Influence of surface pretreatment on the bonding mechanism and mechanical properties of AA5052/CFRP friction stir spot welded joint

Hybrid structure of aluminum alloy and carbon fiber reinforced polymer (CFRP) has been widely applied in the automotive industry for energy saving, emission reduction and structural light weighting. The large differences in physicochemical properties and metallurgical incompatibility between metals and CFRP easily generate interface separation defect at the joint interface. In this study, sulfuric acid anodizing (SAA), sandblasting (SB), and sandblasting + sulfuric acid anodizing (SB + SAA) pretreatments were used to modify the surface of 5052 aluminum alloy (AA5052). Friction stir spot welding was employed to join the modified AA5052 to carbon fiber reinforced polyether ether ketone composites (CF-PEEK). The tensile shear load for SAA, SB, and SB + SAA pretreatment joints were 2076 N, 3112 N, and 2798 N, respectively. SB pretreatment eliminated the interface separation defect and obtained hybrid joints with excellent mechanical properties. The results of microstructural analysis revealed that SB pretreatment removed the oxide layer on the surface of aluminum alloy and increased the surface roughness, which promoted the mechanical interlocking and adhesive bonding effect between AA5052 and CF-PEEK. The distribution of alloying elements showed that a diffusion layer of Al and C formed at the joint interface. This demonstrated the good bonding of aluminum alloys with CF-PEEK. Except specimens after SB pretreatment, the other specimens fractured along the interface separation defect. SB pretreatment eliminated the interface separation defect and changed the fracture path, leading to the excellent mechanical properties of the joint.