A. D. PRADEEP, T. RAMESHKUMAR: EFFECT OF HEAT TREATMENT ON METALLURGICAL AND MECHANICAL PROPERTIES ...
555–560
EFFECT OF HEAT TREATMENT ON METALLURGICAL AND
MECHANICAL PROPERTIES OF AN ALUMINIUM 6061 HYBRID
COMPOSITE
VPLIV TOPLOTNE OBDELAVE NA METALUR[KE IN MEHANSKE
LASTNOSTI HIBRIDNEGA KOMPOZITA S KOVINSKO OSNOVO
IZ ZLITINE Al-6061
Pradeep Devaenthiran
*
, Rameshkumar Thirupathi
Department of Mechanical Engineering, Bannari Amman Institute of Technology, Sathyamangalam, Tamilnadu 638401, India
Prejem rokopisa – received: 2024-04-05; sprejem za objavo – accepted for publication: 2024-07-22
doi:10.17222/mit.2024.1149
Aluminium hybrid metal matrix composites (Al-HMMCs) are fabricated to investigate the influence of heat treatment on metal-
lurgical and mechanical properties. Aluminium 6061, known for its heat treatability and versatile applications is used as the base
material, and it is reinforced with tungsten carbide nanoparticles and graphite. Stir casting is used for fabricating four composite
samples. The prepared samples are subjected to a microscopic analysis and mechanical testing. The microscopic analysis shows
segregation along the grain boundaries, which are refined upon heat treatment. FESEM images with EDX mapping reveal an
even distribution of reinforcement particles without any agglomeration. Two different intermetallic phases are formed in the
shapes of plates and scripts, some of which dissolve to form precipitate-resisting grain growth and dislocation movement upon
heat treatment, enhancing the mechanical properties. The hardness of heat-treated composites is, on average, 33 % higher than
that of non-heat-treated composites. The yield strength and ultimate tensile strength of the heat-treated Al-HMMC are improved
by 43 % and 38 %, respectively, compared to the Al6061 alloy. The ductility of Al-HMMC is preserved even with the addition
of hard ceramic reinforcement, as evidenced by the percentage elongation of heat-treated Al-HMMC and supported by
fractographic analysis.
Keywords: hybrid metal matrix composites, tungsten carbide nanoparticles, graphite, heat treatment
Avtorji so izdelali in preiskovali hibridne kompozite z matrico iz Al zlitine vrste 6061 (Al-HMMC; angl: Aluminium Hybrid
Metal Matrix Composites). Analizirali so vpliv toplotne obdelave na njihove metalur{ke in mehanske lastnosti. Al zlitina vrste
6061 se dobro toplotno obdeluje in uporablja za razli~ne namene. Kovinsko osnovo iz Al 6061 so avtorji oja~ali z nano delci
volfram karbida (WC) in grafita z izbranim metalur{kim postopkom. V ta namen so za izdelavo {tirih razli~nih kompozitov
uporabili t.i. postopek ume{avanja delcev v kovinsko talino (angl.: stir casting process). Izdelane vzorce kompozitov so
metalografsko pregledali pod vrsti~nim elektronskim mikroskopom na emisijo polja (FESEM) in izdelali preizku{ance za
mehanske preiskave. Mikroskopska analiza je pokazala segregacije vzdol` mej kristalnih zrn, ki pa so se zmanj{ale s toplotno
obdelavo kompozitov. Pregled FESEM posnetkov z mikro-kemijsko linijsko (angl.: mapping) EDX analizo so celo odkrili
podro~ja enakomerne porazdelitve delcev oja~itve brez njihove aglomeracije. Tvorili sta se dve razli~ni intermetalni fazi v
obliki plo{~ic in "~rk", ki so se med toplotno obdelavo delno raztopili in tvorili izlo~ke. Ti so prepre~evali rast kristalnih zrn in
gibanje dislokacij ter s tem izbolj{ali mehanske lastnosti izdelanih kompozitov. Trdota toplotno obdelanih kompozitov je bila
pribli`no za 33 % vi{ja kot trdota toplotno neobdelnih vzorcev. Meja plasti~nosti in kon~na natezna trdnost izdelanih
kompozitov se je izbolj{ala za cca 43 % oziroma 38 % v primerjavi s ~isto Al 6061 zlitino. Ohranili so tudi njihovo duktilnost,
razvidno tudi iz SEM prelomov preizku{ancev, kljub dodatku trdih kerami~nih delcev.
Klju~ne besede: hibridni kompozit s kovinsko matrico, nano delci volframovega karbida, grafit, toplotna obdelava
1 INTRODUCTION
Metal matrix composites (MMCs) with reinforce-
ments of varying weight percentages, shapes, and sizes,
have become indispensable materials.
1
Hybrid metal ma-
trix composites (HMMCs) with more than one reinforce-
ment provide superior mechanical properties and adapt-
ability compared to single reinforcement composites.
2
The third most abundant material on the Earth, alu-
minium, is well-known for its light weight, strength, du-
rability, and malleability. Alloyed with other metals, alu-
minium 6061 is the most widely consumed material for
engineering products like bicycle frames, food contain-
ers, automotive wheels and aircraft components such as
bulkheads, fuselages, stabilizers and wings. Heat-treat-
able aluminium alloys hold particular prominence due to
their aplicability across engineering domains and alu-
minium alloy Al6061 stands out, often enhancing the
properties of its equivalents in the Al2XXX series.
Cao Fenghong et al. reinforced aluminium 6061 with
equal proportions of tungsten carbide (WC) and silicon
carbide. Hardness, ultimate tensile strength, yield
strength and compressive strength of the hybrid compos-
ite increase with the increase in the weight percentage of
reinforcement, but percentage of elongation decreases.
3
Suresh et al. studied mechanical properties of silicon car-
bide and tungsten carbide reinforced Al6061 hybrid
metal matrix composites. The hybrid metal matrix com-
Materiali in tehnologije / Materials and technology 58 (2024) 5, 555–560 555
UDK 546.62:539.4.016:621.785 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(5)555(2024)
*Corresponding author's e-mail:
pradeep@bitsathy.ac.in (Pradeep Devaenthiran)

posites had better mechanical properties with a reduced
particle size, improving the wettability and homogeneous
dispersion in the matrix.
1
Ebenezer Jacob Dhas et al. ex-
amined the effect of silicon carbide and tungsten carbide
reinforced aluminium metal matrix composites with
graphite as the solid lubricant. Though the ductility of
tungsten carbide reinforced composites is found to be in-
ferior, its hardness, tensile and compressive strength are
superior to silicon carbide reinforced composites.
4
Swamy et al. analysed the effect of tungsten carbide
reinforced metal matrix composites and found that their
properties improved with an addition of 3 w/% of tung-
sten carbide, beyond which they started to deteriorate.
5
Veeresh Kumar and Pramod fabricated Al6061 compos-
ites with tungsten carbide (1, 2 and 3) w/% as the rein-
forcement using powder metallurgy. An increase in the
tensile strength and hardness is observed with an addi-
tion of the reinforcement at the expense of ductility,
which is found to decrease by nearly 60 % compared to
the base material.
6
Swamy et al. investigated individual
effects of graphite and tungsten carbide on Al6061 based
metal matrix composites. The hardness, ultimate tensile
strength and compressive strength of 3 w/% tungsten car-
bide reinforced MMC are found to be the best and upon
further increase in the weight percentage they decrease.
Percentage elongation (ductility) decreases upon an addi-
tion of tungsten carbide, but it increases with an addition
of graphite. While ultimate tensile strength and compres-
sive strength increase with the addition of graphite, the
hardness decreases.
7
Shubhajit Das et al. investigated the effect of nano re-
inforcement on the properties (physical and mechanical)
of hybrid metal matrix composites by fabricating com-
posites with nano and micro reinforcements. It was
found that there was no significant change in the density
and porosity among the prepared composites. A signifi-
cant improvement in the hardness, tensile and impact
strengths were observed in nano-particle reinforced com-
posites compared to micro reinforcements.
8
Better hard-
ness and tensile strength were achieved with an addition
of 1.5 w/% of nano Al
2
O
3
reinforcement to Al7075 alu-
minium alloy, beyond which they started decreasing.
9
Vignesh Kumar et al. investigated the mechanical prop-
erties of aluminium (AA7075) composites prepared by
stir casting with tungsten carbide nanoparticles and mo-
lybdenum disulphide (MoS
2
) as the reinforcements.
Nano-sized particles allowed better mechanical proper-
ties at a lower percentage of reinforcement compared to
micron-size particles. Beyond 1.5 w/% of tungsten car-
bide nano-particles, tensile, compressive strength and
hardness were reduced considerably.
10
A compromise is
necessary when selecting the percentage of reinforce-
ment to be added for enhancing the mechanical proper-
ties of the composite without sacrificing its ductility.
2 EXPERIMENTAL DETAILS
2.1 Specimen fabrication
Table 1: Compositions and weight percentages of cast specimens
Specimen designation
Al6061
(w/%)
Graphite
(w/%)
Nano WC
(w/%)
Al6061 100 0 0
Al6061 + Gr 98 2 0
Al6061 + WC 95.5 0 1.5
Al-HMMC 93.5 2 1.5
Aluminum alloy 6061 is the base material; tungsten
carbide nanoparticles with an average particle size of 30
nm and graphite particles are used as the reinforcement.
FESEM images of the reinforcements are shown in Fig-
ure 1. Four different samples including re-melted and
as-cast Al6061 alloy with the weight percentages speci-
fied in Table 1 were fabricated using stir casting. In the
fabrication process, the reinforcing particles were mixed
with the molten metal using a stirrer operating at
400 min
–1
for up to 6 min at two-thirds depth from the
molten metal surface.
11,12
Additionally, 1 w/% magne-
sium and 1 w/% degassing powder were introduced to
the molten matrix to enhance the wettability and remove
gases from the melt.
13
Using the bottom pouring mecha-
nism, the molten metal was transferred to a circular
A. D. PRADEEP, T. RAMESHKUMAR: EFFECT OF HEAT TREATMENT ON METALLURGICAL AND MECHANICAL PROPERTIES ...
556 Materiali in tehnologije / Materials and technology 58 (2024) 5, 555–560
Figure 1: FESEM images of: a) tungsten carbide nanoparticles,
b) graphite

mould with a diameter of 25 mm. The cast specimens
were machined to obtain a near-net shape to meet the
ASTM standards for hardness and tensile testing.
2.2 Heat treatment
To enhance their properties, a T-6 heat treatment pro-
cess was applied to 50 % of the composite specimens
fabricated.
14
This involved the initial solution heat treat-
ment, where the samples were heated up to 530 °C and
maintained for 8 h. The heated samples were then
quenched in water. Subsequently, solution-treated sam-
ples were subjected to artificial aging at 165 °C for 6 h,
followed by cooling in air.
2.3 Testing methods
A Carl Zeiss Axiotech 100HD-3D microscope was
used to record the optical microstructures of the cast
composites. Sample preparation included initial polish-
ing with 600-grit sheets, followed by finish polishing
with 6 μm diamond paste on a disc polisher, and etching
with Keller’s etchant. A field emission scanning electron
microscope, Carl Zeiss Sigma 300, was utilized to record
surface morphology, reinforcement particle size, and
conduct fractographic analysis. A Rockwell hardness
tester was used to measure the hardness on the B scale as
per ASTM E18 standards. A computerised tensile testing
machine with a loading rate of 0.5 mm/min and linear
displacement resolution of 0.01 mm was used for tensile
tests as per ASTM B577M standards.
3 RESULTS
3.1 Microstructural analysis of the composites
The optical microstructures of the as-cast Al6061 al-
loy (Figure 2a) and Al-HMMC (Figure 2b) show
equiaxed grains and segregations of secondary phase
particles along the grain boundaries. The reinforcements
added to the composites are evenly distributed and no
agglomeration is observed inside the grains. After the T6
heat treatment, optical micrographs of the as-cast Al6061
alloy (Figure 2c) and Al-HMMC (Figure 2d) show dis-
continuous grain boundaries. Undesirable secondary
phase segregations along the grain boundaries dissolve
into the matrix, resulting in discontinuous grain bound-
aries. Dissolved atoms diffuse to form precipitates
throughout the matrix due to the aging process. Fine pre-
cipitates resist the grain growth and movement of dislo-
cations, resulting in better properties compared to the
non-heat-treated samples.
15
3.2 FESEM and EDX analysis of the composites
Figure 3 shows FESEM and EDX mapping of tung-
sten carbide and graphite reinforced Al-HMMC before
heat treatment. Intermetallic phases are formed due to
the interaction between the alloying elements of Al6061
like magnesium (Mg), iron (Fe) and silica (Si). Inter-
metallic phases in two different shapes are observed: a)
plate-like ones, most of which dissolve upon heat treat-
ment, and b) script-like ones, which remain undisturbed
after heat treatment. Intermetallic phases with Fe-Al-Si
and Mg-Si are formed during casting along the grain
boundaries and inside the grains, respectively.
16,17
The
Fe-Al-Si intermetallic phase is undesirable and becomes
dissolved during heat treatment, leading to discontinuous
grain boundaries. Dissolved atoms diffuse to form more
Mg-Si precipitates both along the grain boundaries and
within the grains, resisting the grain growth and disloca-
tion movement. The reinforcements (graphite and tung-
sten carbide) are evenly distributed throughout the ma-
trix and do not react during the formation of the
intermetallic due to their thermal stability.
A. D. PRADEEP, T. RAMESHKUMAR: EFFECT OF HEAT TREATMENT ON METALLURGICAL AND MECHANICAL PROPERTIES ...
Materiali in tehnologije / Materials and technology 58 (2024) 5, 555–560 557
Figure 2: Microscopic images of a) as-cast Al6061 before heat treatment, b) Al-HMMC before heat treatment, c) as-cast Al6061 after heat treat-
ment, d) Al-HMMC after heat treatment

3.3 Effect of the reinforcements and heat treatment on
the hardness
Variation in the hardness of the composites is shown
in Figure 4. The addition of graphite reduced the hard-
ness of the base alloy and the same effect is observed
with Al-HMMC, when adding graphite to the tungsten
carbide reinforced Al6061 aluminium alloy. This may be
due to the reduced density of graphite in comparison to
the base Al6061 alloy. On the other hand, the addition of
tungsten carbide nanoparticles improved the hardness
significantly due to their higher density and nano-parti-
cle size, even when added in a very small percentage
(1.5 w/%).
7
The increase in the hardness may be attrib-
uted to multiple factors, such as the inclusion of hard ce-
ramic particles, or the constraining effect provided by the
hard reinforcement particles and intermetallic phases.
The hardness of heat treated samples is found to be 33 %
higher on average, compared to non-heat treated sam-
ples. The hardness of Al-HMMC (heat treated and
non-heat treated) is found to be 45 % higher when com-
pared to the base Al6061 alloy.
3.4 Effect of the reinforcements and heat treatment on
the tensile strength
Figure 5 shows the tensile strength (yield and ulti-
mate) of composites before and after heat treatment. The
uniform distribution of reinforcing particles (WC) rein-
forces the metal matrix alloy by impeding the plastic
flow, resulting in an overall boost in the tensile strength.
The rise in the ultimate tensile strength and yield
strength is attributed to the strong interfacial bonding be-
tween the soft aluminium matrix and tough reinforce-
ment particles. Heat treated composites exhibit better
strength compared to non-heat treated composites due to
the differences between thermal expansion coefficients
and improved interfacial bonding between the alloy and
particles.
18
These strengthening mechanisms impede dis-
location movement, thereby increasing the strength of
the composite material.
19–20
A. D. PRADEEP, T. RAMESHKUMAR: EFFECT OF HEAT TREATMENT ON METALLURGICAL AND MECHANICAL PROPERTIES ...
558 Materiali in tehnologije / Materials and technology 58 (2024) 5, 555–560
Figure 3: Al-HMMC before heat treatment: a) FESEM image, b) EDX mapping of elements
Figure 5: Tensile strength of Al6061 and Al-HMMCs Figure 4: Hardness of Al6061 and Al-HMMCs

The yield strength and ultimate tensile strength of
Al-HMMC improved by 65 % and 36 %, respectively,
upon heat treatment. For the heat treated Al-HMMC,
they were found to be 43 % and 38 % higher than those
of the heat treated Al6061 aluminium base alloy.
3.5 Effect of the reinforcement and heat treatment on
percentage elongation
Table 2: Percentage elongation of Al6061 and Al-HMMCs
Specimen
Before heat
treatment
After heat
treatment
Al6061 11.62 11.78
Al6061 + Gr 11.35 11.47
Al6061 + WC 10.72 10.85
Al-HMMC 10.60 10.68
The Al6061 base alloy is ductile in nature, exhibiting
a percentage elongation of 11.62 % during the tensile
test. The addition of hard ceramic tungsten carbide
nanoparticles reduced the percentage elongation to a
large extent as shown in Table 2, compared to the addi-
tion of graphite. Heat treated samples respond better to
heat and exhibit better percentage elongation than that of
non-heat treated samples. Even though percentage elon-
gation is reduced after the reinforcement addition, the
ductile nature is maintained as evidenced by the lowest
percentage elongation of 10.60 % for the non-heat
treated Al-HMMC. The addition of a small amount of re-
inforcement (1.5 w/% WC) in the nano form preserves
the percentage elongation of Al-HMMC, which differs
from what is reported in the literature.
5–7,21–22
3.6 Fracture surface analysis of the Al-HMMC tensile
specimen
The fracture surface of heat treated Al-HMMC sub-
jected to uniaxial tensile testing is shown in Figure 6.
The fractograph consists of dimples, tearing edges and
small voids, indicating that the fracture of the tensile
specimen is ductile in nature.
19
Even though the addition
of the hard ceramic reinforcement reduced the percent-
age elongation (10.68 % for heat treated Al-HMMC)
compared to that of the base Al6061 alloy (11.78 % for
heat treated Al6061), the ductility of Al-HMMC is not
lost as evidenced by the fractograph analysis.
4 CONCLUSIONS
Composite specimens are fabricated using stir casting
and half of the samples are heat treated. The effects of
heat treatment and reinforcement addition are investi-
gated and the major conclusions are as follows:
• The microstructure analysis revealed segregations
along the grain boundaries and equiaxed grains,
which were refined upon heat treatment.
• The FESEM analysis shows the formation of
intermetallic phases (Fe-Al-Si and Mg-Si), some of
which dissolve upon heat treatment throughout the
matrix, improving the mechanical properties.
• The hardness increases with the addition of reinforce-
ment, which is mainly attributed to the addition of
tungsten carbide, rather than graphite that is found to
reduce hardness due to its lower density. Heat treated
samples are found to have better hardness (a 33-% in-
crease) due to the heat treatability of the Al6061 al-
loy.
• The tensile strength of the reinforced and heat treated
composites is better than that of the base aluminium
alloy. The yield strength and ultimate tensile strength
of Al-HMMC are improved by 65 % and 36 %, re-
spectively, upon heat treatment due to various
strengthening mechanisms and reinforcement addi-
tions.
• The ductility of Al-HMMC is preserved as evidenced
by percentage elongation and fractograph analysis.
This may be attributed to the optimal percentage of
reinforcement improving the strength without losing
the ductility.
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A. D. PRADEEP, T. RAMESHKUMAR: EFFECT OF HEAT TREATMENT ON METALLURGICAL AND MECHANICAL PROPERTIES ...
560 Materiali in tehnologije / Materials and technology 58 (2024) 5, 555–560