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Friction spot joining is an alternative technique to produce metal-composite overlap joints. The main process parameters are tool rotational speed, plunge depth, joining time and joining force. In this study, the individual effect of the process parameters on the microstructure and mechanical strength of hybrid AA6181-T4/CF-PPS double lap joints was investigated using Taguchi method and analysis of variance (ANOVA). Produced joints presented mechanical performance from 2107 N to 3523 N. Joints failed by brittle fracture at the interface between aluminum alloy and composite, with displacement-at-peak load values from 0.7 mm to 0.9 mm. Tool rotational speed was the parameter with the largest influence on the joint shear resistance, followed by the joining time, plunge depth and joining force. Higher strength was correlated to the extension of the bonding area and macro-mechanical interlocking related to the formation of a metallic indentation (metallic nub) slightly inserted into the composite. Larger bonding areas were shown to be related to higher heat input (as a result of longer joining times and intermediate rotational speeds) leading to larger consolidate polymeric layers at the metalcomposite interface. Higher macro-mechanical interlocking was obtained at larger plunge depths. Joining force was shown to be related to crevice and pore filling of the metal surface by supporting spreading of the molten polymer. Higher joining forces led to better wetting of the interface increasing adhesive forces and joint mechanical performance. Nevertheless excessive joining forces caused squeezing flow of the molten layer reducing joint strength, since a large adhesive area was lost.

a b s t r a c tThe effects of friction spot joining process parameters on the bonding area and mechanical performance of single lap joints were investigated using full-factorial design of experiments and analysis of variance. On one hand, the main process parameters with significant influence on the bonding area were joining pressure, tool rotational speed and joining time. On the other hand, tool rotational speed and joining pressure displayed the highest influence on the lap shear strength of the joints followed by tool plunge depth, whereas the joining time was not statistically significant. The interaction between the rotational speed and joining time was the only interaction with a significant effect on the mechanical performance. Joints with ultimate lap shear forces varying between 1698 ± 92 N and 2310 ± 155 N were obtained. It was observed that generally a larger bonding area as a result of higher heat input leads to an increased mechanical performance of the joints. The generated regression model by the analysis of variance was used to identify an optimized set of parameters for increasing the lap shear strength of the joints to 2280 ± 88 N. Furthermore, the process temperature was monitored, which varied in the range of 370-474°C.

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