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5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,
IIT Guwahati, Assam, India
28-1
Effect of tool geometry and process parameters on the material
flow of friction stir welding.
Arun Kumar Kadian1, Gautam Puri2, Suman Das2, Pankaj Biswas3*
1Mechanical Engineering, IIT Guwahati, 781039, arun.kadian@iitg.ernet.in 2Mechanical Engineering, IIT Guwahati, 781039,g.puri@iitg.ernet.in
2Mechanical Engineering, IIT Guwahati, 781039,suman.das@iitg.ernet.in 3*Mechanical Engineering, IIT Guwahati,781039,pankaj.biswas@iitg.ernet.in
Abstract
The material flow behavior of friction stir welding is an emerging research area in past few years. In this present
work two different tool geometries has been considered to study the material flow patterns of the welding
process. A 3-D CFD analysis is performed with suitable boundary conditions to study the nature of material
flow behavior of FSW process. A comparative study has been done based on the results obtained from
numerical analysis for the two different tool geometries. The parameters used for the analysis of welding
process also varied one by one for both the cases to achieve a good comparison on the effect of tool geometry on
the material flow. The estimated material flow behavior compared well with those of the published results thus
validating the various assumptions made in the work. It was observed that in FSW tool geometries has
significant effect on material flow behavior. Keywords: Finite Element Analysis, Transient Thermal Analysis, Combined Stick & Slip Condition, Friction Stir Welding.
1 Introduction
Friction Stir Welding (FSW) is a solid state
welding method having a non-consumable tool
without using filler material. The process was
invented by The Welding Institute (TWI), Cambridge,
UK and patented in 1991(W.H. Thomas). During this
process the rotating tool moves material from
advancing side to retreating side. So, it is important to
know the manner in which the material behaves when
it gets plasticized. The way in which the material
flows in a friction stir welding process is need to
understand to know the quality of the weld. The
different parameter of the welds leads to different
material flow behavior such as rotational speed,
traverse speed etc. and a lot more depends upon the
base material to be welded and its material properties
at room temperature and when it reaches plastic state.
The tool geometry is also considered the most
important factor on which the material flow depends.
The material flow directly influences the quality of
weld or the kind of defect may arise after the welding.
Since the material flow is not visible to naked eyes or
any instrument in an experiment. It is very difficult to
understand what is actually happening inside the
welding process by experimental methods. So far, a
very few attempts have been made to study the
material flow behavior of Friction stir welding. E.
Feulvarch et al. (2012)proposed a simple and robust
moving mesh technique for finite element simulation
of friction stir welding in which the heat transfer
analysis and a material flow analysis can be achieved
with a trigonal pin geometry. H. W. Zhang et al.
(2007), (2004)H. Jamshidi Aval et al.
(2011)investigated the material flow using
temperature dependent material properties with the
help of a finite element code using ABAQUS
package. Some researchers like Lorrainet al.
(2010),WU Chuan-songet al.(2012),Carter Hamiltonet
al.(2008),K. Kumaret al. (2008) made an attempt to
understand the flow of material experimentally for an
unthreaded cylindrical tapered tool and studied the
microstructure of the weld zone. Rodrigueset
al.(2014) worked on polymethyl methacrylate
(PMMA) to investigate the material flow and thermo-
mechanical analysis of friction stir welding.S.D.
Jietal.(2012) used different threaded tool geometry to
investigate the flow of material inside the base
material in ANSYS Fluent package. They investigated
the effect if tool shoulder on material flow. R.
Kovacevic et al. (2003) introduced a moving
coordinate in a transient 3D heat transfer analysis with
the help of a mathematical model and analyzed a 3D
model for different process parameter in commercial
Finite Element package ANSYS considering sliding
condition. Similar type of work was performed by G.
buffa et al.(2011,12),D. Trimble et al. (2007) to
predicted the residual stresses distribution with a 3D
elasto plastic model Finite element model in
DEFORM-3D™ and achieved a reduction in
computational time.D. Jacquin et al. [17](2011)
worked on a 3D thermo-mechanical model based on
the model proposed by Heurtier et al. (2006) based on
Eularian approach. Dongun Kim et al.(2010) utilized
FVM code in STAR-CCM+ (based on eularian
formulation) to study the temperature histories at
different welding parameters. So far not much work is
done on material flow. It is an important area to work
Effect of tool geometry and process parameters on the material flow of friction stir welding.
28-2
on so we have made an attempt to study the material
flow behavior with four different tool geometries. It is
a three dimensional numerical model which is
implemented in ANSYS Fluent 14.0.
2 Finite volume models
The tool geometry consists of mainly two parts
i.e. the tool shoulder and the tool pin. In this finite
volume model the shoulder is taken as flat cylindrical
while pin is varied. The two tool pin geometries taken
for the analysis are the cylindrical and the conical
shown in figure 1. The pin base diameter for
cylindrical case is taken as 6mm, the shoulder
diameter is as 25mm and the depth of pin is as 2.5
mm. For conical pin geometry the base diameter is
taken as 6mm while the pin tip diameter is taken as
3mm. the shoulder diameter is also varied to
understand the flow in a better way. The workpiece is
also modelled and then meshed for further analysis.
The dimensions of the workpiece are 150mm length,
50mm width and 3mm height. The two pin geometries
are taken as cavities in the workpiece in the FVM
model in ANSYS Fluent 14.0.
Figure 1: The model of the tool (a) cylindrical and
(b) conical
2.1 Boundary conditions
In the simulation of the welding process the tool
movement is assumed to be form positive to negative
x direction so the fluid flow was given in the opposite
direction. The rotation of the tool is assumed to be in
positive z direction e.g. anti-clockwise direction. To
find out the effect of process parameters on material
flow three different traverse speed of tool (1.8mm/s,
2mm/s and 2.2mm/s) and three different tool rotation
speed (1000rpm, 1200rpm and 1400rpm) is
considered. The temperature 1000Kis given to pin
wall. In this process the metal material is considered
as a highly viscous non-Newtonian fluid. The contact
between the rotational tool and material is considered
to be no-slip contact.The material is considered to be
in liquid in state with temperature dependent
viscosity. The input is given as moving wall with
velocity 120mm/min and the other is considered as
the output wall. The simulation is performed in
FLUENTpackage. For simulation of FSW of the
model, the RNG k-ϵ model is used. The RNG model
is suitable for this case as it was developed using Re-
Normalisation Group (RNG) in order to renormalize
the Navier-Stokes equations so that effects of smaller
scales of motion can be properly calculated.
Figure 2: (a) Enlarged view showing conical pin
cavity, (b) Enlarged view showing the cylindrical
pin geometry.
2.2 Material properties
The workpiece material is Aluminium 6061 alloy,
the composition of the alloy is given in table 1.
Table 1: The composition of AA6061 alloy
Comp
onent
Wt. % Comp
onent
Wt. % Comp
onent
Wt.
%
Al 95.8-
98.6
Mg 0.8-1.2 Si 0.4-
0.8
Cr 0.04-
0.35
Mn Max
0.15
Ti Max
0.15
Cu 0.15-
0.4
Other,
each
Max
0.05
Zn Max
0.25
Fe Max
0.7
Other,
total
Max
0.15
The density is 2700kg/m3 and it is assumed to be
constant. The conductivity and specific heat is
considered to be temperature dependent as the
following table 2.
(a)
(b)
(a)
(b)
5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,
IIT Guwahati, Assam, India
28-3
Table 2: The temperature dependent properties of
AA6061 alloy
Temp (K) K (W/m-K) Cp (J/kg-K)
293 195 870
373 195 950
473 203 980
573 211 1020
673 212 1060
773 225 1150
873 200 1160
915 90 1170
973 91 1170
1073 92 1170
However the main problem is with the value of
viscosity. If a constant viscosity of high value is given
throughout the whole inlet material that will result in
erroneous values of temperature distribution and
velocity magnitude as the temperature is high only the
region close to the rotating tool and it decreases as the
distance increases away from the tool. So the viscosity
is different in different regions of the weld. Colegrove
et al. (2005) approximated the viscosity of the
plasticized metal using a ratio of effective stress and
the strain rate as:
μ =��
���, where µ = viscosity of material, �= effective
stress and =̅ strain rate (1)
The value of � can be obtained by Zener-
Hollomon equation:
11 2 2
1ln 1
n n
R
Z Z
A Aσ
α
= + +
and
expQ
ZRT
ε
= (2)
Where, Q = activation energy, R = universal gas
constant, T = Temperature, Z = Zener-Holloman
parameter, α, A and n are constants related to the
material.
The values of these parameters are obtained
experimentally by Tello et al. (2010) and are listed for
AA6061 as follows: A = 1.63 x 1013 s-1, α = (1/60.7)
Mpa-1, n = 5.33, Q = 191 kJ/mol.
2.2 Governing equations
In FSW the material near the rotating pin gets
plasticised due to frictional heat produced by the pin
and tool shoulder. Considering the material a highly
viscous fluid the finite element model can be made
using commercial software. However to simplify the
model simulation some assumption were considered
(a) There is no tilt angle given to the spindle of tool
e.g. tool rotates perpendicularly inside the workpiece.
(b) The effect of tool shoulder is omitted. (c) In
contact between the surface of pin and workpiece is
no slip condition. (d) The molten material hasisotropic
viscosity and is an incompressible fluid. The process
of FSW follows the equations of mass conservation,
momentum conservation and energy conservation. As
the mass of material is constant so the mass
conservation equation is given by:
0u v w
x y z
∂ ∂ ∂+ + =
∂ ∂ ∂ , (3)
where u, v, w is the velocity in x, y, z direction
respectively. The momentum conservation equation is
given by the Navier-Stokes equation: 2 2 2
2 2 2
1x
u u u u p u u uu v w F
t x y z x x y zµ
ρ
∂ ∂ ∂ ∂ ∂ ∂ ∂ ∂+ + + = − + + +
∂ ∂ ∂ ∂ ∂ ∂ ∂ ∂ 2 2 2
2 2 2
1y
v v v v p v v vu v w F
t x y z y x y zµ
ρ
∂ ∂ ∂ ∂ ∂ ∂ ∂ ∂+ + + = − + + +
∂ ∂ ∂ ∂ ∂ ∂ ∂ ∂ 2 2 2
2 2 2
1z
w w w w p w w wu v w F
t x y z z x y zµ
ρ
∂ ∂ ∂ ∂ ∂ ∂ ∂ ∂+ + + = − + + +
∂ ∂ ∂ ∂ ∂ ∂ ∂ ∂ (4)
Where Fx, Fy and Fz are the force in x, y, z
direction respectively; p is the static pressure of the
flow field; µ is the viscosity of fluid and ρ is the
density of material.
The energy conservation equation is given by: 2 2 2
2 2 2
T T T T T T Tc u v w
t x y z x y zρ λ
∂ ∂ ∂ ∂ ∂ ∂ ∂+ + + = + +
∂ ∂ ∂ ∂ ∂ ∂ ∂ (5)
Here, c = specific heat of material, λ = thermal
conductivity of material, T = temperature of fluid.
3 Results and discussions
3.1 Effect of rotational speed
In order to analyze the influence of the any
parameter on material flow various section of the
weld plates observed. For better results three planes
perpendicular to tool with normal parallel to the axis
of the tool and one plane with normal perpendicular to
the axis of the tool are considered. The surface 1 lies
just below the tool shoulder (0.2mm below from top
surface), surface 2 lies between the tool pin top and
bottom (1.55mm from top surface), surface 3 lies just
below the pin bottom ( 2.7mm from top surface).
Surface 4 is parallel to the inlet surface and located at
the mid-point of the rotating tool.
The tool traverse speed is taken constant as
2mm/s and the rotational speed is varied as 1000rpm,
1200rpm and 1400 rpm. For surface 3 the velocity
vector is plotted coloured by velocity magnitude
shown in figure 3. At all the surfaces it is noted that
materials which are near to the rotating tool had very
high speed and the direction of rotation is same as the
tool i.e. anti-clockwise in the present case. It is
observed that as the rpm of the pin is increased the
speed of particles near the tool pin is also increased.
High rotation velocity generates high deformation
among the material near to weld region which
ultimately leads to high strain rate values. Moreover,
the region of deformation also increased with the
Effect of tool geometry and process parameters on the material flow of friction stir welding.
28-4
increase in rpm value which is clear by the figure. At
the pin bottom surface a swirl zone is present in which
material flow had a rotating velocity. Particles that are
closer to the outer side of the pin radius have higher
value of rotation than that are just directly below the
pin center.
Figure 3: Material flow variation with rpm at
surface 3.
3.2 Effect of translational velocity
For the effects of tool traverse speed on material
flow the tool rotational speed is taken constant as
1200rpm and the rotational speed is varied as
1.8mm/s, 2mm/s and 2.2 mm/s. When material flow
velocity vector by magnitude was plotted for surface
1, 2 and 3 it was found that for each surface the
material flow pattern remains almost same for all the
tool traverse speed. Material near the pin has same
speed for all the cases as tool rotation speed is
constant. As the tool speed increases the material flow
near the pin rotates more for any surface. So the grain
microstructure for higher tool speed will be more
rotated then lower tool speed. Also the material flow
width decreases as tool speed increases so less amount
of material is stirred as shown in figure 4. At surface 1
the mixing of stirred material at the retreating side is
found be good at 2mm/s speed but not at the other two
speed of tool so to avoid a void optimum tool travel
speed should be used.
3.3 Effect of tool pin geometry
The material flow varies with the tool geometry
in respect of maximum velocity of the material around
the pin and the stir zone. Velocity magnitude vectors
at surface 1, 2 & 3 and velocity contour at surface 4
for the three different tool travel speed for both
cylindrical and conical pin tool are observed and
found that as the tool rotational speed remains
constant, the maximum material flow velocity
remained same for all the three tool travel speed in
both type of pin tool. For cylindrical pin tool the
maximum velocity is 0.286 m/s and for conical pin
tool the maximum velocity is 0.305 m/s.No noticeable
change in velocity contour in surface 4 for both the
cases. This implies for a small change in tool travel
speed the material flow remains almost same for both
type of pin type.
Figure 4: Material flow variation with tool speed at
surface 3.
The material flow velocity contour is plotted on
surface 4 of both the pin types as shown in figure 5.
The tool travel speed for both the tool type is 2mm/s
and the rotational speed is 1200 rpm. The highest
material flow velocity is same for both the tool
geometry. In both cases the highest material flow is
obtained right below the shoulder surface of
5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,
IIT Guwahati, Assam, India
28-5
tool.There is a significant dissimilarity between the
velocity contours of these two i.e. the weld nugget
produced will not be same for the two tool types.Also
it is seen that the cylindrical pin gives better material
mixing at the lower part of the weld, near pin wall.
Chances of getting weld with lower penetration are
higher in conical tool. This may lead to root defect.
Figure 5: The difference of material flow as the pin
type changes form (a) cylindrical to (b) conical
From the velocity contours it is clear that
maximum velocity attained by the material is same for
both pin types. But that is only at the surface just
below the tool shoulder. To study further the velocity
vectors are plotted here for surface 2 is shown in
figure 6. For conical pin, material just around the pin
wall have velocity of 0.0156m/s while for cylindrical
pin material just near the pin wall have velocity of
0.0776m/s, which is much higher.
The viscosity of the material changes with
temperature it is lower when the temperature is high
and higher when the temperature is low. So, the
material away the tool rotation region is taken as
highly viscous fluid i.e. effectively as solid but the
viscosity near pin rotation region is considered to be
low thus giving a better result. The variation of
viscosity is shown in figure 7. Near the pin region and
under the shoulder the viscosity is 0.01kg/m-s but just
outside the tool the viscosity changes to 4kg/m-s and
more as moves further away from the tool. The effect
of variation of viscosity on temperature profile is
shown in figure 8 and 9.
It can be seen from the picture that temperature
produced in cylindrical pin tool is greater, as though
heat generation by shoulder is same for both type of
tools, the heat generation due to pin is greater for
cylindrical tool. Also the temperature distribution is
also differed especially in the retreating side because
of the tool geometry difference.
Figure 6: Difference in velocity vector at surface 2
for cylindrical and conical tool.
Figure 7: The variation of viscosity with position
3.4 Validation of model
For validating the model developed using
FLUENT, it is necessary to correlate the developed
model with the published results. For this purpose, the
model is verified with experimental results obtained
by V. Soundararajan et al.(2005).
The model used for validation has the same
dimension that was used by them for experimental
research. Two workpiece of Al 6061-T6 alloy with
each dimension of 200x50x6.4 mm3 are butt welded.
The tool shoulder radius is 12mm, pin radius is 3mm
and pin height is 5.9mm.
(a)
(b)
Effect of tool geometry and process parameters on the material flow of friction stir welding.
28-6
The temperature was measured along a line
whose distance from weld line is 6mm as in the
experiment the thermocouple was also placed at 6mm
distance from weld line. The tool travel speed is
133mm/min and tool rotation is 344 rpm.
Figure 8: Plot of simulated result with
experimental (triangular blue) result for
model(circular red) validation
The simulate results are plotted against the
experimentally observed results which shows a good
fit. So the FLUENT model is used for further analysis.
4 Conclusions
While simulating FSW it is important to consider
heat produced by plastic deformation as it has
significant contribution to the total heat generated at
higher temperatures when deformation is significant.
It is extremely important to consider the sticking
condition in the analysis and thus the subsequent
decrease in heat generation. The asymmetry in the
temperature profile due to material flow can be
estimated by assuming a simplified flow model and
linking the heat absorbed to the time spent in the shear
layer but an exact analysis requires a complete 3-D
velocity field definition. Higher rotational speed leads
to higher area of deformation and due to high
rotational values near the tool surface, higher values
of strain rates are observed. This is helpful from the
thermal aspect of the process. This is probably caused
by the increased friction heat generated. Thus from an
isolated thermal stand point, increasing the rotational
speed improves the heat generated and thus makes the
weld better and thus a high value of maximum
temperature is observed in case of high rotational
values.
Increasing the transverse velocity of the tool
reduces the temperature attained by the workpiece.
This is probably because of the reduction in the
deformation zone observed with higher values of
traverse speed, though the velocity does not affect the
power generated in the workpiece, it reduces the time
the tool is present at a local position. Thus, reducing
decreasing the heat generated at a particular spot.
Figure 9: the temperature profiles of variation of
viscosity model (top and front view)
Figure 10: Difference of temperature contour for
(a) cylindrical and (b) conical pin tool
Thus from an isolated thermal standpoint it is
better to perform the FSW process with low
transverse velocities. The stirred material zone
decrease slightly for increase in tool traverse speed at
a constant rotating speed of tool. However for higher
traverse speed the amount of rotation of plasticised
material also increases so the grain structure will be
similar to that obtained by slower traverse speed but
more rotated. Also an optimum tool travel speed
should be used to avoid bad quality weld at retreating
side. The FLUENT model developed incorporates the
0
100
200
300
400
500
600
700
800
0 20 40 60 80
Tem
per
atu
re (
K)
Time (sec)
Graph for model validation
(a)
(b)
5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,
IIT Guwahati, Assam, India
28-7
material flow phenomenon also and so it gives more
accurate results. The effect of tool travel speed and
tool rotation on material flow and temperature history
is modelled. Increasing tool travel speed or decreasing
tool rotational speed decreases maximum temperature
attained by the weld. Tool speed has less effect in
material flow than tool rotation.The effect of tool pin
geometries of temperature contour and material flow
is discussed. Taper pin tool gives less amount of
material mixing than cylindrical tool and also
decreases the maximum temperature attained by the
weld.
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