Product Code: ICA13_1909

The Damage Characteristics and Mechanism of Cfrp during Laser Joining of Cfrp/Mild Steel Dissimilar Joint
Authors:
Xianghu Tan, Tsinghua University; Beijing Peoples Republic of China
Zhang Jing, General Motors(China) Investment Company Limited; Shanghai Peoples Republic of China
Shan Jiguo, Tsinghua University; Beijing Peoples Republic of China
Shanglu Yang, General Motors(China) Investment Company Limited; Shanghai Peoples Republic of China
Ren Jialie, Tsinghua University; Beijing Peoples Republic of China
Presented at ICALEO 2013

The reliable joining of carbon fiber reinforced polymer (CFRP) and metal is the key problem to its widespread application. And laser joining technology is considered to be a feasible solution. However, the laser beam not only promotes the formation of physical and chemical bonding at the CFRP/metal interface, but also causes thermal damage to the CFRP which will weaken the mechanical properties of the joint. This paper presents an investigation on the fiber laser joining between CFRP and mild steel with chromium electroplating. The interface microstructure and fracture morphology of CFRP/mild steel joint were observed by optical microscope and scanning electron microscopy to investigate the damage characteristics of CFRP during laser joining process. The thermal properties of CFRP have been investigated by differential scanning calorimetry (DSC) and the combined technique thermogravimetry-mass spectrum analysis (TG-MS) to reveal the damage mechanism of CFRP. And based on the damage mechanism, the suppressing of the damage was explored.
The results show that laser heat causes nonuniform thermal damage of CFRP at the interface. In the center region of the fracture, CFRP endures serious ablation characterized with the formation of large long cavities and the shear failure happens at the interface. In the edge region of the fracture, a large amount of pores with micron size are observed and the shear failure happens at the porosities zone in the CFRP. According to the thermochemical analysis of CFRP, the critical temperatures for resin matrix decomposition, carbon residue oxidation and carbon fiber oxidation are 350℃, 484℃ and 650℃. When the temperature is higher than 350℃, the resin matrix of CFRP decomposes and a large number of gaseous products such as CO2, NH3 and the water vapor are produced and stay in CFRP, forming the pores. When the temperature is higher than 484℃, the residual carbon will be oxidized and ablation happens. Thermal history measurement at different locations of bonding interface shows that Gaussian distribution of the laser beam results in nonuniform interface temperature field. The peak temperature in the center of the fracture is 654℃ which is higher than CFRP oxidation temperature and CFRP in this region endures severe thermo-oxidation ablation. In the edge region of the fracture, the peak temperature drops from 484℃ to 350℃ rapidly with the increase of distance from the centerline, and CFRP in this region endures decomposition. Through strict control of laser heat input, thermal ablation can be completely eliminated, and the number and size of the pores in edge region can be reduced and the shear strength can be improved to 27.2MPa. Therefore, using the laser with uniform intensity distribution and controlling the temperature of interface lower than CFRP decomposition temperature is the possible approach to suppress the thermal damage to CFRP.