Materials Transactions, Vol. 46, No. 2 (2005) pp. 298 to 302
#2005 The Japan Institute of Metals
Effect of Heating Rate and Phase Transformation
on the Dimensional Control of Ferrous PIM Compacts
Guo-Jiun Shu* and Kuen-Shyang Hwang
Department of Materials Science and Engineering, National Taiwan University,
1, Roosevelt Rd., Sec. 4, Taipei 106, Taiwan, R. O. China
Powder injection molded (PIM) parts usually show large amounts of shrinkage after sintering due to the low powder loading, resulting in
poor dimensional stability. This problem is further aggravated when a high shrinkage rate occurs or when the furnace temperature is not uniform.
To alleviate this dimensional control problem, the effects of the phase transformation, sintering temperature, and heating rate were investigated.
The results show that when an abrupt volume change occurs, as happens during the ! phase transformation of iron, the dimensional
stability deteriorates. This problem gets worse when the density of the part is low. By slowing down the heating rate in the region where the high
shrinkage rate occurs, avoiding the phase changes, and adding alloying elements to broaden the temperature range of the phase transformation,
the dimensional control of ferrous PIM compacts can be improved.
(Received October 8, 2004; Accepted December 24, 2004)
Keywords: powder injection molding, metal injection molding, dimensional stability, heating rate, phase transformation
1.
Introduction
The powder injection molding (PIM) process is advantageous in fabricating the small and complex-shaped parts with
high densities, good mechanical properties, and fine surface
finishes. However, one weakness of this technology is its
poor dimensional stability. The main reason is that many
processing steps are involved. Any mis-conducted procedures could lead to poor final dimensions. The other reason is
that fine powders and low powder loadings are used in the
feedstock, resulted in large amounts of shrinkage during
debinding and sintering.
The origins of the dimensional stability problem have been
addressed in several previous studies.1–4) Using a noncontacting laser dilatometer, the in-situ dimensional change
of compacts during solvent debinding indicated that the
binder swelled when in contact with the solvent, and this
swelling behavior was affected by the processing parameters
and material’s characteristics. Thus, when the binder amount,
binder type, debinding temperature, and solvent type are not
selected carefully, distortions and inconsistent dimensional
changes will occur. Previous studies on thermal debinding
also showed that abrupt shrinkages and expansions of the
compacts could occur during thermal debinding, and impaired the dimensional stability.5) In addition to the dimensional control problems that occur during debinding, large
amounts of dimensional change of the PIM parts arise in the
sintering process. Takekawa showed that fast heating rates,
particularly near the final sintering temperature, lead to poor
dimensional stability.6) Other studies also demonstrate
several causes of the dimensional problem, such as the
gravity effect, low powder loading, surface roughness of the
setter plate, and non-uniform green density within the
compact.7–10) Large parts and coarse powders also caused
more severe distortions as compared to small parts and fine
powders.11) Despite these developments and insights into the
dimensional stability problem of PIM parts, a large amount of
*Graduate
Student, National Taiwan University
room for improvement still remains. The purpose of this
study is thus to further investigate the effects of sintering
parameter, alloying composition, and phase transformation
on the dimensional control of PIM parts.
2.
Experimental Procedure
A carbonyl iron powder with an average particle size of
4.2 mm was selected as the base powder in this study. The
characteristics and the morphology of the powder are given in
Table 1 and Fig. 1(a), respectively. To understand the effect
of alloying elements on the tolerance control, 8 mass% Ni
and 4.5 mass% Fe3 P (the phosphorus content in Fe3 P is
15.6 mass%), which are austenite and ferrite stabilizers,
respectively, were added. Table 2, Fig. 1(b), and Fig. 1(c)
show, respectively, the characteristics and the morphologies
of these alloying powders. To prepare the feedstock, the iron
powder was mixed with a wax-based binder using a sigma
blade kneader. The powder loading was 61.3 vol%. After
kneading, the feedstock was molded into rectangular specimens of 2 mm 10 mm 100 mm.
A two-stage debinding process was employed. For solvent
debinding, the compact was immersed in heptane at 318 K
until about 80% of the soluble binders had been removed.
Table 1
The characteristics of the carbonyl iron powder used in this study.
Type
Carbonyl iron powder
Designation
Average particle size
CIP-S-1641
D(10) 2.05 mm
(Laser scattering
D(50) 4.24 mm
method)
D(90) 8.81 mm
Shape
Pycnometer density
Spherical
7:57 103 kg/m3
C, mass%
0.741
S, mass%
0.003
N, mass%
O, mass%
0.658
0.812
Supplier
ISP
Effect of Heating Rate and Phase Transformation on the Dimensional Control of Ferrous PIM Compacts
(a)
Table 2
The characteristics of the Ni and Fe3 P powders used in this study.
Powder
Fe3 P
Average particle size
D(10) 2.3 mm
D(10) 1.9 mm
D(50) 3.6 mm
D(50) 3.2 mm
method)
D(90) 6.9 mm
D(90) 5.5 mm
Spiky
8:89 103 kg/m3
Chunky
6:69 103 kg/m3
C, mass%
0.081
0.002
S, mass%
0.001
0.003
N, mass%
0.002
0.002
O, mass%
0.196
0.004
Supplier
INCO
F. W. Winter
the sintering temperature and time were adjusted for each
group of specimens. The data reported are the standard
deviations of the compact lengths and are averages of a
minimum of eight compacts.
To understand the effect of sintering parameters and
alloying elements on the sintering behavior of iron compacts,
a thermal dilatometer (SETSYS TMA 16/18, SETRAM Co.,
Caluire, France) was employed to monitor the in-situ
dimensional changes of the specimens. The dilatometer
analysis was carried out using the same parameters as those
employed for the sintering runs.
3.
(c)
Carbonyl Ni
(Laser scattering
Shape
Pycnometer density
(b)
299
Results
3.1 Effect of heating rate
Figure 2 shows that the dimensional stabilities of the
injection molded specimens were good after molding, solvent
debinding, and thermal debinding. However, the standard
deviation increased significantly after sintering at 1623 K for
9 ks. With the three heating rates employed, 0.083, 0.167, and
0.333 K/s, the slowest heating rate of 0.083 K/s produced the
best results. To determine the cause of such differences,
dilatometry analysis was carried out. Figure 3(a) shows that
all three curves were inflected at about 1185 K, when the
Fig. 1 Morphologies of the metal powders used in this study (a) Fe (b) Ni
(c) Fe3 P.
During the subsequent thermal debinding, the solvent debound compacts were heated at 0.083 K/s to 923 K and then
held at that temperature for 3.6 ks in a hydrogen atmosphere
to remove the remaining binder and reduce the intrinsic
carbon in the iron powder to less than 0.01 mass% so that the
carbon would not complicate the analysis when the effects of
the addition of Ni and Fe3 P were examined.
Sintering was also carried out under a hydrogen atmosphere. To attain the same relative density so that the
dimensional stability could be compared on the same basis,
Standard Deviation (%)
0.08
0.083K/s
0.167K/s
0.06
0.333K/s
0.04
Fe
Sintered Density = 95%
H2 , 1623K
0.02
0.00
after
after
molding SD
after
TD
after
sintering
SD: solvent debinding
TD: thermal debinding
Fig. 2 The effect of heating rate on the standard deviation of the length of
PIM iron compacts.
300
G.-J. Shu and K.-S. Hwang
0
Shrinkage (%)
-2
-4
Standard Deviation, (%)
(a)
Fe
1623K, H2
-6
0.083K/s
-8
0.167K/s
-10
0.333K/s
-12
-14
-16
400 600 800 1000 1200 1400 1600
1173K, 3.6ks
0.05
1173K, 3.6ks 1193K cooling
1623K, 9ks
0.04
0.03
Fe, 0.167K/s
Sintered Density = 95%
H2
0.02
0.01
0.00
after
after after
molding SD
TD
Temperature, T/K
Shrinkage Rate, r/ % s -1
0.06
after
sintering
(b)
SD: solvent debinding
TD: thermal debinding
0.00
Fig. 4 Standard deviations of the lengths of PIM iron compacts that were
sintered using three different sintering schedules.
Fe
1623K, H2
-0.01
0.083K/s
-0.02
0.167K/s
0.333K/s
-0.03
800
1000
1200
1400
1600
Temperature, T/K
Fig. 3 (a) The dimensional changes and (b) shrinkage rates of iron
compacts heated at 0.083, 0.167, and 0.333 K/s, respectively, to 1623 K in
hydrogen.
! phase transformation occurred. Figure 3(b) further
shows that the shrinkage rate slowed down significantly after
the phase transformation. This was due to the decreased
diffusion rate in the phase and the exaggerated grain growth
that accompanied the phase changes.12) It seemed that the
larger standard deviation from the heating rate of 0.333 K/s
was related to either the higher shrinkage rate during heating
in the phase or the more abrupt shrinkage at the phase
transformation. To separate these two effects, a set of
specimens was heated to 1173 K and then immediately
cooled to the room temperature. The results were compared
to the results from those heated to 1623 K. Table 3 shows that
the faster heating also caused larger amounts of standard
deviation at the point when the parts had reached 1173 K. The
differences between the numbers in the rows for 1173 and
1623 K, respectively, show that the faster heating rate also
caused larger amounts of standard deviation when phase
transformation occurred.
Table 3 The standard deviations of compacts heated to 1173 and 1623 K
using heating rates of 0.083, 0.167, and 0.333 K/s, respectively.
0.083 K/s
0.167 K/s
0.333 K/s
1173 K, 1
0.025
0.030
0.036
1623 K, 2
0.037
0.045
0.064
2 1
0.012
0.015
0.028
3.2 Effect of ! phase transformation
The results above, using three heating rates and without
holding at the final temperature, show that the abrupt
dimensional change at 1185 K impaired the dimensional
control. To further verify that, a group of specimens were
sintered below the phase transformation temperature, at
1173 K using a heating rate of 0.167 K/s. The sintering time
to attain 95% density, which was the density obtained after
sintering at 1623 K for 9 ks, was only 2.7 ks. This is because
of the high diffusion rate of iron in the phase and the
elimination of the exaggerated grain growth. Figure 4 shows
that the specimens thus sintered had a smaller standard
deviation than those sintered at 1623 K. This is most likely
attributed to the elimination of the sudden volume change at
1185 K, as shown by the smooth shrinkage rate curve in
Fig. 5.
To further understand whether the density at which the
phase transformation occurs will influence the dimensional
control, another group of specimens were first sintered to
95% density at 1173 K for 2.7 ks and then heated to 1193 K
before being furnace cooled immediately afterward. The
standard deviation, as shown in Fig. 4, was almost the same
as that for specimens sintered isothermally at 1173 K without
further heating to 1193 K. These results suggest that the
phase transformation does not affect the dimensional stability
when the compact is already dense. It could, however, impair
the dimensional consistency of the specimen when the
density of the part is low, as in the case when the high heating
rate was used to heat the parts to 1623 K.
3.3 Effect of alloying additions
The above results show that to improve the dimensional
stability, the iron compact can be sintered below 1185 K to
avoid the phase transformation or, alternatively, heated
slowly so that the compact is relatively dense when the phase
transformation occurs. These methods, however, only work
for pure iron, which is frequently used for soft magnets.
However, for most other applications, such as for structural
(a)
0.000
-0.005
Shrinkage, (%)
Shrinkage Rate, r/ % s -1
Effect of Heating Rate and Phase Transformation on the Dimensional Control of Ferrous PIM Compacts
1185K
-0.010
Fe, 0.167K/s
H2
-0.015
1173K
301
0
-2
0.167K/s, H2
-4
Fe-0.7P
-6
Fe-8Ni
-8
Fe
-10
-12
-14
1473K
-0.020
-16
3000
4000
5000
6000
7000
8000
400 600 800 1000 1200 1400 1600
Time, t/s
Temperature, T/K
4.
Discussion
The results above show that, when the phase transformation occurs during sintering, the dimensional stability of iron
compacts deteriorates significantly. This is because the
compact shrinks suddenly during the phase transformation,
particularly when the compact is still loosely sintered and the
interparticle bonding is still weak. Since the theoretical
volume decrease of 100% dense iron is quite large, at about
1.4%, or 0.46% linearly, when the phase transforms into the
(b) 0.005
0.000
-0.005
0.167K/s, H2
-0.010
Fe-0.7P
-0.015
Fe-8Ni
Fe
-0.020
800
1000
1200
1400
1600
Temperature, T/K
Fig. 6 (a) The dimensional changes and (b) the shrinkage rates of Fe, Fe0.7 mass%P, and Fe-8 mass%Ni compacts heated at 0.167 K/s in hydrogen.
Standard Deviation, (%)
parts that contain some alloying elements, the sintering
behaviors are quite different. To understand the effect of the
alloying elements, Fe-0.7 mass%P and Fe-8 mass%Ni were
examined.
Figure 6(a) shows that when 0.7 mass% P was added in the
form of Fe3 P, the shrinkage curve became very smooth,
without any inflection at 1185 K. This indicated that most of
the phosphorous was dissolved into the matrix, and the whole
sintering cycle proceeded in the phase.13) For Fe-8 mass%Ni, the phase transformation was noticed at a lower temperature, and the inflection was not significant. This is because
nickel has a slower diffusion rate than phosphorous and thus
was less homogenized during heating. This non-uniform
nickel distribution caused the phase transformation to occur
continuously in a temperature range below 1185 K.13,14) In
the case when the phase transformation occurred in a wide
temperature range, the effect of the phase transformation was
not so significant. As shown in Fig. 6(b), a much less abrupt
change in the shrinkage rate than that of the pure iron
compact was noticed around 1185 K. Furthermore, the
shrinkage rate after 1185 K was faster. Since both specimens
were in the phase above 1185 K, this suggests that no
exaggerated grain growth occurred and finer grains must
have been retained during sintering between 1185 and
1623 K in the Fe-8Ni compact.
Figure 7 illustrates that when there was no phase transformation, such as in the case of Fe-0.7 mass%P, the
dimensional stability was the best. The Fe-8 mass%Ni
compact also showed improved results compared to that of
the pure iron. This is because the phase transformation,
though still noticeable, occurred in a continuous mode, and
the abrupt dimensional change was avoided.
Shrinkage Rate, r/ % s -1
Fig. 5 The shrinkage rates of iron compacts heated to 1173 K and 1623 K,
respectively.
0.06
Fe-8Ni
0.05
Fe
0.04
Fe-0.7P
0.03
Sintered Density = 95%
0.167K/s
H2
0.02
0.01
0.00
after
after
molding SD
after
TD
after
sintering
SD: solvent debinding
TD: thermal debinding
Fig. 7 The effect of Ni and P additions on the standard deviations of the
length of PIM compacts.
phase,15) this sudden volume change is over the range of the
elastic deformation and could thus cause loosely bonded
particles to be plastically deformed. Since the composition
and the temperature are not perfectly uniform within the
compact, it is very likely that the phase transformation occurs
302
G.-J. Shu and K.-S. Hwang
2000
0.000
B
-0.005
A
-0.010
1500
0.167K/s
0.167K/s
-0.015
A
0.083K/s
B
-0.020
1000
Temperature, T/K
Shrinkage Rate, r/ % s -1
at slightly different times at different locations within the
part. These local deformations could then cause anisotropic
shrinkages of the part. Furthermore, the temperature is not
uniform inside the furnace hot zone either. As a result, the
dimensional stability from part to part deteriorates. However,
if the compact has already attained a high sintered density
and has strong interparticle contacts, such as after being
sintered at 1173 K for 2.7 ks, the phase transformation that
occurs at 1185 K has much less effect on the dimensional
stability, as shown in Fig. 4.
To improve the dimensional control of iron compacts, the
effect of the sudden volume change at the phase transformation must be alleviated. This can be achieved by
eliminating its occurrence. For example, when an phase
stabilizer such as phosphorus is added, the phase transformation of iron is prevented and the dimensional stability
thus improved. The other method is to broaden the temperature range in which the phase transformation occurs. One
way to do this is to add nickel, which is a phase stabilizer.
When nickel is added into iron powders, the sintering curve
becomes smooth. However, this is not due to the elimination
of the phase transformation, but the phase transformation
occurs in a wider temperature range. Since the abrupt volume
change is prevented, dimensional control is also improved.
With this clearer understanding of the effects of heating
rates and phase transformations on the dimensional stability,
an experiment was carried out to illustrate the benefits of
combining the slow heating rate and the phase sintering.
The specimens used were Fe-0.7 mass%P. As shown in
Fig. 8, the heating rate of the first schedule was 0.167 K/s
throughout the heating period to a peak of 1373 K. For the
second schedule, the heating rate was adjusted to 0.083 K/s
between 973 K and 1373 K so that its maximum shrinkage
rate was only half of that measured from the first schedule.
Table 4 shows that the standard deviation of the Fe-0.7P
heated according to the first schedule was only 0.027%,
Fe-0.7P
-0.025
4000
6000
8000
10000 12000
Time, t/s
Fig. 8 The shrinkage rates of Fe-0.7 mass%P PIM compacts with different
sintering conditions.
Table 4 The amounts of shrinkage and standard deviation of Fe and Fe0.7 mass%P PIM compacts prepared using different sintering parameters.
Fe
(0.167 K/s)
Fe-0.7P
(0.167 K/s)
Fe-0.7P
(0.083 K/s)
Shrinkage, %
14.08
14.22
14.12
Standard deviation, %
0.045
0.027
0.021
improved from the 0.045% of the pure iron. When the second
schedule was employed, the standard deviation further
decreased to 0.021%.
5.
Conclusions
This study shows that the dimensional stability of PIM iron
compacts deteriorates when a high shrinkage rate occurs,
such as when a high heating rate is employed or when the
phase transformation occurs. When the phase transformation
occurs, the large volume change impairs the interparticle
bonding and causes the plastic deformation. This becomes
worse when a high heating rate is employed, as the density of
the compact is still low and the interparticle bonding is weak.
The addition of phosphorous, which is an phase stabilizer,
and the use of a low heating rate improve the standard
deviation of the length of the iron compact from 0.045 to
0.021%. The addition of nickel causes the phase transformation to occur in a wider temperature range and thus
alleviates the problem by preventing the sudden volume
change. These results suggest that good dimensional stability
can be attained by selecting the proper alloying elements and
heating schedules.
Acknowledgements
The authors wish to thank the National Science Council of
the Republic of China for their support of this project under
contract NSC92-2216-E002-008. We also thank F. W.
Winter Co. for supplying the Fe3 P powder.
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