In this study, a new method was proposed to reduce the keyhole volume with friction stir spot welding process and improve the lap joint shear load-carrying capacity of the weld by analyzing the effects of tool rotation speed, plunge depth and dwell time on the weld. Single lap shear tests were carried out to determine the influences of the welding parameters on the mechanical behavior of the welds. The quality of the joint was evaluated by examining the characteristics of the joint as a result of the lap joint shear load. For friction stir spot welding of the acrylonitrile butadiene styrene samples, the experiments were designed according to Taguchi’s L9 orthogonal array in a randomized way. From the analysis of variance and the signal-to-noise ratio, the significant parameters and the optimum combination level of the parameters were obtained. It was found that using a tool rotation of 1000 rpm, plunge depth 11.5 mm and dwell time of 40 s, an improved joint strength can be obtained. The results showed that joint strength was improved by an amount of 20% as compared with the optimum welding parameters to the initial welding parameters. Macrostructure examination plays an important role to determine the joint strength and evaluate the influences of each welding parameters. So, weld morphology was investigated by morphological analysis and visual comparisons. It was also observed failure modes for fractured samples having the highest, moderate and lowest lap joint shear load.
