The by fusion process [2]. A novel technique to

The world demand of energy necessitates the proper selection of
materials or alloy development to achieve low cost of production or exploration
of fossil fuels (due to fluctuations in the prices of oil and gas in the
market), reduced failure of materials and less environmental degradation. Oil
and gas materials are subjected to severe environments (high temperature and
high pressure fields) which lead to catastrophic failure and endangering human
lives and environment at large. Metallic alloys with high strength and
toughness with excellent fatigue life are needed to achieve proper materials
design for intended use in harsh conditions. Steel is the most widely used
alloy in oil and gas industries but has serious challenges in welding
dissimilar alloys which sometimes serves as failure due to corrosion 1. Welding
plays an extremely important role in oil and gas platforms which is a highly
corrosive environment. The production, processing, storage and transportation
of crude oil occur in environments where stress and corrosion is high with
sulphur and hydrogen sulphide (H2S) present. Titanium is highly
resistant to seawater, carbon dioxide and H2S corrosion but there is
yet no consolidated experience on behaviour of the FSW process for titanium
alloys in these environments. Titanium is generally weldable through
conventional welding methods but the problem of workpiece distortion and poor weld
quality occurs when these methods are being used. More advanced titanium alloys
can be difficult to weld by fusion process 2.

A novel technique to join materials in solid-state known as
Friction Stir Welding (FSW) was invented at The Welding Institute (TWI),
Cambridge, UK in 1991 3, 4. To join materials through FSW, there is a subdivision
of grains, break-down of oxide particles and removal of porosity by inserting a
rotating pin with a shoulder travelling through the material. The figure below
shows a demonstration of the FSW process. In this process, softening of the
material being welded occurs through the heat being generated by friction at
the shoulder and to a lesser extent at the pin surface. Translation of the tool
along the welding direction at high rotational speed always lead to severe
plastic deformation and flow of this plasticized metal occurs. The modification
of microstructure and refinement of grains in metals could be easily achieved
through FSW. The process is considered sustainable and environmentally friendly
as it produces no emissions and uses friction as heat source to achieve deformation
and recrystallization. No melting occurs during FSW neither is there a
requirement for filler wire or shielding gas. It is a cost effective mechanical
joining process which leads to less microstructural changes and better
mechanical properties as compared to conventional fusion welding process 5.

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Another critical component in this process is the tool materials
because it determines the success of the process. The tool is distinctively
comprised of a rotating round shoulder and sometimes a threaded cylindrical pin
that heats the workpiece due to friction by moving the softened alloy around it
to form a joint. A tool pin and shoulder profiles are key elements in
determining joining performance and quality. The tool affects the joint’s
static strength, fatigue strength, corrosion resistance, translational force
requirements, processing speeds and control of the metal flow. When FSW is used
to weld hard alloys such as steels and titanium alloys, the tool is subjected
to severe stress and high temperatures and the high cost and short life of
these FSW tools is limited to its commercial viability. Factors influencing
tool material selection are: coefficient of thermal expansion, its hardness,
its ductility, and reactivity with workpiece material 6, 7. Limited
literature exists on the use of FSW to join titanium alloys compared to
aluminium and steel hence a need to replicate this processing technique and
prove its worth in order to enhance its general acceptability thereby
increasing its efficacy and reliability.