This work deals with the joining of 6 mm thick dissimilar aluminium plates (AA5083-H111 and AA6082-T6) using a friction stir welding method and by varying the process parameters. Test experiments were performed to identify the influence of process parameters on the joint efficiency of the weldments. The process parameters such as tool rotation speed and tool pin profile were varied; whereas, tool travel speed, tilt angle, and axial force were kept constant for all weldments. Microstructure evaluation was carried out using light optical and scanning electron microscopes, which exposed the grain refinement in the nugget zone (NZ) and thermo-mechanical affected zone (TMAZ). Mechanical property tests for tensile strength, hardness, and bending were performed to understand the influence of the parameters over the weldments. Heat development between the tool shoulder and workpiece was analyzed by calculating the heat flow and heat flux. The thermal diffusivity of AA5083-H111 and AA6082-T6 were calculated to understand the influence of heat distribution in the joint efficiency of the weldments. It is inferred from the current study that the threaded cylinder tool at a rotational speed of 900 rpm achieved the highest tensile strength, hardness, and bend strength over the combination of other parameters.