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Materials and Manufacturing Processes
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The Effect of Direct Thermal Method, Temperature and
Time on Microstructure of a Cast Aluminum Alloy
A. H. Ahmad
a b c
, S. Naher
a b d
& D. Brabazon
a b
a
School of Mechanical and Manuf act uring Engineering , Dublin Cit y Universit y , Dublin ,
Ireland
b
Advanced Processing Technology Research Cent re , Dublin Cit y Universit y , Dublin , Ireland
c
Facult y of Mechanical Engineering , Universit i Malaysia Pahang , Pekan , Malaysia
d
School of Engineering and Mat hemat ical Sciences , Cit y Universit y London , London ,
Unit ed Kingdom
Published online: 04 Mar 2014.
To cite this article: A. H. Ahmad , S. Naher & D. Brabazon (2014) The Ef f ect of Direct Thermal Met hod, Temperat ure
and Time on Microst ruct ure of a Cast Aluminum Alloy, Mat erials and Manuf act uring Processes, 29: 2, 134-139, DOI:
10. 1080/ 10426914. 2013. 822980
To link to this article: ht t p: / / dx. doi. org/ 10. 1080/ 10426914. 2013. 822980
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Materials and Manufacturing Processes, 29: 134–139, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6914 print/1532-2475 online
DOI: 10.1080/10426914.2013.822980
The Effect of Direct Thermal Method, Temperature
and Time on Microstructure of a Cast Aluminum Alloy
A.H. AHMAD1,2,3, S. NAHER1,2,4, AND D. BRABAZON1,2
1
Downloaded by [Dublin City University], [Dermot Brabazon] at 09:58 06 March 2014
School of Mechanical and Manufacturing Engineering, Dublin City University, Dublin, Ireland
2
Advanced Processing Technology Research Centre, Dublin City University, Dublin, Ireland
3
Faculty of Mechanical Engineering, Universiti Malaysia Pahang, Pekan, Malaysia
4
School of Engineering and Mathematical Sciences, City University London, London, United Kingdom
The direct thermal method is used for the creation of globular microstructures suitable for semi-solid metal forming. In this paper, both simulation
and experimental results using direct thermal method are presented. ProCAST® software was used to estimate temperature distribution inside the
aluminum billet. In validation work, molten aluminum A356 was poured into metallic copper tube molds and cooled down to the semi-solid temperature before being quenched in water at room temperature. The effect of pouring temperatures of 630°C, 650°C, 665°C, 680°C and holding times of
45 s and 60 s on the microstructure of aluminum A356 alloy were investigated. The simulation results showed that the average temperature rate within
the copper mold, from initial pouring temperature to just before quenching, was approximately 1°C/s. Examination of the solidified microstructures
showed that the microstructure was more spherical when lower pouring temperatures and holding periods were used. From the micrographs it was
found that the most globular and smallest structures were achieved at processing parameters of 630°C and 45 s.
Keywords Aluminum; A356; Direct Thermal Method; Holding Time; Pouring Temperature; Semi-Solid.
INTRODUCTION
Semi-solid metal processing has been commercially used
for the casting of various sizes of components on an industrial scale. Commercial interests in semi-solid metal processing are mainly in the automotive, aerospace, ICT,
plumbing and military supplier industries. This processing
technique has become a common casting method for aluminum and magnesium alloys for producing high density and
high strength products [1]. Some current research is also
investigating ways to make this process feasible for semisolid steel forming [2]. The effect of microstructure on fluidity which allows for semi-solid processing was discovered
by Flemings and Spenser in Massachusetts Institute of
Technology 40 years ago [3]. In the initial stage, Flemings
and his co-workers discovered that the viscosity of the
stirred material while in the semi-solid forming was much
lower than the viscosity without stirring. It was also found
that this lower viscosity was due to the generation of a
spherical grained structure in the material from the stirred
action during solidification. Interlocking network of dendrite
grains in the unstirred material on the other hand generates a
high viscosity fluid.
Semi-solid metal exhibits not only a shear rate thinning
but also a time dependency. This fluid is therefore thixotropic
in nature. This time dependency is mainly due to disagglomeration of particles clusters after increased shear rate
processing or due to agglomeration of particles after reduced
shear rate processing [3]. The particles have a natural affinity for each other and therefore agglomerate during periods
of the shearing. However, particle bonds are re-melted during deformation as a result of shear stresses, which allows
the material to flow with greater ease. The flow resistance of
semi-solid metal is strongly depended upon processing history. The resulting microstructure (average grain size and
shape and the distribution of grain size) in semi-solid metal
state and the degree of particle agglomeration appear to be
the most significant material characteristics contributing to
this process history dependent flow behavior.
Thermal treatment processing to obtain the required
microstructure for semi-solid metal forming is the most
commonly used commercial method to provide the required
globular microstructure. In order to achieve this, the creation
of many small nucleation sites is important. The higher the
number of nuclei, the smaller is the average distance between
nuclei and the smaller are the resultant grains.
The new-rheocasting process by UBE Machinery, Inc. is
one of the better known examples of this thermal method
applied commercially. In order to generate more nuclei, the
molten metal is chilled quickly from above the liquidus to
below the liquidus and held for a short holding at a semisolid metal temperature. This processes results in a very
Received March 6, 2013; Accepted July 3, 2013
Address correspondence to S. Naher, School of Mechanical and
Manufacturing Engineering, Dublin City University, Dublin, Ireland;
E-mail: sumsun.naher@city.ac.uk.
Color versions of one or more of the figures in the article can be found
online at www.tandfonline.com/lmmp.
134
135
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MICROSTRUCTURE OF CAST ALUMINUM ALLOY
fine grained microstructure. In the new-rheocasting process,
the semi-solid metal temperature is controlled with induction heating to maintain the desired fraction solid [4]. The
treated semi-solid metal is then injected into the mold within
the die casting machine. The success of the new-rheocasting
process technique depends on the initial structure obtained
in the material. There are few more techniques that use a
similar principle in order to provide the required starting
material. These include the swirled enthalpy equilibrium
device [5], the continuous rheoconversion process [6], low
superheat pouring with a shear field [7] and the direct thermal method [8].
In direct thermal method, liquid metal is poured into a
thin cylindrical metallic mold that has low thermal mass and
high conductivity [8]. Heat matching between molten metal
and mold allow for pseudo-isothermal hold within the solidification range. This allows for the alloy to remain in a
steady semi-solid state where the temperature is held
between the solid and liquid phases of the alloy. This is
achieved by the very low rate of heat loss to the environment
through the mold, which is copper tubing with a metal base
plate. The advantage of this technique is that the process
uses the natural solidification principle and is low cost. The
low superheat of the alloy is extracted into the mold such
that the alloy rapidly cools into the semi-solid zone. The initial rapid cooling gives rise to a high rate of nucleation
events in the alloy. After a short holding period this microstructure spherodises. In order to capture this microstructure, the mold is quenched into water at room temperature
while the alloy is still within the semi-solid metal range.
Simulation using finite element method to examine the
heat distribution during solidification was carried out by
Wang et al. [9]. This research work showed that the combination of simulation and experimental techniques were
used to optimize the product design in order to avoid the
casting defects. Publications in recent years have shown
that the globular microstructure can be achieved by using
direct thermal method [8]. The combination of accurate
pouring temperature and holding time significantly affect
the globular structure creation. It was suggested from previous research that a lower pouring temperature would create smaller primary and secondary phases within the
microstructure [10, 11]. Finite element method was seen in
their work as a useful tool to predict the heat distribution
during solidification process. However the effect of these
parameters on the resultant microstructure is still not fully
understood due to the vast range of possible casting conditions in terms of alloy, holding time, pouring temperature,
mold materials and quenching medium used. This work
focused on investigating the effect of pouring temperature
and time on the microstructure of A356. In particular, simulation and experimental focused on a pouring temperature range of 630°C to 680°C and specific holding times of
45 s and 60 s.
SIMULATION AND EXPERIMENTAL
Simulation
The purpose of the simulation was to investigate the heat
distribution inside the billet during solidification. The simulation results were used to estimate the temperature of the
billets at various times during the holding period. The solidification process of the alloys was simulated using finite
element analysis software ProCAST®. Standard heat transfer via Fourier modeled was used [9]. The Fourier heat
conduction equation used is represented as follows:
.
dT
(1)
= ∇( k ∇T ) + Q
dt
where T is the temperature, t is the time, ρ . is density, c is
specific heat, k is thermal conductivity and Q is the internal
power source. The calculations are based on Cartesian coordinates x, y, z of the part geometry such that Eq. (1) can be
re-written as follows:
ρc
ρc
dT d dT d dT d dT .
k
= k
+ k
+
+Q
dt dx dx dy dy dz dz
(2)
The simulation work began with the development of the mold
geometry which was of the same geometry as that used to
produce the billets experimentally. These cylindrical billets
were 25 mm in diameter and 75 mm in height. The model was
transferred to the GEOMESH Environment software for the
meshing process. The finite element mesh of the ingot consisted of 70,752 nodes and 396,795 tetrahedral elements.
Aluminum A356 was used for the ingot (billets) and copper for the mold. The initial conditions of the mold were set
at 680°C for the ingot and 24°C for the mold. Other boundary conditions and thermo-physical properties which were
used in the model are summarized in Table 1. The molds
were filled via pouring under gravity with the crucible lip
position 25 mm above the copper mold during pouring.
Experimental
A 1 kg aluminum A356 ingot was placed in a graphite
crucible and was heated to a temperature 720°C using induction furnace. Once the desired temperature of the melt was
TABLE 1.—Boundary conditions and thermo-physical
properties of A356 used in the simulations.
Property
Liquidus temperature
Solidus temperature
Pouring temperature
Mold temperature
Latent heat
Conductivity
Density
Heat transfer coefficient
Value
616°C
556°C
680°C
24°C
389 kJ/kg
151 W/mK
2670 kg/m3
14.26 kW/m2 [12]
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A.H. AHMAD ET AL.
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was reported as 538°C to 614°C. Figure 1 shows the fraction
solid vs. temperature curve of A356 at 0.6°C/s cooling rate
[13]. This fraction solid curve was used to estimate the
amount of solid before quenching.
The samples were sectioned 5 mm from the bottom of the
cast billets and mounted in Bakelite and microstructure
analysis was performed by optical microscope after etching
the surface of the polished molded samples with Keller’s
reagent. A Reichert light optical inverted microscope with
Me F2 universal camera connected to a PC was used for
image capture. Microstructures were examined with Buhler
Omnimet Enterprise software.
FIGURE 1.—A fraction solid vs. temperature curve for A356 at a cooling rate
0.6°C/s [13].
obtained, it was poured into a cylindrical copper mold of
1 mm wall thickness, 25 mm in diameter and 75 mm in
height.
The different pouring temperatures were set at 630°C,
650°C, 665°C and 680°C. After pouring, the molten metal
was held in the mold for 45 s or 60 s, at semi-solid temperatures, before quenched into room temperature water. A billet
with pouring temperature of 665°C was also allowed to
solidify via natural solidification.
The semi-solid temperature range of A356 alloy is quoted
with different solidus and liquidus temperatures depending
on the source of reference. In one previous work this range
RESULTS AND DISCUSSION
Simulation Results
The temperature profile within the billet from 2.5 s to 61 s
after pouring is shown in Fig. 2. The simulation results show
that the mold was fully filled with molten metal 2.5 s after
pouring, see Fig. 2(a). In the initial solidification stage, the
temperature started to drop first in the lower region of the
mold as shown in Fig. 2(b).
In the simulation, when the pouring process completed,
the bottom part showed lower temperature than the upper
part of the mold. This is may be due to the fact that the bottom part of the mold is farthest away from the hot metal that
just transferred from the crucible and also has a longer
period of time in contact with the mold walls compared to
the melt within the upper regions of the mold.
FIGURE 2.—Temperature distributions within aluminum billet from (a) 2.5 s to (g) 61 s after pouring.
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MICROSTRUCTURE OF CAST ALUMINUM ALLOY
This temperature difference between the top and bottom
regions became evident after 21 s pouring time (Fig. 2(c)).
The effects of pouring temperature on heat distribution were
also observed by other in previous work [14].
The temperature continued to drop until 61 s with a drop rate
of 3°C/s in the simulation and the average temperature drop
during the previous experimental findings was 1°C/s [12]. This
is come to the agreement that the simulation results were close
to the previous experimental result, supporting the accuracy of
simulation result.
From the simulation, after 45 s and 60 s, the temperatures
were approximately 638°C and 622°C, respectively. The
equivalent recorded temperatures from the experimental
work conducted were 635°C and 620°C at 45 s and 60 s,
respectively. By correlating with Fig. 1, these results could
then be used to estimate the fractions solid of the billets
before quenching.
In direct thermal method, the temperature cycle was influenced by the material and wall thickness of the mold [12].
The temperature drops within a copper were higher than a
steel mold as steel has the higher volumetric heat capacity.
Furthermore, the greater selection of a wall thickness is
needed for a steel mold.
137
dendritic microstructure was produced. The microstructure of
the billets with different pouring temperatures and holding
times are presented in Figs. 4 and 5. The lower pouring temperatures would be expected to have a significant effect not
just for the finer grain size but also the morphology of the
grain [11]. As per previous findings, the lower pouring temperatures were found to produce more globular and finer
microstructures [15]. The primary phase microstructures
were also found within this work to be more spherical when
produced at lower pouring temperatures as shown in Fig. 4(a).
Lower pouring temperatures lead to higher cooling rates
from above the liquidus to below the liquidus as less superheat has to be extracted. Furthermore, when the cooling rate
is higher, the undercooling of the alloy becomes larger. A
raise in undercooling increases the amount of nucleation
which ultimately results in a smaller grain size from this
larger crystallization driving force [16].
Experimental Results
Effects of pouring temperature. The billet produced with
665°C pouring temperature and with natural solidification to
room temperature produced a dendritic microstructure, see
Fig. 3, as would be expected under this conventional type of
solidification. Even though the molten metal was poured into
the copper mold, without a controlled period for holding
within the semi-solid temperature region, a ripened type of
Effects of Holding Time
The role of holding time was to ensure an adequate
fraction solid before quenching. A longer holding time for a
specific constant pouring temperature produced larger
primary phase grains. Figure 5 shows the microstructures
for the samples produced with 630°C, 650°C and 680°C
pouring temperatures and with 60 s holding time. These
have larger grain sizes compared with the microstructures of
the samples shown in Fig. 4 produced with 45 s holding
time.
Formation of globular microstructure was influenced by
fraction solid [17]. The sample with a 630°C pouring temperature was quenched at 570°C, equivalent to 0.65 fraction solid.
The corresponding microstructure is shown in Fig. 5(a). For the
FIGURE 3.—Dendritic microstructure formed within billet after solidification
from pouring temperature of 665°C under air cooling.
FIGURE 4.—Microstructures from a 45 s holding time and with pouring
temperatures of (a) 630°C, (b) 650°C and (c) 680°C.
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138
A.H. AHMAD ET AL.
FIGURE 6.—Microstructure of the sample produced with a 45 s holding time and
a pouring temperature of 630°C.
FIGURE 5.—Microstructures from a 60 s holding time and with pouring
temperatures of (a) 630°C, (b) 650°C and (c) 680°C.
sample with 650°C pouring temperature, with microstructure
shown in Fig. 5(b), was quenched at 590°C (0.4 fraction solid).
The microstructure was less globular compare with the sample
produced with a pouring temperature of 630°C. This was confirms that size of primary grains was affected with a low fraction
solid used [17].
The sample with 680°C pouring temperature was
quenched at 620°C (Fig. 5(c)). This sample was therefore
still in the liquid condition upon quenching. The microstructure results from this sample showed a primary phase which
was too large and irregular compared to what would typically be required for semi-solid metal forming.
In order to understand the relationship between pouring
temperature and viscosity, the microstructure of the respective billets should be related to the material fluidity.
Viscosity is an important indicator of the material capability
to fill the die cavity during casting. Lower viscosity produced better movement of the material and allows the semisolid metal to move into the complex geometries such as
thin die cavity sections.
The two important phases in the microstructure of A356
are the primary aluminum phase which solidifies first and
the secondary aluminum–silicon phase which solidifies second, shown in Fig. 6. In general, a microstructure with more
globular primary grains and more liquid phase, seen as secondary upon solidification, will have higher fluidity.
Pouring temperature is one of the important parameters
that affect the evolution of the primary phase during solidification [18]. Low pouring temperature is a key factor that
establishes higher temperature gradients within the semisolid metal which encourage the formation of a multitude of
nuclei and subsequently a more globular microstructure
upon holding within the semi-solid state.
Relationship between the pouring temperature and
microstructure formation is determined by the undercooling
temperature. The undercooling is the difference between the
equilibrium temperature and temperature which the material
cools, before the start of solidification [19]. The undercooling is influenced by the solidification rates which depend on
the type of a mold material, mold thickness, etc. The formation of a microstructure which evolves within material
depends on the degree of undercooling. During solidification process, as the undercooling temperature and time
increase, the melt potential nucleation decreases resulting in
a coarse-grained structure deformation. Higher cooling rate
is therefore associated with a finer grain size and a globular
microstructure. The higher pouring temperature lead to a
higher cooling rate as it need more time to cool from the
liquidus to solidus temperature. For this reason, dendritic
microstructure occurs.
The formation of a globular microstructure in these
experiments was influenced by the volume fraction of solid.
In semi-solid metal processing, the volume of the fraction
solid play the important rules in determined the successful
of the process. The fraction solid which is used in semi-solid
processing is normally in the range of 0.30 to 0.7 depending
on the type of the material used [20]. During rapid solidification, the nuclei start to deform from the liquid condition.
The nuclei evolved and impinged to each other within the
material and later produced a dendritic microstructure as in
typical solidification process. During this process (from the
liquid to dendritic microstructure formation), the fraction of
solid which occurs inside the material is increased as it
approaching the solidus temperature. The formation of the
nuclei which is small at this stage becomes larger due to
the increment of fraction solid volume. The quenching technique which was applied in this period captured a finer globular shape microstructure. For this reason, the proper
MICROSTRUCTURE OF CAST ALUMINUM ALLOY
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selection of a holding time in direct thermal method is
crucial to allow the formation of a desired microstructure
feature due to the fraction solid effect.
CONCLUSION
The simulation of the heat distribution inside the billet
and direct thermal method experiment was successfully carried out. The simulation provided useful information about
solidification temperature and time prediction before
quenching. The copper mold was only fully filled 2.5 s after
pouring. The A356 billet temperature decreased at approximately 1°C/s from the initial pouring temperature. The
experimental work showed that a fine spherical microstructure can be achieved with the correct selection of pouring
temperature and holding time. A lower pouring temperature
contained more primary particles which were also more
globular than those formed from higher pouring temperatures. The spherical microstructures were dominant at the
lower pouring temperature (630°C) regardless the holding
time of either 60 s or 45 s. The longer holding time produced
larger primary phase particles which were also less globular.
The results of this work showed that the best pouring temperature and time for the A356 alloy were at 630°C and 45 s,
respectively. In direct thermal method, the combination of
proper pouring temperature and holding time are necessary
in order to produce the microstructure required for semisolid metal forming.
FUNDING
The authors would also like to acknowledge the support
from the Ministry of Higher Education, Malaysia, and
Dublin City University for funding this work.
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