Friction riveting of aluminium alloy 6056-T6 with short-glass-fiber-reinforced polyamide 6 composite

Abstract

Friction Riveting (FricRiveting) is a relatively new joining technique for metal-polymer hybrid structures. This master thesis was carried out to investigate the FricRiveting process for polyamide 6 reinforced with 30 wt% short glass fiber (PA6-30GF) and aluminum alloy 6056-T6. These materials were selected because of their current joint use in automotive structures. AA6056-T6/PA6-30GF friction-riveted joints were successfully produced. Peak temperatures monitored for the process achieved between 323 °C and 399 °C leading the plastic deformation and thus anchoring of the rivet. The metallic rivet had its microstructure changed during the process, with dynamic recovery and recrystallization being observed in the anchoring zone. Microhardness in the metallic rivet decreased by 40 % of the base material hardness in the anchoring zone, due to a possible dissolution of the precipitates in the aluminum matrix and dynamic recovery and recrystallization. Polyamide 6 degradation was investigated by viscosity measurements and ATR/FT-IR, the joint with the highest level of degradation showed a reduction of 19 % on viscosity average molecular weight and an increase of 2.4 % of the carbonyl index in relation to the base material. Despite this reduction of properties, the friction-riveted joints had a good mechanical performance under tensile loading. Two joining conditions fractured through the metallic rivet outside the composite plate, achieving 92 % of the ultimate tensile strength of the metallic rivet. The influence of the process parameters on the process temperature, viscosity average molecular weight and ultimate tensile force were studied through Box-Behnken Design of experiment, response surface methodology and analysis of variance. Regression equations for these responses were estimated and validated, and an optimized condition was selected. Post joining heat treatment was performed on the optimized joining condition, resulting in an increase of the joint ultimate tensile force up to 99 % of the metallic rivet ultimate tensile force. The durability of the optimized joint was evaluated through natural weathering. An expected negative effect of weathering on the joint ultimate tensile force was observed, strength decrease of 8.4 % after 6 months and 15.5 % after 12 months of exposure.