I. DINAHARAN, E. T. AKINLABI: INFLUENCE OF THE TOOL ROTATIONAL SPEED ON THE MICROSTRUCTURE ...
791–796
INFLUENCE OF THE TOOL ROTATIONAL SPEED ON THE
MICROSTRUCTURE AND JOINT STRENGTH OF FRICTION-STIR
SPOT-WELDED PURE COPPER
VPLIV HITROSTI VRTENJA ORODJA NA MIKROSTRUKTURO IN
TRDNOST TORNO VRTILNO TO^KASTO ZVARJENEGA SPOJA
^ISTEGA BAKRA
Isaac Dinaharan, Esther T. Akinlabi
University of Johannesburg, Department of Mechanical Engineering Science, Auckland Park, Kingsway Campus,
Johannesburg 2006, South Africa
dinaweld2009@gmail.com
Prejem rokopisa – received: 2015-09-21; sprejem za objavo – accepted for publication: 2015-10-05
doi:10.17222/mit.2015.301
Copper is very difficult to be spot welded with conventional fusion-welding techniques due to a high thermal diffusivity. Fric-
tion-stir spot welding (FSSW) is a novel solid-state welding process, suitable and effective for spot welding copper.
Commercially pure copper sheets of 3 mm thickness were spot welded using an industrial friction-stir welding machine. The
spot welds were made by varying the tool rotational speed at three levels. The spot welds were characterized using light
microscopy. The shear-fracture load was evaluated using a computerized tensile-testing machine. The results revealed that the
tool rotational speed remarkably influenced the microstructure, the shear-fracture load and the mode of fracture.
Keywords: copper, friction-stir spot welding, microstructure, shear load
Zaradi velike toplotne prevodnosti se baker zelo te`ko to~kasto vari pri obi~ajnih postopkih varjenja z zlivanjem. Torno vrtilno
to~kasto varjenje (FSSW) je nov na~in varjenja v trdnem stanju, ki je primerno in primerljivo s torno vrtilnim to~kastim
varjenjem bakra. Bakrene plo~evine, debeline 3 mm, iz ~istega komercialno dostopnega bakra, so bile to~kasto zvarjene s torno
vrtilnim to~kastim varjenjem, z uporabo industrijske naprave za tovrstno varjenje. To~kasti zvari so bili izdelani pri treh
hitrostih vrtenja orodja. Karakterizirani so bili z uporabo svetlobne mikroskopije. Stri`na trdnost je bila ocenjena z uporabo
ra~unalni{ko vodenega nateznega stroja. Rezultati so pokazali, da hitrost vrtenja orodja mo~no vpliva na mikrostrukturo, stri`no
trdnost in na~in preloma.
Klju~ne besede: baker, torno vrtilno to~kasto varjenje, mikrostruktura, stri`na obremenitev
1 INTRODUCTION
Pure copper is extensively used in the optical and
electronic industries owing to its excellent properties
such as good ductility, high electrical, thermal conduc-
tivity and good corrosion resistance. The welding of
copper is often encountered in the electrical, nuclear and
automobile industries.1,2 Spot welding of pure copper is
generally hard to do with conventional fusion welding
because of the high thermal diffusivity, which is about 10
to 100 times higher than in many steels and nickel alloys.
The heat input required is much higher than in almost
any other material. Further, pure copper is susceptible to
solidification cracking and blowhole formation.3,4 Fric-
tion-stir spot welding (FSSW) is a novel solid-state
welding technique, promising for spot welding copper
without the problems associated with fusion-welding
techniques.5
FSSW is a derivative process based on friction-stir
welding (FSW) which was developed at The Welding
Institute in 1991. There are several differences between
FSSW and FSW. One distinct difference is that there is
no translation of tool during FSSW. FSW is commonly
employed to join metallic plates in a butt configuration
along the line of contact. On the other hand, FSSW is
performed on thinner plates kept in a lap configuration.
A rotating, non-consumable, cylindrical-shouldered tool
with a pin is plunged, at a predetermined feed rate, into
the overlapping plates to a depth slightly shorter than the
total thickness of both plates. Frictional heat is generated
between the plate material and the rotating tool, which
plasticizes the material. The rotating action of the pin
induces the material flow in both the circumferential and
axial directions. The axial force applied along the tool
axis forges the plasticized material and forms an annular,
solid-state bond around the pin. At this moment, the
rotating tool is retracted, leaving the exit hole behind.
The major process parameters, which influence the joint
strength, are the tool geometry, the tool rotational speed,
the tool penetration depth and the dwell time. FSSW
exhibits key advantages such as excellent mechanical
properties, a low distortion, ease of handling, low cost,
and clean working environment.6–8
The FSSW technique has been successfully used to
spot weld aluminum9, magnesium10, steel11 and plastics.12
Both similar and dissimilar spot welds were reported in
Materiali in tehnologije / Materials and technology 50 (2016) 5, 791–796 791
UDK 621.663:67.017:621.791.05:669.3 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(5)791(2016)
the literature13,14. FSSW of copper and its alloys is least
explored. Z. Barlas15 spot welded pure copper and brass
sheets using FSSW and analyzed the influences of the
tool rotational speed, dwell period and material location
on the microstructure and the tensile shear-fracture load
(TSFL). He found that the welding parameters had an
important influence on the TSFL and the failure mode.
The TSFL increased with an increase in the tool rota-
tional speed and dwell time. R. Heideman16 spot-welded
AA6061 and pure-copper sheets and studied the effects
of the tool pin length, the shoulder plunge depth, the
welding time and the tool rotational speed on the TSFL.
Literature on FSSW of pure copper is scantily. There-
fore, the present work focuses on spot welding copper
sheets of 3 mm using FSSW and analyzes the influence
of the tool rotational speed on the TSFL.
2 MATERIALS AND METHODS
Commercially available pure copper sheets of 3 mm
were used in this study. The optical photomicrograph of
the as-received copper sheet is shown in Figure 1. Lap
joint configuration was used to fabricate the spot welds
where the rolling direction of the material was kept
parallel to the loading directions and the joint was
initially obtained by securing the sheets in position using
mechanical clamps. A non-consumable tool made of
high-carbon steel was used to fabricate the joints. The
tool had a shoulder of 18 mm and a conical profile of a
diameter varying from 5 mm to 3 mm along the length of
pin. The pin length was 5.7 mm. An industrial-purpose
FSW machine (I-STIR), depicted in Figure 2, was used
for FSSW. The FSSW procedure and the welding cycle
are depicted in Figure 3. The tool rotational speed was
varied at three levels of 1200 min–1, 1600 min–1 and 2000
min–1. Other parameters were kept constant. The dwell
period and axial force were 3 s and 10 kN, respectively.
Spot welds were made on two sheets clamped in the lap
configuration. Sufficient cooling was allowed between
successive spot welds. The spot-welded copper sheet is
shown in Figure 4. Six spot welds were made for each
set of the process parameters. Specimens were machined
from the welded sheets and polished semi-automatically
in a polishing machine (Struers LaboPol 25). The
mirror-polished specimens were etched with a color
etchant containing 20 g of chromic acid, 2 g of sodium
I. DINAHARAN, E. T. AKINLABI: INFLUENCE OF THE TOOL ROTATIONAL SPEED ON THE MICROSTRUCTURE ...
792 Materiali in tehnologije / Materials and technology 50 (2016) 5, 791–796
Figure 3: Schematic illustration of the friction-stir spot-welding
process
Slika 3: Shematski prikaz postopka torno vrtilnega to~kastega varjenja
Figure 1: Light micrograph of pure copper
Slika 1: Svetlobni posnetek mikrostrukture ~istega bakra
Figure 2: I-STIR friction-stir welding machine
Slika 2: I-STIR naprava za torno vrtilno varjenje
Figure 4: Friction-stir spot-welded sheet
Slika 4: Torno vrtilno to~kasto zvarjena plo~evina
sulfate and 1.7 mL of HCl (35 %) in 100 mL distilled
water. The macrostructure was recorded using a stereo
microscope (OLYMPUS SZX16). The microstructure
was observed using a metallurgical microscope (OLYM-
PUS BX51M). The TSFL was evaluated using a compu-
terized tensile tester (INSTRON 1195) at a crosshead
speed of 2 mm/min.
3 RESULTS AND DISCUSSION
Macrographs of the copper friction-stir spot-welded
as a function of the tool rotational speed are presented in
Figure 5. It is possible to spot weld successfully using
the chosen parameters. The weld zones are almost sym-
metrical with respect to the axis of the keyhole. The tool
rotation generates frictional heat, which plasticizes the
copper. The force applied along the axis of the tool
promotes the vertical motion of the plasticized copper.
The axial force further consolidates the plasticized
copper and a spot joint is formed. The joint width is
clearly visible on all the joints. Considerable areas of
both the top and bottom sheets are bonded together. The
hook does not extend to the keyhole area, which indi-
cates bonding between the sheets. The bond width was
measured and was found to be (7.5, 8.5 and 10) mm, res-
pectively, at the selected tool rotational speeds. An
increase in the tool speed improves the bond width
because the frictional-heat generation depends upon the
tool rotational speed.17 The higher the tool rotational
speed, the higher is the heat generation. Hence, more
copper is plasticized and the bond width increases.
Regions with different microstructures were observed
on these macrographs. They are the stir zone (SZ), the
thermomechanically affected zone (TMAZ), the heat-
affected zone (HAZ) and the base copper. These regions
are presented in the micrographs in Figure 6 as a func-
tion of the tool rotational speed. The stir zone is adjacent
to the keyhole area and displays very fine grains. FSSW
was derived from FSW. The plasticized material in FSW
undergoes dynamic recrystallization, which results in the
formation of fine grains. The width of the stir zone
reduces as the tool rotational speed is increased due to a
higher heat input. The TMAZ region presents slightly
elongated grains with an array of oxide particles. The
shearing of the plasticized material from the advancing
side to the retreading side causes the grains in this region
to elongate. The width of the TMAZ is found to reduce
with an increase in the tool rotational speed. The third
I. DINAHARAN, E. T. AKINLABI: INFLUENCE OF THE TOOL ROTATIONAL SPEED ON THE MICROSTRUCTURE ...
Materiali in tehnologije / Materials and technology 50 (2016) 5, 791–796 793
Figure 6: Light micrographs of the transition zone of friction-stir
spot-welded copper at: a) 1200 min–1, b) 1600 min–1 and c) 2000 min–1
Slika 6: Svetlobni posnetki mikrostrukture prehodne cone torno
vrtilno to~kasto zvarjenega bakra pri: a) 1200 min–1, b) 1600 min–1 in
c) 2000 min–1
Figure 5: Macrographs of friction-stir spot-welded copper at:
a) 1200 min–1, b) 1600 min–1 and c) 2000 min–1
Slika 5: Makro posnetek torno vrtilno to~kasto zvarjenega bakra pri:
a) 1200 min–1, b) 1600 min–1 in c) 2000 min–1
region, the HAZ, contains coarse grains. The coarseness
of the grains increases with an increase in the tool
rotational speed as presented in Figure 7.
A higher frictional heat causes the grains to become
coarse. The grain size in the HAZ region is higher com-
pared to the grain size of the as-received pure copper in
Figure 1. This region is clearly influenced by the
frictional heat generated during FSSW, leading to the
grain growth. The stir zone is very thin at the higher tool
rotational speed of 2000 min–1. The higher heat causes
the recrystallized grains to grow further and diminishes
the width of the stir zone. The microstructure of the un-
bonded zone is presented in Figure 8. The grains of the
upper and lower sheets are not uniform. The grains in the
upper sheet are coarser than those in the lower sheet.
This can be attributed to the proximity of each sheet to
the tool shoulder. The upper sheet receives more heat
input compared to the lower sheet, causing the grains to
become coarser.
The influence of the tool rotational speed on the
TSFL is depicted in Figure 9. It is evident from the
figure that the increase in the tool rotational speed
increased the TSFL. Although the increase in the tool
rotational speed caused grain growth in different regions
across the joint, the TSL improved upon the increased
tool rotational speed because the TSFL depends upon the
joint width and the hook position from the stir-zone area.
A hook is formed in a FSSW joint as the material from
the bottom sheet flows upwards during the process. The
distance of the hook from the stir-zone area determines
the load-carrying capacity of the joint. The hook is not
I. DINAHARAN, E. T. AKINLABI: INFLUENCE OF THE TOOL ROTATIONAL SPEED ON THE MICROSTRUCTURE ...
794 Materiali in tehnologije / Materials and technology 50 (2016) 5, 791–796
Figure 8: Light micrographs of the unbonded zone of friction-stir
spot-welded copper at: a) 1200 min–1, b) 1600 min–1 and c) 2000 min–1
Slika 8: Svetlobni posnetki nezvarjenega podro~ja torno vrtilno
to~kastega zvara bakra pri: a) 1200 min–1, b) 1600 min–1 in c) 2000 min–1
Figure 7: Light micrographs of the heat-affected zone of friction-stir
spot-welded copper at: a) 1200 min–1, b) 1600 min–1 and c) 2000 min–1
Slika 7: Svetlobni posnetki mikrostrukture toplotno vplivane cone
torno vrtilno to~kasto zvarjenega bakra pri: a) 1200 min–1, b) 1600 min–1
in c) 2000 min–1
clear in the macrograph in Figure 1 due to the annealing
effect of copper during FSSW.
The other factor is the bond width. The bond width
increased with an increase in the tool rotational speed.
This provided more tangential area to resist the external
load. Hence, the TSFL increased as the tool rotational
speed was increased. The photographs of failed speci-
mens are shown in Figure 10. The failed specimens de-
monstrate different modes of failure. The nugget pull-out
failure is observed at the tool rotational speeds of
1200 min–1 and 1600 min–1. It can be inferred from the
I. DINAHARAN, E. T. AKINLABI: INFLUENCE OF THE TOOL ROTATIONAL SPEED ON THE MICROSTRUCTURE ...
Materiali in tehnologije / Materials and technology 50 (2016) 5, 791–796 795
Figure 12: Images of tensile shear specimens during loading as
marked in Figure 11
Slika 12: Posnetki nateznih stri`nih vzorcev med obremenjevanjem,
kot je prikazano na Sliki 11
Figure 9: Effect of tool rotational speed on tensile shear load of
friction-stir spot-welded copper
Slika 9: Vpliv hitrosti vrtenja orodja na natezno stri`no obremenje-
nega torno vrtilno zvarjenega bakra
Figure 11: Load-versus-displacement graph for a tensile shear-tested
specimen at the tool rotational speed of 1600 min–1
Slika 11: Odvisnost obremenitve in raztezka pri nateznem stri`nem
preizkusu vzorcev pri hitrosti vrtenja orodja 1600 min–1
Figure 10: Images of failed sheets at tool rotational speeds of: a) 1200
min–1, b) 1600 min–1 and c) 2000 min–1
Slika 10: Posnetki poru{enih plo~evin pri hitrosti vrtenja orodja:
a) 1200 min–1, b) 1600 min–1 in c) 2000 min–1
photographs (Figures 10a and 10b) that the nugget
pull-out originated from the bottom of the keyhole area.
The diameter of the hole in the upper failed sheet is
higher at 1500 min–1 than 1200 min–1. This is due to an
increase in bond width, which provided a wider me-
tallurgical bonding. The mode of failure at the tool rota-
tional speed of 2000 min–1 is tearing. The tearing started
circumferentially around the unbonded region and could
not pull the nugget out because the higher bond width at
2000 min–1 caused the sheet to tear like a tensile speci-
men.
The load-versus-displacement curve for the tensile
shear-tested specimen at the tool rotational speed of
1600 min–1 is presented in Figure 11. The load increases
with a constant slope until point šc’ is reached. Corres-
ponding photographs in Figure 12 show details of the
points marked in Figure 11. As the load is increased to
point šb’, the specimen bends. The axis of the normal
load does not coincide with the centre of the tested spe-
cimen, which creates a small moment causing the bend-
ing. The specimen starts to yield, i.e., the nugget pull-out
commences at point šc’. The load does not drop sharply
to zero, unlike in the cases of tensile failures, because the
nugget pull-out is not instant. The nugget pulls out
gradually as the separation increases until point šg’ is
reached. The load drops to zero, which indicates a com-
plete separation of both sheets.
4 CONCLUSIONS
Commercially pure copper sheets were successfully
spot welded using the novel FSSW technique. The
influence of the key processing parameter, the tool
rotational speed, on the microstructure and TSFL was
analyzed. The increase in the tool rotational speed
increased the bond width and coarsened the grains in the
stir zone and HAZ. The TSFL improved as the tool rota-
tion increased. The nugget pull-out failure took place at
1200 min–1 and 1600 min–1 and the tear-out failure took
place at 2000 min–1.
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I. DINAHARAN, E. T. AKINLABI: INFLUENCE OF THE TOOL ROTATIONAL SPEED ON THE MICROSTRUCTURE ...
796 Materiali in tehnologije / Materials and technology 50 (2016) 5, 791–796