Friction absenteeism of solidification process. Among the many

Friction
Stir Welding of Dissimilar metals  

 

 

ABSTRACT:

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            Friction stir Welding is an upgraded
version of conventional welding process. It is considered one of the most
important one during the last two decades. Friction
stir welding (FSW) is a revolutionary green manufacturing technique due to its
energy efficiency and environmental friendliness. Friction
Stir Welding is more of a forging process than an actual welding process, and
simply uses a spinning pin tool and axial forge force to create a bond between
two pieces. FSW
parameters such as tool design, tool pin offset, rotational speed, welding
speed, tool tilt angle and position of workpiece material in fixture for
dissimilar Cu-Al system are summarized in the present review paper.
Additionally, welding defects, microstructure and intermetallic compound
generation.

Keywords: Friction Stir welding (FSW), TMAZ (Thermo-Mechanically
Affected Zone), Heat Affected zone (HAZ),

 

INTRODUCTION:

 

Friction
Stir Welding is one of the new off-center welding techniques which has made extraordinary
progress in Welding technology. Friction Stir Welding was invented at The
Welding Institute (TWI) in United Kingdom in 1991 as a solid state welding
technique and was primarily applied to Al alloys. Prevalence of non-heat
treatable and powder metallurgy Al alloys to which fusion welding cannot be
applied. The spot light has been turned to Friction Welding as the joining
technology that do not have welds normally associated with fusion welding
process 3. Al alloys to which fusion welding cannot be applied. FSW offers a
lot of advantage over conventional fusion welding process because of its low
heat input and absenteeism of solidification process. Among the many
manufacturing technologies joining has been identified as key enabling
technology to innovation and sustainable manufacturing.

As
FSW is a solid state welding process it has good dimensional stability, no loss
of alloying element, excellent mechanical properties in the mechanical area,
fine recrystallized microstructure, no solidification cracks.

Joining
of dissimilar materials by any welding process is always difficult because of
the enormous differences in mechanical and metallurgical properties. The joints
of dissimilar materials are increasingly employed in different sectors of
industries due to its technical and economic advantages.

FSW
has environmental benefits as no shielding gas is required, no filler material
needed, no detrimental emissions, abolishes grinding waste, etc.

 

 

             

Fig. 1: Principle of Friction
stir welding/processing.

 

Literature review

FRICTION
STIR WELDING PROCESS:

            It consists of a rotation tool which
is pressed against the surface of two abutting or overlapping plates, between
two severely clamped substrates on a backing plate support.  Friction stir welding can be divided into
three stages: Plunge, Dwell and Traverse. A cylindrical tool rotating at high
speed is slowly plunged into the plate material, until the shoulder of the tool
touches the upper surface of the material. After piercing the tool at the
required depth is held in that position for 5-20 seconds. A downward force is
applied to maintain the contact. Frictional heat, generated between the tool
and the material, causes the plasticized material to get heated and softened,
without reaching the melting point.  The
tool is then traversed along the joint line, until it reaches the end of the
weld. As the tool is moved in the direction of welding, the leading edge of the
tool forces the plasticized material, on either side of the butt line, to the
back of the tool. In effect, the transferred material is forged by the intimate
contact of the shoulder and the pin profile. It should be noted that, in order
to achieve complete through-thickness welding, the length of the pin should be
slightly less than the plate thickness, since only limited amount of
deformation occurs below the pin. The tool is generally tilted by 2-4°, to
facilitate better consolidation of the material in the weld. Upon reaching the
end of the weld, the tool is withdrawn, while it is still being rotated. As the
pin is withdrawn, it leaves a keyhole at the end of the weld.  This is the main disadvantage of FSW and few
variants are being used to overcome this aspect.

           

MICRO-STRUCTURE:

            Joint material can be separated into
four visually distinct micro-structural zones according to how they are
affected:

These
zones can be famed as follows:

1.
SZ: Stirred Zone or weld nugget, which is the central area where the pin
traverse between both sheets. This region is characterised by a fine grained
dynamically recrystallized structure.

2.
TMAZ: Thermally Mechanically Affected Zone, on each side of SZ (these zones
seem to be more narrow in the unconventional – bobbin tool – conditions).

3.
HAZ: Heat-Affected Zone, situated between the minimum of hardness and the base
material.

Fig. 2: Schematic of the
cross-section of the friction stir weld showing the various characteristic
regions

 

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