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Friction Stir Welding
1. Introduction
The difficulty of making high-strength, fatigue and fracture resistant welds in
aerospace aluminium alloys, such as highly alloyed 2XXX and 7XXX series, has long
inhibited the wide use of welding for joining aerospace structures. These aluminium alloys
are generally classified as non-wieldable because of the poor solidification microstructure
and porosity in the fusion zone. lso, the loss in mechanical properties as compared to the
base material is !ery significant. These factors make the joining of these alloys by
con!entional welding processes unattracti!e. "ome aluminium alloys can be resistance
welded, but the surface preparation is e#pensi!e, with surface o#ide being a major problem.
$riction stir welding %$"&' is a no!el solid state joining process. (ne of the main
ad!antages of $"& o!er the con!entional fusion joining techni)ues is that no melting occurs.
Thus, the $"& process is performed at much lower temperatures than the con!entional
welding. t the same time, $"& allows to a!oid many of the en!ironmental and safety issues
associated with con!entional welding methods. *n $"& the parts to weld are joined by
forcing a rotating tool to penetrate into the joint and mo!ing across the entire joint.
+esuming, the solid-state joining process is promoted by the mo!ement of a non-consumable
tool %$"& tool' through the welding joint.
$"& consists mainly in three phases, in which each one can be described as a time
period where the welding tool and the work piece are mo!ed relati!e to each other. *n the first
phase, the rotating tool is !ertically displaced into the joint line %plunge period'. This period
is followed by the dwell period in which the tool is held steady relati!e to the work piece but
still rotating. (wing to the !elocity difference between the rotating tool and the stationarywork piece, the mechanical interaction produces heat by means of frictional work and
material plastic deformation. This heat is dissipated into the neighbouring material,
promoting an increase of temperature and conse)uent material softening. fter these two
initial phases the welding operation can be initiated by mo!ing either the tool or the work
piece relati!e to each other along the joint line. $ig. illustrates a schematic representation of
the $"& setup.
$ig. $riction stir welding setup
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The $"& tool consists of a rotating probe %also called pin' connected to a
shoulder piece, as shown in $ig. 2. uring the welding operation, the tool is mo!ed
along the butting surfaces of the two rigidly clamped plates %work piece', which are
normally placed on a backing plate. The !ertical displacement of the tool is controlled
to guarantee that the shoulder keeps contact with the top surface of the work piece.The heat generated by the friction effect and plastic deformation softens the material
being welded. se!ere plastic deformation and flow of plasticized metal occurs when
the tool is translated along the welding direction. *n this way, the material is
transported from the front of the tool to the trailing edge %where it is forged into a
joint'.
The half-plate in which the direction of the tool rotation is the same as the
welding direction is called the ad!ancing side, while the other is designated as
retreating side. This difference can lead to asymmetry in heat transfer, material flow
and in the mechanical properties of the weld.
$ig. 2 "chematic illustration of the $"& process
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2. Literature Survey
$riction stir welding %$"&' was in!ented at The &elding *nstitute %T&*' of / in
00 as a solid-state joining techni)ue, and it was initially applied to aluminium alloys 1,2.
The basic concept of $"& is remarkably simple. non-consumable rotating tool with a
specially designed pin and shoulder is inserted into the abutting edges of sheets or plates to be
joined and tra!ersed along the line of joint %$ig. '.
$ig. . "chematic drawing of friction stir welding.
The tool ser!es two primary functions3 %a' heating of work piece, and %b' mo!ement
of material to produce the joint. The heating is accomplished by friction between the tool and
the work piece and plastic deformation of work piece. The localized heating softens the
material around the pin and combination of tool rotation and translation leads to mo!ement of
material from the front of the pin to the back of the pin. s a result of this process a joint is
produced in 4solid state5. 6ecause of !arious geometrical features of the tool, the material
mo!ement around the pin can be )uite comple# 1. uring $"& process, the material
undergoes intense plastic deformation at ele!ated temperature, resulting in generation of fine
and e)uia#ed recrystallized grains 1897. The fine microstructure in friction stir welds
produces good mechanical properties. $"& is considered to be the most significant
de!elopment in metal joining in a decade and is a 44green55 technology due to its energyefficiency, en!ironment friendliness, and !ersatility. s compared to the con!entional
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welding methods, $"& consumes considerably less energy. :o co!er gas or flu# is used,
thereby making the process en!ironmentally friendly. The joining does not in!ol!e any use of
filler metal and therefore any aluminium alloy can be joined without concern for the
compatibility of composition, which is an issue in fusion welding. &hen desirable, dissimilar
aluminium alloys and composites can be joined with e)ual ease 1;9
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3. Process parameters
$"& in!ol!es comple# material mo!ement and plastic deformation. &elding
parameters, tool geometry, and joint design e#ert significant effect on the material flow
pattern and temperature distribution, thereby influencing the microstructural e!olution of
material. *n this section, a few major factors affecting $"&A$"= process, such as tool
geometry, welding parameters, joint design are addressed.
3.1. Tool geometry
Tool geometry is the most influential aspect of process de!elopment. The tool
geometry plays a critical role in material flow and in turn go!erns the tra!erse rate at which
$"& can be conducted. n $"& tool consists of a shoulder and a pin as shown schematically
in $ig. 2. s mentioned earlier, the tool has two primary functions3 %a' localized heating, and
%b' material flow. *n the initial stage of tool plunge, the heating results primarily from the
friction between pin and workpiece. "ome additional heating results from deformation of material. The tool is plunged till the shoulder touches the workpiece. The friction between the
shoulder and workpiece results in the biggest component of heating. $rom the heating aspect,
the relati!e size of pin and shoulder is important, and the other design features are not critical.
The shoulder also pro!ides confinement for the heated !olume of material. The second
function of the tool is to 4stir5 and 4mo!e5 the material. The uniformity of microstructure and
properties as well as process loads are go!erned by the tool design. Denerally a conca!e
shoulder and threaded cylindrical pins are used.
$ig. 2. "chematic drawing of the $"& tool.
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$ig. . &orlT> and >X TrifluteT> tools de!eloped by The&elding *nstitute
%T&*', / %Eopyright 2 with the flute lands being flared out %$ig. 8' and
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-skewT> with the pin a#is being slightly inclined to the a#is of machine spindle %$ig. ?'
were de!eloped for impro!ed )uality of lap welding. The design features of the $lared-
TrifuteT> and the -skewT> are belie!ed to3 %a' increase the ratio between of the swept
!olume and static !olume of the pin, thereby impro!ing the flow path around and underneath
the pin, %b' widen the welding region due to flared-out flute lands in the $lared-Trifute
T>
pinand the skew action in the -skewT> pin, %c' pro!ide an impro!ed mi#ing action for o#ide
fragmentation and dispersal at the weld interface, and %d' pro!ide an orbital forging action at
the root of the weld due to the skew action, impro!ing weld )uality in this region. Eompared
to the con!entional threaded pin, $lared-TrifuteT> and -skewT> pins resulted in3 %a' o!er
pin produced a
slight downturn at the outer regions of the o!erlapping plateAweld interface, which are
beneficial to impro!ing the properties of the $"& joints. Thomas and olby suggested that
both $lared-TrifuteT> and -skewT> pins are suitable for lap, T, and similar welds where
joining interface is !ertical to the machine a#is.
$ig. 8. $lared-TrifluteT> tools de!eloped by The &elding *nstitute %T&*', /3
%a' neutral flutes, %b' left flutes, and %c' right hand flutes.
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$urther, !arious shoulder profiles were designed in T&* to suit different materials and
conditions. These shoulder profiles impro!e the coupling between the tool shoulder and the
work pieces by entrapping plasticized material within special re-entrant features.
Eonsidering the significant effect of tool geometry on the metal flow, fundamental
correlation between material flow and resultant microstructure of welds !aries with each tool.
critical need is to de!elop systematic framework for tool design. Eomputational tools,
including finite element analysis %$B', can be used to !isualize the material flow and
calculate a#ial forces. "e!eral companies ha!e indicated internal +I efforts in friction stir
welding conferences, but no open literature is a!ailable on such efforts and outcome. *t is
important to realize that generalization of microstructural de!elopment and influence of
processing parameters is difficult in absence of the tool information.
$ig. ?. -"kewT> tool de!eloped by The &elding *nstitute %T&*', /3 %a' side !iew,
%b' front !iew, and %c' swept region encompassed by skew action.
3.2. Welding parameters
$or $"&, two parameters are !ery important3 tool rotation rate %!, rpm' in clockwise
or counter clockwise direction and tool tra!erse speed %n, mmAmin' along the line of joint.
The rotation of tool results in stirring and mi#ing of material around the rotating pin and the
translation of tool mo!es the stirred material from the front to the back of the pin and finishes
welding process. Jigher tool rotation rates generate higher temperature because of higher
friction heating and result in more intense stirring and mi#ing of material as will be discussed
later. Jowe!er, it should be noted that frictional coupling of tool surface with workpiece is
going to go!ern the heating. "o, a monoonic increase in heating with increasing tool rotation
rate is not e#pected as the coefficient of friction at interface will change with increasing tool
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rotation rate. *n addition to the tool rotation rate and tra!erse speed, another important
process parameter is the angle of spindle or tool tilt with respect to the work piece surface.
suitable tilt of the spindle towards trailing direction ensures that the shoulder of the tool holds
the stirred material by threaded pin and mo!e material efficiently from the front to the back
of the pin. $urther, the insertion depth of pin into the work pieces %also called target depth' isimportant for producing sound welds with smooth tool shoulders. The insertion depth of pin
is associated with the pin height. &hen the insertion depth is too shallow, the shoulder of tool
does not contact the original workpiece surface. Thus, rotating shoulder cannot mo!e the
stirred material efficiently from the front to the back of the pin, resulting in generation of
welds with inner channel or surface groo!e. &hen the insertion depth is too deep, the
shoulder of tool plunges into the workpiece creating e#cessi!e flash. *n this case, a
significantly conca!e weld is produced, leading to local thinning of the welded plates. *t
should be noted that the recent de!elopment of 4scrolled5 tool shoulder allows $"& with
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Thus, it is difficult to produce continuous defect-free weld. *n these cases, preheating
or additional e#ternal heating source can help the material flow and increase the process
window. (n the other hand, materials with lower melting point such as aluminium and
magnesium, cooling can be used to reduce e#tensi!e growth of recrystallized grains and
dissolution of strengthening precipitates in and around the stirred zone.
3.3. Joint design
The most con!enient joint configurations for $"& are butt and lap joints. simple
s)uare butt joint is shown in $ig.@. Two plates or sheets with same thickness are placed on a
backing plate and clamped firmly to pre!ent the abutting joint faces from being forced apart.
uring the initial plunge of the tool, the forces are fairly large and e#tra care is re)uired to
ensure that plates in butt configuration do not separate. rotating tool is plunged into the
joint line and tra!ersed along this line when the shoulder of the tool is in intimate contactwith the surface of the plates, producing a weld along abutting line. (n the other hand, for a
simple lap joint, two lapped plates or sheets are clamped on a backing plate. rotating tool is
!ertically plunged through the upper plate and into the lower plate and tra!ersed along
desired direction, joining the two plates %$ig. @d'. >any other configurations can be produced
by combination of butt and lap joints. part from butt and lap joint configurations, other
types of joint designs, such as fillet joints %$ig. 7g', are also possible as needed for some
engineering applications. *t is important to note that no special preparation is needed for $"&
of butt and lap joints. Two clean metal plates can be easily joined together in the form of butt
or lap joints without any major concern about the surface conditions of the plates.
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4. Development o riction stir processing
$riction stir welding has a number of attributes that can be used to de!elop a generic
tool for microstructural modification and manufacturing. $riction stir processing was
de!eloped based on basic concept of $"&. This has led to se!eral applications for
microstructural modification in metallic materials, including superplasticity, surface
composite, homogenization of nanophase aluminum alloys and metal matri# composites, and
microstructural refinement of cast aluminum alloys.
4.1. Superplasticity
*t is well known that two basic re)uirements are necessary for achie!ing structural
superplasticity. The first is a fine grain size, typically less than ? mm. The second is thermal
stability of the fine microstructure at high temperatures. Eon!entionally, thermo-mechanical
processing %T>=' is used to produce fine-grained microstructure in commercial aluminium
alloys. typical T>= for heat-treatable aluminium alloys consists of solution treatment,o!eraging, multiple pass warm rolling %2
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4.2. Surace composites
Eompared to unreinforced metals, metal matri# composites reinforced with ceramic
phases e#hibit high strength, high elastic modulus, impro!ed resistance to wear, creep and
fatigue, which make them promising structural materials for aerospace and automobile
industries. Jowe!er, these composites also suffer from a great loss in ductility and toughness
due to incorporation of nondeformable ceramic reinforcements, which limits their
applications to a certain e#tent. $or many applications, the useful life of components often
depends on their surface properties such as wear resistance. *n these situations, it is desirable
that only the surface layer of components is reinforced by ceramic phases while the bulk of
components retain the original composition and structure with higher toughness.
4.3. !icrostructural modiication
l97 wt.F "i9>g alloys are widely used to cast high-strength components in the
aerospace and automobile industries because they offer a combination of high strength withgood casting characteristics. Jowe!er, some mechanical properties of cast alloys, in
particular ductility, toughness and fatigue resistance, are limited by porosity, coarse acicular
"i particles, and coarse primary aluminium dendrites
Narious modification and heat-treatment techni)ues ha!e been de!eloped to refine the
microstructure of cast l9"i9>g alloys. The first category of research is aimed at modifying
the morphology of "i particles. There are some drawbacks with these modifiers. $or sodium,
the benefits fade rapidly on holding at high temperature and the modifying action practically
disappears after only two remelts. $or strontium, the density of microshrinkage porosity is
increased after the addition of strontium due to owing to increased gas pickup from the
dissolution difficulty and a depression in the eutectic transformation temperature $or
antimony, en!ironmental and safety concerns ha!e precluded its use in most countries.
lternati!ely, heat treatment of cast alloys at high temperature, usually at the solid solution
temperature around ?8
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". #dvantages and Disadvantages
".1 #dvantages$
. Cow distortion, e!en in long welds.
2. B#cellent mechanical properties as pro!en by fatigue, tensile and bend test.. :o arc, fume, spatter and porosity.8. Cow shrinkage.?. Ean operate in all positions.@. :on consumable tool.7. :o filler wire, gas shielding.;. Cow en!ironmental impact.
".2 Limitations$
. &ork piece must be rigidly clamped.2. 6acking bar re)uired %e#cept for self reacting and directly opposed tools'.. /ey holes at end of each weld.8. Eannot make joints that re)uire metal deposition.?. Cess fle#ible than manual and arc process.
%. #pplications
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pplication of $"& includes !arious industries including few of following3-
• "hipping and marine industries3 - "uch as manufacturing of hulls, offshore
accommodations, aluminium e#trusions, etc.
• erospace industries3 - for welding in l alloy fuel tanks for space !ehicles,
manufacturing of wings, etc.
• +ailway industries3 - building of container bodies, railway tankers, etc.
• Cand transport3 - automoti!e engine chassis, body frames, wheel rims, truck bodies,etc.
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&. 'ase Study$
$riction stir welding of aluminium alloys has been cited by many users as a cost-
sa!ing process. This is in part due to the elimination of consumable costs, but is also due to
the ability to make most welds in one or two passes, e!en in thick material. *t is also a !ery
efficient process in terms of energy consumption, which can also lead to significant cost
sa!ings. The a!oidance of multiple passes eliminates the need for inter-run cleaning, back
gouging, etc., and the a!oidance of spatter means that post weld dressing is reduced or not
re)uired. The fully automated nature of the process also reduces labour costs.
>any companies ha!e reported significant sa!ings due to the considerable reduction in repair
and re-work, low distortion, and general e)uipment fle#ibility.
The following comments on cost sa!ings were published by users of the $"& process andspeak for themsel!es3
• The 6oeing Eompany reported that Othe $"& specific design of elta *N and elta **
achie!ed @
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(. Summary and uture outloo)
*n this re!iew article current de!elopments in process modeling, microstructure and
properties, material specific issues, applications of friction stir weldingAprocessing ha!e been
addressed.
Tool geometry is !ery important factor for producing sound welds. Jowe!er, at the present
stage, tool designs are generally proprietary to indi!idual researchers and only limited
information is a!ailable in open literature. $rom the open literature, it is known that a
cylindrical threaded pin and conca!e shoulder are widely used welding tool features. 6esides,
tri-fluted pins such as >X TrifuteT> and $lared-TrifuteT> ha!e also been de!eloped.
&elding parameters, including tool rotation rate, tra!erse speed, spindle tilt angle, and target
depth, are crucial to produce sound and defect-free weld.
s in traditional fusion welding, butt and lap joint designs are the most common jointconfigurations in friction stir welding. Jowe!er, no special preparation is needed for the butt
and lap joints of friction stir welding. Two clean metal plates can be easily joined together in
the form of butt or lap joints without concern about the surface conditions of the plates.
*t is widely accepted that material flow within the weld during $"& is !ery comple# and still
poorly understood. *t has been suggested by some researchers that $"& can be generally
described as an in situ e#trusion process and the stirring and mi#ing of material occurred only
at the surface layer of the weld adjacent to the rotating shoulder.
$"& results in significant temperature rise within and around the weld. temperature riseof8
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the base material. The fracture toughness of friction stir welds is obser!ed to be higher than
or e)ui!alent to that of base material. s for corrosion properties of friction stir welds,
contradicting obser!ations ha!e been reported. &hile some studies showed that the pitting
and "EE resistances of $"& welds were superior or comparable those of the base material,
other reports indicate that $"& welds of some high-strength aluminum alloys were moresusceptible to intergranular attack than the base alloys with preferential occurrence of
intergranular attack in the JM adjacent to the T>M.
*n addition to aluminum alloys, friction stir welding has been successfully used to join other
metallic materials, such as copper, titanium, steel, magnesium, and composites. 6ecause of
highmelting point andAor low ductility, successful joining of high melting temperature
materials by means of $"& was usually limited to a narrowrange of $"¶meters.
=reheating is beneficial for impro!ing theweld )uality as well as increase in the tra!erse rate
for high melting materials such as steel.
6ased on the basic principles of $"&, a new generic processing techni)ue for microstructural
modification, friction stir processing %$"=' has been de!eloped. $"= has found se!eral
applications for microstructural modification in metallic materials, including microstructural
refinement for high- strain rate superplasticity, fabrication of surface composite on aluminum
substrates, and homogenization of microstructure in nanophase aluminum alloys, metal
matri# composites, and cast l9"i alloys. espite considerable interests in the $"&
technology in past decade, the basic physical understanding of the process is lacking. "ome
important aspects, including material flow, tool geometry design, wear of welding tool,
microstructural stability, welding of dissimilar alloys and metals, re)uire understanding.
Jowe!er, as pointed out by =rof. Thomas &. Bagar of >assachusetts *nstitute of Technology,44:ew welding technology is often commercialized before a fundamental science
emphasizing the underlying physics and chemistry can be de!eloped55. This is )uite true with
the $"& technology. lthough it is only 8 years since $"& technology was in!ented at The
&elding *nstitute %Eambridge, /' in 00, )uite a few successful industrial applications of
$"& ha!e been demonstrated.
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*. 'onclusion$
There is no doubt that the use of $"& will open up new markets and new opportunities as the
technology gets wider recognition as a welding process that can produce superior welds, of
impro!ed reliability and of increased producti!ity. The $"& process is already in commercial
use and has been found to be a robust process tolerant, techni)ue that has much to offer.
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1+. ,eerences
1 &.>. Thomas, B.. :icholas, K.E. :eedham, >.D. >urch, =. Templesmith, E.K. awes,
D.6. =atent pplication :o.
02?07;.; %ecember 00'.
12 E. awes, &. Thomas, T&* 6ulletin @, :o!emberAecember 00?, p. 28.
1 6. Condon, >. >ahoney, 6. 6ingel, >. Ealabrese, .&aldron, in3 =roceedings of the
Third *nternational "ymposium on $riction "tir &elding, /obe, Kapan, 2792; "eptember,
2ahoney, &.J. 6ingel, +.. "purling, E.E. 6ampton, "cripta >ater.
@ %007' @0.
1? D. Ciu, C.B. >urr, E.". :iou, K.E. >cElure, $.+. Nega, "cripta >ater. 7 %007' ??.
1@ /.N. Kata, ".C. "emiatin, "cripta >ater. 8 %2ater. +es. *nno!at. 2 %00;' ?urr, K. >ater. "ci. Cett. 0 %2odern &elding Technology, =rentice-Jall, :ew Kersey, 2ishra, in3 /.N. Kata, >.&. >ahoney, +.". >ishra, ".C.
"emiatin, T. Cienert %Bds.', $riction "tir &elding and =rocessing **, T>", 2
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12 www.hilda-europe.euA$"&=rocessApplicationsBconomicsAtabidA827Aefault.asp#
122 www.boeing.comAnewsAfrontiersAarchi!eA2