Influence of laser welding power on steel/CFRP lap joint fracture behaviors

Dual-phase steel DP590 and carbon fibre reinforced polymer (CFRP) were successfully joined by laser welding. Experiments and numerical simulations were performed to clarify the influence of laser power on the joint characteristic, bonding mechanism and fracture behavior. The results indicate that the melting width and depth of CFRP increased with the enhancement of laser power. With relatively low laser power, un-bonded steel/CFRP interface was formed due to the insufficient melting of resin matrix and interfacial residual stress resulted from thermal expansion mismatch between steel and CFRP. Compact-bonding was produced with the laser power range of 400-700 W. Decomposition of CFRP at the interface occurred with the further increase of laser power to 800 W due to excessively high peak temperature. The highest tensile-shear peak load of 3855 N was produced at 700 W laser power. Three different fracture modes were found in the joints with different laser powers, i.e., interfacial fracture (IF) at 300 W and 400 W, cohesion fracture (CF) in CFRP at 500 W and 600 W, and interface fracture at inner region together with cohesion fracture at outer region of the bonding interface (IF + CF) at 700 W and 800 W. X-ray photoelectron spectroscopy results showed that C-M and O-M chemical bonds were formed at the interface due to the reaction between resin matrix and DP590. Stronger O-M and C-M chemical bonds produced at higher laser power were beneficial for the improvement of fracture load.