Achieving high strength friction lap spot joints of carbon fiber reinforced thermosetting composite to aluminum alloy with additional thermoplastic interlayer

The dissimilar materials of the 5182-aluminum alloy and continuous carbon fiber reinforced bismaleimide (CFBMI) were joined by friction lap spot joining (FLSJ) directly and via the addition of the polyamide 6 (PA6) interlayer, and the effect of interlayer on the joining mechanism and the joint mechanical property was investigated. It was found that no effective joints were achieved by FLSJ directly, which was mainly attributed to the poor re-forming ability and liquidity of the thermosetting BMI, and its difficulty in reacting to metals. After the addition of the PA6 interlayer, the effective dissimilar joints were successfully realized by FLSJ. The maximum average tensile shear force of the joint reached 3.68 kN, with the average normal tensile shear strength of 20.9 MPa at the optimum parameter with the rotation rate of 2000 rpm, dwell time of 7 s, plunge depth of 0.7 mm and the PA6 interlayer of 0.3 mm. A new C-O-Al chemical bond was found at the interface of the PA6 interlayer and aluminum alloy, which was the result of the chemical reaction between the amide polar function of PA6 and surface oxide of aluminum alloy. The great increase in the joint strength by the addition of the PA6 interlayer was mainly attributed to the great reduction of the interface defects and the formation of the chemical bonding, as the result of the great improvement of the interface fluidity and chemical reaction ability of PA6. This study provides an effective way to achieve high strength metal/continuous carbon fiber reinforced thermosetting composite joints.