Improving bonding strength of Al/CFRTP hybrid joint through modifying friction spot joining tools

Pinless friction spot joining (FSpJ) has proven to be a competitive friction-based joining technique for fabricating metal/carbon fiber-reinforced thermoplastics hybrid joints. The present study investigated the feasibility of tool modification to improve the property of the silane-coated 6061-T6 Al alloy and carbon fiber-reinforced polyamide-6 (CF/PA6) FSpJ joint. The interfacial structure, bonding strength and fracture surface of the joints obtained by four tools, viz, old flat (OT), new flat (NT), concave-shaped (CT) and ring-shaped tools (RT) with different rotation speeds, were analyzed and compared. An experimentally validated three-dimensional finite element model was also built to help understand FSpJ process. Despite modifying the tool shape, covalent bonding predominated the interfacial bonding force where the formation of nanoscale diffusion layers was witnessed with no new type of covalent bonds generated. The resultant fracture surface was dominated by the cohesive failure mode, suggesting strong chemical bonding between the re-solidified resin and silanized Al surface. The decrease in tensile shear force in the case of NT was due to the extensive formation and coalescence of microscale voids and porosities. This was mitigated by using CT and RT tools, from which the heat input was controlled leading to a more homogenized heat field. From the combined evaluation of thermal process and mechanical characteristics, CT tool was recommended for the FSpJ of Al and CF/PA6 because it offered a more uniform temperature distribution and improved macro-mechanical interlocking at the interface with good wetting conditions, sufficient chemical reaction and effective joining area, and lower tensile residual stresses, in comparison with other tools, and hence showed the highest shear force.