Abstract
It was found that the ultrasonic spot welding may serve as an efficient method to join relative large thin-walled parts made of fiber-reinforced thermoplastics. In this study, a new control method for the ultrasonic spot-welding process was investigated. It was found that, when welding fiber-reinforced thermoplastic laminates without energy directors, overheating and decomposition of the polymer at the weld spot occurred. The occurrence of the overheating took place at unpredictable times during welding. It was observed that the time trace of the consumed power curve by the welder follows a similar pattern as the time trace of the temperature in the weld spot center. Based on this observation, a control system was developed. The time derivative of the welder power was monitored in real time and, as soon as it exceeded a critical value, the ultrasonic vibration amplitude was actively adjusted through a microcontroller. The controlling of the ultrasonic welding process forced the temperature in the weld spot to remain in an adequate range throughout the welding duration for the polymer diffusion to occur. The results of the controlled welding process were evaluated by means of weld temperature measurements, computed tomography scans, and microscopic analysis of the weld spot fracture surfaces.
Highlights
The well-established methods used in joining of fiber-reinforced polymers in general are mechanical fastening, adhesive bonding, and fusion bonding [1,2]
Energy directors are intentionally inserted in the weld interface at the required joint location to focus the ultrasonic vibration energy [4]
The dashed line is the time trace of the temperature measured in the weld spot for one of the repetitions, and the solid line is the corresponding consumed weld power curve
Summary
The well-established methods used in joining of fiber-reinforced polymers in general are mechanical fastening, adhesive bonding, and fusion bonding (sometimes referred to as welding) [1,2]. Benatar and Gutowski [5] investigated the ultrasonic welding process of carbon fiber-reinforced polyetheretherketone (PEEK) with the presence of energy directors. They concluded in their work that a good bond quality was achieved when the melt fronts of the adjacent energy directors met. Li et al [8] investigated a spot welding method through focusing the ultrasonic energy between the mating fiber-reinforced thermoplastic plates with the absence of the energy directors. Zhi et al [9] monitored the quality of the ultrasonic welds done without energy directors between carbon fiber-reinforced thermoplastic plates through the horn displacement and dissipated power. Several methods for controlling and monitoring the ultrasonic welding process were proposed and investigated in the past decades; the need for an active process control system is essential
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