Estimation Of Cooling Rates In Suction Casting And Copper-Mould Casting Processes

Abstract

The cooling rates associated with suction and copper-mould casting of ø2, ø3 and ø5 mm rods made in Fe-25wt%Ni and Al-33wt%Cu alloys were determined based on their cellular and lamellar spacings, respectively. The work showed that the temperature profile in cylindrical samples can not be determined merely by microstructural examination of eutectic sample alloys. A concave solidification front, as a result of eutectic transformation, caused decrease of a lamellar spacing while approaching to the rod centre. The minimum axial cooling rates, estimated based on the cellular spacing in the Fe-25wt%Ni alloy, were evaluated to be about 200 K/s for both ø2 and ø3 mm and only 30 K/s for the ø5 mm suction cast rods. The corresponding values were slightly lower for the copper-mould cast rods.

Full text

ARCHIVES OF METALLURGY AND MATERIALS
Volume 60 2015 Issue 2
DOI: 10.1515/amm-2015-0204
T. KOZIEŁ∗,]
ESTIMATION OF COOLING RATES IN SUCTION CASTING AND COPPER-MOULD CASTING PROCESSES
OSZACOWANIE SZYBKOŚCI CHŁODZENIA STOPÓW W METODACH SUCTION CASTING ICOPPER-MOULD CASTING
The cooling rates associated with suction and copper-mould casting of ø2, ø3 and ø5 mm rods made in Fe-25wt%Ni
and Al-33wt%Cu alloys were determined based on their cellular and lamellar spacings, respectively. The work showed that
the temperature profile in cylindrical samples can not be determined merely by microstructural examination of eutectic sample
alloys. A concave solidification front, as a result of eutectic transformation, caused decrease of a lamellar spacing while
approaching to the rod centre. The minimum axial cooling rates, estimated based on the cellular spacing in the Fe-25wt%Ni
alloy, were evaluated to be about 200 K/s for both ø2 and ø3 mm and only 30 K/s for the ø5 mm suction cast rods. The
corresponding values were slightly lower for the copper-mould cast rods.
Keywords: suction casting, copper mould casting, cooling rate, cellular solidification; eutectic solidification
Na podstawie analizy wielkości dendrytów komórkowych w stopie Fe-25Ni i odległości międzypłytkowych w stopie
Al-33Cu zostały oszacowane szybkości chłodzenia w trakcie odlewania stopów metodami suction casting i copper-mould
casting. Badania wykazały, że rozkład szybkości chłodzenia w cylindrycznych próbkach nie może być oszacowany w stopach
z krystalizacją eutektyczną. W tym przypadku bowiem dochodzi do zmniejszania odległości międzypłytkowej w miarę zbliżania
się do osi pręta, ze względu na wklęsły charakter frontu krystalizacji.
Minimalna szybkość chłodzenia w osi prętów odlanych za pomocą metody suction casting, wyznaczona w oparciu
o pomiary wielkości dendrytów komórkowych w stopie Fe-25wt%Ni, wyniosła ok. 200 K/s dla stopów o średnicy ø2 i ø3 mm,
i tylko 30 K/s dla stopów o średnicy ø5 mm. W przypadku stopów odlanych metodą copper-mould casting oszacowane wartości
były nieznacznie mniejsze.
1. Introduction
The critical cooling rate Rc, required to hinder the crys-
tallization process, depends mostly on the alloy composition.
The first metallic glass, reported in 1960, was made of Au-Si
binary system with a cooling rate in the range of 106to 107
K/s [1]. High cooling rates limited the thickness of the first
synthesized metallic glasses to several microns. Since then,
amorphous materials in larger sizes were made by improving
glass forming ability (GFA) at lower cooling rates and this
led to the development of bulk metallic glasses (BMG’s), with
thicknesses greater than 1 mm. The best glass former reported
up to date is the Pd-Cu-Ni-P system, with critical cooling rate
below 1 K/s [2,3]. BMG’s exhibit superior properties such as
a high elastic limit and strength or excellent soft magnetic
properties in the Fe-based systems [4,5]. However until de-
velopment of relatively low-cost CuZr-based alloys [6-8] the
widespread commercialization of BMGs was not possible.
Rcis effective indicator of GFA, but it is very difficult
to be measured accurately. Therefore several different para-
meters, based on the thermal analysis at constant heating rate
of the glassy alloy, has been proposed in order to infer the
relative GFA among BMG’s [9]. Formation of the amorphous
phase requires application of a casting technique that allows
reaching cooling rates above critical cooling rate.
Bulk metallic glasses can be fabricated with different
forms and shapes using various rapid solidification techniques,
e.g.suction casting, copper-mould casting and die pressure
casting [10-12]. Most of these processes utilize copper moulds
as a heat sink. In the suction casting method, an arc-melted
alloy is sucked into a copper mould, due to a negative pressure
in the mould relative to the main chamber. Moreover cooling
rate of suction-cast alloys depends on the casting tempera-
ture, interfacial heat transfer, mould temperature and mould
geometry or configuration [13].
The copper-mould casting process relies on induction
melting of the alloy in quartz crucible with small orifice in
the bottom, and using pressurised gas to eject the melt into
the cavity of a copper block.
In order to obtain a homogeneous glassy structure, the
cooling rate should be higher than Rcon the entire cross sec-
tion of the as-cast alloy. Thermocouples can only measure
moderate cooling rates on the surface of a cast [12]. Pyromet-
∗AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF METALS ENGINEERING AND INDUSTRIAL COMPUTER SCIENCE, AL. A. MICKIEWICZA 30, 30-059 KRAKÓW, POLAND
]Corresponding author: tkoziel@agh.edu.pl

768
ric measurements are also excluded if the melt is poured into
a casting form.
Alternatively, indirect methods based on the microstruc-
tural features of the as-cast alloys, can be used to esti-
mate cooling rates during solidification. However, the re-
sults obtained from microstructural features of the suction-cast
Fe-25Ni [14] and Al-33Cu [12] alloys were completely differ-
ent. In case of the suction-cast 2 and 4 mm rods, the cooling
rate was about of 5·103K/s close to the rod surface which was
sharply decreased to ∼102K/s in the centre due to a radial
cooling [14]. On the other hand in the Al-33Cu eutectic alloy,
no evidence for the radial cooling was noticed. The cooling
rate near the bottom of the 3 mm thick rod was in the range
of 50 to 220 K/s and decreased along the axis towards the
40 to 125 K/s [12]. Such discrepancies may result from dif-
ferent types of equipment and/or different suction parameters
(pressure difference, suction force etc.).
In this work the cooling rates were estimated by exam-
ining the microstructure of the suction cast and copper-mould
cast Fe-25wt%Ni and Al-33wt%Cu rods.
2. Experimental procedure
The Fe-25wt%Ni (hereinafter referred to as Fe-25Ni)
and Al-33wt%Cu (Al-33Cu) alloy ingots were synthesized by
arc melting of a mixture of elements (with 99.9% or high-
er purity) under Ti-gettered argon atmosphere. The ingots
were re-melted four times in order to ensure its homogeneity.
The Arc Melter AM (Edmund B¨
uhler GmbH) with a spe-
cial water-cooled suction casting unit was used. This unit is
adjusted to the copper plate, with hole in its central part, en-
abling formation of rods by suction of the liquid alloy into the
two-parts copper form.
The chamber was evacuated to 6·10−5mbar and then filled
with high purity argon up to 800 mbar. Two vacuum tanks
were evacuated to 1·10−4mbar and used to produce required
suction force. Three rods with 2, 3 and 5 mm diameter and
an identical length of 55 mm were produced.
The copper-mould cast rods (ø2 and ø5×55 mm) were
synthesized using Melt Spinner HV (Edmund B¨
uhler GmbH)
in which the 2-part copper form was mounted just below the
quartz crucible orifice, instead of the spinning wheel. The
chamber was evacuated to the pressure of about 10−4mbar
and then filled with argon to the pressure of 200 mbar. After
melting, the alloy was ejected through a round nozzle into the
cavity of the copper block, by applying a gas pressure of 1000
mbar into the quartz tube.
The Fe-25Ni and Al-33Cu microsections were ground,
polished and then etched in 2% Nital and Keller’s reagents,
respectively. Light microscopy (Leica DM LM) enabled obser-
vations up to a magnification of 1000x. More detailed investi-
gations were conducted in the Al-33Cu alloy, using scanning
electron microscope (FEI Nova NanoSEM 450).
In order to evaluate the superficial and axial cooling rates,
the microstructure analysis was carried on the near-to-surface
(<100 um depth) and in central part of rods at three position
from the rod bottom end, namely at x=5 mm (5 mm from
the end – “foot”), x=27.5 mm (middle) and x=50 mm (5 mm
from the top end – “head”), as shown in Figure 1.
Fig. 1. A suction-cast ø3×55 mm Fe-25Ni rod
For the Fe-25Ni alloy, the cooling rate εwas estimated
based on relationship between measured dendrite arm spacing
λ[14,15]:
ε= λ
B6!−1
n(1)
where:
B6– constant equal 60 µm(K/s)nfor the Fe-25wt%Ni
alloy [14,15],
n – constant equal 0.32 for the Fe-25Ni alloy [14,15].
In case of the eutectic Al-33wt%Cu alloy, the cooling
rate was estimated by measuring the interlamellar spacing.
The relationship between solidification front velocity v and
interlamellar spacing λwas determined from [12]:
v=K
λ2(2)
where:
K – the constant equals 27.5·10−12 cm3s−1obtained from
unidirectional solidification
experiments for the Al-Al2Cu eutectic system [12].
Cooling rate (ε) was estimated from the following equa-
tion [12]:
ε=λhf/cp(2v/R)(3)
where:
∆hf– latent heat,
cp– specific heat,
R – radius of cylinder.
3. Results and discussion
The Fe-25Ni alloy
Typical micrographs of the 3 mm diameter suction cast
and copper-mould cast rods, are shown in Figs. 2 and 3, re-
spectively. The cellular morphology, with most cells oriented
perpendicular to the rod surface, indicates, that radial heat
flux during solidification was dominant, independently of the
casting process. The observed cellular-dendritic structure in
the centre of the rods was due to high constitutional super-
cooling. The X-ray diffraction studies (not shown) revealed
the presence of two-phase structure; BCC martensite and FCC
austenite. Martensitic transformation takes place at concentra-
tions below 40 at% Ni and at high cooling rates [16].

769
Fig. 2. Cross-sectional microstructure of the suction cast ø3 mm
Fe-25Ni alloy at x=5 mm: a) near the rod surface, b) in the rod axis
(light microscopy, 1000x)
Fig. 3. Cross-sectional microstructure of the copper-mould cast ø3
mm Fe-25Ni alloy at x=5 mm: a) near the rod surface, b) in the rod
axis (light microscopy, 1000x)
The results of microstructure investigations are summa-
rized in Tables 1-2 and in Fig. 4. The cell widths close to the
rod surface (<100 µm depth), are about 3-4 µm. The corre-
sponding superficial cooling rates are higher than 1000 K/s
(Fig. 4a). In this range, the cooling rate strongly depends on
the cell width. The increase in interlamellar spacing λfrom
2,7 to 3.9 µm indicated a reduction in the cooling rate from
15710 K/s to 1380 K/s.
However, in synthesising BMGs, lowest cooling rate
across profile is most important factor. A mean cellular spac-
ing, measured in the rod axis in the suction cast Fe-25Ni rods
of diameter 2 and 3 mm, are of about 10 µm with no signif-
icant change along the rod (Table 1). These values represent
a cooling rate of about 200 K/s. On the other hand, for the
5 mm diameter suction cast rod significant increase of mean
cell width led to much lower cooling rates in the range of
30-50 K/s (Fig. 4b).
TABLE 1
Results of cellular spacing measurements in Fe-25wt%Ni suction
cast rods
Distance
from the
foot (x),
mm
Suction cast Fe-25Ni - cellular spacing (λ), µm
ø2 mm ø3 mm ø5 mm
Surface Axis Surface Axis Surface Axis
5 2.9±1.5 9.4±3.4 4.3±1.6 10.8±3.9 4.8±1.7 17.1±5.6
27.5 3.3±1.4 10.9±4.5 4.2±1.4 10.6±3.3 4.3±2.3 16.5±5.7
50 2.8±1.3 10.7±4.6 4.7±1.8 11.1±3.3 5.5±2.2 19.8±7.3
In case of the copper-mould casting process, the mould
was not fully filled with molten alloy and shrinking-induced
holes close to the rod head were formed. The axial cooling
rates of the copper-mould alloys are lower than those of the
suction cast. In case of the 5 mm rods, at distance x =5 mm,
the lowest estimated cooling rate was 11 K/s (Fig. 4b). How-
ever, close to the rod surface the cooling rates was higher
compared to the suction casting process (Fig. 4a).
TABLE 2
Results of cellular spacing measurements in Fe-25wt%Ni
copper-mould cast rods
Distance
from the
foot (x),
mm
Copper-mould cast Fe-25Ni -
cellular spacing (λ), µm
ø2 mm ø5 mm
Surface Axis Surface Axis
5 3.9±1.8 13.3±4.5 3.8±2.0 27.4±7.6
27.5 3.4±1.8 10.4±4.2 5.9±2.9 20.4±7.2
50 2.7±1.6 hole 5.9±2.8 hole
Fig. 4. Estimated a) superficial and b) axial cooling rates based on
observed cellular spacing in Fe-25Ni alloy
The Al-33Cu alloy
Microstructures of the 3 mm diameter suction cast
Al-33Cu rod, close to the surface and at axis, are shown in Fig.
5 in which typical lamellar eutectic morphologies, composed
of the α-Al and CuAl2phases, can be seen. The mean values
of interlamellar spacing λclose to the rod axis at positions
x=5 mm, 27.5 mm and 50 mm were 111.9 nm, 122.3 nm and
102.4 nm, respectively. Based on the Eq. (3) the corresponding
cooling rate should be about 643.3 K/s, 539.1 K/s and 770.0
K/s. Unexpectedly, in the rod axis the lamellar spacing was
smaller compared to those observed on the surface. For 3 mm
diameter suction cast rod, the λwas estimated to be about

770
90.0 nm, 105.5 nm and 85.1 nm at x=5, 275 and 50 mm,
respectively.
Fig. 5. Microstructures of ø3 mm Al-33Cu rod suction-cast at x
=27.5 mm: a) close to the rod surface, b) in the rod axis (SEM,
30000x). Inset shows a schematic illustration of lamellar elimination
caused by a concave perturbation of the eutectic front [17]
Corresponding superficial and axial cooling rates in the
3 mm diameter suction cast Al-33Cu alloy are presented in
Fig. 6. These results indicate than the axial cooling rate is
higher than cooling rate on the surface, hence radial heat trans-
fer could not be the dominant mechanism of heat removal.
Fig. 6. Estimated superficial and axial cooling rates in Al-33wt%Cu
(ø3×55 mm) suction cast alloy
Estimation of cooling rates based on the microstructural
features in Fe-25Ni and Al-33Cu alloys led to inconsistent re-
sults, despite using the same casting process. Results obtained
for the Al-Cu alloy indicated that the interlamellar spacing,
controlled by the magnitude of undercooling, was also influ-
enced by other factors. Karma and Plapp [17] have explained
a lamellar elimination due to a concave perturbation of the
eutectic front, as shown in Fig. 5b (inset). The envelope of
the composite interface is shown as a dashed line that passes
through trijunctions. The arrows denote a motion of the tri-
junctions of the central βlamella normal to this envelope and
lateral motion of the junctions in the direction of increasing
spacing, respectively. For the lamellae growing locally perpen-
dicular to the envelope of the eutectic front, lateral displace-
ment of the trijunctions to the local slope of the envelope of
the eutectic front occurs. This means that the lamellar spacing
decreases in the concave region of the envelope. The smaller
lamellar spacing cause the front temperature to drop further
and hence finer lamellar structure is formed [17].
The main outcome of this work is that the eutectic alloys
should not be used to estimate the cooling rate or temperature
profile across cylindrical samples. In the Ref. [12], the authors
have estimated the cooling rate along the axis of samples, but
did not consider the radial cooling rates.
The Fe-25Ni alloy exhibited cellular solidification, with
most cells oriented perpendicular to the rod surface. Estimat-
ed superficial cooling rates are much higher than those in the
rod axis, which confirms that the radial cooling is dominant
during suction casting process. The cooling rate estimated in
this work is consistent with the published values [14].
In order to compare suction casting and copper-mould
casting processes, some factors must be taken in account. First-
ly, in the former the sample was arc melted, while in the latter
induction heating was used. Thus, the casting temperature and
heat content are higher in the suction casting method than in
the copper cast samples. On the other hand, the heat removal
strongly depends on the equipment setup. Secondly, in case
of the copper-mould casting the heat was removed from the
alloy only through the copper, while in the suction casting, a
water-cooled system was used. Estimated axial cooling rates
obtained for the suction cast Fe-25Ni alloys are slightly higher
than those of the copper-mould cast, but the results have the
same order of magnitude.
It is expected that the cooling rate for the glass-forming
systems to be higher compared to those obtained from the con-
ventional solidification due to absence of the latent heat during
glass formation. Other factors that should be considered are
(1) much lower thermal conductivity of BMG’s, and (2) heat
flow affected by the interfacial resistance at the mould-metal
interface. Finally, it should be noted that the cooling rates
were calculated using the equations adopted from unidirec-
tional solidification experiments, which was not the process
used in this work.
4. Conclusions
Having measured the cellular spacing in Fe-25wt%Ni al-
loy, superficial and axial cooling rates were evaluated. Cooling
rate estimated for the suction cast and copper-mould cast alloys
were comparable, although the former had a slightly high-
er cooling rates due using a water cooled system. Estimated
minimum axial cooling rates, as the most important parameter
in synthesising BMGs, were about 200 K/s for the ø2 and ø3
mm, and about 30 K/s for the ø5 mm suction cast rods. The
outcome of this work clearly showed that the eutectic alloys
should not be used to estimate temperature profile (e.g.cooling
rates) in cylindrical samples, since the interlamellar spacing is
affected by the concave perturbation of the solidification front.
Acknowledgements
This work was financially supported by the National Science
Centre (NCN) under contract No. 2011/03/D/ST8/04131. Valuable
contribution of Dr. Jerzy Latuch (Warsaw University of Technology)
to the experimental work is also acknowledged.
REFERENCES
[1] W. Klement, R.H. Willens, P. Duwez, Non-crystalline structure
in solidified gold-silicon alloys, Nature 187, 869-870 (1960).

771
[2] A. Nishiyama, A. Inoue, Stability and nucleation behavior of
glass-forming Pd-Cu-Ni-P alloy with a critical cooling rate of
0.067 K/s, Intermetallics 10, 1141-1147 (2002).
[3] H. Chakrabarti, C.B. Chaudhuri, B. Kanjilal, Glass transition
temperature of Pd-Cu-Ni-P thin film metallic glass – a 2D ap-
proach, J. Non-Cryst. Solids 359, 51-55 (2013).
[4] J. Gondro, J. Zbroszczyk, W. Ciurzyńska, J. Olszewski, M.
Nabiałek, K. Sobczyk, J. Świerczek, A. Łukiewska, Struc-
ture and Soft Magnetic Properties of Bulk Amporphous
Fe0.61Co0.10Zr0.025W0.02Hf0.025Ti0.02B0.20)96Y4 Alloy,
Arch. Metall. Mater. 55, 85-90 (2010).
[5] M. Nabiałek, J. Zbroszczyk, W. Ciurzyńska, J. Olszewski, S.
Lesz, P. Brągiel, J. Gondro, K. Sobczyk, A. Łukiewska, A.
Świerczek, Microstructure, Some Magnetic and Mechanical
Properties of Amorphous Fe60Co10Zr2.5Hf2.519W2Y2B21
Plates, Arch. Metall. Mater. 55, 195-203 (2010).
[6] D.C. Hofmann, Bulk metallic glasses and their composites: a
brief history of diverging fields J. Mater. 2013, 1-8 (2013).
[7] Y. Li, W. Zhang, F.X. Qin, A. Makino, Mechanical properties
and corrosion resistance of a new Zr56Ni20Al15Nb4Cu5 bulk
metallic glass with a diameter up to 25 mm, J. Alloys Compd.
615, S71-S74 (2014).
[8] N. Hua, T. Zhang, Glass-forming ability, crystallization kinetics,
mechanical property, and corrosion behavior of Zr-Al-Ni-Ag
glassy alloys, J. Alloys Compd. 605, 339-345 (2014).
[9] B.S. Dong, S.X. Zhou, D.R. Li, C.W. Lu, F. Guo, X.J. Ni, Z.C.
Lu, A new criterion for predicting glass forming ability of bulk
metallic glasses and some critical discussions, Prog. Nat. Sci.
– Materials International 21, 164-172 (2011).
[10] W. Pilarczyk, Preparation and characterization of Zr-based
bulk metallic glasses in form of plate, J. Alloys Compd. 615,
S132-S135 (2014).
[11] F.G. Coury, L.C.R. Aliaga, C.R.M. Afonso, C. Bolfarini, W.J.
Botta, C.S. Kiminami, Comparative study between two die
cast methods for processing Cu-Zr-Al bulk metallic glasses,
J. Mater. Res. Tech 2, 125-129 (2013).
[12] R.M. Srivastava, J. Eckert, W. L¨
oser, B.K. Dhindaw, L. Schultz,
Cooling rate evaluation for bulk amorphous alloys from eu-
tectic microstructures in casting processes, Mater. Trans. 43,
1670-1675 (2002).
[13] K.J. Laws, B. Gun, M. Ferry, Influence of casting parameters
on the critical casting size of bulk metallic glass, Metall. Mater.
Trans. A 40A, 2377-2387 (2009).
[14] P. Pawlik, K. Pawlik, A. Przybył, Investigation of the cooling
rate in the suction casting proces, Rev. Adv. Mater. Sci. 18,
81-84 (2008).
[15] H. Jones, Rapid solidification of metals and alloys, Monograph
No. 8, Institution of Metallurgists, London (1982).
[16] L.J. Swartzendruber, V.P. Itkin, C.B. Alcock in H. Okamoto
(ed.), Phase Diagrams of Binary Iron Alloys, Materials Infor-
mation Soc., Materials Park, Ohio (1993).
[17] A. Karma, M. Plapp, New insights into the morphological sta-
bility of eutectic and peritectic coupled growth, JOM 56, 28-32
(2004).
Received: 20 April 2014.