Abstract
D.Phil. (Mechanical Engineering)
Welding is a crucial joining technique that is generally employed in the fabrication industry. The integrity of weld joints is of great importance, because of its safety and the economic implications. This research provides a glimpse into the basic concept of weld reliability, failure prediction, mechanical and microstructure characterization, with respect to friction stir welding (FSW) of similar and dissimilar Aluminium alloys. The establishment of this concept will enhance welds in service performance and their application in setting criteria for the evaluation of materials’ integrity. Friction stir welding (FSW) of a 6 mm thick AA6082-T6 Aluminium alloy was performed for the preliminary experiment. Three welding speeds of 90 mm/min, 120 mm/min and 150 mm/min at a tool-rotational speed of 950 rpm were used. A solution heat treatment carried out on the specimen revealed a drastic response by an over 120 % increment in strength. The welds at welding speed 150mm/min and the tool-rotation speed of 950rpm (i.e. sample S3) have maximum UTS values of 101MPa at as-weld and 234MPa after heat treatment (i.e. sample H3). The lowest values of strength were observed at 84MPa, at as-weld (i.e. sample S2) and 167MPa when heat-treated (i.e. 90mm/min and 950rpm, sample H1), respectively. The higher the heat input the softer the weld joints. However, the as-weld samples (S1-S6) deviate from the conventional trend unlike the heat treated samples (H1-H6). This is due to overlapping of the temperature field for friction stir bead at the end of each welds.
All the samples (i.e. similar welds) fractured at the heat-affected zone (HAZ), due to the reduction in hardness, as a result of a dissolution mechanism. Aside from the point of fracture, the disparities in composition evolution and physical integrity of the weld joints are similar. Ductility is a material property that is inversly related to the strength of the materials. However, a solution heat treatment of the samples (H1-H6) has an increment of over 120% in UTS...