FRICTIONAL STIR WELDINGSOLID STATE JOINING PROCESS

BAPATLA ENGINEERING COLLEGEBAPATLA

Author:

Sudheer.chMechanical Engineering III/IV B.Tech Email: sudheer_unicon@yahoo.co.in

Mobile: 9391673727

Hardware Requirement: Computer, a Data projector, a laser light.

Abstract:

Friction Stir Welding (FSW),

a derivative of conventional friction

welding, was invented at The Welding

Institute; U.K. has been shown to

produce superior as-welded

mechanical properties when compared

to typical arc welding processes in

aluminum alloys. As with other

welding processes, it is used primarily

to achieve metallurgical joining of

materials; however, secondarily this

process also enhances properties of

materials. It uses the friction of the

rotating tool to heat, soften, and then to

stir together the materials to be joined.

Most of the other welding processes,

such as gas metal arc welding, electron

beam welding, laser welding and

resistance welding achieve a weld by

applying direct thermal energy to the

materials causing the materials to melt

and fuse which may cause distortion

due to thermal stress. FSW achieves

results and over come the problems,

using mechanical energy rather than

direct thermal energy. The main

advantages which made FSW to adapt

is lower energy input results in low

distortion, it can work on sheet

materials, tubes, extrusions, or

complex castings or forgings.

Recently, a new technology called

friction stir spot welding (FSSW) has

been developed that has several

advantages over the electric resistance

welding process widely used in

automotive industry in terms of weld

quality and process efficiency. FSW

has been used on a wide range of

materials, including aluminum, copper,

bronze, lead, magnesium,

thermoplastic resin; and even titanium

and steel. FSW has found its greatest

application in aluminum. Even energy

consumption drops nearly 99% for

aluminum and 80% for steel.

Equipment costs also drop by 40% as

there's no longer a need for large-scale

sources of electricity and specialized

joining equipment.

Key words: Non consumable

electrode, solid state joining process,

Introduction:

Friction Stir Welding is a solid state

joining process, in which a cylindrical

shouldered tool with a profiled pin is

inserted into the joint line between two

pieces of material. Frictional heat is

created between the wear resistant pin

and the two work pieces, which are

butted together and clamped onto a

backing bar.

The heat causes the materials to soften,

without reaching melting point, and

allows the pin to traverse along the

joint. As the tool moves along, the

material is plasticized by the frictional

heat at the front of the rotating pin and

transported to the back. Here it

consolidates and cools down to form a

solid state weld. The tool has a circular

section except at the end where there is

a threaded probe or more complicated

flute; the junction between the

cylindrical portion and the probe is

known as the shoulder. The probe

penetrates the work piece whereas the

shoulder rubs with the top surface. The

heat is generated primarily by friction

between a rotating--translating tool, the

shoulder of which rubs against the

work piece. There is a volumetric

contribution to heat generation from

the adiabatic heating due to

deformation near the pin. The welding

parameters have to be adjusted so that

the ratio of frictional to volumetric

deformation--induced heating

decreases as the work piece becomes

thicker. This is in order to ensure a

sufficient heat input per unit length.

The technique uses a non-consumable

tool to generate frictional heating at the

point of welding and to induce gross

plastic deformation of the work piece,

resulting in complex mixing across the

joint. Friction stir welds have been

fabricated in a variety of aluminum

alloys up to 50 mm thick, titanium

alloys and steels up to 25 mm thick.

The welds can be made in any position

at welding speeds of a few inches per

minute.

In friction stir welding, the plates to be

joined are placed on a rigid backing

plate, and clamped in a manner that

prevents the abutting joint faces from

being forced apart. A cylindrical-

shouldered tool, with a specially

profiled projecting pin with a screw

thread, is rotated and slowly plunged

into the joint line. The pin length is

similar to the required weld depth. The

shoulder of the tool is forced against

the plates. When the rotating pin

contacts the work piece, it causes

friction heating of the plates which

lowers their mechanical strength. The

threads on the pin assist in ensuring

that the plastically deformed material

flows around the pin as the tool

advances along the joint line. As the

tool proceeds along the joint line, it

causes friction heating just head of it to

a plastic state. It subsequently

pulverizes the joint line and stirs and

recombines the plasticized material to

the trailing side of the tool where the

material cools to form a solid state

weld. At the end of the weld, the tool is

retracted from the plate and leaves a

hole at the end of the weld.

Circumferential welds have been made

in aluminum alloys by withdrawing the

tools slowly after a complete rotation.

Recently, Friction Stir Spot Welding

(FSSW) has been developed that has a

several advantages over the electric

resistance welding process widely used

in automotive industry in terms of

weld quality and process efficiency.

This welding technology involves a

process similar to FSW, except that,

instead of moving the tool along the

weld seam, the tool only indents the

parts, which are placed on top of each

other as illustrated

The FSSW process consists of three

phases; plunging, stirring, and

retraction as shown in figure. The

process starts with spinning the tool

and slowly plunging it into a weld spot

until the shoulder contacts the top

surface of work piece during plunging

require amount of pressure is applied

by the tool on the work such that it

penetrates into the work to be weld.

The penetrating pressure depends on

material properties. Then, the stirring

phase enable the materials of two work

pieces mix together. The time for

stirring mainly depends on the density

of the work material and its thermal

properties. Lastly, once a

predetermined penetration is reached,

the process stops and the tool retract

from the work piece. The resulting

weld has a characteristic hole in the

middle.

Microstructure Classification of

Friction Stir Welds:

A schematic diagram is shown in the

below Figure which clearly identifies

the various regions. The process not

only generates a heat-affected zone

(HAZ), but within this HAZ near the

weld nugget a thermo-mechanically

affected zone (TMAZ) is also

produced. TMAZ is a result of the

severe plastic deformation and the

temperature rise in the plate from the

friction heating. The friction stir weld

appears broad at the top surface with a

smaller well-defined weld nugget in

the interior. The weld nugget

corresponds to the tool probe that

penetrates through the plate thickness,

whereas the broader surface

deformation and subsequent

recrystallization are associated with the

rotating tool shoulder.

The system divides the weld zone into distinct regions as follows:

A. Unaffected material

B. Heat affected zone (HAZ)

C. Thermo-mechanically affected

zone (TMAZ)

D. Weld nugget (Part of thermo-

mechanically affected zone)

Unaffected material or parent metal:

This is material remote from the weld,

which has not been deformed, and

which although it may have

experienced a thermal cycle from the

weld is not affected by the heat in

terms of microstructure or mechanical

properties.

Heat affected zone (HAZ): In this

region, which clearly will lie closer to

the weld centre, the material has

experienced a thermal cycle which has

modified the microstructure and/or the

mechanical properties. However, there

is no plastic deformation occurring in

this area. In the previous system, this

was referred to as the "thermally

affected zone". The term heat affected

zone is now preferred, as this is a

direct parallel with the heat affected

zone in other thermal processes, and

there is little justification for a separate

name.

Thermo-mechanically affected zone

(TMAZ): In this region, the material

has been plastically deformed by the

friction stir welding tool, and the heat

from the process will also have exerted

some influence on the material. In the

case of aluminium, it is possible to get

significant plastic strain without

recrystallisation in this region, and

there is generally a distinct boundary

between the recrystallised zone and the

deformed zones of the TMAZ.

Aluminium behaves in a different

manner to most other materials, in that

it can be extensively deformed at high

temperature without recrystallisation.

In other materials, the distinct

recrystallised region (the nugget) is

absent, and the whole of the TMAZ

appears to be recrystallised. This is

certainly true of materials which have

no thermally induced phase

transformation which will in itself

induce recrystallisation without strain,

for example pure titanium, b titanium

alloys, austenitic stainless steels and

copper. In materials such as ferrite

steels and a-b titanium alloys

understanding the microstructure is

made more difficult by the thermally

induced phase transformation, and this

can also make the HAZ/TMAZ

boundary difficult to identify precisely.

Weld Nugget: The recrystallised area

in the TMAZ in aluminium alloys has

traditionally been called the nugget.

Although this term is descriptive, it is

not very scientific. However, its use

has become widespread, and as there is

no word which is equally simple with

greater scientific merit, this term has

been adopted. It has been suggested

that the area immediately below the

tool shoulder (which is clearly part of

the TMAZ) should be given a separate

category, as the grain structure is often

different here. The microstructure here

is determined by rubbing by the rear

face of the shoulder, and the material

may have cooled below its maximum.

It is suggested that this area is treated

as a separate sub-zone of the TMAZ.

The simultaneous use of two or more

friction stirs as welding tools:

The concept involved a pair of tools

applied on opposite sides of the work

piece slightly displaced in the direction

of travel. The contra-rotating

simultaneous double-sided operation

with combined weld passes has certain

advantages such as a reduction in

reactive torque and a more

symmetrical weld and heat input

through-the-thickness. The probes need

not touch together but should be

positioned sufficiently close that the

softened 'third-body' material around

the two probes overlaps near the probe

tips to generate a full through-

thickness weld. To avoid any problems

associated with a zero velocity zone in

mid-thickness, the probes can be

displaced slightly along the direction

of travel. Common to all such

simultaneous contra-rotating

techniques is a reduction in the reactive

forces on the work holding fixtures

owing to the reduction or elimination

of reactive torque. Moreover, for

certain applications, the use of purpose

designed multi-headed friction stir

welding machines can increase

productivity, reduce side force

asymmetry, and reduce or minimize

reactive torque.

Parallel twin-stir:

The Twin-stir TM parallel contra-

rotating variant enables defects

associated with lap welding to be

positioned on the 'inside' between the

two welds. For low dynamic volume to

static volume ratio probes using

conventional rotary motion, the most

significant defect will be 'plate

thinning' on the retreating side. With

tool designs and motions designed to

minimize plate thinning, hooks may be

the most significant defect type. The

Twin-stir method may allow a

reduction in welding time for parallel

overlap welding. Owing to the

additional heat available, increased

travel speed or lower rotation process

parameters will be possible.

Tandem twin-stir:

The Twin-stir tandem contra-rotating

variant can be applied to all

conventional FSW joints and will

reduce reactive torque. More

importantly, the tandem technique will

help improve the weld integrity by

disruption and fragmentation of any

residual oxide layer remaining within

the first weld region by the following

tool. Welds have already been

produced by conventional rotary FSW,

whereby a second weld is made over a

previous weld in the reverse direction

with no mechanical property loss. The

preliminary evidence suggests that

further break-up and dispersal of

oxides is achieved within the weld

region. The Twin-stir tandem variant

will provide a similar effect during the

welding operation. Furthermore,

because the tool orientation means that

one tool follows the other, the second

tool travels through already softened

material. This means that the second

tool need not be as robust. It is noted

that under certain circumstances these

tools need not always be used in the

contra-rotation mode and their

rotational speed can also be varied.

Staggered twin-stir:

The staggered Twin-stir means that an

exceptionally wide 'common weld

region' can be created. Essentially, the

tools are positioned with one in front

and slightly to the side of the other so

that the second probe partially overlaps

the previous weld region. This

arrangement will be especially useful

for lap welds, as the wide weld region

produced will provide greater strength

than a single pass weld, given that the

detail at the extremes of the weld

region are similar. Residual oxides

within the overlapping region of the

two welds will be further fragmented,

broken up and dispersed. One

particularly important advantage of the

staggered variant is that the second tool

can be set to overlap the previous weld

region and eliminate any plate thinning

that may have occurred in the first

weld. This will be achieved by locating

the retreating side of both welds on the

'inside'.

Friction Stir Welding - Joint

geometries

The process has been used for the

manufacture of butt welds, overlap

welds, T-sections and corner welds.

For each of these joint geometries

specific tool designs are required

which are being further developed and

optimized. The FSW process can also

cope with circumferential, annular,

non-linear, and three dimensional

welds. Since gravity has no influence

on the solid-phase welding process, it

can be used in all positions. It can be

used to eliminate porosity and other

defects from castings with minimal

property changes. FSW does not take

the materials to the melting point, only

to sufficiently plastic condition to

enable appropriate stirring.

Tool requirements:

Because the peak temperatures

experienced during friction stir

welding are lower than those of fusion

welding processes distortion may be

reduced and micro structural changes

associated with the welding thermal

cycle are minimized. Characteristics

such as these make friction stir welding

an attractive process for welding a

variety of high temperature alloys and

metal matrix composites.

For these alloys, however, the

selection of materials for the rotating

nonconsumable tooling is crucial to

successful deployment. Properties that

are likely to be important for tool

materials include strength, fatigue

resistance, wear resistance, thermal

conductivity, toughness, and chemical

stability. High strength relative to base

materials is an absolute necessity for

tools.

To with stand these adverse conditions

the best option is ceramics and

Cermets of tungsten carbide bonded

with 10 wt% Co, WC10Co, and a gas-

pressure-sintered silicon nitride

(Kyocera SN282).

Material

W/m-K

MPa

Thermal

conductivity

W/m-K

MPa

Strength

Fracture

toughness

6061+20%

Al2O3

130 359

H13 steel 20 2000

WC-

10%Co100 2100

Si3N4 35 800

Advantages:

The only energy consumed with

friction stir welding is the electricity

needed to rotate and apply force to the

welding tool. The process eliminates

the need for the large current and

coolant/compressed air that

conventional resistance welding

requires. Energy consumption drops

nearly 99% for aluminum and 80% for

steel. Equipment costs also drop by

40% as there's no longer a need for

large-scale sources of electricity and

specialized joining equipment. The

process also produces no weld spatter,

which makes for a cleaner and safer

assembly line. High strength

aluminium alloy (2024) is used for

aircraft applications due to its high

strength to weight ratio. Currently, the

major joining process employed is

riveting. Friction stir welding (FSW) is

a new solid state joining technique

which offers substantial improvements

over riveting in terms of weight saving

and mechanical integrity.

Even high strength materials like

Austenitic stainless steels which can

easily be welded using conventional

arc welding and other processes.

However, FSW can offer lower

distortion, lower shrinkage and

porosity as the maximum temperature

reached is of the order of 0.8 of the

melting temperature. More important is

the avoidance of fumes containing

hexavalent chromium which is

carcinogenic. In addition, chemical

segregation effects associated with

welding processes involving

solidification are avoided.

FSW has unique ability to retain near-

parent metal properties across the

weld, especially strength and ductility.

Because it does not melt the material,

there is minimal property change due

to a heat-affected zone, and

contaminant inclusions are minimized

versus other welding techniques. It has

greater tolerance than all other welding

techniques for butt weld gaps, poor

sheet edge conditions, lubricants,

oxides, and other contaminants.

FSW does not require ventilation and it

requires much lower energy inputs. It

is used to both seam weld and to spot

weld. Lower energy input results in

low distortion seam weld types

including butt weld, lap weld, or

penetration weld. It can work on sheet

materials, tubes, extrusions, or

complex castings or forgings. It can be

used to eliminate porosity and other

defects from castings with minimal

property changes.

FSW covers a wide range of materials

which can be successfully welded

include Copper and its alloys (up to

50mm in one pass)Lead, Titanium and

its alloys, Magnesium alloys, Zinc

Plastics, Mild steel, Stainless steel

(austenitic, martensitic),Nickel alloys.

Diverse materials: Welds a

wide range of alloys, including

previously unweldable and

composite materials.

Durable joints: Provides twice

the fatigue resistance of fusion

welds and no keyholes.

Retention of material

properties: Minimizes material

distortion.

Safe operation: Does not create

hazards such as welding fumes,

radiation, high voltage, liquid

metals or arcing.

FSW – Applications:

Shipbuilding and marine industries

The shipbuilding and marine industries

are two of the first industry sectors

which have adopted the process for

commercial applications. The process

is suitable for the following

applications:

Aerospace industry

At present the aerospace industry is

welding prototype and production parts

by friction stir welding. Opportunities

exist to weld skins to spars, ribs, and

stringers for use in military and civilian

aircraft. In which a high proportion of

the rivets are replaced by friction stir

welding, has made many certification

flights. This offers significant

advantages compared to riveting and

machining from solid, such as reduced

manufacturing costs and weight

savings. Longitudinal butt welds in Al

alloy fuel tanks for space vehicles have

been friction stir welded and

successfully used. The process could

also be used to increase the size of

commercially available sheets by

welding them before forming. The

friction stir welding process can

therefore be considered for:

Railway industry

The commercial production of high

speed trains made from aluminium

extrusions which may be joined by

friction stir welding has been

published. Applications include:

Land transportation

The friction stir welding process is

currently being used commercially,

and is also being assessed by several

automotive companies and suppliers to

this industrial sector for its commercial

application. Existing and potential

applications include:

Limitations:

However, FSW produces a

heterogeneous microstructure in the

weld zone, causing corrosion

problems. The variation of

microstructure is caused by the

different frictional heat input

determined by welding parameters,

especially travel and spindle speeds.

Steel can be friction stir welded but the

essential problem is that tool materials

wear rapidly. Indeed, the wear debris

from the tool can frequently be found

inside the weld.

FSW uses forces, which are

significantly higher relative to arc

welding. Therefore, the design of the

joint and the fixture, as well as the

rigidity of the equipment required, are

factors to be considered.

However, the main limitations of the

FSW process are at present:

Work pieces must be rigidly

clamped due to high forces

involve in welding

Backing bar required (except

where self-reacting tool or

directly opposed tools are used)

Keyhole at the end of each

weld

Cannot make joints which

required metal deposition (e.g.

fillet welds)

Conclusion:

FSW mechanical properties were

found to have greater strength and

twice the ductility when compared to

conventional GMAW properties.

Reference:

www.twi.co.uk

www.msm.cam.ac.uk

Modern welding techniques by