Vamsi Krishna. R12ETMM10

Int. M.Tech / Ph.DMaterials Engineering

SEST, UoH

Overview Of Friction Stir Welding

contents Welding Types Of Welding Introduction To Friction Stir Welding Principle Of Operation Weld Structure Analysis Important Welding Parameters Welding Forces Advantages & Disadvantages Applications Of FSW Conclusion References

WELDING

Welding is a materials joining process by heating them to suitable temperatures with or without the application of pressure with or without the use of filler material.

Welding is used for making permanent joints.

It is used in the manufacture of automobile bodies, aircraft frames, railway wagons, machine frames, structural works, tanks, furniture, boilers, general repair work and ship building.

TYPES

• Non fusion welding or Plastic Welding or Pressure WeldingThe piece of metal to be joined are heated to a plastic state

and forced together by external pressure(Ex) Resistance welding

• Fusion Welding or Non-Pressure WeldingThe material at the joint is heated to a molten state and

allowed to solidify(Ex) Gas welding, Arc welding

Classification of welding processes

(i). Arc welding Carbon arc Metal arc Metal inert gas Tungsten inert gas Plasma arc Submerged arc Electro-slag

(ii). Gas Welding

Oxy-acetylene Oxy-hydrogen

(iii). Resistance Welding

Butt Spot Seam Projection

(iv)Thermit Welding

(v)Solid State Welding

Friction Stir WeldingUltrasonicDiffusion

(vi)Newer Welding

Electron-beamLaser

Introduction to FSW

Welding using friction as the major resource No filler material involved Welds created by,

a) Frictional heating b) Mechanical deformation

Friction-stir welding (FSW)

It is a solid-state joining process (the metal is not melted)

In the process a rotating FSW tool is plunged between two clamped plates. The frictional heat causes a plasticised zone to form around the tool. The rotating tool moves along the joint line. A consolidated solid-phase joint is formed.

Friction Stir Welding transforms the metals from a solid state into a plastic state, and then mechanically stirs the materials together under pressure to form a welded joint

Principle Of Operation

In friction stir welding (FSW) a cylindrical, shouldered tool with a profiled probe is rotated and slowly plunged into the joint line between two pieces butted together.

The parts have to be clamped onto a backing bar in a manner that prevents the abutting joint faces from being forced apart.

Frictional heat is generated between the wear resistant welding tool and the material of the work pieces.

Principle Of Operation(contd.)

This heat causes the material to soften without reaching the melting point and allows traversing of the tool along the weld line.

The maximum temperature reached is of the order of 0.8 of the melting temperature of the material.

It leaves a solid phase bond between the two pieces.

The process can be regarded as a solid phase keyhole welding technique since a hole to accommodate the probe is generated, then filled during the welding sequence.

Weld Structure AnalysisA. Unaffected material B. Heat affected zone (HAZ) C. Thermo-mechanically affected zone (TMAZ) D. Weld nugget (Part of thermo-mechanically affected zone)

Important welding parameters

Tool rotation and traverse speedsTool tilt and plunge depthTool design

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.

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 minimize the forces acting on the tool.

Tool rotation and traverse speeds(contd.)

If the material is too cold then voids or other flaws may be present in the stir zone and in extreme cases the tool may break.

Tool tilt and plunge depth

Plunging the shoulder below the plate surface increases the pressure below the tool and helps ensure adequate forging of the material.

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.

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

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 the material surface.

The traverse force acts parallel to the tool motion

The lateral force may act perpendicular to the tool traverse direction

Torque is required to rotate the tool, the amount of which will depend on the down force and friction coefficient and the flow strength of the material in the surrounding region.

AdvantagesLow distortion and shrinkage, even in long welds

Excellent mechanical properties in fatigue and tensile tests

No arc or fumes

No porosity

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

Energy efficient

One tool can typically be used for up to 1000m of weld length in 6XXX series aluminium alloys

No filler wire required

No gas shielding is also required for welding

Disadvantages

Exit hole left when tool is withdrawn.

Less flexible than manual and arc processes

Work pieces must be rigidly clamped

Often slower traverse rate than some fusion welding techniques.

Cannot make joints which required metal deposition (e.g. fillet welds)

Shipbuilding and Marine Construction

Panels for decks, sides, bulkheads and floorsHelicopter landing platformsMarine and transport structures

Aerospace Industry

Wings, fuselagesCryogenic fuel tanks for space vehiclesAviation fuel tanksExternal throw away tanks for military aircraftMilitary and scientific rockets

Land Transportation

Wheel rimsTruck bodies & tail lifts for lorriesMobile cranesFuel tankersCaravans

Railway Industry

Rolling stock of railways and underground carriagesRailway tankers and goods wagonsContainer bodies

Industrial Applications

Conclusion FSW is a solid state welding process.

Friction is the major resource.

Makes joint with the application of pressure and frictional heat.

Overcomes many of the problems by traditional welding processes.

FSW has more demand in many industrial applications

An alternative to fusion welding

References

R.S. Mishraa, Z.Y. Mab Friction stir welding and processing REPORTS(a review journal) R 50 (2005) 1–78

Friction Stir Welding And Processing by Rajiv.S.Mishraa and Murray.W.Mahoney ASM text book.

http://wikipedia.org/Friction_stir_welding

http://www.twi.co.uk

THANK YOU

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 at the welding temperature.

Further it should have a good oxidation resistance and a low thermal conductivity.

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 compositesor higher melting point materials such as steel or titanium.

History

Invented and patented by The Welding Institute, a British research and technology organization,

Friction Stir Welding (FSW) was invented by Wayne Thomas at TWI Ltd in 1991 and overcomes many of the problems associated with traditional joining techniques

Microstructural features

The stir zone (also called weld nugget,)is a region of heavily deformed material that roughly corresponds to the location of the pin during welding.

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.

The heat-affected zone (HAZ) is common to all welding processes. The temperatures are lower than those in the TMAZ but may still have a significant effect if the microstructure is thermally unstable.

Barriers for FSW

Special clamping system necessary Only for simple joint geometries (e.g. butt joint) License required from TWI Few applications in the construction industry Corrosion protection is needed