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Saturday, 13 July 2013

Friction stir welding by moses dhilip kumar

Friction stir welding 


Abstract:
Friction stir welding is a promising solid state joining process and is widely being considered for aluminium
alloys. In this work, the micro structural and corrosion properties of friction stir welded 7075 Al alloy were
studied. The microstructures of the base metal, bore metal, thermo-mechanically affected zone (TMAZ) and
Weld region were characterized by optical microscopy and transmission electron microscopy. Micro-hardness
profile was obtained across the weld. The pitting corrosion properties of the weldments were studied in
3.5%NaCl solution. Friction stir welding of this alloy resulted in fine recrystallized grains in weld nugget which
has been attributed to frictional heating and plastic flow. The process also produced a softened region in the
Weld nugget, which may be due to the dissolution and growth of possible precipitates, identified as Mg32 (Al,
Zn) 49. Corrosion resistance of weld metal has been found to be better than that of TMAZ and base metal.
INTODUCTION:
The process of Friction Stir Welding has Been
widely used in the aerospace, Shipbuilding,
automobile industries and in many applications of
commercial importance. This is because of many of
its advantages over the conventional welding
techniques some of which include very low
distortion, no fumes, porosity or spatter, no
consumables (no filler wire), no special surface
treatment and no shielding gas requirements. FSW
joints have improved mechanical properties and are
free from porosity or blowholes compared to
conventionally welded materials. However along
with these advantages there are a few
disadvantages, which also need to be mentioned. At
the end of the welding process an exit hole is left
behind when the tool is withdrawn which is
undesired in most of the applications. This has been
overcome by providing an offset in the path for
continuous trajectory, or by continuing into a
dummy plate for non-continuous paths, or simply
by machining off the undesired part with the hole.
Large down forces and rigid clamping of the plates
to be welded are a necessity for this process, which
causes limitation in the Applicability of this
process to weld jobs with certain geometries. In
FSW, a cylindrical-shouldered tool, with a profiled
threaded/unthreaded probe or pin is rotated at a
constant speed and fed at a constant traverse rate
into the joint line between two pieces of sheet or
plate material, which are butted together as The
parts have to be clamped rigidly onto a backing bar
in a manner that prevents the abutting joint faces
from being forced apart. The length of the pin is
slightly less than the weld depth required and the
tool shoulder should be in intimate contact with the
work piece surface. The pin is then moved against
the work piece, or vice-versa. Frictional heat is
generated between the wear resistant welding tool
shoulder and pin, and the material of the workpieces.
This heat, along with the heat generated by
the mechanical mixing process and the adiabatic
heat within the material, cause the stirred materials
to soften without reaching the melting point (hence
cited a solid-state process). As the pin is moved in
the direction of welding the leading face of the pin,
assisted by a special pin profile, forces plasticized
material to the back of the pin whilst applying a
substantial forging force to consolidate the weld
metal. The welding of the material is facilitated by
severe plastic deformation in the solid state
involving dynamic recrystallization of the base
material.
The friction stir welded plates were taken for nondestructive
evaluation comprising of the die
penetration test and X-ray Radiography. Only those
sample plates that qualified in the aforementioned
tests Were taken for detailed micro structural
Characterization. For the optical microscopy the
samples were cut in a direction perpendicular to the
welding direction. Some of these samples were
given a post weld heat treatment schedule
consisting of solution zing. The micro hardness
measurements were taken on the cross section
perpendicular to the welding direction using an
indenter with A load of 50 gf for a dwell period of
10s.To evaluate the mechanical properties standard
tensile specimens were fabricated in a direction
perpendicular to the welding Direction having a
gauge length of 25 mm.These were then tested on a
screw driven Instron Machine at strain rates of 10-3
sec-1 The fractured surfaces were examined using
an SEM.for different periods of 4, 8, 12 and 18
hrs.These samples were then grinded successively
on Sic papers of grit 220 to 600. After which they
were polished on a fine cloth using a 1μm diamond
paste to obtain a mirror finish.
In the nugget region, which experiences higher
temperatures than the remaining regions, the
dissolved precipitates do reprecipitate
subsequently. Here the precipitates are finer and
uniformly distributed in the nugget region. The
micro hardness values for the PWHT Specimen
show no such characteristic Distribution and are
more or less uniform throughout. The micro
hardness values of the solution zed parent material
and those of the parent material in the T6 temper
National Conference on RIME 2013
Mailam Engineering College, Mailam
are shown by horizontal dotted and continuous
lines respectively for comparison.
Machining off the undesired part with the
Hole. Large down forces and rigid clamping of the
plates to be welded are a necessity for this process,
which causes limitation in the applicability of this
process to weld jobs with certain geometries. In
FSW, a cylindrical-shouldered tool, with a profiled
threaded/unthreaded probe or pin is rotated at a
constant speed and fed at a constant traverse rate
into the joint line between two pieces of sheet or
plate material, which are butted together The parts
have to be clamped rigidly onto a backing bar in a
manner that prevents the abutting joint faces from
being forced apart. The length of the pin is slightly
less than the weld depth required and the tool
shoulder should be in intimate contact with the
work piece surface. The pin is then moved against
the work piece, or vice-versa.
Frictional heat is generated between the
wear resistant welding tool shoulder and pin, and
the material of the work-pieces. This heat, along
with the heat generated by the mechanical mixing
process and the adiabatic heat within the material,
cause the stirred materials to soften without
reaching the melting point hence cited a solid-state
process. As the pin is moved in the direction of
welding the leading face of the pin, assisted by a
special pin profile, forces plasticized material to the
back of the pin whilst applying a substantial
forging force to consolidate the weld metal. The
welding of the material is facilitated by severe
plastic deformation in the solid state involving
dynamic recrystallization of the base material.
Friction stir welding is not used for hard alloys
because of premature tool failure. A scheme is
created that exploits the physical three-dimensional
heat and mass flow models, and implements them
into a fast calculation algorithm, which, when
combined with damage accumulation models,
enables the plotting of tool durability maps that
define the domains of satisfactory tool life. It is
shown that fatigue is an unlikely mechanism for
tool failure, particularly for the welding of thin
plates. Plate thickness, welding speed, tool
rotational speed, shoulder, and pin diameters and
pin length all affect the stresses and temperatures
experienced by the tool. The large number of these
variables makes the experimental determination of
their effects on stresses and temperatures
intractable and the use of a well-tested, efficient
friction stir welding model a realistic undertaking.
An artificial neural network that is trained and
tested with results from a phenomenological model
is used to generate tool durability maps that show
the ratio of the shear strength of the tool material to
the maximum shear stress on the tool pin for
various combinations of welding variables. These
maps show how the thicker plates and faster
welding speeds adversely affect tool durability and
how that can be optimized.
Friction Stir Welding is the most recent upgrade to
the Space Shuttle’s gigantic External Tank, the
largest element of the Space Shuttle and the only
element not reusable. The new welding
technique—being marketed to industry—utilizes
frictional heating combined with forging pressure
to produce high-strength bonds virtually free of
defects. Friction Stir Welding transforms the metals
from a solid state into a "plastic-like" state, and
then mechanically stirs the materials together under
pressure to form a welded joint. Invented and
patented by The Welding Institute, a British
research and technology organization, the process
is applicable to aerospace, shipbuilding, aircraft
and automotive industries. One of the key benefits
of this new technology is that it allows welds to be
made on aluminum alloys that cannot be readily
fusion arc welded, the traditional method of
welding. In 1993, NASA challenged Lockheed
Martin Laboratories in Baltimore, Md., to develop
a high-strength, low-density, lighter-weight
replacement for aluminum alloy Al 2219–used on
the original Space Shuttle External Tank. Lockheed
Martin, Reynolds Aluminum and the labs at
Marshall Space Flight Center in Huntsville, Ala.,
were successful in developing a new alloy known
as Aluminum Lithium Al-Li 2195, which reduced
the weight of the External Tank by 7,500 pounds
(3,402 kilograms). Today, the External Tank
project uses the new alloy to build the Shuttle’s
Super Lightweight Tanks. The lithium in the new
lighter-weight material—aluminum lithium alloy
Al-Li 2195—made the initial welds of the External
Tank far more complex. The repair welds were
difficult to make and the joint strength of the
External Tank had much lower mechanical
properties. This drove up production cost on the
tank. In an effort to mitigate the increased
production cost and regain the mechanical
properties of the earlier Al 2219 External Tank the
project began researching alternative welding
techniques. Because Friction Stir Welding produces
stronger welds—that are easier to make—the
External Tank Project Managers chose to use the
process on its Super Light Weight Tank, which is
made from Al-Li 2195. The Friction Stir Welding
process produces a joint stronger than the fusion
arc welded joint, obtained in the earlier Light
Weight Tank program. A significant benefit of
Friction Stir Welding is that it has significantly
fewer process elements to control. In a Fusion
weld, there are many process factors that must be
controlled–such as purge gas, voltage and
amperage, wire feed, travel speed, shield gas, arc
gap. However, in Friction Stir Weld there are only
three process variables to control: rotation speed,
travel speed and pressure, all of which are easily
controlled. The increase in joint strength combined
National Conference on RIME 2013
Mailam Engineering College, Mailam
with the reduction in process variability provides
for an increased safety margin and high degree of
reliability for the External Tank. How does Friction
Stir Welding work? First, a dowel is rotated
between 180 to 300 revolutions per minute,
depending on the thickness of the material. The pin
tip of the dowel is forced into the material under
5,000 to 10,000 pounds per square inch (775
to1550 pounds per square centimeter) of force. The
pin continues rotating and moves forward at a rate
of 3.5 to 5 inches per minute (8.89 to 12.7
centimeters per minute). As the pin rotates, friction
heats the surrounding material and rapidly produces
a softened "plasticized" area around the pin. As the
pin travels forward, the material behind the pin is
forged under pressure from the dowel and
consolidates to form a bond. Unlike fusion
welding, no actual melting occurs in this process
and the weld is left in the same fine-grained
condition as the parent metal. One of the early
drawbacks to the friction stir process was the fixed
pin, because it limited welding to materials with a
constant thickness. The Shuttle’s External Tank
project developed a through-spindle retractable pin
tool that can retract or expand its pin tip within the
material. This allows for changes in thickness such
as on the tank’s longitudinal barrel. The viability of
the technology was demonstrated when NASA’s
Marshall Center used the retractable pin tool to
weld a full-scale External Tank hydrogen barrel.
The External Tank project will implement Friction
Stir Welding on the longitudinal barrel welds on
both the liquid oxygen and hydrogen tanks.
External Tank 134—scheduled to fly in January
2005—will be the first tank to incorporate the
process. The Marshall Center is NASA’s lead
center for development of space transportation and
propulsion systems, including the development of
the Space Shuttle’s External Tank, Solid Rocket
Boosters, Reusable Solid Rocket Motors and Main
Engines.
CONCLUSION
The overall conclusion of our paper is that
friction stir welding is the technique
mainly implies over the subtractions of the
materials and the implementation of the
materials in eager technology and the cost
of production is low and production is
high the material can be easily saved the
distortion of the materials is less when
compared to the other sections .easier in
the operations system over all the friction
stir is the best eroding in all the way.

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