Acta Materialia 52 (2004) 3019–3025
www.actamat-journals.com
On the role of magnesium and nitrogen in the infiltration
of aluminium by aluminium for rapid prototyping applications
T.B. Sercombe, G.B. Schaffer
*
Division of Materials, School of Engineering, The University of Queensland, Frank White Building, Brisbane, Qld. 4072, Australia
Received 10 December 2003; received in revised form 26 February 2004; accepted 2 March 2004
Available online 2 April 2004
Abstract
Selective laser sintering has been used to fabricate an aluminium alloy powder preform which is subsequently debound and
infiltrated with a second aluminium alloy. This represents a new rapid manufacturing system for aluminium that can be used to
fabricate large, intricate parts. The base powder is an alloy such as AA6061. The infiltrant is a binary or higher-order eutectic based
on either Al–Cu or Al–Si. To ensure that infiltration occurs without loss of dimensional precision, it is important that a rigid
skeleton forms prior to infiltration. This can be achieved by the partial transformation of the aluminium to aluminium nitride. In
order for this to occur throughout the component, magnesium powder must be added to the alumina support powder which
surrounds the part in the furnace. The magnesium scavenges the oxygen and thereby creates a microclimate in which aluminium
nitride can form. The replacement of the ionocovalent Al2 O3 with the covalent AlN on the surface of the aluminium powders also
facilitates wetting and thus spontaneous and complete infiltration.
Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Aluminium; Solid freeform processes; Liquid infiltration; Nitrides
1. Introduction
Rapid prototyping is a manufacturing methodology
in which energy and/or material is delivered to a point to
produce a solid. A series of lines are then traced out to
make a layer and a series of layers formed to make a
three-dimensional part. Parts of any shape can be produced directly from a computer solid model without the
use of a tool or die. Many materials have now been
fabricated this way. There is substantial demand for an
aluminium system, particularly from the automotive industry, because it is both a large user of rapid prototyping and it is a major, growing user of aluminium.
Indirect rapid prototyping of aluminium components is
possible but typically requires the production of a lost
wax model and subsequent investment casting. While
this uses rapid prototyping techniques, it is not a true
rapid prototyping technology because it still requires the
*
Corresponding author. Tel.: + 61-7-3365-4500; fax: + 61-7-33653888.
E-mail address: g.schaff[email protected] (G.B. Schaffer).
fabrication of a mould. Aluminium tooling has been
produced from tape by ultrasonic consolidation [1],
while parts have been produced from powder by extrusion free form fabrication [2,3] or laser sintering [4–6].
These powder parts were fabricated as a polymer/aluminium composite and post-processed by burning out
the polymer and sintering of the remanent metal powder
to full or near-full density, in a manner similar to that
used in powder injection moulding. However, it is extremely difficult to maintain dimensional accuracy during sintering of such a powder preform because of
density gradients in the green part and geometrical
constraints. While uniform shrinkage can be incorporated into the initial model, non-uniform shrinkage is
more difficult to control reproducibly and to accommodate by design. Because dimensional accuracy is a critical
criterion for any rapid prototyping system, the inability
to accurately sinter large parts is fatal. Only small aluminium parts can be made this way: the limit is 1 cm3 .
The dimension problem is much reduced by infiltration. Here, a loosely formed powder body is lightly
pre-sintered and the porous mass is subsequently
1359-6454/$30.00 Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.actamat.2004.03.004
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T.B. Sercombe, G.B. Schaffer / Acta Materialia 52 (2004) 3019–3025
Turbula mixer for 30 min. All compositions are given in
wt% unless stated otherwise. The powder details are
provided in Table 1. During the initial stages of this
work, parts were manufactured by heating the powder
mixture in a small mould to 220 °C for 30 min. This
melted the nylon powder resulting in a resin-bonded
green body that simulated processing by rapid protoTable 1
Details of the powders used
Fig. 1. Hypothetical thermal cycles for the post-processing of steel and
aluminium preforms. There is a very small process window for aluminium which complicates alloy design.
infiltrated by a liquid at a temperature between the
melting point of the infiltrant and the base metal. Because there is so little sintering, there is negligible dimensional change between the preform and the finished
part. Numerous systems have been fabricated by the
rapid prototyping-infiltration route to date, including
stainless steel–bronze [7–9], mullite–aluminium [10],
ZrB2 –Cu [11] and SiC–Mg [12]. An aluminium-based
system is a conspicuous omission. There are two factors
specific to aluminium which render the development of
an infiltration system particularly problematical. The
first is related to the oxide film present on the surface of
all metal powders. Because of the thermodynamic stability of aluminium sesquioxide, it is difficult to remove
or avoid. The second problem arises from the relatively
low melting point of aluminium. There is then a narrow
infiltration window between the burnout of the polymeric resin which is used to bind the powder into the
required shape and the melting of the aluminium matrix.
This is illustrated in Fig. 1.
We have recently described a new infiltration route
which facilitates the direct rapid manufacturing of aluminium [13]. It involves the formation of an unconstrained, resin-bonded aluminium powder part, the
burnout of the resin, the partial transformation of the
aluminium into a rigid aluminium nitride skeleton by
reaction with the atmosphere under a magnesium/alumina blanket and the subsequent infiltration with a
second aluminium alloy. Here, we provide a detailed
description of the formation of the skeleton and an
analysis of both the role of magnesium in the gettering
blanket and the need for nitrogen.
2. Experimental
The feed stock was prepared by mixing pre-alloyed
AA6061 powder, 2% Mg and 4% (10 vol%) nylon in a
Powder
Composition
Particle size
(lm)
Source
Al
AA6061
Mg
Nylon
Al2 O3
99.7% pure
Al–1Mg–0.8Si–0.3Cu–0.1Cr
99.6% pure
Co-polyamide 6–12
99.9% pure
)75 + 15
)75 + 15
)45
10
)63
Alpoco
Alpoco
Alpoco
Atofina
Reade
Alpoco: The Aluminium Powder Company Ltd., Forge Lane,
Minworth, Sutton Coldfield, B76 1AH, UK.
Atofina: Atofina Chemicals, Inc. 2000 Market St., Philadelphia, PA
19103, USA.
Reade: Reade Advanced Materials, Post Office Drawer, 12820
Reno, NV 89510-2820, USA.
Fig. 2. The laser sintered structure. The dark background is the nylon
binder, the white particles are aluminium and the textured particle in
the centre of the image is Mg.
Fig. 3. Schematic illustration of the infiltration configuration. The part
to be infiltrated is placed in contact with a tab of the same material. A
block of solid infiltrant is placed at the other end of the tab and the whole
assembly is placed in a crucible and covered with a support powder.
T.B. Sercombe, G.B. Schaffer / Acta Materialia 52 (2004) 3019–3025
typing techniques. The green microstructure is shown in
Fig. 2. Parts were also produced using Selective Laser
Sintering (SLS) on a SinterStation 2500+ machine using
a part bed temperature of 120 °C, feed temperatures of
60 °C and laser power of 28 W. The shaped part was
placed in a crucible with a mass of solid infiltrant at one
end and covered with a support powder that consisted of
either pure alumina or alumina powder with 1%Mg
mixed with it. The infiltration configuration is shown
schematically in Fig. 3. Four infiltrants were used: Al–
12Si, Al–14.7Si–4.3Mg, Al–10.5Si–10Zn–5.5Ni and Al–
33Cu. These were all close to eutectic compositions, with
melting points of approximately 580, 560, 550 and 548
°C, respectively. Infiltration was performed in a controlled atmosphere horizontal tube furnace. The parts
were heated under flowing nitrogen at 90 °C/min and
then isothermally treated at 540 and/or 570 °C (590 °C
for Al–12Si) for up to 6 h at each temperature. Parts
were then furnace cooled to below 300 °C before being
removed from the furnace.
Samples for metallographic examination were
mounted in epoxy and polished using standard techniques. They were left unetched and examined either
optically or in a Phillips XL30 scanning electron microscope with a LaB6 filament and fitted with an Oxford
energy dispersive X-ray spectrometer (EDS) detector.
Thermogravimetric analysis was performed on a Netszch 409 STA using a heating rate of 5 °C/min. Approximately 250 mg of AA6061–2%Mg powder was
evacuated to below 5 102 mbar and back-filled with
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either high purity Ar or N2 , after which a flow of 60 cm3 /
min was used for the remainder of the test.
3. Results and discussion
Dimensional stability is extremely difficult to maintain during infiltration of a freestanding part (i.e., one
Fig. 4. Optical micrograph of a AA6061 sample infiltrated with an Al–
14.8Si–4.2Mg alloy. The sample was heated to 570 °C and held for 6 h.
Without prior skeleton formation, the original aluminium powder
particles are destroyed, grain growth results and the microstructure is
reminiscent of a sintered material.
Fig. 5. Optical micrographs showing the development of a percolating skeleton in 6061-xMg powder after 6 h at 540 °C: (a) 0 Mg, (b) 1 Mg, (c) and
(d) 2 Mg. The image in (d) is post-infiltration by Al–14.8Si–4.2Mg at 570 °C, showing that the original Al particle size can be preserved if a skeleton
forms prior to infiltration.
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T.B. Sercombe, G.B. Schaffer / Acta Materialia 52 (2004) 3019–3025
not confined within a mould) if a rigid skeleton does not
form prior to infiltration or if the skeleton is attacked by
the liquid infiltrant. The problem is apparent in Fig. 4,
which shows the microstructure of a 6061 preform infiltrated with Al–14.8Si–4.2Mg at 570 °C for 6 h. Although spontaneous infiltration has occurred, there is
excessive grain growth and the structure is typical of a
sintered material. There was little dimensional stability
as a consequence.
A rigid skeleton can form if there is a thermal arrest
at 540 °C before progressing to the infiltration temperature, Fig. 5. It is also apparent from these micrographs
that adding magnesium to the aluminium powder increases the amount of skeleton which forms and this
skeleton prevents grain growth during infiltration. The
skeleton also coarsens over time, Fig. 6. Thermogravimetry of an AA6061 alloy powder with 2%Mg at 540 °C
showed a minimal weight gain under Ar but a 16% increase in weight when processed with identical conditions under N2 . That both gases had similar oxygen
contents suggests that the weight gain observed under
N2 is not due to oxidation but may be due to nitridation.
The EDS maps in Fig. 7 indicates that the skeleton
contains both aluminium and nitrogen, suggesting that
it may indeed be AlN.
The high thermodynamic stability of Al2 O3 means
that it will form in preference to AlN unless the oxygen
partial pressure is exceedingly low [14]. This requires the
use of an oxygen getter. It has recently been shown that
lightly compacted aluminium powder can act as its own
getter when sintered under flowing nitrogen at 620 °C
[15]. Others have shown that direct nitridation can occur
at lower temperatures but requires either a magnesium
containing alloy [16,17] or a pressurised nitrogen atmosphere [18] or an external magnesium getter [19,20].
Aluminium nitride has also been observed in the infiltration of alumina by aluminium when a magnesium
getter is used [21,22].
The rate of nitride growth increases with both time
and temperature, as shown in Fig. 8. Here, it is assumed
that the nylon binder is completely removed and the
weight gain is solely a result of nitride formation. At
540 °C, the initial rate is reasonably slow, but accelerates
after approximately 4 h. The nitride growth rate is faster
at 570 than 540 °C, while a 2-h hold at 540 °C prior to
the hold at 570 °C has no effect on the rate of growth.
There is, however, an increase in the quantity of nitride
that forms. That is, after 2 h at 540 °C, the overall
weight gain is 2%. After increasing the temperature to
570 °C and holding for a further 2 h, the weight gain is
8.5% (i.e., an increase of 6.5%). Without the hold at
540 °C, the weight gain after 2 h at 570 °C is only 5%.
Thus, the intermediate hold has increased the weight
gain above that of the sum of the individual holds. This
suggests that there is an incubation period prior to the
formation of the nitride which may be a consequence of
Fig. 6. The growth of the nitride in AA6061–2Mg after: (a) 0 h, (b) 4 h
and (c) 6 h at 540 °C.
the time required to reduce the oxygen content in the
atmosphere to a sufficiently low level that nitride growth
is possible. Thus, when using a dual hold, the oxygen
depletion period does not exist for the second temperature, and therefore the nitride growth can occur
continuously.
The physical location of the Mg and the quantity
used affects the extent of infiltration, Fig. 9. In Fig. 9(a),
where there is no magnesium present anywhere in the
T.B. Sercombe, G.B. Schaffer / Acta Materialia 52 (2004) 3019–3025
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Fig. 7. SEM-EDS maps from infiltrated AA6061–2Mg/Al–14.8Si–4.2Mg showing the AlN skeleton.
macroscopic yellow magnesium nitride deposit is often
apparent [25], it has been suggested [26,27] that infiltration is dependant on a reaction between Mg and N to
form magnesium nitride:
3Mg þ N2 ! Mg3 N2
ð1Þ
which then reacts with the aluminium to form aluminium nitride:
Mg3 N2 þ 2Al ! 2AlN þ 3Mg
Fig. 8. The effect of time and temperature on the weight gain due to
nitride growth of AA6061–2Mg powder. All data points are included,
the line is the best fit through the median values and is used as a guide
to the eye. The heating rate between 540 and 570 °C was 1.5 °C/min.
See text for further details.
system, some infiltration has occurred (lower right corner of Fig. 9(a)). However, the infiltrant has not penetrated all the way to the edge of the sample, where the
aluminium powders are oxidised and there is no AlN.
Pre-alloying the Mg into either the base powder, as in
Fig. 9(c), or into the infiltrant as in Fig. 9(g) or admixing elemental Mg powder to the 6061 base powder as
in Fig. 9(e), has marginal impact. Adding Mg into all
three locations simultaneously also has little effect,
Fig. 9(i). However, adding a small (1%) amount of
magnesium powder to the alumina support powder has
a significant effect and facilitates infiltration to the very
edge of the sample, Fig. 9(b), (d), (f), (h) and (j).
Based on the observation that Mg is required to infiltrate alumina with aluminium [23,24] and that a
ð2Þ
In this scenario, the nitrogen is a catalyst for the distribution of magnesium. However, Mg has a higher
vapour pressure than Mg3 N2 [28] and nitrogen is
therefore not required for vapour transport of the
magnesium [15]. The results presented in Fig. 9(a),
where no magnesium is present anywhere in the system
but where some, albeit limited, infiltration occurs, suggests that the magnesium is not essential to infiltration.
It does facilitate complete infiltration but it is not a requirement for infiltration.
An alternative explanation for the role of the magnesium is that it acts simply as an oxygen scavenger. The
Mg in the support powder removes the oxygen from the
air trapped in the interstices between the alumina particles and also from the fresh nitrogen gas that impinges
on the surface of the sample. The difference between
infiltrating alumina and infiltrating aluminium is that
the aluminium can self-getter. This reduces the oxygen
partial pressure without the need for magnesium. Alumina, however, is not self-gettering and therefore magnesium is required for infiltration. In both cases, any
residual magnesium will subsequently react with the
nitrogen to form magnesium nitride as per reaction (1),
which will ultimately decompose by reaction with water
vapour on removal from the furnace:
Mg3 N2 þ 3H2 O ! 2NH3 þ 3MgO
ð3Þ
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T.B. Sercombe, G.B. Schaffer / Acta Materialia 52 (2004) 3019–3025
Fig. 9. Optical micrographs of Al or AA6061 powder with and without 2Mg infiltrated with either Al–12Si, Al–10.5Si–10Zn–5.5Ni or Al–14.7Si–
4.3Mg under a blanket of either Al2 O3 or Al2 O3 –1wt%Mg. Some infiltration occurs in each case but complete infiltration only occurs when the
alumina blanket is doped with magnesium powder.
which accounts for the strong odour of the freshly infiltrated samples. The magnesium nitride is simply a byproduct. Given an absence of oxygen, the magnesium
will react with nitrogen but it is not a necessary precursor reaction to infiltration.
In contrast to the formation of magnesium nitride,
the formation of aluminium nitride is critical to successful infiltration. It has been reported [29] that the
contact angle of Al on AlN is 41° at 1100 °C, which is
half that of Al on Al2 O3 . It is, therefore, important to
form AlN throughout the part if infiltration is to occur
everywhere. Hence, the role of the nitrogen maybe that
of a wetting agent in that it facilitates the formation of
AlN [30,31]. Without Mg in the alumina support blanket, the outer areas of the preform become heavily oxidised and do not nitride. These areas do not infiltrate
because they are not wet by the infiltrant. It is not sufficient to have Mg admixed into the preform, or prealloyed into the base powder or the infiltrant.
The curves of Fig. 8 can also be used to estimate the
time it takes to infiltrate a part. By measuring the net
weight gain of the infiltrated part (i.e., final mass of the
part less initial mass of infiltrant) and comparing it to
the respective curves in the figure, an estimate of the
T.B. Sercombe, G.B. Schaffer / Acta Materialia 52 (2004) 3019–3025
infiltration time can be made. For example, the specimen labelled 1, which had an original mass of 2.7 g was
sintered for 2 h at 540 °C and infiltrated for 4 h at
570 °C. It has a net weight gain of 5.5% which corresponds to an infiltration time of 1 h.
4. Conclusions
A new rapid manufacturing process for the production of aluminium components has been developed in
which a porous, resin-bonded green part is first formed
by a rapid prototyping technique such as selective laser
sintering and then debound and infiltrated during a
single furnace cycle. A key factor for the maintenance of
dimensional control and hence successful processing is
the development of a rigid skeleton that provides
structural support during the infiltration stage. This is
achieved through the partial transformation of the aluminium powder into a percolating AlN structure by
reaction with a nitrogen atmosphere. The AlN also
improves the wettability and facilitates spontaneous infiltration. The AlN forms when the oxygen partial
pressure is exceedingly low. This can be achieved
through self-gettering by the aluminium powder. While
magnesium is not necessary for infiltration, full infiltration only occurs under an alumina blanket to which
magnesium powder is added. Adding magnesium to the
aluminium powder or the infiltrant is beneficial but not
sufficient.
Acknowledgements
This work was supported by 3D Systems, Inc., and
The Aluminium Powder Company Ltd.
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