To improve the strength of the adhesive-bonded carbon fiber reinforced polymer (CFRP) joints, atmospheric pressure plasma treatment (APPT) was used to treat a CFRP substrate surface. This study investigated the effects of nozzle distance (i.e., the distance between plasma nozzle and CFRP substrate) and nozzle speed (i.e., the moving speed of plasma nozzle relative to CFRP substrate) of APPT on the lap-shear strength of adhesive-bonded CFRP joints. Results show that the lap-shear strength of plasma-treated CFRP joints increased to a peak value and then decreased as the nozzle distance increased, and the nozzle distance associated with the peaked joint strength depends on the applied nozzle speed. The lap-shear strength of plasma-treated adhesive-bonded CFRP joints reaches up to 31.6 MPa, compared to 8.6 MPa of the as-received adhesive-bonded CFRP joints. The surface morphology of plasma-treated CFRP substrates was investigated by scanning electron microscope observation, and the mechanism associated with the improved joint strength after applying APPT was revealed through surface chemistry analysis. It is found that APPT not only effectively removed the content of Si element and –CH₃ (i.e., the main compositions of release agent) from the as-received CFRP substrate surface, but also generated many polar groups (i.e., –NH₂, –OH, –COOH, etc.), which has a positive effect on increasing the wettability and interfacial bonding strength of CFRP substrates and consequently results in a significant improvement of lap-shear strength of plasma-treated CFRP joints. In addition, the result of differential scanning calorimetry (DSC) test shows that the surface temperature of CFRP substrate should not exceed 175.3 °C during APPT. In this study, an empirical model governing temperature, nozzle distance and nozzle speed was established to guide the selection of atmospheric pressure plasma treatment process parameters in industrial manufacture.

To improve the strength of the adhesive-bonded aluminum alloy, atmospheric pressure plasma treatment (APPT) was conducted on aluminum AA5052 with three different gases (i.e., argon, nitrogen and air). The contact angle and surface free energy of the as-received and treated AA5052 were measured and mechanical testing of the adhesive-bonded joints were performed. Test results indicated that air and nitrogen plasma treatments significantly improved the wettability of AA5052, and consequently resulted in the increase of joint strength. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used for the characterisation of the aluminum surface physical and chemical changes induced by plasma treatment. It was found that APPT with gases used in this study hardly induced noticeable difference on the surface morphology of aluminum AA5052. The results of XPS indicated that argon plasma treatment removed the residual carbon contamination without significant affecting the chemical state of oxide layer. A thick aluminum oxide layer with adsorption of polar groups (i.e., -NH₂, -NO₃, -OH) was formed on the nitrogen and air plasma-treated surfaces of aluminum AA5052, which was likely a major contributing factor to the improved wettability and bond strength in adhesive bonding of aluminum AA5052.

A new one-sided joining method, friction stirring blind riveting (FSBR) was successfully implemented to form lap-shear joints for dissimilar metals from pairs of 3.05 mm thick cast Mg AM60, rolled 1.5 mm thick Al AA6022, and extruded 3.15 mm thick Al AA6082 specimens. The concept of this process is riveting the two workpieces with reduced force under frictional heat and fastening the workpieces through blind riveting once the rivet is fully inserted. In this research, the process was experimentally analyzed and optimized for four joint combinations. It was demonstrated that switching the positions of Mg and Al alloy specimens has a significant effect on the process window and maximum tensile load of the joints. Three quality issues of the FSBR joints were observed and discussed. During tensile testing, the sheet closer to the rivet tail work-hardens due to tail forming process but has worse loading condition than the sheet closer to the rivet head. For AA6xxx sheets, precipitate hardening due to frictional heat is another strengthening mechanism in FSBR compared to the conventional riveting process, which leads to higher tensile loads in FSBR joints.