Abstract
In this study, the effect of rotational speed and traverse speed on the micro – and macrostructure, and mechanical properties (tensile and microhardness properties) of friction stir butt-welded 6061-T6 aluminium alloy has been investigated. A number of research studies have been conducted on friction stir welding of various aluminium alloys, the rotational and traverse speeds were noticed to have a greater influence on the formation of a quality weld. In this study, welds were fabricated from different parameter combinations by varying the rotational and traverse speeds during the welding procedure. The rotational speeds employed representing the low, medium and high settings are 700, 900, and 1100 rpm respectively while the traverse speeds utilised were 60, 80, and 100 mm/min traverse speeds.
To ascertain the joint integrities, the welds were characterised through hardness, microstructure, and tensile tests. The hardness test was performed along the cross-section of the welds. The changes in the microstructure and hardness were analysed and further correlated to the tensile strength of the 6061-T6 aluminium alloy. Optical microscope and Scanning Electron Microscope were used for microstructural analysis. Instron machine and Vickers hardness machine were used to perform tensile and hardness tests, respectively.
The results showed that the grain size decreased from the heat affected zone (HAZ) towards the centre of the nugget zone (NZ) due to the stirring during the FSW process. The average hardness in the NZ decreased when the rotational speed varied from 700 rpm to 900 rpm, and then increased with a further increase in the rotational speed to 1100 rpm at constant traverse speeds of 60, 80 and 100 mm/min.
Moreover, the ultimate tensile strength increased with an increase in traverse speed at constant rotational speeds of 700, 900, and 1100 rpm. In addition, the tensile results showed that fracture occurred in the relatively weak region which is the HAZ of the advancing side of the weld under a ductile-mode fracture. Additionally, the HAZ was found to be the soft zone in the weld due to thermal action of the FSW process and also due to the heat treatability of the aluminium alloy under investigation. It was observed that welds produced at 1100 rpm and 100 mm/min has the optimum weld quality and can be recommended for future welds in typical applications.
M.Ing. (Mechanical Engineering)