FSW Final Report RS_1458202652971

8/19/2019 FSW Final Report RS_1458202652971

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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

Dept. Of ME, Dr. AIT Page 1


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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

Dept. Of ME, Dr. AIT Page #


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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.

Dept. Of ME, Dr. AIT Page 1%


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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 $"&parameters.

=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.

Dept. Of ME, Dr. AIT Page 1"


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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.

Dept. Of ME, Dr. AIT Page 1#


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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

FSW Final Report RS_1458202652971

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