Friction stir welding

ISHTIAQUE AHMEDROLL: 113217

MECHANICAL ENGINEERINGNATIONAL INSTITUTE OF TECHNOLOGY, WARANGAL.

FRICTION STIR WELDING

CONTENTS:

INTRODUCTIONCLASSIFICATION OF WELDINGWORKING PRINCIPLE OF FSW STEPS OF FSWTOOL USEDPIN DESIGNMICROSTRUCTURE ANALYSISEFFECT OF ROTATIONAL SPEED ON WELD QUALTYPLUNGE DEPTH AND TILT ANGLE ADVANTAGESDISADVANTAGESCHALLENGESAPPLICATIONSCONCLUSION REFERENCES

INTRODUCTION:

FSW is a solid state welding process performed attemperatures lower than the melting point of the alloy.

Friction Stir Welding (FSW) was invented by The

Welding Institute (TWI) in England in 1991.

Weld is created by means of friction heating and

mechanical deformation.

Unlike fusion welding here no filler material is used.

Commonly used for aluminum and its alloys.

CLASSIFICATION OF WELDING

• Mainly welding is classified into two categories. They are:

a)Fusion welding :* heated to molten state* no pressure required* Example: Gas welding, Arc welding

b) Plastic welding:*heated to plastic state*pressure required*Example: friction welding, forge welding

WORKING PRINCIPLE OF FSW

WORKING PRINCIPLE OF FSW(continued):

WORKING PRINCIPLE OF FSW(continued):

FSW a cylindrical, shouldered tool with a profiled probe/pin is rotated andslowly plunged into the joint line between two pieces butted together.

The parts have to be suitably clamped rigidly on a backing bar to prevent theabutting joint faces from being forced apart

The length of the pin is slightly less than the required weld depth. Theplunging is stopped when the tool shoulder touches the surface of the job

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

The plasticized material is transferred the front edge of the tool to back edgeof the tool probe and it’s forged by the intimate contact of the tool shoulderand pin profile.

As the tool is moved along the seam the desired joint is created.

STEPS OF FSW PROCESS:

a) 1. Plunge stageb) 2. Dwell stagec) 3. Welding stage d) 4. Pull out stage.

STEPS OF FSW PROCESS(continued):

MICROSTRUCTURE ANALYSIS OF ESW:

The macro and microstructural investigation reveal that the friction stir weldment is composed of four different regions namely

1. Weld Nugget (WN) or stirred zone, 2. Thermo-Mechanically Affected Zone (TMAZ), 3. Heat Affected Zone (HAZ) and 4. Unaffected material.

MICROSTRUCTURE ANALYSIS OF ESW(continued):

Unaffected material or parent metal

This is material remote from the weld, which has not been deformed.

it may have experienced a thermal cycle from the weld .but it is not affected by the

heat in terms of microstructure or mechanical properties.

Heat affected zone (HAZ)

In this region 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.

MICROSTRUCTURE ANALYSIS OF ESW(continued):

Thermo-mechanically affected zone (TMAZ)

In this region, the material has been plastically deformed by the friction stir welding tool.

In the case of aluminum, there is generally a distinct boundary between the recrystallized zone and the deformed zones of the TMAZ.

In other materials, the distinct recrystallized region (the nugget) is absent, and the whole of the TMAZ appears to be recrystallized.

Weld Nugget

The recrystallized area in the TMAZ in aluminum alloys has traditionally been called the nugget.

this term is descriptive, it is not very scientific.

TOOL USED:

Wear resistance Chemical stability Should withstand high temperature. Commonly used tool materials:

• High speed steel• EN Steel• PCBN

PIN DESIGN:

Tool pin the most important part of FSW arrangement. It mainly perform the stirring function and also frictional heating. Some of popular pin profile is described below:

a) Round bottom cylindrical pinb) Flat bottom cylindrical pinc) Truncated cone pind) Whorl pine) MX Triflute pin

PIN DESIGN(continued):

a) Round bottom cylindrical pin:

The pin consists of a cylindrical threaded pin with a round bottom. Threads are used to transport material from the shoulder down to the

bottom of the pin. A round or doomed end to the pin tool reduces the tool wear upon plunging

and improves the quality of the weld root directly underneath the bottom of the pin.

b) Flat bottom cylindrical pin:

The bottom pin is currently the most commonly used pin design. Flat bottom pin has higher surface velocity than round bottom pin. so it has

higher affect on the material below it. It is easier to machine.

PIN DESIGN(continued):

PIN DESIGN(continued):

c) Truncated cone pin: Cylindrical pins were found to be sufficient for aluminium plate

up to 12mm thick, but to weld thicker plates at fast travel speeds it is not sufficient.

This can be done by a simple modification of the cylindrical pin i.e, Truncated cone pin.

Truncated cone pins have lower traverse loads (when compared to a cylindrical pin) and the largest moment load on a truncated cone is at the base, where it is the strongest.

PIN DESIGN(continued):

d) Whorl pin: The whorl pin reduces the displaced volume of a cylindrical pin

of the same diameter by 60%. Reducing the displaced volume also decreases the traverse

loads, which enables faster tool travel speeds. The key difference between the truncated cone and the whorl

pin is the design of the helical ridge on the pin surface. In the case of a whorl pin, the helical ridge is more than an external thread, but the helical ridge acts as an auger, producing a clear downward movement.

e) MX Triflute pin: The MX Triflute pin is a further refinement of the whorl pin. In

addition to the helical ridge, the MX Triflute pin contains three cut into the helical ridge.

The flutes reduced the displaced volume of a cylindrical pin by 70% and supply additional deformation at the weld line

PIN DESIGN(continued):

EFFECT OF ROTATIONAL AND TRAVEL SPEED:

An optimum range of rotational speed exists for better quality of the weld (For commercial aluminium 1200-1400 RPM is the range)

If the speed is lower than this value the friction heat produced is less sufficient and quality of weld decreases.

If the speed is higher than the optimum range excessive heat produced results in cracking and quality of weld decreases.

TOOL TILT ANGLE AND PLUNGE DEPTH:

• 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, has been found to

assist this forging process.

ADVANTAGES:

• Good mechanical properties .• Improved safety due to the absence of toxic fumes or the spatter of molten material.• No consumables 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• Generally good weld appearance and minimal thickness under/over-matching, thus reducing the need for expensive machining after welding.• Low environmental impact.

DISADVANTAGES:

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). Initial cost of the machine is very high compared to fusion

welding

CHALLENGES OF FSW:

• Main applications of FSW remain limited toAluminium and its alloys.

• It also faces challenges from welding ofdissimilar metals.

• Recently researches have been made on steeland dissimilar metals by changing the processparameters. The results are satisfactory.

APPLICATION:

Shipbuilding and OffshoreAerospaceAutomotiveRailwaysFabricationRoboticsPersonal Computers

CONCLUSION:

Friction Stir Welding is a promising process and has clearadvantages in terms of the mechanical properties of the weldedmaterial. FSW has already found great applications in theAluminium industry. Therefore it becomes imperative to try touse it with other materials including stainless steels.

REFERENCES:

• Welding and Welding technology, Richard.L.Little, Tata McGraw hill

• Friction Stir Welding and Processing by Rajiv S. Mishra, Murray W. Mahoney

• A Text book of welding technology, O.P.Khanna, Dhanapath Rai publications