FRICTION STIR WELDING

FRICTION STIR WELDING

Seminar report submitted by

NAME- SHRILAY THAKUR

BRANCH- INDUSTRIAL AND PRODUCTION ENGINEERING

CLASS – VII SEMESTER

ROLL NO. - 13

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1.0 Principle of operation

In FSW, a cylindrical-shouldered tool, with a profiled threaded/unthreaded probe (nib

or pin) is rotated at a constant speed and fed at a constant traverse rate into the joint

line between two pieces of sheet or plate material, which are butted together. The

parts have to be clamped rigidly onto a backing bar in a manner that prevents the

abutting joint faces from being forced apart. The length of the nib is slightly less than

the weld depth required and the tool shoulder should be in intimate contact with the

work surface. The nib is then moved against the work, or vice versa.

Frictional heat is generated between the wear-resistant welding tool shoulder and

nib, and the material of the work pieces. This heat, along with the heat generated by

the mechanical mixing process and the adiabatic heat within the material, cause the

stirred materials to soften without reaching the melting point (hence cited a solid-

state process), allowing the traversing of the tool along the weld line in a plasticised

tubular shaft of metal. As the pin is moved in the direction of welding, the leading

face of the pin, assisted by a special pin profile, forces plasticised material to the

back of the pin while applying a substantial forging force to consolidate the weld

metal. The welding of the material is facilitated by severe plastic deformation in the

solid state, involving dynamic recrystallization of the base material.

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2.0 Microstructural features

asymmetric nature, results in a highly characteristic microstructure. The

microstructure can be broken up into the following zones:

 The stir zone (also nugget, dynamically recrystallised zone) is a region of

heavily deformed material that roughly corresponds to the location of the pin

during welding. The grains within the stir zone are roughly equiaxed and often an

order of magnitude smaller than the grains in the parent material. A unique

feature of the stir zone is the common occurrence of several concentric rings

which has been referred to as an ‘onion-ring’ structure. The precise origin of

these rings has not been firmly established, although variations in particle

number density, grain size and texture have all been suggested.

 The flow arm zone is on the upper surface of the weld and consists of material

that is dragged by the shoulder from the retreating side of the weld, around the

rear of the tool, and deposited on the advancing side.

 The thermo-mechanically affected zone (TMAZ) occurs on either side of the

stir zone. In this region the strain and temperature are lower and the effect of

welding on the microstructure is correspondingly smaller. Unlike the stir zone the

microstructure is recognizably that of the parent material, albeit significantly

deformed and rotated. Although the term TMAZ technically refers to the entire

deformed region it is often used to describe any region not already covered by

the terms stir zone and flow arm.

 The heat-affected zone (HAZ) is common to all welding processes. As

indicated by the name, this region is subjected to a thermal cycle but is not

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deformed during welding. The temperatures are lower than those in the TMAZ

but may still have a significant effect if the microstructure is thermally unstable. In

fact, in age-hardened aluminium alloys this region commonly exhibits the poorest

mechanical properties.

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3.0 Important welding parameters

3.1 Tool rotation and traverse speeds

There are two tool speeds to be considered in friction-stir welding; how fast the tool

rotates and how quickly it traverses the interface. These two parameters have

considerable importance and must be chosen with care to ensure a successful and

efficient welding cycle. The relationship between the welding speeds and the heat

input during welding is complex but, in general, it can be said that increasing the

rotation speed or decreasing the traverse speed will result in a hotter weld. In order

to produce a successful weld it is necessary that the material surrounding the tool is

hot enough to enable the extensive plastic flow required and minimise the forces

acting on the tool. If the material is too cool then voids or other flaws may be present

in the stir zone and in extreme cases the tool may break.

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At the other end of the scale excessively high heat input may be detrimental to the

final properties of the weld. Theoretically, this could even result in defects due to the

liquation of low-melting-point phases (similar to liquation cracking in fusion welds).

These competing demands lead onto the concept of a ‘processing window’: the

range of processing parameters that will produce a good quality weld. Within this

window the resulting weld will have a sufficiently high heat input to ensure adequate

material plasticity but not so high that the weld properties are excessively reduced.

3.2 Tool tilt and plunge depth

The plunge depth is defined as the depth of the lowest point of the shoulder below

the surface of the welded plate and has been found to be a critical parameter for

ensuring weld quality . Plunging the shoulder below the plate surface increases the

pressure below the tool and helps ensure adequate forging of the material at the rear

of the tool. Tilting the tool by 2-4 degrees, such that the rear of the tool is lower than

the front, has been found to assist this forging process. The plunge depth needs to

be correctly set, both to ensure the necessary downward pressure is achieved and to

ensure that the tool fully penetrates the weld. Given the high loads required the

welding machine may deflect and so reduce the plunge depth compared to the

nominal setting, which may result in flaws in the weld. On the other hand an

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excessive plunge depth may result in the pin rubbing on the backing plate surface or

a significant undermatch of the weld thickness compared to the base material.

Variable load welders have been developed to automatically compensate for

changes in the tool displacement while TWI have demonstrated a roller system that

maintains the tool position above the weld plate.

3.3 Tool design

The design of the tool is a critical factor as a good tool can improve both the quality

of the weld and the maximum possible welding speed.

It is desirable that the tool material is sufficiently strong, tough and hard wearing, at

the welding temperature. Further it should have a good oxidation resistance and a

low thermal conductivity to minimise heat loss and thermal damage to the machinery

further up the drive train. Hot-worked tool steel such as AISI H13 has proven

perfectly acceptable for welding aluminium alloys within thickness ranges of 0.5 –

50 mm  but more advanced tool materials are necessary for more demanding

applications such as highly abrasive metal matrix composites or higher melting point

materials such as steel or titanium.

Improvements in tool design have been shown to cause substantial improvements in

productivity and quality. TWI has developed tools specifically designed to increase

the depth of penetration and so increase the plate thickness that can be successfully

welded. An example is the ‘whorl’ design that uses a tapered pin with re-entrant

features or a variable pitch thread in order to improve the downwards flow of

material. Additional designs include the Triflute and Trivex series. The Triflute design

has a complex system of three tapering, threaded re-entrant flutes that appear to

increase material movement around the tool. The Trivex tools use a simpler, non-

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cylindrical, pin and have been found to reduce the forces acting on the tool during

welding.

The majority of tools have a concave shoulder profile which acts as an escape

volume for the material displaced by the pin, prevents material from extruding out of

the sides of the shoulder and maintains downwards pressure and hence good

forging of the material behind the tool. The Triflute tool uses an alternative system

with a series of concentric grooves machined into the surface which are intended to

produce additional movement of material in the upper layers of the weld

4.0 Welding forces

During welding a number of forces will act on the tool:

 A downwards force is necessary to maintain the position of the tool at or

below the material surface. Some friction-stir welding machines operate under

load control but in many cases the vertical position of the tool is preset and so the

load will vary during welding.

 The traverse force acts parallel to the tool motion and is positive in the

traverse direction. Since this force arises as a result of the resistance of the

material to the motion of the tool it might be expected that this force will decrease

as the temperature of the material around the tool is increased.

 The lateral force may act perpendicular to the tool traverse direction and is

defined here as positive towards the advancing side of the weld.

 Torque is required to rotate the tool, the amount of which will depend on the

down force and friction coefficient (sliding friction) and/or the flow strength of the

material in the surrounding region (sticking friction).

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In order to prevent tool fracture and to minimize excessive wear and tear on the tool

and associated machinery, the welding cycle should be modified so that the forces

acting on the tool are as low as possible, and abrupt changes are avoided. In order

to find the best combination of welding parameters it is likely that a compromise must

be reached, since the conditions that favour low forces (e.g. high heat input, low

travel speeds) may be undesirable from the point of view of productivity and weld

properties.

5.0 Flow of material

alloy, which had a different contrast to the normal material when viewed through a

microscope, in an effort to determine where material was moved as the tool

passed. The data was interpreted as representing a form of in-situ extrusion where

the tool, backing plate and cold base material form the ‘extrusion chamber’ through

which the hot, plasticised material is forced. In this model the rotation of the tool

draws little or no material around the front of the pin instead the material parts in

front of the pin and passes down either side. After the material has passed the pin

the side pressure exerted by the ‘die’ forces the material back together and

consolidation of the join occurs as the rear of the tool shoulder passes overhead and

the large down force forges the material.

More recently, an alternative theory has been advanced that advocates considerable

material movement in certain locations. This theory holds that some material does

rotate around the pin, for at least one rotation, and it is this material movement that

produces the ‘onion-ring’ structure in the stir zone. The researchers used a

combination of thin Cu strip inserts and a ‘frozen pin’ technique, where the tool is

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rapidly stopped in place. They suggested that material motion occurs by two

processes:

1. Material on the advancing front side of a weld enters into a zone that rotates

and advances with the pin. This material was very highly deformed and

sloughs off behind the pin to form arc-shaped features when viewed from

above (i.e. down the tool axis). It was noted that the copper entered the

rotational zone around the pin, where it was broken up into fragments. These

fragments were only found in the arc shaped features of material behind the

tool.

2. The lighter material came from the retreating front side of the pin and was

dragged around to the rear of the tool and filled in the gaps between the arcs

of advancing side material. This material did not rotate around the pin and the

lower level of deformation resulted in a larger grain size.

The primary advantage of this explanation is that it provides a plausible explanation

for the production of the onion-ring structure.

6.0 Generation and flow of heat

minimise the heat input as this will increase productivity and possibly reduce the

impact of welding on the mechanical properties of the weld. At the same time it is

necessary to ensure that the temperature around the tool is sufficiently high to permit

adequate material flow and prevent flaws or tool fracture.

When the traverse speed is increased, for a given heat input, there is less time for

heat to conduct ahead of the tool and the thermal gradients are larger. At some point

the speed will be so high that the material ahead of the tool will be too cold, and the

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flow stress too high, to permit adequate material movement, resulting in flaws or tool

fracture. If the ‘hot zone’ is too large then there is scope to increase the traverse

speed and hence productivity.

The welding cycle can be split into several stages during which the heat flow and

thermal profile will be different :

 Dwell. The material is preheated by a stationary, rotating tool in order to

achieve a sufficient temperature ahead of the tool to allow the traverse. This

period may also include the plunge of the tool into the workpiece.

 Transient heating. When the tool begins to move there will be a transient

period where the heat production and temperature around the tool will alter in a

complex manner until an essentially steady-state is reached.

 Pseudo steady-state. Although fluctuations in heat generation will occur the

thermal field around the tool remains effectively constant, at least on the

macroscopic scale.

 Post steady-state. Near the end of the weld heat may ‘reflect’ from the end of

the plate leading to additional heating around the tool.

Heat generation during friction-stir welding arises from two main sources: friction at

the surface of the tool and the deformation of the material around the tool. The heat

generation is often assumed to occur predominantly under the shoulder, due to its

greater surface area, and to be equal to the power required to overcome the contact

forces between the tool and the workpiece. The contact condition under the shoulder

can be described by sliding friction, using a friction coefficient μ and interfacial

pressure P, or sticking friction, based on the interfacial shear strength &tor; at an

appropriate temperature and strain rate. Mathematical approximations for the total

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heat generated by the tool shoulder Qtotal have been developed using both sliding

and sticking friction models :

 (Sliding)

 (Sticking)

where ω is the angular velocity of the tool, Rshoulder is the radius of the tool shoulder

and Rpin that of the pin. Several other equations have been proposed to account for

factors such as the pin but the general approach remains the same.

A major difficulty in applying these equations is determining suitable values for the

friction coefficient or the interfacial shear stress. The conditions under the tool are

both extreme and very difficult to measure. To date, these parameters have been

used as 'fitting parameters' where the model works back from measured thermal

data to obtain a reasonable simulated thermal field. While this approach is useful for

creating process models to predict, for example, residual stresses it is less useful for

providing insights into the process itself.

7.0 Advantages and disadvantages

welding methods since any problems associated with cooling from the liquid phase

are immediately avoided. Issues such as porosity, solute redistribution, solidification

cracking and liquation cracking are not an issue during FSW. In general, FSW has

been found to produce a low concentration of defects and is very tolerant to

variations in parameters and materials.

Nevertheless, FSW is associated with a number of unique defects. Insufficient weld

temperatures, due to low rotational speeds or high traverse speeds, for example,

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mean that the weld material is unable to accommodate the extensive deformation

during welding. This may result in long, tunnel-like defects running along the weld

which may occur on the surface or subsurface. Low temperatures may also limit the

forging action of the tool and so reduce the continuity of the bond between the

material from each side of the weld. The light contact between the material has given

rise to the name 'kissing-bond'. This defect is particularly worrying since it is very

difficult to detect using nondestructive methods such as X-ray orultrasonic testing. If

the pin is not long enough or the tool rises out of the plate then the interface at the

bottom of the weld may not be disrupted and forged by the tool, resulting in a lack-of-

penetration defect. This is essentially a notch in the material which can be a potent

source of fatigue cracks.

A number of potential advantages of FSW over conventional fusion-welding

processes have been identified :

 Good mechanical properties in the as welded condition

 Improved safety due to the absence of toxic fumes or the spatter of molten

material.

 No consumables - A threaded pin made of conventional tool steel, e.g.

hardened H13, can weld over 1000m of aluminium, and no filler or gas shield is

required for aluminium.

 Easily automated on simple milling machines - lower setup costs and less

training.

 Can operate in all positions (horizontal, vertical, etc.), as there is no weld pool.

 Generally good weld appearance and minimal thickness under/over-matching,

thus reducing the need for expensive machining after welding.

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 Low environmental impact.

However, some disadvantages of the process have been identified:

 Exit hole left when tool is withdrawn.

 Large down forces required with heavy-duty clamping necessary to hold the

plates together.

 Less flexible than manual and arc processes (difficulties with thickness

variations and non-linear welds).

 Often slower traverse rate than some fusion welding techniques although this

may be offset if fewer welding passes are required.

8.0 Applications

Friction stir welding was used to prefabricate the aluminium panels of the Super

Liner Ogasawara at Mitsui Engineering and Shipbuilding

The FSW process is currently patented by TWI in most industrialised countries and

licensed for over 183 users. Friction stir welding and its variants friction stir spot

welding and friction stir processing are used for the following industrial applications:

Shipbuilding and Offshore

 Fish freezer panels at Sapa Group and Riftec

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 Deck panels and helicopter landing platforms at Marine Aluminium and

Bayards

 Hull section of the ocean viewer vessel "The Boss" at Research Foundation

Institute

 Super Liner Ogasawara at Mitsui Engineering and Shipbuilding

 Sea Fighter at Nichols Bros

 Aluminium panels for Littoral Combat Ship USS Freedom by Friction Stir Link 

 Houbei class missile boat at China Friction Stir Centre

 HMNZS Rotoiti (P3569) at Donovans

 Armor plating for amphibious assault ships 

Longitudinal and circumferential friction stir welds are used for the Falcon 9 rocket

booster tank at the SpaceX factory

Aerospace

 Main Space Shuttle external tank at NASA

 Ares I and Orion Crew Vehicle test article at NASA

 Boeing Delta II and Delta IV expendable launch vehicles at Boeing

 Falcon 1 and Falcon 9 rockets at SpaceX

 Wings and fuselage panels of the Eclipse 500 aircraft at Eclipse Aviation

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 Toe nails for ramp of Boeing C-17 Globemaster III cargo aircraft by Advanced

Joining Technologies

 Cargo barrier beams for Boeing 747 Large Cargo Freighter

 Floor panels for Airbus A400M military aircraft by Pfalz Flugzeugwerke

 Legacy 450 and 500 Jets of Embraer

The centre tunnel of the Ford GT is made from two aluminum extrsuions friction stir

welded to a bent aluminium sheet and houses the fuel tank

Automotive

 Engine tunnel of Ford GT at Tower Automotive

 Suspension struts at Showa Denko and Tower Automotive for the

stretched Lincoln Town Car at Tower Automotive

 Tailor welded blanks for the Audi R8 at Riftec

 Rear seats for the Volvo V70 at Sapa

 HVAC pistons at Halla Climate Control

 Friction stir spot welding of bonnet and rear doors of the Mazda RX-8

 Flatbed trailer "Revolotion" of Fontaine Trailers

 Wheels at Simmons Wheels, UT Alloy Works and Fundo

 Automotive components at Friction Stir Link

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 Aluminium sheet to galvanised steel brackets for boot lid of the Mazda MX-5

 Friction stir spot welding of the boot lid of the Toyota Prius

The high-strength low-distortion body of Hitachi's A-train British Rail Class 395 is

friction stir welded from longitudinal aluminium extrusions

Railway Rolling Stock

 Roof panels for DSB class SA-SD trains of Alstom LHB at Hydro Marine

Aluminium

 Side panels for Bombardier's Electrostar trains at Sapa Group

 Side panels for Alstom's British Rail Class 390 Pendolino trains at Sapa

Group

 Japanese commuter and express A-trains and British Rail Class 395 trains

by Hitachi

 Friction stir spot welded roof panels by Kawasaki

 Curved Side and roof panels for the Victoria Line trains of London

Underground at Sapa

 Shinkansen floor panels by Sumitomo Light Metal

 IGBT coolers at Sapa Group

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 Heat sinks at Sykatek, EBG, Austerlitz Electronics, Sapa  and Rapid Technic

The lids of 50mm thick copper canisters for nuclear waste are attached to the

cylinder by friction stir welding at SKB

Fabrication

 Cathode sheets at AMAG and Hammerer Aluminium Industries

 Bizerba meat slicers, HVAC units and Siemens X-ray vacuum vessels at

Riftec

 Vacuum valves and vessels at Japanese and Swiss companies

 Encapsulation of nuclear waste at SKB

 Pressure vesssels from ø1m semispherical forgings of 38.1mm thick

aluminium alloy 2219 at Advanced Joining Technologies and Lawrence

Livermore Nat Lab.

 Hunting knives by DiamondBlade

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