Charax of Flow Behavior of Slurries in HPDC

Available online at www.sciencedirect.com
Acta Materialia 61 (2013) 6267–6275
www.elsevier.com/locate/actamat
Characterization of flow behavior of semi-solid slurries containing
low solid fractions in high-pressure die casting
S. Janudom a , J. Wannasin a,⇑ , J. Basem a , S. Wisutmethangoon b
a Department of Mining and Materials Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand
b Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand
Received 21 April 2013; received in revised form 30 June 2013; accepted 6 July 2013
Available online 27 July 2013
Abstract
Understanding the flow behavior of semi-solid slurries containing low solid fractions is key to the success in applying this process in
the die casting industry. With these low initial solid fractions, the flow behavior of semi-solid slurries is quite complicated, making it
difficult to model accurately. This present work developed and studied characterization methods for the flow behavior of semi-solid slurries
at low solid fractions in high-pressure die casting. A new parameter, the ratio of gate speed to initial solid fraction (V g /f s ), was proposed
to be correlated to the normalized flow interface length, blister area and tensile properties. Results from the flow pattern analysis
suggest that the flow behavior can be controlled to achieve laminar flow by varying the initial solid fraction. Blister test results show the
trend that slurry die casting conditions with high V g /f s values exhibit high blister areas. Die casting conditions with excessively high gate
speeds and insufficient solid fractions result in turbulent flow patterns and high levels of blister defect. The results of tensile test and fracture
surface analysis are consistent with other analysis results. The samples formed by liquid die casting and slurry die casting with high
V g /f s values have gas porosity due to turbulent flow pattern during die filling. On the other hand, the samples formed by slurry die casting
with too low V g /f s values contain shrinkage porosity. This is because of insufficient time for shrinkage feeding due to a combination of a
high solid fraction and a low gate speed. This study has demonstrated that die casting with slurries containing low initial solid fractions
gives die casters another process parameter to adjust, which can help reduce and control the gas and shrinkage porosities.
Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Die casting; Semi-solid metal; Slurry; Rheocasting; Flow characterization
1. Introduction
High-pressure die casting of liquid aluminum alloys has
been extensively used to mass-produce a variety of aluminum
components in automotive, electronic and agricultural
applications. It is a cost-effective process that produces
parts with accurate dimensions and good surface finish.
However, two key inherent defects are often faced in this
process: gas and shrinkage porosity defects. These defects
often cause rejects in a secondary process such as machining
or painting. They also limit die cast components from
being used in high performance and safety applications.
⇑ Corresponding author. Tel.: +66 7428 7312; fax: +66 7455 8834.
E-mail address: jessada@alum.mit.edu (J. Wannasin).
Semi-solid metal forming has been applied in high-pressure
die casting for more than 30 years to solve these problems
and to yield other benefits such as die life extension
[1]. In this process, semi-solid metal at a high solid fraction,
generally in the form of a slug, is transferred to a shot system
and then injected into the die. Improved quality with
high mechanical properties has been reported [2]. However,
it is quite difficult and expensive to form semi-solid metal in
the slug form since the process requires major modifications
to the machine and process [3]. Therefore, recent
studies have shifted the focus to injection of semi-solid slurries
at low solid fractions since semi-solid slurries can be
directly poured into the current shot system. The aim of
slurry die casting process is to reduce the cost of the system
and the needs to modify the machine, die and processes.
1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.actamat.2013.07.010

6268 S. Janudom et al. / Acta Materialia 61 (2013) 6267–6275
The process also yields several cost benefits such as die life
extension, cycle time reduction, lubricant usage reduction
and melting energy reduction. Several applications are
being commercially applied worldwide using this slurry
die casting approach [4].
Since slurry die casting is formed at a low solid fraction
and with minor modifications to the die, the slurries may
still flow in a turbulent manner if the process is not properly
controlled. The process may then result in entrapped
air and oxide films in the parts. In addition, shrinkage
porosity may also be present if the processing conditions
are not optimized. To implement slurry die casting successfully
in the industry, it is thus important to understand the
flow behavior of the slurries. In literature, all the studies of
semi-solid metal die filling were conducted with high initial
solid fractions in the range of 30–60% [1,5,6]. In these studies,
the effects of key processing parameters such as gate
velocity and solid fraction were systematically investigated
using numerical simulation methods [7–10]. In most cases,
fill test or short shot techniques were used to verify the
models [5,6,11–14]. For semi-solid slurry with low initial
solid fractions of less than 15%, however, no studies have
been reported. With these very low initial solid fractions,
the flow behavior of semi-solid slurries is quite complicated.
The initial solid fraction starts from a very low fraction
in the range of 3–15% when it is poured into a shot
sleeve. The solid fraction then abruptly increases to a
higher value in the shot sleeve and then in the die. The evolution
and variation of the solid fraction in the slurries during
the die casting process make it difficult to model
accurately. Thus, this present work developed and studied
characterization methods for the flow behavior of semisolid
slurries at low solid fractions in high-pressure die
casting. A new parameter, the ratio of gate speed to initial
solid fraction (V g /f s ), was proposed to be correlated to the
normalized flow interface length, blister area and tensile
properties.
2. Experimental
2.1. Semi-solid slurry die casting
The secondary aluminum 356 alloy (Al–7Si–0.3Mg–Fe
or JIS AC4C) was used in this study. The chemical composition
of this alloy is given in Table 1. To produce semisolid
slurries at low solid fractions, the gas-induced semisolid
(GISS) technique was used [15]. A schematic drawing
of the GISS technique is shown in Fig. 1. The process creates
semi-solid slurries by injecting fine inert gas bubbles
through a cold graphite diffuser into a melt held at a temperature
slightly above the liquidus temperature. Vigorous
convection and localized cooling result in numerous solid
particles in a short time [15].
In the slurry die casting process, a steel ladle was used
to transfer the molten metal from the melting furnace.
Before pouring the slurry into the shot sleeve, the ladle
was paused and the GISS process was applied to convert
the molten aluminum alloy into semi-solid slurry containing
different initial solid fractions [4]. The slurry was then
poured into the shot sleeve and injected into the die. Schematic
illustration of the process is given in Fig. 1. In the
experiments, the temperatures of the shot sleeve and the
die were at 250 °C and 180 °C, respectively. The weight
of the aluminum per shot was 300 g. The die casting
machine had the clamping force of 80 t. The key parameters,
gate speed and initial solid fraction were controlled
by the following procedures.
2.1.1. Gate speed (V g )
The casting parts in this study as shown in Fig. 2 had the
dimensions of 70 mm in width, 100 mm in length and 8 mm
in thickness. The thickness at the gate was 6 mm.
Three levels of gate speeds were obtained by varying the
plunger speeds. The gate speeds were calculated by assuming
flow continuity of the fluid using the following equation
[16]:
A g V g ¼ A ss V ss
where V g and V ss are the speeds at the gate and shot sleeve,
respectively, and A g and A ss are cross-sectional areas of the
gate and shot sleeve, respectively.
2.1.2. Initial solid fraction (f s )
Different levels of initial solid fractions were obtained by
varying the time that the graphite diffuser was immersed in
the molten metal, called rheocasting time. In this study, the
molten metal was initially at 620 °C (7 °C superheat)
before the immersion of the graphite diffuser. Two rheocasting
times of 5 and 10 s were used to achieve two levels
of initial solid fractions.
To analyze for the initial solid fraction, the rapid
quenching method was used. In this method, the rapid
quenching mold, shown schematically in Fig. 3, captured
the microstructure at an instant in time by applying vacuum
to draw some semi-solid slurry into a thin-channel
copper mold, which helped to freeze the slurry [17,18].
The width, height and thickness of the channel were 31,
125 and 1 mm, respectively. A detailed description of the
method can be found elsewhere [17,18].
The microstructures at different conditions at the center
location of the casting sample were examined using an optical
microscope, see Fig. 2. Standard metallurgical proce-
ð1Þ
Table 1
Chemical composition of the aluminum 356 alloy used in this study.
Alloy Si Cu Mg Mn Fe Zn Ti Al
356 6.93 0.10 0.39 0.05 0.47 0.01 0.05 Balance

S. Janudom et al. / Acta Materialia 61 (2013) 6267–6275 6269
Fig. 1. Schematic illustration of the GISS slurry die casting process [3].
The blister area (A B ;mm 2 ) for each condition was calculated
using the following equation:
A B ¼
X 5
i¼0
p d2 i
f 4 i
N
where d i is the diameter of each blister size, f i is the frequency
of each blister size and N is the total number of assessed
samples.
ð2Þ
Fig. 2. Illustration of the die cast parts used in this study (dimensions are
in mm).
dures were used. The samples were etched with Keller’s
reagent.
2.2. Flow behavior characterization
To characterize the flow behavior of semi-solid slurries
at low initial solid fractions, three analysis and testing
methods – flow pattern analysis, blistering test and tensile
test – were used.
2.2.1. Flow pattern analysis
The fill test or short shot method was used to analyze
the flow pattern of the metal front interface during die filling.
In this technique, the injection piston was interrupted
at a prescribed position [9]. Observation of the solidified
die cast parts from this short shot study can reveal the flow
pattern during the die filling.
2.2.2. Blistering test
Blisters can be found on the casting surface after a heat
treatment process. They are caused by expansion of gases
in the porosities beneath the surface during solution heat
treatment. The level of blister area on the casting surface
can be used to analyze the flow behavior during die filling.
In this study, five samples per condition were assessed
for blisters after the solution treatment at 540 °C for 4 h.
2.2.3. Tensile test
A characterization method for flow behavior that can
reveal internal defects is tensile test. Die cast parts containing
porosity defects will have low ultimate tensile strength
and elongation. Fracture surfaces can also be observed to
evaluate the types of defects present in the castings.
Die cast samples after T6 heat treatment were machined
for tensile test specimens following the ASTM B557M
standard as shown in Fig. 4. The T6 heat treatment process
included solution treatment at 540 °C for 4 h, water
quenching and artificial aging at 165 °C for 18 h [19]. Five
samples were tested for each casting condition.
Fracture surfaces of the tensile specimens were observed
under a scanning electron microscope. Three locations
were analyzed: T, M and C, as illustrated in Fig. 4b.
3. Results and discussion
3.1. Microstructures
Rapidly quenched microstructures of the slurries processed
by the GISS process with the rheocasting times of 5
and 10 s are given in Fig. 5. Primary a particles (bright
phase) are dispersed in the matrix of finer secondary a phase
and very fine eutectic structure. Results of quantitative analysis
of the fraction of primary a phase are given in Table 2.
The initial solid fractions of the slurries with the rheocasting
times of 5 and 10 s are 3.5% and 11.5%, respectively.
Representative microstructures of the liquid die cast
and slurry die cast parts are given in Fig. 6. These

6270 S. Janudom et al. / Acta Materialia 61 (2013) 6267–6275
Fig. 3. Schematic illustration of the rapid quenching method used to determine the initial solid fraction [17].
T
M
Fig. 4. Illustrations of (a) the tensile specimen and (b) locations of SEM analysis on the fracture surface.
Fig. 5. Representative micrographs of the rapidly quenched semi-solid slurries at the rheocasting times of (a) 5 and (b) 10 s.
Table 2
Summary of the process parameters in this study.
Condition Rheocasting time (s) Plunger velocity (m s 1 ) f s (%) V g (m s –1 ) V g /f s
Liquid 0 0.20 – 1.05 N/A
S-5-10 5 0.10 3.5 0.52 14.95
S-5-15 5 0.15 3.5 0.79 22.43
S-5-20 5 0.20 3.5 1.05 29.90
S-10-10 10 0.10 11.5 0.52 4.55
S-10-15 10 0.15 11.5 0.79 6.83
S-10-20 10 0.20 11.5 1.05 9.10
microstructures consist of a phase particles and very fine
dendritic microstructure. The micrographs clearly show
that slurry die casting samples have coarser a phase
particles, which is due to the long growth time during the

S. Janudom et al. / Acta Materialia 61 (2013) 6267–6275 6271
In the practice of aluminum die casting, the viscosity of
liquid aluminum is roughly constant at 0.001 Pa s [7].
Thus, in liquid die casting with constant die design and
melt temperature, the key process parameter to control
the flow behavior is the gate speed. For semi-solid slurries,
on the other hand, the viscosity can greatly vary from
0.1 Pa s to 1 MPa s, depending on their microstructure
and local solid fraction [7]. Therefore, in slurry die casting,
besides the gate speed, the viscosity is also an important
parameter. However, the viscosity of semi-solid slurries
greatly depends on the solid fraction and shear rate [2].
These parameters vary significantly during the die casting
process. So it is not simple to determine the viscosity of
semi-solid slurries. To simplify the analysis to be industrially
practical, this work defined a new process parameter
to control the flow as the ratio of the gate velocity to the
initial solid fraction, V g /f s . Table 2 summarizes the values
of this process parameter. In this work, the parameter
V g /f s is correlated to three output parameters: the normalized
flow interface length, blister area and tensile properties.
These parameters are briefly described in this section.
3.2.1. Flow pattern analysis
Die cast samples from the short shot experiments are
summarized in Fig. 7. It is evident from the results that
the liquid die cast samples and the slurry die cast samples
with V g /f s of 29.90 have a non-laminar flow pattern.
To quantify the results, an output parameter, normalized
flow interface length, is introduced in this study. It is
defined as:
Fig. 6. Representative microstructures of as-cast samples from (a) liquid
die casting, and slurry die casting at the initial solid fractions of (b) 3.5%,
and (c) 11.5%.
rheocasting process. However, in general, the results show
that all the processing conditions produce quite uniform
microstructures.
3.2. Flow behavior
To produce casting parts without entrapped air and
oxide films, it is important to ensure that the melt flows
in a stable and planar manner by applying appropriate processing
conditions [7]. For fluid flow, the parameter commonly
used to determine whether the flow is laminar is
Reynolds number (Re), which is given as:
Re ¼ q V D
ð3Þ
g
where q, V and g are the density, velocity and viscosity of
the fluid, respectively, and D is the diameter of the flow
channel [20].
L N ¼ L f
W
where L f is length of the flow front interface (mm) and W is
the width of the flow channel (mm). An ideal laminar flow
with planar interface will have the L N value equal to 1. A
very turbulent flow will have a very large value of L N .
The analysis results of the normalized flow interface
length as a function of V g /f s are given in Fig. 8. During
the initial die filling (fill fractions of 0.3 and 0.5), liquid
die casting and slurry die casting with V g /f s of 29.90 have
the L N > 2. For other cases of slurry die casting, the value
of L N < 2. The results show that liquid die casting and
slurry die casting with high V g /f s have unstable flow interface
during the die filling. This is an indication of turbulent
flow. The results suggest that the flow behavior can be controlled
by varying the initial solid fractions. With a high
gate speed of 1.05 m s –1 , die casting with liquid aluminum
clearly yields turbulent flow. Also, in slurry die casting,
the initial solid fraction of 3.5% is not sufficient to yield
laminar flow (V g /f s = 29.90). However, with the initial solid
fraction of 11.5% (V g /f s = 9.10), the flow behavior clearly
becomes laminar.
3.2.2. Blistering test
Quantitative analysis results of blister area are tabulated
in Table 3 and plotted in Fig. 9. Liquid die cast samples
ð4Þ

6272 S. Janudom et al. / Acta Materialia 61 (2013) 6267–6275
Liquid
V g /f s =29.90
V g /f s =22.43
V g /f s =9.10
V g /f s =4.55
Fig. 7. Short shot samples showing the flow patterns during die filling.
Normalized Flow Interface Length
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Fill Fraction 0.3
Fill Fraction 0.5
Fill Fraction 0.8
Fill Fraction 0.3 (Liquid)
Fill Fraction 0.5 (Liquid)
Fill Fraction 0.8 (Liquid)
0 5 10 15 20 25 30 35
V g /f s
Table 3
Summary of results from the blistering and tensile tests.
Sample V g /f s Blister area
(mm 2 )
Ultimate tensile
strength (MPa)
Elongation (%)
Liquid N/A 37.07 289.9 ± 38.5 13.3 ± 4.2
S-5-10 14.95 2.32 307.2 ± 4.4 9.5 ± 0.5
S-5-15 22.43 3.38 299.6 ± 5.2 10.1 ± 1.3
S-5-20 29.90 5.56 287.7 ± 14.0 9.7 ± 1.4
S-10-10 4.55 1.96 290.0 ± 9.8 8.7 ± 0.6
S-10-15 6.83 2.24 307.2 ± 5.2 11.4 ± 0.7
S-10-20 9.10 3.22 312.6 ± 5.0 13.5 ± 1.5
Fig. 8. Analysis results of the normalized flow interface length as a
function of V g /f s .
have a very high value of blister area of 37.07 mm 2 . Slurry
die cast samples have significantly lower blister area, in the
range of 1.96–5.56 mm 2 . Results show the trend that slurry
die casting conditions with high V g /f s values have high blister
area. The data clearly show that the blister area results
are consistent with the results of normalized flow interface
length. Die casting conditions with excessively high gate

S. Janudom et al. / Acta Materialia 61 (2013) 6267–6275 6273
Blister Area (mm 2 )
40
35
30
25
20
15
10
5
SSM
Liquid
0
0 5 10 15 20 25 30 35
V g /f s
Fig. 9. Blister area on the surface of the samples after T6 heat treatment.
Ultimte Tensile Strength (MPa)
340
330
320
310
300
290
280
270
260
250
Slurries-UTS
Liquid-UTS
Slurries-Elongation
Liquid-Elongation
240
6
0 5 10 15 20 25 30 35
V g /f s
Fig. 10. Ultimate tensile strength and elongation data of the die cast
samples after T6 heat treatment at different values of V g /f s .
24
22
20
18
16
14
12
10
8
Elongation (%)
speeds and insufficient solid fractions result in turbulent
flow patterns and high levels of blister defect.
3.2.3. Tensile test
Ultimate tensile strength and elongation results are
listed in Table 3 and plotted as a function of V g /f s in
Fig. 10. For liquid die casting, the results clearly show that
the die cast parts have a large variation of ultimate tensile
strength and elongation values (large standard deviation
values). This result indicates a high degree of randomness
of defects in the liquid die cast parts, which suggests that
the flow pattern is turbulent.
For slurry die casting, results show a clear trend that the
conditions with higher V g /f s values than 9.10 yield lower
ultimate tensile strength and elongation. The data are consistent
with the flow pattern and blister test results, which
suggest turbulent flow with high V g /f s values for slurry
die casting. The results also show that slurry die casting
with low V g /f s values also yield low mechanical properties.
This may be because of insufficient time for shrinkage feeding
due to a combination of a high solid fraction and a low
gate speed.
Representative secondary electron micrographs of the
tensile fracture specimens are given in Fig. 11. Fracture
surfaces of the liquid die casting specimens and slurry die
casting specimens with V g /f s of 29.90 are given in
Fig. 11c and d, respectively. Gas and shrinkage porosities
are clearly observed at the M and C positions, as indicated
by the arrows. For the slurry die casting samples with V g /f s
of 9.10, no porosity defects are observed. This is consistent
with the highest mechanical properties of 312 MPa in ultimate
tensile strength and 13.5% in elongation of this processing
condition. For lower values of V g /f s such as the
T
M
P
P
P
P
P
C
P
P
P
P
(a) V g /f s = 4.55 (b) V g /f s = 9.10 (c) V g /f s = 29.90 (d) Liquid
Fig. 11. Representative scanning electron micrographs of the fracture surfaces of the tensile samples.

6274 S. Janudom et al. / Acta Materialia 61 (2013) 6267–6275
case of 4.55, shrinkage porosities are clearly present at the
M and C positions, see Fig. 11a. This defect leads to lower
values of ultimate tensile strength and elongation, as shown
in Fig. 10.
The results of fracture surface analysis are consistent
with other analysis results. The samples formed by liquid
die casting and slurry die casting with high V g /f s values
have gas porosity due to turbulent flow patterns during
die filling. On the other hand, the samples formed by slurry
die casting with too low V g /f s values contain shrinkage
porosity. This is because of insufficient time for shrinkage
feeding due to a combination of a high solid fraction and
a low gate speed.
4. Conclusions
Several conclusions can be derived from this work,
including the following:
1. Semi-solid slurries containing low initial solid fractions
can be produced and controlled by varying
the rheocasting times. With the current processing
conditions, slurries having 3.5% and 11.5% solid
were created with the rheocasting times of 5 and
10 s, respectively.
2. The microstructures of all the die cast samples
consist of a phase particles and very fine dendritic
microstructure. The micrographs clearly show
that slurry die casting samples have coarser
a phase particles, which is due to the long growth
time during the rheocasting process. However,
in general, the results show that all the processing
conditions produce quite uniform
microstructures.
3. The results from the flow pattern analysis suggest
that the flow behavior can be controlled by varying
the initial solid fraction. With a high gate speed of
1.05 m s –1 , die casting with liquid aluminum clearly
yields turbulent flow. Also, in slurry die casting, the
initial solid fraction of 3.5% is not sufficient to yield
laminar flow (V g /f s = 29.90). However, with the
initial solid fraction of 11.5% (V g /f s = 9.10), the
flow behavior clearly becomes laminar.
4. Blister test results show the trend that slurry die
casting conditions with high V g /f s values have a
high level of blister area. The data clearly show
that the blister area results are consistent with
the results of normalized flow interface length.
Die casting conditions with excessively high gate
speeds and insufficient solid fractions result in turbulent
flow patterns and high levels of blister
defect.
5. The results of tensile test and fracture surface analysis
are consistent with other analysis results. The
samples formed by liquid die casting and slurry
die casting with high V g /f s values have gas porosity
due to turbulent flow pattern during die filling. On
the other hand, the samples formed by slurry die
casting with too low V g /f s values contain shrinkage
porosity. This is because of insufficient time for
shrinkage feeding due to a combination of a high
solid fraction and a low gate speed.
6. This study shows that for each die design, there is a
range of optimum values of V g /f s to yield cast parts
with minimum defects. With a given solid fraction,
the gate speed can be adjusted to a value that will
not result in turbulent flow and insufficient time
for shrinkage feeding. In other words, with a given
gate speed, the initial solid fraction may also be
adjusted.
7. The results from this study demonstrate that die
casting with slurries containing low initial solid
fractions gives die casters another process parameter
to adjust, which can help reduce and control the
gas and shrinkage porosities.
Acknowledgements
This work was supported by the Higher Education
Research Promotion and National Research University
Project of Thailand, Office of the Higher Education Commission
(Contract No. ENG540551a), the Royal Golden
Jubilee PhD Program (Grant No. PHD/0173/2550) and
the Thai Research Fund (Contract No. MRG5280215).
The authors also would like to thank the Innovative Metal
Technology (IMT) team and GISSCO Co. Ltd for kind
support.
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