The damage characteristics and mechanism of CFRP during laser joining of CFRP/mild steel dissimilar joint

This paper presents an investigation on the thermal damage characteristics and mechanism of carbon fiber reinforced polymer (CFRP) during fiber laser joining between CFRP and mild steel with Cr electroplating. The results show that laser heat causes nonuniform thermal damage of CFRP at the bonding interface. In the center region of the fracture after shear testing of the joint, CFRP endures serious ablation characterized with the formation of large cavities and the joint failure happens at the bonding interface. In the peripheral region of the fracture, a large amount of micron porosities are observed and the joint failure happens at the porosity zone in CFRP. According to the thermochemical analysis of CFRP, the critical temperatures of resin matrix decomposition, carbon residue oxidation and carbon fiber oxidation are 350°C, 484°C and 650°C respectively. When the temperature is higher than 350℃, the resin matrix of CFRP decomposes, producing gaseous products such as CO2, NH3 and the water vapor, which form the porosities in CFRP. When the temperature is higher than 484℃, the residual carbon is oxidized and ablation happens. The thermal histories at different locations of bonding interface show that Gaussian distribution of the laser beam results in nonuniform temperature distribution in the bonding interface. The peak temperature in the center of bonding interface is 654℃, higher than the CFRP oxidation temperature. CFRP in this region endures severe thermo-oxidation ablation. In the peripheral region of the bonding interface, the peak temperature drops from 484°C to 350°C sharply with the increase of distance from the centerline and CFRP in this region endures decomposition. Through strict control of laser heat input, the thermal ablation can be completely eliminated and the amount of the porosities can be reduced. Thus the shear strength of the joint is apparently improved from 16.8MPa to 27.2MPa. Therefore, using the laser with uniform intensity distribution instead of fiber laser and controlling the temperature of interface lower than CFRP decomposition temperature is a promising approach to suppress the thermal damage in CFRP.