4th International Symposium on Friction Stir Welding, Park City, Utah, USA, 14-16 May 2003
FLAWS IN FRICTION STIR WELDS
A J Leonard and S A Lockyer
TWI Ltd, Granta Park, Great Abington, Cambridge, CB1 6AL, UK
Abstract
One of the major drivers for using friction stir welding for aluminium fabrication is the low
incidence of weld flaws compared to that produced by conventional arc welding. However,
the process does have its own characteristic flaws. A number of different process variables
affect the quality of a joint produced by friction stir welding: tool design, tool rotation and
travel speeds, tool heel plunge depth and tilt angle, welding gap, thickness mismatch and
plate thickness variation. Successful, reproducible welds may be produced by operating
within process “windows”. However, problems may arise when the welding conditions
deviate from the standard operating window. In such circumstances, flaws may be
generated. In the current work, a number of flaws encountered in friction stir welds, in
particular voids, joint line remnants and root flaws were generated in an Al-Cu-Mn-Si-Mg
alloy by using welding parameters outside of the established tolerance box for producing
flaw-free welds. The welds were characterised using X-ray and ultrasonic non-destructive
testing techniques and by metallographic sectioning. The causes of such features are
described and recommendations are made to prevent their occurrence.
INTRODUCTION
Flaws are inherent in most materials joining processes. For example, a number of flaw types
are associated with fusion welding of aluminium alloys, most notably weld metal porosity
(1), Fig.1, and in certain alloys, weld metal solidification cracking and heat affected liquation
cracking (2), Fig.2. Such problems have contributed to the view that some aluminium alloys,
in particular some of the 2xxx and 7xxx series alloys, are difficult to fusion weld
successfully. Friction stir welding, being a solid state process, has overcome the problems of
porosity and hot cracking encountered in fusion welds. In addition, being a largely automated
process, it is possible to produce defect-free welds reliably when operating within an
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Fig.1 Porosity in an aluminium arc weld.
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Fig.2 Hot cracking in an aluminium arc weld.
established parameter, or “weldability” envelope (3). The precise details of any operating
envelope will depend on the material being welded and the actual quality control criteria
specified. The rapid commercial application of the process is testimony to its robustness
within defined operating parameters; to date, the authors are aware of no major problems that
4th International Symposium on Friction Stir Welding, Park City, Utah, USA, 14-16 May 2003
have been reported with regard to the occurrence of defects. However, it is inevitable that, as
the process becomes more widely used, and operating conditions are pushed to their limits to
satisfy needs for improved joint properties and productivity, flaws will be introduced.
Two previous studies have reported on flaws in friction stir welds (4,5), in which tool rotation
speed and welding speed were evaluated. The authors identified void formation and a root
flaw. These were attributed to high travel speeds coupled with slow tool rotation speeds.
Inadequate tool plunge depth was attributed to the cause of the root flaw. The objective of
the current paper is to characterise the flaws that may, typically, be introduced into friction
stir welds when operating outside of the “standard” set of optimised welding parameters. This
will assist end users of the technology with an awareness of how process variation may
introduce flaws and to enable appropriate process modifications to be made to eradicate them.
The results reported were all generated on welds made in 6mm thick plate from an Al-CuMn-Si-Mg alloy 2014A, produced using altered conditions derived from, but outside of, the
original TWI development work. Specifically, flaws were generated by the variation of
welding speed, welding force (forging pressure), tool pin height and surface oxide thickness.
Table 1 Summary of X-ray, ultrasonic and metallographic inspection of each of the welds in
alloy 2014A.
Weld
W9
Welding
speed
mm/min
90
W10
90
W11
90
W12
180
Other details
Shortened pin
used
2mm shim
inserted
between
plates
Total pressure
limited to
500psi
Two large voids,
65mm and 140mm
long
-
No defects
recorded
W13
250
-
W14
300
-
W15
400
-
W17
90
20µm
anodised
surface on
plates
X-ray result
No defects
recorded
No defects
recorded
Ultrasonic
inspection result
No defects recorded
Sectioning results
Root flaw
No defects recorded
No flaws found
Two voids: No 1,
5mm deep, 45mm in
length No 2, 4.9 to
5.6mm deep 120mm
in length
No defects recorded
Large voids on
advancing side of
the weld
No defects
recorded
No defects
recorded
No defects recorded
Large void and
associated small
voids at stop end of
weld: area affected
85mm
Feint linear
indications full
weld length
Void 4.0m deep
11mm in length
No defects recorded
Intermittent
reflector full weld
length, 4.5mm deep
Root flaw + void
beneath surface of
weld
No flaws found
Small voids
beneath surface of
weld
Large void on
advancing side of
weld
Oxide inclusions
(joint line remnant)
4th International Symposium on Friction Stir Welding, Park City, Utah, USA, 14-16 May 2003
Characterisation was performed using two non-destructive techniques: X-ray radiography and
ultrasonic inspection, the latter being performed manually utilising a compression wave at
5MHz with a 10mm diameter probe; metallographic sections were also prepared to illustrate
the features found by non-destructive examination. Table 1 summarises each of the welds
produced and the flaws encountered. Three main flaw types are discussed: voids, root flaws
and joint line remnants.
VOIDS
A number of the welds contained voids, which ran along the advancing side between the weld
nugget and the remainder of the thermomechanically affected zone (TMAZ). Limiting the
forging pressure generated a void along the entire length of weld W11, which was
predominantly, although not exclusively, surface-breaking (Fig.3). If welding pressure were
to vary momentarily during welding, voids may be generated that visual appraisal alone may
not detect. A similar void in terms of appearance was generated in a weld produced at a speed
of 400mm/min, over a factor of four times faster than a conventional friction stir weld in
alloy 2014A using established technology. In this particular case, void formation was, in part,
caused by the plates moving apart and the tool lifting (Fig.4).
Advancing side
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Fig.3 A void on the advancing side of a weld (W11, mm scale).
67363_01
Fig.4a A surface-breaking void in a weld produced at 400mm/min (W15).
4th International Symposium on Friction Stir Welding, Park City, Utah, USA, 14-16 May 2003
Advancing side
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Fig.4b A surface-breaking void in a weld produced at 400mm/min (W15, mm
scale).
The location and shape of the voids provide useful information concerning the development
of the characteristic microstructure observed in aluminium alloy friction stir welds. In each
case, the voids were on the advancing side of the weld and showed a clear delineation
between the region of the nugget and the remainder of the TMAZ. The fine-grained nugget
exhibited clear scallops (Fig.5) which indicated that this region was formed by stirring and
forging material either side of the joint line. The almost vertical edge of the advancing side
of the void which was not at the original joint interface, indicates that material had been
swept away by the rotation of the tool, the final joint being then formed by forging the nugget
material into the created void. The tool shoulder plays an important part in forging the
material that has been plasticised by the rotating pin in the joint line. If insufficient forging
pressure is applied to the tool, the workpiece may ride up and prevent complete
consolidation. When welding at higher travel speeds, the material receives less work per unit
of weld length, i.e. fewer tool rotations per mm. Under such conditions, the plasticised
material may be cooler, and less easily forged by the shoulder, resulting in voids remaining
unconsolidated.
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Fig.5 A void on the advancing side of weld W11.
Bendzsak et al (6) and Colegrove (7) have developed mathematical models for material flow
during friction stir welding. Both models predict a region of transition between a region of
4th International Symposium on Friction Stir Welding, Park City, Utah, USA, 14-16 May 2003
rotational motion of material immediately beneath the tool shoulder and a region in which
material is extruded past the rotating tool pin. Bendzsak et al (6) described the motion of the
transition region as chaotic. Both models predict flow singularities in this region on the
advancing side of the weld. Bendzsak et al attributed these to be the source of weld defects
(voids). The current work supports the theoretical models in that voids were generated at the
locations in which they were predicted to occur.
Small voids were also observed elsewhere in the welds. In some instances, they were due to
inadequate forging. Others were present intermittently, below the top of the weld when the
welding speed was increased (Fig.6). The features corresponded to the region in which the
tool shoulder left a mark on the top surface, in the form of a series of advancing semi-circles.
It may be noted that all of the voids in the current work, with the exception of the small voids
below the weld surface, were detected by conventional X-ray radiography and ultrasonic
inspection. The small voids below the top of the weld may be removed by light machining of
the weld surface.
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Fig.6 A void beneath the upper surface of a weld produced at 180mm/min (W12).
Christner and Sylva (8) investigated the effect of the presence of a joint gap on the
mechanical properties of friction stir welds in 6.4mm thick alloy 2014A-T6. As in the
current work, joint gaps were achieved by the insertion of shims at each end of the weld panel
assembly. The length of weld produced and the distance between the shims was not
presented, but it was recorded that a joint gap of 2.3mm, which represented 36% of the plate
thickness, could be tolerated without a significant reduction in joint strength. A joint gap of
3.2mm, or 50% of the thickness, resulted in incomplete joint consolidation and the presence
of a cavity along the advancing side of the weld. In the current work, a joint gap of 2mm
(33% of the plate thickness) was tolerated, over a span of ~260mm, without the presence of
any discernible flaws. This is consistent with the findings of Christner and Sylva, although in
the current work the actual joint properties were not measured.
ROOT FLAWS AND JOINT LINE REMNANTS
Two principal examples of joint line remnants were generated in the current work. Firstly, by
use of a shortened pin, a root flaw was created. During the formation of the friction stir weld,
it is vital that the oxide interface between the two butting plates is adequately disrupted in
order to form a bond. The correct depth of penetration of the tool pin is essential to ensure
4th International Symposium on Friction Stir Welding, Park City, Utah, USA, 14-16 May 2003
that this occurs. Where a shortened pin is used, where the tool plunge depth is incorrectly set,
or where there is poor alignment of the tool relative to the joint line, a root flaw is produced;
the original plate interface is only partially disrupted and can still be identified on a crosssection, Fig.7b. In one weld, which essentially had the correct pin height and plunge depth, a
root flaw was still produced. In this instance, either the plunge depth cannot have been
adequate, or the joint line was badly misaligned relative to the tool axis of rotation,
generating the flaw.
Advancing side
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Fig.7a A root flaw in weld W9 (arrowed, mm scale).
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Fig.7b Higher magnification photomicrograph of weld W9, showing the root flaw (arrowed).
Such root flaws do not necessarily represent an absence of any bond, indeed some regions of
weak bonding may be present. However, work to date at TWI (9) on welds in alloy 5083H321, which contained similar features, did show a reduction in tensile properties and a loss
in fatigue strength. Therefore, such features should not be regarded as innocuous. Currently,
NDT techniques such as radiography and dye penetrant inspection, as well as visual
examination, are not reliable for detecting root flaws, and, indeed, the root flaws were not
detected in the current work. The only definitive method that is currently available is a
destructive bend test with the root in tension (10), although efforts are being applied to the
4th International Symposium on Friction Stir Welding, Park City, Utah, USA, 14-16 May 2003
development of NDT techniques that are capable of detecting them. In the case of critical
applications, machining the weld root may be an effective measure to remove such features.
A second type of joint line remnant is the distribution of oxide particles through the thickness
of the weld, Fig.8. In the current work, the oxide coating, which normally covers the surface
of aluminium, was deliberately thickened by anodising. In this instance, a visible dispersed
oxide line was produced in the weld, which was also of sufficient size to be detected by both
radiography and ultrasonic inspection. Previous work at TWI (9) produced joint line remnant
flaws in welds in alloys 5083-O and 6082-T6. In the former alloy, the feature was introduced
by increasing the welding speed, resulting in less disruption of the oxide per mm advance of
the tool; in the case of the latter, an oversize tool shoulder was employed, resulting in more
surface oxide being swept into the weld. In each of these welds the tensile properties were
not affected by the features, with the exception of a small drop in ductility in the 5083 weld.
The fatigue strength did not appear to be affected by the presence of the flaws in the welds
examined and fracture mechanics calculations were conservative in comparison with the test
results. The TWI work concluded that such a flaw resulting in a loss in mechanical
properties should be viewed as a crack and therefore not tolerable. Even if there is no loss in
mechanical properties, the feature is clearly undesirable and is best minimised by careful
machining of the butting faces to reduce the quantity of oxide prior to welding and control of
welding speed. Improvements in tool design may also be of benefit in disrupting oxidised
layers.
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Fig.8a
Advancing side
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Fig.8b Joint line remnant in weld W17 (mm scale).
PRACTICAL IMPLICATIONS
Table 2 summarises the main flaw types in friction stir welds identified in this work and other
published information. It also includes details of the factors causing their formation. It
should be noted in the case of voids produced in the current work that only some of the
welding parameters were varied, namely welding pressure, travel speed and joint fit-up; other
parameters, such as tool design and rotation speed, may also influence flaw formation.
4th International Symposium on Friction Stir Welding, Park City, Utah, USA, 14-16 May 2003
Comprehensive mathematical modelling of the welding process, as adopted by Bendzsak et al
(6) and Colegrove (7), may result in a greater understanding of how individual welding
parameters interact, permitting weld procedures to be developed without recourse to a large
matrix of test welds in order to establish a tolerance window. The Table provides an initial
resource for the identification of problems encountered in production welds, and an aid to
their eradication.
Table 2 Flaws that may be present in friction stir welds and their causes.
Flaw Type
Void
Location
Advancing side at edge of weld 1.
nugget.
2.
3.
Causes
Reduced forging pressure.
Welding speed too high.
Plates not clamped close
enough together. Joint gap
too wide.
Welding speed too high
1. Inadequate removal of oxide
from plate edges.
2. Inadequate disruption and
dispersal of oxide by tool.
3. Increase in welding speed.
4. Increase in tool shoulder
diameter.
1. Tool pin too short.
2. Incorrect tool plunge depth.
3. Poor joint to tool alignment.
Void
Joint Line Remnant
Beneath top surface of weld
Weld nugget, extending from the
root of the weld at the point
where the original plates butted
together.
Root flaw
Weld nugget, extending from the
root of the weld at the point
where the original plates butted
together.
Currently, no national or international inspection criteria exist for evaluating the quality of
friction stir welds. Although, an AWS standard is in preparation (10). Lloyds Register of
Shipping has issued guidance notes for weld qualification (11), which have been based
largely on the requirements for arc welds in British Standard BS EN288 part 4 (12). This
latter document specifies 100% visual examination, 100% radiographic or ultrasonic
inspection and 100% penetrant inspection, together with bend tests, tensile tests and
metallography. Lloyds have specified penetrant inspection on only the weld root for friction
stir welds. The acceptance levels for weld imperfections in BS EN288 Part 4 are described as
level B – stringent, as defined in BS EN30042 (13). In the current work, flaws were identified
by a combination of non-destructive and metallographic inspection techniques. No attempt
was made to determine the limits of detectability of flaws by these techniques. However, if
weld qualification standards are to be developed for friction stir welds, further work will be
necessary in determining these limits.
CONCLUSIONS
1. Voids may be formed when insufficient forging pressure is applied to the weld. Welding
too fast may also result in similar effects.
2. A joint gap of up to 2mm may be tolerated when welding 6mm thick 2014A plate without
the formation of weld flaws.
4th International Symposium on Friction Stir Welding, Park City, Utah, USA, 14-16 May 2003
3. Joint line remnants in the form of inadequately dispersed oxide may be produced if
inadequate pre-weld cleaning is performed or the welding speed is too high. Evidence
suggests that joint line remnants may be tolerated in certain circumstances, but are best
avoided. Machining the plates prior to welding and control of welding speed are effective
in restricting their appearance.
4. Joint line remnants in the form of root flaws are introduced when either insufficient pin
depth or tool plunge depth is selected for the joint, or when there is poor tool to joint
alignment. These flaws have been found to be damaging to both tensile and fatigue
properties. Appropriate selection of the above parameters may eliminate these flaws.
However, for critical applications, machining of the weld root may be advisable.
5. Cross weld tensile testing and root bend tests are recommended to identify damaging joint
line remnants and root flaws.
ACKNOWLEDGEMENTS
This work was funded by Industrial Members of TWI as part of the Core Research
Programme. The authors are grateful for the assistance of colleagues at TWI who assisted
with the work.
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4th International Symposium on Friction Stir Welding, Park City, Utah, USA, 14-16 May 2003
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