Process parameter optimization of continuous ultrasonic welding of thermoplastic composites: Insights into mechanical strength and weld quality

Abstract

Continuous ultrasonic welding is one of the new, high-speed and cost-effective methods for joining thermoplastic composites. The purpose of this research is to conduct a quantitative and qualitative study of the process in order to identify and solve challenges in order to achieve high-quality continuous ultrasonic welding in thermoplastics reinforced with unidirectional continuous fibers; specifically GF/PA6. In a quantitative study using the response surface method (RSM), with welding power, speed, and pressure as input parameters and lap shear strength as the response, a statistical design of welding experiments was carried out to obtain the appropriate mathematical equation governing the process and to determine the effect of these factors on shear strength. In a qualitative study, continuous ultrasonic welds were classified based on shear strength into three categories: weak, average, and strong. Using microscopic images of the fracture surface and the cross-sectional area of overlapping adherends, phenomena such as penetration amount, uniformity, voids, and failure mechanisms in joints were studied. Additionally, the relationship between process parameters and these phenomena was investigated. The results showed that the two-factor interaction (2FI) model is suitable (R² = 0.9061) and the power-pressure interaction being the most effective factor. In contrast, the pressure factors and power-speed interaction were found to be ineffective. Additionally, setting the welding power between 1000–1200 W, the speed at around 30 mm/sec, and the pressure between 0.5–1 MPa leads to high penetration and uniform, high-quality welding. High penetration in strong welds allows the fibers to bear the load, increasing weld strength.