effect of tool geometry and process parameters on the ... stir welding (fsw) is a solid state ......

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, [email protected] 2Mechanical Engineering, IIT Guwahati, 781039,[email protected]

2Mechanical Engineering, IIT Guwahati, 781039,[email protected] 3*Mechanical Engineering, IIT Guwahati,781039,[email protected]

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)



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


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



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)


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)



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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effect of tool geometry and process parameters on the ... stir welding (fsw) is a solid state ......

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