Influence of rotational speed on the microstructure and mechanical performance of friction-riveted thermosetting composite joints

Facing the actual demand for efficient joining technologies for multi-materials structures, friction riveting was shown to be an alternative joining technology for thermoset composite profiles in civil infrastructure. This process is based on plasticizing and deforming the tip of a rotating metallic rivet within a polymeric component through frictional heating. The feasibility of friction-riveted hybrid joints of Ti-6Al-4 V/glass-fibre reinforced thermoset polyester was already demonstrated in a separate work. This paper complements this study by analyzing the rivet rotational speed effect on the process temperature, joint microstructure and the local and global mechanical properties of the joint. Joints were produced using two different levels of rotational speed: 9000 and 10,000 rpm (the other parameters were kept constant). The results showed process temperatures (655–765 °C) up to 96% higher than the onset decomposition temperature of the polyester matrix (370 °C); this led to severe degradation of the composite in the joint area. The increase in rotational speed, and therefore in heat generation, led to a statistically insignificant increase of the rivet penetration depth and the rivet diameter widening. However, the extension of the degraded composite area increased 47% which was responsible to deteriorate in 50% the joint tensile strength (from 4.0 ± 1.2 kN to 2.0 ± 0.7 kN). Moreover, the microhardness map of the joined rivet evidenced possible phase transformations in the alloy, favouring the material hardening by increasing in rotational speed. However, no correlations could be established between the changes in hardness and the joint tensile strength since the joints majority failure by full rivet pull-out. Thereby, for the improvement of friction-riveted Ti-6Al-4 V/ glass-fibre reinforced thermoset polyester joints, the optimization of rotational speed is essential. This can guarantee the formation of efficient anchored joints and wider rivet tip deformation, concomitantly with the minimizing of the extension of the matrix degradation and finally leading to better tensile strength of the joints.