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Comparative Evaluation of Tensile Properties and
Microstructural Behaviour of Friction Stir Welded Butt and
Lap Joints of AA2014-T6 Aluminum Alloy
1C. Rajendran, 2K. Srinivasan, 3V. Balasubramanian, 4H. Balaji, 5P. Selvaraj
1Assistant Professor, Department of Mechanical Engineering,
Sri Krishna College of Engineering and Technology, Coimbatore, India-641008 2Assistant Professor, 3Professor
Centre for Materials Joining and Research, Annamalai University, India-608002 4 Scientist D, 5Scientist-F
Aeronautical Development Agency, Bangalore, India
Email: [email protected]
.
Abstract Age hardening aluminum alloys such as 2XXX and 7XXX series are
suitable for parts and structures requiring high strength to weight
ratio and are commonly used in aircraft fuselage and wing skins. The
structures are conventionally joined by rivets. It is difficult to join
these aluminum alloys especially 2XXX series by traditional joining
process due to break up the temperature of Al2O3 which usually
results in solidification cracking, burns through and porosity. Hence to
overcome such problems solid-state welding technique is chosen.
Friction Stir Welding (FSW) is one such promising process, which can
be effectively applied to weld these alloys for aircraft application with
butt joint configuration. Hence, in this investigation, an effort has
International Journal of Pure and Applied MathematicsVolume 119 No. 12 2018, 2079-2091ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu
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been made to evaluate the tensile and microstructural characteristics
of lap and butt joint of 3 mm thick plate welded by FSW, at constant
tool shoulder diameter (D) to plate thickness (T) ratio of 3 and tool tilt
angle of 2˚. It is observed that the butt joint fabricated with a D/T ratio
of 3 exhibited superior tensile and microstructural properties
compared to the lap joint fabricated with D/T ratio of 3. The width of
the thermo-mechanically affected (TMAZ) region in the lap joint was
comparatively higher than the butt joint. Hence the lap joints yielded
inferior tensile properties than the butt joint due to the enhanced grain
coarsening produced in the TMAZ region of the lap joint. In addition,
the hook formation on the AS and RS of the lap joint was bending
downwards, which is different from the ideal condition.
Keywords: Friction Stir Welding, Al-Cu alloys, Microstructure, Tensile
properties
1. Introduction
The precipitation hardening aluminum alloys such as 2XXX, 6XXX and 7XXX series
are ideal for aircraft structural applications. Most aircraft and aerospace sheets
involve butt and lap joint welds. Conventional joints are produced using rivets,
fusion welding limited to GMAW and GTAW [1]. It is very challenging to weld
aluminum due to the high temperature involved in breaking up of the oxide film.
The friction stir welding (FSW), a new solid-state welding, offers several benefits
over fusion welding processes. Some of the benefits are low heat input which
eliminates the melting and solidification process thus resulting in good dimensional
stability, improved mechanical properties, lesser weld defects and low internal
stress [2]. Hence, the use of FSW in place of riveting for joining Al alloys in lap
configuration could help reduce the weight and overall cost with enhanced
mechanical properties and manufacturing complexities. The FSW process
development for lap (LJ) and butt joints(BJ) are quite different from each other. The
removal of the Al2O3 at the interface of lap joint is very difficult than BJ [3]. In
general, the FSW tool penetrates the abutting faces in a BJ; whereas, in LJ, a non-
consumable rotating tool is made to pierce into lower sheet up to certain plunge
depth. The original joint line with severe plastic deformation (OJLwSPD) [4] kissing
bond [5] and interface [6] of the weld bends upward or downward with respect to the
movement of material around the pin resulting in the hook formation [7]. The hook
formation in an ideal LJ joint is a downward bending hook on the AS side and an
upward bending hook on the RS which are more beneficial for load carrying capacity
[8]. Cederqvist and Reynolds [9] investigated the parameters affecting the
properties of friction stir LJ between Al alloy 2024-T3 and 7075-T6. A joint
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efficiency of 78% was reported in LJ which is much higher than what could be
achieved with riveting and electrical resistance spot welding. Several researchers
have highlighted the significance of process parameters on mechanical and
metallurgical properties. In the current study, comparative evaluation of friction
stir butt and lap joints, in terms of (1) microstructural behaviour and (2) Tensile
properties have been made.
2. Experimental work
The rolled sheets of 3 mm thickness, made of aluminum alloy AL clad 2014-T6,
were utilized in this work. The chemical proportion and mechanical properties of
the base metal are listed in Tables 1 and 2 respectively. The dimensions of BJ and
LJ were 150 mm x150 mm x 6 mm. The sides of the plates were machined using a
milling machine, and butt and lap joints were prepared by conducting FSW normal
to the rolling direction using CNC controlled FSW machine. The FSW tool was
prepared of super HSS with threaded taper cylindrical pin of length 2.8 mm for BJ
and 5.75mm for LJ with a concave profile head of 3o on 9 mm and 18 mm tool
shoulders. The tool details are listed in Table 3. The position of tool was kept at 2°
for each condition. Two unequal axial forces were attained by controlling the pierce
depth of welding tool since both the specimens had different thickness. In order to
calculate the efficiency of the LJ, plain sheet specimen (100 mm long, 10 mm wide
and 3 mm thick) was taken from the base material in T6 condition and loaded in
tension to failure. The tension to failure load of the base metal in T6 condition was
measured. The process parameters used in this study are presented in Table 4. The
metallographic observation was carried out by optical microscopy (OM) and
Scanning Electron Microscopy (SEM). The samples for OM were milled, polished
and etched using Keller’s etchant for macro and microstructure. The microhardness
was noted using microhardness tester applying 50 N force for 15 sec. In order to
obtain fracture location of the butt and lap joints, room temperature tensile tests
were carried out according to ASTM-E8 M-04 and ANSI/AWS/SAE/D8.9-97.
3. Results and Discussion 3.1 Macrostructure
Optical macrographs showing the cross-section of the FSW joints at different
configurations (type of joint) of BJ and LJ are shown in Fig.2 and Fig.3, it can be
observed that both the welded joints are sound and defect free at constant D/T ratio
of 3 and tool tilt angle of 2°. All joints showed elliptical weld nugget [13] with wider
nugget zone(NZ) at the top than the bottom. The top surface had direct contact with
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the shoulder, therefore, experienced more frictional heating and plastic flow which
resulted in wider weld zone than the bottom surface.
Table 1 Chemical composition (wt. %) of base metal
Si Fe Cu Mn Mg Zn Cr Ti Al
0.874 0.135 4.815 0.813 0.734 0.063 0.005 0.011 92.45
Table 2 Mechanical properties of base metal
Material
Yield
stress
( MPa)
Ultimate
tensile
stress (MPa)
Elongation
50 mm
gauge
length
(%)
Micro
hardness
50 N, 15 sec
(VHN)
Shear load
(kN)
AA2014-T6 431 455 10 163 17
Table 3 Geometry of used FSW tool
Joint
configuration
Pin
description
Pin diameter Pin
length
(mm)
Taper
angle
in
pin
diameter
(o)
Thread
pitch
(mm) Major
diameter
(mm)
Minor
diameter
(mm)
Butt joint Threaded
taper pin 3.0 2.5 2.75 5.19 0.75
Lap joint Threaded
taper pin 6.0 5.0 5.75 4.96 0.75
Table 4 Process parameters of butt and lap joint
Joint
configuration
Tool
rotational
speed (rpm)
Welding
speed
(mm/min)
Tool
Shoulder
diameter
(mm)
Tool tilt angle
(deg)
Butt joint 1400 50 9.0 2.0
Lap Joint 700 50 18.0 2.0
The bottom surface which was in contact with the backing plate extracted heat from
the bottom area of the joint which in turn contracted the lower portion of the weld
nugget.
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ig.1 Schematic diagram of the FSW tool used in this investigation
Fig.2 butt joint configuration and tensile
specimen extraction
Fig.3 Lap joint configuration and tensile
specimen extraction
D- Tool shoulder diameter (mm)
d1 - Major diameter of pin (mm)
d2 - Minor diameter of pin (mm)
L- Length of pin (mm)
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a. Fabricated Butt joint tools b. Fabricated lap joint tools
c. Fabricated butt joints d. Fabricated lap joints
e. Tensile specimen(Before testing) f. Lap shear specimen(Before testing)
g. Tensile specimen (After testing) h. Lap shear specimen(After testing)
a. Fractured butt joint b. Fractured lap joint
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Fig.4 Photograph of FSW butt and lap joint
In case of FSW joints of AA2014-T6, the dimension of each joint of weld nugget
closely matched with the dimension of tool ie. Shoulder diameter at the top surface
(10.1 and 20.3 mm) and pin diameter at the bottom (2.6 and 5.65mm) of both joints.
NZ at the top surface was wider than at the bottom surface, as the upper surface
was in contact with the tool shoulder. In FSLW a visible bright contrast line seen
along the original sheet interface in Fig. 5(b) is the al clad layer. The interface line
enters into the SZ and bends toward the bottom on the AS, whereas in RS the line
terminates at the TMAZ/SZ interface. In this investigation, the material flow in and
around the pin is seen in the macrostructure showed maximum width in SZ bottom
than the top. The reason for this problem may be the restriction of material flow
around the pin caused by the al clad layer between two sheets thus pushing the
material back to the SZ bottom, so the size was bigger.
3.2 Microstructure
The formation of NZ is due to the collective effect of thermal and mechanical
stresses caused by stirring action of the tool and axial force. TMAZ and HAZ in joint
are shown in Fig. 5. The micrographs of the center of weld NZ for both the joints are
shown in Fig. 5(c) and 5(d). All weld nuggets invariably showed fine grains
produced by severe plastic distortion and high temperature resulting in dynamic
recrystallization. Due to the revolution and movement of FSW tool during welding,
the coarser grains are converted into fine grain structure in the NZ. Fewer
strengthening precipitates of CuAl2 were observed in NZ as broken down and
uniformly distributed by the stirring tool. The LJ showed three distinct
microstructural regions SZ, TMAZ and HAZ. Very fine and recrystallized grains
were observed in the SZ of both the butt and lap joint Fig. 5(c) and 5(d), towards the
welded bottom distinct onion ring pattern, were seen in both the welds.The TMAZ
in both the case showed severely deformed un recrystallized grains, no significant
grain coarsening in the HAZ was observed in any of the welds. Within the region
covered by tool shoulder, two distinct bond region were observed in lap welds, such
as partially bonded region and fully bonded region. The partially bonded region has
been described some of few researchers such as an OJLwSPD [4], kissing bond line
[5] and Interface [6]. The upper and lower sheets closely mate with each other. They
are separated by a thin layer beginning somewhere near the tool shoulder mark on
either side of the weld and extend inwards, ending either at the TMAZ/SZ interface
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or SZ. In this weld, the partially bonded region was observed to extend into
TMAZ/SZ Fig. 5(b).
a. Macrostructure- butt joint b. Macrostructure-lap joint
c. Stir zone(Butt joint) d. Stir zone (lap joint)
e. Interface SZ/TMAZ- AS f. Interface SZ/TMAZ-AS
g. Interface SZ/TMAZ-RS h. Interface SZ/TMAZ-RS
50µm
50µm
50µm
50µm 50µm
50µm
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i. Thermo mechanical affected zone j. Thermo mechanical affected zone
Fig.5 Microstructure of various regions of butt and lap joints
a. Hook formation on RS b. Hook formation on AS
Fig.6 Microstructure of hook formation on lap joint
a. Base metal
b. FSW Butt joint
b. lap joint
Fig.7 Fractographs of base metal, FSW butt and Lap joint
50µm 50µm
50µm 50µm
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The second one was in the fully bonded region, the region within SZ, where the
upper and lower sheet gets metallurgically bonded to each other with no discernible
original interface line in the weld. The width of the fully bonded region is an
important consideration in lap joint. It was found that the width of bond in FSLW
was less than the butt joint. When the thread rotates in the favorable direction it
can result in such a strong downward metal flow [10-12]. The reason for lower lap
shear strength of AA2014-T6 aluminum alloy is due to different relative speeds of
plastic material on AS and on RS which results in different structures [14]. It was
found on the AS; the speed gradient is greater than the RS. Microstructure changes
rapidly and there was lack of necessary transition. In friction stir BJ, Fig.5 (e)
shows the microstructure in the weld nugget-TMAZ on the AS, the grain in the SZ
is finer than in the TMAZ. It can be seen from Fig.5 (e) that microstructures change
smoothly from SZ to TMAZ because of sufficient plastic material flow due to little
speed gradient. The grains are smaller and microstructure is even. The size of grain
changes gradually from SZ to TMAZ and this will not affect the tensile properties
very much. But an obvious boundary can be seen between SZ and TMAZ.
3.3 Tensile Properties
Table 5 shows tensile test results of the joints with different weld configurations.
Experimental results showed that FSW butt joint had significant efficiency than LJ.
The efficiency of BJ and LJ was inferior to the base material. Tensile strength,
hardness, and elongation were lower than the base material, the tool rotational
speed of 1400 rpm, welding speed of 50mm/min, D/T ratio of 3.0 and tilt angle 2.5
yielded the maximum tensile strength of 339 MPa. The increase in tool rotational
speed and welding speed with constant D/T ratio of 3.0 and tool tilt angle 2.5, was
found to reduce the ultimate tensile strength due to high heat input and improper
material flow resulting in a defect which was observed either on advancing side or
retreating side because of the grain coarsening occurring in the TMAZ/SZ interface
region. The process parameters influenced the heat input per unit length of the
weld, which controlled the degree of softness and flows ability of the plasticized
material. At low speed of tool, the amount of heat supplied to the deforming
material in weld area was greater and therefore wider softened region around the
stirring tool lead to more homogeneity of the NZ, which resulted in the inferior
ultimate tensile strength of welded joint having higher heat input per unit length
achieved at higher welding speed. The lower heat input per unit length of the weld,
resulting from reduced stirring of material and flow in weld area resulted in the
poor ultimate tensile strength of friction stir welded joint of AA2014-T6.
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For LJ, the average failure load measured on the base metal in T6 condition was 17
kN. This value was used to calculate the joint efficiency of weld specimens under lap
shear loading using the following formula, which was proposed by Cederqvist and
Reynolds [9]:
Weld failure load (kN)
Joint Efficiency [FSLW] = ------------------------------- X100% (1)
Base metal failure load (kN)
The lap shear testing conducted on the joint in alloy AA2014 sheet produced using
FSW is given in Table 5, the joint efficiency also included in Table 5. The efficiency
of LJ made with threaded tapper cylindrical tool (52%) was less than that of butt
joint (78%). The butt joint configuration efficiency thus confirmed its superiority
overlap joint. Examination of the fractured LJ specimen showed that the weld made
with the threaded taper cylindrical pin with the originating fracture at the
SZ/TMAZ interface on AS, and both the weld fractured on the top sheet. The lap
shear specimen was loaded on advancing side in the upper sheet and on the RS in
the lower sheet, since the hook is bent downward on both the side in the weld, since
the upper sheet is indeed the critical location for failure, this explains why the lap
joint fractured on the AS in the upper sheet. Fractographs of the transverse tensile
and lap sheared samples are presented in Fig.7, fracture surface is invariably
characterized by dimples of varying size and shape, which an indication of ductile
failure, welds produced in FSW butt joint exhibited finer and deeper dimple and
rupture features, as compared to the shallow and coarse dimples in the case of laps
joint. The average hardness in the SZ on butt and lap joints were found to be 135
HV and 115 HV respectively.
4 Conclusion
1. The best quality of weld was obtained at tool rotational speed of 1400 rpm,
welding speed of 50 mm/min, D/T of 3.0 and tool tilt angle of 2o for butt joint, and
tool rotational speed of 700 rpm, welding speed of 50 mm/min, D/T ratio of 3.0 and
tool tilt angle of 2o for lap joint of AA2014-T6 aluminum alloy.
2. The tensile properties of the butt joint and lap joint are lower than that of the
base material, and a maximum joint efficiency attained in terms of TS and TSFL is
78% and 52% respectively.
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3. In both the cases, crack propagated along the interface between nugget zones
(NZ) and TMAZ, due to the dissimilarity of grains observed in the microstructure of
the interface.
Acknowledgement
The authors are indebted to Aeronautical Development Agency (ADA), Bangalore, for
financial support through project FSED 83.07.03.
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