Advances in Materials and Processing Technologies

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Metal Matrix Composites Fabricated by Stir

Casting Process–A Review

Arvind Sankhla & Kaushik M Patel

To cite this article: Arvind Sankhla & Kaushik M Patel (2021): Metal Matrix Composites Fabricated
by Stir Casting Process–A Review, Advances in Materials and Processing Technologies, DOI:
10.1080/2374068X.2020.1855404

To link to this article: https://doi.org/10.1080/2374068X.2020.1855404

Published online: 18 Jan 2021.

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ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES
https://doi.org/10.1080/2374068X.2020.1855404

REVIEW ARTICLE

Metal Matrix Composites Fabricated by Stir Casting Process–A

Review
Arvind Sankhla and Kaushik M Patel
Mechanical Engineering Department, Institute of Technology, Nirma University, Ahmedabad, Gujarat, India

ABSTRACT ARTICLE HISTORY

This paper presents a detailed review of the fabrication of metal matrix Accepted 21 November 2020
composites through the stir-casting process and its processing issues.
KEYWORDS
Metal matrix composites are prevalent materials among researchers Metal matrix composites;
and industries owing to their specific properties. These materials also stir-casting reinforcement;
offer the scope of tailoring the desired properties. There are several porosity; hardness; strength
applications where metal matrix composites are superior to other
conventional metal and alloys. However, it is evident that each type
of manufacturing or fabrication process faces inherent problems and
issues. The difference in properties of each constituent material such as
density, melting point, strength, and severity of plastic deformation
poses limitations to each processing method, either stir casting or
powder metallurgy. This study attempts to reveal such processing
issues of metal matrix composites by stir-casting processes.

1. Introduction
Newer research and ever-emerging technologies bring many challenges to the existing
materials. It becomes a need of the day to develop novel materials to shape up the technol­
ogies into real-time products and applications. Composite materials are such materials which
can meet many challenges to shape up new things, i.e. whether it is an engineering product or
component such as piston, turbine blades, or structural items of advanced aircraft, spacecraft,
or missile systems, or some sporting goods such as golf stick or frame of the bicycle. Superior
mechanical and thermal properties, resistance to corrosion and wear, improved fatigue, and
creep behaviour, good damping characteristics are few examples that justify the increased
attention towards composite materials, mostly metal matrix composites. Mother Nature
encompasses many examples of composite material to inspire the development of new
composite materials. Since ancient times wood straws are used along with cow’s dung,
which serves as an excellent raw fuel and pasting materials. Bones are another example of
composite materials, and wood is also a classic example of such material where long cellulose
fibres are held by a substance called lignin. It is interesting to note that these two materials are
never so strong individually as they are vital in a combined form. Many types of wood have
been used by mankind for many structural and non-structural applications. Composite
materials are of mainly three types based on their primary material or matrix material such
as polymer-based, metal-based, or ceramics based. The second material is known as

CONTACT Arvind Sankhla arvind.sankhla@nirmauni.ac.in Mechanical Engineering Department, Institute of

Technology, Nirma University, Ahmedabad, Gujarat 382481, India
© 2021 Informa UK Limited, trading as Taylor & Francis Group
2 A. SANKHLA

reinforcement, which can be long or short fibres or maybe particles. The general classification
of composite materials is shown in Figure 1.
Metal matrix composites are such material where the base material is a conventional
metal or alloy, and it is reinforced by a secondary material. The secondary material or
reinforcement material can be a metallic compound, metals in powder or fibre form, or
oxides or ceramic particles. In most of the cases, a continuous metallic matrix is combined
with some ceramic or refractory material, where the metallic matrix is ductile and does have
good formability and machinability. The second phase usually is hard, brittle, and does
have a high capacity of load-bearing. For example, aluminium is soft and ductile can be
embedded with hard particles or fibres of SiC or aluminium oxide. However, the develop­
ment of metal matrix composites is hindered by the high cost of material and processing
equipment and the cost of reinforcement material and their availability.
Depending upon the characteristics of matrix material and reinforcement material,
various processing or fabrication methods are employed, and it is possible to process
these materials in all three states, i.e. solids, liquids, and gases. Recent development included
newer methods for processing of MMCs which includes laser sintering, spark plasma
sintering, rapid prototyping, etc., following section deal with conventional methods of
processing of metal matrix composites, namely, stir casting and powder metallurgy [1,2]. It
is worthwhile to know that physical and mechanical properties like melting point, the
density of composite materials decides the service applications. In general, metal matrix
composites possess high strength and modulus apart from a higher level of toughness and
impact strength. These properties make them suitable for designing structures where owing
to less density, the weight of the structure is reduced, and section size can be increased by
the designer to take advantage of flexural strength without increasing the weight, and this
becomes of paramount importance while designing components for automobile and aero­
space applications. In the case of high modulus composite, a significant role is played by
matrix material; it retains the properties of wrought material, so that it is possible to alter
the properties of composite material. It is expected that when the load is applied during
service application, interfacial properties should be stable and consistent; otherwise, it
becomes difficult to for the composite material to exhibit the desired properties as
a strong interaction takes place between the matrix and reinforcement, which does not
normally happen in resin-based composite material.

Figure1 Types of composite materials [1].

 ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 3

In view of the above facts, advanced technologies, processing equipment, and material
characterisation equipment are required to develop target properties-based metal matrix
composite materials [2]. Hence comparatively, metal matrix composite material is always
costlier than other types of composite materials. Processing methods like diffusion
bonding, hot isostatic pressing (HIP), liquid metal infiltration are few examples of
processes that are technology-intensive, and resultant composite material is costlier
than those produced by stir casting and powder metallurgy processes. Products and
components related to segments like traffic engineering, sporting goods always demand
for process economy; hence it becomes mandatory to develop components made from
MMC to be cost-effective. However, despite many challenges, still metal matrix compo­
site materials are of significant research interest. Figure 2 shows the demand pattern of
MMC in recent years, which seems to be ever-increasing.
The metal matrix composites offer various advantages such as increased yield strength,
increase in fatigue, and creep resistance, especially at higher temperatures, the better
thermal coefficient for expansion, yet weight and density of MMCs can be managed at
lower levels. Consequently, these advantages have drawn the attention of many research­
ers and industries like automotive and aerospace. Hence, the need for metal matrix
composite is ever emerging, and there are so many companies, research institutes are
involved in the production of MMCs.
A survey is presented in Table 1 for the need for metal matrix composites.
Application of Metal Matrix Composites: There are many areas where the applica­
tion of metal matrix composite can be found automotive industries, aircraft and space
crafts, sporting goods, electronics industries are few examples where components made
from various types of metal matrix composites have been used. Piston for engines,
connecting rods, drive shafts, turbine blades and rotors, bicycle frame, putter of golf
stick are some examples, to name a few. Chawla et al [3]. reported that connecting rod
made from Al/SiCp composite material can reduce 12–20% secondary shaking force, and
it improves the balancing of IC engines and also has a positive impact on crankshaft
durability. Figure 3–7 depict various applications of MMC.

Figure 2. Global demand pattern of metal matrix composite materials [4].

4 A. SANKHLA

Table 1. Market projection of MMC [4].

Sr
no. Attribute Remark
1 Key Trend An increase in the number of industries involved in the production of MMC will increase
availability and demand for MMC may increase
2 Market Driver MMC reduces the curb weight of vehicle hence most attractive for the automotive sector
3 Impact on 10–12% reduction in weight of passenger cars and light-duty truck can reduce carbon
environment emission significantly
4 Business Forecast The global market can outreach by 10.77 kilotons by 2021

Figure 3. Special structural components made from MMC (a) structure of space shuttle and (b)
antenna waveguides [5,6].

Figure 4. Fittings made from MMC (a) truss node and (b) cast fitting [5,6].

The other applications also reported by other researchers like MMC can also be used
to detect pollutant elements in aquatic environments since lead is very harmful to the
human brain; hence, MMC can help in the effective detection of such elements using UV
spectrometry or Anodic Stripping Voltammetry (ASV). Conductor cables have been
developed by many industries that are used in electrical power transmission, and these
cables are light in weight yet strong than steel; and moreover, they carry a large amount of
current without losses. Carbon Nanotube-based MMCs are being developed and still in
the research phase, and these materials may be used in the development of parts of hybrid
vehicles, army vehicles, and special diodes for electronic industries. Table 2 shows the
applications of MMC in various sectors, and Table 3 shows multiple companies and
 ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 5

Figure 5. (a) MMCs for electronic packaging applications and (b) various industrial products made
from metal matrix composites [6].

Figure 6. Engine parts made from metal matrix composites; these components were reported for
superior stretch, stiffness, and high wear resistance [7].

Figure 7. Engine block made with aluminium-based MMC [8].

6 A. SANKHLA

Table 2. Various companies involved in manufacturing of MMC [2].

Name of sector System/product Name of component
Thermal management Motorola power chip Power semiconductor
Recreation Specialised Stump-Jumper Bicycle frame
Disney Thunder Mtn- thrill ride Brake Fins
Automotive Toyota Altezza (Asian- Exhaust valves
market Engine block cylinder liner
Honda Prelude Brake rotor
Plymouth Prowler Pistons
Honda Connecting rod
Chevy Corvette Pickup Driveshaft
Aero- propulsion F-16 fighter jet of US air force Ventral Fin
Fuel access door covers
Aero-structure Eurocopter EC-120, Rotor blade sleeve
N-4
Space Hubble space telescope waveguide mast
Antenna
Commercial LEO satellites Microwave thermal packaging
Commercial GEO comsats Power semiconductor base

industries involved in the manufacturing of MMC [4–9]. Various processes used by

different companies to produce MMC components are shown in Table 3.
Matrix Material for MMC: To fabricate any type of composite material, a matrix
material is needed. Conventional metal and alloy can be used as matrix material and can
be used in various forms to prepare composite material of intended properties.
Aluminium and its alloys, copper magnesium, titanium, and iron are some examples
of matrix materials are widely used. The most widely used matrix material is aluminium,
and its alloys, low density, strength, a fair amount of toughness, and resistance to
corrosion makes it a suitable candidate. The low density is one of the reasons that
aluminium-based MMC has been developed by many industries and is used mainly by
automobile and aerospace industries. Aluminium in conventional form, alloy form, and
particular alloy form has been used by many researchers and industries [10]. Table 4
presents various forms of aluminium alloy for MMC applications.
In some of the cases, lithium was added to aluminium as an alloying element, and it
results in much increase in elastic modulus and slightly decreases the density. Also the
presence of lithium in Al-Li Alloy makes it precipitation-hardenable and behave similarly
to Al-Cu-Mg and Al-Zn – Mg-Cu alloys. However, the precipitation hardening sequence
in Al-Li alloys is much complex as compared to other alloys. Aluminium and its alloy can
be processed quickly through various processes like stir casting, powder metallurgy,

Table 3. List of various companies involved in the manufacturing of MMC [2].

Name of Manufacturer/Company Type of process
3 M Company (USA) Pressure infiltration, squeeze casting
Cycotech Pty Ltd (USA) Stir casting
Mahle GmbH (Germany) Gravity casting
Hitchiner Manufacturing Company, Inc Counter gravity investment casting
O’Fallon Casting, USA Investment castings
Toyota Motor Corporation Infiltration, squeeze casting
Metal Matrix Cast Composites (USA) infiltration
Rio Tinto Alcan (Canada) Stir casting
Intelligent Composites, USA Stir casting, infiltration, squeeze casting
PCC Advanced Forming Technology Infiltration
 ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 7

Table 4. Various aluminium alloy for matrix material [7].

Conventional cast alloys Conventional wrought alloys Special alloys
AlSi12CuMgNi, AlSi9Mg, AlMgSiCu (6061), AlCuSiMn (2014), Al–Cu–Mg–Ni–Fe-alloy (2618), Al–Cu–Mg–
AlSi7 (A356) AlZnMgCu1.5 (7075) Li-alloy (8090)

squeeze casting, etc., post-processing like extrusion is also possible, and tooling is
comparatively less complex and cost-effective. Copper is also used as matrix material
but owing to higher density as compared to aluminium; it finds limited applications. It is
mainly employed to develop electrical components like rotors of electrical motors, etc.
Copper has high thermal and electrical conductivity, and niobium-based superconduc­
tors have been developed using copper as matrix material.
Titanium is yet another metal that can be used as matrix material. As compared to
aluminium, however, its density is higher than that of aluminium, i.e. 4.5 g/cc; hence,
lightweight MMC cannot be fabricated, but it is rather suitable for aerospace industries
owing to the high melting point. Titanium can retain strength at elevated temperatures
and does have adequate corrosion resistance. Hence, titanium-based alloys and MMC are
used in jet engines, fuselage, and supersonic aircraft and space crafts. Cruising at speed
more than Mach 2, outer skin heats up very high; in this case, titanium-based MMCs are
the right choice compared to aluminium-based MMC. Titanium has an excellent affinity
for nitrogen, oxygen, and hydrogen. Interstitials, even at parts per million, can alter the
properties of titanium, specially embrittlement. Hence, titanium is preferred as matrix
material compared to the direct fabrication of components by welding process [11].
Magnesium and its alloys are yet another choice available as matrix material, and it is
a lightweight material too. It is used for making parts of assemblies or systems that their
weight contribution can at minimum, but still, they are functional as per design criteria.
Magnesium-based MMCs are used in the gearboxes of aircraft, the housing of chain saw.
In both cases, being a lightweight components is of advantages. AZ91 is one of the most
popular magnesium-based alloys when used as a matrix material for MMC. It exhibits
excellent corrosion resistance, relatively higher strength. Its mechanical properties can be
easily enhanced by plastic forming [12–14]

2. Reinforcement for MMC

As mentioned earlier, a secondary material is added to the primary material in order to
fabricate MMC. This secondary material, in turn, is referred to as reinforcement. The
reinforcement material can be metallic or non-metallic such as carbides or oxides. The
selection of reinforcement for a given MMC is governed by many factors. The low
density is always desired owing to strength to weight ratio. Chemical stability and
compatibility, thermal stability are few criteria that affect the selection of reinforcement
when MMC is to be used in a varying temperature profile, and the environment is such
that it promotes corrosion [15,16]. High strength in tension or compression and high
modulus is always desired to make MMC superior on the ground of specific properties.
Factor such as cost, process economy, and availability are some other criteria that can
readily affect the selection of reinforcement [17].
8 A. SANKHLA

Reinforcement to MMCs is available mainly in three forms, particles, short fibres or

whiskers, and Continuous fibre. The aspect ratio of particles lies in the range of 1–4 while
it is 10–1000 for whiskers and continuous fibres have aspect ratio more than 1000. As far
as size is concerned, they are available in the diameter range of 0.1 µm–150 µm. Silicon
carbide, aluminium oxide, boron nitride, boron carbide [18], graphite, and tungsten are
some examples of reinforcement used in the preparation of MMC. Other materials can
also be considered as reinforcement to the MMC, but the interface between parent
material, i.e. matrix material and reinforcement, is of paramount importance. Hence,
in the majority of the cases, ceramic particles are used as reinforcement. The interaction
between matrix and reinforcement may be a kind of mechanical locking or chemical
bonding; hence, the reinforcement material must respond well during the processing of
MMC. Interfacial reactions always determine the load-carrying capacity of an MMC.
Wettability and surface tension play an essential role in achieving bonding and homo­
geneity between matrix and reinforcement, especially when matrix material is processed
in the liquid or semi-solid phase. Here, particles’ surface energy can be altered by
applications of metallic coatings to the non-metallic particles, and issues related to
wettability and surface tension can be addressed. Recently extensive research is being
carried out on nano-sized reinforcement, damping property, wear resistance, and elec­
trical conductivity. Carbon nanotubes are popular due to their characteristics of uniform
dispersion [19]. Thermal conductivity is improved to a great extend by the addition of
carbon nanotubes.
Stir Casting for MMC: Among the available conventional processing methods, stir
casting is a method of processing the composite material in the liquid stage. The matrix
material is cast conventional by melting and moulding it into a suitable shape. The
reinforcement material is so chosen that its melting point is always higher than the
matrix material; hence, reinforcement material mostly in the particle or whisker form is
added in the liquid metal, and mechanical agitating is continuously provided to achieve
uniform distribution of reinforcement [20]. After that, the liquid mix is introduced in
a sand or metallic mould. If required, suitable post-processing can be applied to improve
appearance or dimensions. The basic setup for stir casting involves a melting furnace and
crucible with a mechanical stirrer arrangement provided with it. Since some metals are
highly reactive to atmospheric gases, there can be an arrangement of purging inert gases
in the melt. Figure 8 shows the schematic diagram of the stir-casting set up.
Mixing of reinforcement material with matrix material may require preheating of rein­
forcement material; hence, secondary furnace or additional heating devices are needed along
with the basic set up. Ultrasonic vibrations may be coupled with the setup to improve the
distribution of secondary material. However, it increases the cost of setup, and coupling the
ultrasonic transducer with the crucible or melting pot is a tedious task, and it is very cost-
intensive. All mostly all types of matrix material like copper aluminium, titanium, magne­
sium, and nickel can be processed through this method. Temperature controlling devices,
pouring of the liquid metal mix are other important aspects of a good stir-casting setup.
Several researchers reported that stir-casting works well for a maximum of 30% reinforce­
ment; beyond that, it is too difficult to ensure uniform mixing of support and its presence.
Segregation is inevitable, and with the increased content of secondary material, it should be
given due importance. The stir-casting process can be extended to Rheo-casting if stirring is
done in a semi-solid state of the metal. However, a special stirrer would be required to do so.
 ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 9

Figure 8. Schematic diagram of stir-casting set up [6].

It is worth noting that composite material properties can be heavily dependent on the
distribution of reinforcement in the metal matrix. Various parameters that affect MMC’s
target properties are stirring velocity, the geometry of stirrer, stirring time, temperature of
the melt, density of liquid metal, wettability, the surface energy of reinforcement particles,
the surface tension of liquid metal, and scope of the chemical reaction. Some of the
researches have reported modifying the stir-casting process into a hybrid stir-casting process.
Dasgupta [21], in her experimental investigation of wear behaviour of Al-MMC, did
synthesis of MMC with stir-casting process in which the first reinforcement was introduced
in liquid metal and stirred continuously for uniform distribution after that poured into
preheated metal moulds. Singla et al. [22] fabricated their stir caster with a stirring speed of
600 rpm and heating capacity of 1100°C. Hossein et al. [23]fabricated nano MgO reinforced
aluminium-based MMC through stir-casting processes and suggested that low cost, process
flexibility, and continuous matrix media are favourable features of the stir-casting process.
Ramachandra et al [24] used stir-casting method to introduce fly ash in the matrix of
aluminium and melting. They carried out subsequent stirring out at 800°C and 720°C,
respectively, and fabricated fly ash reinforced Al-MMC successfully [25].

3. Stirrer and its effect on MMC/Melt quality/Composite slurry

Stirrer is an essential part of any stir-casting setup, and it plays a vital role in the distribution
of reinforcement in the matrix material. The material of stirrer, its design, number of blades
in impeller, blade geometry, and speed of rotation are key variables that affect the quality of
stir cast MMC. Singla et al [22] fabricated a mild steel stirrer and impeller with four blades.
They kept the blade angle at 45°. The main reason behind keeping the blades at 45° is to
ensure that 65% of the material remains above the stirrer, and the remaining 35% should
stay above the impeller to impart maximum shear during stirring. It is required that the
impeller should overcome the viscous resistance of liquid metal to ensure uniform dis­
tribution of reinforcement. The temperature of metal should be monitored and maintained
10 A. SANKHLA

that it is always higher than the melting point matrix material to ensure lower viscosity of
the melt. Abhijat et al [26] reported to use steel impeller at 750 rpm for 10 minutes of
stirring, and the vortex was created in the melt pool to achieve maximum homogeneity.
While Brabazon et al [27] proposed a groove-based design of impeller, which was found
effective in inducing more shear during metal stirring. The stirrer was made of Reaction
Bonded Silicon Nitride (RSBN), which proved to be suitable shock resistant, retaining
strength at high temperature and inert to melt. The schematic of the set is shown in Figure 9
(a) and impeller with groove is shown in Figure 9(b).
Hossien et al [23] also fabricated Al-MMC by stir-casting process, and melt tempera­
ture was kept in the range of 800–950°C. The rotational speed was 420 rpm, and stirring
was carried out for 14 minutes. No major interface of melt material and stirrer was
reported; at the same time, the density of cast MMC was in agreement with the
theoretical density. The close agreement between theoretical density and experimental
density was attributed to the proper distribution of reinforcement in the matrix with very
few local clusters of reinforcement [28]. Hashim et al [29] reported importance of particle
dispersion number (PDN) to achieve uniformity of mixing achieved through stirring.
According to them, if PDN is more significant when it is more than one, then axial
velocity is more than settling velocity, and particles are carried from bottom to top.
A PDN, more than four, will always ensure homogenous dispersion. A higher speed of
rotation can break the agglomeration which in turn releases the entrapped liquid and
increase the fraction of solid, causing more uniform mixing. The effect of the higher
speed of impeller reduces the apparent viscosity of composite slurry due to increased
shear rate. Subhranshu et al [30] successfully fabricated the Al-MMC with stir-casting
process wherein treatment of impeller was emphasised. The mild steel impeller was given
a heat treatment at 850°C for 3 hours and thereafter, air-cooled. This was done to
minimise the chemical reaction between steel impeller and liquid metal. The stirrer
was attached with four blades of 20 mm width and 4 mm thickness at a gap of 5 mm.
Uniformity of distribution of particles was also observed in microstructures show in
Figure 10.

Figure 9. (a) Schematic of set up and (b) groove-based impeller of ceramic material [27].
 ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 11

Figure 10. Homogenous microstructure of Al-MMC [30].

A stirrer with twisted blades was fabricated by Tony et al [31] in order to improve the
flow of molten metal. A set of two anti-parallel blade was used in the impeller to form
a random flow pattern during stirring. They reported that it resulted in uniform mixing
of SiC particles uniformly in the aluminium; also, the formation of clusters was reduced
to a great extent. They also found that an increase in the blade area can result in larger
stirring force, and as a result, an attraction between reinforcement, i.e., SiC particle got
reduced. Stainless steel impeller was used by Poddar et al [32] to fabricate magnesium-
based MMC, which was reinforced by SiC. The Microstructure of MMC obtained by
them did not show any local reason for the interfacial region, confirming that stainless
steel can be the right choice for impeller or stirrer material.
Rohatagi et al [33] reported that the stir-casting process is the most cost-effective
fabrication route for the fabrication of MMC, especially for two-phase combinations
where density difference prevails. They raised the concern about the uniform distribution
of hard particles like SiC in the matrix of aluminium, especially when MMCs are
produced by stir-casting method in large quantity. According to them, dimensions of
the vessel, design, and dimensions of the impeller, and its position in the vessel do have
an immediate impact on the uniformity of slurry to be cast as MMC. In order to
investigate the effect of impeller design, they used two different types of the impeller,
i.e. one with three blades and another one with four blades. All the impeller had different
values of blade angel, i.e. 30°,45°, and 60°. With this setup water model of SiC flow
behaviour was investigated. For positive blade angel higher speed of impeller rotation
was found favourable for homogeneity of the mixture as compared to negative angle, and
an increase in angular flow was reported as blade angle increase from 30° to 90°. For
a non-baffled tank, the depth of vortex and shape was found much dependent type of
impeller, while for a baffled tank produced more homogenous mixture for the same
impeller design and geometry. This experimental study successfully predicated the flow
behaviour of SiC particles in a liquid medium like molten aluminium at higher speed and
with baffled tank residence time of ceramic particles and ensured the availability of
reinforcement during poring of slurry in the mould and the same can be confirmed in
the microstructure. Ghosh et al. [34] suggested that apart from the type of impeller and
its rotational speed, the height ratio is also a key parameter to affect the uniformity of
12 A. SANKHLA

reinforcement while preparing an MMC slurry. They suggested a value of 0.81 of h/H,
where h is the position of the impeller from the bottom, and H is the total depth of liquid
slurry in the melting pot. Dehghan et al [35] demonstrated that the compressive strength
of SiC-reinforced A356 alloy was very much dependent on the stirring rate, and they
observed diminishing effect due to increase speed of stirrer. Agarwala et al [36] reported
the effect of a reduced amount of graphite particles in the stir-casting process for reduced
temperature of the melt. Constant speed of the stirrer did not affect the performance, but
the process was significantly found affected by increased viscosity of the melt. However,
preheating of graphite powder and mould was reported favourable for retaining the
reinforcement as shown in Figure 11.
Atunya et al [37] employed a double-feeding stir-casting method to synthesise and
evaluate Al-Cu-Mg alloy, which was reinforced by nanoparticles [38]. Preheating of LM6
alloy and SiC particle with stirring speed resulted in uniform dispersion of particles, as
shown in Figure 12, and MMC was found resistant and machinable.
Hashim et al [39] reported about the glycerol-based simulation of stir-casting process
and found a turbulent zone at the bottom of the container can help a random distribution
of particles. During stirring, a dead zone can be found where the minimal effect of the
increase of impeller speed takes place. The proposed pattern as shown in Figure 13(a) was
confirmed and observed during experimental simulation of the stirring process with
glycerol Figure 13(b). However, a change in the position of the impeller immediately
affects the mixing process. Impeller with a blade angle of 45°and made up of zirconia
coated steel was used in the experimental study by Afsnhe et al. [40]. They kept impeller
speed 500 rpm, and the ultrasonic head supplied additional energy.
Researchers obtained encouraging results as cast MMC was better in wear tests
performed by them. Gao et al [41] found a reduction in clusters and agglomeration
with the increase of impeller speed. However, the non-wetting part contributed to some
void formation, and it was seen as shrinkage cavities in microstructure, as shown in
Figure 14.

4. Wettability and porosity

Since stir-casting deals with molten metal and two different materials are mixed together,
mostly one is in liquid form, and the other is in solid form. Hence, apart from the
uniform distribution, the affinity of reinforcement material with matrix material becomes
of paramount importance. Wettability determines the degree of intimateness, and thus

Figure 11. Effect of preheating and mould temperature on graphite retention [36].
 ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 13

Figure 12. Uniform distribution at constant stirring speed [38].

Figure 13. Flow pattern in stirring [39].

Figure 14. Presence of shrinkage voids in the microstructure of MMC [41].

14 A. SANKHLA

strength, the hardness of MMC are directly affected. Physical and chemical properties of
both matrix material and reinforcement are important to induce intimacy between the
material being mixed together. In most of the cases, MMCs are reinforced by the
particulates. Hence particle size is another important factor in achieving good wettability.
A small particle does have a larger surface area, and it promotes agglomeration by
obstructing the homogenous dispersion. Poddar et al [32] suggested to introduce man­
ganese. It improves the wettability got magnesium-based MMC, which was reinforced by
SiC particles. Ramachandra et al. also [24] added magnesium in the melt to enhance the
wettability of fly ash particles.
Several wetting agents like Mg, Ti, Ca, and Zr were suggested by the [40–43] to
improve wettability; these wetting agents directly affect the surface tension of liquid
metal and results in improvement of wettability. Several researchers report that [40–44]
wetting agent creates a temporary layer between matrix and the reinforcement, coating of
reinforcement particles also found to be effective in order to promote interfacial reactions
and surface energy is altered, and it results into improved wettability. In many cases [43],
enhancing the surface energy of the reinforcement particle and reducing the surface
tension of liquid metal were two key findings reported. In the case of Al-MMC, the use of
magnesium has been suggested by many researchers. Few researchers [45] proposed to
use a large quantity of magnesium when particle size is finer. Magnesium tends to reduce
the oxidation of aluminium, thereby improves the wettability. Ghosh et al [34] also
proposed that the operating parameter of stir caster like stirring speed and stirring time
also affects the wettability. Amirkhanlou et al. reported that [46] wettability was
improved when instead of untreated SiC particles, pretreated, i.e. the milled composite
powder was used, and as a result, some reduction in the SiC particle size was observed.
The distribution of the reinforcement particles in the solidified matrix was found
improved. It was reported that casting in a semi-solid state (compo casting) rather
than in a fully liquid state (stir casting) resulted in improved wettability and distribution
of SiC particles [47]. The presence of a layer of oxides on droplets of meatal can be
a concern as it prevents it from establishing contact with another material present; hence,
wettability is immediately affected [48]. To improve wettability up to some extent hybrid
approach like ball milling of powder mix was adopted, and its effect on tensile strength
was studied [49]. Porosity is an inherent feature of any casting process; several mechan­
ical properties are influenced by the presence of porosity in the composite material.
Porosity, which is usually observed, maybe due to gas entrapment during stirring, or it
may be due to the formation of air envelop around the reinforcement. While water
vapour on the surface of the particles, hydrogen evolution, and solidification shrinkage
might also be the reasons for porosity and moisture absorption tendency. Amirkhanlou
et al [46] found that the addition of SiC particles in the form of (Al-SiCp) composite
powder and casting in a semi-solid state increases the hardness of the composites by 10%
and decreases the porosity by 68%. The impact energy is slightly influenced by the form
of the reinforcement addition, but the casting method does not influence the impact
energy. The smaller reinforcement size also increases the dislocation density due to the
greater mismatch in the thermal expansion of the matrix and the reinforcement at the
particles–matrix interfaces [50]. Preheated moulds also help in the reduction of porosity.
It is also prudent that the pressure gradient, which is built up during solidification of
 ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 15

composite material, and dissolved gases, also gives rise to porosity. Porosity can be found
either in the vicinity of the particle or cluster [51,52].

5. Mechanical properties of stir-cast MMC

Mechanical properties of composite material are always of particular interest since
a composite material has to be superior to its constituent materials. Sajjadi et al [53]
found the immediate effect of micro and nanoparticles on strengthening mechanisms
and reported an increase in strength, hardness, and ductility. Distribution of hard
particles, grain refinement, mismatch in elastic moduli, and other parameters like
a mismatch of coefficient of thermal expansion were reported as essential aspects of
resultant mechanical properties of a composite material. Prabhu et al [54] revealed that
stirring speed, stirring time can be investigated and optimised further to influence the
mechanical properties of composite material.
Several researchers have found the effect of types of reinforcement and its amount, its
reinforcement particle size. In the experimental study carried out by Yang et al. [55], it
was observed that reinforcement material plays an essential role, and the addition of 3.5%
bauxite in aluminium matrix resulted in an 8% enhancement improvement in tensile and
yield strength. However, increased proportion of reinforcement did not show any
encouraging result. Hardens of the composite material were found to be increased, and
it was reported in the range of 87–100HRB. Mehedi et al [56] reported that the wear
behaviour Al-MMC was found to be improved with the combined action of SiC and Al2
O3 in the AL-Mg alloy. Hence, the presence of hard particles and its bonding with matrix
material makes an MMC material wear resistance. Their experimental study also estab­
lished the scope of reinforcing a single material with two different types of reinforcement.
Amirkhanlu et al. [46] reported that the hardness of Al-MMC could be further
increased by giving a pre-treatment to reinforcement and compo-casting acting improves
the hardness of MMC as compared to conventional stir casting. They reported hardness
of Al-MMC in the range of 60–68 HRB, which is 2–3 times than commercial aluminium.
Further to it 10% improvement was found in the hardness. The impact energy was in line
with the trend of hardness, i.e. increasing, but there was little or no effect of casting
method was reported.
Brabazon et al. and other researchers [57–59] suggested that impeller design and
operating parameters have a direct impact on mechanical properties. They used a groove-
based design of impeller in stir casting of Al-MMC. The toughness of MMC was found
better with increased value impact energy. The stir-casting method was found to be an
improved method over gravity casting. It was also observed that due to intense stirring,
less porosity was found in MMC. Reduction in porosity signifies intact bonding hence
better mechanical properties. Goswami et al. carried out the extrusion of cast Al-MMC to
analyse the impact of mechanical working on the properties. The extrusion ratio of 16
was found promising and along with the increase of reinforcement and extrusion
together resulted in improved mechanical properties. Cast structure was also examined
and found that it was a completely broken new microstructure with fine grains was
obtained. Porosity was eliminated as fine grains were observed in the microstructure.
Ramchandra et al [24] reported the effect of fly ash on the ultimate tensile strength of
Al-Si alloy. Fly ash was added in the range of 5–15%, and UTS was found to improve by
16 A. SANKHLA

7%. It was 9.7% and 19% compared to that of base alloy, i.e. 255. 6MPa.The coefficient of
friction was also reported higher than base alloy; hence, MMC can be treated as wear-
resistant material. Ezatpour et al. [60] reported a correlation of hardness with stirring
speed for stir-cast MMC. They proposed that uniform distribution can be achieved with
an increase in stirring speed, and the hardness of cast MMC was reported reasonably
higher with an increase of reinforcement and stirring speed. However, after a limit
stirring speed becomes detrimental, as due to density difference, reinforcement particles
can segregate, and dispersing is affected to a large extend. Even with an increased content
of reinforcement, higher stirring speed does not hold together; thus, hardness may
decrease beyond appointing of saturation. Compressive strength improves as load trans­
fer characteristics along with the presence of hard particles. The improvement can be
attributed to grain refinement also, and it becomes in agreement with the Hall Path
equation. Orowan strengthening [53] and secondary processing [61] also plays an
important role in the improvement of the compressive strength. They also reported
that a reduction in grain size always increases the hardness of MMC. Abhilash et al [26]
investigated the effect of SiC particle on magnesium-based aluminium alloy. An
increased amount of reinforcement showed a linear increase in yield strength only in
elastic reason, and ultimate tensile strength did not show an appreciable increase along
with yield strength. However, a small increment in UTS was reported due to the presence
of hard particles, which restrict the flow of dislocations. Elimination of finer cracks
supports the argument. Hasan et al. [62]found improvement in wear characteristics of
Al-alloy with increased content of SiC, and improvement in UTS was also reported [63].
Higher hardness is one of the desirable properties of MMC materials, especially those
that are made up of soft matrix, Kassem et al. [64] reported improvement in the hardness
of Al-MMC made by stir-casting method. The hardness improved from 113HV to
150HV with increase content of reinforcement. Since a little brittleness may be induced
along with the hardness and it was observed in the reduction of UTS. Tensile strength
was found affected by particle size as Atuanya et al [37] examined the effect of bean pod
ash(BPA) nanoparticles on the tensile strength of Al-Cu-Mg alloys. According to them
the strength of MMC was better than unreinforced alloy. They concluded that a degree of
work hardening behaviour contributed to the enhanced strength. Hard particles were
assumed to prevent plastic deformation of the matrix material. Dislocation generation
and their uniform distribution were another reason proposed by them [65].Interfacial
reaction of aluminium with silica and titanium were found of profound interest to
investigate the behaviour of stir-cast MMC reinforced by secondary material in powder
form [66,67].
The effect of Al2O3 reinforcement on aluminium alloy Al-6061 was experimentally
investigated by Bharat et al [68]. The tensile strength of MMC was reported to be
superior to the base Al-6061 alloy and was found increasing with a weight content of
reinforcement, which was added in the range of 3–12% by weight. The UTS of MMC was
reported in the range 167 Mpa to 193 Mpa, which was higher than base alloy, i.e. 149.76
MPa. As shown in Figure 15, Considerable improvement in tensile strength in the range
of 12.1%–29.5% was found. However, elongation, or in other words, ductility, was found
to be affected to a little extent, which was in agreement with increased hardness. Wear
test of cast MMC gave encouraging results, and weight loss was reduced with a higher
content of Al2O3, even at a higher sliding distance.
 ADVANCES IN MATERIALS AND PROCESSING TECHNOLOGIES 17

Figure15 Effect of Al2O3 reinforcement on tensile strength and elongation [68].

Yuyung et al [69] obtained TiB2 reinforced Al-MMC through stir-casting process,

suggesting that the stir-casting process can be effective if interfacial reactions are favour­
able and result in good bonding. TiB2 was found most suitable for in-situ reinforcement
in the aluminium matrix; MMC’s strength was at par with those who were processed with
secondary processes like extrusion. Due to favourable interfacial reactions, hardness was
appreciably increased, i.e. from 54 BHN to 84.6 BHN, and modulus improved from 73.7
GPa to 91.4GPA. Interestingly, there was little or no effect on ductility. In the other study,
Tamo et al. [70] reported a positive impact of preheating the particles to improve the
hardness of MMC. Saheb et al. [71] investigated the effect of graphite and SiC on the
hardness of stir-cast MMC and found Sic-based MMC harder than that of graphite based.
Chawla et al [3] revealed that micro plasticity in MMC happens at very low stress, and the
same can be confirmed by little offset in the strain stress curve. Due to micro plasticity,
stress concentration can be observed at various locations in the microstructure, especially
when reinforcement particles have sharp corners. The small size of particles always
contributes to an increase in ductility. A higher value of stiffness for reinforcement
also increases the creep resistance. Creep behaviour of SiC-reinforced Al-6061 alloy
was found superior to the base alloy. The random distribution of the particles, along
with micro plasticity, can bring in enhancement in the strength and hardness [72]. Some
of the other factors they reported which to affect MMC’s behaviour are (i) control of the
location of the second phase and (ii) control of segregation can significantly help to
characterise the strength of stir-cast MMC [73]. TiB2 was used by Lee et al. [74] as
reinforcement in an Al-Alloy to improve yield strength and tensile strength, but they
observed that the presence of an Al-Ti-B system could have an adverse effect on ductility.
Interfacial reactions participated in the improvement of strength. Das et al [75] added
zircon sand particles in a copper-based alloy of aluminium by stir-casting process. They
found that zircon particles can be uniformly distributed in the matrix; the stir-casting
route and recovery were dependent on size and amount of zircon particle. The presence
of zircon particle was found responsible for increased hardness and wear resistance. In
another study of hardness analysis [76], A359 researchers reinforced the aluminium alloy
by Al2O3, and hardness was found to be increased linearly with the weight content of
reinforcement. Around 8% content reported maximum hardness value, and tensile
18 A. SANKHLA

strength was found in the range of 108–148 MPa. The study was found in line with other
researchers like looney et al [29]. Raja Kumar et al. studied the incorporation of fly ash for
wear resistance [77] in the range of 5%–10%. Fly ash can improve wear resistance of the
Al-Alloy-based matrix [78], but not comparable to those MMC, which are reinforced by
SiC or Al2O3.

6. Conclusion
MMC materials are new generation material that finds wide applications in structural,
automotive, and special engineering. It is evident that aluminium-based metal matrix
composites (Al-MMC) are the most popular material amongst all other types of
MMCs.
Al-MMC offers an extensive range of superior specific properties, which greatly
benefit components of automotive engines and sporting industries. Various advantages
of MMC have been discussed, and stir casting, which is a very widely used process for Al-
MMC fabrication, has been discussed. It has been found that type of impeller, its
material, and geometry can affect the quality of cast MMC. Types of reinforcement and
size of the particle are few other parameters which can readily affect the properties of Al-
MMC for a target application. Porosity and wettability are the two major issues that have
been revealed which affect the material behaviour most, and several findings have been
discussed. Applications of Al-MMC and future demand profiles also have been discussed
apart from mechanical properties.

Disclosure statement
No potential conflict of interest was reported by the author.

ORCID
Arvind Sankhla http://orcid.org/0000-0001-5167-9437

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