Interfacial structure and bonding mechanism of AZ31/carbon-fiber-reinforced plastic composites fabricated by thermal laser joining

Multi-material joining is attracting the attention of the automotive industry given its potential to realize lighter vehicles, and therefore fuel savings and reduced emissions. The aim of the present study was to understand the bonding mechanisms whereby metal and plastic/composites are joined and to improve the multi-material joint strength. In this study, the effect of thermal oxidation of Mg alloy sheets on the strengths of Mg–CFRP (carbon-fiber-reinforced plastic) lap joints prepared using laser-assisted metal and plastic joining technique was investigated. Characterization techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro-computed tomography (MCT), x-ray photoelectron spectroscopy (XPS), and atom probe tomography (APT) were used to study the underlying mechanisms of the thermal oxidation. The formation of bubbles, mechanical interlocking and chemical reactions at the joint interface were found to be the three key factors influencing the strength of joints. Thermal oxidation increased the joint strength significantly through the suppression of bubble formation, CFRP decomposition and the creation of mechanical interlocking effects at the joint interface. Moreover, MgCO3, MgO1+x, and Mg(OH)2 phases were detected by XPS analysis at the joints prepared using thermally oxidized Mg alloy sheets. The presence of the high O/Mg ratio phases was also confirmed by APT analysis. The formation of these phases confirmed the occurrence of chemical reactions between the MgO and CFRP matrix at the nanometer level, which are regarded as contributing to the increase in the joint strength.