Friction stir welding

1. FRICTION STIR WELDING
MADAN PATNAMSETTY
KHASHAYAR KHANLARI

2. FRICTION
• FRICTION is the force resisting the relative motion of solid surfaces, fluid layers, and material
elements sliding against each other
• when surfaces in contact move relative to each other, the friction between the two surfaces
converts kinetic and heat energy

3. FRICTION WELDING
• The process that use machines that are designed to convert mechanical
energy into heat at the joint to be welded.
• This process accomplishes welding by bringing atoms of the materials
being joined to equilibrium spacing principally through plastic
deformation due to the application of pressure at temperatures below
the melting point of the base materials, without the addition of filler
that melts.
• This process rely on friction to cause heating and bring atoms or
molecules together by microscopic plastic deformation to produce a
weld

4. INTRODUCTION
• FRICTION STIR WELDING (FSW) TECHNIQUES,
INVENTED AT THE WELDING INSTITUTE OF THE
UNITED KINGDOM IN 1991
• A CYLINDRICAL TOOL WITH A PIN LIKE
ATTACHMENT IS ROTATED AND SLOWLY INSERTED
INTO THE RIGIDLY CLAMPED JOINT TO BE WELDED.
• THE FRICTIONAL AND DEFORMATIONAL EFFECTS
DUE TO THE ROTATING TOOL SURFACE IN
CONTACT WITH THE WORK PIECE CAUSE
PLASTICIZATION OF THE METALS TO BE JOINED.
• THUS IT MAY BE CALLED AS DEFORMATION
PROCESS USED TO JOIN METALS.
http://products.asminternational.org/hbk/content/V06A/D03/graphics/inline/i00

5. ZONES AFFECTED
• THE TOOL ROTATION DIRECTION HAS
SIMILAR SENSE TO THE TOOL
TRANSLATION DIRECTION, IS KNOWN
AS THE ADVANCING SIDE OF THE
WELD
• THE TOOL ROTATION IS OPPOSITE TO
THE TOOL TRANSLATION DIRECTION,
IS KNOWN AS THE RETREATING SIDE
OF THE WELD
• ZONE IN CONTACT WITH SHOULDER
IS SHOULDER EFFECTED ZONE AND
ZONE IN CONTACT WITH PIN IS PIN-
AFFECTED ZONE
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ne/i0082748.jpg

6. THERMAL ASPECTS
• THE WELDING CYCLE IS SPLIT IN FOUR STAGES DURING WHICH THE HEAT FLOW AND THERMAL
PROFILE WILL BE VARIED.
1. DWELL: THE MATERIAL IS PREHEATED BY A STATIONARY, ROTATING TOOL TO ACHIEVE A SUFFICIENT TEMPERATURE
AHEAD OF THE TOOL TO ALLOW THE TRAVERSE. THIS PERIOD MAY ALSO INCLUDE THE PLUNGE OF THE TOOL INTO
THE WORK PIECE.
2. TRANSIENT HEATING: WHEN THE TOOL BEGINS TO MOVE THERE WILL BE A TRANSIENT PERIOD WHERE THE HEAT
PRODUCTION AND TEMPERATURE AROUND THE TOOL WILL ALTER IN A COMPLEX MANNER UNTIL AN ESSENTIALLY
STEADY-STATE IS REACHED.
3. PSEUDO STEADY-STATE: ALTHOUGH FLUCTUATIONS IN HEAT GENERATION WILL OCCUR THE THERMAL FIELD
AROUND THE TOOL REMAINS EFFECTIVELY CONSTANT, AT LEAST ON THE MACROSCOPIC SCALE.
4. POST STEADY-STATE: NEAR THE END OF THE WELD HEAT MAY "REFLECT" FROM THE END OF THE PLATE LEADING TO
ADDITIONAL HEATING AROUND THE TOOL.
• HEAT IS GENERATED PRIMARILY FROM TWO SOURCES
1. FRICTIONAL
2. PLASTIC DEFORMATION
• FRICTIONAL COMPONENT IS AN OUTCOME OF ROTATIONAL MOVEMENT OF TOOL SHOULDER AND
PIN SURFACE AND IS ALSO THE PRIMARY HEAT SOURCE.
• THE FRICTIONAL HEAT PRODUCED SOFTENS THE METAL WHICH CAUSES PLASTIC DEFORMATION.
• DURING PSEUDO-STEADY STEADY STATE WELDING BOTH FRICTIONAL AND PLASTIC DEFORMATION
CONTRIBUTE TO OVER ALL HEAT GENERATION.

7. HEAT GENERATION
• The heat generation is assumed to occur predominantly under the shoulder,
due to its greater surface area, and to be equal to power required
to overcome the contact forces between the tool and work piece.
• The contact condition under the shoulder can be described by sliding friction,
using a friction coefficient μ and interfacial pressure P, based on the interfacial
shear strength at an appropriate temperature and strain rate.
• The mathematical equation for total heat generated by the tool shoulder is Qtotal
have been developed from
• Q1 = Heat generated at the tool shoulder,
• Q2 = Heat generated at the tool pin,
• Q3 = Heat generated at the tool pin tip

8. HEAT GENERATION
dQ = ω r dF
Where Q1 = 2π(1 + tan α )τshear (R3
shoulder- R3
pin )
Q2 = 2 π τshear R2
pin ω
Q3 = 2/3. π. τshear ω R3
pin
Qtotal = Q1+Q2+Q3
α is the inclination of conical surface of the shoulder
Image courtesy from P.S. De, N. Kumar, J.Q. Su, and R.S. Mishra, Fundamentals of
Friction Stir Welding

9. MATERIAL FLOW
• With respect to all the thermal aspects in FSW, Plastic flow plays an important role in
heat generation.
• Experiments on Material flow in FSW are categorised as
1. Marker studies: A small quantity of dissolvable materials are placed in grooved along
the welding path and thus the FSW tool passing displaces the markers from which the
material is reconstructed
2. Dissimilar welding: Materials those are differently etched are welded together, with the
material flow is reconstructed by post welding etching. And also the microstructural
changes after the FSW are used to understand the material movement.
• The basic material flow characteristics from the above kind of experiments are
1. Near the shoulder the atoms from retreating side are diffused or dragged towards the
centre weld line and deposited on advanced side
2. Atoms from the line intersecting pin diffuses and deposit behind the tool through the
retreating side for not more than 1 pin diameter, exception with welds of lower
thickness for which the shoulder has its domination.
3. Flow pattern similar to shoulder area is visible at pin bottom but at very low scale(area).
4. Tool run out can vary material flow in addition to causing periodic variation in welding

10. MATERIAL FLOW
• Distribution showing the position of AA5454-O markers
(white region) in AA2195-T8 alloy after the passage of
friction stir tool pin. The markers placed perpendicular to
the weld path moved backward by a distance equal to the
chord length intersecting the pin circle, and oriented
parallel to welding direction. The double arrow gives a
measure of the pin diameter
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/i0082750.jpg
http://products.asminternational.org/hbk/content/V06A/D03/graphics/inli
ne/i0082751.jpg
• Difference in grain size within the ring patterns
observed in the horizontal-section microstructure of a
bead-on-plate run of cast A356 alloy. The tool
advance per revolution (APR) was 0.1 mm (0.004 in)
• Onion rings type of bands are formed alternating with
different grain size and particle size distribution in the
weld nugget.
• Bands are separated by an equal distance or less than
tool advance per revolution.

11. MATERIAL FLOW
A schematic view of the material
transport in the shoulder/work piece
interface region
(a, b) and the pin/work piece interface
region (c, d)
Image courtesy P.S. De, N. Kumar, J.Q. Su, and R.S. Mishra,
Fundamentals of Friction Stir Welding

12. FRICTIONAL COEFFICIENT
• According to Schmidt and Hattel’s simulation, during the pseudo- steady-state
period, the contribution of friction to generate heat is reduced by 25% of total
heat generated, Thus for an accurate FSW process simulation, a proper
assessment of frictional heat contribution is essential.
• Frictional forces are classified into two types:
1. Coulomb’s frictional force: The frictional force may be expressed as
σ = μP
And μ is Coulomb’s co-efficient of friction and p is pressure applied.
2. constant shear Force: The frictional stress for constant shear model is
σ = m(σ0/√3)
m = ionic factor and σ0 = flow stress of the material

13. FRICTIONAL COEFFICIENT
• But after the modification of the same theory there are three conditions assumed to be
existed. They are sticking, sliding, and partial sliding/sticking
conditions.
• For sliding condition m value is 0 and for sticking condition the m value is 1, at times the
co-efficient of friction is greater than 1 (1.3)
Variation of coefficient of friction with torque for
AA5182- and F-357-type aluminium alloys
http://products.asminternational.org/hbk/content/V06A/D03/graphics/inline/
i0082753c.jpg

14. DEFECTS
• Defects mainly occur due to flow related or geometric.
• There are various types pf defects which includes
1. wormhole or voids: is the tunnel of inadequately consolidated and forged
material running in the longitudinal direction which is formed due to
excessive heat input due to high rotational speed low transverse feed
2. Lack of penetration:
3. Lack of fusion:
4. Surface lack of fill: due to insufficient forge pressure
5. Root-flow defect :
6. Nugget collapse: A unique defect which is caused high welding speed and
excessive metal flow to stir zone and excessive hot weld.
7. Surface galling: sticking of material at the tip of the pin causes surface
galling which is tearing of metal on top surface of weld.
8. Ribbon flashing: excessive expulsion of material on the surface of welding
9. Scalloping: defect in which the series of small voids located in the
advancing side interleaving the stir zone along the weld

15. DEFECTS
Image courtesy from 6A, ASM
Handbook, ASM International,
2011, p 186–200
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i0082754.jpg

16. STRAIN AND STRAIN RATE
• Since the plastic deformation its is obvious to experience the strain and strain rate
through the process.
• There is a continuous rise in strain and strain rate as it approaches the shear zone with a
maximum strain of at the shear zone boundary
• The deformation in FSW is similar to torsional which leads to an expression for strain rate.
έ = Rm.2πre/Le  C.I. CHANG, C.J. LEE, AND J.C. HUANG, SCR. MATER., VOL 51, 2004, P 509–514
Where Rm is average material flow rate re is the radius of the dynamically recrystallized zone,
and le is the depth of the dynamically recrystallized zone.
• The strain in shear zone is expressed as
ε = ln(l/APR)+|ln(APR/l)| T. LONG, W. TANG, AND A.P. REYNOLDS, SCI. TECHNOL. WELD. JOIN., VOL 12, 2007, P
311–317
Where l = 2 r Cos-1[(r-x)/r] and APR is advance per revolution of tool
R, r and x are the radius of pin and distance of the streamline from
the retreating side of the tool

17. STRAIN AND STRAIN RATE
• The estimated strain distribution in
the processed zone for an APR of 0.5
mm/rev and a pin diameter of 10 mm
is presented in where strain on the
retreating side is observed to be zero
and reaches a maximum (~8) on the
advancing side
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/i0082762.jpg
• The calculated average strain and
strain rate variation with tool
rotation is presented in where
both the parameters increases
with increase in tool rotational rate
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18. MICROSTRUCTURAL FEATURES
• The microstructure depends on alloy
composition, initial
material temperature, welding parameter,
tool geometry, and cooling rate
• The asymmetric nature of FSW resulted in high
characteristic microstructure, the zone can be
broken up as, with different microstructures in
each zone
1. THE STIR ZONE (NUGGET): The zone which
experiences extreme deformation and is a
consequence of rotating tool. And under some
processing conditions, an onion ring structure
develops. The interface between the parent
material is relatively diffusive and it’s a quite
sharp towards the advancing side.
2. THERMO-MECHANICALLY AFFECTED
ZONE (TMAZ): A special zone in between nugget
and HAZ which experiences both temperature
http://products.asminternational.org/hbk/content/V06A/D03/graphics/inline/i
0082764.jpg
Typical macrograph showing various microstructural
zones in friction stir welded 7075Al-T651.

19. MICROSTRUCTURAL FEATURES
• THE STIR ZONE (also nugget, dynamically
recrystallized zone) heavy deformation is
occurred and fine-grained microstructure is formed.
And this region is also named as dynamically
recrystallized zone. And the grains found are
equi-axed
• The THERMO-MECHANICALLY AFFECTED ZONE (TMAZ)
in this zone the grains deform in an upwards pattern
around the nugget. and the full recrystallization
doesn’t occur due to insufficient deformation strain.
The grains are highly dense in sub boundaries
• The HEAT-AFFECTED ZONE (HAZ) is common to all
welding processes. As indicated by the name, this
region is subjected to a thermal cycle but is not
deformed during welding. The temperatures are lower
than those in the TMAZ(Or lower than solidus
temperature) but may still have a significant effect if
the microstructure is thermally unstable. In fact, in
http://products.asminternational.org/hbk/content/V06A/D03/graphics/inline
/i0082765.jpg
Grain structure variations across
friction stir welded 7050-T651
aluminium alloy. (a) Unaffected parent
material. (b) Heat-affected zone. (c)
Thermo mechanically affected zone.

20. MICROSTRUCTURAL FEATURES
Material
Grain size
μm
7xxx 0.1–7.5
6xxx 5.9–17.8
5xxx 3.5–6
2xxx 0.5–12
1xxx 0.5–20
Al-Cu-Mg-Ag-T6 5
Al-Li-Cu 9
Al-4Mg-1Zr 1.5
Al-Zn-Mg-6.7Sc 0.68
AZ31 0.085–50
AZ91 15
Mg-6Al-3Ca-0.5Re-0.2Mn 0.9
Mg-5.5Y-4.3Zn 1
• GRAIN SIZE IN NUGGET ZONE OF
FRICTION STIR WELDED ALUMINIUM AND
MAGNESIUM ALLOYS
NUGGET GRAIN SIZE:
• Grain size in nugget zone is influenced by
processing parameters, tool geometry,
composition of work piece, temperature of
the work piece, vertical pressure, and active
cooling.
• So variation in grain size(finer) can be
achieved by external cooling

21. NUGGET ZONE MICRO STRUCTURE
• The microstructure is not uniform
in this zone
• The grain size tend to increase in
the top side and tend to decrease
in either side of the centre line,
this is due to the variation in
temperature.
Grain size distribution in various locations of a 7050Al weld
nugget
http://products.asminternational.org/hbk/content/V06A/D03/graphics/inline
/i0082766.jpg

22. INFLUENCING WELDING PARAMETERS
• TOOL ROTATION AND TRAVERSE SPEEDS:
1. Increase in rotation speed and decrease in traverse speed the result will be hotter
weld and for a better weld it is necessary to maintain a proper heat which cause
better diffusion.
2. The high heat may also loose the weld properties.
3. The tool may break the tool in extreme cases(if its exposed to cooler environment).
• TOOL TILT AND PLUNGE DEPTH
1. The plunge depth may be defined as the lowest depth the tool is permitted to
penetrate and plunging the tool shoulder below the surface increases the pressure
below the tool and thus adequate forging is done inside the material at rear side of
tool
2. Tilting about 2-4 degrees such the rear part of tool is lower than the front will
assist a good kind and gives the torsion elliptically.
3. The high load may create weld deflections thus the plunge depth to adjusted

23. INFLUENCING WELDING PARAMETERS
• TOOL DESIGN:
• the tool to be designed with sufficient strength, toughness and hard wearing at
welding temperatures.
• To be good corrosion resistance and low thermally conductive material
Alloy
Thickness
mm Tool materials
Aluminium alloys <12 Tool steel, WC-Co
<26
MP159(nickel cobalt based
multi phased alloy)
Manganese alloys <6 Tool steel, WC
Copper and
copper alloys <50 Nickel alloys, tungsten alloys
<11 Tool steel
Titanium alloys <6 Tungsten alloys
Stainless steels <6 PCBN, tungsten alloys
low carbon steels <10 WC, PCBN
nickel steels <6
PCBN(Poly crystalline cubic
boron nitrides)

24. TOOL DESIGN
Tool Cylindrical Whorl™ MX triflute™ Flared
triflute™
A-skew™ Re-stir™
Schematics
Tool pin
shape
Cylindrical
with threads
Tapered with
threads
Threaded,
tapered with
three flutes
Tri-flute
with flute
ends flared
out
Inclined
cylindrical
with threads
Tapered with
threads
Ratio of pin
volume to
cylindrical
pin volume
1 0.4 0.3 0.3 1 0.4
Swept
volume to
pin volume
ratio
1.1 1.8 2.6 2.6 Depends on
pin angle
1.8
Rotary
reversal
No No No No No Yes
Application Butt welding;
fails in lap
welding
Butt welding
with lower
welding
Butt welding
with further
lower
Lap welding
with lower
thinning of
Lap welding
with lower
thinning of
When
minimum
asymmetry
R.S. Mishra, Z.Y. Ma / Materials
Science and Engineering R 50
(2005) 1–78

25. ADVANTAGES AND LIMITATIONS
ADVANTAGES
• Good mechanical properties in the as-welded condition
• Improved safety due to the absence of toxic fumes or the spatter of molten material.
• No consumables — a threaded pin made of conventional tool steel, e.G., Hardened H13, can weld
over 1 km (0.62 mi) of aluminium, 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.), As there is no weld pool.
• 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).
• Often slower traverse rate than some fusion welding techniques, although this may be offset if
fewer welding passes are required.

26. APPLICATIONS
• SHIPBUILDING AND OFFSHORE: HULLS AND SUPERSTRUCTURES, HELICOPTER
LANDING PLATFORMS
• AEROSPACE: WINGS, FUSELAGES, EMPENNAGES, CRYOGENIC FUEL TANKS FOR SPACE
VEHICLES
• AUTOMOTIVE: Aluminium engine cradles and suspension struts for stretched Lincoln
town car(Ford car) were the first automotive parts that were friction stir at Tower
Automotive
• RAILWAY ROLLING STOCK: RAILWAY TANKERS AND GOODS WAGONS, ROLLING
STOCK OF RAILWAYS, UNDERGROUND CARRIAGES, TRAMS
• FABRICATION: FAÇADE panels and ATHODE sheets are friction stir welded at AUSTRIA
METALL AG and HAMMERER Aluminium Industries including friction stir lap welds of
copper to aluminium
• ROBOTICS: KUKA ROBOT group has adapted its kr500-3mt heavy-duty robot for
friction stir welding via the deltan fs tool
• PERSONAL COMPUTERS: Apple applied friction stir welding on the 2012 i-Mac to
effectively join the bottom to the back of the device

27. https://www.youtube.com/watch?v=aNbQH8XBgxQ
COURTESY BY CRIQ

29. REFERENCES
• IMAGE COURTESY: HTTP://SEMINARLINKS.BLOGSPOT.FI/2014/04/FRICTION-STIR-WELDING-FSW-
SEMINAR.HTML
• IMAGE COURTESY : HTTP://GIPHY.COM/SEARCH/WELDING?SORT=RECENT
• IMAGE COURTESY : HTTP://WWW.PICGIFS.COM/JOB-GRAPHICS/WELDER/
• HTTPS://WWW.YOUTUBE.COM/WATCH?V=ANBQH8XBGXQ
• HTTP://EN.WIKIPEDIA.ORG/WIKI/FRICTION_STIR_WELDING
• HTTP://EN.WIKIPEDIA.ORG/WIKI/FRICTION_WELDING
• P.S. DE, N. KUMAR, J.Q. SU, AND R.S. MISHRA, FUNDAMENTALS OF FRICTION STIR WELDING,
• R.S. MISHRA, Z.Y. MA / MATERIALS SCIENCE AND ENGINEERING R 50 (2005) 1–78
• ASM HANDBOOK ONLINE VOLUME 6A