Friction Stir Welding (FSW) has gained an increase in popularity over the years since it was first invented at 'The Welding Institute' in the year 1991. Since then, industries and researchers have been developing sophisticated joints by using advanced welding techniques such as FSW to develop optimum quality weld joints for use in modern industrial applications. Dissimilar material welding has enabled the incorporation of multiple properties within a particular structural component to achieve high-level functionality in industrial equipment’s. As a solid-state process, FSW plays a major role in welding of dissimilar materials as it eliminates the difficulties posed by traditional fusion welding due to defects such as porosity, blowholes, solidification cracks etc. 

The information in this thesis only accounts for two metals namely electrolytic tough pitch(ETP) pure copper (Cu) and 304L stainless steel (SS). These two metals have certain characteristics that make it an excellent choice for use in industrial applications. Cu has numerous properties such as high electrical conductivity, high thermal conductivity and corrosion resistance. On the other hand, a widely available metal, SS has excellent corrosion resistance, high static and dynamic strength, and high toughness. Owing to these properties Cu-SS weld joints are widely used in fusion reactors, power generation, aerospace, heat exchangers etc. 

Adequate information has been evaluated for the FSW process in the last few years but limited research has been conducted on the present system of heat and cooling-assisted sources applied for weld joint fabrication. Hence, this thesis provides information on the effect of heat and cooling-assisted FSW to investigate the microstructural evolution and the mechanical properties. As an assisted heating source, a gas tungsten arc welding torch is applied at different preheating currents and similarly for assisted cooling source, compressed air and water is applied to fabricate the necessary weld joints. The experimental work results reveal that normal FSW proved to be the superior weld quality after comparing the macrostructure, microstructure and mechanical properties to heat-assisted friction stir welding (HFSW) and cooling-assisted friction stir welding (CFSW). An extreme level of dynamic recrystallization was observed for all the welded samples due to intense plastic deformation. The defects such as voids and cracks were mostly present around the dispersed SS particles. The weld nugget consists of a stirred zone with large SS particles and Cu matrix. Evaluating the mechanical properties of the welded samples it was found that normal friction stir welding had the highest hardness and tensile strength compared to other weld conditions. Cooling-assisted joints reported higher hardness compared to heat-assisted FSW. It was concluded that the tensile properties deteriorated drastically for CFSW with HFSW providing better results. However, the tensile strength also worsened as the heating current increased.