Post on 10-Dec-2021
Analysis of microstructural, mechanical and thermal behaviourof thixocast LM25-10wt% SiC composite at different processingtemperatures
ANOOP KUMAR TULI1, PRADEEP SINGH2,* , S DAS3, D P MONDAL3 and J P SHAKYA2
1S.V. Polytechnic College, Shyamla Hills, Bhopal 462002, India2Department of Mechanical Engineering, Samrat Ashok Technological Institute, Vidisha 464001, India3CSIR-Advanced Materials and Processes Research Institute (AMPRI), Bhopal 462026, India
e-mail: erpradeep3408@gmail.com
MS received 15 March 2021; revised 30 June 2021; accepted 13 September 2021
Abstract. In this study, composite was made by the addition of 10 wt% SiC particles in the LM25 matrix
through gravity casting and thixocasting processes. Thixocasting temperature was varied to investigate its effect
on the microstructure, mechanical and thermal properties. Properties of thixocast samples were also compared
with gravity cast ones. XRD analysis was done to analyse the phases present in the samples. Optical microscope
and scanning electron microscope were used to record the microstructures of gravity cast as well as thixocast
samples. Tensile, compressive and flexural tests were performed on universal testing machine with the prepared
composite samples according to the ASTM standards. Thermal diffusivity tests of gravity cast and thixocast
samples were done on laser flash diffusivity instrument. Results reveal that thixocast samples are sounder than
the gravity cast samples. The microstructure of a-Al phase transformed from dendritic to globular shape due to
the thixocasting. Also, volume fraction of eutectic phase in the sample increased with the increment of thixocast
temperature. Tensile strength, compressive strength and flexural strength were found higher in the case of
thixocast samples as compared to gravity cast. Thermal diffusivity considerably increased due to thixocasting,
while its value slightly reduced with increment of thixocast temperature.
Keywords. Thixocasting; LM25 alloy; mechanical properties; thermal properties; composite.
1. Introduction
The trend of light-weight materials in the automobile and
aerospace industry is increasing day by day due to fuel
economy concern. Al alloys have higher specific strength,
good weldability, lower melting point and noble thermal
conductivity [1–3]. These are the most appealing properties
which are well suited for the synthesis of various compo-
nents in automotive industries. Vehicle frame, cylinder
heads, engine blocks, piston, steering and brake compo-
nents, rotors etc. are being manufactured by the use of Al
alloys [4–6]. Due to lower melting point and good forma-
bility of the Al alloys, number of components are processed
through casting or forging techniques. Components made of
forging techniques have better mechanical properties than
the parts prepared through casting process [7, 8]. However,
higher cost and limited shape and size of the components
restrict the forging process for the wider applications.
Casting technique provides easiness in manufacturing of
different shape and size components, but mechanical
properties are inadequate as compared to the forged com-
ponents. This is due to the various defects like dendritic
grains, voids, inclusions, inappropriate dimensions associ-
ated in the cast parts [9].
Defects in the products synthesized through casting
techniques can be minimized by the adoption of semi-
solid casting process. Semisolidcasting process is of
growing industrial significance as it offers several
advantages over the conventional casting owing to lower
porosity and forming temperatures, improved flow prop-
erties, reduced process force, near net-shape forming,
better mechanical properties. In this process, metal is
heated at semi solidus temperature to obtain the globular
solid phase in the certain fraction of liquid phase
[10, 11]. Movement of globular solid particles becomes
easy in the liquid phase which in turn minimize the pores
and voids in the product. Because of the semi-solid state
and globular solid particles, dendritic structure of the
material can be minimized which is beneficial to improve
the mechanical properties. Through this technique,
porosity free complex parts with tight tolerance and
excellent mechanical properties can be obtained.*For correspondence
Sådhanå (2021) 46:219 � Indian Academy of Sciences
https://doi.org/10.1007/s12046-021-01757-3Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)
Semi-solid metal processing technique can be catego-
rized in two different techniques; known as rheocasting and
thixocasting. In rheocasting technique, a metal alloy is
heated at a semi-solidus temperature to achieve slurry of
liquid and globular solid phases. The weight fraction of
liquid and solid phase in the slurry depends on the pro-
cessing temperature in the semisolid region as shown in
figure 1a. With increment of processing temperature in the
semi-solidus region, the amount of liquid phase present in
the slurry increases (Lever Rule). In rheocasting process,
slurry is directly poured in the mould and allowed to
solidify to obtain the final product. On the other hand, in
thixocasting process, slurry is solidified in the form of ingot
and then ingots are cut into small pieces, melted again in
the semi solidus region to find the desired viscosity, stirred,
filled in the predefined mould cavity and allowed to cool
[12]. Thixocast products have extremely high quality in the
form of close tolerance, voids free and high mechanical
properties. However, the manufacturing cost of the thixo-
cast products is higher than the rheocast products.
Thixocasting process is more suitable for the larger
products made of Al alloys. In the conventional casting
process, more heat is required to extract from the
conventional casting as compared to thixocasting process.
Therefore, solidification time is higher in cast product
which is responsible for the dendritic grain structure, hence
reduces the mechanical properties. In thixocasting, the heat
energy associated with the slurry is much lower than its
melt, causing rapid solidification as compared to the con-
ventional casting. Consequently, globular microstructure of
the grains retained without entrapment of gases and
improved the properties of the final product.
Mechanical properties of Al alloys can be improved by
the addition of ceramic materials which have excellent
strength and high elastic modulus. Hard ceramic materials
are reinforced in the form of particulates or whiskers in
highly tough and ductile matrix of Al alloys to make Al
matrix composite (AMC). Carbon nano tube (CNT), alu-
mina (Al2O3), silicon carbide (SiC), titanium boride (TiB2),
graphite (Gr) are the most commonly used ceramic mate-
rials which are reinforced in the Al alloy matrix [13–17]. In
the present time, researchers are trying to develop ceramic
reinforced Al alloy composites which associate with high
strength along with toughness for the aerospace and auto-
mobile applications.
The aim of the present study is the synthesis of LM25
alloy composite reinforced with silicon carbide (SiC) par-
ticulates through thixocasting method. Thixocasting tem-
perature was optimized to find the best mechanical
properties of the synthesized composite. Different thixo-
casting temperatures were selected from the Al-Si phase
diagram which are located in the semi-solid region as
shown in figure 1b [18].
2. Experimental procedure
2.1 Materials and methods
LM25 alloy was used as the base material. The chemical
composition of LM25 alloy was determined by the optical
emission spectrometer (model: SPECTRO Max LMF05,
Germany) and presented in table 1. From table 1, it is
confirmed that silicon (Si) is the main alloying element in
the LM25 alloy with weight percentage 7.46. The average
size of SiC particles was analysed by the laser scattering
particle size analyser (model: Horiba LA-950) and it was
measured as 25 lm. LM25-10SiC composite was made by
the addition of 10 wt% preheated SiC particles in the LM25
alloy melt. The synthesis procedure of LM25-10SiC com-
posite is represented by the schematic diagram as shown in
figure 2. Initially, LM25 alloy was melted in the electric
Figure 1. (a) Represents the schematic phase diagram showing
semisolidous or thixocast temerature, (b) shows the split Al-Si
phase digram.
Table 1. Chemical Composition (wt%) of LM25 Alloy.
Si Mg Cu Mn Zn Pb Sn Fe Ni Ti Al
7.5 0.2 0.1 0.3 0.1 0.1 0.05 0.5 0.1 0.2Remainder
219 Page 2 of 12 Sådhanå (2021) 46:219
resistance furnace at temperature range of 700–720�C and
then 10 wt% preheated SiC particles (SiC particles were
preheated at 600–700�C for 6 hours to avoid splashing and
cracking in the SiC particles due to temperature difference
during adding in the LM25 alloy melt) were added in the
LM25 alloy melt. Mixture was simultaneously stirred using
mechanical stirrer up to 3 minutes for the proper distribu-
tion of SiC particles in the melt. Coveral 11 was used as
cover flux and dry nitrogen gas as degasser. Small amount
of Al – TiB2 master alloy was added in composite melt
prior to pouring into the die for casting to achieve relatively
globular dendritic structure and grain refinement of matrix
material. The liquid mixture was poured in the cast iron
finger-shaped mould cavity and allowed to solidify.
For the thixocasting, finger-shaped cylindrical billets
were cut in to the small pieces. These pieces were again
partially melted in a cylindrical die of inner diameter 40
mm and height 115 mm. For partial melting, the selected
temperatures of the pieces were kept in semi-solid region
according to the Al-Si phase diagram [18]. To investigate
the effect of processing temperature, pieces were heated at
three different semi solidus temperatures (590 �C, 600 �Cand 610 �C). Finally, semi-solid pieces were pressurised
within the same cylindrical die by using a 400 ton pressure
die casting machine. After solidification, thixocast samples
were removed from the die. The samples thixocast at 590
�C, 600 �C and 610 �C are assigned as Thixo 590, Thixo
600 and Thixo 610, while gravity cast LM25-SiC sample as
As Cast.
2.2 Characterization techniques
For the microstructural analysis, thixocast samples of each
group were radially cut into small pieces and cold mounted
using resin and hardener for the handling purpose during
polishing. Mounted samples were polished by the use of
sandpapers of 100 to 1600 grits. After sandpaper polishing,
samples were mirror polished using cloth on disc polisher.
Microstructure of mirror polished samples were recorded
Figure 2. Schematic diagram of synthesis of thixocast samples.
Figure 3. Universal testing machine (UTM) with their respective test samples of tensile (a), compressive (b) and flexural (c).
Sådhanå (2021) 46:219 Page 3 of 12 219
by the use of Scanning Electron Microscope (model: JEOL
5600). XRD analysis was performed using XRD diffraction
machine (Rigaku Japan: miniflex ii) of CuKa radiation with
scan rate 0.2�/s. Different elements and phases present in
the samples were determined by matching the d value of
each peak with the standard JCPDS file. Density was cal-
culated by the measurement of weight and volume of the
each sample.
The tensile test of gravity cast and thixocast samples was
carried out at a strain rate of 0.01/s on standard tensile
specimens (as per ASTM standard B557) [19]. During the
test, the samples were pulled until failure. The stress and
strain were recorded in the system interfaced with UTM
(Instron: Model 8801). The UTM machine and standard
tensile specimen is shown in figure 3a. The compression
test of gravity cast and thixocast samples was carried out at
a strain rate of 0.01/s on cylindrical samples of 12 mm
diameter and 20 mm length on the same UTM machine as
displayed in figure 3b. During the compression test,
surfaces between the anvil and specimen were lubricated
with MoS2 to avoid friction and proper deformation of the
contact surfaces. To investigate the flexural behaviour of
the composite, three point bending test was performed on
the same UTM with cross head speed 1 mm/min or 0.004/s
strain rate. The UTM with bending specimen is shown in
figure 3c. The specimens used for bending test were pre-
pared in the rectangular die during thixocasting to obtain
rectangular bar shape. The span length of the rectangular
specimen was 70 mm, while breadth and thickness were 55
mm and 4.2 mm, respectively [20].
Thermal diffusivity (aT) of the as cast and thixocast
samples were determined by a vertical laser flash diffusivity
instrument (model: LFA 467 Hyper flash) at different
temperatures (room temperature and 50 �C to 300 �C with
an alternate temperature of 50 �C). Disc specimens with
diameter 12 mm and thickness 2 ± 0.5 mm were prepared
for this test. The experiment was done in the vacuum shield
chamber of the instrument with graphite heater. Specimens
were contained in a graphite coated sapphire cup while
laser passes through the transparent top striking the upper
surface, resulting in temperature rise in the rear face
monitored by infrared detector.
3. Results and discussion
3.1 Density analysis
The actual density (qaÞ of machined as cast as well as
thixocast billets was calculated by Archimedes principle
according to the ASTM B862-17 [21]. The theoretical
density was also measured to investigate porosity in the
samples. It was measured by the equation of rule of mixture
as given below;
1
qT¼ mLM25
qLM25
þ mSiC
qSiCð1Þ
where qT is the theoretical density of the composite,
mLM25 and mSiC are the mass fractions of LM25 alloy and
SiC particulates used to prepare composite, qLM25 and qSiCare densities of LM25 and SiC particulates. From equation
(1), theoretical density of LM25-10SiC composite was
obtained as 2.73 g/cm3.
Porosity present in the samples was determined by the
following equation:
Porosity %ð Þ ¼ 1� qaqT
� �� 100 ð2Þ
Density variation (actual) of as cast and thixocast sam-
ples is shown in figure 4a. From the figure, it can be
depicted that due to the thixocasting, improvement in
density takes place. For the sample Thixo 590, density is
about 1.5% higher than the As Cast sample. In the group of
thixocast samples, the Thixo 590 sample has slightly higher
Figure 4. Variation of density (a) and porosity (b) for the As
Cast and thixocast samples.
219 Page 4 of 12 Sådhanå (2021) 46:219
density than Thixo 600 and Thixo 610 samples. Porosity
present in the different samples are shown in figure 4b. This
figure indicates that As Cast sample has about 63% higher
porosity than the thixocast samples. This indicates that the
soundness of the product prepared by the thixocasting
method is better than the gravity cast products.
3.2 XRD analysis
Figure 5 represents the Xray diffraction pattern for the
LM25 as cast and Thixo 590 samples. In LM25 alloy, Si is
abudantly present with 7.5 wt%. Therefore, peaks of Si can
be easily detected. The Al-Si phase diagram suggests that
the solubility of Si in a-Al is very low [22]. At 577�C, only1.65 wt% Si can be totally dissolved in the a-Al. The sol-
ubility limit also reduces with the reduction of temperature.
Also, any intermetallic or precipitate of Al and Si can not
be formed. Therefore, these metals present as the a-Al andthe eutectic Si particles. Mg2Si phase is also detected with
the base materials. Diffracction pattern of Thixo 590 sam-
ple is also represented in figure 5. The diffraction pattern of
Thixo 590 contains the peaks of Al, Si, Mg2Si and the SiC.
Actually, Al, Si and Mg2Si particles present due to the
LM25 alloy and SiC peaks were present due to the addition
of SiC particles as a reinforcement in the LM25 base alloy.
The observed peaks of the thixocast samples are similar to
the as cast samples made by the use of same alloy. It
indicates that due to thixocasting, the phase formation does
not change. Additionally, the increment of thixocast tem-
perature does not alter the formed phases.
Figure 5. XRD analysis for the as cast LM25 and thixocast composite sample at 590 �C.
Figure 6. Surface microstructures of the different samples. (a),
(b), (c) and (d) represent the microstructure of as cast and
thixocast samples processed at 590 �C, 600 �C and 610 �C,respectively.
Sådhanå (2021) 46:219 Page 5 of 12 219
3.3 Microstructural, grain size and phase analysis
Cross sectional microstructures of As Cast and Thixo 590,
Thixo 600 and Thixo 610 samples are shown in figure 6.
Figure 6a represents the surface microstructure of the As
Cast sample in which dendritic-shaped a-Al phase was
observed. Eutectic Si phase is uniformly distributed in the
microstructure. Microstructure for the Thixo 590 sample is
shown in figure 6b. As can be seen in this figure that
dendritic a-Al transforms in the globular shape due to
thixocasting. Eutectic Si phase (black spots) is also present
in the microstructure. From the inset microstructure, it can
be clearly seen that SiC particles embedded in the a-Al aswell as eutectic Si phase. With the increment of thixocast
temperature from 590 �C to 600 �C, level of sphericity of
globular a-Al increases (figure 6c). However, for Thixo 610sample, sphericity of the a-Al phase reduces and globular
shape changes in the dendritic type of structure (figure 6d).
In the previous work, it was found that with the increment
of semisolid temperature, the fineness and globular shape of
a-Al decline and size with dendritic shape of the grain
increases [23]. Similar kind of behaviour of a-Al phase wasobserved in the present work.
Intercept method (ASTEM E112-13) was used to analyse
the average grain size of the each sample [24]. In this
method, number of randomly oriented straight lines was
drawn on the micrograph and then actual length of the line
is divided by the number of intercepts cut by the line on the
grain boundary. Aspect ratio of the grains was determined
by the average ratio of major and minor dimensions of 500
grains. The obtained average grain size and aspect ratio for
the as cast and thixocast samples are shown in figure 7a, b,
respectively. It can be seen that average grain size of As
Cast is about 150 lm. After thixocasting, the average
grain size reduces significantly with attaining minimum
value about 50 lm for Thixo 590 sample (figure 7a). The
aspect ratio of the As Cast sample is much higher than the
other thixocast sample. It is due to the dendritic shape of
the grains. In the group of thixocast samples, aspect ratio
is minimum for the Thixo 590, while it increases slightly
with the increment of thixocasting temperature from 590
�C to 600 �C and 610 �C (figure 7b). In the thixocasting
process, recrystallization and reorientation take place and
formation of new grains occurs. The grain growth depends
on the viscosity of the formed slurry at a certain thixo-
casting temperature. For higher viscosity, the outer pres-
sure on the surface of newly formed grain will be more
which depresses the grain growth after attaining a definite
size. With increment of thixocasting temperature, viscos-
ity of the slurry reduces [25] and therefore, pressure
applied by the viscous fluid on the new born grain reduces,
causing increment in grain size and aspect ratio.
Volume fractions of formed eutectic phase in the sam-
ples were obtained by the evaluation of area fraction of
eutectic phase in the recorded micrograph of each sample.
The area fraction of eutectic phase was calculated by the
use of Image J software through image threshold method
as shown in figure 8. Figure 8a represents the original
micrograph, while figure 8b shows the threshold micro-
graph. Threshold eutectic phase is shown by the red col-
our, while white colour shows the a-Al phase. The area
fraction of red colour in the image is about 30%. This is
how eutectic phase was measured for as cast and thixocast
samples and shown by a histogram in figure 8c. From this
figure, it can be determined that volume fraction of
eutectic phase is higher for the thixocast samples. With the
increment of thixocast temperature, the content of eutectic
phase increases, while a-Al phase decreases (figure 8c, d).Amount of eutectic phase present in the thixocast sample
depends on the extent of liquid metal in the slurry during
the semisolid processing. As suggested by the Al-Si phase
diagram, amount of liquid in the slurry increases with the
increment of thixocast temperature, therefore volume
fraction of eutectic phase also increases in the final
product.
Figure 7. Variation of grain size and aspect ratio obtained by the
microstructures of different samples. (a) represents the variation of
grain size, while (b) shows the change in aspect ratio.
219 Page 6 of 12 Sådhanå (2021) 46:219
3.4 Analysis of tensile test and fractography
The tensile stress-strain curve of as cast and thixocast
samples is shown in figure 9. The ultimate tensile strength
(UTS), yield stress at 0.2% strain and percentage elongation
of each sample is also given in table 2. It can be observed
that thixocast samples have comparatively higher UTS than
the gravity cast or as cast sample. UTS of gravity cast
sample is about 27% to 37% lower than the thixocast
samples. Also, the elongation improves due to the thixo-
casting. For the As cast, Thixo 590, Thixo 600 and Thixo
610 samples, the elongation at the point of ultimate tensile
strength was measured as 0.7 %, 1%, 1.2% and 1.2%,
respectively. The reason of increment of elongation due to
thixocasting may be the good bonding among the rein-
forced SiC particles with the eutectic silicon phase and
globular a-Al phase. The increment in ultimate tensile
strength of the thixocast samples is due to the evolution of
primary a-Al from dendritic to globular shape. The den-
dritic microstructure of a-Al in as cast sample influences
the soundness of the product. During solidification, for-
mation of dendritic structure may cause the generation of
micro-porosity along the interface of pre-solidified phase
and liquid phase. Also, dissolved gases evaporates from the
casting during the solidification which create micro-
porosity. In the thixocast samples, these micro-porosities
minimized in the influence of globular microstructure of a-Al and application of pressure. Presence of micro-porosity
Figure 8. Original microstructure (a) and threshold microstructure through image J software to investigate the volume fraction of
different phases (b). (c) Represents the variation of volume fraction of eutectic phase, while (d) shows the variation of volume fraction of
a-Al phase for different samples.
Sådhanå (2021) 46:219 Page 7 of 12 219
in the material considerably affect the mechanical proper-
ties [26, 27]. In the group of thixocast samples, it may be
observed that tensile strength influences by the thixocasting
temperature. For Thixo 590 and Thixo 600 samples the
variation of UTS is minute. But for Thixo 610 sample, it is
about 6 to 7 % lower. For the uniform distribution of SiC
particles, an optimized viscosity of the slurry is required for
the proper movement of the mixture due to mechanical
stirring during the time of thixocasting. At the higher
thixocasting temperature the viscosity of the slurry
decreases, therefore the chance of agglomeration of SiC
particles may increase. Hence, strength reduces with
increment of processing temperature. It is demonstrated
that it is better to conduct thixocasting of composite at
lower temperature (590�C).
The tensile fractography of as cast and thixocast samples
is shown in figure 10. Figure 10a1, a2 are the fractograph for
As Cast sample at lower and higher magnifications
respectively. In figure 10a1, the presence of voids can be
clearly seen. These voids may arise due to the dendritic
structure of a-Al and evolution of dissolved gases from the
casting during solidification. The agglomerated SiC parti-
cles (circled in figure 10a1) were also detected in the
fractography and the distribution of SiC particles in the
matrix is uneven. It is due to the difference in density of
SiC particles and matrix material (LM 25). There is more
chance of drainage of heavy SiC particles along the wall of
the container because of the vortex formed in the influence
of mechanical stirring of the melt during casting. It causes
the SiC particles agglomeration and nonuniform distribu-
tion. Probably, this is also a major reason of weakening of
as cast sample. In Figure 10a2, thick plates with ridges can
be easily detected. Some dimples were also found in the as
cast sample. This reveals that behaviour of the tensile
failure of the as cast sample is the combination of ductile
and the brittle nature. The reason of ductile failure is the
presence of soft a-Al phase, while brittle failure might
takes place at the interface of SiC particles and the matrix.
Figure 10b1, b2 represent the fractography of Thixo 590
sample. In figure 10b1, it can be observed that SiC particles
uniformly distributed in the matrix. Also, fine plates with
ridges and more number of tiny dimples present in the
Thixo 590 as compared to as cast sample (figure 10b2). The
fineness of the plates is caused due to the shearing among
the fine and globular a-Al and SiC particles, while more
number of dimples narrates the ductile nature of the
thixocast material. Figure 10c1, c2 are the fractograph of
Thixo 600 sample. In this sample, fractured particles are
bigger in size than the Thixo 590 (figure 10c1). Also,
cleavage steps are found with thick as well as fine ridges
Figure 9. Tensile stress-strain diagram for the gravity cast and
thixocast samples.
Table 2. Tensile, compressive and flexural properties of the gravity cast (As Cast) and thixocast samples.
Sample name Engineering stress at yield (ryt) (MPa) Engineering strain at yield (et) (%) Ultimate strength (rut) (MPa))
Tensile propertyAs Cast 100 ± 5 4.87 ± 0.24 123 ± 6.15
Thixo 590 145 ± 7 5.90 ± 0.29 170 ± 8.50
Thixo 600 130 ± 7 5.08 ± 0.25 152 ± 7.60
Thixo 610 125 ± 6 5.08 ± 0.25 142 ± 7.10
Compressive propertyAs Cast 243 ± 13 3.5 ± 0.17 347 ± 17.35
Thixo 590 337 ± 18 5.4 ± 0.27 473 ± 23.65
Thixo 600 333 ± 18 4.4 ± 0.22 467 ± 23.35
Thixo 610 314 ± 16 4.3 ± 0.21 461 ± 23.05
Flexural propertyAs Cast 219 ± 10.95 2.12 ± 0.10 79 ± 4
Thixo 590 338 ± 16.90 2.35 ± 0.11 121 ± 6
Thixo 600 303 ± 15.50 3.84 ± 0.19 120 ± 6
Thixo 610 301 ± 15.05 2.68 ± 0.13 112 ± 5
219 Page 8 of 12 Sådhanå (2021) 46:219
(figure 10c2). The fractographs of Thixo 610 are near
similar to the fractographs of as cast sample, but the ridges
are fine and dimples are more in numbers. It is due to the
tending of a-Al phase towards the dendritic shape.
3.5 Compression test Analysis
Figure 11 shows that the compressive strength for the
thixocast composite samples improved considerably as
compared to that of gravity cast samples. The value of yield
stress, ultimate compressive stress (UCS) and elongation
are given in table 2. It was noted that the strength of
thixocast samples varies with the thixocasting temperature.
At higher temperature a-Al become coarser and Si becomes
finer. But at lower temperature a-Al become finer and Si
needles become coarser. At higher temperature more SiC
pushed by the dendrites which may get either embedded in
a-Al matrix or segregated to the inter-dendritic region. The
improvement in strength and strain at failure is due to
microstructural modifications, better bonding of SiC within
the matrix and uniform distribution of SiC particles.
Coarser dendrite and Si needles with lower amount of a-Alcauses decrease in strength when processing temperature is
increased. Also, the strength noted to be decreased with an
increase in thixocasting temperature.
3.6 Analysis of three point bending test
The flexural stress-strain curves for the different thixocast
and as cast composite samples made of LM25 and 10 wt%
SiC are shown in figure 12. The trend of the stress-strain
curve is similar as discussed above in the tensile and
compressive stress-strain diagram. Flexural properties are
also recorded in table 2. The flexural properties for the
thixocast composite samples found to be improved
Figure 10. Microstructures of fractured surfaces of the various
samples subjected to the tensile test.
Figure 11. Compressive stress-strain diagram for the gravity cast
and thixocast samples.
Figure 12. Flexural stress-strain diagram for the gravity cast and
thixocast samples.
Sådhanå (2021) 46:219 Page 9 of 12 219
considerably as compared to that of gravity cast samples. It
is caused due to the change in microstructure from the
dendritic to the globular in shape. The uniform distribution
of SiC particles in the eutectic phase also improves the
bonding among the particles. Also, the flexural strength of
thixocast samples found to be changed with the change in
thixocasting temperature. The improvement in flexural
strength of thixocast composite is due to microstructural
modifications and better bonding between SiC and the
matrix. Thixocasting results in uniform distribution of SiC
particles which also would cause better flexural properties.
There is a decrease in flexural strength with an increase in
thixocasting temperature due to coarser dendrite, coarser Si
needles and lower amount of a-Al.
The fracture surface of LM25-10SiC gravity cast composite
showed fracture along the agglomerated SiC particles,
because these are the weaker regions (figure 13a). The parti-
cles also get sheared during deformation. The matrix contains
dendrites of a-Al and eutectic silicon. As a result, finer den-
drites get sheared and ridges formed around dendrites. These
lead to flower type fracture of matrix (figure 13b). The ridges
and shearing of matrix demonstrate brittle type fracture. The
same composite when thixocast, the particles also moved and
the dendrites get finer. Due to this, materials developed some
kind of ductility. Fracture surface showed some dimples.
However, at the same time, particle shearing and debonding
took place (figure 13c). In addition to this, shearing of den-
drites and SiC particles took place (figure 13d). This
Figure 13. Microstructures of fractured surfaces of the various samples subjected to the flexural test.
219 Page 10 of 12 Sådhanå (2021) 46:219
figure also showed debonding of particles. When thixocast at
higher temperatures, because of more amount of liquid phase,
the particle may move and agglomerate. The fracture may
occur through these agglomerated particles through bonding
of particles in the inter-dendritic region (figure 13e). Here
also, micro-cracks are formed, dendrites get sheared and inter-
dendritic regions get fractured through formation of ridges
(figure 13f). All these characteristics dictate that the fracture
occurs mainly through brittle mode. However, thixocast
sample (when thixocast at lower temperature) exhibits some
extent of ductility. The ductility of composite reduces when
thixocast at higher temperature.
3.7 Analysis of thermal diffusivity
Measured thermal diffusivity of as cast and thixocast samples
at different temperatures is shown in figure 14. With incre-
ment of temperature, thermal diffusivity reduces for all the
samples. The reduction of thermal diffusivity is almost linear.
In the previous works done by researchers, it was mentioned
that thermal diffusivity reduces with temperature [28]. The
figure shows that thermal diffusivity of the thixocast sample
is higher than the as cast sample. It is about 67 mm2/s for the
Thixo 590 which is 18% higher than the As Cast sample at
room temperature. It is due to the less porosity and globular
a-Al with fine eutectic silicon phase present in the thixocast
sample [29]. Thermal diffusivity of Thixo 590 is also higher
than Thixo 600 and Thixo 610. It indicates that higher
thixocasting temperature is unfavourable for the thermal
diffusivity. Payandesh et al stated that higher volume fraction
of solid phase in the material is beneficial to improve its
thermal behaviour [30]. Also, the dendritic nature of a-Alphase increases, while eutectic silicon phase becomes thick
with increment of thixocast temperature which may cause
reduction in the thermal diffusivity.
4. Conclusions
LM25-10wt% SiC composites were prepared through grav-
ity cast (as cast) and thixocast method at different thixo-
casting temperature. Microstructural evolution, mechanical
properties and thermal behaviour were compared for the as
cast and thixocast samples. Also, the effect of thixocasting
temperature on these entities were analysed. According to
the results and discussion, following conclusions are made.
• Density of the thixocast samples is considerably higher
than the density of gravity cast samples. With incre-
ment of thixocasting temperature, slight reduction in
the density of thixocast sample was observed.
• Gravity cast sample contains dendritic a-Al phase,
while due to thixocasting, dendritic a-Al phase trans-
forms in globular shape. The volume fraction of
eutectic silicon phase in the thixocast samples
increases with the thixocasting temperature.
• Thixocast composite (LM25-10SiC) samples were
found with significant improvement in mechanical
properties (tensile, compressive and flexural strengths)
as compared to the gravity cast sample due to fine and
globular microstructure. However, mechanical proper-
ties slightly degrade with the increment of thixocasting
temperature from 590 �C to 610 �C.• Tensile fracture analysis reveals that decohesion of SiC
particles takes place for the gravity cast sample. But in
thixocast sample, particle shearing is the major mech-
anism of fracture. The SiC particles distribution and
their interface bonding with matrix are much better
when thixocast at 590�C. This is also true for the
flexural behaviour of the material.
• Thermal diffusivity of thixocast samples is considerably
higher than the gravity cast sample. With increment of
thixocasting temperature, thermal diffusivity slightly
reduces. Also, the value of thermal diffusivity decreases
linearly with the increment of operating temperature.
Acknowledgements
The authors are thankful to the Director, CSIR-AMPRI,
Bhopal for providing necessary equipment facility to
perform this work.
References
[1] Dursun T and Soutis C 2014 Recent developments in
advanced aircraft aluminium alloys. Mater. Des. 56:
862–871. https://doi.org/10.1016/j.matdes.2013.12.002.
[2] Chua S F, Chen H C and Bi G 2019 Influence of pulse energy
density in micro laser weld of crack sensitive Al alloy sheets.
J. Manuf. Process. 38: 1–8. https://doi.org/10.1016/j.jmapro.
2018.12.035
Figure 14. Variation of thermal diffusivity for the gravity cast
and thixocast samples at different operating temperature.
Sådhanå (2021) 46:219 Page 11 of 12 219
[3] Surappa M K 2003 Aluminium matrix composites: Chal-
lenges and opportunities. Sadhana. 28: 319–334[4] Guo B, Zhang W, Li S and Wang X 2017 High temperature
low cycle fatigue and creep-fatigue behavior of a casting Al-
9Si-CuMg alloy used for cylinder heads. Mater. Sci. Eng. A.700: 397–405. https://doi.org/10.1016/j.msea.2017.05.111
[5] Lombardi A, Sediako D, Machin A, Ravindran C and
MacKay R 2017 Effect of solution heat treatment on residual
stress in Al alloy engine blocks using neutron diffraction.
Mater. Sci. Eng. A. 697: 238–247. https://doi.org/10.1016/j.msea.2017.05.026
[6] Zhang S and Feng S 2011 Friction and wear performances of
brake material dry sliding against a composite with a semi-
interpenetrating network structure of ceramics and Al-alloy.
Tribol. Int. 44: 248–257. https://doi.org/10.1016/j.triboint.
2010.10.029
[7] Li P, Li H, Liang X, Che Y, Zhan X and Huang L 2018
Effect of isothermal and non-isothermal aging on the low
cycle fatigue behavior of an Al–Cu–Mg–Si forging alloy.
Mater. Charact. 144: 378–386. https://doi.org/10.1016/j.
matchar.2018.07.027
[8] Kumar N, Owolabi G and Jayaganthan R 2019 Al 6082 alloy
strengthening through low strain multi-axial forging. Mater.Charact. 155: 109761. https://doi.org/10.1016/j.matchar.
2019.06.003
[9] Rosso M 2012 Thixocasting and rheocasting technologies,
improvements going on. J. Achiev. Mater. Manuf. 54: 110–119[10] Flemings M, Riek R and Young K 1976 Rheocasting. Mater.
Sci. Eng. 25: 103–117. https://doi.org/10.1016/0025-
5416(76)90057-4
[11] Birol Y 2009 Response to artificial ageing of dendritic and
globular Al–7Si–Mg alloys. J. Alloys Compd. 484: 164–167.https://doi.org/10.1016/j.jallcom.2009.05.043
[12] De Freitas E, Ferrante M, Ruckert C and Filho W W B 2008
Thixocasting of an A356 alloy: Fluidity, porosity distribution
and thermomechanical fatigue behavior. Mater. Sci. Eng. A479: 171–180. https://doi.org/10.1016/j.msea.2007.06.037
[13] Yadav B, Muchhala D, Singh P, Venkat A C and Mondal D
2019 Synergic effect of MWCNTs and SiC addition on
microstructure and mechanical properties of closed-cell Al–
SiC-MWCNTs HCFs. Compos. B. Eng. 172: 458–471.
https://doi.org/10.1016/j.compositesb.2019.05.041
[14] Shrivastava V, Gupta G K and Singh I B 2019 Heat
treatment effect on the microstructure and corrosion behavior
of Al-6061 alloy with influence of a-nanoalumina reinforce-
ment in 3.5% NaCl solution. J. Alloys Compd. 775: 628–638.https://doi.org/10.1016/j.jallcom.2018.10.111
[15] Yadav B, Muchhala D, Singh P, Gupta G, Venkat A C and
Mondal D 2019 Compressive Deformation Behavior of Al–
SiC–MWCNTs Hybrid Composite Foam Through Factorial
Design of Experiments. Trans. Indian Inst. Met 73: 223–234.https://doi.org/10.1007/s12666-019-01825-7
[16] Lee T and Lee C 1999 Microstructure and mechanical
properties of TiB2/TiAl composites produced by reactive
sintering using a powder extrusion technique. J. Mater. Sci.Lett. 18: 801–803. https://doi.org/10.1023/A:1006645302032
[17] Baradeswaran A and Perumal A E 2014 Wear and mechan-
ical characteristics of Al 7075/graphite composites. Compos.
B. Eng. 56: 472–476. https://doi.org/10.1016/j.compositesb.
2013.08.073
[18] Ikhmayies S 2019 Phase Diagrams of Al–Si System, in:
Energy Technology. Springer, 2019, pp. 231–237. https://
doi.org/10.1007/978-3-030-06209-5_24
[19] A Standard, B557: Standard methods of tension testing
wrought and cast aluminium and magnesium alloy products.
Annual Book of ASTM Standards, 2 (1993)
[20] Su M N, Young B and Gardner L 2014 Deformation-based
design of aluminium alloy beams. Eng. Struct. 80: 339–349.https://doi.org/10.1016/j.engstruct.2014.08.034
[21] Shrivastava V, Singh P, Gupta G K, Srivastava S K and
Singh I 2021 Synergistic effect of heat treatment and
reinforcement content on the microstructure and corrosion
behavior of Al-7075 alloy based nanocomposites. J. AlloysCompd. 857: 157590. https://doi.org/10.1016/j.jallcom.2020.
157590
[22] Urrejola E, Peter K, Plagwitz H and Schubert G 2010 Al-Si
alloy formation in narrow p-Si contact areas, in: 2ndWorkshop on Metallization of Crystalline Silicon SolarCells, 2010
[23] Fan C, Chen Z, He W, Chen J and Chen D 2010 Effects of
the casting temperature on microstructure and mechanical
properties of the squeeze-cast Al–Zn–Mg–Cu alloy. J. AlloysCompd. 504: L42–L45. https://doi.org/10.1016/j.jallcom.
2010.06.012
[24] Bi G, Guan Y, Chen X, Tan W, Huang W, Gao Y and Wang
M 2020 Evaluation of uncertainty in determining average
grain size by ASTM E112 standard, in: IOP ConferenceSeries: Materials Science and Engineering, IOP Publishing,
pp. 012045
[25] Sołek K P, Rogal L and Kapranos P 2017 Evolution of
globular microstructure and rheological properties of stel-
liteTM 21 alloy after heating to semisolid state. J. Mater. Eng.Perform. 26: 115–123. https://doi.org/10.1007/s11665-016-
2421-9
[26] Singh P, Abhash A, Yadav B, Shafeeq M, Singh I and
Mondal D 2019 Effect of milling time on powder charac-
teristics and mechanical performance of Ti4wt% Al alloy.
Powder Technol. 342: 275–287. https://doi.org/10.1016/j.
powtec.2018.09.075
[27] Lee C 2007 Effects of microporosity on tensile properties of
A356 aluminum alloy. Mater. Sci. Eng. A 464: 249–254.
https://doi.org/10.1016/j.msea.2007.01.130
[28] Agazhanov A S, Abdullaev R, Samoshkin D and Stankus S
2020 Thermal conductivity and thermal diffusivity of Li-Pb
eutectic in the temperature range of 293–1273 K. FusionEng. Des. 152: 111456. https://doi.org/10.1016/j.fusengdes.2020.111456
[29] Chen J, Hung H, Wang C and Tang N 2017 Effects of casting
and heat treatment processes on the thermal conductivity of an
Al-Si-Cu-Fe-Zn alloy. Int. J. Heat Mass Transf. 105: 189–195.https://doi.org/10.1016/j.ijheatmasstransfer.2016.09.090
[30] Payandeh M, Sjolander E, Jarfors A and Wessen M 2016
Influence of microstructure and heat treatment on thermal
conductivity of rheocast and liquid die cast Al-6Si-2Cu-Zn
alloy. Int. J. Cast Met. Res. 29: 202–213. https://doi.org/10.1080/13640461.2015.1125990
219 Page 12 of 12 Sådhanå (2021) 46:219