Elucidation of interface bonding mechanism and novel-tool strengthening principle in friction assisted joining of Al alloy to CFRTP

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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 study in the past one year investigated the feasibility of tool modification to improve the property of the silane-coated 6061-T6 aluminum alloy (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. From the combined evaluation of thermal process and mechanical characteristics, the CT tool was recommended for the FSpJ of Al and CF/PA6 because it offered more uniform temperature distribution and improved macro-mechanical interlocking at the interface with good wetting conditions, sufficient chemical reaction and effective joining area, and a lower tensile residual stress, in comparison with other tools, and hence showed the highest shear force.