2007-01-1704
Fatigue behaviour of friction stir joined aluminium alloy
NG5754 and AA6111 sheets
L. Han*, N. Blundell
Warwick Manufacturing Group, University of Warwick, Coventry, CV4 7AL, UK
Z. Lu, M. Shergold
Jaguar & Land Rover, Gaydon Engineering centre, Lighthorne, CV35 0RR, UK
A. Chrysanthou
School of Aerospace, Automotive and Design Engineering, University of Hertfordshire, Herts, AL10 9AB, UK
ABSTRACT
A study examining the fatigue behaviour of spot friction
stir joined (SFJ) aluminium NG5754 and AA6111 sheets
was carried out. Different friction stir holding times were
applied to the NG5754 alloy, whilst paint bake cycles
were applied to the AA6111 joints in order to examine
the effects of these variables on the fatigue behaviour of
the joints. Static and fatigue tests were performed on all
samples and the results were compared. It was
demonstrated that the material flow characteristics were
affected by the holding time leading to different joint
attributes, which in turn contributed to different fatigue
behaviour. Paint bake cycle affected the fatigue life,
particularly at low applied loads.
INTRODUCTION
The increase in use of light weight materials, in
particular aluminium alloys, for vehicle body applications
has impelled the development of new joining techniques.
Traditional joining methods such as spot-welding and
arc-welding are being challenged. Self-piercing riveting,
as one of the alternatives to spot-welding, has been
introduced and used in vehicle body assembly [1]. Its
superior advantages such as, no pre-drilled hole,
capable of joining similar or dissimilar materials and
combinations have attracted considerable interest from
the automotive industry. However, the use of rivets adds
weight and cost to the process. Manufacturers are
therefore looking for other alternatives. Spot Friction Stir
Joining (SFJ) as another alternative to spot-welding has
thus emerged. SFJ is derived from the established
technology of linear Friction Stir Welding (FSW), which
was invented at The Welding Institute (TWI) in 1991. It is
a solid phase process and particularly suitable for joining
lightweight sheet materials, such as aluminium, copper,
lead and plastics to produce straight-line welds. The
principle of the SFJ process is similar to FSW and has
all the advantages of the process. A backing anvil is
used to replace the backing plate for FSW. The pin is
rotated and then plunged into the sheet material to be
joined. Instead of moving in a straight-line making a butt
joint, the pin is kept in the same place as it is
continuously rotating. At the same time a relatively high
pressure is applied between the pin and anvil to hold the
work-pieces together. Frictional heat is generated and
the materials are therefore heated and softened. The
high pressure brings the softened material together as
the material flows. The pin is released and a permanent
point joint is formed. There are four major stages that
can be simply described as Rotate, Plunge, Hold and
Release, involved in the process, as shown in Fig. 1.
The process is attracting interest from the automotive
industry due to a number of advantages including low
cost, low power demand, no emission of fumes and
radiation [2, 3, 4]. In addition, as a solid-phase joining
method, many of the problems associated with liquidphase welding of aluminium alloys are avoided in SFJ.
The surface oxide layer can be effectively broken and
dispersed throughout the weld due to the weld action.
SFJ has been reported to join thin sheet material for
automotive closures [5]. It is also reported by Lin et al
that a fatigue crack growth model based on the Paris law
was created and adopted to predict the fatigue lives of
spot friction stir joined AA61111-t4 sheets for automotive
application [6, 7]. However, as an emerging technology,
the mechanical behaviour of SFJ joints has not been
well understood, in particular the effect of the process
variables on the fatigue behaviour of such joints.
Freeney et al [8] reported the effect of the rotation rate
and plunge depth on the static behaviour of spot friction
stir joined 5052 aluminium alloy. The present authors
reported the effect of paint bake cycle on static
behaviour of such joints [9]. The aim of the study that is
reported here was therefore to examine the effect of
holding time and paint bake cycles on the fatigue
behaviour of SFJ joints.
Rotate
Plunge
Hold
was employed as the applied load. The stress ratio was
R=0.1 and a frequency of 5~15Hz were used. Three
load levels that had maximum axial loads ranging from
50-80% of the static ultimate shear load were used. In
total 12 samples were tested for each group. The failure
criterion was that the specimen reached 1.0mm
extension or to 2,000,000 cycles, which was defined as
run out.
Release
Figure 1: Schematic Spot friction stirring process
EXPERIMENTAL PROCEDURES
The materials used for this study were NG5754 and
AA6111 aluminium alloy sheets in 2.0mm thickness. The
NG5754 alloy sheets were previously pre-treated with a
thin chromate-free film and a wax-based solid lubricant.
The AA6111 alloy sheets were in the T4 condition and
had a mill finish surface only. The nominal compositions
and mechanical properties of the NG5754 and AA6111
aluminium alloy sheet materials are listed in Table 1 and
2 respectively. All the samples were made using a robot
mounted SFJ machine built by Kawasaki Heavy
Industries and supplied by Kawasaki Robot UK,
featuring 2.2kW motors for both the rotation and plunge
of the weld tool. The welding is controlled via a force
control system, where a programmed force is applied for
a pre-determined amount of holding time. The weld tool
used was a Kawasaki standard SFJ tool designated
‘KN07’. All samples were manufactured using two
coupons with dimensions of 40mm x 100mm. The
specimen dimensions for both lap shear and fatigue
tests are the same and are shown in figure 2. For the
NG5754 alloy sheets, three groups of samples were
created and designated as 4s, 5s and 6s, that simply
indicated the holding time of 4, 5 and 6 seconds
respectively. The holding pressure and rotation speed
for all three groups of samples remained the same. Thus
the results obtained from the three groups of samples
would only reflect the effect of holding time on the
fatigue behaviour of the joints. For the AA6111 alloy
sheets, one group of samples was manufactured under
an identical setting condition and then it was divided into
two groups. One group of samples was subjected to a
paint bake cycle at 180°C for 30min, whilst the other
group was not. At least 5 samples were tested in the
shear condition from each group in order to obtain the
ultimate shear load under static conditions. The strength
values presented in the paper are average values of the
5 samples. Fatigue tests were carried out on an Instron
servo-hydraulic test machine with a load capacity of
10kN at room temperature. A cyclic tension–tension load
with a sinusoidal waveform and constant load amplitude
Figure 2: Geometry and dimensions of shear and
fatigue samples
Table 1: Compositions and mechanical properties of
NG5754 alloy
Young’s
Modulus
(GPa)
70
Si
0-0.40
MECHANICAL PROPERTIES
Tensile
Elongation
Hardness
strength
(HV)
(MPa)
240
22%
63.5
nominal composition(Balance Al)
Fe
Cu
Mn
Mg
0-0.40
0-0.10
0-0.50
2.60-3.60
Table 2: Composition and Mechanical Properties of
AA6111
Young’s
Modulus
(GPa)
70
Si
0.201.70
MECHANICAL PROPERTIES
Tensile
Elongation
Hardness
strength
(HV)
(MPa)
308
26%
93
Nominal compositions
Fe
Cu
Mn
Mg
Al
0.70
0.90
0.80
0.101.40
Balance
RESULTS AND DISCUSSION
Figure 3 shows the shear test results for the NG5754
and AA6111 samples. For the NG5754 samples, the 5s
group had the highest shear strength, whilst the 4s
group exhibited the lowest strength of all three groups.
In addition, the 5s group had the lowest strength
variation and the 6s group had the greatest strength
variation of the three groups of samples. These results
indicated the effect of holding time on shear behaviour
and the longer holding time does not necessarily provide
the joints with higher shear strength. For the AA6111
group, the baked samples had higher shear strength
than the unbaked samples indicating that the paint
baking cycle led to an increase in the shear strength of
the SFJ joints.
Shear Ultimate Load (N)
4500
4s
5s
6s
Baked
Unbaked
4000
3500
3000
2500
2000
calculated at any load amplitude. At a load amplitude Fa
of 0.5kN, the baked samples could sustain 1,166,090
cycles before failure, whilst the unbaked samples could
reach 1,622,631 cycles until failure. The paint bake cycle
led to a 28% reduction in the fatigue life of the samples
at this load level. However, at 1.0kN, following paint
baking, the fatigue life was increased by 10% from
50,269 to 55,179 cycles. It has to be noted here that,
statistically, the experimental data did not differ
significantly. Although based on the power law, the
fatigue life for the two groups can be calculated at any
load amplitude. Due to the feature of scattered data for
fatigue test and the use of progression method for the
power law, the calculation can only indicate a trend.
1500
1000
500
0
NG5754 samples
AA6111 samples
Figure 3: Shear test results for NG5754 and AA6111
samples
Failure examination revealed that following fatigue
testing only one failure mode was observed for the
AA6111 samples. As shown in Figure 5, nugget failure
with an eyebrow crack formed at the top sheet was
observed for both the baked and unbaked AA6111
samples. Fretting indicated by black fretting debris was
also observed to occur for all the samples.
FATIGUE OF THE AA6111 SAMPLES
Figure 4 shows fatigue test results for the AA6111
samples. Although the fatigue data are scattered
indicating that process optimization is necessary, the
trend is that the unbaked samples had longer fatigue life
than the baked samples. The lower shear strength for
the unbaked samples is not accompanied with lower
fatigue strength. This suggested that for a SFJ joint, high
shear strength is not necessarily accompanied with high
fatigue strength and the paint bake cycles had a minor
effect on the fatigue strength.
1
Unbaked
-0.2272
Load amplitude [KN]
y = 11.949x
-0.1995
y = 8.6678x
6111(2.0)/6111(2.0) Un-baked
6111(2.0)/6111(2.0) Paint-baked
0.1
10000
100000
1000000
10000000
N [cycles]
Figure 4: Fatigue results for the AA6111 samples
Baked
As shown in Fig. 4, within the life interval of 10,000 and
2,000,000 cycles the fatigue life for both baked and
unbaked samples can be expressed by a power law:
Fa = CN m
where Fa is the load amplitude, N is fatigue life in cycles,
C and m are constants as shown in Fig. 4. Based on the
power law, the fatigue life for the two groups can be
Figure 5: Fatigue failure mode that occurred for the
AA6111 baked and unbaked samples
FATIGUE OF THE NG5754 SAMPLES
Based on the shear test results, a range of maximum
applied loads from 1.0kN to 2.7kN were adopted in
fatigue tests for the NG5754 samples. Figure 6 shows
the fatigue test results for the three groups of samples.
Similar to the AA6111 group, the data are scattered and
statistically the differences between samples with
different holding time are not significant. However, the
6s group of samples still exhibited the longest fatigue
endurance, whilst the 4s group had the shortest fatigue
life of all three groups. The increase in the holding time
led to an increase in the fatigue life, although the effect
is not significant.
and some of the 5s group of samples, the main failure
mode was fracture of the sheet material, as shown in
Fig. 7b. Fretting was also observed at the interface
between the two joined sheets for all samples following
the fatigue test.
1
Load amplitude [KN]
y = 3.9523x -0.146
y = 4.24x -0.1568
-0.1591
y = 4.2046x
Bottom sheet
Botto
Top sheet
(a)
3
12
5754(2.0)/5754(2.0) 4s
5754(2.0)/5754(2.0) 5s
9
5754(2.0)/5754(2.0) 6s
0.1
10000
6
100000
1000000
10000000
N [cycles]
Figure 6: Fatigue test results for the three groups of
samples
Figure 6 also shows evidence that within the life interval
of 10,000 and 2,000,000 cycles, the fatigue life of all
three groups of samples can also be expressed by a
power law relationship:
Fa = CN m
Based on the power law, the fatigue life for the three
groups can be calculated. At 1.0kN load amplitude, the
4s, 5s and 6s groups had a fatigue life of 8324 cycles,
10024 cycles and 12247 cycles respectively. The
increase by 1s holding time from 4s to 5s led to a 20%
increase in fatigue life, whilst from 5s to 6s increase in
the holding time resulted in 22% increase in fatigue life.
The increase in the holding time led to a corresponding
increase in the fatigue life. From 4s – 6s, the longer the
holding time, the longer the fatigue life. This is different
from what was observed during shear test, in which the
5s group had the highest shear strength, rather than 6s
group. It has to be emphasised again, similar to the
AA6111 group, due to great scatter of the data and the
use of progression method for the power law, any
increase in the fatigue life as the holding time increased
only indicated a trend.
Examination of the failed samples reveals that two
failure modes were involved following the fatigue test.
For the 4s group and most of the 5s group of samples
interface failure or nugget failure accompanied with
eyebrow crack on the bottom sheet of the joint was the
main failure mode, as shown in Fig. 7a. For the 6s group
Bottom sheet
Top sheet
(b)
Figure 7: Failure modes that occurred in
fatigue tests
Figure 8 shows the surface of a fractured nugget with
the top and bottom sheets for a sample from the 4s
group. The nugget fractured along the interface between
the two joined sheets leaving a fractured nugget in both
the joined sheets. Fretting debris appeared around the
12 o’clock position and part of the fractured nugget
around the 12 o’clock position was also covered by
fretting debris. The eyebrow crack was observed to
initiate at the 12 o’clock position and propagated
outwards. It seemed that for the nugget failure, the crack
initiated at the 12 o’clock position of the nugget and
propagated outwards along the keyhole circumference
and into the shank of the nugget. This led to fracture of
the nugget and eventually separation of the two jointed
sheets. For sheet material failure, the top sheet fractured
along the 9 and 3 o’clock positions. It seemed that the
crack initiated at the 9 and 3 o’clock positions and
propagated outwards leading to fracture of the top sheet,
whilst the other half of the nugget and part of the top
sheet remained intact, as shown in Fig. 7(b).
the baked AA6111 samples, than for the NG5754
curves. Consequently, as the applied load decreased,
the difference in the fatigue life between the AA6111 and
NG5754 samples was reduced.
12
9
3
6
Top sheet
12
9
3
6
Bottom sheet
Figure 8: Fractured surface of a 4s group of samples
COMPARISON OF FATIGUE PERFORMANCE FOR
THE NG5754 AND AA6111 SAMPLES
1
-0.2272
y = 11.949x
-0.1995
Load amplitude [KN]
y = 8.6678x
y = 4.24x -0.1568
-0.1591
y = 4.2046x
y = 3.9523x -0.146
5754(2.0)/5754(2.0) 4s
5754(2.0)/5754(2.0) 5s
5754(2.0)/5754(2.0) 6s
6111(2.0)/6111(2.0) Un-baked
6111(2.0)/6111(2.0) Paint-baked
0.1
10000
100000
1000000
10000000
N [cycles]
Figure 9: Comparison fatigue results for the NG5754
and AA6111 groups
Fig. 9 shows the comparison of fatigue strength for the
NG5754 and AA6111 samples. Although the data are
scattered as mentioned earlier, at high applied load, the
AA6111 group exhibited longer fatigue life than the
NG5754 samples. However, the S-N curves for the
AA6111 group had more negative slopes, in particular
Fatigue fracture usually occurs on a cross-sectional
discontinuity due to high stress concentration. For a spot
friction stirred joint, the keyhole creates a cross-sectional
discontinuity leading to high stress concentration around
the keyhole circumference. Fatigue cracks therefore are
likely to initiate in this region. Under the fatigue loading,
the top sheet was subjected to the highest tensile stress
at the 3 and 9 o’clock positions. This may lead to crack
initiation and eventually failure of the top sheet at the 3
and 9 o’clock positions. In addition, secondary bending,
which is an inherent feature of single lap joints like those
that were used throughout this project, also played an
important role in the fatigue failure mechanism.
Secondary bending introduces a bending stress, which
can locally exceed the axial stress by several times at
cross-sectional discontinuity to the joined sheet material,
whilst a tensile force is also introduced to the nugget.
This resulting bending stress at the cross-sectional
discontinuity combined with normal tensile stress led to
crack initiation and propagation at the cross-sectional
discontinuity, which were the 6 and 12 o’clock positions.
Additionally, the resulting tensile stress for the nugget
was the highest at the 12 o’clock position for the top
sheet and the 6 o’clock positions for the bottom sheet.
This led to crack initiation at the 12 o’clock position for
the nugget. Following the crack initiation, a new interface
at the nugget was generated. Fretting fatigue, as a kind
of wear and corrosion phenomenon, occurs when two
contacting components are subjected to an oscillatory
load. The appearance of fretting debris on part of
surface of the fractured nugget indicated the occurrence
of fretting. Under continuous fatigue loading, the crack
propagated along the circumference of the nugget and
penetrated into the shank of the nugget leading to
fretting damage and fracture of the nugget. The AA6111
alloy has higher strength than the NG5754 alloy. If the
sheet material fracture led to failure of the joints then the
AA6111 group would have higher fatigue strength than
the NG5754 group. However, from the experimental
results, most of the samples failed by nugget fracture
suggesting that the sheet material was stronger than the
nugget itself. The fact that the AA6111 group exhibited
longer fatigue life than the NG5754 group at high applied
load and failed by nugget fracture only indicated that the
nugget for the AA6111 group was stronger than the
NG5754 group. This was proved true by the shear test
results in which the AA6111 group had higher shear
strength than the NG5754 group. The high strength of
the AA6111 alloy may have contributed to this result.
However, at low applied load, the fatigue strength of the
AA6111 and NG5754 groups tended to be similar. This
may be attributed to the heat-treatable feature, which
allows participant hardening to occur during the joining
process leading to even more brittle of the nugget.
For the NG5754 group of samples, longer holding time
produced not only deeper welds and thinner upper
sheet, but also changed the position of triple point,
where the two sheets meet with the stir zone boundary.
These contributed to the different fatigue life and failure
mode. As shown in Fig. 10, for the 4s group the triple
point, as circled, was almost at the same level of the
interface between the two joined sheets. A very sharp
interface was formed leading to sharp notch effect. The
cross-section of the nugget at the interface between the
two joined sheets sustained most of the loading.
Therefore fracture of the nugget occurred leaving broken
nugget in both the joined sheets for the 4s group of
samples. In contrast, for the 6s group, this triple point
was much higher than the interface between the two
joined sheets and smoother than that for the 4s group.
The sharp notch effect was therefore minimized and a
weak point of the top sheet at the triple point was
created due to thinning of the top sheet during nugget
formation. As a result, the top sheet material fracture
occurred along the weak points leading to failure of the
joints for the 6s group of samples. The geometry for the
5s group was similar to the 4s group leading to nugget
failure.
4s
6s
Figure 10: Micrograph of etched sections of the 4s
and 6s group of samples
CONCLUSION
The results reported here are from initial primary trials.
The scattered data suggested that further process
optimization is necessary in order to improve process
sustainability. Based on this experimental result, the
following trends may be predicted:
1.
The paint bake cycle – of 180°C for 30min - had a
minor effect on the fatigue performance of the spot
2.
3.
4.
friction joined AA6111 sheets, in particular at low
applied loads.
Nugget failure was the only failure mode that
occurred for the AA6111 joints tested during this
investigation.
The holding times of 4-6s affected the fatigue life of
the SFJ joined NG5754 sheets, but the effect was
not significant during this examination.
Both nugget failure and sheet material fracture were
observed to occur during fatigue testing for the
NG5754 samples due to the effect of holding time
on material flow and geometric formation of the
nugget.
ACKNOWLEDGMENTS
The authors wish to thank Advanced West Midlands for
funding the project.
REFERENCES
1. L. Han, Y. K. Chen, A. Chrysanthou and J. M.
O’Sullivan, “Self-pierce riveting – a new way for
joining structures”, ASME, 2002; PVP-Vol.
446(2): 123-127.
2. Pan, T., Joaquin, A., Wilkosz, D. E.,
Reatherford, L.and Nicholson, J. M., 2004, Spot
Friction Welding for Sheet Aluminum Joining.
Proceedings of the 5th International Symposium
of Friction Stir Welding, Metz, France, Sep. 1416, 2004.
3. Kallee. S. W and Nicholas. E. D, “Application of
friction stir welding to lightweight vehicles”, SEA
Technical Paper, 982362, 1998.
4. Waldron. D. J, Roberts. R. W, Dawes. C. J,
Tubby. P. J, “Friction stir welding-A revolutionary
new joining method”, SAE Technical Paper,
982149, 1998.
5. Sakano R, Murakami K, Yamashita K, Hoye T,
Fujimoto M, Inuzuka M, Nagao Y, Kashiki H,
“Development of Spot FSW Robot System for
Automobile Body Members”, Proceedings of the
3rd International Symposium of Friction Stir
Welding. Kobe, 2001.
6. Lin, P.-C., Pan, J. and Pan, T., “Fracture and
Fatigue Mechanisms of Spot Friction Welds in
Lap-Shear Specimens of Aluminum Alloy 6111T4 Sheets”, SAE Technical Paper No. 2005-011247.
7. Lin, P.-C., Pan, J. and Pan, T., “Fatigue Failures
of Spot Friction Welds in Aluminum 6111-T4
Sheets Under Cyclic Loading Conditions”, SAE
Technical Paper, 2006-01-1207.
8. T. A. Freeney, S. R. Sharma and R. S. Mishra,
“Effect of welding parameters on properties of
5052 aluminum friction stir spot welds”, SAE
Technical Paper, 2006-01-0969.
9. N. Blundell, L. Han, K. W. Young and R. Hewitt,
“The influence of Paint Bake Cycles on the
Mechanical Properties of Spot Friction Joined
Aluminium Alloys”, SAE Technical paper, 2006 –
01 – 0967
CONTACT
Corresponding author:
Dr. L. Han
Warwick Manufacturing Group,
University of Warwick,
Coventry, CV4 7AL, UK
Tel: +44(0)2476575385, Fax: +44(0)2476575366,
E-mail: li.han@warwick.ac.uk