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The aim of this investigation is to study the effect of geometric tool parameters on mechanical and microstructural behaviours during dissimilar friction stir welding of 5 mm thick plates of AA 5086-O and AA 6061-T6. Three tool geometries were used, including a tool with a concave shoulder and a conical probe with three grooves, flat shoulder and threadless cylindrical probe, and a tool with a flat shoulder and a threaded cylindrical probe. It was found that the tool with a concave shoulder and a conical probe with three grooves produces higher heat input and temperatures that provides more homogeneous stir zones than the other tools. In addition, the grain sizes of the stir zone in the AA 6061 side of all the welds are finer than those produced in the AA 5086 side, and decreasing the weld pitch, i.e. a ratio of welding to rotational speed, results in coarser grain structures in the stir zone.

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A prominent benefit of friction stir welding process is to join plates with dissimilar material. In this study, an attempt is made to find effects of tool offset, plunge depth, welding traverse speed and tool rotational speed on tensile strength, microhardness and material flow in dissimilar friction stir welding of AA1100 aluminium alloy and A441 AISI steel plates. Here, one factor at a time experimental design was utilised for conducting the experiments. Results indicated the strongest joint obtained at 1.3 mm tool offset and 0.2 mm plunge depth when the tool rotational speed and linear speed were 800 rev min 21 and 63 mm min 21 respectively. The maximum tensile strength of welded joints with mentioned optimal parameters was 90% aluminium base metal. Fracture locations in tensile test at all samples were in aluminium sides. Owing to the formation of intermetallic compounds at high tool rotational speed, the microhardness of joint interface goes beyond that of A441 AISI steel.