The Effect of Adhesive Curing Condition on Bonding Strength in Auto Body Assembly

The bonding strength of metal-to-metal lap joining of a two-part epoxy-based adhesive employed in an automotive assembly line was investigated under different heating rates (10 to 6000°C/min), peak temperatures (room to 250°C), and holding times. The results indicate that bonding strength is controlled mainly by the peak curing temperature and the heating rate. The maximum bonding strength appears between 70 and 110°C, but the value of it depends on the heating rate. At heating rates of 10, 50, and 100°C/min, the peak strength decreases with increasing heating rate. However, a further increase in heating rate to 2000–6000°C/min resulted in higher peak bonding strength. The microstructures and fractured surfaces after shear testing were examined by a scanning electron microscope. The results revealed that many gas bubbles (voids) were formed during the adhesive curing process, and the fracture process was controlled by the link of the voids. At low heating rates (10–100°C/min), the mean void size and volume fraction increase with heating rate and peak temperature, causing the weakening of the bonding strength. However, at very high heating rates (2000–6000°C/min), the rapid hardening of the adhesive suppressed the development of gas bubbles, so that the mean void size and volume fraction were low, and the bonding strength was high. This result indicates that to effectively improve the adhesive bonding strength, both the chemical reaction (degree of cure) and physical response (gas bubble formation) need to be optimized.