse rupture
strengths (Fig. 5), but more importantly resulted
in a 50 to 80% improvement in the impact energy (Fig. 6) when compared to the 7.4 g/cm3
density level (Ref 10). This study demonstrated
the potential for significantly improved mechanical properties of P/M materials via DP/DS
of a warm-compacted component. The resultant
(a) MPIF designations, based on MPIF Standard 35, 1997 edition. (b) Hoeganaes Corporation prealloyed powder with nominal
1.5% Mo
Siniered
Warm compacted at 150 °C
I
I
I
1~
7.3
sequently, copper-containing premixes are not
considered ideal candidates for warm compaction (Ref7).
Rotating bending fatigue testing was performed on a variety of warm-compacted materials in both the as-sintered and heat-treated conditions; Table 3 summarizes the available data
(Ref 8, 9). As expected, increasing the density
increased the fatigue endurance limit; however,
higher-temperature sintering did not consistently
improve the fatigue endurance limit. Reviewing
Table 3, it is observed that no generalized correlation exists between the fatigue endurance limit
and the tensile strength of P/M materials. It is
recommended that designers use available data
when specifying the fatigue endurance limit of
P/M components.
Double pressing of conventionally compacted
parts results in improved part densities and mechanical properties. In experimental work per-
7
7.1 7.2 7.3
Sintered density, g/cm3
1100 ~
7.4
Sintered transverse rupture strength of diffusionbonded 4% Ni, 1.5% Cu, and 0.5% Mo with
0.6% pmrnixed graphite
50% fatigue
endurance Ihnit
MPa
ksi
99% fatigue
endurance limit
MPa
kd
MI~
k.4
207
225
216
260
234
243
230
242
217
227
399
409
233
262
207
256
253
247
239
242
270
403
449
316
330
336
368
374
185
197
210
234
175
193
181
192
172
185
317
332
189
241
165
201
222
219
227
210
234
353
410
276
283
279
315
316
365
403
403
476
596
621
710
798
814
925
249
1327
64|
693
652
710
632
672
621
856
917
1211
1349
1193
1131
1151
1192
1303
53
58
59
69
86
90
103
115
118
134
18t
193
30
32.7
31.4
37.7
33.9
35.3
33.3
35.3
31.5
32.9
57.9
59.3
33.8
38.0
30.0
37.1
36.7
35.8
34.6
35.1
39.2
58.5
65.1
45.8
47.9
48.8
53.4
54.2
26.8
28.6
29.7
34
25.4
28.0
26.3
27.8
24.9
26.9
46.0
48.1
27.4
35.0
24.0
29.2
32.2
31.7
32.9
30.4
34.0
51,2
59.4
40.1
41.1
40.4
45.7
45.9
Temne
93
101
95
103
91
98
90
124
133
176
196
176
164
167
173
189
378 / Shaping and Consolidation Technologies
mechanical properties of such parts are equivalent to the properties of ductile cast irons and
machined carbon steel forgings.
Green Strength Enhancement. Warmcompaction processing provides improved green
strength of the as-compacted component. This
increase in the green strength results from the
synergy of greater powder particle deformation
with enhanced particle welding during compaction, plus the presence of the unique binder and
lubricant utilized in the ANCORDENSE material (Ref 2, 5). The improved green strengths are
realized at densities significantly below the porefree density (Fig. 7). These data imply the potential of warm compaction being used in lowerdensity applications where the enhanced green
strength reduces part breakage or part chipping
of fragile features.
A consequence of the enhanced green strength
via warm compaction is the ability to green machine the as-compacted part. This concept has
been used in a commercial application of a P/M
safety locker part (Ref 13). After compaction,
the component was milled in the green condition, thus reducing the overall part cost. A machinability study utilizing a drilling test was conducted on a molybdenum prealloyed material
premixed with 2% Ni, 0.5% graphite, and 0.6%
lubricant (Ref 14). This study concluded that
satisfactory surface finishes are achieved with
machining conditions using high speeds and
high feed rates. Additionally, modifications of
the standard drill bit geometry from a standard
38o
34O
2300 °i= (1260 °C'), DPDS
e 320
&--'f
2160
I
3oo - - 2300 OF(1260 °C), SP
~. 280
I~
2452 ~
2316 ~
2044
1908 ~.
L ~ 5 0 1 1 * F (1120 °C), DPDS 1772 ~
m 260
/
>m 240 2050 °Fl(1120 °C),I S P - ~
1636
220
1500
7.4
7.45
7.5
7.55
7.6
IDensity, g/cm 3
~
I
Fig. 5
Transverse rupture strength of diffusion-bonded
4% Ni, 1.5% Cu, and 0.5% Mo prernixed with
0.3% graphite subsequently carburized and tempered
90 ° chisel bit to a 135 split-point drill bit en-
hanced the as-machined surface finish. Prior to
establishing green-machining parameters, it is
recommended that testing be conducted to examine the effects of tool-bit geometry, machining feed rate, and machining speed. Green machining of P/M parts in combination with sinter
hardening offer the part designer greater flexibility in part design and material selection.
M a g n e t i c Applications
The use of warm compaction of P/M magnetic
alloys effects higher sintered densities with corresponding higher saturation induction levels
and higher permeabilities with no change in the
coercive force (Ref 15). Sintered densities in excess of 7.4 g/cm3 are possible using warm compaedon of iron-phosphorus alloys. At this density
level, the magnetic and mechanical performances of this family of materials are equal to
the properties of a low-carbon steel forging. Table 4 summarizes the mechanical and magnetic
property data for a 0.45% P/M phosphorus steel
processed to 7.4 g/cm 3 and an AISI 1008 steel
forging. From these data, the P/M material is a
suitable replacement for the wrought steel.
Warm-compaction processing enables the introduction of a new class of P/M materials for
use in alternating current (ac) magnetic applications (Ref 15-17). These materials utilize a
high-strength polymer and warm-compaction
processing to produce components that do not
require sintering. The polymer acts to both electrically insulate the powder particles and provide
strength without the need for sintering. As-compacted green densities in excess of 7.2 g/cm 3 are
possible. Manufacturing flexibility can produce
a variety of material options with unique magnetic performance. Applications for these materials include automotive ignition coils and stators for high-speed eleclric motors.
5O
¢= 40
63.6
- 2300 °F (1260 °C) sinter 7
cm 30
Table 4 Magnetic and mechanical
properties of a warm-compacted
iron-phosphorus steel versus AISI 1008
Property
Fe-0.45wt% P
Density, g/cm3
Sintering temperature,
7.35
1120 (2050)
N/A
N/A
0.2% yield strength,
MPa (ksi)
Tensile strength,
MPa(ksi)
Elongation, %
Maximum permeability
Coercive force, Oe
Saturation at 15 Oe, G
285 (42)
285 (42)
405 (59)
383 (56)
12
2700
1.9
15,000
37
1900
3.0
14,400
oC(°F)
N/A, not applicable
AIS11008
~. 2o
g
10
7.4
DPDS
/
s. ~ ' _ . 2 "°s
o!,,1oo0,isn.
7.45
7.5
7.55
Sintered density, g/cm 3
Table 5 summarizes these ac magnetic materials and their magnetic performance. These materials are ideally suited for applications with operating frequencies above 400 Hz. Optimizing
the amount and type of insulation produces components that can operate at frequencies up to
50,000 Hz. The unique three-dimensional structure of these materials can be used to carry magnetic flux in any direction. The strength of these
materials in the as-compacted condition is approximately 103 MPa (15 ksi) transverse rupture
strength. Employing a 315 °C (600 °F) thermal
treatment to the as-compacted part raises the
transverse rupture strength to approximately 240
MPa (35 ksi).
C o m m e r c i a l Powder Heating
and Delivery Systems
Successful utilization of the warm-compaction
process necessitates that the powder, powder
shuttle, and compaction tooling be heated to the
proper temperature. Recommended temperature
control of the heated powder and tooling is _+2.5
°C. It is imperative that the ~mperature of the
powder not exceed 170 °C (320 °F); above this
temperature the lubricant and binder degrade,
resulting in diminished powder flow. Heating of
the tooling is accomplished using cartridge heaters embedded in the stress ring of the die. Typically, eight to twelve 500 W cartridge heaters are
required to heat the tooling to 150 °C (300 °F) in
approximately 30 min. Heating of the powder
shuttle is necessary to maintain the powder temperature during the transfer of the powder into
the die cavity. Top-punch heating is recommended to eliminate the possibility of a tool
binding between the top punch and core rod(s).
Heating the core rod and lower punches is not
necessary; where practical, incorporating a car-
135
7.2
53.6 .~"
~6.8
8884
23.6 g
13.6
7.6
Fig.
6 Impact energy for diffusion-bonded 4% Ni, 1.5%
Cu, and 0.5% Mo premixed with 0.3% graphite
subsequently carburized and tempered
J
7.0
43.6 =~
33.6 ~
Compacting pressure, MPa
205
275
345
- Green density I \
/
f
~"
3000
v
"~
(27.5) ~,
(20.5)
2oo0
g 6.2
m
(13.75)
O 6.0
5"810
Fig. 7
415
4000
15
20
25
Compacting pressure, tsi
.~
10oo o
3~(7"0)
Green density and green strength of warm-compacted FN0205 at low compacting pressures
Table 5 Magnetic performance of insulated iron p o w d e r s
Material
Iron powder with 0.6% plastic
Iron powder with 0.75% plastic
Iron powder with oxide coating and 0.75% plastic
Initial
permeability
120
100
80
Ma.~mum
permeability
425
400
210
Coercive
farce,Oe
4.7
4.7
4.7
Induction
40 Oe, G
11,200
10,900
7,700
Warm Compaction / 379
tridge heater in the core rod will provide greater
temperature uniformity.
Currently, three commercial powder heating
and delivery systems are available. Each system
is capable of delivering heated powder at the
proper temperature. Additionally, each has the
capability of heating and controlling temperatures in the die, the punches, and the powder
shuttle system. The three systems are:
• Cincinnati Inc. El Temp System
• Abbott Furnace Company Thermal Powder
Processor
• Slotheater
The Cincinnati Incorporated E1 Temp system
utilizes an auger to both heat and transport the
powder from the powder feed hopper to the
heated shuttle (Ref 18). The auger operates
within a resistively heated shell; additionally, the
auger is hollowed, allowing preheated air to provide for additional heating capability. The
amount of powder heated is determined by the
part weight and press speed. Production systems
are available that heat up to a maximum of 9
kg/min (20 lb/min). A unique feature of the E1
Temp system is its direct interface with the Cincinnati computer operating system of the press,
allowing for control of all press and heating
functions from a single touch screen.
The Abbott Furnace Company Thermal Powder Processor, TPP 300 (patent pending) uses a
low-pressure fluidizing air 35 kPa (5 psi) to heat
the powder within a sealed reactor. Heating of
the powder is accomplished in a stream of air
that passes across resistively heated elements. As
powder is withdrawn from the bed into the delivery system, additional powder is drawn into the
reactor. This system uses a stand-alone programmable logic controller (PLC) controller to heat
the powder, die, and powder shuttle. Units are
available that can deliver powder up to 3.5
kg/min (8 lb/min) and 3.5 to 9 kg/min (8 to 20
lb/min). The TPP 300 is portable and can be
adapted to any press. These units have no moving parts, thus minimizing maintenance.
The Slotheater uses the principle of direct contact of the powder with the heated surfaces of an
oil-filled slotted heat exchanger (Ref 4). The
powder flows via gravity from the press feeder
hopper into the slot heater where it is heated and
then flows into the powder delivery system. The
temperature of the heated oil is controlled to a
temperature approximately 4 °C (7 °F) hotter
than the desired temperature of the powder. To
achieve uniform temperature of the powder, the
residence time of the powder in the heater must
be at least 5 min. Commercial units are available
that can deliver 3.5 kg/min (8 lb/min) of hot
powder. However, the design is scalable to
achieve up to 9 kg/min (20 lb/min).
Considerable attention has been given to the
actual mechanism of heating the powder; however, attention must also be given to the powder
shuttle system. Although no commercial systems
exist, it is a relatively easy task to design and
build a hot powder feed shoe. Heating of the
feed shoe is accomplished by embedding car-
tridge heaters and a thermocouple in the aluminum feed shoe. Temperature control of the feed
shoe is necessary to prevent any heat loss during
the residence time of the powder in the shoe.
Both a closed shoe and an open shoe have been
successfully used. Unlike conventional powder
shoes, the amount of powder in the feed shoe is
critical. Excessive amounts of powder in the feed
result in a long residence time within the feed
shoe, possibly resulting in a temperature drop
causing excessive part-to-part weight variations.
~
~/
i
0.3
02
i
t~
--
0
-0.2
-03
Conventional
~
6.8
_ _ Sintered dimensional change _
6.9
7.0
I
7.1
I
7.2
I
7.3
7.4
Density, g/cm 3
Tooling Design for
Warm Compaction
[::|o.R
V
Green expansion and sintered dimensional
change of warm-compacted material relative to
conventional compaction techniques
~n~
The tooling design for warm compaction is
essentially the same as for regular compaction
with typical radial tooling clearance of 0.01 to
0.02 mm (0.0004 to 0.0008 in.). The choice of
carbide inserts or tool steel inserts is not critical.
Carbide inserts have proven to be successful;
however, the designer is cautioned that additional interference fits are required to compensate for the differential thermal expansion of the
carbide insert compared to the steel stress ring.
One word of caution in the design of tooling is
the stress involved during the compaction to
near-pore-free densities. As the density increases, the tooling loads increase rapidly. This
increase in tooling pressure necessitates that
thicker stress rings be used and the allowances
made for the greater tool deflections. Shown in
Fig. 8 is the green expansion as a function of the
green density Of powder compacts compacted
using both conventional room-temperature compaction in addition to warm-compaction conditions. Note that the green expansion at equivalent density is lower for the warm-compacted
material. The rationale for the lower green expansion for the warm compacted material is explained by the fact that lower compacting pressure was required to achieve this same density;
thus the tooling load was decreased. However, as
the green density increases to near-pore-free
density, the green expansion increases dramatically. With this increased density, the tooling
loads increase, resulting in greater expansion of
the part.
This increased green expansion can cause microlaminations in the compacted part. These microlaminations are serious problems because
they reduce the structural integrity of the sintered component. In multilevel parts, these microlaminations usually occur at the transition
from one level to another. Incorporating toppunch hold-down during the ejection cycle often
prevents these cracks from occurring. However,
even top-ptmch hold-down is not sufficient to
prevent microcracking if the density of the part
exceeds 98% of the theoretical density.
part variability of the warm-compaction process
is equivalent to conventional compaction (Ref
10, 19, 20). Equal press speeds were achieved
with the warm-compaction process compared to
conventional compaction. The limiting feature in
part production is the rated capacity of the powder heating system and the part weight. Although conventional compacting presses are
used, attention must be given to prevent the heat
generated in the tooling from reaching critical
bearing components. Cincinnati Inc. recommends that stainless steel adapter plates be used
to minimize the flow of heat to the critical components (Ref 18). Additionally, incorporating an
air gap between the die body and die pot within
the press minimizes the transfer of heat.
Part Processing Considerations
Because warm compaction is a single-press
and single-sinter process, the process is ideal for
complex multilevel P/M parts that require high
mechanical properties that cannot be obtained at
Studies conducted by Hoeganaes, Presmet,
QMP, and others demonstrated that the part-to-
Effects of Prolonged Time at Temperature
and Regrinding of Green Parts. Laboratory
testing performed by Hoeganaes demonstrated
that binder-treated powder can be reheated to
warm-compaction temperatures a maximum of 4
cycles with minimal loss of powder flow and
compressibility (Ref 4). Additionally, the powder can be held at temperature up to 4 h with no
degradation of the apparent density, flow, green
density, and green strength.
Although powder metallurgy is considered a
low-scrap manufacturing process, nonusable
parts are generated during the setup stage. To
address this issue of potential green scrap, laboratory work was initiated at Hoeganaes Corporation to study the effects of adding reground
warm-compacted powder into new premixes.
This work demonstrated that additions up to 5%
regrind can be successfully compacted without
any degradation in the strength or flow characteristics of the premix. Although it is not recommended that regrind additions be made to critical
components, this work demonstrated that additions can be made without any loss in powder or
sintered properties.
Potential Applications
of Warm Compaction
380 / Shaping and Consolidation Technologies
REFERENCES
Table 6 Density and processcomparison between warm and cold compaction
Base
powder
Graphite
Distaloy AE(a)
0.5%
Distaloy DC(b)
0.5%
DistaloyAE
0.8%
Lubricant
0.7% Kenolube
0.6% Densmix
0.6% Kenolube
0.6% Densmix
0.6% Kenolube
0.6% Densmix
0.6%Kenolube
0.6% Densmix
Compaction
Sintered
density, g/cm3
Simermg
600 MPa cold compaction
600MPa warm compaction
650 MPa cold compaction
500 MPa warm compaction
650 MPa cold compaction
500 MPa warm compaction
600+500MPacoldcompacdon
(DPDS)(c)
700 MPa warm compaction
1120 °C, 30 rain, Endogas
1120 °C, 30 min, Endogas
1120 °C, 30 rain, 90% N2/10% H2
1120 °C, 30 min, N2/10% H2
1120 °C, 30 rain, 90% N2/10% H2
1120 °C, 30 min, 90% N2/10% H2
750+1120°C,20+30min,
90% N2/10% 1-12
1120 °C, 30 rain, 90% N2/10% H2
7.07
7.31
7.1
7.1
7.1
7.1
7.3
7.3
(a) Distaloy AE is a diffusion bonded powder utilizing pure iron with 4.0% Ni, 1.5% Cu, and 0.5% Mo. (b) Distaloy DC is a diffusion bonded powder utilizing a prealloy 1.50% Mo powder with 2.0% Ni. (c) DPDS, double-press double sinter. Source: Ref
21
Table 7 Comparisonof warm-compacted materials to selectedwrought and cast alloys
Material
Ylekl
areagth
....MPa
AIS11020
AIS11050
AIS18620
AIS18620 Heat treat
Ductile iron 120-90-02
Powder forged F-0005
Powder forged FL-4605
FLN-4205 at 7.39 g/cm3
FIMM05 at 733 g/cm 3
7.80
7.60
345
427
358
1390
860
765
1172
1220
938
~
MPa
50
62
52
202
125
111
170
177
136
440
745
635
1482
965
827
1455
1503
1248
Warm comp~ed turbinehub
7.40
7.20
~b 7 00 -
::t
6.00 '
0
Conventionallycompactedturbine hub
'
'
10
'
20
Y,
30
'
40
'
50
Distance along spllnefrom bottom,mm
60
Fig. 9
Variation in sintered density and dimensional
change of turbine hub processed by conventional
P/M and warm compaction
conventional compaction densities. Higher density (or equivalent density at lower compaction
pressures) can be achieved with warm compaction as compared with cold compaction (Table
6).
Recent articles have demonstrated the usefulness of the warm-compaction process in the fabrication of an automotive turbine hub for highperformance engines (part weight 1100 g), the
manufacture of helical gears with gear densities
in excess of 7.3 g/cm,3 lock components (part
weight 27 g), and gearing with complex gear
forms or spiral gears requiring high gear densities (Ref 22-24). The current production parts
made by warm compaction are parts with a complex shape that are not adaptable for double
pressing and double sintering. Warm compaction
offers a simplified manufacturing process with
Tensile
strength
I~
64
108
92
215
140
120
211
218
181
Eiongatiaa,
%
35
20
26
10
2
10
9.5
1.9
1.7
hardness
77 HRB
96 HRB
90 HRB
45 HRC
36 HRC
27 HRC
47 HRC
42 HRC
41 HRC
resulting mechanical properties that met or surpassed the part specification.
Mechanical properties of warm-compacted steel
powders were compared to selected wrought and
forged alloys (see Table 7). Note that the yield
and tensile strengths of the warm-compacted alloys were equivalent to those of wrought alloys.
Thus it would seem that components made from
these alloys are suitable candidates for the
warm-compaction process. It must be noted that
the elongation of the P/M materials is significantly lower than the wrought alloys chosen (except for the heat-treated ductile iron). Thus,
proper application of the warm-compaction
process must consider the reduced elongation
and impact energy of the P/M part.
One significant advantage of the warm-compaction process is the increased density uniformity achieved in the compacted part (Ref 22, 23).
Quantitative metallographic techniques demonslrated this feature in both a helical gear and an
automotive turbine hub. Figure 9 demonstrates the
greater uniformity of sintered density achieved
with a turbine hub compared to a conventionally
compacted part. This enhanced density uniformity results in increased load-carrying capacity
with reduced dimensional variations because of
the uniform density.
Future applications of the warm-compaction
process will exploit the ability to achieve higher
green densities at lower compaction pressures,
thus minimizing the tooling stresses. Additionally, with the increased interest in the sinterhardening process, warm compaction offers the
potential to green machine these components
prior to sintering and subsequent hardening.
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22.
23.
24.
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Advertising Literature on W~a'rn Compaction,
Porite Taiwan Company, LTD