Owing is used for the production of large aluminum

Owing
to the fact that, it is a mechanical solid state welding technique, it can also
be applied under water. The welding speed is dependent on the thickness of the
plate to be welded. For thicker plates double sided friction stir welding can
be applied. (Refer fig.3)11

The
desire for better standard of living resulted in continuous development of the
existing manufacturing technologies. Particularly friction stir welding was
developed for welding non-weldable metals and alloys. The metals which cannot
be welded by conventional technique. Apart from that friction stir welding had
an enormous growth as we can weld similar as well as non-similar metals,
polymers, 3D printed objects, etc.

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Trends in Friction Stir Welding:

Very
suitable for robotics and automation applications.

            High strength alloys like 2XXX and
7XXX series alloys are classified as non-weldable by fusion welding technique.

So when FSW was invented, it opened new opportunities to weld high strength
aluminum alloys. Rajeev Mishra FSP has replaced riveting technology. Friction
Stir Spot welding is used in Automobile technology. Friction Stir Welding has
grown into an important industrial process that has led to many worldwide
applications, predominantly in the fabrication of aluminum components and
panels. FSW is being used in all transport industries including shipbuilding,
automotive, rail and aerospace industries 10. In shipbuilding industry, the
FSW process is used for the production of large aluminum panels, which are made
from aluminum extrusions. Large tanks for satellite launch vehicles are
fabricated by FSW from high-strength aluminum alloys for the aerospace
industry. Several companies (The Boeing Company, Eclipse Aviation Corporation,
Airbus etc.) manufacture lightweight aluminum airframe structures, fuselage and
wing applications using FSW for commercial and military aircrafts. The railway
industry uses FSW for the production of large prefabricated aluminum panels,
which are made from aluminum extrusions. The automotive industry uses FSW in
the production of components like light alloy wheels and fuel tanks.

Weld
ability is one of the most important factors determining the application of
novel materials.

APPLICATIONS:

 

            Corrosion may be a serious issue
during FSW, particularly during unprotected dissimilar alloy combinations. This
is because metallurgical and microstructural zonal heterogeneities developed
during welding process. The development of pronounced zonal heterogeneities (or
distinctly different metallurgical and microstructural zones) will, most
likely, lead to the formation of micro-galvanic corrosion cells in the weld.

Pronounced zonal heterogeneities is inevitable in welds involving dissimilar
materials. The galvanic-coupling effect between the zones can be catastrophic
if the materials are both conductive and have widely separated corrosion
potentials. Consequently, it is very important to establish the extent of zonal
heterogeneities in the microstructures of welds/dissimilar welds relative to
the welding parameters, the processing history and the compositions of the
alloys involved in order to improve in service performance where no additional
corrosion protection is present (e.g. paint scheme) 2.

CORROSION
SUSCEPTIBILITY:

 

 

The
process of friction stir welding is reasonably free from defects which are inexorable
to occur in fusion welding processes 1. Even though defects in friction stir
welded samples are experienced which are the outcome of improper selection of
combination of different process parameters, the defects found in friction stir
welded samples are mostly subsurface 2. This brings the limitation to visual
inspection methods for identification of defects in the welded samples.

Available non-destructive methods are applicable in detection of internal
defects but these methods need high investment and precise experience for the
analysis of the collected information. Most of the researchers contributed
towards finding the optimum range of different process parameters to avoid
defects. Contributions toward detection of defects in friction stir welding
though non-destructive manner is less. Hence, in the present research work, an
attempt has been made to develop procedure for defect identification in
friction stir welded samples. Vertical force, traverse force, tool rotational
speed and main spindle motor current signatures are the four process signals
acquired and analyzed for the same. Signal information extracted though fractal
theory will provide an indicator based on which conclusions can be drawn for
samples to be defective or defect free

 

Defects:
6

 

 

One
of the important characteristics of FSP is the different relative speed of
plastic material from advance side and retreating side which results in
different microstructure. Advance side the speed is greater than the retreating
side, microstructure changes rapidly. Due to lack of necessary transition, the
zone between the nugget and TMAZ often has poor mechanical properties, which
can be tested by mechanical examination. Pin Geometry has significant effect on
mechanical properties and joint structure. Each pin has different transfer
manners. It can be seen from the experiment that Void defects were created when
welded by the pin without screw threads under this weld parameter. After
tensile test cracks often arise upside voids.5

Influence
of Pin Geometry on Bonding, Mechanical Properties and Material Flow:

 

·        
Relatively
low speed of welding.

·        
Powerful
plates are required for thicker plates.

·        
Plates
have to be held firmly.

·        
Cannot
be applied for every material. It can only be applied for materials having low
strength and low melting point( higher melting point materials require special
tools).

·        
Exit
holed left when the tool is withdrawn

·        
Critical
tolerances

·        
Less
flexibility as compared to that of arc welding, i.e. difficulties associated
with non-linear welds

DIS-ADVANTAGES:

 

 

·        
A green
process Friction Stir Welding is environmentally friendly, with a process that
features low energy input and requires no consumables, flux, filler material,
or shielding gases to run, like conventional welding methods. Friction Stir
Welding also does not emit smoke, fumes, or gases that need to be exhausted on
the back end.

·        
Join
dissimilar alloys Friction Stir Welding may be used to weld dissimilar alloys – even
combinations not compatible with conventional welding methods.

·        
Superior mechanical characteristics Friction Stir Welding
produces a weld with high weld strength and toughness, plus a fine grain
structure that resists fatigue stress. Due to the low heat and small
heat-affected zone, there is minimal distortion of the joined parts, reducing
the costs associated with preparing the part for subsequent use.

·        
Limitless panel length and width for large projects: The
flexibility of our Friction Stir Welding process means we can accommodate the
welding of large parts.

·        
Virtually
defect-free bonding: As a solid-state process, Friction Stir Welding eliminates many of the
defects associated with conventional fusion welding techniques such as
shrinkage, solidification cracking, and porosity. The bond between the two
pieces is made solely of the original material, giving it similar strength,
bending, and fatigue characteristics of the parent material.

·        
Provides
opportunities for new solutions to old joining problems: The leading-edge technology
of Friction Stir Welding allows us to continually identify new joining
applications for extrusions, castings, plate, and sheet for customers ranging
from railcars to aerospace.

Advantages:

 

Defects:
Insufficient weld temperature, resulting from lower rotation speed and high
transverse speed, long tunnel which may be produced on the surface or
subsurface. Lower temperature may also reduce the forging action of the tool
and reduce the continuity of the bond from each side of the weld. Light contact
between the materials has given rise to the name kissing bond for such cases.

This type of defect is extremely difficult to detect by non-destructive
testing. Lack of penetration defect

            Friction Stir welding has several
advantages over fusion welding methods as any probable defects from cooling
from liquid phase are immediately avoided. FSW has found to be to produce low
concentration of defects and vary tolerant in variation of parameters and
materials. 

ADVANTAGES
AND DISADVANTAGES OF FRICTION STIR WELDING:

 

During FSW/P, high temperature and workload
experienced by the tool material results in high amount of material wear and
considerable damage to material. Failure of the tool as well as tool wear are
the most common issues observed in fabrication of surface composites due to
hard reinforcement particles during Additive Friction stir Processing (AFSP).

Tungsten-rhenium (W-Re), Iridium Rhenium, cobalt alloys, tungsten carbide and
Polycrystalline Boron Nitride (PCBN) are employed successfully for hard alloys
such as brass, Titanium based alloys and steels due to its high thermo
mechanical performance.  However, low wear
and good weld quality along with some other factors such as strength, hardness,
reactivity with work material and ductility that may influence tool material
selection.

Tool material

 

Tool
pin is responsible for plasticized material flow by stirring action in the
joint area. Pin diameter, surface profile and pin length are important parts of
the tool pin. Pin length affects the penetration level of plasticized material
in nugget/ stir zone. Tool pin length is generally kept 0.2 to 0.3 mm less than
the workpiece thickness so that the shoulder can get proper contact with the
workpiece by giving ppropriate axial plunge load 5. Pin diameter and surface
profile features affect the size of stir zone, microstructure and material
flow. Zhao et al. 38 studied experiments with three different tool pin
profiles, threaded cylindrical, taper cylindrical and straight cylindrical and
concluded that, the taper pin is most effective to attain high strength
dissimilar Cu-Al FSW joint. Selection of the optimum tool pin profile and its
dimension is one of the active research area for dissimilar Cu-Al FSW
joint.  The relation between pin and
shoulder dimension is defined as shoulder to pin diameter ratio (SPR). SPR of
dissimilar Cu-Al FSW system depends on the type and thickness of alloys being
joined. However, general range of 2:1 to 5:1 is noted by researchers (refer
Table 1). This mentioned range of SPR is relatively higher than the similar
material FSW system. In a dissimilar system of Cu and Al materials (which is
having different thermal conductivities and specific heats), the SPR should be
such that the thermal input and the distribution of it can be maintained. So,
by keeping relatively higher SPR, the thermal input can be raised in an
appropriate way, and at the same time, the distribution of it can be managed by
the other process parameters such as tool pin offset, position of the workpiece
material, rotational speed and welding speed. In addition to this, higher
Downloaded by University of Colorado – Health Science Library at 04:18 31
March 2015 thickness of workpiece materials requires larger SPR because pin is
more responsible for thermal input in case of higher thickness system.

Tool Pin

 

22,
29, 69, 70. Conical angle depends on the thickness of workpieces and the
diameter of the shoulder. Optimum shoulder design for dissimilar Cu-Al FSW system
may still consider for strenuous research interest because of limited research
articles. 

material
downward, which, in turn give good surface morphology. But, large amount of
IMCs are formed when scrolled profile is used. These IMCs are responsible to
increase hardness and brittleness in the stir zone and that also causes
defects. So, scroll surface profile should not be recommended to achieve sound
defect free dissimilar Cu-Al FSW joint. Conical and flat surface shoulder
features are favourable profiles for dissimilar Cu Al FSW system. Conical shape
around 2-10? cavity helps to push the material downward through centrifugal
force which gives the proper material flow for joint formation 12-14,

scrolled,
ridges, grooves, concentrating circles (shown in Fig. 2) can be provided to
increase material deformation and uniform mixing in FSW 5. Selection of
proper shoulder geometry/ feature depends on the workpiece and tool materials
as well as workpiece thickness. For dissimilar Cu-Al FSW system, the tool
shoulder geometries and profile affects the material flow, formation of IMCs
and mechanical properties of the joint. Galvao et al. 13 reported that,
scrolled shoulder is used to force Cu-Al mixed

In
FSW, the shoulder diameter is maximum responsible for heat generation. It has
been found that the shoulder generates around 87% heat by rubbing action
between the shoulder surface and the workpiece 9. Tool shoulder diameter and
geometry/surface features affect the quality of weld in FSW as it contributes
to maximum heat generation. For achieving good quality FSW joint, the optimum shoulder
diameter is one of the important parameters needed into consideration before
the welding 5. Tool shoulder diameter affects the peak temperature variation,
material deformation, plunge load variation, mechanical properties,
microstructural variation and formation of intermetallic compounds (IMCs) in
dissimilar Cu-Al FSW system. Akinlambi et al. 29 claimed that uniform mixing
between Cu and Al with a proper material flow pattern can be obtained with 15
mm and 18 mm diameters while improper material mixing was observed with 25 mm
diameter for 3.175 mm thick dissimilar (AA5754-C11000) FSW system. Maximum
tensile strength (of 208 MPa) and a minimum tensile strength (of 171 MPa) are
reported at 18 mm and 25 mm shoulder diameters respectively. Moreover, the
layer of IMCs is found thicker when the larger shoulder diameter (i.e. 25 mm)
was applied. Higher hear Downloaded by University of Colorado – Health Science
Library at 04:18 31 March 2015 input is responsible for the thick IMCs layer,
material deformation and mechanical properties which consequently deteriorate
the material flow of Cu fragments and Al matrix. So, the micro-hardness varies
as the shoulder diameter changes. Three shoulder geometries such as concave,
convex, flat with special profile features like

Tool Shoulder:

 

parts
are shoulder diameter, shoulder surface angle, pin geometry, including its
shape and size, and the nature of tool surfaces 11. Tool design and geometry
affects the heat input, force & torque variations and plasticized material
flow in FSW technology 7. Different tool designs and geometries for
dissimilar Cu-Al FSW system are discussed as below.

The
FSW tool has two basic parts: (I) Pin and (II) shoulder. Important elements of
these

Tool Geometry:

 

Tool
Shoulder-the region of tool in contact with work piece surface. To enhance the
material flow, the tool shoulder can have negative or positive scrolls.

Negative Scrolls is a depression in shoulder surface and work piece material
fills this. A positive scroll is protrusion on shoulder surface. Tool pin also
called as probe is inserted in workpiece and influences horizontal material
flow from front to back as well as vertical material flow from top to bottom. Advancing
side tool pin surface rotation direction and traverse direction have same
vectorial sense. Because of the tools forward movement, the material wants to
flow back, but the pins surface rotation opposes the flow on this side of the
tool. Retreating side: the tool pin surface rotational direction and tool
traverse direction have opposite vectorial sense. The material flow is easier
in this side of the tool pin as the pin surface helps the tool material flow backwards.

Friction Stir Welding of aluminum alloys. Aluminum is the third largest element
available on the Earth’s crust and second highest consumed element by weight.

Low density combined with attractive strength properties of modern Al alloys
makes it ideal for structural applications. There are different combinations of
elements used in aluminum alloys used for strengthening. Mainly they can be
classified as age hardening, work hardening and casting alloys. Principle
Friction Stir Welding variables which are under operator control include tool
design and tool movement parameters. Machine characteristic, workpiece thickness
and control mechanism will also affect the quality of the weld.