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Resistance and friction stir spot welding of
DP600: a comparative study
M. I. Khan*
1
, M. L. Kuntz
1
,P.Su
2
, A. Gerlich
2
, T. North
2
and Y. Zhou
1
Efforts to reduce vehicle weight and improve crash performance have resulted in increased
application of advanced high strength steels (AHSS) and a recent focus on the weldability of
these alloys. Resistance spot welding (RSW) is the primary sheet metal welding process in the
manufacture of automotive assemblies. Friction stir spot welding (FSSW) was invented as a novel
method to spot welding sheet metal and has proven to be a potential candidate for spot welding
AHSS. A comparative study of RSW and FSSW on spot welding AHSS has been completed. The
objective of this work is to compare the microstructure and mechanical properties of Zn coated
DP600 AHSS (1.2 mm thick) spot welds conducted using both processes. This was
accomplished by examining the metallurgical cross-sections and local hardnesses of various
spot weld regions. High speed data acquisition was also used to monitor process parameters and
attain energy outputs for each process. Results show a correlation found among microstructure,
failure loads, energy requirements and bonded area for both spot welding processes.
Keywords: Resistance spot welding, Friction stir spot welding, Advance high strength steel, DP600, Microstructure, Mechanical properties
Introduction
Interest in the application of advance high strength
steels (AHSS), particularly within the automotive
architecture, has resulted in increasing demands for
reliable spot welding methods. The mechanical and
metallurgical changes in AHSS after the resistance spot
welding (RSW) operations are well documented in the
literature;
1–3
however, much work is left to be done. The
feasibility of automotive joining of AHSS using friction
stir spot welding (FSSW) has been recently consid-
ered,
4,5
but the performance of welded joints is still
generally unknown. The increased use of AHSS grades
in the automotive architecture has emphasised the need
to examine how FSSW joining directly compares with
RSW.
Advance high strength steels sheet has been intro-
duced into auto body closures and suspension compo-
nents resulting in a recent focus on the weldability of
these alloys.
6
Resistance spot welding is the primary
sheet metal welding process in the manufacture of
automotive assemblies. The embedded infrastructure
coupled with its superior surface finish makes RSW the
economically and aesthetically desirable process. The
microstructure of DP600 grade AHSS results in
mechanical properties that are ideal for automotive
applications with a high strength to weight ratio;
however, microstructural changes during RSW drama-
tically affect mechanical properties by transforming the
base metal (BM) microstructure. To date, the micro-
structures and failure mechanisms in resistance spot
welded DP600 have not been examined in great detail.
This is essential to the integration of dual phase sheet
material in today’s automobiles. For example, inter-
facial fracture, which is believed to have detrimental
effects on the crashworthiness of vehicles, is a common
occurrence when resistance spot welding dual phase
steels.
7
The friction stir welding process was developed by
TWI, Abington, UK
8
in 1991 as a novel method for
joining aluminium alloys. Since that time the welding
process has been employed in aerospace, rail, auto-
motive and marine industries for joining aluminium,
titanium, magnesium, zinc and copper alloys, as well as
steel and thermoplastics in thicknesses ranging from 1 to
50 mm. During FSSW the rotating tool penetrates the
sheets being welded and is then retracted producing a
stir zone region that comprises a fine dynamically
recrystallised microstructure. Experimental results in
aluminium and magnesium alloy sheets have shown that
a combination of plastic deformation and viscous
dissipation during spot welding produces very high
power densities (y10
10
Wm
23
),
9,10
heating rates from
275 to 400 K s
21
depending on the rotational speed
used
11
and peak temperatures, which approach the
solidus temperature of the material being fabricated.
12,13
The resistance and friction stir spot welding processes
are leading candidates for spot welding AHSS. The
objective of this work was to compare the microstruc-
tural features of welds produced using these two
processes. Metallurgical cross-sections and local hard-
ness of various spot weld regions were examined. Data
1
Centre for Advanced Materials Joining, University of Waterloo, 200
University Ave. Waterloo, ON N2L 3G1, Canada
2
Department of Materials Science and Engineering, University of Toronto,
184 College Street, Toronto, ON M5S 3E4, Canada
*Corresponding author, email Ibraheem@rogers.com
ß2007 Institute of Materials, Minerals and Mining
Published by Maney on behalf of the Institute
Received 8 June 2006; accepted 10 September 2006
DOI 10.1179/174329307X159801 Science and Technology of Welding and Joining 2007 VOL 12 NO 2175
acquisition (DAQ) systems were used to measure the
energy input required to achieve desired failure loads for
both processes. The fracture mechanisms in overlap
tensile shear testing were studied and the relationship
among peak failure load, energy input and bonded area
was determined. The comparison of AHSS weld proper-
ties enables a benchmark for the evaluation of the weld
performance of both the RSW and FSSW processes,
which is important in weld process selection and welding
procedure design.
Experimental procedure
The chemical properties of the material used in this
study are summarised in Table 1.
Resistance spot welding
The RSW samples were produced using a pneumatically
operated single phase RSW machine (CenterLine Ltd,
250 kVA) with constant current control and a frequency
of 60 Hz. A truncated class 2 electrode with 6.0 mm face
diameter was used as per AWS standards for 1.22 mm
thick sheet.
14
Cooling water flowrate and hold time also
followed AWS recommendation of 4 L min
21
and
5 cycles respectively. The RSW machine was fully
equipped with a DAQ system capable of recording
load, displacement (¡0.01 mm), current and voltage
simultaneously as a function of time. A linear transducer
mounted to the top electrode measures the displacement
while a calibrated coil collects the dI/dt, which is
conditioned to attain current as a function of time.
The load cell located under the bottom electrode
measures the force applied by the overhead cylinder.
The data acquisition rate was 25 000 points s
21
(pps).
Figure 1 shows the typical DAQ output for a resistance
spot weld.
Resistance spot welding samples were produced over a
range of force, current and time parameters. A weld-
ability test was conducted to determine weld lobes which
produce acceptable weld quality as determined by AWS
standards.
12
The weld current was varied from 7 to
9 kA, the weld force ranged from 3.5to5
.5 kN, and the
weld time was between 10 and 20 cycles. The weld
parameters optimised for tensile shear strength and
button size were 8 kA, 3.5 kN and 20 cycles. The weld
samples were subjected to overlap tensile shear testing,
coach peel testing and metallographic examination. A
total of 11 tests were conducted per condition including
five tensile tests, five peel tests and one sample for
metallographic preparation.
The energy supplied Q
RSW
during RSW is a product
of the weld power and time, where the weld power is a
product of the measured current Iand voltage V. In this
work, Q
RSW
was calculated using equation (2) where
1(n(Nand Dtis the sampling time
15
QRSW~X
N
n~1
I(n)
jj
:V(n)
jj
Dt(1)
Friction stir spot welding
Friction stir spot welding welds were produced using a
StirSpot welder (Friction Stir Link Inc.). Capabilities of
this particular machine include tool rotational speed of
up to 3000 rev min
21
, an axial load of 14 kN and
plunge rates (tool displacement rate) from 0.1to
25 mm s
21
. The W–25 wt-%Re tool used for the spot
welds had a truncated cone geometry, a pin diameter of
4to5
.1 mm, a pin length of 1.7 mm, and a shoulder
diameter of 10 mm.
Samples were produced for mechanical testing used
using a range of plunge rate and plunge depth settings.
The plunge rates were 0.5 and 1 mm s
21
while the
plunge depths varied from 1.7to2
.1 mm. In all cases the
tool penetration depth was measured using a linear
transducer with an accuracy of ¡0.01 mm while the
spindle revolution per minute was measured using a
shaft encoder, which had an accuracy of
¡30 rev min
21
. The axial load and the torque values
were measured using a six axis load cell, which was
coupled with a DAQ system so that the axial force,
torque and penetration depth values were recorded
simultaneously during each spot welding operation.
Figure 2 shows the typical output produced during
FSSW of the DP600 sheet. For an explanation of the
observed changes in axial force and torque during the
FSSW operation the literature should be referenced.
12,16
During linear friction stir welding, where the rotating
tool is traversed across the joint, inert gas is required to
Table 1 Material properties of DP600
Steel Thickness, mm Coating
Alloying elements, %
CMn Mo Cr Si
DP600 1.22 Hot dip galvanised 0.11
.523 0.195 0.197 0.156
1 Data acquisition output for RSW
Khan et al. Resistance and friction stir spot welding of DP600
Science and Technology of Welding and Joining 2007 VOL 12 NO 2176
shield the weld. Experimental results have shown that
shielding gas is not required for spot welds where the
tool is stationary (i.e. does not traverse parallel to the
sheet). There was no difference between welds with and
without Ar gas shielding, except for minor oxidation of
the keyhole and upper surface of the weld occurring
after tool retraction upon completion of the weld. After
this, no gas shielding was used for the samples prepared
for metallographic inspection and mechanical testing.
The energy applied during FSSW (Q
FSSW
) is the sum
of the normal and vertical components. Equation (2)
was used along with the experimentally measured
normal force Fand displacement x; and, the axial
torque Tand angular velocity v.
17
The sampling time
was Dtand 1(n(N
QFSW~X
N
n~1
F(n):x(n){x(n{1)½zX
N
n~1
T(n):v(n):Dt(2)
During metallographic examination all test sections
were etched using Lepera’s reagent to distinguish the
different phases in the fusion zone (FZ), heat affected
zone (HAZ), stir zone and thermomechanically affected
zone (TMAZ) for both RSW and FSSW processes.
18
When this particular etchant is used, martensite is etched
white, a-ferrite is tan coloured and bainite is black.
Joint mechanical properties were evaluated by mea-
suring the peak load to failure during overlap tensile
shear testing. Table 2 shows specimen dimensions. Care
was taken to maintain coplanar alignment during
mechanical testing. The fracture surfaces of broken
overlapped shear test specimens were examined using
SEM fractography. Detailed examination of failure
mechanism was also facilitated by interrupting the
loading cycle during overlap shear testing, i.e, by halting
the tensile testing machine when the welded section had
only partially failed. This technique allowed detailed
examination of the nature of failure propagation during
failure of particular joints. Finally, the projected cross-
sectional area of the bonded region was measured by
digital image analysis.
Microhardness testing of DP600 steel sheet was
carried out using a Shimadzu HMV-2 Vickers micro-
hardness testing machine with a 200 g load and a
holding time of 10 s. Microhardness testing with 0.2mm
grid spacing revealed the hardness distribution and the
individual hardness values in selected regions of welded
joints. Different sample were used for RSW and FSSW
welds in order to accommodate the sample fixtures used.
Other work has shown that increasing the sample
dimensions in FSSW has no effect on properties.
Results
Resistance spot welding
A representative RSW weld cross-section is shown in
Fig. 3 (8 kA, 3.5 kN, 20 cycles). The FZ, HAZ and BM
can be clearly observed. Microstructural observations of
these regions are shown in Fig. 3a–d. A hardness profile
of the weld region is shown in Fig. 4. This profile shows
the hardness in the BM, HAZ and FZ regions.
The BM in Fig. 3ashows the typical finely dispersed
martensite particles (white) surrounded by a ferrite
matrix (fawn) that are characteristic of the automotive
dual phase steels. Peak temperatures during welding in
this region are typically below the martensite tempering
temperature (i.e. ,200uC). Hardness values in the BM
range from 150 to 200 HV, which is an indication of the
mainly ferritic nature of the microstructure.
Table 2 Specimen dimensions
Specimen Length, mm Width, mm
RSW 120 40
FSSW 100 25
abase metal; bHAZ; ccoarse grain region; dfusion
zone
3 Microstructure for different weld sites in RSW
2 Data acquisition output for FSSW
Khan et al. Resistance and friction stir spot welding of DP600
Science and Technology of Welding and Joining 2007 VOL 12 NO 2177
In the HAZ, the volume fraction of martensite
increased. The peak temperature during welding in this
region ranges from martensite tempering temperatures
to just below the liquidus. Figure 3bshows a transitional
region from the intercritical (IC) to the fine grained
region (FG) within the HAZ. Peak temperatures in the
IC region are between the Ac1 and Ac3, resulting in a
coarsening of the martensite phase. Within the FG
region, temperatures exceed the Ac3 resulting in
complete austenitisation. The austenite is inhomogenous
owing to the nature of segregation within the DP
microstructure and short time above Ac3; resulting in
the banding nature of martensite and the formation of
fine grained ferrite. Hardness values in this area exceed
that of the BM, 230–280 HV, which indicates an
increase in the volume fraction of martensite with a
ferrite matrix.
Within the HAZ, the peak temperature is well above
Ac3, resulting in complete austenitisation and grain
growth. The grain coarsened (GC) region consists of
prior austenite grains about 10–15 mm in diameter. The
microstructure in the GC region is blocky martensite, as
shown in Fig. 3c, with a hardness in excess of 350 HV
(Fig. 4).
The FZ shown in Fig. 3dis characterised by the
columnar nature of solidification. The microstructure
consists of large equiaxed columnar martensite grains.
From Fig. 4 it can be seen that hardness values are
similar to those in the HAZ region, ranging above
350 HV.
Friction stir spot welding
Figure 5 shows the microstructural features observed at
different locations relative to the keyhole centreline in a
FSSW spot weld in DP600 sheet. The BM was similar to
the RSW case, with martensite islands in a ferrite matrix
with a hardness up to 200 HV. The IC region of the
HAZ, as shown in Fig. 3b, is significantly wider than the
RSW case owing to the longer weld times. The hardness
in this region was y220 HV, as shown in Fig. 6.
Beyond the HAZ and towards the keyhole, in the
TMAZ region the microstructure is comprised of a
mixture of lath martensite and fine acicular ferrite, as
shown in Fig. 5c. This region is subject to temperatures
above Ac3 and high strain rates, resulting in dynamic
recrystallisation and grain growth. The prior austenite
grain size in this location was markedly increased. The
4 Microhardness maps of RSW cross-section
abase metal; bHAZ; cTMAZ; dstir zone
5 Microstructure for different weld sites in FSSW
Khan et al. Resistance and friction stir spot welding of DP600
Science and Technology of Welding and Joining 2007 VOL 12 NO 2178
hardness in the TMAZ was y300 HV (Fig. 6). In
addition to the lathy martensitic microstructure, fine
particles or rods of martensite which are ,1mmin
diameter are observed, which will be the subject of
further communications.
Immediately beside the keyhole periphery the stir zone
microstructure is comprised of very fine grain martent-
site that could not be observed using optical microscopy
(Fig. 5d). The grain size in the stir zone of friction stir
spot welds are typically ,10 mminFSSW.
12
The
hardness in this particular location was y350 HV, as
shown in Fig. 6.
The top surface of the FSSW welds shows a poor
finish with a keyhole resulting from the pin, and an
indentation surrounded by expulsion, or debris, caused
by the tool shoulder. Discoloration of the surface is a
result of oxidation; however, this may be prevented by
Ar shielding if desired. Figure 7bshows a typical surface
for FSSW. Compared with FSSW, the typical surface
appearance for RSW is considered more acceptable for
automotive applications. As shown in Fig. 7a,the
surface is smooth with a slight indentation and
discoloration of the galvanised coating resulting from
the thermal effects of the welding process.
Tensile shear testing
Cross-sections of partial tensile shear test specimens can
be used to observe failure propagation. Figure 8 shows a
partially failed tensile shear test result for a RSW sample
welded under the condition of 8 kA current, 3.5kN
force and 20 cycles weld time. The fracture extends from
the faying surface interface at the fusion boundary into
the FZ along the weld centreline. Figure 9 shows an
FSSW cross-section along the side where crack initiation
occurs for partial tensile test specimen. This weld
was produced under optimal welding condition
(3000 rev min
21
,2
.1 mm penetration, 0.5mms
21
plunge rate). The fracture initiates from the tip of the
unbonded interface, and propagates through the stir
zone in the upper sheet.
Figure 7c–fshows the fracture surfaces produced in
failed overlap shear testing specimens made using high
and low energy input values. Low and high energy
inputs produced an interfacial fracture through the FZ
of the resistance spot weld (Fig. 7cand e). When a low
energy input was applied during FSSW the welded joint
failed across the weld zone at the interface of the two
sheets.When high energy inputs were applied the mode
of failure involved a partial pull-out (Fig. 7f). There was
6 Microhardness maps of FSSW cross-section
ab
cd
ef
aRSW weld surface; bFSSW weld surface; clow
energy RSW input; dlow energy FSSW input; ehigh
energy RSW input; fhigh energy FSSW input
7 Weld and fracture surface for RSW and FSSW
8 Partial tensile of RSW cross-section
Khan et al. Resistance and friction stir spot welding of DP600
Science and Technology of Welding and Joining 2007 VOL 12 NO 2179
no change in failure mechanism observed with changes
in heat input.
Discussion
Microstructure and hardness
The microstructure and hardness in the weld regions of
both RSW and FSSW had similar microstructural
features. The IC and FG regions of the HAZ consist
of a mixture of martensite and ferrite. The hardness in
these regions reaches up to 280 HV.
The FSSW weld contains unique regions that are not
found in the RSW case. The RSW weld microstructure
shows a CG region beside to the fusion boundary. The
microstructure is predominately martensite as indicated
by the high hardness values (350 HV). The FSSW weld
microstructure contains a region termed the TMAZ
beside the HAZ. This region consists of a mixture of
hard martensite rods, lathy martensite, and bainite or
acicular ferrite. The hardness in this region is 300 HV,
slightly lower than the CG region in the HAZ owing to
the microstructure mixture. In both the TMAZ of
FSSW and CG regions of the RSW welds respectively,
the prior austenite grain size is large.
The FZ in the RSW weld consists of a hard
martensitic columnar microstructure in excess of
350 HV. This compares well with the fine grained
FSSW stir zone microstructure hardness of 350 HV.
There is a gradual transition in hardness from the stir
zone through the TMAZ and into the HAZ of the
FSSW weld (Fig. 6). The hardness gradient is much
steeper in the RSW weld as a result of the smaller HAZ
size (Fig. 4). The heat input for the FSSW process is
higher than the RSW process, resulting in a HAZ that is
significantly larger than the RSW weld. The hardness
gradient and size of the HAZ have no effect on weld
tensile properties at low strain rates.
The keyhole produced by the stir tool decreases the
bonded area for a given weld size. This necessitates a
comparison of the weld size based on bonded area rather
than weld diameter. The bonded area as a function of
total weld energy is shown in Fig. 10. There is a clearly
increasing trend in weld area with increasing weld
energy. Furthermore, the ratio of weld area to total
energy is similar for both the RSW and FSSW processes.
This shows that the weld efficiency of both processes is
similar.
Fracture analysis and mechanical properties
Taking into account the unbonded region located near
the TMAZ, both FSSW and RSW welds are bonded
with a zone consisting predominately of martensite, with
hardness values .350 HV. Partial tensile results from
Figs. 8 and 9 show the location of fracture initiation
within the weld. From Fig. 8 the fracture for RSW welds
initiates at the interface and continues through the
coarse grain region towards the centreline structure.
This sequence indicates fracture occurring through a
brittle median. A similar brittle path is followed with
FSSW welds where the crack initiates at the tip of the
unbonded region and propagates through the stir zone
under the shoulder (Fig. 9aand b).
Figure 11 shows the relationship between the failure
load during overlap shear testing and the bonded area in
completed welds. The failure load increased when the
aas welded; bpartial fracture
9 Partial tensile of FSSW cross-section
10 Bonded area versus energy
Khan et al. Resistance and friction stir spot welding of DP600
Science and Technology of Welding and Joining 2007 VOL 12 NO 2180
bonded area increased for both processes. Similar test
output has been found during FSSW of both Al alloy
and Mg alloy sheet materials.
14
This indicates similar
material properties in terms of tensile strength and
hardness within the stir zone and FZ for FSSW and
RSW welds respectively.
Figure 12 shows the relationship between energy
input during FSSW and the failure load during overlap
shear testing. Higher failure loads were produced when
the energy input during spot welding increased. In this
connection, it is worth noting the parallel relationship of
energy input and failure load between the two welding
processes. Differences in bonded area of fracture
surfaces of high and low energy welds are shown in
Fig. 7c–e. Friction stir spot welding welds produced
both pull-out and interfacial fracture for high and low
energy input welds.
The bonded area seems to provide a reasonable basis
for comparing the RSW and FSSW outputs (Fig. 11). A
similar trend is observed between RSW and FSSW
welds. Peak failure loads of 15 and 11 kN were obtained
for RSW and FSSW welds respectively. Owing to the
geometry of the unbonded regions failure loads are
greater for RSW welds. Energy requirements also show
a comparable trend (Fig. 12). Resistance spot welding
welds show a relatively linear relationship with energy
and failure loads. With lower energy input FSSW welds
show a non-linear relationship. This is likely due to the
change from only pin penetration to a combination of
pin and shoulder penetration. Higher failure load energy
inputs are relatively similar to RSW energy inputs.
Additional factors
Aesthetically, the surface finish of welds produced using
RSW are superior to those produced using FSSW. In
some automotive application the smooth surface finish
produced by RSW is desired, and the FSSW keyhole
would be considered unacceptable. Recent advance-
ments in tool design have produced a retractable pin
tool (RPT) which refills the remaining keyhole without
affecting the mechanical properties of the spot weld.
19
This, however, increases the cost and complexity of
FSSW equipment. Discoloration of spot welds in steel is
not considered a problem, because the surface is
typically finished by painting. If required, oxidation of
the surface in FSSW can be prevented by adequate
shielding using inert gas, but this too increases the cost
of the process.
Prevention of galvanic corrosion for this particular
sheet of DP600 is facilitated with a hot dip galvanised
zinc coating. Studies have shown during RSW of Zn
coated steel annular zinc braze forms at the faying
surface around the FZ.
17
Furthermore during the first
two cycles of welding the molten Zn coating is pushed
away from the FZ.
20
The mechanical nature of FSSW
limits interference of Zn coatings during FSSW.
Previous studies show the peak temperatures reaching
0.9T
m
of the BM,
12,13
which in the case for DP600 is well
above the melting temperature of Zinc. Figure 7cand d
shows evidence of an annular braze forming around the
RSW and FSSW weld.
The effect of the zinc coating on the RSW process is
known to significantly reduce electrode contact tip life.
Studies have shown that electrode degradation may be
delayed through the use of conductive coatings that
prevent alloying between the copper electrode and zinc
coating.
21
The net result of the zinc coating is an increase
in consumable cost. The FSSW process does not appear
to be affected by this as there are no studies to show an
observable effect of the zinc coating on tool wear.
Results from the energy calculations show that energy
consumption is similar for both processes. Faster cycle
times in RSW result in a greater productivity than
FSSW. Owing to the widespread implementation of
RSW, the infrastructure and support system is already
in place. The FSSW process is not yet used extensively in
the automotive industry, and as a result the infrastruc-
ture and support is not yet in place. Furthermore, the
FSSW process requires additional fixtures owing to its
mechanical nature. At the moment RSW is a more cost
effective process; however, further advancements in
FSSW will make it more economically competitive.
It must be stressed at the outset that the objective in
this particular study involves comparing the factors
which determine the overlap shear strength properties of
11 Failure load versus bonded area
12 Failure load versus total energy
Khan et al. Resistance and friction stir spot welding of DP600
Science and Technology of Welding and Joining 2007 VOL 12 NO 2181
RSW and FSSW spot welded joints in dual phase steel.
A single pulse welding schedule was used in the RSW
case, and optimising the welding schedule to minimise
centreline growth of the columnar grains may further
improve mechanical properties. Also a smooth pin
FSSW tool design was used when welding dual phase
sheet a limited range of welding parameter settings. As a
result, the output produced in this paper should be
regarded only as a nominal starting point for future
research. The fracture loads produced during overlap
shear testing of spot welded dual phase sheet should
therefore are not indicative of the highest values which
can be obtained when the steel sheet is spot welded.
Conclusions
In this study, mechanical and metallurgical properties of
RSW and FSSW DP600 welds were compared. A
correlation was found among failure loads, energy
requirements and bonded area for both processes.
Also, partial tensile shear can be used to understand
the initiation and propagation of cracks. From this it
can be concluded that:
1. The microstructure of the HAZ is similar in both
RSW and FSSW welds in DP600 AHSS. The IC and FG
subregions consist of a mixture of martensite and ferrite.
The martensite occurs as islands in a ferrite matrix, with
increasing martensite volume fraction towards the weld
centreline.
2. The FSSW weld microstructure consisted of a
TMAZ between the HAZ and the stir zone. This region
consists of a mixture of lathy martensite, bainite and
ferrite. Martensite is observed in both the FZ and stir
zone of the RSW and FSSW welds respectively. The
morphology of the microstructure, however, is very
different for both processes.
3. The microstructure hardness is similar in the FZ
and stir zone for RSW and FSSW respectively. The
hardness decreases from the weld centreline into the
BM; however, the HAZ in RSW case was narrower.
4. In the case of RSW, fracture initiates between the
two sheets and propagates through the interface of the
material. For the FSSW welds, fracture initiates at the
unbonded region and propagates through the upper
sheet just under the shoulder, suggesting a need for
further optimisation of tool geometry.
5. The failure load increased when the bonded area
increased for both processes. Failure loads also
increased with increasing energy input into the weld.
6. The weld efficiency of both processes is similar for
DP600 sheet steel when compared on a basis of fracture
load versus energy or bonded area.
Acknowledgement
The authors would like to acknowledge the funding
and support from Auto21 (www.auto21.ca), one of the
Networks of Centres for Excellence supported by the
Canadian Government.
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