Improving the joint strength of laser-welded CFRTP/TC4 joints via appropriate groove design

Laser surface treatment can enhance the joint strength of reinforced thermoplastic composites (CFRTP) and metal joints. In this study, an ultraviolet picosecond laser system grooved the Ti-6Al-4V (TC4) surface, and the influence of grooves on the CFRTP/TC4 joints was investigated. 0 degrees parallel, 90 degrees parallel, 45 degrees parallel, 0/90 degrees crossed and +/- 45 degrees crossed grooves were created on TC4 surface. Compared with the joint without grooves, the joint strength was improved by all groove patterns. The grooves improve the thermal contact resistance at the interface, and buffer the energy input of the CFRTP side, inhibiting the pyrolysis of the CFRTP and eliminating the bubble defects. Meanwhile, the groove enhances mechanical interlocking. The joint with +/- 45 degrees crossed grooves was the strongest, being 120.1 % stronger than the joint without grooves (ultimate shear force of 1552 N). Apart from the mechanical interlocking, the compound layer consisting of the CTi0.42V1.58 phase was confirmed and a novel chemical bonding, TiOS, was also discovered at the interface. However, no significant effect of the grooves on chemical bonding was found. The increase in thermal contact resistance and the mechanical interlocking reinforcement after cooling were the main strengthening mechanisms of the grooves.