N. GANGIL et al.: SURFACE NANOCOMPOSITE FABRICATION ON AA6063 ALUMINIUM ALLOY ...
77–82
SURFACE NANOCOMPOSITE FABRICATION ON AA6063
ALUMINIUM ALLOY USING FRICTION STIR PROCESSING: AN
INVESTIGATION INTO THE EFFECT OF THE TOOL-SHOULDER
DIAMETER ON THE COMPOSITE MICROSTRUCTURE
IZDELAVA NANOKOMPOZITA NA POVR[INI ALUMINIJEVE
ZLITINE AA6063 Z UPORABO VRTILNO-TRENJSKEGA
PROCESA: RAZISKAVE VPLIVA PREMERA DR@ALA ORODJA NA
MIKROSTRUKTURO KOMPOZITA
Namrata Gangil1, Sachin Maheshwari1, Arshad Noor Siddiquee2
1Netaji Subhas Institute of Technology, Department of Manufacturing Processes and Automation Engineering, New Delhi 110078, India
2Jamia Millia Islamia, Department of Mechanical Engineering, New Delhi 110025, India
namrata.gangil@gmail.com
Prejem rokopisa – received: 2017-10-09; sprejem za objavo – accepted for publication: 2017-11-14
doi:10.17222/mit.2017.172
In this work, surface metal matrix composites (SMMCs) were fabricated on AA6063 base metal through friction stir processing
(FSP). In all the samples for surface composites, grooves of 2 mm × 2 mm were made along the centreline of plates and TiB2
powder (~80 nm) was filled and compacted in these grooves. A pinless tool was employed to initially cover and compact the
grooves filled with TiB2 particles to prevent it from sputtering during FSP. Tools of different shoulder diameters (16, 18, and 20)
mm with anti-clockwise scrolls on the shoulder surface were used for the FSP with constant pin diameter and pin length. The
tool rotational speed of 900 min–1, traversing speed of 40 mm/min and tilt 2°, respectively, kept constant for all the experiments.
Macro, optical micro images and micro hardness tests were used to evaluate the particle distribution. Powder agglomeration was
observed in the retreating side of samples processed with 16 mm and 18 mm shoulder diameter tools. On the other hand,
significant improvement in particle distribution and excellent bonding with the substrate were observed for the sample processed
with 20 mm shoulder diameter tool. The findings of this investigation are important and provide knowledge for better tool
design and effective tool selection to bring out better distribution in a single pass.
Keywords: friction stir processing, aluminium alloy, shoulder diameter, microstructure
V tem prispevku avtorji opisujejo izdelavo in raziskavo izdelanega nanokompozita s kovinsko osnovo (angl.: SMMCs; Surface
Metal Matrics Composites) na povr{ini aluminijeve zlitine vrste AA6063, izdelanega s pomo~jo vrtilno (rotacijsko) trenjskega
procesa (angl.: FSP; Friction Stir Processing). Vsi vzorci povr{inskega kompozita {irine 2 mm in globine 2 mm so bili izdelani
vzdol` sredi{~ne linije kovinske plo{~e. Pri tem so bili med FSP dodajani pribli`no 80 nm delci TiB2 prahu v nastajajo~e brazde.
Za za~etno prekrivanje in kompaktiranje brazd napolnjenih s TiB2 so uporabili orodje brez trna in s tem prepre~ili razpr{evanje
delcev med FSP. Za izdelavo SMMCs so uporabili dr`ala razli~nih premerov (16, 18, in 20) mm z valj~ki, name{~enimi na
povr{ini dr`al, ki so se vrteli v nasprotni smeri urnega kazalca. Pri tem so za FSP uporabili trn s konstantnim premerom in
dol`ino. Pri vseh preizkusih so za izbrani FSP uporabili hitrost vrtenja 900 min–1, vzdol`no hitrost potovanja orodja 40 mm/min,
in nagib 2°. Da bi ugotovili porazdelitev delcev TiB2 v izdelanih SMMCs so izvedli metalografske preiskave in dolo~ili
mikrotrdoto kompozitov. Opazili so aglomeracijo pra{nih delcev na vzorcih izdelanih s 16 mm in 18 mm premerom dr`ala
orodja. Po drugi strani pa so dosegli pomembno izbolj{anje porazdelitve pra{nih delcev in njihovo odli~no vezavo s kovinsko
osnovo pri vzorcih, ki so bili izdelani z 20 mm premerom dr`ala orodja. Ugotovitve te raziskave omogo~ajo bolj{e oblikovanje
in u~inkovito izbiro orodja za doseganje optimalne porazdelitve nanodelcev v kovinski osnovi pri FSP z enim samim prehodom
F.
Klju~ne besede: proces rotacijskega trenja, zlitina na osnovi aluminija, premer dr`ala, mikrostruktura
1 INTRODUCTION
FSP is based on the principles of Friction Stir Weld-
ing (FSW) developed at "The Welding Institute (TWI),
UK" in 1991.1 In FSP a cylindrical shouldered tool with
a profiled probe or pin is rotated and plunged into base
metal (BM) and traversed on the workpiece surface in
the processing direction (Figure 1). The rubbing action
of the tool shoulder generates frictional heat and softens
the material under the shoulder, which also undergoes
severe plastic deformation at high strain rate by the ro-
tating pin (called stirring).2,3 During FSW/FSP, material
is subjected to a combination of metal working pro-
cesses, e.g., friction, extrusion and forging.3–5 FSP is
evolving as a promising surface modification technology
for surface composite fabrication mainly because it is a
solid-state process and a green process by virtue of being
free from use of consumables and evolution of effluent.
One of the major challenges of the process, however, is
the inhomogeneous distribution of reinforcement part-
icles. A large number of research works have been
focused on achieving a homogeneous distribution of
particles, elimination of agglomeration of particles, over-
coming of tunnel-like defects and achieving a wide
Materiali in tehnologije / Materials and technology 52 (2018) 1, 77–82 77
UDK 669.715:621.9:621.9.041 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(1)77(2018)
composite zone by utilizing various strategies such as
applying multiple number of FSP passes, change of tool
rotation between passes, process hybridization such as
electric current assisted FSP, etc.6–10 All these, result in
loss of time and energy and often loss of substrate pro-
perties as well. Multiple passes not only increase the
energy input, production time but every pass also lowers
the material properties, especially in case of heat-treated
materials.
Studies with a specific focus on material flow and
reinforcement particle distribution have been reported in
which the effects of pin profile have been consi-
dered.8,11–12 Few researches have used concave and
scrolled shoulder for surface composite fabrication with-
out investigating specifically the effect of the shoulder
diameter on the response.9–10,13 Tool shoulder and pin
control the heat generation caused by friction during
FSP. Of the two, the shoulder is the main heat generation
source since the contact area between shoulder and BM
is higher than the pin contact area.14 It is pertinent that
the tool shoulder diameter is important to materials
movement as well and hence an important factor in ob-
taining adequate particle distribution. AA6063 is a
material of choice in naval and automobile applications
mainly due to it good formability, specific strength and
resistance to corrosion.15–18 Some researchers have fabri-
cated AA6063/TiB2 composites by using liquid metallur-
gy and FSP process.19–20 However, this study is per-
formed with a specific objective to understand the
distribution of TiB2 particles in the AA6063 alloy for
which the effect of tool shoulder diameter has been
investigated. In this paper tool shoulder diameter was
varied and their effect on particle distribution and
hardness were analysed.
2 MATERIALS AND METHODS
The chemical composition of the 6063-T6 aluminium
alloy used in the investigation is presented in Table 1
and its mechanical properties are presented in Table 2.
The FSP experiments were performed on samples of 170
mm × 50 mm × 4.75 mm dimensions. In all samples for
surface composites, grooves of 2 mm × 2 mm were made
along the centreline of plates and TiB2 powder (~80 nm)
was filled and compacted in these grooves. The scanning
electron micrograph (SEM) and X-ray diffraction (XRD)
pattern of TiB2 reinforcement powder is shown in Fig-
ure 2. A pinless tool was employed to initially cover and
compact the grooves filled with TiB2 particles to prevent
it from sputtering during FSP.
N. GANGIL et al.: SURFACE NANOCOMPOSITE FABRICATION ON AA6063 ALUMINIUM ALLOY ...
78 Materiali in tehnologije / Materials and technology 52 (2018) 1, 77–82
Table 1: Chemical composition of AA6063-T6, in mass fractions (w/%)
Element Al Mg Si Cu Mn Fe Ti Cr Zn Ni
AA6063 98.71 0.499 0.424 0.022 0.034 0.25 0.015 0.005 0.028 0.003
Figure 2: a) SEM micrograph, and b) XRD pattern of titanium diboride (TiB2) nano powder
Figure 1: Schematic diagram of FSP
Table 2: Mechanical properties of as-received AA6063-T6
UTS
(MPa)
Yield strength
(MPa)
Elongation
(%)
Microhardness
(Hv)
220 110 14 72.6
The FSP was carried out on an indigenously
retrofitted FSW machine. FSP tools made of high-carbon
high-chromium (HCHCr) steel as shown in Figure 3
were used in this study. (16, 18 and 20) diameter tools
with anti-clockwise (ACW) scrolled shoulder surface
having cylindrical pin with 6 mm diameter and 2.5 mm
in length were used.
For all the experiments the rotational speed, traverse
rate, tool tilt and tool plunge were fixed at 900 min–1, 40
mm/min, 2° and 2.42 mm respectively. The friction stir
processed plates are shown in Figure 4.
After FSP, microstructural analyses of processed
zone (PZ) were carried out for which specimens were
prepared using a standard metallographic procedure. The
metallographic samples were subsequently etched with
extended flick reagent (15 ml hydrochloric acid, 10 ml
hydrofluoric acid and 90 ml distilled water) for 3 min.
Macroscopic images were taken using a Stereozoom
microscope (Focus, Japan). Microstructural observations
were carried out by employing OM (QS Metrology,
India). The micro-hardness of the samples was measured
under a load of 0.1 N and for a dwell time of 15 seconds
using micro-hardness tester (Mitutoyo, Japan).
3 RESULTS AND DISCUSSION
Macro and micro-structural, bead geometry, analysis
along with indentation test were carried out to investigate
the influence of the tool shoulder diameter on the
fabricated surface composites.
3.1 Macro and micro-structure
Macrograph of cross-section showing reinforced
zone (RZ) of all the samples is shown in Figure 5 and
the microstructure of its various regions is shown in
Figures 6 to 8. It is also evident from the micrographs
that the advancing side (AS) interface in all samples
generally possesses better distribution and good bonding
of the reinforcement with the substrate material. How-
ever, the accumulation of TiB2 particles was witnessed
on the retreating side (RS) interface of the samples
(especially in those which were processed with 16 mm
and 18 mm diameter shoulder tool). Whereas no accu-
mulation was found in the sample that was processed
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Materiali in tehnologije / Materials and technology 52 (2018) 1, 77–82 79
Figure 3: FSP tools having shoulder diameter: a) 16 mm, b) 18 mm,
and c) 20 mm
Figure 6: Microstructure of various regions of sample processed with
16 mm tool shoulder diameter showing: a) AS interface, b) agglome-
rated region in RS side, c) bottom interface and d) SZ
Figure 4: Friction stir processed plates with tool shoulder diameter of:
a) 16 mm, b) 18 mm, and c) 20 mm
Figure 5: Macrograph of cross-section of PZ processed with tool
shoulder diameter of: 16 mm, b) 18 mm and c) 20 mm
with a 20 mm diameter shoulder tool. Significant impro-
vements in terms of particle distribution were achieved
in samples processed with a 20 mm diameter shoulder.
Bands of reinforced and unreinforced regions were found
in samples processed with 16 mm and 18 mm tools, as
shown in Figures 6a and 7c. No such bands were
observed in samples processed with 20 mm tool diameter
and the reinforced zone in these samples exhibited
homogeneous particle distribution as well as good bond-
ing with the substrate material (Figure 8a to 8d). The PZ
dimensions are given in Table 3. The results indicated
that the depth and area of RZ increase with the increase
in the shoulder diameter in all the samples.
The tool shoulder has a direct relation with the heat
generation during processing due to the higher frictional
contact area. For a larger shoulder diameter the heat
generation is higher because of the larger contact area
and vice versa.21 Small shoulder diameter produces in-
adequate frictional heat and consequently provides
insufficient plasticized base material flow under the
shoulder. Also, the shoulder is responsible for nearly
one-third of the material transport in the upper portion of
processing zone.22 Kumar and Kailas demonstrated that
the combined effect produced by shoulder and pin driven
material flow is responsible for the resultant particle
distribution and the microstructure of SZ. The shoulder
facilitates bulk material transfer, while the tool pin is
responsible for layer-by-layer material flow.23 The addi-
tional features on the shoulder surface such as scrolls
also play an exceptionally important role by providing
additional frictional treatment and better material
flow.24–25 In the present investigation, the combined
effect of shoulder-driven and pin-driven flow has been
observed in all samples. It can be inferred that the tool
shoulder diameter of 16 and 18 mm could not generate
sufficient frictional heat and material flow, which results
in the agglomeration of reinforcement particles in RS of
SZ due to higher flow stresses and poor material flow.
However, 20 mm shoulder diameter tool was able to
generate sufficient frictional heat, lower flow stresses,
proper consolidation of material behind the tool and
better material flow, therefore, it results in a homo-
geneous particle distribution in the SZ, larger reinforced
zone depth and area without any microscopic defect.
3.2 Microhardness
The microhardness profile was generated along the
width of PZ at 1 mm equidistant point and the same is
shown in Figure 9. The microhardness profile was traced
1 mm below the top surface.
The average micro-hardness of PZ of samples pro-
cessed with 16, 18, and 20 mm diameter shoulder along
the horizontal direction of the cross-section were found
to be 107.65, 111.33, and 113.27 HV, respectively.
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Figure 9: Hardness profiles
Figure 8: Microstructure of various regions of sample processed with
20 mm tool shoulder diameter showing: a) AS interface, b) RS inter-
face, c) bottom interface and d) SZ
Table 3: Processed zone dimensions
Tool shoulder
diameter
(mm)
Reinforced
zone depth
(mm)
Particle
agglomerated
area (mm2)
Reinforced
zone area
(mm2)
16 2.43 1.03 19.37
18 2.67 1.35 21.74
20 2.71 – 26.62
Figure 7: Microstructure of various regions of sample processed with
18 mm tool shoulder diameter showing: a) AS interface, b) agglome-
rated region in RS side, c) bottom interface and d) SZ
Narimani et al.20 also fabricated AA6063/TiB2
composite and reported an increase in hardness to a
significant value due to higher inherent hardness (2500
kg/mm2) of the reinforcement particles. Fine-grained
matrix and the Orowan strengthening are the two major
contributors to the improvement of the micro-hardness of
fabricated composites.19,20 Surface composites (SCs)
fabricated in this study exhibited significantly higher
hardness than the base metal due to the presence of hard
TiB2 particles. The average micro-hardness of the sample
processed with 20 mm diameter tool is highest among all
the samples. The highest and more uniform distribution
of hardness profile of this sample was attributed to
homogeneous distribution of TiB2 particles in processed
zone.
4 CONCLUSION
The AA6063/TiB2 composites were fabricated
successfully using FSP. The effect of tool shoulder
diameter on the microstructure and micro-hardness of
the metal matrix composite was studied. The obtained
results can be summarized as follows:
• The hardness of processed zone is increased in all the
samples due to inherent higher hardness of the
reinforcement.
• The reinforced region depth and area are increased
with an increase in the diameter of tool shoulder.
• No agglomeration of particles was observed in the
sample processed with a 20 mm diameter shoulder
tool.
• The composite processed with 20 mm diameter tool
shoulder exhibited excellent particle distribution,
superior micro-hardness and good bonding with the
substrate material.
Acknowledgements
The authors wish to thank the University Grants
Commission (UGC) for its financial assistance (vide
sanction order No. F.3-40/2012(SAP-II)) under its SAP
(DRS-I) sanctioned to Department of Mechanical
Engineering, Jamia Millia Islamia, New Delhi for the
project entitled "Friction Stir Welding, Ultrasonically
Assisted Machining".
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