Overview of friction stir welding

Vamsi Krishna. R
12ETMM10
Int. M.Tech / Ph.D
Materials 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 Welding
The 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 Welding
The 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 Welding
Ultrasonic
Diffusion
(vi)Newer Welding
Electron-beam
Laser

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 Analysis
A. 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 speeds
Tool tilt and plunge depth
Tool 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.

Advantages
Low 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
floors
Helicopter landing platforms
Marine and transport structures
Aerospace Industry
Wings, fuselages
Cryogenic fuel tanks for space vehicles
Aviation fuel tanks
External throw away tanks for military aircr
Military and scientific rockets
Land Transportation
Wheel rims
Truck bodies & tail lifts for lorrie
Mobile cranes
Fuel tankers
Caravans
Railway Industry
Rolling stock of railways and underground carriages
Railway tankers and goods wagons
Container 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

Overview of friction stir welding

Tool design
The design of the toolis 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

Overview of friction stir welding

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Overview of friction stir welding