<p>Fusion welding of steel to aluminium is prone to weld defects. Friction stir welding (FSW) has been useful in this regard. Butt FSW of spring steel, EN42J to a commercially pure aluminium alloy, AA 1100, having applications particularly in Chemical, Food processing and Aerospace industries, has not been reported before. This work explores FSW of 2&#xa0;mm thick sheets of these materials with an H13 steel tool, with a goal to determine optimum welding and rotational speeds that gives good joint efficiency maintaining productivity, without additional post-processing like heat treatment. A face-centred central composite design of experiment and desirability function approach was used for the multi-objective parametric optimisation. A good joint efficiency of 86% and microhardness of 408% compared to the weaker base metal, AA 1100 aluminium alloy (96&#xa0;MPa strength and 28 HV average microhardness), were obtained. However, the elongation shown by the transverse tensile test samples was only 26% of AA 1100 (AA 1100 showed 12.9% elongation at break). UTS, % elongation of the transverse tensile specimens and microhardness of the weld region were found to be optimum at the middle level of rotational speed range (600-900 RPM) and welding speed range (30-60&#xa0;mm/min). The weld characteristics obtained with a cemented carbide tool using the same process parameters were poor compared to the weld characteristics obtained with an H13 steel tool because of faster heat extraction from the weld region due to higher thermal diffusivity of cemented carbide. Metallographic characterisations including Electron Backscattered Diffraction were carried out to explain the experimental results. Fe₂Al₅ and AlFe intermetallics were observed in the XRD analysis of the fractured transverse tensile test specimens of the welds, which partly contributed to the lower percentage of elongation.</p>