Abstract
Due to the increased demand for various engineering applications such as heat exchangers, tubing, valves, and environmental protection equipment for radioactive nuclear waste; there is also an increased demand for welding copper. Copper and its alloys are among the most widely utilized structural materials within industries because of their excellent properties such as high thermal and electrical conductivity, corrosion resistance, and wear resistance. Unfortunately, the joining/welding of copper is usually tricky by conventional fusion welding techniques because of its rapid heat dissipation, high thermal diffusivity, high thermal conductivity, and melting point resulting in weld defects. Alternatively, through Friction Stir Welding, a defect-free weld with improved material properties, operation and investment cost savings, and weight reduction and the welding process is environmentally friendly. The process parameters determine the weld integrity of a Friction Stir Welded joint. These variables control the weld's heat input, metal flow, development of the microstructure, and mechanical properties. Although FSW produces exceptionally sound welds, copper joints require higher heat input because of its high thermal diffusivity. Some degree of heat input is therefore required to improve the weld integrity; also, the tool's material flow behaviour and heat generation are vital factors that decide the quality of the weld. In this study, an added heat input (preheating) through the copper metal was implemented before welding to explore the effect of preheating on the properties of copper joints.
The work presented in this dissertation is an experimental study of Friction Stir welded copper joints with the effect of preheating. Four objectives have been persuaded within the study. The first was to optimise process parameters and preheat and weld the samples. The second objective was to characterize welded joints through metallurgical techniques (microstructural evaluation). The third mechanical objective was to conduct mechanical testing on the welded joints. Lastly, the fourth objective was to analyse the metallurgical and mechanical techniques and provide conclusions and recommendations on the findings. Four butt weld joints were fabricated using Friction stir welding. The first three butt welds were preheated before welding, while the other joint was fabricated without preheating. The optimised process parameters varied from 250-400 mm/min transverse speed, a plunge depth of 2-3.5mm, and a constant rotational speed of 600 RPM.
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The microstructural analyses showed that the stir zone of the preheated samples had finer grains than the not preheated welds. There was a decrease in the ultimate tensile strength of the preheated sample compared to the parent material. The Normally welded sample followed hooks law, and had the highest tensile strength with a ductile fracture, while all the preheated welds had a brittle failure within the joints. The Vickers hardness results indicated that the stir zone (SZ) was found to have the highest value compared to the other welded zones. Additionally, preheated welded joint number 3(PRW3) was recorded as the highest value produced on the stir zone (SZ).