Materials Science and Engineering A 478 (2008) 251–256
Dimensional behaviour of aluminium sintered in different atmospheres
T. Pieczonka a , Th. Schubert b,∗ , S. Baunack c , B. Kieback b
a
AGH University of Science and Technology, Mickiewicza 30, 30-059 Kracow, Poland
Fraunhofer-Institute for Manufacturing and Advanced Materials, Department of Powder Metallurgy and Composite Materials,
Winterbergstr. 28, D-01277 Dresden, Germany
c IFW Dresden, Leibniz-Institut für Festkörper- und Werkstoffforschung Dresden, Postfach 270116, D-01171 Dresden, Germany
b
Received 26 January 2007; received in revised form 30 May 2007; accepted 1 June 2007
Abstract
The sinterability of pure aluminium powder was controlled in different sintering atmospheres, i.e. nitrogen, argon, nitrogen/hydrogen and nitrogen/argon gas mixtures, and also in vacuum. Dimensional changes occurring during sintering were monitored by dilatometry. Thermogravimetric
analysis (TG) was used to recognize possible interactions between aluminium and nitrogen. Pure nitrogen was found to be the only active sintering
atmosphere for aluminium, promoting shrinkage, associated with a weight gain by binding nitrogen, and enhancing mechanical properties of
the sintered compacts. Hydrogen lowers the sinterability of aluminium very strongly, even when present in small concentrations in a nitrogen
atmosphere. Auger electron spectroscopy was used to characterize the surface layers on aluminium powder particles, fractured green compacts
and sintered samples. Distributions of aluminium, nitrogen, oxygen and other elements, contained as impurities, were obtained by depth profiling
measurements on this surfaces. Indications are that enhanced concentration of magnesium within the powder particle surface film promotes sintering
of aluminium.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Sintering of aluminium; Sintering atmosphere; Dilatometry; AES
1. Introduction
During the last decade, interest in sintered aluminium-based
structural parts for the automobile industry has dramatically
increased [1], as evidenced by the number of scientific papers
and successful use-oriented developments. Prospective conventional press-and-sinter aluminium components include camshaft
bearing caps and the rotor and sprocket for a camphaser system.
Industrial development of aluminium powder technology is
impeded by the very stable Al2 O3 layer on powder particles,
which cannot be reduced during conventional sintering. Successful sintering of aluminium alloys can only be achieved
through the formation of a liquid phase—able to disrupt the
stable oxide skins [2]. Copper is a well-known alloying element
for liquid phase activation [3,4]. An important processing factor, more complex, as compared with, e.g. iron, is the sintering
atmosphere. The best-sintered properties of aluminium-based
compacts are achieved by sintering in dry nitrogen, mostly
∗
Corresponding author. Tel.: +49 351 2537 346; fax: +49 351 2537 399.
E-mail address: [email protected] (Th. Schubert).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2007.06.002
evaporated liquid nitrogen [5–10]. The interactions between
aluminium-based compacts and the sintering atmosphere have
been analysed, but unanswered questions remain, one of which is
the role of hydrogen. Some researchers claim that hydrogen had
little influence [11], while various investigations have shown
that it had a detrimental effect on sintering of aluminium and
its alloys [5,12–16], because it strongly reduces the sintering
shrinkage. The main purpose of this work was to investigate
the influence of the sintering atmosphere on the dimensional
changes of pure aluminium compacts during solid state sintering. High purity aluminium powders were used to eliminate the
effect of alloying additions and a liquid phase on the sintering
behaviour.
2. Experimental procedure
The raw material was an air atomized, 99.5% purity aluminium powder, delivered by Fluka (impurities: Si, 0.15; Fe,
0.15; Mg, 0.02; Cu, 0.03 wt.%; particle size: 28% <20 ␮m/23%
20–40 ␮m/49% >40 ␮m).
For dilatometry investigations rectangular (14 mm × 4 mm ×
4 mm) green compacts were produced by uniaxial cold press-
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T. Pieczonka et al. / Materials Science and Engineering A 478 (2008) 251–256
Fig. 1. Dilatometry traces for compacts made of Al powder sintered in nitrogen/hydrogen atmospheres (e.g. N95H5 = N2 /5% H2 ).
Fig. 3. Terminated shrinkage for Al compacts by atmosphere change: nitrogen → N2 /5% H2 .
ing at 200 MPa with a glycerol die lubrication. In addition,
some rectangular bend test samples (60 mm × 5 mm × 5 mm)
were prepared by double action pressing at 200 MPa with a die
lubrication. The green density of the samples was about 89%
of the theoretical density; the low value was selected in order
to enhance the effects of possible gas/solid interactions during sintering. Consequently further work is necessary to effect
technology transfer to high density aluminium-based alloys.
The sintering was performed in a dilatometer using the
following sintering atmospheres: nitrogen, argon, vacuum
(10−3 Pa) and additionally nitrogen/hydrogen mixtures and
nitrogen/argon mixtures.
The gas flow rate through the dilatometer’s furnace tube having an inner diameter of 35 mm was 100 ml/min. The exact
compositions of the sintering atmospheres in vol.% are given in
Figs. 1–4. The nitrogen, hydrogen and argon used in the current
investigations were high purity gases with a dew point below
−70 ◦ C (which was the lowest point on a dew-point-meter’s
scale), and an oxygen content below 5 ppm.
Dimensional changes were monitored during the entire
temperature–time programme using Netzsch Dil 402E dilatometers: heating at 0.33 K/s to the isothermal sintering temperature
of 600 ◦ C, at which the samples were held for 2 h, and cooling at 0.33 K/s to the room temperature. The sample holder was
alumina.
The bulk density of the sintered compacts was measured by
using a method based on Archimedes’ law and compared with
the theoretical density of aluminium.
To enhance a possible nitride formation by increasing the
gas/solid interface, a uncompacted Al powder layer (about
50 mm × 50 mm × 50 mm in size) formed in a steel crucible was
sintered in nitrogen. The temperature–time profile was the same
as specified above.
Nitrogen and oxygen contents of the as-delivered powder
and the sintered specimens were measured by hot extraction
with the LECO O/N-analyser TC 600. In addition, to reveal
possible changes in oxygen and nitrogen concentrations through
the sintered specimens, samples sintered in nitrogen were cut
Fig. 2. Dilatometry traces for compacts made of Al powder sintered in nitrogen/argon atmospheres (e.g. N95Ar5 = N2 /5% Ar).
Fig. 4. Terminated shrinkage for Al compacts by atmosphere change: nitrogen → N2 /50% Ar.
T. Pieczonka et al. / Materials Science and Engineering A 478 (2008) 251–256
253
Table 1
Nitrogen and oxygen contents in the materials investigated
Material
Sintering atmosphere
Sintered density (g/cm3 )
Nitrogen (wt.%)
Oxygen (wt.%)
Fluka Al99.5 powder, as delivered
–
–
0.005
0.36
Sintered compact
N2
N2 /5% H2
Ar
Vaccum
2.47
2.41
2.42
2.42
0.94
0.001
0.001
0.003
0.59
0.80
0.71
0.50
Sintered compact
Surface layer (0–300 ␮m)
Center of the sample
N2
2.47
0.44
0.83
0.70
0.53
Sintered specimen of loose Al powder
Surface layer (0–300 ␮m)
Center of the sample
N2
3.59
5.45
1.35
0.33
into discs, parallel to the long side of a sample, of about 300 ␮m
thickness.
The surface analysis of “fresh” fractures of sintered compacts
was made by Auger electron spectroscopy (AES) on a Scanning Auger Microprobe PHI660 (Physical Electronics, USA)
with primary electrons of 10 keV, 40 nA and an energy resolution of E/E = 0.6% with the beam diameter about 400 nm.
The sintered compacts were fractured under ultra high vacuum conditions using the impact fracture stage of the PHI660.
For depth profiling, the samples were sputtered by 1.5 keV
argon ions with an equivalent sputtering rate in SiO2 of about
3 nm/min.
Bend test specimens were sintered in a laboratory tube furnace with the same temperature–time programme. The sintering
atmospheres were: nitrogen, nitrogen/5 vol.% hydrogen and
argon. Three-point bend tests were performed on green and
sintered compacts.
3. Results and discussion
Preliminary experiments using different atmospheres (argon,
vacuum, nitrogen) showed that only nitrogen causes a detectable
shrinkage of the samples during sintering. The addition of hydrogen, even in small amounts, to nitrogen is detrimental because
it strongly decreases the sintering shrinkage (Fig. 1). The addition of argon to nitrogen also reduces shrinkage, but only at
much higher contents than hydrogen (Fig. 2). Shrinkage may
be quickly interrupted by a change of the atmosphere during
sintering (Figs. 3 and 4).
The data collected in Table 1 and the TG curve in Fig. 5 clearly
indicate that the sintering shrinkage is associated with nitride formation while an increase in the oxygen content is simultaneously
observed. Thus the assumption of a possible Al2 O3 reduction by
nitrogen [9,10] seems questionable.
The very high shrinkage of loose packed Fluka Al powder
particles (estimated to be about 40%) is accompanied with a
very strong nitriding compared to the green compacts (Fig. 5).
The X-ray phase analysis showed aluminium nitride AlN as a
product of Al powder–nitrogen atmosphere interactions. The
oxygen distribution over the cross section of the sintered powder,
shown in Table 1 demonstrates the self-gettering mechanism,
(Not measured)
Fig. 5. TG curve for uncompacted and compacted Al powder heated in nitrogen.
which reduces the oxygen partial pressure in deeper regions to
a very low level [10].
The proposed reduction of the Al2 O3 by gaseous nitrogen at
this low oxygen pressures [10], however, is not a condition for
the nitride formation. The higher nitride content in the interior
should be explained by the constant thickness of the oxide layer,
while the growing oxide layers at the surface of the specimen hinder the nitrogen diffusion. There is still a lack of understanding
about the details of nitride formation and its positive influence
on the sintering behaviour. The strength data in Table 2 indicate
that stronger bonds between Al-particles are produced in nitrogen atmosphere. The comparison to the transverse strength of
aluminium-based P/M alloys which are in the range of 400 MPa
(Al–0.2Cu–0.8Si–1Mg) to 550 MPa (Al–1Cu–7Zn–2.5Mg) [17]
can give some indication of the interparticle strength of the sintered pure aluminium specimen. Solid solution strengthening
and precipitation hardening can contribute to the higher strength
values of the commercial alloys.
Table 2
Three-point bend test results for specimens made of Fluka Al powder
No.
Sintering atmosphere
Bending strength (MPa)
1
2
3
4
Nitrogen
Nitrogen/5 vol.% hydrogen
Argon
Green compacts
243
123
165
21.5
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T. Pieczonka et al. / Materials Science and Engineering A 478 (2008) 251–256
Fig. 6. AES concentration depth profiles for a Al powder particle (a), a fracture surfaces of a compact sintered for 2 h at 600 ◦ C in nitrogen (b), and a compact, heated
to 400 ◦ C, then fractured (c). The dashed line marks the steady state Mg concentration of about 5%. The estimated sputter depth in aluminium oxide films is given
for orientation only.
Fluka Al contains about 200 ppm Mg as impurity and consequently no Mg was detected on and near the surfaces of untreated
powder particles by AES (Fig. 6a). Kimura et al. [18] and Kondoh et al. [19,20] showed that magnesium atoms have a tendency
to migrate from the inner region to the surface of Al particles at
temperatures above 500 ◦ C. Using X-ray photoelectron spectroscopy, they also observed Al in the metallic state on the
surfaces, which was attributed to the reduction of Al2 O3 by
Mg. Metallic Al can directly react with gaseous nitrogen and
aluminium nitride is formed in situ.
The AES results presented here confirm that Mg atoms
migrate to the surface region of the particles. We found Mg
always in the oxidic state, as concluded from the position of the
Mg(KLL) peak. For the sintered compact up to 10 at.% Mg were
found on the fracture surface (Fig. 6b). This level is sufficiently
high to reduce Al2 O3 [18–21] and to degrade the oxide layer
locally. With sputtering, the Mg concentration diminishes to a
steady state value of about 5 at.%. The same level was already
found after thermal treatment at 400 ◦ C (Fig. 6c), i.e. prior to the
uptake of nitrogen by aluminium, which starts at about 520 ◦ C.
Sputter removal of thin layers depends inter alia on composition, angle of ion incidence and surface roughness. It should be
pointed out that on rough surfaces the sputter removal takes place
laterally non-homogeneous. Shadowing of the ion beam and
scattering of electrons on rough surfaces could be responsible
for the detection of oxides in groves and holes after long sputter times. Additionally, small precipitates (see Fig. 7) have much
smaller sputter rates than flat surfaces. Consequently the retarded
sputter removal could be interpreted as a thicker layer. Therefore, a definite sputter rate for surfaces of fractured samples and
powders can hardly be given. In Fig. 7, the estimated sputter
depth in aluminium oxide films is given only as an orientation.
If the constant Mg signal in the steady state of sputtering
(Fig. 6b and c) is caused by complete segregation of Mg into the
surface region of a powder particle, the observed concentration
of 5 at.% would require the formation of an Mg-rich shell of
about 30 nm thickness.
It is obvious that the revealed incorporation of nitrogen atoms
enhances the destruction of Al2 O3 and can subsequently result in
the formation of AlN. After a short-time sintering (at 600 ◦ C for
T. Pieczonka et al. / Materials Science and Engineering A 478 (2008) 251–256
Fig. 7. SEM picture of fractured Al99.5 compact sintered at 600 ◦ C for 10 min.
10 min) of aluminium compacts, some rosette-like precipitates
form as the first reaction products decorating the powder particle
surfaces (Fig. 7). The local chemical surface analysis of these
precipitations made by AES on the not sputtered surface reveals
a composition of about Al–12Mg–51O–9N (in at.%).
In the depth profile (Fig. 6b) the concentrations of oxidized
Al, O and N decay similar. From this behavior the formation of
an oxynitride as an intermediate phase can be speculated about.
In the depth profiles Al was detected in the metallic and oxidic
state as revealed by the position of the Al(KLL) Auger peak. For
AlN the position of the Al(KLL) Auger peak is reported to be
between the two positions [22,23], but this peak was not found
in the depth profiles. No peak positions of the Al(KLL) peak in
Al oxynitride were found in the literature. A direct proof of the
existence of a Al(–Mg)–O–N phase by XRD data of the samples
does not exist until now.
Obviously, all of these reactions:
3Mg + 4Al2 O3 → 3Al2 MgO4 + 2Al
2Al + N2 → 2AlN
may support diffusion processes of aluminium, which could be
responsible for the shrinkage observed when pure nitrogen atmosphere was used or at least attack the oxide layers at the particle
surface.
The concentration of hydrogen in a nitrogen/hydrogen gas
mixture influences not only the amount of sintering shrinkage,
but also the incubation time necessary to begin the shrinkage
(Fig. 1). A concentration of 5 vol.% hydrogen in the mixture
should be considered as a critical level, above which no sintering shrinkage occurs. However, shrinkage observed in the lower
hydrogen content atmospheres suggests that hydrogen does not
concentrate preferentially on the Al surface and its amount is
not high enough to hinder totally the reaction between nitrogen
and Al, producing shrinkage. Even a small amount of hydrogen in the sintering atmosphere nevertheless lowers the rate of
Al nitride formation, which results both in a longer shrinkage
incubation time and a lower shrinkage, when compared with the
hydrogen free atmosphere.
255
It has been suggested [15] that the sintering process is
impeded by hydrogen due to the formation of gaseous Al
hydrides. As no liquid phase is available during sintering of
pure aluminium, however, no effect on pore filling can be proposed. In addition, hydrogen is also deleterious to the sintering
of aluminium in argon/hydrogen [15]. This suggests, that the
detrimental effect of hydrogen is not connected with the active
nitrogen atmosphere and the formation of AlN. Although the solubility of hydrogen in solid aluminium and its alloys is small,
the protons can be entrapped by lattice defects, e.g. vacancies,
dislocations, grain boundaries [24]. A coupling of hydrogen diffusion and vacancy migration was already proposed in [25] for
high temperatures. Recent studies [26] have shown, that the activation energy for vacancy diffusion is increased by 0.4–0.5 eV if
hydrogen is dissolved in aluminium. Details of the mechanism
are still under discussion, but the decrease of the self-diffusion
coefficient caused by this increase of the activation energy could
explain the slow-down of shrinkage if hydrogen enters the sintering atmosphere.
4. Conclusions
Pure and sufficiently dry nitrogen is the only active sintering shrinkage-producing atmosphere for pure aluminium.
The formation of aluminium nitride is thereby a key effect. A
direct reduction of Al2 O3 by gaseous nitrogen seems unlikely,
because magnesium, even in small amounts present in Al powder, concentrates on the surface of powder particles and enhances
sintering by local reduction of Al2 O3, prior to the incorporation
of nitrogen.
In contrast, hydrogen strongly counteracts the sintering
shrinkage of pure aluminium. A volume content of 5% hydrogen
in the nitrogen/hydrogen mixture seems to be the critical level,
above which no sintering shrinkage occurs or a running shrinkage stops by a change of the sintering atmosphere to N2 /5%
H2 .
Although this work contributes some additional observations
of the effect of the atmosphere on the solid state sintering of pure
aluminium, the development of an accepted mechanism for the
detrimental influence of hydrogen on the sintering response of
aluminium needs further investigations.
Acknowledgements
This work was supported by the Alexander von Humboldt
Foundation which is gratefully acknowledged.
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