O. O. ABEGUNDE et al.: MICROSTRUCTURAL EVOLUTION AND MECHANICAL CHARACTERIZATIONS OF AL-TiC ...
297–306
MICROSTRUCTURAL EVOLUTION AND MECHANICAL
CHARACTERIZATIONS OF AL-TiC MATRIX COMPOSITES
PRODUCED VIA FRICTION STIR WELDING
KARAKTERIZACIJA RAZVOJA MIKROSTRUKTURE IN
MEHANSKIH LASTNOSTI KOMPOZITA Al-TiC IZDELANEGA
S TORNIM VARJENJEM Z ME[ANJEM
Oluwatosin Olayinka Abegunde, Esther Titilayo Akinlabi, Daniel Madyira
University of Johannesburg, Faculty of Engineering and Built Environment, Department of Mechanical Engineering Science,
Auckland Park Kingsway Campus, 2006 Johannesburg, South Africa
oabegunde@uj.ac.za
Prejem rokopisa – received: 2016-02-09; sprejem za objavo – accepted for publication: 2016-03-21
doi:10.17222/mit.2016.033
A study was conducted on the material characterization of aluminium (Al) and titanium carbide (TiC) metal-matrix composites
produced via friction stir processing. Different process parameters were employed for the welding process. Rotational speeds of
1600 min–1 to 2000 min–1, at an interval of 200 min–1 and traverse speeds of 100 mm/min to 300 mm/min, at an interval of 100
mm/min were employed for the welding conducted on an Intelligent Stir Welding for Industry and Research (I-STIR) Process
Development System (PDS) platform. The characterizations carried out include light microscopy and the scanning electron
microscopy analyses combined with Energy-Dispersive Spectroscopy (SEM/EDS) techniques to investigate the particle
distribution, microstructural evolution and the chemical analysis of the welded samples. Vickers microhardness tests were used
to determine the hardness distribution of the welded zone and tensile testing was conducted to quantify the strength of the
welded area compared to the base metal in order to establish the optimal process parameters. Based on the results obtained from
the characterization analysis, it was found that the process parameters played a major role in the microstructural evolution. A
homogenous distribution of the TiC particles was observed at a high rotational speed of 2000 min–1 and a low traverse speed of
100 mm/min. The highest hardness value was measured at the stir zone of the weld due to the presence of the TiC reinforcement
particles. The tensile strength also increased as the rotational speed increased and 92 % joint efficiency was recorded in a
sample produced at 2000 min–1 and 100 mm/min. The EDS analysis revealed that Al, Ti and C made up the composition formed
in the stir zone. The optimum process parameter setting was found to be at 2000 min–1 and 100 mm/min and can be
recommended.
Keywords: aluminium, friction stir welding, mechanical properties, metal matrix composite, microstructural evolution, titanium
carbide
V tem raziskovalnem delu je bila izvedena obse`na {tudija karakterizacije kovinskega kompozita aluminija (Al ) in titanovega
karbida (TiC) izdelanega z me{alno tornim varjenjem. Za postopek varjenja so bili uporabljeni razli~ni procesni parametri.
Rotacijski hitrosti 1600 min–1 do 2000 min–1, v razmaku po 200 min–1, in pre~nih hitrostih od 100 mm/min do 300 mm/min, v
intervalu 100 mm/min, je bilo uporabljeno za varjenje na industrijski platformi za razvoj in raziskave (PDS) sistema inteligent-
nega varjenja z me{anjem (I-Stir). Izvedena karakterizacija vklju~uje svetlobno mikroskopijo in vrsti~no elektronsko
mikroskopijo v kombinaciji z energijo disperzijsko spektroskopijo (SEM/EDS), za preiskavo porazdelitve delcev, razvoja
mikrostrukture in kemijsko analizo zvarjenih vzorcev. Za dolo~itev optimalnih procesnih parametrov je bil uporabljen Vickers
preizkus mikrotrdote, s katerim je bila dolo~ena porazdelitev trdote na podro~ju zvara, z nateznim preizkusom pa je bila
dolo~ena trdnost zvara v primerjavi z osnovnim materialom. Na osnovi rezultatov, dobljenih z analizo, je bilo ugotovljeno, da so
procesni parametri igrali pomembno vlogo pri razvoju mikrostrukture. Homogena porazdelitev TiC delcev je bila opa`ena pri
visokih hitrostih vrtenja (2000 min–1) in nizki pre~ni hitrosti (100 mm/min). Najve~ja vrednost trdote je bila izmerjena v me{alni
coni zvara zaradi prisotnosti delcev TiC za oja~anje. Natezna trdnost se je pove~ala tudi pri pove~anju hitrosti vrtenja in 92 %
skupne u~inkovitosti spoja je bila zabele`ena pri vzorcu, izdelanem pri 2000 min–1 in pre~ni hitrosti 100 mm/min. EDS-analiza
je pokazala, da Al, Ti in C povzro~ijo sestavo kompozita, ki je nastal v podro~ju me{anja. Priporo~ljiva in optimalna nastavitev
procesnih parametrov je 2000 min–1 in pre~na hitrost 100 mm/min.
Klju~ne besede: aluminij, torno varjenje z me{anjem, mehanske lastnosti, kompozit s kovinsko osnovo, mikrostruktura, titan
karbid
1 INTRODUCTION
Metal-matrix composites (MMCs) reinforced with
ceramic phases exhibit high stiffness, high elastic
modulus, improved resistance to wear, creep and fatigue,
which make them promising structural materials for the
aerospace and automobile industries compared to mono-
lithic metals. However, these composites also suffer from
a significant loss in ductility and toughness due to the
incorporation of non-deformable ceramic reinforce-
ments, which limit their application, especially where the
ductility of the material is a determinant factor in the
material selection.1 Aluminium metal-matrix composites
(AMCs) are variants of MMCs that have the potential to
replace many conventional engineering materials. AMCs
have already found commercial applications in the
defence, aerospace, automobile and the marine industries
due to their favourable metallurgical and mechanical
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UDK 620.1:66.017:621.791 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(2)297(2017)
properties.2–5 The metal matrix is aluminium, while the
reinforcement can be any ceramic particles compatible
with the metal matrix.
In recent years, several techniques have been reported
for manufacturing AMCs, these include; plasma air
spraying, stir casting, squeeze casting, molten metal
infiltration and powder metallurgy. These techniques
have been reported for producing bulk composites, while
high-energy laser melt injection, plasma spraying, cast
sinter and electron beam irradiation have been used for
producing surface AMCs.6,7 Nevertheless, it should be
pointed out that most of these existing processing tech-
niques for forming composites are generally based on
liquid-phase processing at high temperatures. In this
case, it is hard to avoid the interfacial reaction between
the reinforcement and the metal matrix and the formation
of some detrimental phases.1
Friction stir welding (FSW) is a solid-state welding
process developed by TWI for welding aluminium and
its alloys.8 It has been used to successfully weld alumi-
nium alloys9–11 and also used to weld other metals like
magnesium, copper and titanium.12–14 FSW is an emerg-
ing potential technique that can be employed for pro-
ducing AMCs.15–18 Since the process is a solid-state
welding process, it is envisaged to alleviate the problems
associated with interfacial reaction, the melting of
ceramics and the formation of detrimental phases during
the manufacture of AMCs.
Research studies have been reported on the friction
stir processing (FSP) of aluminium matrix compo-
sites.19–22 These studies concluded that grain refinement
was achieved using the FSP process. An improved
particle distribution and better mechanical properties
were also observed. Also reported is that the process
parameters used for welding and the tool geometry
played a major role in the final outcome. Based on the
available literature, previous research studies have been
limited to surface composites using the FSP process for a
modification of the surface properties.
In this study, AMCs were produced using FSP and
titanium carbide (TiC) particles were used as the rein-
forcement. The addition of the TiC ceramic particles is
due to its favourable mechanical properties, which
include a high melting point, favourable fracture and
tribological properties. The preference of FSP for the
production of Al-TiC composite is to avoid delamination
(a failure when laminated material becomes separated,
perhaps due to poor processing during production,
impact on service, or some other factors that may lead to
the separation of layers of reinforcement), debonding
(when two materials stop adhering to each other), incom-
patible mixing of base materials and filler materials, the
presence of porosity, inhomogeneous distribution
(clustering), the segregation of grains at boundaries, the
wetting of the particles, excess eutectic formation, melt-
ing of ceramic particles and the formation of undesirable
deleterious phase usually experienced in other techni-
ques. FSP is also advantageous due to the rapid removal
of reaction products from the interface, which enhances
further reaction
The effect of process parameters on the stir zone’s
microstructure, microhardness and tensile behaviour was
studied and the optimal process parameters were estab-
lished.
2 EXPERIMENTAL PART
2.1 Preparation, dimensions and composition of work-
pieces
Aluminium 1050 alloy sheets of dimensions 300 mm
× 200 mm × 3 mm with a smooth surface finish were
used for this research work. The chemical composition
of the aluminium as per manufacturer Material Safety
Data Sheet (MSDS) is shown in Table 1.
Before the welding process, V grooves with depth of
1.5 mm and a width of 3 mm were made on all the
aluminium sheets using a milling machine and the
titanium carbide particles were filled and compacted into
the grooves using a tool with only the shoulder, as
illustrated schematically in Figure 1.
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Figure 1: Schematic illustration of FSW of Al/TiC
Slika 1: Shematski prikaz FSW Al / TiC
2.2 FSW tool
The FSW tool used is a cylindrical H13 steel tool
hardened to 52 HRC shown in Figure 2.
A basic tool geometry was used with a tool length of
5.7 mm and a tool diameter of 6 mm. The tool shoulder
diameter is three times the pin diameter (18 mm) and
with a concave geometry to exert pressure on the work-
piece during welding.
2.3 Friction stir welding platform
The experimental setup of the samples properly
positioned and firmly clamped on the backing plate is
shown in Figure 3. The process was performed on an
Intelligent Stir Welding for Industry and Research
(I-STIR) Process Development System (PDS) at the
eNtsa of Nelson Mandela Metropolitan University, Port
Elizabeth, South Africa. Table 1 summarizes the diffe-
rent welding parameters used to produce the welds. A tilt
angle of 3° was kept constant and used for all the
different welding parameters.
Table 1: FSW process parameters
Tabela 1: FSW-procesni parametri
Weld
number
Rotational
speed
(min–1)
Traverse
speed
(mm/min)
Weld
Interface
Weld pitch
(mm/ min–1)
A1 1600 100 With TiC 0.063
A2 1600 200 With TiC 0.125
A3 1600 300 With TiC 0.188
B1 1800 100 With TiC 0.056
B2 1800 200 With TiC 0.111
B3 1800 300 With TiC 0.167
C1 2000 100 With TiC 0.050
C2 2000 200 With TiC 0.100
C3 2000 300 With TiC 0.150
D1 1600 200 WithoutTiC 0.125
D2 1800 200 WithoutTiC 0.111
D3 2000 200 WithoutTiC 0.100
A backing plate made of mild steel was positioned
between the bed of the FSW platform and the workpiece.
The choice of the backing plate is for a proper dissipa-
tion of heat during the welding process. A supporting
sheet of the same thickness was placed underneath the
upper plate to help align and stabilize the sheets to be
joined during welding.
2.4 Metallographic sample preparation and mechani-
cal testing
Before sectioning the samples for various characte-
rizations with a water-jet cutting machine, the flashes
created during welding were removed from the weld
seams. The metallographic sample preparation was done
in accordance with ASTM E3-95 for microstructure
analyses.23 The samples were sectioned perpendicular to
the weld direction. Grinding and polishing were care-
fully done on the samples to obtain mirror-finished
samples. Keller’s reagent was used to etch the samples
for the proper observation of the grains. A DP25 Olym-
pus optical microscope and a scanning electron micro-
scope with energy-dispersive spectrometry (SEM +
EDS) were used for the microstructural analysis. To eva-
luate the mechanical properties, Vickers microhardness
and Instron tensile testing were used. The Vickers
hardness was done in accordance with the ASTM
E92-82E3 standard.24 A load of 100 g and a dwell time
of 10 seconds were used. The tensile tests were carried
out using a load cell capacity of 100 kN at a crosshead
rate of 1 mm/min. No fewer than three lap tests were
made for each process parameter. Since there is no test
standard for friction stir lap joints, ASTM E8/E8M-13a
and ASTM D100225,26 for shear strength of a single lap
joint adhesively bonded metal specimen (tension loading
of metal-to-metal) were used as the reference test
standard for the lap shear tests. Fractography was
performed on the fractured surface of the tensile samples
to determine the mode of failure.
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Figure 3: Experimental weld setup of FSW platform
Slika 3: Eksperimentalna postavitev FSW platforme za varjenje
Figure 2: FSW Tool
Slika 2: Orodje za FSW
3 RESULTS AND DISCUSSION
3.1 Weld surface visual observation
The top surfaces of the welded samples under diffe-
rent welding process parameters are shown in Figure 4.
The samples are labelled according to the designations
indicated in Table 1. The visual assessment of the weld
surfaces show no typical physical defects like worm-
holes, cracks or voids.
The shape of the weld seam is slightly convex in
nature, which is caused by the design of the tool
shoulder. A semi-circular ripple effect caused by the
action of the tool shoulder was also observed. This effect
is referred to as the wake effect. Keyholes were seen at
the end of the weld seam. The depth of these keyholes
shows the extent of the penetration of the pin from the
top to the bottom sheet. Flashes were observed for all the
process parameters used and more on the welds
produced without reinforcement particles. Most of the
flashes were located on the retreating side of the weld
due to the movement of the materials from the advancing
side of the weld to the retreating side.
3.2 Microstructural evolution
Macrostructure
Table 2 summarizes the macrostructure pictures at
the cross-section of the weld zone for different process
parameters.
From Table 2 it is clear that the process parameters
have a significant effect on the orientation of the FSP
macrostructure. As the traverse speed increased from
100 mm/min to 300 mm/min using the same tool geo-
metry, the geometry of the nugget zone changed from an
elliptical shape to a basin-like shape. It is important to
note that the formation of the basin shape is due to the
effect of thermal heat transfer from the shoulder of the
tool to the sheets. At a high traverse speed of 300
mm/min, the heat generated is lower and most of the heat
built up at the top sheet with a minimal proportion of the
heat sink into the bottom sheet. This makes the top sheet
undergo more thermal cycles by direct contact with the
tool shoulder and severe plastic deformation than the
bottom sheet, causing the basin-like shape to form. The
intense plastic deformation and high-temperature
exposure experienced at the lower traverse speed resulted
in the elliptical shape.
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Figure 5: SEM photomicrograph of TiC Powder
Slika 5: SEM-posnetek prahu TiC
Figure 4: Top view of the processed FSW welds
Slika 4: Pogled od vrha na FSW-zvare
Table 2: Macrostructural features for different process parameters
Tabela 2: Izgled makrostrukture pri razli~nih procesnih parametrih
Weld number Macrostructure Nugget shape
A1 Elliptical
A2 Basin
A3 Basin
B1 Elliptical
B2 Elliptical
B3 Basin
C1 Elliptical
C2 Basin
C3 Basin
TiC Powder
The micrograph of the TiC powder under the SEM of
the TiC powder used as the reinforcement in this
research study is illustrated in Figure 5.
The morphology of the TiC powder is irregular
shaped ball milled powder with a grain size of about 2
microns.
Microstructure
The pictorial overview of the microstructural evolu-
tion across different zones after FSP is presented in
Figure 6. All the four zones, namely the base metal
(BM) close to the heat-affected zone (HAZ), the HAZ
that is sandwiched by the BM and the thermo-mecha-
nically affected zone (TMAZ), TMAZ found on both
sides of the stir zone (SZ) and the SZ were exhibited in
the micrographs taken from the processed zones. The
BM retains its original microstructural features. The
TMAZ and HAZ were formed on both the retreating and
advancing sides of the welds. The grain structure in the
HAZ shows elongated grain growth that is slightly diffe-
rent from the base material. The temperature experienced
in the HAZ was enough to thermally activate the grain
growth, but not sufficient to plastically deform the grain.
In TMAZ, severely deformed grains are found, which are
induced by drastic plastic deformation of the SZ during
the FSP. In the SZ, the microstructure is characterized by
dynamically recrystallized fine equiaxed grains owing to
the drastic deformation induced by the sufficient stirring
during welding of the top and bottom sheets. The distri-
bution of the TiC reinforcement particles is a salient
feature observed between the top and the bottom sheets
around the SZ. At the top SZ, the presence of TiC is
negligible and scanty, but a significant distribution was
found at the bottom of the sheet. This indicates that
during the welding process, the reinforcement particles
experienced both downward and horizontal flow around
the stir zone. Grains in the upper SZ are coarser than
those in the bottom SZ. The heat during the FSP mainly
originates from the tool shoulder friction with the surface
of the top sheet. Additionally, the heat in the bottom SZ
can easily transfer into the bottom sheet and the backing
plate. Therefore, the heat cycle of the bottom SZ is
relatively lower. The grains in the upper SZ have more
time to grow due to the higher heat input.
Another observation from the microstructure is the
transition region on the advancing side (AS) and the
retreating side (RS), which is illustrated in Figure 7. On
the AS, the transition region is sharper and well defined
and on the RS, the transition region diffuses into the
parent material. On the AS, the plastic deformation
direction of the processed zone and the BM are in
opposite directions, which resulted in an enormous
relative deformation and the homogenous distribution of
the TiC particles between the BM and the processed
zone at the AS, but the BM distorted and diffused
smoothly together with the processed zone at the RS,
resulting in clustering of the reinforcement.
It can be observed that the TiC reinforcement within
the processed zone had undergone intense mixing and
stirring, resulting in breakup of the coarse TiC mor-
phology. As the rotational speed of the weld increased
from 1600 min–1 to 2000 min–1, the distribution of TiC
becomes more homogenous, as shown in Figure 8. At a
rotational speed of 1600 min–1, the particles clustered
together around the bottom sheet and at 2000 min–1, the
particles were uniformly distributed around the stir zone.
The contribution of intense deformation and high-
temperature exposure within the stir zone resulted in
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Figure 6: Microstructural evolution at different zones: A) thermo-
mechanical affected zone, B) upper stir zone, C) heat-affected zone,
D) lower stir zone E) macrostructure of the weld zone at 2000 min–1
and 300 mm/min
Slika 6: Razvoj mikrostrukture na razli~nih podro~jih: A) termo-
mehansko vplivano podro~je, B) zgornje me{alno podro~je, C)
toplotno vplivano podro~je, D) spodnje me{alno podro~je, E) makro-
struktura zvara pri 2000 min–1 in pre~ni hitrosti 300 mm/min
Figure 7: Transition zone: A) retreating side and B) advancing side
Slika 7: Prehodno podro~je: A) umikajo~a stran in B) napredujo~a
stran
fragmentation, recrystallization and the development of
refined texture within and around the stir zone at a
rotational speed of 2000 min–1. In addition, an increase
in the traverse speed caused the particles to agglomerate
in the stir zone. As the traverse speed decreased, the
grain size also decreased in the composite but increased
in the pure aluminium samples without the reinforce-
ment. This might be due to the high heat input associated
with the low traverse speed. The significant effect of
particle reinforcement on the grain size refinement of the
matrix is reported as a pinning effect. According to the
pinning effect, the grain refinement by reinforcement
particles increases with a decrease in the particle size
and an increase in the volume fraction of the particles.
Sufficient heat input and stirring are responsible for the
deformation and recrystallization of the matrix with the
reinforcement. At a higher rotational speed of 2000
min–1 and a traverse speed of 100 mm/min, a more
uniform distribution of the TiC particles was found.
Energy Dispersive Spectroscopy Results
EDS analyses were performed on all the welds with
reinforcement. The uniformly distributed particles were
confirmed to be titanium and carbon, as shown in Fig-
ure 9, which is a scan of the weld interface of the sample
produced at a rotational speed of 2000 min–1 and a tra-
verse speed of 100 mm/min.
The elemental composition by atomic weight at the
stir zone is confirmed to be 72.04 % of aluminium,
23.71 % of carbon and 4.34 % of titanium.
3.3 Microhardness Profiling
The Vickers hardness distribution is illustrated in
Figure 10. The shape of the hardness distribution is a
"W-sinusoidal". The lowest hardness value was found at
the HAZ and the highest hardness value at the SZ. The
hardness value of the SZ increased by 58 % when
compared to the base metal for sample C1. Thangarasu21
suggested four methods of hardening in FSW MMC:
• Orowan strengthening.
• Grain and substructure strengthening.
• Quench hardening resulting from the dislocations
generated to accommodate the differential thermal
contraction between the reinforcing particles and the
matrix.
• Work hardening due to the strain misfit between the
elastic reinforcing particles and the plastic matrix.
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Figure 10: Hardness profile of the FSL welds
Slika 10: Profil trdote preko FSL-zvarov
Figure 9: EDS from the weld interface at a rotational speed of 2000
min–1 and a traverse speed of 100 mm/min
Slika 9: EDS iz stika z zvarom pri hitrosti vrtenja 2000 min–1 in
pre~ni hitrosti 100 mm/min
Figure 8: SEM micrograph at a traverse speed of 100 mm/min and rotational speeds of: a) 1600 min–1 b) 1800 min–1 and c) 2000 min–1
Slika 8: SEM posnetek pri pre~ni hitrosti 100 mm/min in hitrosti vrtenja: a) 1600 min–1, b) 1800 min–1 in c) 2000 min–1
The increment in the hardness value at the SZ is
attributed to grain refinement and the presence of rein-
forcement particles. The fragmentation of larger TiC
particles gave rise to dislocation density and dynamic
recrystallization during welding, thereby producing a
finer grain size in the stir zone. These factors are respon-
sible for higher hardness values in the stir zone of welds
with the reinforcement particles. The minimum hardness
value appeared at the HAZ. This is due to the thermal
history experienced at this zone, which resulted in the
coarsening of the precipitates.
Because of the distribution and the deposition of the
TiC particles around the AS of the welded zone, the AS
size shows higher hardness values than the RS due to the
fact that materials on the RS have a shorter time to rotate
since the current flow of material is directly proportional
to the time of flow on this side.
3.4 Tensile behaviour
In order to quantify the mechanical resistance of the
FSWed joints, the ratio between the maximum trans-
ferred load by the specimens in shear test to the width of
the specimen itself was considered. In this way, the
values are shown for all the considered cases. The
average results of the three replica samples carried out
are reported. Every sample was tested to failure. The
shear fracture load per unit width of the FSWed Al with
and without the TiC composite for different process
parameters are presented in Table 3.
From the results obtained, it was found that the
maximum shear strength was observed at a rotational
speed of 2000 min–1 and a traverse speed of 100
mm/min, and the minimum was observed at a rotational
speed of 1600 min–1 and a traverse speed of 300
mm/min. Both the maximum and the minimum shear
strengths were observed when the TiC reinforcement
particles were added, but at different rotational and
traverse speeds, respectively. It can be concluded from
the results that the relationship between the fracture load
and the traverse speed is inversely proportional. An
increase in the traverse speed causes the fracture load to
decrease. A shorter reaction time and a lower reaction
temperature are associated with a higher traverse speed
and this led to a decrease in the stirring period and the
vertical movement of the material with the reinforce-
ment, thereby affecting the strength of the bonding at the
interface. It is obvious that the fracture load increases
with an increase in the rotational speed for all the
samples with reinforcement. As the rotational speed
increased from 1600 min–1 to 2000 min–1, a substantial
increase in the fracture load was observed. A higher
rotational speed generated a higher heat input because of
the higher friction heating, which resulted in more
intense stirring and mixing of the material.
It should be noted that the fracture load behaviour
that occurred in the samples without reinforcement is the
reciprocal of results obtained from the samples with
reinforcement. The absence of the ceramic particle along
the path of the weld seam exposed the weld interface to a
higher degree of thermal reaction, thereby making it
sensitive to temperature changes. As the rotational speed
increases, the temperature around the weld zone in-
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Table 3: Shear strength and joint efficiency of the welds
Tabela 3: Stri`na trdnost in skupna u~inkovitost razli~nih zvarov
Weld number Rotational speed(min–1)
Traverse speed
(mm/min)
Weld pitch
(mm/ min)
Average shear fracture
load per unit width
(N/mm)
Joint efficiency
(%)
A1 1600 100 0.063 159 79.4
A2 1600 200 0.125 150 60.8
A3 1600 300 0.188 132 52.97
B1 1800 100 0.056 185 90.51
B2 1800 200 0.111 178 77.53
B3 1800 300 0.167 170 68.60
C1 2000 100 0.050 218 92.14
C2 2000 200 0.100 213 85.96
C3 2000 300 0.150 175 69.44
D1 1600 200 0.125 201 81.67
D2 1800 200 0.111 187 81.45
D3 2000 200 0.100 173 69.81
Figure 11: Fracture load against the process parameters
Slika 11: Odvisnost obremenitve pri poru{itvi od parametrov procesa
creases, causing a highly turbulent mixture and stirring
of the materials. Since there is no intermediate particle to
balance the temperature change, the strength of the
bonding will be compromised. However, once a suffi-
cient rotational speed is achieved, a further increase is
not beneficial to the mechanical properties for welds
produced without reinforcement particles.
Figure 11 shows the effect of the reinforcement
particles on the fracture load. As can be seen, the pre-
sence of the TiC reinforcement particles contributed an
appreciable strength change to the fracture load at a
higher rotating speed of 2000 min–1 and does not show a
remarkable improvement in the fracture load at rotational
speeds of 1600 min–1 and 1800 min–1, respectively,
instead, it had an inverse effect on the strength.
At a rotational speed of 2000 min–1, the TiC homo-
genously mixed with the Al alloy properly, thereby
forming a well-bonded matrix that yielded a higher frac-
ture strength. The presence of the ceramic particles
constrained the easy failure of the material when under
loading, thereby improving the mechanical strength of
the matrix.
Joint efficiency
To estimate the joint efficiency of the FS welds, the
ratios of the tensile strength of the lap shear specimens
were compared to the tensile strength of the base metals.
According to studies,27 the tensile strength of the lap
shear specimen is derived from the fracture load per unit
width to the effective sheet thickness (EST) as shown in
Equation (1):
Tensile strenth of
lap shear specimen
=
Fracture load per unit width
EST
(1)
The EST is defined as the minimum sheet thickness
determined by measuring the smallest distance between
any un-bonded interface and the top surface of the upper
sheet or the bottom surface of the lower sheet and it
varies with the process parameter, depending on the de-
gree of bonding that exists between the weld interfaces.
These phenomena should have apparent influences on
the bearing load of FSW lap-welded joints. Also pre-
sented in Table 3 is the joint efficiency of the processed
samples for different process parameters. From the re-
sult, the joint efficiency ranges from 52 % to 92 %. The
highest was found at a rotational speed of 2000 min–1
and a traverse speed of 100 mm/min.
The effect of the traverse speed on the EST was
studied. Figure 12 shows the graphical relationship
between the EST versus traverse speed. The traverse
speed exhibits a linear relationship with the EST. As the
traverse speed increased, the dimension of the EST also
increased, thereby reducing the area of the metallurgical
bond that exists at the processed interface. Since the
strength of the weld interface depends on the area of the
metallurgical bond during the welding process, it is
apparent that the relationship between the EST and the
overall strength of the processed zone is exponential.
Fracture behaviour
Four different modes of failure were observed at the
joint interfaces, as illustrated in Figure 13. They are the
fracture mode (FM) 1, the shear fracture that occurred
due to a lack of joint formation along the original inter-
O. O. ABEGUNDE et al.: MICROSTRUCTURAL EVOLUTION AND MECHANICAL CHARACTERIZATIONS OF AL-TiC ...
304 Materiali in tehnologije / Materials and technology 51 (2017) 2, 297–306
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 13: Fracture mode of the welded samples for different process
parameters
Slika 13: Na~in zloma vzorcev, zvarjenih pri razli~nih procesnih para-
metrih
Table 4: Mode of fracture for different process parameters
Tabela 4: Na~in zloma pri razli~nih procesnih parametrih
Weld number Rotationalspeed (min–1)
Traverse speed
(mm/min) Fracture mode
A1 1600 100 FM 1/FM 2
A2 1600 200 FM 2
A3 1600 300 FM 1
B1 1800 100 FM 3
B2 1800 200 FM 3
B3 1800 300 FM 4
C1 2000 100 FM 3/FM 4
C2 2000 200 FM 3
C3 2000 300 FM 4
D1 1600 200 FM 3
D2 1800 200 FM 4
D3 2000 200 FM 4
Figure 12: EST against the traverse speed
Slika 12: Odvisnost med EST in pre~no hitrostjo
face of the two sheets. This led to a pseudo metallurgical
bond between the two sheets and the bond shear under
tensile loading. Fracture mode 2 occurred on the
advancing size hooking in which the crack initiates from
the tip of the hook on the AS, propagates upward along
the SZ/TMAZ interface and finally, the fracture at the
SZ. Fracture mode 3 was noticed on the retreating side
softening initiated from the hook and then linked to the
pores on the bottom plates caused by the diffusion of the
bottom plate with the backing plate. The crack follows
the sharp end of the grove to the other end. Fracture
mode 4 failure took place close to the base metal, but the
weld actually failed at the HAZ on the advancing side of
the weld. Table 4 lists the failure modes observed for
each process parameter combination.
FM 1 was found at a low rotational speed of 1600
min–1 and a high traverse speed of 300 mm/min. The
dominant fracture modes are FM 3 and FM 4.
FM 1 is observed at a a low rotational speed and high
traverse speed. This process condition is associated with
the low heat input that resulted in insufficient deforma-
tion and a flow of the material forming the pseudo weld.
The crack initiation occurs through the gap tip of the
unwelded area and went through the stir zone, making
the weld shear into two at the welded area. This usually
occurs when an insufficient metallurgical bond is formed
between the sheets.
FM 2 and FM 3 are the most dominating failure
modes. The fracture mode is similar to the normal tensile
behaviour of the aluminium alloy. The material went
through necking for a period before eventually fracturing
at the weakest zone.
The SEM images of the fracture surfaces were taken
to determine the mode of fracture. Figure 14 illustrates
the typical fractography features of the failure surfaces.
The morphology of the failure mode shows a large
number of fine dimples, which confirms the amount of
plastic flow prior to the failure under tensile loading. The
fine dimple features observed indicate that the behaviour
of the fracture is ductile, which implies that the lap joints
exhibited ductile fracture during the lap shear tests.
4 CONCLUSION
Based on the observations from the results, the
followings conclusions can be drawn:
• The microstructural evolution correlates with the
process parameters employed to produce the welds in
this study. It was found that as the traverse speed
increases, the evolving microstructure changed from
elliptical to a basin-like shape at the interface.
• The microstructure revealed that the majority of the
TiC particles were transported from the weld
interface and deposited in the bottom sheet.
• The highest tensile value of 218 N/mm and the joint
efficiency of 92 % were recorded for a weld
produced at a high rotational speed of 2000 min–1 and
a low traverse speed of 100 mm/min. This parameter
combination setting can be recommended.
• The maximum hardness occurred at the stir zone and
the minimum at the HAZ. The advancing side
exhibited a higher hardness distribution compared to
the retreating side of the welds.
Acknowledgements
Mr Abegunde (co-author) would like to acknowledge
the University of Johannesburg under the Global Excel-
lence Stature (GES) award scholarship of the Post-
graduate Research Centre for their financial support,
Prof Esther Akinlabi (co-author) acknowledges the
Johannesburg Institute of Advanced Study (JIAS) for the
writing fellowship award during which she was able to
contribute to this manuscript and finally, the eNtsa
Research Group of Nelson Mandela Metropolitan Uni-
versity (NMMU), Port Elizabeth, South Africa is
acknowledged for allowing us to use their facility to
produce the welds.
5 REFERENCES
1 R. Mishra, Z. Ma, Friction stir welding and processing. Materials
Science and Engineering: R: Reports 50 (2005) 1, 1–78, doi:10.1016/
j.mser.2005.07.001
2 K. Kainer, Metal matrix composites: Custom-made materials for
automotive and aerospace engineering. John Wiley & Sons, 2006
3 Fabrication of near-net shape graphite/magnesium composites for
large mirrors, Orlando’90, International Society for Optics and
Photonics, 1990, 16–20
4 D. Miracle, Metal matrix composites–from science to technological
significance, Composites Sci. Technol. 65 (2005) 15, 2526–40,
doi:10.1016/j.compscitech.2005.05.027
5 S. Rawal, Metal-matrix composites for space applications, Journal of
Minerals, Metals and Materials Society, 53 (2001) 4, 14–7,
doi:10.1007/s11837-001-0139-z
6 L. Santo, D. Paulo, Surface engineering techniques and applications:
Research Advancements, 2014, 1–347
7 P. Babalola, C. Bolu, A. Inegbenebor, K. Odunfa, Development of
aluminium matrix composite, International Journal of Engineering
and Technology Research, 2 (2014) 1, 1–11
O. O. ABEGUNDE et al.: MICROSTRUCTURAL EVOLUTION AND MECHANICAL CHARACTERIZATIONS OF AL-TiC ...
Materiali in tehnologije / Materials and technology 51 (2017) 2, 297–306 305
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 14: SEM images of the fracture surface for the weld produced
at a rotational speed of 2000 min–1 and a traverse speed of 100 mm/min
Slika 14: SEM-posnetek povr{ine preloma zvara, izdelanega pri
hitrosti vrtenja 2000 min–1 in pre~ni hitrosti 100 mm/min
8 W. Thomas, E. Nicholas, J. Needham, M. Murch, P. Temple Smith,
C. Dawes, Inventors, Friction stir welding, UK Patent Office,
London patent, December 6, 1991.
9 D. Klobcar, L. Kosec, A. Pietras, A. Smolej, Friction-stir welding of
aluminium alloy 5083, Mater. Tehnol., 46 (2012) 5, 483–488
10 S. Kastelic, J. Tu{ek, D. Klob~ar, J. Medved, P. Mrvar, AA4130 and
AA1050 Joined with friction-stir welding, Mater. Tehnol., 47 (2013)
2, 195–198
11 R. Palanivel, P. Mathews, The tensile behaviour of friction-stir-
welded dissimilar aluminium alloys, Mater. Tehnol., 45 (2011) 6,
623–626
12 E. Akinlabi, Characterisation of dissimilar friction stir welds bet-
ween 5754 aluminium alloy and C11000 copper, Dissertation
submitted to Nelson Mandela Metropolitan University, Port Eliza-
beth, South Africa 2011
13 D. Sanders, Development of friction stir welding combined with
superplastic forming processes for the fabrication of titanium
structures. Dissertation submitted to University of Washington,
Washington, United States 2008
14 A. Elrefaey, M. Gouda, M. Takahashi, K. Ikeuchi, Characterization
of aluminum/steel lap joint by friction stir welding, 2004
15 R. Mishra, Z. Ma, I. Charit, Friction stir processing: A novel tech-
nique for fabrication of surface composite. Materials Science and
Engineering, 341 (2003) 1, 307–310, doi:10.1016/S0921-5093(02)
00199-5
16 Z. Ma, Friction stir processing technology: A review. Metallurgical
and Materials Transactions, 39 (2008) 3, 642–658, doi:10.1007/
s11661-007-9459-0
17 A. Smolej, D. Klob~ar, B. Skaza, A. Nagode, E. Sla~ek, V. Drago-
jevi}, S. Smolej, The superplasticity of friction stir processed al-5Mg
alloy with additions of scandium and zirconium, International
Journal of Materials Research, 105 (2014) 12, 1218–1226,
doi:10.3139/146.111141
18 A. Smolej, Superplastic behaviour of AA5083 aluminium alloy with
scandium and zirconium, Materials Science ForumTrans Tech Publ,
2012, doi:10.4028/www.scientific.net/MSF.706-709.395
19 V. Sharma, U. Prakash, B. Kumar, Surface composites by friction stir
processing: A review, J Mater Process Technology, vol. 706–709,
2015,395
20 E. Akinlabi, R. Mahamood, S. Akinlabi, E. Ogunmuyiwa, Processing
parameters influence on wear resistance behaviour of friction stir
processed al-TiC composites, Advances in Materials Science and
Engineering, 2014, doi:10.1155/2014/724590
21 A. Thangarasu, N. Murugan, I. Dinaharan, S. Vijay, Microstructure
and microhardness of AA1050/TiC surface composite fabricated
using friction stir processing, Sadhana 37 (2012) 5, 579–586,
doi:10.1007/s12046-012-0097-x
22 S. Jerome, S. Bhalchandra, S. Babu, B. Ravisankar, Influence of
microstructure and experimental parameters on mechanical and wear
properties of al-TiC surface composite by FSP route, Journal of
Minerals and Materials Characterization and Engineering, 11 (2012)
05, 488–493, doi:10.4236/jmmce.2012.115035
23 Standard A. E3, standard guide for preparation of metallographic
specimens, ASTM International, West Conshohocken, PA 2001
24 Standard A. E92-82 E3. 1997: Standard test method for vickers
hardness of metallic materials, Annual Book of ASTM Standards,
ASTM International, West Conshohocken, PA ASTM
25 Standard A. D1002-01, 2001, Standard Test Method for Apparent
Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Spe-
cimens by Tension Loading (Metal-to-Metal), ASTM International,
West Conshohocken, PA, 2001, doi:10.1520/D1002-10.
26 Standard A. E8/E8M-13a, standard test method for determining
volume fraction by systematic manual point, ASTM International,
West Conshohocken, PA 2013
27 X. Xu, X. Yang, G. Zhou, J. Tong, Microstructures and fatigue
properties of friction stir lap welds in aluminum alloy AA6061-T6,
Journal of Materials and Design 35 (2012) 5, 175–83, doi:10.1016/
j.matdes.2011.09.064
O. O. ABEGUNDE et al.: MICROSTRUCTURAL EVOLUTION AND MECHANICAL CHARACTERIZATIONS OF AL-TiC ...
306 Materiali in tehnologije / Materials and technology 51 (2017) 2, 297–306
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