2022 7th International Conference on Electric Vehicular Technology (ICEVT)

September 14-16, Bali, Indonesia

Effect of Process Parameters on AZ91/SiC Surface

Composites for Lightweight E-vehicles
2022 7th International Conference on Electric Vehicular Technology (ICEVT) | 978-1-6654-0942-1/22/$31.00 ©2022 IEEE | DOI: 10.1109/ICEVT55516.2022.9924699

Roshan Vijay Marode Srinivasa Rao Pedapati Tamiru Alemu Lemma

Mechanical Engineering Mechanical Engineering Mechanical Engineering
Universiti Teknologi PETRONAS Universiti Teknologi PETRONAS Universiti Teknologi PETRONAS
Bandar Seri Iskandar, Perak, Malaysia Bandar Seri Iskandar, Perak, Malaysia Bandar Seri Iskandar, Perak, Malaysia
roshan_20001046@utp.edu.my srinivasa.pedapati@utp.edu.my tamiru.lemma@utp.edu.my

Abstract—High specific strength, superior damping introduced magnesium to the automobile industry for the first
capability, and excellent specific stiffness make magnesium and time with their Beetle model, using 22 kg of metal in each
its alloys a great alternative for usage in the automotive and vehicle; in 1928, Porsche created the first magnesium engine
transportation industries, particularly for electric cars. The [3]. AZ91 contains a significant fraction of the
AZ91 is a desirable option among magnesium alloys for intermetallic compound of the secondary β phase and often
different applications due to its superior casting qualities.; occurs around the grain boundary of the α-Mg. The
however, the main obstacles to their widespread usage are their hexagonal close-packed (HCP) crystal lattice with low
poor surface qualities and lack of corrosion resistance. Thus, the symmetry has a strong basal texture, which limits its strength,
current study aims to enhance surface strength by fabricating decreases fatigue resistance, and creep resistance due to the
AZ91/SiC surface composites. Friction stir processing (FSP) is limited number of active slip systems [10]. They also have
utilized to refine the microstructures and fabricate the surface
the lowest standard in the EMF family and have low
composites without introducing any intrinsic flaws. Tool
Rotational Speed (TRS), Tool Traverse Speed (TTS), Tool Pin
ionization energy, which makes them very corrosive [11],
length (PL), and Plunge Depth (PD) were chosen as parameters [12]. Due to the thin surface coating formed by magnesium
to investigate their effect on processed materials. Range of 500- and its alloy, the outer layer is not thick and is thus more
1500rpm and 20-60 mm/min of TRS and TTS were found to be susceptible to corrosion. This is seen by the low Pilling-
effective respectively. It was discovered that PL of 3 mm with Bedworth ratio (0.81) of the material. Further, the high
3.3 mm PD was successful in producing surface composites with corrosion rate deters mechanical strength and hardness.
no flaws. The microstructure of the composite band lowers the These surface material shortcomings limit their broad range
granule size from 70 μm to 10 μm. Subsequently, the of structural applications when weight is the key design issue
strengthening mechanism was attributed to the enhancement of and may be remedied by grain refinement to adjust the
hardness and achieved 15% higher than that of BM. distribution and shape of the β-phase by adding reinforcing
materials [13], [14]. Some severe plastic deformation (SPD)
Keywords— electric vehicles, friction stir processing, techniques have been studied to modify the surface
magnesium alloys, metal matrix composites, microstructure, characteristics with microstructure refinement; however, the
surface properties. post-processed materials are unfinished, facing problems
with segmentation and cracking. To overcome these
I. INTRODUCTION disadvantages, the researchers looked for other novel
techniques and developed friction stir processing (FSP) using
Recent environmental protection regulations emphasize
friction stir welding (FSW) as a foundation. FSP is a solid-
the requirement for vehicular weight reduction to improve
state surface engineering technology widely used to modify
fuel economy and minimize the effect of greenhouse
microstructures without inherent defects. It provides more
emissions from the automotive industry [1]–[3]. Researchers
severe plastic deformation and greater strain rates than other
have called attention to the usage of light structural alloys as
conventional procedures, making it an effective grain
a technique for increasing fuel economy and lowering
refining technique. Recently, it has become an adaptable
greenhouse gas emissions. Magnesium, 4.5 times lighter than
practice for producing surface composites. Morisada et al.
steel, 1.7 times lighter than aluminium, and just slightly
[15] were the one to inaugurate in creation of a AZ31 Mg
heavier than carbon fibre, is now the lightest available
alloy-multi walled carbon nanotubes (MWCNTs) composite
structural metal. As a result, applications include switching
by FSP using the groove filling approach. They examined the
out steel and aluminium in the aerospace and automotive microstructure and mechanical characteristics. By using
industries and using polymers in the computer and electrical
sandwich techniques, Mertens et al. [16] introduced the
industries [4]. Since their high specific strength will reduce carbon into the pure magnesium matrix and showed how
an automobile’s bulk and boost its power-to-weight ratio, the
important the carbon is for grain refining. By grooving, Lee
automotive industry uses them more often [5]. According to
et al. [17] implanted SiO2 in AZ61 with altering the number
estimates, a typical new automobile emits 156g of CO2 per of passes and vol percent. While the mechanical parameters
kilometer; however, magnesium technology could lower this
in terms of yield and ultimate strength were in the range of
to around 70g [6]–[8]. Due to its significant castability, rapid 225 MPa and 251 MPa, respectively, the average grain size
heat dissipation, and high strength-to-weight ratio, AZ91 is
of 0.5-2 μm was produced with the base doubled the base
the most preferred magnesium alloy in the series. It is a good material’s hardness.
choice for use in automobile construction [9]. Companies that
produce cars have significantly benefited from research and Nevertheless, due to its weak formability and ductility,
development on magnesium and its alloys. Volkswagen the FSP of AZ91 is difficult.; little research on the synthesis

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 of mono and hybrid surface composites has been recorded. To generate a finer grain structure in the processing zone,
FSP with subsequent aging was applied by Feng and Ma[18] it was advised to keep the shoulder diameter to pin diameter
and obtained a substantial rise in the tensile properties (D/d) as an aspect ratio larger than 3 [22]. Before clamping,
compared to the untreated material due to the dissolution of a the BM for processing must be degreased with acetone to
network like β-phase and grain refinement. Hemendra et al. remove any foreign materials. The plates were measured and
[19] incorporated the B4C in AZ91 and exhibited superior drilled with 23 linear blind holes with an interspacing of 4 mm
tribological and mechanical properties. Yousefpour [20] packed with SiC as reinforcing particles to accommodate the
recently fabricated the AZ91 surface hybrid composites of 12 vol.%. Drilled holes were 2 mm in diameter and 4 mm in
hydroxyapatite and silver as reinforcements for improving
depth. To lock and avoid the scattering of the reinforcement
energy absorption capability and mechanical properties for
from the filled holes, a pin-less tool with a shoulder diameter
biomedical applications.
of 20 mm was used, termed the capping process. Constant
The influence of several FSP process parameters, machine parameters used for the capping process are depicted
including tool rotational speed, traverse speed, plunge depth, in Table II.
and tool pin length, were examined for producing a processed
material free of defects. AZ91/SiC surface composite was TABLE II. CAPPING PROCESS PARAMETERS
considered to examine the impact of the process parameter on
Parameters Opted values
mechanical and microstructural properties. A detailed Tool Rotational Speed (ω) 1000 rpm
experimental investigation was conducted to determine the Tool Traverse (v) and Plunging Speed (vp) 40 mm/min
best range of various process variables for fabricating defect- Tool Tilt Angle (TTA) 0ᵒ
free surface composites with enhanced mechanical Tool Plunge Depth (PD) 0.2mm
properties. Axial Force (Pa) 4 kN

II. MATERIALS AND METHOD Subsequently, the mono composite was processed using a
pinned tool with a constant load of 5 kN and 40 mm/min
The AZ91 magnesium alloy plates as a base material (BM) plunging speed with a single pass. In contrast, the varying
of specification 150 mm x 100 mm x 6.35 mm (from Samnai process parameters are tabulated in Table III. The complete
Energy and Engineering SDN. BHD., Malaysia) were used for friction stir processing with the hole method as a
this study. The average grain size of the BM was about 70 μm reinforcement deposition technique is depicted in Fig. 2.
with a microhardness value of 62 HV. The chemical
composition of AZ91 is given in Table I. The Silicon Carbide
(SiC) ceramic powder supplied by NovaScientific Malaysia TABLE III. PROCESS PARAMETERS USED DURING EXPERIMENTS
was used as a reinforcing material to fabricate the AZ91/SiC Parameters Opted values
surface composite. The SiC particulates used were of particle Tool Rotational Speed (ω) 400, 500, 1000, 1500, and 1600 rpm.
size 100-200nm, >99% purity with 3C beta (β) cubic crystal Tool Traverse Speed (v) 10, 20, 40, 60, and 70 mm/min
structure polytype. This type of SiC particle owes more Tool Pin Length (PL) 4.5, 4, 3.5, and 3 mm.
surface area than alpha hexagonal polytype particles. An Tool Plunge Depth (PD) 4.84, 4.34, 3.85, and 3.3 mm
Tool Tilt Angle (TTA) 0ᵒ and 2.5ᵒ
automatically operated CNC FSW machine (From Beijing
FSW Technology Co., Ltd., China) was used to process the
samples. A cylindrical tool of H13 grade steel with a 20 mm
shoulder diameter, pin diameter of 6 mm, and pin length of
4.5, 4, 3.5, and 3 mm was utilized. The tool was hardened by
heat treatment as per Guanghua et al. [21] to achieve the
hardness value of 55HRC, as shown in Fig. 1.

Fig. 2. Setup of FSP with hole method

Samples measuring 2 cm x 2 cm were wire cut using EDM

for microstructural examination, and the surface was polished
using silicon carbide sheets with grit values ranging from 200
to 1200. The specimen was rotated 90 degrees in each grit
Fig. 1 Heat Treated H13 tool steel increment before being polished with a diamond slurry. After
polishing, specimens were etched with a picral reagent as per
ASTM E407-07 (10 ml acetic acid, 6 g picric acid, and 100
TABLE I. CHEMICAL COMPOSITION OF AZ91
mL ethanol). SEM and optical microscopy (OM) were used to
Chemical composition (wt.%) record the microstructural characteristics (SEM). Mechanical
Al Zn Mn Si Fe Cu Ni Mg testing for surface hardness was done as per ASTM E384-17
8.3-9.7 0.35-1 0.15 0.10 0.005 0.030 0.002 Bal.
by using Vicker’s hardness tester (LM 247AT- Leco, USA)
with a load of 200 gf for the dwell time of 10 sec. The

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 indentation was measured using the square base pyramid's two the composite band was observed, and thus TTS was
diagonals (D1 and D2). Few measurements were taken on the restricted to 60 mm/min.
SZ, and the average reading was utilized to analyze hardness
further.
III. RESULTS AND DISCUSSIONS

A. Effect of Process Parameters

Processed material characteristics and process efficacy are
determined by their microstructure, which can typically be
modified by altering process parameters. As a result, process
parameters must be selected to provide the optimal amount of
heat to achieve the best possible microstructure with no
inherent flaws. FSP necessitates considerable effort to Fig. 4. Low and High-end processing windows for TRS and TTS
regulate the complicated process parameters relating to
material flow, heat generation, tool variables and design, and Fig. 5 shows the friction stir processed bands for AZ91/SiC
mechanical forces. Various process elements are analyzed in composites without any macro-defects. Although some flash
the following sections. was observed for higher TRS of 1500 rpm, there is a
successful establishment of reinforcement with the uniform
1) Tool Rotational Speed and Traverse Speed composite band. TRS of 500 rpm must be considered the
minimum speed required for successfully consolidating
The permitted ranges of traversal speeds, which correspond reinforcements inside the AZ91. Due to inadequate heat and
to the revolving speeds at which the materials are processed the tool's reduced stirring action, less than 500 rpm causes
without acquiring any abnormalities, are shown. in Fig. 3. The more surface cracking and failure composite bands. TTS of
maximal traversing speed necessary to produce a specimen 20 mm/min is considered to be the minimum speed to avoid
free of flaws increases as the rotation speed rises. Low rotating over-stirring due to the TRS; TTS reflects the rate of heating,
speeds do not provide enough heat to sufficiently soften the so lower than 20 mm/min causes more intense heat, and
material for use at high traversal rates. However, increasing higher rotating action of the tool leads to intense flash/
the traverse and rotational speed beyond the optimal range scattering of reinforcements and weakening the AZ91 base
causes the tool to oversteer, and it may lead to the instability plate. Thus, the allowable traverse speeds range correlating
of the workpiece with significant defects. A drop in TRS and with the rotational speeds see Fig. 3. FSP process parameters
a rise in TTS do not produce enough heat to allow the process window for unreinforced material would not be the same for
to become plastic. As a result, both the rotating and traversing the reinforced materials as the reinforcement increases the
speeds should be chosen so that enough heat is produced for requirement of torque and other machine parameters.
plasticity but not so much that the material gets softened to
melt.

Fig. 5. FSPed composite tracks for 500 rpm-20 mm/min; 1000 rpm-40
mm/min and 1500 rpm-60mm/min without any macro-defects

Fig. 3. Permissible range of TRS and TTS 2) Tool Pin length and Plunge Depth

Fig. 4 depicts the low- and high-end processing window for The pin length of the tool and plunge depth are both
AZ91/SiC composite. It can be visualized that insufficient interrelated to each other in deciding the frictional heat
heat was generated for 400 rpm and 10 mm/min, which generated during the process. Because of the result of the
restricts thorough material mixing between the AZ91 and SiC increased friction, they immediately affect the heat intake
particles, owing to which the holes made for reinforcement into the workpiece. As a result of this relation, it can also alter
deposition got uncovered, and surface cracking was observed. the properties of processed materials. They are also attributed
In contrast to the lower end, for higher processing values of to deciding the depth of composite to be processed on the
1600 rpm and 70 mm/min, BM becomes unstable, causing the BM. Magnesium alloys are brittle and very temperature-
tool to wobble and stick inside the material due to over- sensitive due to their hexagonal closed pack (HCP) structure.
stirring; thus, a dent in the AZ91 was observed. Moreover, A brittle fracture happens when penetration depths are too
with 70 mm/min, clamping on the machine could not shallow because there isn't enough heat generated by the
withstand high jerks and vibrations; hence, misalignment in shoulder-workpiece interface, to properly soften the material.

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 However, when the tool penetration depth was excessively
increased, the workpiece stuck to the tool and formed flaws.
Therefore, the optimum PL concerning the PD must be
obtained. All trials of PL and PD were made at 1000 rpm, and
40 mm/min was considered one of the optimal parameters
obtained from the TRS and TTS section. In this article, PD is
considered from the top surface of BM to the rear surface of
the tool pin, as highlighted in Fig. 9. Fig. 6 demonstrates that
for 4.84 mm PD with 4.5 mm pin length, the specimen was
buckled, bent, and cracked from the top and bottom end due
to the superposition effect of PL and PD. Even for the lower
value of 4 mm, PL with 4.34 mm, the specimen damage from
the bottom and heavy flash was detected to the absence of
Fig. 7. Effect of 3.5 mm and 4 mm PL with 3.85 mm and 4.34 mm PD
sliding condition as shown in Fig. 7. In other words, the respectively on composite bands
previously sliding friction now acts as a sticking friction. In
the sliding friction mode, the real contact area between the
workpiece and the tool shoulder is less than the potential
contact area. Since the amount of friction force depends on
the real contact area, increasing the normal force between the
tool and the workpiece increases both the actual contact area
and the friction force [23]. When the real connection region
becomes close to the perceived contact area, there is an
increase in pressure between the tool and the base material.
In this situation, sticking friction takes control over sliding
friction. The shearing force of the softer material, which is
independent of the normal force, then equalizes the friction
force. With excessively high tool penetration depths, sticking
between the tool and the workpiece occurs due to increased
pressure on the contact surface. Fig. 8. Effect of 3 mm PL with 3.3 and 3.85 mm PD

From Fig. 8, when the PD raised from 3.3 mm to 3.85 mm; 3) Tool Tilt Angle
it could be seen that for PL 3 mm and PD 3.3 mm, the
processed AZ91/SiC composite band shows no cracks and The tool is tilted to the FSP machine’s axis in friction stir
any macro defects. However, heavy flash was noticed with processing. TTA between 1 to 3ᵒ is regarded as effective and
the same pin length, increasing the PD to 3.85 mm and optimal, according to research, and hence ensures excellent
constantly putting it constantly for the rest of processing material processing [24]. Fig. 9 depicts the tilt angle assigned
without any cracks and defects. As discussed earlier, this may to the tool during FSP.
be due to the dominating nature of sticking conditions over
the sliding. To summarize, the base plate was observed with
macro defects, buckled, bent, cracked composite bands, and
tunnel defects in all cases. Therefore, 3 mm pin height/length
with 3.3 mm PD was observed to be free from defects. Thus,
for the fabrication of AZ91/SiC composite, 3 mm and 3.3 mm
PL and PD must be opted for further investigations.

Fig. 9. TTA and Plunge Depth

TTA for the current study was investigated for 0ᵒ and 2.5ᵒ
with constant 3.3 mm PD, 1000 rpm TRS and 40 mm/min as
TTS. It was found that by reducing the angle from 2.5ᵒ to 0ᵒ,
the tool plunge depth decreases. Plastic deformation and edge
friction contribute to the heat created during the FSP.
Temperature and heat generation substantially impact the
microstructure of processed samples and the product’s
mechanical qualities. High defects such as tunnels,
microvoids, and fractures were produced at 0ᵒ TTA at 1000
rpm and 40 mm/min because the heat was detected purely by
the shoulder workpiece interface without material mixing.
Fig. 6. Effect of 4.5 mm PL and 4.84 PD on AZ91/SiC
Additionally, changes in PD and the potential for magnesium
to adhere to the shoulder at lower tilt angles fosters defect
development more severely.

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 Material mixing at 2.5ᵒ contributed effectively to the The linear intercept method was employed to measure the
process, which was attributed to eliminating these defects. grain size of the base material and the treated magnesium
Sound processing was observed, as shown in Fig. 10. It was alloy using ImageJ analysis software (
also intended to allow the volume of plasticized material to Fig. 12). In the treated zone, the grain size is decreased
be housed underneath the tool, allowing the back element of from 70µm to 10µm, as can be seen in Fig. 13 and Fig. 14.
the tool shoulder to push out this material. It lowers the tool Moreover, the transition phase from the SZ to TMAZ shows
wear and defects due to excellent material mixing and heat the elongated grains. However, there is little difference
generation. It also assists in reducing the force of reaction between the HAZ and BM due to the less deformation
generated by friction from the tool’s engagement with the subjected as compared to the SZ and TMAZ. SiC material
workpiece. Moreover, during the surface composite analysis, flow is seen in the macrographs and is more clearly aligned
the tool tilt angle benefits the uniformity. It facilitates the with the advancing side (AS) than the retreating side (RS).
thorough dispersion of reinforcement in the base material, Material shearing is shifted more on the AS due to the high
making it easier to move about during the stirring process. plastic deformation and high output velocities with advancing
The findings obtained were in good agreement with movement. It can be explained in AS, where the materials are
Veerendra et al [25] investigations. subjected to more displacement than in RS, as the
components on both sides of the tool have different speeds
and feed rates. The output speeds are estimated by adding
TRS and TTS if they are in the same direction. However, if
their directions are reversed or opposite to each other, the
output of speeds can be computed by subtracting the velocity
component. This knowledge shows that the material particles
on the RS are subjected to two opposing flows, which causes
higher friction. SiC particles served as strong, load-bearing
components. Furthermore, nanoparticles were equally
Fig. 10. Processed Band for 0ᵒ and 2.5ᵒ TTA distributed on the surface bed of the base material, which
improved the microstructure and minimized the grain size
brought on by the pinning action of reinforcement
B. Microstructure and Microhardness
particles. SiC's pinning action slowed down grain
The base material and treated samples will be polished, development, which decreased the boundaries of grain and
cleaned, and etched in order to investigate how FSP with reinforced the material in agreement with the Hall-Petch
various process parameters affects microstructure. The relation [26]. The formability and super-plastic behavior,
microstructure morphology is examined using optical
including the hardness and strength, are all improved by the
microscopy and field emission scanning electron microscopy
finely tuned and equiaxed grain structure. The friction stir
(FESEM). Three unique zones have been found based on
processing of the tool's rapid mixing and stirring effectively
examination of grains, precipitates, temperature, strain, and
strain rate: a stirred or processed zone (SZ/NZ), a heat- distributed the reinforcing particles in SZ. Over the top bed
affected zone (HAZ), and a thermo-mechanically impacted of the base material, SiC particles are evenly distributed and
zone (TMAZ). Fig. 11. depicts the different zones in the
friction stir processed (FSPed) material. Due to material flow
and high heat generation during processing, which fosters
dynamic recrystallization, high plastic deformation occurs in
the nugget zone. TMAZ experiences less recrystallization
than the nugget zone, which results in less plastic deformation
in this area. HAZ, on the other hand, does not suffer plastic
deformation and only experiences heat-related deformation.
The nugget zone has a finer grain size than the TMAZ and
HAZ. The change in microstructure modification
significantly impacts the mechanical properties of post-FSP
materials. The BM had grains that were, on average, around
70µm in size.

dispersed in all directions.

Fig. 12. OM image of the BM at 50X magnification

Fig. 11. Macrograph image at cross-section for 1000 rpm and 40 mm/min

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 Fig. 13. Macro and Micro-images SEM at 1000 rpm TRS, 40 mm/min TTS , 2.5ᵒ TTA and 3.3 mm PD with 3 mm PL.

C. Strengthening Mechanisms associated with FSPeds

AZ91
The matrix's solid solution, grain boundary strengthening,
and the second phase strengthening provided by the β-phase
are some of the processes that regulate the strength of the
AZ91. Greater Hall-Petch effect as a result of grain refining;
the strengthening of the grain boundary was enhanced by the
grain size reduction from 62 μm to 10 μm. Due to the
significant Hall-Petch coefficient and high grain boundary
pining factor in magnesium alloys, grain refinement may
significantly increase the alloy's strength.
Fig. 14 SEM image of SZ

Furthermore, the FSP tool plunge throughout the operation

reached the desired 3.3 mm out of the 6.35 mm thickness,
which helps split the material into the processed and
unprocessed layers. Thus, the treated material’s top layer
consistently exhibits high hardness, but the SZ, the TMAZ,
and HAZ vary cross-sectionally. As the secondary phase is
distributed we ll in BM, this is attributed to the increase in the
surface microhardness up to a maximum of 71 Hv compared
to the base material (62Hv). The increase in microhardness
results from the strengthening coupled with particulate- Fig. 16. Optical microscope showing α and β-Mg phases
reinforced metal matrix nano-composite. Examples include
how significant grain size refinement affects the Hall-Patch Owing to the inclusion of the coarse β-phase in the matrix,
connection and how the strengthening of the Orowan the plastic deformation restriction plays a major role in
mechanism results from the dispersion of reinforcing determining how as-cast solid AZ91 may be. Although the β-
particles [27]. Higher microhardness values were found in the phase may be refined to an ultra-fine scale, doing so can cause
SZ of several specimens. the particle-dislocation interaction (in this instance, the
Orowan mechanism). By distributing ultra-fine β-phase
particles throughout the matrix, the Orowan strengthening
mechanism becomes dominant. Increased Al and Zn levels in
the matrix are brought about by the dissolution of the
intermetallic complex β-phase Mg17Al12 (β-phase) and the
homogeneity action of the FSP. This may increase the FSP-
treated alloy's strength [28].
IV. CONCLUSIONS
FSP was an effective technique for producing surface
composites and refining the microstructure with enhanced
mechanical properties. Using FSP and the permitted range of
Fig. 15. Effect of TRS and TTS on Microhardness process parameters, the fabrication of AZ91/SiC was

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