Decomposition of ultrasonically welded carbon fiber/polyamide 66 and its effect on weld quality

Ultrasonic welding of 4 mm thick carbon fiber/polyamide 66 (C-f/PA 66) composite samples in a lap configuration was completed to characterize the effect of weld energy on weld strength. In general, it was seen that weld strength was proportional to weld energy; however, with excessive energy, the weld strength decreased. Weld indentation had little influence on joint strength, whereas thermal decomposition of C-f/PA 66 composite was the main reason for the low joint strength. Higher weld energy caused thermal decomposition of C-f/PA 66 composite and degradation of tensile strength of welded workpieces. A decomposition index of the joint was proposed and calculated to predict weld quality. When the decomposition index was small, joint strength increased with the weld area while high decomposition index of polyamide 66 (i.e. > 30.5%) led to the decrease in joint strength based on the experimental data. The optimal decomposition index was found in the range of 12-30.5% depending on joint strength.