The world demand of energy necessitates the proper selection ofmaterials or alloy development to achieve low cost of production or explorationof fossil fuels (due to fluctuations in the prices of oil and gas in themarket), reduced failure of materials and less environmental degradation. Oiland gas materials are subjected to severe environments (high temperature andhigh pressure fields) which lead to catastrophic failure and endangering humanlives and environment at large. Metallic alloys with high strength andtoughness with excellent fatigue life are needed to achieve proper materialsdesign for intended use in harsh conditions.
Steel is the most widely usedalloy in oil and gas industries but has serious challenges in weldingdissimilar alloys which sometimes serves as failure due to corrosion 1. Weldingplays an extremely important role in oil and gas platforms which is a highlycorrosive environment. The production, processing, storage and transportationof crude oil occur in environments where stress and corrosion is high withsulphur and hydrogen sulphide (H2S) present. Titanium is highlyresistant to seawater, carbon dioxide and H2S corrosion but there isyet no consolidated experience on behaviour of the FSW process for titaniumalloys in these environments. Titanium is generally weldable throughconventional welding methods but the problem of workpiece distortion and poor weldquality occurs when these methods are being used. More advanced titanium alloyscan be difficult to weld by fusion process 2.
A novel technique to join materials in solid-state known asFriction Stir Welding (FSW) was invented at The Welding Institute (TWI),Cambridge, UK in 1991 3, 4. To join materials through FSW, there is a subdivisionof grains, break-down of oxide particles and removal of porosity by inserting arotating pin with a shoulder travelling through the material. The figure belowshows a demonstration of the FSW process. In this process, softening of thematerial being welded occurs through the heat being generated by friction atthe shoulder and to a lesser extent at the pin surface. Translation of the toolalong the welding direction at high rotational speed always lead to severeplastic deformation and flow of this plasticized metal occurs.
The modificationof microstructure and refinement of grains in metals could be easily achievedthrough FSW. The process is considered sustainable and environmentally friendlyas it produces no emissions and uses friction as heat source to achieve deformationand recrystallization. No melting occurs during FSW neither is there arequirement for filler wire or shielding gas. It is a cost effective mechanicaljoining process which leads to less microstructural changes and bettermechanical properties as compared to conventional fusion welding process 5.
Another critical component in this process is the tool materialsbecause it determines the success of the process. The tool is distinctivelycomprised of a rotating round shoulder and sometimes a threaded cylindrical pinthat heats the workpiece due to friction by moving the softened alloy around itto form a joint. A tool pin and shoulder profiles are key elements indetermining joining performance and quality. The tool affects the joint’sstatic strength, fatigue strength, corrosion resistance, translational forcerequirements, processing speeds and control of the metal flow. When FSW is usedto weld hard alloys such as steels and titanium alloys, the tool is subjectedto severe stress and high temperatures and the high cost and short life ofthese FSW tools is limited to its commercial viability.
Factors influencingtool material selection are: coefficient of thermal expansion, its hardness,its ductility, and reactivity with workpiece material 6, 7. Limitedliterature exists on the use of FSW to join titanium alloys compared toaluminium and steel hence a need to replicate this processing technique andprove its worth in order to enhance its general acceptability therebyincreasing its efficacy and reliability.