Effect of anodic oxide layer on friction stir spot welding between anodized A5052 and CFRTP

Dissimilar joining between carbon fiber reinforced thermoplastic (CFRTP) and metal has attracted attention of transportation equipment industry for reduction of weight and cost by multi-material structure. Friction stir spot welding (FSSW) between anodized A5052 (AO-A5052) and carbon fiber reinforced thermoplastic (CFRTP) was investigated for the improvement of joining strength. Average tensile shear strength of AO-A5052/CFRTP joint was 2505 N and more than 500 N higher than that of untreated-A5052/CFRTP joint. In the AO-A5052/CFRTP joint, probe plunge induced cracks and microwrinkles in the anodic oxide layer on the weld surface. Matrix resin was immersed into the cracks and the microwrinkles underneath of the shoulder of the welding tool. Reinforced carbon fiber and matrix resin was immersed into large crack at the outer peripheral corner of probe. Thus, those increased the joining strength by mechanical anchoring effect. Thermal stress test and machining stress test were performed for revealing the mechanism of cracking of anodic oxide layer. Although thermal stress did not generate cracks in the anodic oxide layer, machining stress by probe plunge generated cracks in the anodic oxide layer. The generated cracks in anodic oxide layer underneath of the shoulder were distorted to the rotating direction of welding tool by the friction stirring.