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Materials Today: Proceedings 5 (2018) 6009–6017 www.materialstoday.com/proceedings

ICMPC 2017

Fabrication of magnesium matrix composites using powder

metallurgy process and testing of properties
Rajesh Purohit1, Yogesh Dewang2, R.S. Rana3, Dinesh Koli4 and Shailendra Dwivedi5
1
Associate Professor, Department of Mechanical Engineering, Maulana Azad National Institute of Technology, Bhopal, 462003, India
2
Assistant Professor, Department of Mechanical Engineering, Lakshmi Narain College of Technology, Bhopal, 462021, India
3
Assistant Professor,Department of Mechanical Engineering,Maulana Azad National Institute of Technology, Bhopal, 462003, India
4
Associate Professor, Department of Mechanical Engineering, Sagar Institute of Research and Technology, Bhopal, India
5
Associate Professor, Department of Mechanical Engineering, Lakshmi Narain College of Technology, Bhopal, 462021, India

Abstract

Magnesium based metal matrix composites produced by powder metallurgy are finding applications for automotive, aerospace,
defence and other industries. These composites exhibit high hardness, wear resistance, low coefficient of thermal expansion
along with light weight. The metal matrix composites (MMCs) are currently experiencing active developments all over the world.
In the present work, magnesium reinforced with SiC particulate composites are fabricated using powder metallurgy process. The
physical and mechanical properties like density, porosity, tensile strength, compressive strength and hardness were measured, so
that this information can prove helpful in assessing their readiness for near-term commercial implementation in the automotive
and other industries. The microstructure of the Mg-SiCp composites was also studied using scanning electron microscopy.

© 2017 Published by Elsevier Ltd.

Selection and/or Peer-review under responsibility of 7th International Conference of Materials Processing and Characterization.

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1. Introduction

The unique structures and characteristics of composites provide enhanced structural and mechanical properties
which are the primary reason for its consideration in diverse engineering applications. Not surprisingly, this aspect
of composite materials performance has been well researched. However there still remain barriers for the adaptation
of magnesium alloys for widespread engineering applications. The present work deals with the fabrication and
testing of lightweight, high strength and wear resistant magnesium matrix composites.

* Corresponding author.
E-mail address: avi.borgaonkar@gmail.com

2214-7853© 2017 Published by Elsevier Ltd.

Selection and/or Peer-review under responsibility of 7th International Conference of Materials Processing and Characterization.
6010 Rajesh Purohit et al./ Materials Today: Proceedings 5 (2018) 6009–6017

The metal matrix composites (MMCs) combine a metallic base material with a reinforcing constituent, which is
usually a ceramic material in the form of fibers, whiskers or particulates. The metal matrix composites are often
produced by combining two pre-existing constituents i.e. a metallic matrix material and ceramic particulates
reinforcement. Processes commonly used include powder metallurgy, diffusion bonding, liquid phase sintering,
squeeze-infiltration and stir casting. Alternatively, the typically high reactivity of metals at high processing
temperatures can be exploited to form the reinforcement and/or the matrix in situ, i.e. by chemical reaction within a
precursor of the composite. With regard to industrial applications, MMCs now have a proven track record as
successful “high-tech” materials in a wide range of applications, bringing significant benefits (in terms of energy
savings or component lifetime) and having documented engineering viability. These often relate to specific
applications, where achievable property combinations (e.g. high specific stiffness and weldability, high thermal
conductivity and low thermal expansion or high wear resistance and light weight with high thermal conductivity) are
attractive for the component concerned. The potential of metal matrix composites for significant improvements in
performance over conventional alloys has been widely recognized [1]. The density of magnesium is approximately
two thirds of that of aluminum, one quarter of zinc and one fifth of steel [2]. Magnesium metal matrix composites
(Mg-MMCs) have been receiving attention in recent years as an attractive material for aerospace applications
because of their low density and superior specific properties. When alloyed, Mg has the highest strength-to-weight
ratio of all the structural metals [3]. Advanced materials like magnesium matrix composites, having low density,
high specific strength and stiffness and good dimensional stability offer design flexibility and new opportunities to
develop more sophisticated remote sensing instruments [4]. Pure Mg-30 Volume % SiCp composites were fabricated
by melt stir technique without use of flux or protective gas atmosphere. After hot extrusion with extrusion ratio of
13, Mg-30 Vol. % SiCp composites were found to have 40 and 30 % higher stiffness and ultimate tensile strength
respectively as compared to un-reinforced pure magnesium [5]. Unidirectional solidification of Mg–2 wt.% Si
alloys yields Mg2Si/Mg composites with a mechanical strength as high as industrial magnesium cast alloys (AZ63)
with a 100 times higher damping capacity. Moreover, Mg-2 wt. % Si alloys reinforced with long carbon fibers have
a Young’s modulus of about 200 GPa with a damping capacity of 0.01 for strain amplitude of 10−5 [6]. Carbon nano
tube (CNT) reinforced AZ91 magnesium matrix composites were fabricated by squeeze infiltration method. The
properties of magnesium alloy were found to improve by impurity reduction, surface treatment and alloy design [7].
The damping behaviour of powder metal magnesium matrix composites reinforced with copper coated and uncoated
SiC particulates was studied by Jinhai et. al. [8]. The un-reinforced magnesium was found to have better damping
behaviour then composites at low temperature however at high temperature magnesium matrix composites have
better damping behaviour than pure magnesium. Jayakumar et al. [9] reviewed the various challenges and
development in synthesis of magnesium matrix nano composites. Dey and Pandey [10] also reviewed about
magnesium metal matrix composites. Balakrsihnan et al. [11] utilized friction stir processing for synthesis of
AZ31/TiC magnesium matrix composites. The objective of the present work is to fabricate magnesium metal matrix
composite using powder metallurgy process and testing of develoded composite material in terms of density,
porosity, hardness, compressive strength, tensile strength and microstructures.

2. Fabrication of Mg-SiCp Composites by powder metallurgy process

Powder metallurgy is a fabrication technique consisting of three major processing stages. First, the primary material
is physically powdered or divided into small individual particles using ball mills or by other process. Next, the
powder is injected into a mould or passed through a die to produce a weakly cohesive structure (via cold welding)
very near to the dimensions of the object ultimately required. Pressures of 10-50 tonnes per square inch are
commonly used. Also, to attain the same compression ratio across more complex pieces, it is often necessary to use
lower punch as well as an upper punch. Finally, this green compact is subjected to high temperature called sintering
below its melting point (during which self-welding occurs).

2.1 Sieve analysis of powders

The SiC particulates were placed on the top of a sieve shaker that contains a series of sieves one upon the other with
gradually decreasing mesh sizes. The sieves were shaken continuously for a period of 15 minutes. After this shaking
operation, the sieves were taken apart and the sand left over on each sieve was carefully weighed. The same
 Rajesh Purohit et al. / Materials Today: Proceedings 5 (2018) 6009–6017 6011

procedure was repeated for Mg powder. The grain fineness number (GFN) of Mg and SiC particulates was
calculated as 0.23 mm and 0.26 mm respectively.

2.2 Ball milling

The horizontal ball mill consists of a cylindrical container, which rotate about a horizontal axis partially filled with
the material to be ground and the grinding medium. The different materials are used as grinding media including
ceramic balls, flint pebbles and stainless steel balls. An internal cascading effect reduces the material to a fine
powder. Industrial ball mills can operate continuously fed at one end and discharged at the other end. Large to
medium-sized ball mills are mechanically rotated on their axis, but small ones normally consist of a cylindrical
capped container that sits on two drive shafts (pulleys and belts are used to transmit rotary motion). The grinding
works on principle of critical speed. The critical speed can be understood as that speed after which the steel balls
(which are responsible for the grinding of particles) start rotating along the direction of the cylindrical device; thus
causing no further grinding. Ball mills are used extensively in the mechanical alloying process in which they are not
only used for grinding but also for cold welding as well, with the purpose of producing alloys from powders. The
grinding media used here were stainless steel balls. The final particle size of Mg powder was 0.15 mm and SiC
particulate was 0.20 mm as shown in fig.1. The three samples of Mg powder with 10, 20 and 30 wt. % of SiC
particulates were prepared. 2 wt. % of zinc stearate was also added as die lubricant.

Fig.1 The finely grained mixture of Mg, SiC and Zinc Stearate (final particle size of Mg = 0.15 mm, SiC = 0.20 mm)

2.3 Die compaction

A die and punch were designed and fabricated for cold die compaction of ball milled powders. The material used for
die and punch was high carbon high chromium steel. The die and punch were hardened and ground after machining.
The dimensions of the die and punch are given in Table 1. The die was filled with weighed amount of Mg-SiCp
composite powder mixture. The die and punch set-up was placed on a 100 Ton hydraulic press and compaction was
done at a pressure of 400 MPa for 60 seconds. Fig.2 shows the samples of various compositions after die
compaction but before sintering.

Table 1 The dimensions of the die and punch used for compaction
S. No. Parameters of die and punch Dimension, mm
1. Outer diameter of die 80
2. Inner diameter of die 20
3. Length of the die 108
4. Diameter of the punch 19.8
5. Length of the punch 110
6012 Rajesh Purohit et al./ Materials Today: Proceedings 5 (2018) 6009–6017

Fig .2 Photograph of the Green compacts of Mg-SiCp

composites
2.4 Sintering
The green compacts of Mg-SiCp composites with 10, 20 and 30 wt. % of SiCp were subjected to solid state sintering
in inert (Argon) atmosphere furnace at a temperature of 460° C for 30 minutes as shown in fig.3. During sintering
bonding takes place between the aggregate powder particles. The word solid state in solid state sintering simply
refer to the state in which the material is when it bonds, solid meaning the material was not turned molten to bond
together while sintering. To allow efficient stacking of product in the furnace during sintering and prevent parts
sticking together, many manufacturers separate articles using ceramic powder separator sheets. These sheets are
available in various materials such as alumina, zirconia and magnesia. They are also available in fine medium and
coarse particle sizes. By matching the material and particle size to the ware being sintered, surface damage and
contamination can be reduced while maximizing furnace loading.

Fig.3. High temperature muffle furnace

2.5 Grinding and polishing

The sintered Mg-SiCp composite samples were ground using emery paper of gradually increasing fineness and then
polished on polishing machine for hardness test and microstructural analysis. The polished samples were then
lapped on polishing machine using diamond-lapping paste and velvet cloth for about 30 minutes so that mirror finish
is obtained on the samples.

3. Testing of properties

3.1 Density
The density of Mg-SiCp composites was determined by measuring the weight and volume of the specimens. The
volume was determined by measuring the accurate dimensions of the powder metal specimens. The theoretical
density was also determined by comparing the sum of volume (weight divided by the density) of constituents and
the volume of composite. For example, the density of Mg-10 wt. % SiCp composites was determined as follows:

Density of SiC = 3210 Kg/ m3, Density of Magnesium = 1770 Kg/ m3

(1)
ρ
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which gives the theoretical density (ρt ) for Mg-10 wt. % SiCp composites = 1853.13 kg/ m3, similarly the theoretical
densities of other compositions of Mg-SiCp composites were determined.

3.2 Porosity
The porosity was determined using the following formula:

E = 1–ρs/ρt ................. (2)

Where, E = Porosity (%)
ρs = Density of sintered part (Kg/ m3)
ρt = Theoretical density (Kg/ m3)

3.3 Rockwell hardness test

The Rockwell hardness test (C-scale) was performed on the polished surface of Mg-SiCp composites using digital
Rockwell hardness testing machine. The spheroconical diamond indenter (Brale indenter) with a load of 150 Kg was
used to perform the hardness test.

3.4 Compression test

The compression tests were performed on cylindrical samples of Mg-SiCp composites with L/D ratio of 3 using
universal testing machine (UTM) of 100 Ton capacity as shown in fig.4. The load was applied till fracture of the
specimens. The compression tests were performed on three samples of each composition of Mg-SiCp composites.

Fig.4. Compression of powder to form a weakly cohesive structure on UTM

3.5 Tensile test

The tensile tests were performed on cylindrical samples (15 mm diameter) of Mg-SiCp composites on universal
testing machine (UTM) of 100 Ton capacity. The load was applied till fracture of the specimens. The tensile tests
were performed on three samples of each composition of Mg-SiCp composites.

3.6 Microstructural analysis

The microstructures of the powder metal Mg-SiCp composites were studied using scanning electron microscopy
(SEM). The polished samples of Mg-SiCp composites were etched with 5 % NaOH solution for about 45 seconds
and washed with distilled water before the microstructural analysis. A number of scanning electron micrographs of
Mg-SiCp composite samples with 10 to 30 weight % of SiCp were taken and studied for microstructural analysis.
6014 Rajesh Purohit et al./ Materials Today: Proceedings 5 (2018) 6009–6017

4. Results & discussions

4.1 Density
The densities of Mg-SiCp composites are shown in Fig. 5. The fig. 5 shows that the theoretical as well as measured
density of Mg-SiCp composites increases with increase in wt. % of SiCp. This is because the SiC particles have
higher density then the magnesium. Theoretical densities of Mg-SiCp composites are more than the measured
densities, which is due to the porosity in the specimens.

Theoretical
2.1 Calculated 16
2.05 14
2 12
1.95 10

Porosity %
Density

1.9 8
1.85 6
1.8 4
1.75 2
1.7 0
0% 10% 20% 30% 40% 0% 10% 20% 30% 40%
Composition Composition

Fig. 5 Comparison of densities through theoretical and calculated Fig.6 Variation of porosity with different composition

4.2 Porosity
The porosity of Mg-SiCp composites are shown in Fig. 6. The fig. 6, shows that the porosity of Mg-SiCp composites
increases with increase in wt.% of SiCp which is due to the increase in percentage of coarser component (SiC
particles). Porosity of 10, 20 and 30 wt. % SiCp samples are 5.94%, 10.28% and 14.42% respectively. The increase
in porosity with increase in wt. of SiC particles have a neutralising effect on the increase in measured density of Mg-
SiCp composites.

4.3 Rockwell hardness test

The Rockwell hardness of Mg-SiCp composites are shown in Fig.7. Fig. 7 shows that the Rockwell hardness of Mg-
SiCp composites increases with increase in wt. % of SiCp. Rockwell hardness of 10, 20 and 30 wt. % SiCp samples
are 67 HRB, 86 HRB and 90 HRB respectively.

4.4 Compression test

The compressive strength of Mg-SiCp composites are shown in Fig.8. The fig.8 shows that the compressive strength
of Mg-SiCp composites increases with increase in wt. % of SiCp. Compressive Strengths of 10, 20 and 30 wt. %
SiCp samples are 78.66 MPa, 109.70 MPa and 122.71 MPa respectively. This was attributed to the uniform
dispersion and mechanical interlocking of SiC particles in the magnesium matrix obtained during mechanical
alloying process, which strengthened the consolidated specimens with increasing weight percent of SiCp.
 Rajesh Purohit et al. / Materials Today: Proceedings 5 (2018) 6009–6017 6015

100
140

Compressive Strrength (MPa)

90
Rockwell Hardness (HRB)

80 120
70 100
60
80
50
40 60
30 40
20
10 20
0 0
0% 10% 20% 30% 40% 0% 10% 20% 30% 40%
Composition Composition

Fig.7.Variation of porosity with different composition Fig.8.Variation of compressive strength with different composition

4.5 Tensile test

The tensile strength of Mg-SiCp composites are shown in Fig 9. The figure 9 shows that the tensile strength of Mg-
SiCp composites increases with increase in wt. % of SiCp. Tensile Strength of 10, 20 and 30 wt. % SiCp samples are
61.40 MPa, 69.25 MPa and 87.38 MPa respectively. The increase in the tensile strength of Al-SiCp composites with
increasing wt. % of SiCp is due to the increase in the modulus of elasticity and the elastic limit of the material. The
tensile strengths of powder metal Al-SiCp composites are quite less, which is due to the inherent porosity of the
powder metal compacts. Porosity is required for oil impregnation, which impart self-lubrication properties to the
components.
100
Tensile Strength (MPa)

80

60

40

20

0
0% 10% 20% 30% 40%
Composition

Fig.9.Variation of tensile strength with different composition

4.6 Microstructural analysis

The microstructures of the powder metal Mg-SiCp composites with 10 to 30 wt. % SiCp were studied using scanning
electron microscopy (SEM). The figure 10, 11 and 12 show the SEM micrographs of the Mg-SiCp composites with
10, 20 and 30 wt. % of SiC particles. The micrographs show that the SiC particles are uniformly distributed in the
magnesium matrix. SiC particles are visible in the micrograph. The micrograph shows that the bonding has taken
place between magnesium and SiC particles after sintering. Some amount of porosity is also visible in the
micrographs.
6016 Rajesh Purohit et al./ Materials Today: Proceedings 5 (2018) 6009–6017

Fig. 10. SEM micrograph of Mg-10 wt. % SiCp composite

Fig. 11. SEM micrograph of Mg-20 wt. % SiCp composite

Fig. 12. SEM micrograph of Mg-30 wt. % SiCp composite

5. Conclusions
In the present work Mg matrix-SiCp composites have been successfully fabricated using powder metallurgy process.
Testing of properties such as density, Rockwell hardness, compressive strength and tensile strength of fabricated
magnesium metal matrix composites have been carried out. The microstructure of the fabricated magnesium
composite was analyzed using scanning electron microscopy. The following conclusions were drawn from the
experimental results:
• The density, Rockwell hardness, compressive strength and tensile strength of Mg-SiCp composites made by
powder metallurgy process increases with increase in wt. % of SiCp from 10 to 30 weight percent.
• The porosity of Mg-SiCp composites was found to increases with increase in weight percent of SiC
particles.
• The mechanical alloying of powders result in improvement in hardness, compressive strength and tensile
strength of Mg-SiCp composites with 10 to 30 weight percent of SiC particles.
• Scanning electron micrographs of powder metal Al-SiCp composites reveals that the sintering in inert
atmosphere results in bonding between magnesium and SiC particles.
• The SEM micrographs show some amount of porosity and uniform distribution of SiC particulates in the
magnesium matrix.
 Rajesh Purohit et al. / Materials Today: Proceedings 5 (2018) 6009–6017 6017

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