A Control Method for the Ultrasonic Spot Welding of Fiber-Reinforced Thermoplastic Laminates through the Weld-Power Time Derivative

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.

A Numerical Analysis of the Temporal and Spatial Temperature Development during the Ultrasonic Spot Welding of Fibre-Reinforced Thermoplastics

“…Using this approach, the positioning of the thermocouple was guaranteed and the effect of the strain concentration around the thermocouple was reduced. Computed tomography scans of the weld spots confirmed that the thermocouples remained unaffected after the welding process (compare [6]). The temperature recording was undertaken using a universal amplifier and a data recorder with a sample rate of 300 Hz.…”

“…When welding fibre-reinforced thermoplastic (FRTP) laminates without energy directors, there is insufficient polymer at the weld zone. The lack of the matrix could result in excessive heating of the weld spot and thus the quality of the weld might be reduced [4][5][6][7]. Even though the ultrasonic spot welding without energy directors requires additional process control, the demands for the preparation of the welding zone and the weldments are simplified significantly [4,6].…”

“…The second phase started as soon as the weld interface softened/melted, then the frictional heating diminished, and a concentrated viscoelastic heating dominated the process. An approach to avoid the overheating of the matrix for this spot welding configuration was discussed by the authors in [4,6]; the weld power time derivative was used to detect the overheating initiation of the weld spot and it was avoided by the controlled reduction of the vibration amplitude. Figure 1.…”

“…Figure 1. Schematic drawing of the differential ultrasonic spot welding configuration, the essential components and the related dimensions [6,8].…”

“…The inverse measurement method was applied to determine the loss modulus E m (T) of the polymeric matrix at the ultrasonic welding frequency range by minimizing the difference between the experimental data and the predictions from implemented numerical model in which the loss modulus was the adjustable parameter. This approach was preferred for two reasons: first, the flexibility that it provided to investigate various physical phenomena, hypotheses and theories of the DUS process; and second, to establish a numerical basis for the improvement of the active control system of the ultrasonic welding, which was presented in [6]. The numerical simulation served to explain the energy direction mechanism during DUS, clarify the causes of the overheating, detect the overheating initiation regions, and thus confirm the findings made in [8].…”