©2009 Old City Publishing, Inc.
Published by license under the OCP Science imprint,
a member of the Old City Publishing Group
High Temperatures-High Pressures, Vol. 38, pp. 221–231
Reprints available directly from the publisher
Photocopying permitted by license only
Density and thermal expansion of the
aluminium alloy Al‑17Si‑4Cu (A390)
in the solid and liquid states*
E. Kaschnitz1,** and R. Ebner2
2
1
Österreichisches Gießerei-Institut, Parkstraße 21, 8700 Leoben, Austria
Materials Center Leoben Forschung GmbH, Roseggerstraße 12, 8700 Leoben, Austria
Received: August 25, 2008. Accepted:
December 4, 2008.
Density and thermal expansion of the aluminium alloy Al-17Si-4Cu
(A390) were measured in the temperature range from room temperature to
740ºC using pushrod and piston dilatometry. Commercial pushroddilatometers (NETZSCH DIL 402E and 402CD) were used for the measurements. The specimens are heated and cooled slowly at controlled rates
in a furnace; the expansion is transferred by one or two long thin rods to
displacement sensors. A graphite tubular body with two graphite pistons
of just sufficient clearance was used to contain the specimen in the mushy
region and in the liquid state.
During the growing of the primary silicon crystal network at solidification, the material does not shrink but expands slightly. Starting with the
aluminium-silicon eutectic at the eutectic temperature, the density incre­
ases rapidly to its solid state behaviour. The linear thermal expansion in
the solid state shows no peculiarities, but is significantly lower than those
of eutectic or hypoeutectic aluminium-silicon alloys.
Keywords: Aluminium alloy A390; Al-17Si-4Cu; density; liquid metal; piston
dilatometry; pushrod dilatometry; thermal expansion.
1 INTRODUCTION
The pushrod dilatometry technique is presently the most straightforward
but nevertheless a very reliable method to measure thermal expansion in
*Paper presented at the 18th European Conference on Thermophysical Properties, Pau, 2008.
**Corresponding author: [email protected]
221
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E. Kaschnitz and R. Ebner
the high-temperature range. The relative expansion of a specimen at high
temperature is transmitted by rods or tubes to a displacement sensor. Typically, linear-variable-differential-transformers (LVDT) are used as extensometer. The measured relative expansion of a specimen is compared to
an appropriate reference material in order to correct for thermal expansion
of the specimen holder as well as intrinsic temperature gradients of the
pushrod [1].
Measurement of thermal expansion of solid materials by pushrod dilatometry technique is an established technique today in academic and industrial
research. There are several commercial manufacturers of pushrod dilatometers; the measurement procedure including data processing is kept relatively
simple. However, thermal expansion measurements performed on liquid
specimens are considerably more difficult for several reasons, e.g., possible
specimen-container reactions, wetting problems, and convective heat loss
effects.
Accurate thermophysical property data of solid and liquid alloys are
urgently needed for the numerical simulation of industrial processes like casting. In recent years, liquid densities of some common aluminium alloys have
been measured by several research groups. Different techniques have been
used to measure density: an x-ray attenuation technique [2], an indirect Archimedean method [3], a shadow imaging technique applied on levitated drops
[4], a diffraction method to study atomic density changes [5], and piston
pushrod dilatometry [6, 7, 8].
However, there is still a lack of reliable thermophysical property data,
especially for newly developed alloys and mould materials. This information
(thermal diffusivity, thermal conductivity, heat capacity, latent heat of solidification, fraction solid, and density) is required to perform accurate simulations. The density change at solidification largely determines the shrinkage
behaviour of a casting; insufficient feeding of the shrinking alloy leads to
internal flaws like porosity. Uneven thermal shrinkage of a casting caused by
different local cooling conditions can lead to internal stress that even can
cause cracks.
This work presents results of density and thermal expansion measurements of the aluminium alloy Al-17Si-4Cu (A390) in the solid and liquid
states over the temperature range from room temperature to 740ºC. This alloy
is typically used for engine blocks and pistons for high performance racing
cars or luxury cars.
2 EXPERIMENTAL
All measurements were performed by two commercial pushrod dilatometers (Model DIL 402E and 402CD, Netzsch Gerätebau GmbH, Selb, Germany). The systems are equipped with high-temperature furnaces for
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Density and Thermal Expansion of A1-17Si-4Cu
223
dynamic and steady-state heating and cooling of the specimen, allowing
mea­sure­ments between room temperature and 1500ºC. The sample chambers are vacuum-tight; rotary vane pumps provide a vacuum of 10–3 mbar.
Alternatively, the experimental chamber can be filled with helium or argon
as an inert gas.
A double pushrod configuration was used for the solid specimens in the
temperature range from room temperature to 450ºC, the specimen holder and
pushrods are made from fused silica. The reference material for calibration of
the dilatometer is NIST SRM 736 (copper) [9], calibration was checked by
measuring platinum (purity 99.99%, supplied by ÖGUSSA, Vienna) and silicon (single crystal 99.999%, provided by Goodfellow Metals, Cambridge,
UK). The reference values of platinum are taken from [10], the values of silicon from [11]. The specimen chamber was purged with helium at a flow rate
of 50 ml·min–1.
For the high temperature range above 450ºC (solid, mushy and liquid
specimens), a single pushrod configuration was used; the specimen holder
and pushrod are made from alumina. The specimen itself was contained in a
tube shaped container with two pistons, all parts made of pure graphite. The
dimensions of the container are: outer diameter, 10.5 mm; inner diameter,
6.5 mm; length 22 mm. The dimensions of the cylindrically shaped pistons
are: diameter, 6.5 mm; length, 8 mm. The container and pistons are very precisely machined; the clearance between container and piston is in the order of
several micrometers.
It should be noted that linear thermal expansion is measured as long as the
specimen is solid. When the specimen collapses in the melting region, the
pistons move into the graphite container and the extensometer reading is correlated to the specific volume of the alloy. That means that the inside diameter
of the container has to be precisely known as function of temperature. This
diameter was measured at room temperature by a calibrated precision internal
bore micrometer (Tesa SA, Renens, Switzerland, Type Micro BAF1). The
temperature dependence of the graphite was determined by measuring a stack
of three pistons in the same dilatometer. From these data, the temperature
dependence of the inner diameter of the container was calculated as function
of temperature.
The reference material for the length calibration of the single pushrod
dilatometer with graphite container and pistons is NIST SRM 738 (austenitic
steel) in annealed condition [12]. The specific volume data was then calculated from the (temperature dependent) inner diameter of the cell and the
measured length. Tests of the measurement equipment and the measurement
procedure with specimens of aluminium (purity 99.999%, supplied by Goodfellow Metals, Cambridge, UK) were carried out to check the reliability of
the experiment. The obtained data for the thermal diffusivity are in good
agreement with the recommended values of [13]. The specimen chamber was
purged with argon at a flow rate of 100 ml·min–1.
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224
E. Kaschnitz and R. Ebner
3 MEASUREMENTS
The Al-17Si-4Cu (A390) was obtained from an aluminum foundry producing for the automotive industry (NEMAK GmbH, Linz, Austria). The specimens were cut from a cylinder head taken from the production after heat
treatment. The chemical composition, measured by ICP-spectroscopy is
given in Table 1. The density at room temperature was measured by an Archimedean balance (Sartorius AG, Göttingen, Germany, Type LA230S and
YDK01). The obtained density at 20ºC is (2710 ±5) kg·m-3. The melting and
solidification behaviour of the Al-17Si-4Cu (A390) alloy was determined by
differential thermal analysis with a heating and cooling rate of 5 K·min-1
(Model DSC 404, Netzsch Gerätebau GmbH, Selb, Germany). The solidus
temperature was obtained at 507ºC, the eutectic temperature at 562ºC, and
the liquidus temperature at 666ºC.
Measurements of the linear thermal expansion of the solid alloy were performed on six specimens in the temperature range from room temperature to
450ºC with the double pushrod dilatometer. The room temperature dimensions of the cylindrical specimens were: diameter, 6 mm; length, 25 mm. Four
of the specimens were repeatedly heated to temperatures of 100ºC, 200ºC,
300ºC and 400ºC, kept for one hour at these selected temperatures and subsequently cooled to the same temperature holding points. Two of the specimens
were heated repeatedly to the maximum temperature of 450ºC with a heating
rate of 2 K·min-1 and subsequently cooled with the same rate. The reproducibility of the measurements in the solid phase for an individual specimen is in
the range of 0.5% (standard deviation), and between different specimens, it is
between 0.5% and 1.2% at 400ºC and 100ºC, subsequently. It should be noted
that the length at room temperature changed slightly with the first heating
cycle (an increase in the order of 0.00008) due to micro-structural changes;
therefore, the results of the first heating/cooling cycle were discarded.
Measurements of the specific volume in the mushy and liquid states were
made on six individual specimens in the temperature range from 570 to 740ºC
with the single pushrod dilatometer. The room temperature dimensions of the
cylindrical specimens were: diameter, 6.4 mm; length, 10 mm. Each specimen was heated to 740ºC with a heating rate of 2 K·min-1, kept at maximum
temperature for 10 min, cooled at the same rate through the solidification
interval to 400ºC; then this heating/cooling cycle was repeated.
TABLE 1
Chemical composition of the Al-17Si-4Cu (A390) aluminium alloy in percent by mass.
Elements
Si
Cu
Mg
Fe
Mn
Zn
Ti
Cr
Ni
Pb
Sn
Al
17.86
4.32
0.47
0.23
0.09
0.06
0.11
<0.01
<0.01
<0.02
<0.02
bal.
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Density and Thermal Expansion of A1-17Si-4Cu
225
The linear thermal expansion of the solid state and the initial mushy phase
in the temperature range from 450ºC to approx. 550ºC was me­a­s­u­red during
the first heating cycle before the collapse of each specimen. These data connect very well to the results obtained by the double pushrod dilatometer and
extend the temperature range close to the eutectic temperature.
The reproducibility of the measurements of the specific volume in the
mushy and liquid states for an individual specimen is between 0.1% and
0.15% (standard deviation), and that between different specimens is 0.3%.
From the measured specific volume, density as a function of temperature was
calculated using the room temperature mass of the specimen. The mass of
each individual specimen was measured by a precision balance (Sartorius
AG, Göttingen, Germany, Type ED224ES) before and after the measurement.
The mass change during the experiment was found to be negligible. A typical
measurement of density during two heating and cooling cycles of an individual specimen is shown in Figure 1. When the specimen collapses during
the first heating cycle close to the eutectic temperature, the pistons move into
the tube-shaped graphite specimen holder. This gives a sharp rise in the apparent density (dotted line with steep rise at Teutectic) and the liquid specimen fills
entirely the container. At further heating (lower solid line), the specimen goes
2550
Density, kg.m-3
2540
piston movement stops
first collaps of the specimen
second collaps of the specimen
cooling
2530
heating
2520
2510
2500
500
Teutectic
Tsolidus
520
540
560
580
Tliquidus
600
620
640
660
680
700
720
740
Temperature, °C
FIGURE 1
Variation of (apparent) density as a function of temperature in the mushy and liquid regions for
a typical experiment on aluminium alloy Al-17Si-4Cu (A390). The specimen is heated and
cooled twice (solid lines represent valid measurement values; dots are artefact due to voids in the
specimen container).
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226
E. Kaschnitz and R. Ebner
through the mushy phase and finally into the liquid. At cooling (upper solid
line), the density in liquid phase follows a linear function. No difference
between heating and cooling was noticed in the obtained values for the liquid
state above approximately 700ºC. At cooling, the specimen goes through the
mush again, but at slightly higher values. A few degrees below the eutectic
temperature, the typical kink can be noticed and the pistons stuck. Below this
temperature, the apparent density values (dotted line) do not have real meaning, as the specimen container is not filled entirely anymore. The specimen
does not contract uniformly but solidifies forming voids usually in the middle
section. This can be seen after the experiment on cold specimens. The second
heating and cooling cycle reproduces the measured values very accurate.
4 RESULTS
The obtained results of linear thermal expansion in the temperature range
from room temperature to 550ºC of the aluminium alloy Al-17Si-4Cu (A390)
are shown in Figure 2. The values were partially fitted by a third order polynomial function using the least-squares method.
The variation of density of mushy and liquid Al-17Si-4Cu (A390) as a
function of temperature for all six specimens is shown in Figure 3. The obtained
0.012
Dilatometer (fused silica holder)
Dilatometer (alumina holder)
Solidus temperature
Eutectic temperature
Linear thermal expansion
0.010
0.008
0.006
0.004
0.002
0.000
0
100
200
300
400
500
600
Temperature, °C
FIGURE 2
Results for linear thermal expansion of the aluminium alloy Al-17Si-4Cu (A390) as a function
of temperature in the solid state and in the initial melting phase.
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Density and Thermal Expansion of A1-17Si-4Cu
227
2550
Density, kg.m-3
2540
2530
2520
2510
Teutectic
Tliquidus
2500
560
580
600
620
640
660
680
700
720
740
Temperature, °C
FIGURE 3
Measured (apparent) density of the aluminium alloy Al-17Si-4Cu (A390) as a function of temperature in the mushy and liquid regions (dotted lines) and least-squares fits to the measured
values (solid line).
data were fitted piecewise with linear functions using the least-squares method.
In the liquid state, the data of the heating cycle close to the liquidus point was
excluded due to considerable rounding of the results. In the mushy region, the
temperature range between 572ºC and 656ºC was used to compute the slope of
the linear fit function, which in turn was connected to the result of the liquid
state at 666ºC. As there is a slight difference between heating and cooling in
the obtained values in the mushy phase, mean values have been taken.
Figure 4 shows the volume expansion and density of Al-17Si-4Cu (A390)
in the entire measured temperature range; in Table 2 the linear thermal expansion, the volume expansion and the density are summarised. At the eutectic
temperature, volume expansion and density are extrapolated at one side from
the values in the initial melting state, at the other side from the mushy state.
5 UNCERTAINTIES
The uncertainty of linear thermal expansion measurement in the temperature
range from room temperature to 550ºC (solid state and onset of melting of the
alloy) was calculated as recommended in Ref. 14. An expanded uncertainty
(multiplied by a coverage factor of 2) of ±0.00004 at 100ºC and 0.00012 at
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228
E. Kaschnitz and R. Ebner
1.10
2750
Teutectic Tliquidus
Tsolidus
1.08
Density, kg.m-3
2650
1.06
2600
Solid line: Density
Dotted line: Volume Expansion
2550
1.04
2500
Volume Expansion
2700
1.02
2450
2400
1.00
0
100
200
300
400
500
600
700
Temperature, °C
FIGURE 4
Results for density (solid line) and volume expansion (dotted line) of the aluminium alloy Al-17Si-4Cu (A390) as a function of temperature in the solid and liquid states.
TABLE 2
Experimental results of linear thermal expansion, volume expansion and density of the Al-17Si4Cu (A390) aluminium from room temperature up to the liquid state.
Temperature
Linear thermal
expansion
Volume
expansion
Density
(ºC)
(-)
(-)
(kg·m-3)
20
0.00000
1.0000
2710
100
0.00147
1.0044
2698
150
0.00247
1.0074
2690
200
0.00352
1.0106
2682
250
0.00461
1.0139
2673
300
0.00573
1.0173
2664
350
0.00686
1.0207
2655
400
0.00801
1.0242
2646
450
0.00916
1.0277
2637
500
0.01029
1.0312
2628
a
507
0.01045
1.0317
2627
507a
0.01096
1.0332
2623
550
0.01150
1.0349
2619
562
0.01165
1.0354
2617
b
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Density and Thermal Expansion of A1-17Si-4Cu
229
Table 2. Continued.
Temperature
Linear thermal
expansion
Volume
expansion
Density
(ºC)
(-)
(-)
(kg·m-3)
562c
1.0732
2525
580
1.0729
2526
600
1.0727
2526
620
1.0724
2527
640
1.0721
2528
660
1.0718
2528
666d
1.0718
2529
680
1.0736
2524
700
1.0762
2518
720
1.0789
2512
740
1.0816
2506
Solidus temperature;
Eutectic temperature, volume expansion and density values extrapolated from the solid;
c
Eutectic temperature, volume expansion and density values extrapolated from the liquid;
d
Liquidus temperature.
a
b
500ºC is obtained in this temperature range. This leads to an expanded uncertainty of ±0.00012 at 100ºC and 0.00036 at 500ºC for the volume expansion.
The expanded uncertainty for the density in the solid state is mainly determined by the uncertainty of the room temperature value and is estimated to
be ±5 kg·m-3 at 100ºC and ±6 kg·m-3 at 500ºC.
A detailed analysis of the uncertainty of density measurement at 700ºC is
given in Table 3. It follows in principle the work of Morrell and Quested [7]
and yields an expanded uncertainty of ±0.8%. This uncertainty varies only a
little over the entire mushy and liquid states of the alloy. By an analogous
calculation, an expanded uncertainty of ±0.008 is estimated for the volume
expansion in the mushy and liquid states (omitting the uncertainty of mass at
room temperature which has no influence on volume expansion).
6 DISCUSSION AND CONCLUSIONS
Considering the amount of literature data on aluminium alloys, there is little
information on thermophysical properties in the liquid range, and data of
Al-17Si-4Cu (A390) does not appear to have been obtained. Published work
by Magnusson and Arnberg [3] reports results of density measurements on
hypoeutectic liquid aluminium-silicon alloys. They derived a linear regres-
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230
E. Kaschnitz and R. Ebner
TABLE 3
Sources of uncertainty for density at 700ºC.
Uncertainty
Probability
distribution
Divisor
ci
Density
uncertainty
(%)
Source
(%)
Specimen mass
0.05
rectangular
1.73
1
0.03
Calibration internal bore micrometer
0.03
rectangular
1.73
2
0.03
Reproducibility measurement inner
diameter
0.08
normal
2
2
0.08
Measurement expansion graphite cell
0.05
rectangular
1.73
2
0.06
Standard reference material
0.01
rectangular
1.73
1
0.01
Reproducibility length measurement
reference
0.05
normal
2
1
0.03
Reproducibility length measurement
specimen
0.10
normal
2
1
0.05
Dilatometer drift
0.01
normal
2
2
0.01
Deviation from cylindrical shape
0.35
normal
2
1
0.18
Temperature measurement
0.20
normal
2
1
0.10
Measurement repeatability
0.12
normal
1
1
0.12
Measurement reproducibility
0.30
normal
1
1
0.30
Total combined uncertainty
normal
0.40
Expanded uncertainty
normal
0.80
sion for density at 700ºC as a function of silicon content in the range from 0
to 12%. Extrapolation of this regression to the silicon content of this work
gives very different values and can not be considered to be valid. This shows
that the density of hypoeutectic aluminium-silicon alloys is not comparable
to those of the hypereutectic state.
A simple mixing rule of atomic volumes and masses of the constituting
elements as given by Smith et al. [2] was used to compute the density of
Al-17Si-4Cu (A390) in the liquid state. The density ρ is calculated as a function of T from their relative fractions xi of Al, Si, and Cu
ρ=
∑x M ,
∑ x V (T )
i
i
(1)
i i
where Mi is the atomic mass and Vi the atomic volume. To compute the atomic
volume and mass, numerical values are taken from Iida and Guthrie [15], but
have to be extrapolated over a wide temperature range for the contributions of
silicon and copper. The results show an exceptional agreement with the measured values. This means the mixing rule can be applied, excess volumes due
to chemical effects might be small.
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Density and Thermal Expansion of A1-17Si-4Cu
231
It should be noted that the specific volume in the range between the liquidus point and the eutectic temperature during solidification is slightly increasing. That means that the growing primary silicon crystal network expands
more than the remaining liquid melt shrinks. Starting with the aluminiumsilicon eutectic at the eutectic temperature, the specific volume decreases rapidly to the solid state value. The volume change at the eutectic temperature is
approximately 3.8%. The small volume change at the solidus temperature is
probably due to a secondary silicon-copper reaction.
ACKNOWLEDGMENTS
This work was partially supported within the “Kplus”, the “COMET”, as well
as the “basis” programs by the Österreichische Forschungsförderungsgesellschaft mbH (FFG), the Province of Styria, the Steirische Wirtschaftsförderungsgesellschaft mbH (SFG), and the Municipality of Leoben, and parts of the
work were also co-financed by the European Regional Development Fund.
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