Development of new feedstock formulation
based on high density polyethylene for MIM of
M2 high speed steels
G. Herranz, B. Levenfeld, A. Várez and J. M. Torralba
A new feedstock formulation for metal injection
moulding (MIM) of M2 high speed steels has been
developed. The binder is a multicomponent system
based on high density polyethylene (HDPE) and
paraffin wax (PW). The compatibility between binder
constituents has been studied by dynamomechanical
thermal analysis (DMTA) showing a partial miscibility between both components. Viscosity measurements of the different binder mixtures at different
shear rates showed that the optimum formulation for
MIM was 50 vol.-% HDPE. With this optimised
binder, several mixtures were prepared with different
powder loadings of M2 grade high speed steel.
Torque measurements of the mixtures indicated that
the maximum amount of metal to be used was
70 vol.-%. The wide distribution of the metal powder
was homogeneously distributed into the polymer
matrix. The polymeric part was driven off by thermal
debinding using a thermal cycle designed on the basis
of a thermogravimetric study of the binder. Finally
the vacuum sintering of the parts allow high quality
parts to be obtained.
PM/1152
At the time the work was carried out the authors
([email protected]) were at the Universidad Carlos
III de Madrid, Departamento de Ciencia e Ingenierı́a
de Materiales, Avda. de la Universidad, 30. 28911
Leganés. Spain. Dr Herranz is now at the
Universidad Castilla La Mancha, Materials Science
and Engineering Area, Avda. Camilo José Cela s/n.
13071 Ciudad Real, Spain. Manuscript received 13
September 2004; accepted 25 November 2004
Keywords: Metal Injection Moulding, High Speed
Steels, HDPE Binder, Rheology
# 2005 Institute of Materials, Minerals and Mining.
Published by Maney on behalf of the Institute.
INTRODUCTION
Metal injection moulding (MIM) is a cost effective
manufacturing technique for relatively small, complex and
high performance metal components.1 This process does
not usually require secondary working operations because
it provides net shape components. The inherent capability
of this technology makes the MIM of high speed steels
(HSS) an attractive fabrication approach and it was
developed in recent years for the production of HSS
components with complex shapes.2–8 For these hard
materials, this technology is considered better than other
manufacturing techniques, such as die compaction, which
are limited to producing parts with simple shapes and low
134
Powder Metallurgy
2005
Vol. 48 No. 2
aspect ratios.1 Moreover, the samples obtained by MIM
present homogeneous carbide distribution, which enhances
the final properties.9
Binder formulation plays a very important role in MIM.
The organic component is the vehicle of flowability during
the moulding step and also provides the shape retention
during the following steps in the global process. In general,
the thermoplastic polymer systems are the most studied and
are used in industry. The high density polyethylene–
wax binder is one of the simplest systems used in powder
injection moulding (PIM).1 The main advantages are low
cost, low molecular weight and good lubricating properties.
Usually, multicomponent binders, comprising of polymer
and other additives, are prepared because they improve
the debinding process. The main advantage of these binders
is the gradual elimination of different components preventing cracks and shape loss. The combination of low
molecular weight and different polymer components allows
a progressive removal as the temperature slowly increases.
The diffusion paths opened by the volatile component
makes it easier to eliminate the polymer at higher
decomposition temperatures. Enlargement of the decomposition temperature range facilitates shape retention and
fast debinding.
The formulation of the feedstock (metal–binder mixture)
is one of the most critical aspects due to its effects on every
step of the MIM process. Some of the main requirements of
the feedstock are good flowability and low viscosity, as
increasing the shear rate results in pseudoplastic behaviour.
The use of low amounts of binder produces high viscosity
feedstock making the moulding process difficult, while large
amounts of binder provide low strength and may produce
heterogeneous green parts.1 The binders used for the
injection moulding of M2 HSS are usually multicomponents based on polypropylene and paraffin wax,2,3 polyacetals5 or different kinds of waxes.7
In this study, a high density polyethylene (HDPE)–wax
based binder was developed due to its simplicity, low cost
and good properties. Initially, the rheological behaviour of
the polymer blends was studied. The optimised blend was
mixed with different amounts of M2 HSS powder. On the
basis of torque and viscosity measurements of the different
mixtures, a suitable volumetric powder loading was established and the mixing process was optimised. Finally, the
thermal debinding cycle was established through thermogravimetric analysis of the binder and the sintering process
was carried out.
EXPERIMENTAL PROCEDURE
The metal powder used for this study was a prealloyed, gas
atomised M2 high speed steel with spherical shape as shown
in Fig. 5a. The chemical composition is given in Table 1.
The density of the M2 HSS was 8.16 g cm23, as measured
with a pycnometer Micrometrics AccuPyc 1330 and the
particle distribution was evaluated using the Fritsch particle
DOI 10.1179/003258905X37828
Herranz et al.
1
Development of new feedstock formulation
135
Particle size distribution of as received M2 HSS powder
2
sizer, Analysette 22 model. Figure 1 shows that 90% of the
particles were ,16 mm.
Developed binders were based on HDPE and paraffin
wax (PW). Firstly, several binder formulations were
prepared by mixing different amounts of HDPE and PW
(50–80 vol.-% HDPE) (Table 2). Then, the feedstocks were
obtained by mixing the optimum binder formulation with
different quantities of metal powder. All the mixing
processes were conducted in a Haake Rheocord Mixer
252p with a pair of roller rotor blades. Mixing conditions
for different binder and feedstock formulations are shown
in Table 2.
The testing samples for debinding and sintering experiments were small disc compacts (25 mm diameter and
2 mm thickness). These specimens were prepared by
pressing feedstock pellets into aluminium moulds at the
mixing temperature.
The rheological behaviour was evaluated by different
instruments depending on the characteristics of each
component. The paraffin wax viscosity was measured in a
concentric cylinder rheometer (Physica) because of its low
viscosity value at 150uC. The diameter of the cylinder was
45 mm. The binder mixtures and the feedstock measurements were performed at 150uC and at 170uC, respectively,
in a capillary rheometer (Kayeness/Dynisco) with a L/D
relation of 30.
The miscibility studies of the binders were carried out
through dynamomechanical thermal analysis (DMTA MkI
Polymer Laboratories) in the single cantilever mode at 1 Hz
and at a heating rate of 4 K min21 from 2135uC to the
softening temperature of the samples.
The elimination of the binder was carried out by thermal
decomposition. The binder degradation study was performed through thermogravimetric measurements in a
Perkin Elmer equipment TGA-7. The experiments were
done under a flux of argon. The temperature was increased
from room temperature (RT) up to 550uC at 5 K min21.
Debinding of the parts was carried out in tubular furnaces
using argon and the sintering process was performed in a
high vacuum furnace (1024 torr) at 1250uC for 1 h.
Microstructures of materials at different stages of the
process (green, brown and sintered parts) were evaluated in
a Philips XL 30 scanning electron microscope equipped
with BSE and EDS (EDAX DX4i) detectors. All the EDS
Thermogravimetric analysis of optimum binder
(designated B3 (50% HDPE–50% PW) and its plain
components under argon atmosphere
spectra were collected at the same voltage (20 kV), working
distance, take off angle and live time (50 s).
The carbon content of different debound samples was
analysed by the infrared absorption method using an
elemental carbon analyser Leco CS-200. The presence of
crystalline oxides in the debound parts was analysed using a
Philips X’Pert automatic diffractometer with (h/2h) Bragg–
Brentano geometry, Cu Ka radiation and a curved graphite
monochromator.
RESULTS AND DISCUSSION
Binder and feedstock characterisation
The choice of two component binders was made to combine
the best properties of both constituents and to favour a
gradual elimination of the organic component. The HDPE
component provides shape retention and mechanical
strength, while the low molecular weight and the low
melting point of the paraffin wax provides high flowability
to the binder, decreasing the viscosity at the processing
temperature. Paraffin wax cannot be used alone because of
the difficulties during its thermal extraction as a consequence of its narrow decomposition range. Moreover, the
Table 2 Mixing parameters of different binder and
feedstock formulations
Component, vol.-%
Binder
80%PEz20%PW
60%PEz40%PW
50%PEz50%PW
Feedstock
50%M2z50%B3
58%M2z42%B3
64%M2z36%B3
66%M2z34%B3
70%M2z30%B3
Speed,
rev min21
Temperature,
uC
40
150
B1
B2
B3
80
170
F1B3
F2B3
F3B3
F4B3
F5B3
Tag
Table 1 Chemical analysis (wt-%) of M2 powder provided by supplier together AISI specification
Present study
AISI specification
C
W
V
Mo
Cr
Mn
0.84
0.80–0.9
6.54
6.00–6.75
1.95
1.75–2.05
4.81
4.75–5.50
3.97
3.75–4.50
0.36
0.10
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Herranz et al.
Development of new feedstock formulation
DMTA analysis of HDPE, paraffin wax and optimum
binder
use of a multicomponent binder normally enlarges the
decomposition temperature range. This is achieved as a
consequence of the big difference between the degradation
temperatures of both components. In Fig. 2 the TGA
curves of plain polymer components and the selected binder
(see above) are shown. The degradation of paraffin started
at 200uC and ended at 350uC, while the HDPE decomposed
between 350 and 500uC. However, the decomposition
window of the blends is slightly enlarged (200–525uC) and
produced in a progressive way.
To obtain feedstocks with high solid powder loading and
suitable rheological properties for injection moulding, the
compatibility between binder constituents (PW and HDPE)
was first studied by DMTA (Fig. 3). The tan d curves of
the HDPE and the PW show the characteristic peaks
associated with glass transition (Tg(HDPE)52104uC and
Tg(PW) 5245uC). In the case of the mixture of both
components, at least three peaks, labelled as arrows, are
distinguished. The main peak appears at a slightly lower
temperature than the major PW peak. A small shoulder of
this peak near to the temperature of the PW can be
observed and finally, the peak corresponding to the HDPE
at 2104uC. This fact should indicate a partial miscibility
between both components, which is beneficial for the binder
elimination.10,11
To evaluate the homogeneity of the different polymer
mixtures, torque measurements were carried out. The mixer
equipment continuously gives the torque value, as a
measure of the resistance on the rotor blades in the
mixture. Before loading each component, the mixer was
heated up to the required temperature. The homogeneity of
the mixtures is achieved when the torque reaches a steady
state value.12 To obtain uniform mixtures, every component was loaded in different steps at the melting temperature of the mixture (150uC) with the torque value stabilised
at a rotation rate of 40 rev min21. First, several binder
formulations were prepared by mixing different amounts of
HDPE and PW (20, 40 and 50 vol.-% PW). The optimum
binder was selected by taking into account the viscosity
values (see below). Later the feedstocks for moulding were
prepared by mixing the optimised binder with different
amounts of powders.
In Fig. 4 the mixing behaviour of the selected binder
formulation (50/50) with different M2 contents is shown. In
these curves the torque value is plotted against the mixing
time. The torque value at the steady state decreases as PW,
with a lower viscosity than the HDPE, was added. After the
addition of all the PW the value of the torque at steady
state was 0.2 Nm.
In the case of the feedstock, the torque value increases
after metal loading and steady state is achieved in a
relatively short time when all the powder is added. For the
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4
Torque measurements of binder with 50% HDPE–50%
PW and three feedstocks with different powder loading, during mixing process. Torque steady state (SS) is
indicated
feedstock with 50% metal powder, the torque value
increases considerably (from 0.2 Nm for the binder to
1.3 Nm). Higher powder loading produced a higher steady
state torque level, indicating the differences in the viscosity
of the mixture. The maximum value achieved corresponds
to 3.9 Nm in the case of the mixture with 70% metal
powder. The homogenisation time for the mixture also
increases with the powder loading as a consequence of
higher resistance on the rotor blades. Moreover, for
mixtures with a metal content higher than 50%, rises in
torque values are detected in each metal loading.
The critical metal loading for this system was about 70%.
The higher amount of powder produced noisy torque
curves as it was more difficult to reach the steady state.
In all the analysed feedstocks, the binder was homogeneously distributed and the green parts presented good
homogeneity (Fig. 5b and c). Considering the binder as
mobile (responsible for flow process) and immobile1 (filler of
voids between particles), the quantity of mobile binder
decreases from F1B3 to F5B3 feedstocks. For formulations
with higher quantities of metal, the approach between
particles clearly decreases the flowability. In the end, all the
binder would be immobilised. However, for 70% M2
content (F5B3) with the maximum metal powder loading,
the amount of mobile binder is enough to allow flowability.
The homogeneous distribution of the different sized
particles is also shown in the scanning electron micrographs
(Fig. 5) allowing an ordered dense packing. In fact, the
broad size distribution of spherical particles enables the
achievement of higher powder loading.13–15
Rheological behaviour of binders and feedstocks
Figure 6 shows the evolution of the viscosity of different
binder formulations with the shear rate at 150uC. The
viscosity of plain constituents is also plotted for comparison. As is shown, the difference in viscosity in both pure
components is considerable, about six orders of magnitude.
In fact for PW, a concentric cylinder rheometer was
required due to the very low viscosity value. In all the
cases, the viscosity is practically independent of shear rate,
indicating a near Newtonian fluid behaviour. When the
amount of PW increases, the viscosity considerably
decreases as a consequence of its lower viscosity and a
slight deviation from this Newtonian behaviour is observed.
After metal loading, the viscosity dramatically increases,
being more important with increasing solid fractions. Large
amounts of the component with the lowest viscosity, PW in
this case, do not maintain the part shape during extraction.
In this sense, binders with a viscosity lower than 100 Pa s at
Herranz et al.
5
Development of new feedstock formulation
137
Scanning electron micrographs of a starting powder; b and c green parts with 50 and 70 vol.-% respectively of metal
powder loading; d brown part
1000 s21 are suitable for mixing with different powder
loadings and feedstocks prepared with these binders,
including those with relatively high powder loading, do
not show any processing problems during injection moulding.16,17 Taking into account these facts, the binder labelled
as B3 (50% HDPE z 50% PW) was selected as the optimum in this study.
The rheological properties of feedstocks are crucial to
evaluate the eligibility of prepared compositions to be
injected. Too a high viscosity of the blend at a working
temperature could obstruct the capillary impeding the
injection process. In contrast, too low a viscosity of the
blend produces green parts with very low strength.
Figure 7 shows the viscosity dependence with the shear
rate of different feedstocks at 170uC. The viscosity of the
feedstocks decreases as the shear rate increases, according
to the pseudoplastic behaviour. The Ostwald and De Waele
power laws describe the pseudoplastic character of fluids,
according to the following equation.
c~ktn
As expected the relative viscosity of the feedstock
increased with metal volume fraction. The values of the
flow index of the feedstocks go from 0.5 to 0.3 as the
powder load decreases, indicating that the pseudoplastic
character is stronger in the case of the highest powder
loaded feedstock.
Taking into account the feedstock requirements needed
for good viscosity, maximum powder loading and the
desirable viscosity values at high shear rate, the 70 vol.-%
formulation was chosen.
Debinding and sintering
The binder was removed by thermal debinding. The design
of the thermal cycle is critical in order to avoid the presence
of defects in the brown and sintered parts. On the basis of a
thermogravimetric analysis of the binder and taking into
account that a high heating rate produces cracks in the
parts, the thermal cycle of Fig. 8 was designed. Up to
200uC, a faster heating rate was applied because no
components start to decompose. From this temperature
where t is the shear stress, c is the shear rate, k is a constant
and n is a flow behaviour exponent.
6
Apparent viscosity versus apparent shear rate of different prepared binders and HDPE and paraffin wax
plain components at 150uC
7
Apparent viscosity versus apparent shear rate of different feedstock formulation at working temperature of
170uC
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Herranz et al.
Development of new feedstock formulation
Thermal debinding cycle applied under argon atmosphere
and sintering process under high vacuum conditions
to 400uC the degradation of paraffin takes place and a slow
heating rate of 1 K min21 was chosen to avoid any loss of
shape. The next step corresponds to the elimination of the
HDPE, which retains the basic shape. For this reason an
extremely slow heating rate (0.33 K min21) was selected.
Finally, the parts were kept at 500uC for 1 h to burn out
completely the organic components.
After debinding, the carbon was almost removed from
the parts (Table 3) as the elimination was more difficult for
samples with a higher amount of binder.
The brown parts presented good homogeneity, as shown
in Fig. 5, and had enough strength to be handled. EDS
analysis on the surface showed the presence of a small
amount of oxygen, although XRD patterns of powder
removed from the surface of the brown part do not present
peaks corresponding to any iron oxide. This indicates that
the oxidation is produced on the surface and in a small
scale.
The sintering process has been studied by many research
workers.18–20 Generally a sintering temperature in the range
1220–1250uC is recommended for this type of steel.
However, the temperature range for an appropriate
microstructure (homogeneous distribution of carbides) is
very narrow. In this case the best microstructure was
obtained at 1250uC, as shown in Fig. 9. For this
temperature a relative density of 98% has been achieved
and the hardness was 621 HV30. This value could be
improved by thermal treatments of the parts.
CONCLUSIONS
A new feedstock formulation composed by HDPE and PW
was developed to produce M2 high speed steel parts by
metal injection moulding. A systematic mixing and
rheological study of the different formulations was carried
out.
The optimum binder mixture was 50/50 owing to its
adequate viscosity value.
On the basis of torque and viscosity values, the
composition with 70 vol.-% metal powder loading was
chosen as the optimum mixture. Torque measurements of
different feedstock formulations showed that this composition was suitable, considering that high powder loading is
beneficial for MIM purposes.
Table 3
Carbon content of starting powder M2 HSS and
debound specimens
Debound Sample M2 HSS F1B3 F2B3 F3B3 F4B3 F5B3
wt-%C
0.87
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0.90
Vol. 48 No. 2
0.90
0.88
0.88
9
Microstructure of sintered part showing homogeneous
carbide distribution. Sintering was performed at
1250uC under high vacuum
The elimination of the organic components was carried out
by thermal debinding. On the basis of the thermogravimetric
study of the binder, the debinding cycle was designed. The
binder was gradually removed in a relatively short time,
obtaining brown parts with enough strength to be handled.
Preliminary results show that well sintered parts with a
favourable microstructure were obtained.
ACKNOWLEDGEMENTS
The authors thank CICYT (MAT2003-03376) and
Comunidad Autónoma de Madrid (3rd Regional Research
Programme ‘Grupos Estratégicos’) for financial support. The
authors would also like to thank the Rheology Group in the
University of the Basque Country for assistance.
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