International Journal of Materials and Mechanics Engineering, Vol. 2, No.

Analysis of plastic flow during friction stir spot welding using finite
element modelling
Z. Gao1, 2, a, F. Krumphals2, b, P. Sherstnev3, c, N. Enzinger2, d, J.T. Niu1, e,
C. Sommitsch2, f
1

School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, Henan,
454000, China
2

Institute for Materials Science and Welding, Graz University of Technology Graz, Austria
2

JOIN 4+ Network of Excellence for Joining, Austria

AIT Austrian Institute of Technology, LKR Leichtmetallkompetenzzentrum Ranshofen GmbH,

mrgaozeng@163.com, bfriedrich.krumphals@tugraz.at, cpavel.sherstnev@ait.ac.at,

norbert.enzinger@tugraz.at, eniujitai@163.com, fchristof.sommitsch@tugraz.at

Keywords: Friction stir spot welding; FEM; Plastic flow; Temperature field

Abstract: Friction stir spot welding (FSSW) as a variant of the linear friction stir welding is
implemented in automotive industry as a partial replacement of resistance spot welding for
aluminium. FSSW as a solid state joining technology, primarily takes advantage of severe
thermoplastic deformation, to achieve the joining between two parts, which can be from the same
material or even dissimilar. In this paper, the coupled thermo-mechanical viscoplastic finite element
formulation is presented based on the character of FSSW. The model was calibrated by comparing
temperature history obtained from the simulation with experimental data and subsequently used to
investigate the effective strain distribution in the weld zone as well as the material flow and the
shape of the stir zone.
Introduction
Friction stir welding (FSW) is a solid state joining process, which uses friction and
deformation heating to produce high quality joints. As a reason that the FSW process usually
operates at temperatures below the melting temperature of the material, it has been proved to be
ideal for producing high quality joints for a number of materials, including those which are
extremely difficult to weld by fusion welding methods. FSW process has been developed to
successfully weld steel, aluminium, copper, titanium, magnesium as well as combinations [1-2].
Recently, a variant of the linear FSW called friction stir spot welding (FSSW) has been
developed and implemented in automotive industry enabling spot welding of aluminium. This
welding technology involves a process similar to FSW, except that, instead of moving the tool along
the weld seam, the tool only indents the parts [3]. The FSSW process consists of three phases:
plunging, stirring and retraction. The process starts with spinning the tool with high rotational speed
and plunging it into a work piece until the shoulder contacts the top surface of the work piece. Then,

International Journal of Materials and Mechanics Engineering, Vol. 2, No. 4

the stirring phase enables the materials of the two work pieces to mix together due to strong
compressive stresses. At this stage, the bonding takes place due to the pressure and temperature that
cause inter-diffusion of material across the interface at atomic level. Lastly, once a predetermined
penetration is reached, the process stops or dwell for a little while and then the tool retracts from the
work piece [4]. During FSSW process, the pin experiences direct contact with the work piece for
longer period as compared to the shoulder. As a result, the friction force between the pin and the
work piece generates most of the heat energy. This characteristic make the FSSW process different
from FSW process [5].
Simulation of the FSSW Process
Assembly - Tool, Work Piece and Back Plate. The commercial FEA software DEFORM-3DTM,
Lagrangian implicit code designed for metal forming processes, has been utilized to model the
FSSW process. The assembly of the model consists of three parts: tool, work piece and back plate,
as shown in Fig. 1. The work piece was modelled as a rigid-viscoplastic material with a thickness of
4 mm and the welding tool as well as the back plate were assumed as rigid bodies. One single sheet
of 4 mm thickness was used as the work piece instead of 2 sheets, in order to avoid contact
instability due to the intermittent contact at the interface between sheets [6]. In order to optimize the
resolution close to the tool and minimize the computational expenses, non-uniform mesh density
with automatic remeshing was applied in the work piece, whose finer mesh size under the tool was
set to 0.75 mm and the other mesh size was set to 1.5 mm. The tool was made of hot work tool steel
H13 with cylindrical pin whose geometric dimensions were given in Fig. 1. Both the experiment
and simulation were based on the following process parameters shown in Tab. 1.
Table 1: Process parameters used in experiment and simulation
Rotational speed
(r/min)

Plunge depth (mm)

Plunge rate (mm/min)

Dwell time (s)

2400

2.2

72

Tool
Work piece
Back plate

Figure 1: Schematic illustration of tool, work piece and back plate

International Journal of Materials and Mechanics Engineering, Vol. 2, No. 4

Material Model. For the work piece of the aluminium alloy 6082-T6, a rigid-viscoplastic
temperature, strain and strain rate dependent material model was utilized. The true stress-strain data
for the material flow were obtained from isothermal compression tests at temperatures from 300 to
550 C and strain rates from 0.001 to 100 s-1. As far as the thermal characteristics of AA6082-T6
concerns, the following constant values were utilized: heat capacity 889 [J/kg K], convection
coefficient 20 [W/m2 K], emissivity 700 [W/m2 K4]. Thermal conductivity was taken into account as
a function of temperature [6]. The utilized tool and back plate were made of hot work steel H13 and
considered as rigid bodies. The default values for both thermal and mechanical properties in
DEFORM were used for H13 in this work.
Friction Model. The friction coefficient between tool and work piece is an input parameter in the
FE model and is used in heat generation formulations. The constant shear friction model was
selected in this study. The friction force is defined by:
f = m k.

(1)

Where f denotes the frictional stress, k the shear yield strength and m the friction factor. This
states that the friction is a function of the yield stress of the deforming body [7].
Results
Temperature Distribution and Verification. As it is known, the plastic deformation and friction
energy are two sources of heat generation during FSSW. The friction energy depends upon the
friction factor, friction area between the work piece and tool and the tools rotational speed. Fig. 2
shows the temperature distribution in work piece at the end of the plunging. It can be seen that the
peak temperature value is 523 C. It is about 0.86 times the homologous melting temperature (in
Kelvin) of the base metal. The temperature distribution is symmetric around the tool and higher
temperature appears at the interface between tool and work piece.

(a)
(b)
Figure 2: Temperature distribution at the end of plunge: (a) isometric view and
(b) cut-open view
During the experiment, temperature history has been recorded at 2 points whose distance to the
edge of the shoulder was 3 mm and 5 mm. By using a shear friction factor of 0.6, a good match
could be achieved between predicted and measured temperature history, see Fig. 3. The temperature

International Journal of Materials and Mechanics Engineering, Vol. 2, No. 4

increases slowly at the beginning of the plunge. The main reason for that is the small friction area
and large distance to the measuring point. After the shoulder contacts the work piece (after 1.67
seconds in this case), the temperature increases rapidly.

(a)

(b)

(c)

Figure 3: Verification of temperature history at two representative points: (a) two points of
temperature measured by thermo-couple; (b) and (c) comparison of calculated and measured
temperature history
Effective Strain. The effective strain is vey important for the FSSW because the materials may not
be welded together if it is not high enough. Fig. 4 (a) shows the effective strain distribution at the
end of plunge. It can be seen that the amount of the effective strain is very high under the tool with
the maximum value of 67.4, which indicates that the deformation during the FSSW process is quite
large. Fig. 4 (b) shows the effective strain history of 5 points. The 5 points are located at the surface
of the work piece and the distance to the weld centre is 0, 1.7, 3.4, 5.1 and 8 mm, respectively. It is
clear that the effective strain increases gradually during the plunge and dwell stage (before 2.84
seconds), and then remain unchanged during the cooling stage (after 2.84 seconds) for all of the
points. After the contact between the shoulder and work piece, the effective strain increases rapidly
for almost all of the points indicating that the shoulder plays a very important effect on the
deformation of the material. The material near to the pins cylindrical surface has a large effective
strain value. Just like point 2 and 3, they are much closer to the cylindrical surface, their strain is
much higher than the else's.

International Journal of Materials and Mechanics Engineering, Vol. 2, No. 4

60

P1 P2 P3 P4 P5

Effective strain

50
P1
P2
P3
P4
P5

40
30
20
10
0

35 mm

(a)

2
Time (s)

(b)

Figure 4: (a) Effective strain distribution at the end of plunge and (b) effective strain history of the 5
points
Material Flow Analysis and the Shape of the Stir Zone. For the investigation of the stir zone
shape, 63 tracking points were defined in the joint cross section. The distance between two points is
0.5 mm in depth direction and 1 mm in radial direction. Half of the stir zone has been utilized to
perform the point tracking because of the symmetry in FSSW. All of the points adhere to the
material. Once the material flow occurs, the points leave their initial position and disappear from
the cross section. Fig. 5 (a) shows that the material flow occurs under the tool. The disappeared
points indicate that these points flow to other places with the material deformation. All other points
stay more or less on their initial position indicating that they have no or very little strain. In the weld
centre, the points are compressed seriously. It is clear that the material in the weld centre mainly
experiences the compression process other than the shear process. By connecting the points at the
border, which have displacement, the shape and size of the stir zone can be estimated. Fig. 5 (b)
compares the shape of the stir zone obtained from simulation and experiment. It is shown that there
is a very good match between the predicted shape of the stir zone and the experimental results. The
rotation of the tool shoulder promotes the material flow onto the top surface. So the material
deformation near the top surface is higher than the one near the bottom. This is the reason for the
conic shape of the stir zone.

(a)

(b)

Figure 5: Shape of the stir zone: (a) simulation result and (b) experimental result

International Journal of Materials and Mechanics Engineering, Vol. 2, No. 4

Conclusions
A fully coupled thermo-mechanical 3D FE modelling of the FSSW process has been developed
by using a DEFORM code. The distribution of the temperature, effective strain and the material
flow can be obtained by numerical simulation. The following conclusions can be drawn from this
work:
(1) The temperature history at the measured points agrees very well between the simulation and the
experiment. The temperature at the contact surface between the tool and work piece is almost
constant at 523 C and gradually decreases towards both the bottom and edge of the work piece.
The peak temperature also appears at the contact surface and the value is equivalent to 0.86 Ts
(Ts is the melting temperature of AA6082-T6 in Kelvin).
(2) During the FSSW process, the effective strain is extremely high due to the high rotational speed.
There is severe plastic deformation in the stir zone. The region with high effective strain is
mainly located at the periphery of the pin.
(3) The material flow can be obtained through the point tracking method. The particle near to the
pin moves more vigorously compared with particles at larger distances. The material in the weld
centre mainly experiences the compression process while the material under the tool
experiences both compression and shear. The material deformation is related to the
microstructure evolution and therefore determines the width of the stir zone.
Acknowledgement
The K-Project Network of Excellence for Joining Technolgoies JOIN4+ is fostered in the
frame of COMET - Competence Centers for Excellent Technologies by BMVIT, BMWFJ, FFG,
Land Obersterreich, Land Steiermak, SFG and ZIT. The programme COMET is handled by FFG.
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