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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 5, May 2012)
212
Simulation of Peak Temperature & Flow Stresses during
Friction Stir Welding of AA7050-T7451 Aluminium
Alloy Using Hyperworks
K.D.Bhatt1, Bindu Pillai
2
1M.Tech.PG student, CIT, Changa, Affiliated to CHARUSAT, Changa
2Assistant Professor, Mechanical Engineering Department, CIT, Changa
Abstract— To overcome limitations of fusion welding of the
AA7050-T7451aluminum alloy friction stir welding (FSW) has
become a prominent process which uses a non-consumable
FSW tool to weld the two abutting plates of the workpiece.
The FSW produces a joint with advantages of high joint
strength, lower distortion and absence of metallurgical
defects. Process parameters such as tool rotational speed, tool
traverse speed and axial force and tool dimensions play an
important role in obtaining a specific temperature
distribution and subsequent flow stresses within the material
being welded. Friction stir welding of AA7050-T7451
aluminum alloy has been simulated to obtain the temperature
profiles & flow stresses using a recent FEA software called
HyperWorks.; the former controlling the microstruture and
in turn, mechanical properties and later, the flow of material
which depends up on the peak temperatures obtained during
FSW. A software based study has been carried out to avoid
the difficulty in measuring the temperatures directly and
explore the capabilities of the same to provide a basis for
further research work related to the said aluminum alloy.
Keywords: AA7050 Aluminum alloy, Flow stresses, Friction
stir welding, Peak temperature, Simulation.
I. INTRODUCTION
Friction stir welding (FSW) was originally invented &
developed by The Welding Institute (TWI) at UK in 1991 by
using Aluminum alloy [1-2]. The FSW involves the use of a
specially designed tool having a shoulder and a protruded pin
with a specific geometry. The tool, being non-consumable, is
rotated at certain rotational speed (rpm) and plunged into the
abutting edges of the two plates to be welded. After the due
rotations at the joining line (given time being called as ‘dwell
time’), the tool is traversed along this line with a certain
welding speed (mm/min) till the end of the plates (Figure - 1).
Thus, it is a solid-state joining technique in which no
melting of workpiece occurs and the required heat to join two
plates is produced by the tool in two fold: (i) friction between
tool shoulder & the workpiece-surface and (ii) heavy plastic
deformation of the workpieces. Thus, during the FSW, fine
equiaxed recrystallized grains result as the material undergoes
tremendous plastic deformation at very high temperature [3-6].
The fine microstructure obtained by FSW process produces
remarkable mechanical properties in welds. The joint is
eventually produced as a result of material movement around
the pin which is found to be very complex due to geometrical
features of the tool [7].
Figure – 1 Schematic of Friction Stir Welding [8]
The Aluminum alloys from 7XXX series are from the non-
weldable class of material having poor solidified
microstructure and porosity in the fusion zone. Also, there is
significant loss of mechanical properties during fusion
welding as compared to their base metals. Both these factors
lead to difficulty in joining these alloys by the conventional
methods of welding [8].
International Journal of Emerging Technology and Advanced Engineering
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As the FSW results in tremendous plastic deformation
around the tool and friction between tool and workpiece, the
temperature rise and its distribution in the weld zones become
responsible for establishing the microstructure of the weld that
includes grain size, grain boundary character, coarsening and
dissolution of precipitates and resulting mechanical properties
of the welds. It is, therefore, necessary to obtain information
regarding temperature distribution during FSW. Of course, the
direct measurements of temperatures within the stirred zone
are quite difficult because of tremendous plastic deformation
taking place during FSW due to rotation and translation of the
tool along the weld center-line. Despite, the attempts have
been made to estimate the maximum temperatures within the
stirred zone from the microstructure of the weld [3, 4, 9] or
record the same by embedding thermocouples in the region
nearer to rotating pin [ 10, 11-13].
Rhodes et al. [3] investigated for microstructural evolution
in AA7075-T651 during FSW that larger precipitates dissolve
and reprecipitate in the weld center. Therefore, they concluded
that maximum process temperatures are between about 400 to
4800C in the AA7075-T651. Murr and coworkers [4, 9]
indicated the non-dissolution of some of the precipitates and
suggested a temperature rise roughly to 4000C for AA6061
alloy during FSW. A study of microstructural evolution of
AA6063 during FSW was done using transmission electron
microscopy (TEM) by Sato et al. [11] and by comparison of
these with those obtained by simulated thermal cycles at
different peak temperatures they concluded that in the regions
of 0 to 8.5, 10.0, 12.5 and 15.0 mm away form weld center,
the temperatures were higher than 402, 353, 3020C and lower
than 2010C respectively. Tang et al. [12] made an attempt to
measure heat input and temperature distribution within friction
stir weld by embedding thermocouples in the region to be
welded for AA6061-T6 Aluminum alloy having thickness of
6.4 mm. They concluded that (i) Maximum peak temperature
was recorded at the weld center and it decreased with
increasing distance from the weld centre-line. (ii) At tool
rotation speed of 400 rpm and a traverse speed of 122
mm/min, a peak temperature of 4500C was observed at the
weld center one quarter from top surface. (iii) The temperature
distribution within stirred zone is relatively uniform. Tang et
al. [12] investigated further that increasing both tool rotation
rate and weld pressure result in an increase in the weld
temperature. Further, they [12] studied the effect of shoulder
on the temperature field and concluded that contact area &
vertical pressure between shoulder and workpiece are much
larger than those between the pin and the workpiece, also,
shoulder has higher linear velocity compared to small radiused
pin. Hashimoto et al. [14] reported that the peak temperature
in the weld zone increase with increasing the ratio of tool
rotation rate to traverse speed for FSW of AA2024-T6,
AA5083-O and AA7075-T6. A peak temperature of > 5500C
was reported in FSW of AA5083-O at a higher ratio of tool
rotation/traverse speed. Frigaad et al. [15] suggested that the
tool rotation rate and the shoulder radius are the main process
variables in FSW, and pressure P can not exceed the actual
flow stress of the material at the operating temperature if a
sound weld without depressions is to be produced. They
performed FSW of AA 6082-T6 and AA7108-T79 at constant
tool rotation rate of 1500 rpm and a constant welding force of
7 kN at three welding speeds of 300, 480 and 720 mm/min.
They [15] revealed that (i) peak temperature of above ~5000C
was recorded in the FSW zone, (ii) peak temperature
decreased with increasing traverse speeds from 300 to 720
mm/min. For a three dimensional thermal model based on
finite element analysis developed by Chao and Qi [16] and
Khandkar & Khan [17] showed reasonably good match
between the simulated temperature profiles and experimental
data for both butt and overlap FSW process. The effect of
FSW parameter on temperature was further examined by
Arbegast and Hartley [18]. They concluded that for a given
tool geometry and depth of penetration, the maximum
temperature was a strong function of the rotation rate (rpm)
while rate of heating was a strong function of the traverse
speed (mm/min). The maximum temperature observed during
FSW of various Aluminum alloys is found to be between
0.6Tm and 0.9Tm which is within the hot working range for
those Aluminum alloys, where Tm is melting point of
material. Ulysee [19] studied the impact of varying weld
parameters on temperature distribution in AA7050-T7451
plate. Khandkar et al. [20] introduced a more comprehensive
model of heat input based on the torque of the FSW tool that
they used to model temperature history of friction stir welded
Aluminum alloy AA6061-T651 plate. The prime objective of
the present paper is to simulate peak temperature and
distribution of flow stresses produced during the FSW of
AA7050-T7451 Aluminum alloy and to compare the same
International Journal of Emerging Technology and Advanced Engineering
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214
with those of the researchers’ results. Use of a recent software
known as HyperWorks9.0 has been explored in the modeling
FSW process. It basically includes a specially designed
module for friction stir welding for modeling.
II. EXPERIMENTAL WORK
Simulations were performed by entering following
properties of the AA7050-T7451 Aluminum alloy as shown in
table – 1. The default tool material available in
HyperWorks9.0 database was used for simulating the FSW
using properties as shown in table – 2.
Table - 1: Physical & Thermal properties of AA7050-T7451
Density 2830 kg/m3
Melting Point 488 - 629°C
Modulus of Elasticity 7.17 x 1010 Pa
Poisons Ratio 0.33
Thermal Conductivity 155 W/m-K
Specific Heat 860 J/kg-K
Volumetric heat source 0.0 W/m3
Table - 2: Physical & Thermal properties of Default tool
Density 2260 kg/m3
Specific Heat 896 J/kg-K
Modulus of Elasticity 2.0 x 1010 Pa
Poisons Ratio 0.35
X – conductivity 198 W/m-K
Y – conductivity 198 W/m-K
Z – conductivity 198 W/m-K
The dimensions of plates of AA7050 alloy were selected
as 381 x 127.5 x 6.4 mm and FSW tool geometry was
selected with cylindrical pin having a shoulder diameter (D),
shoulder length (L), pin diameter (d), and pin length (l) as
shown in table – 3 below:
Table - 3: Dimensions of Default tool
Element D,
mm
L,
mm
d,
mm
l,
mm
Tool 20.3 70.2 7.1 5.76
Table – 4 indicates the FSW process parameters as tool
rotational speed (RS, rpm) and welding speed (WS, mm/min)
and tool tilt angle selected for the simulation in
HyperWorks9.0:
Table - 4: FSW parameters
III. RESULTS & DISCUSSIONS
The graphical results showing temperatures and flow stress
distributions obtained by running the simulations on
HyperWorks9.0 indicate the effects of varying welding
parameters particularly welding speed (WS). At the constant
value of 180 rpm (RS), the peak temperatures obtained at two
values of welding speeds of 51.0 mm/min and 76.2 mm/min
differ only by 100C from the value indicated in literature.
Figure – 2(a) and - 2(b) shows peak temperature values of
3400C and 360
0C for the said alloy at tow welding speeds of
51.0 mm/min and 76.2 mm/min respectively. The peak
temperatures are found as maximum at the tool pin center.
Sr. No. RS,
rpm
WS,
mm/min
Tool tilt angle
1 180 51.0 2.50
2 180 76.2 2.50
International Journal of Emerging Technology and Advanced Engineering
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2(a) At RS = 180 rpm, WS = 51.0 mm/min
2(b) At RS = 180 rpm, WS = 76.2 mm/min
Figure –2: Temperature Distributions at RS & WS shown at (a) & (b)
3(a) At RS = 180 rpm, WS = 51.0 mm/min
Also, the flow stress distribution is shown in figure – 3(a)
and – 3(b) for the same parameters selection. The results
show that at constant RS = 180 rpm the flow stresses are
lower for WS = 51.0 mm/min as compared to those at WS =
76.2 mm/min.
3(b) At RS = 180 rpm, WS = 76.2 mm/min
Figure – 3: Flow Stress Distributions at RS & WS shown at (a) & (b).
IV. CONCLUSION
It can be concluded that at constant tool rotational speed
(RS) and tool with the same geometry; variation in tool
traverse speed has prominent effects on temperature history &
flow stresses developed during FSW of AA7050-T7451
Aluminum alloy. The induced temperatures and flow stresses
are in good confirmation with the results obtained in
literatures. The flow stresses at lower peak temperature of
3400C are as high as 720 MPa but are as low as 680 MPa at
higher peak temperature of 3600C as the flow of material
becomes easier at higher temperatures. It is also observed that
at constant rotational speed the peak temperature has increased
by increasing the welding speed. These temperature profiles
govern viscosity at and ahead of the tool which affects the
flow of material and thus, contribute in establishing a
microstructure which in turn dictates the mechanical
properties of the joint produced. Simulations performed on
computer software opens the new horizon of modeling friction
stir welding process in virtual laboratory and help predict the
mechanical properties of FSW-joints.
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