Friction stir spot welding (FSSW) is performed on transformation-induced plasticity (TRIP) steel (HCT690T) sheets. The influence of parameters: rotational speed, dwell time plunge depth and plunge speed on a mechanical property of lap joints are investigated. An engineering robust design (ERD) approach has been utilized to design the experiment with L9 orthogonal array. In order to minimize the bias, each experiment has been repeated three times, resulting in 27 experiments. For each influence parameter, three levels have been identified (i.e. minimum, average and maximum) based on trial and error experimentation as well as utilizing published research data. The ultimate tensile load capacity (UTLC) of TRIP lap welded joints has been used to assess the joint performance and to calculate the signal-to-noise ratios. Based on the experimental results, optimal parameter levels have been investigated and theoretical joint strength under the optimal parameter settings has been calculated using the additive model proposed in the ERD. A verification experiment has been performed to validate the experimentally investigated optimal settings of FSSW.

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