Frictional Stir Welding

FRICTIONAL STIR WELDING SOLID STATE JOINING PROCESS

BAPATLA ENGINEERING COLLEGE BAPATLA

Author: Sudheer.ch Mechanical Engineering III/IV B.Tech Email: [email protected] Mobile: 9391673727 Hardware Requirement: Computer, a Data projector, a laser light.

a new technology called friction stir

Abstract:

spot 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

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

electricity

large-scale and

sources

specialized

of

joining

equipment.

beam welding, laser welding and resistance welding achieve a weld by applying direct thermal energy to the

Key

words:

Non

consumable

electrode, solid state joining process,

materials causing the materials to melt and fuse which may cause distortion

Introduction:

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,

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,

joint. Friction stir welds have been

without reaching melting point, and

fabricated in a variety of aluminum

allows the pin to traverse along the

alloys up to 50 mm thick, titanium

joint. As the tool moves along, the

alloys and steels up to 25 mm thick.

material is plasticized by the frictional

The welds can be made in any position

heat at the front of the rotating pin and

at welding speeds of a few inches per

transported to the back. Here it

minute.

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

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 cylindricalshouldered 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

parts, which are placed on top of each

retracted from the plate and leaves a

other as illustrated

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

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

deformation

and

subsequent

material properties. Then, the stirring

recrystallization are associated with the

phase enable the materials of two work

rotating tool shoulder.

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

The system divides the weld zone into distinct regions as follows: A. Unaffected material

middle.

B. Heat affected zone (HAZ) C. Thermo-mechanically affected zone (TMAZ)

Microstructure Classification of

D. Weld nugget (Part of thermo-

Friction Stir Welds:

mechanically affected zone)

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

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.

temperature rise in the plate from the

Heat affected zone (HAZ): In this

friction heating. The friction stir weld

region, which clearly will lie closer to

appears broad at the top surface with a

the weld centre, the material has

smaller well-defined weld nugget in

experienced a thermal cycle which has

the

nugget

modified the microstructure and/or the

corresponds to the tool probe that

mechanical properties. However, there

penetrates through the plate thickness,

is no plastic deformation occurring in

interior.

whereas

the

The

weld

broader

surface

this area. In the previous system, this

alloys, austenitic stainless steels and

was referred to as the "thermally

copper. In materials such as ferrite

affected zone". The term heat affected

steels

zone is now preferred, as this is a

understanding the microstructure is

direct parallel with the heat affected

made more difficult by the thermally

zone in other thermal processes, and

induced phase transformation, and this

there is little justification for a separate

can

name.

boundary difficult to identify precisely.

Thermo-mechanically affected zone

Weld Nugget: The recrystallised area

(TMAZ): In this region, the material

in the TMAZ in aluminium alloys has

has been plastically deformed by the

traditionally been called the nugget.

friction stir welding tool, and the heat

Although this term is descriptive, it is

from the process will also have exerted

not very scientific. However, its use

some influence on the material. In the

has become widespread, and as there is

case of aluminium, it is possible to get

no word which is equally simple with

significant

without

greater scientific merit, this term has

recrystallisation in this region, and

been adopted. It has been suggested

there is generally a distinct boundary

that the area immediately below the

between the recrystallised zone and the

tool shoulder (which is clearly part of

deformed

TMAZ.

the TMAZ) should be given a separate

Aluminium behaves in a different

category, as the grain structure is often

manner to most other materials, in that

different here. The microstructure here

it can be extensively deformed at high

is determined by rubbing by the rear

temperature without recrystallisation.

face of the shoulder, and the material

In

distinct

may have cooled below its maximum.

recrystallised region (the nugget) is

It is suggested that this area is treated

absent, and the whole of the TMAZ

as a separate sub-zone of the TMAZ.

other

plastic

zones

strain

of

materials,

the

the

and

also

a-b

make

titanium

the

alloys

HAZ/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

The simultaneous use of two or more friction stirs as welding tools:

TM

The concept involved a pair of tools

The Twin-stir

applied on opposite sides of the work

rotating

piece slightly displaced in the direction

associated with lap welding to be

of

contra-rotating

positioned on the 'inside' between the

simultaneous double-sided operation

two welds. For low dynamic volume to

with combined weld passes has certain

static

advantages such as a reduction in

conventional rotary motion, the most

reactive

significant

travel.

The

torque

and

a

more

variant

volume

parallel contraenables

ratio

defect

defects

probes

will

using

be

'plate

symmetrical weld and heat input

thinning' on the retreating side. With

through-the-thickness. The probes need

tool designs and motions designed to

not touch together but should be

minimize plate thinning, hooks may be

positioned sufficiently close that the

the most significant defect type. The

softened 'third-body' material around

Twin-stir

the two probes overlaps near the probe

reduction in welding time for parallel

tips to generate a full through-

overlap

thickness weld. To avoid any problems

additional heat available, increased

associated with a zero velocity zone in

travel speed or lower rotation process

mid-thickness, the probes can be

parameters will be possible.

method welding.

may

allow

Owing

to

a the

displaced slightly along the direction of

travel.

Common

simultaneous

to

all

such

Tandem twin-stir:

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 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

than a single pass weld, given that the

tool.

been

detail at the extremes of the weld

produced by conventional rotary FSW,

region are similar. Residual oxides

whereby a second weld is made over a

within the overlapping region of the

previous weld in the reverse direction

two welds will be further fragmented,

with no mechanical property loss. The

broken

preliminary evidence suggests that

particularly important advantage of the

further break-up and dispersal of

staggered variant is that the second tool

oxides is achieved within the weld

can be set to overlap the previous weld

region. The Twin-stir tandem variant

region and eliminate any plate thinning

will provide a similar effect during the

that may have occurred in the first

welding

Furthermore,

weld. This will be achieved by locating

because the tool orientation means that

the retreating side of both welds on the

one tool follows the other, the second

'inside'.

Welds

have

already

operation.

up

and

dispersed.

One

tool travels through already softened material. This means that the second

Friction Stir Welding - Joint

tool need not be as robust. It is noted

geometries

that under certain circumstances these tools need not always be used in the contra-rotation

mode

and

their

rotational speed can also be varied.

The process has been used for the manufacture of butt welds, overlap welds, T-sections and corner welds. For each of these joint geometries

Staggered twin-stir:

specific tool designs are required which are being further developed and

The staggered Twin-stir means that an

optimized. The FSW process can also

exceptionally wide 'common weld

cope with circumferential, annular,

region' can be created. Essentially, the

non-linear,

tools are positioned with one in front

welds. Since gravity has no influence

and slightly to the side of the other so

on the solid-phase welding process, it

that the second probe partially overlaps

can be used in all positions. It can be

the

This

used to eliminate porosity and other

arrangement will be especially useful

defects from castings with minimal

for lap welds, as the wide weld region

property changes. FSW does not take

produced will provide greater strength

the materials to the melting point, only

previous

weld

region.

and

three

dimensional

to sufficiently plastic condition to

pressure-sintered

silicon

enable appropriate stirring.

(Kyocera SN282).

nitride

Tool requirements: Because

the

experienced

peak during

temperatures 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

Material W/m-K

Thermal

Strength

conductivity

MPa 6061+20% Al2O3 H13 steel WC10%Co Si3N4

Fracture

W/m-K

toughness

MPa 130

359

20

2000

100

2100

35

800

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-

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 conventional

air

resistance

that 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

greater tolerance than all other welding

aluminium alloy (2024) is used for

techniques for butt weld gaps, poor

aircraft applications due to its high

sheet

strength to weight ratio. Currently, the

oxides,

edge

conditions,

and

other

lubricants,

contaminants.

major joining process employed is riveting. Friction stir welding (FSW) is

FSW does not require ventilation and it

a new solid state joining technique

requires much lower energy inputs. It

which offers substantial improvements

is used to both seam weld and to spot

over riveting in terms of weight saving

weld. Lower energy input results in

and mechanical integrity.

low

distortion

seam

weld

types

including butt weld, lap weld, or Even high strength materials like Austenitic stainless steels which can easily be welded using conventional arc welding and other processes. However,

FSW

distortion,

lower

can

offer

lower

shrinkage

and

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.

porosity as the maximum temperature reached is of the order of 0.8 of the

FSW covers a wide range of materials

melting temperature. More important is

which can be

the avoidance of fumes containing

include Copper and its alloys (up to

hexavalent

is

50mm in one pass)Lead, Titanium and

carcinogenic. In addition, chemical

its alloys, Magnesium alloys, Zinc

segregation effects associated with

Plastics, Mild steel, Stainless steel

welding

(austenitic, martensitic),Nickel alloys.

chromium

processes

which

involving

successfully welded

solidification are avoided. FSW has unique ability to retain near-

•

wide range of alloys, including

parent metal properties across the

previously

weld, especially strength and ductility.

to

a

heat-affected

zone,

and

contaminant inclusions are minimized versus other welding techniques. It has

unweldable

and

composite materials.

Because it does not melt the material, there is minimal property change due

Diverse materials: Welds a

•

Durable joints: Provides twice the fatigue resistance of fusion welds and no keyholes.

•

•

Retention

of

material

and successfully used. The process

properties: Minimizes material

could also be used to increase the size

distortion.

of commercially available sheets by

Safe operation: Does not create

welding them before forming. The

hazards such as welding fumes,

friction

radiation, high voltage, liquid

therefore be considered for:

stir welding process

can

metals or arcing. Railway industry

FSW – Applications:

The commercial production of high

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

speed trains made from aluminium extrusions which may be joined by friction

stir

welding

has

been

published. Applications include: Land transportation

following The friction stir welding process is

applications:

currently being used commercially, and Aerospace industry

is also being assessed by several

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

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,

problems.

The

microstructure different

causing is

corrosion

variation caused

frictional

heat

of

by

the 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

•

www.twi.co.uk

from the tool can frequently be found

•

www.msm.cam.ac.uk

inside the weld.

•

Modern welding techniques by

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: