Temperature Evolution during the Ultrasonic Welding of Thermoplastics

Author: Kaytie Barkley, Department of Mechanical Engineering

Mentor: Dr. Cassandra Birrenkott, Department of Mechanical Engineering

Mentor: Dr. Prasoon Diwakar, Department of Mechanical Engineering

Ultrasonic spot welding (USSW) has become an attractive joining method for thermoplastic polymers (TP) and thermoplastic matrix composites (TPMC). In the USSW process, the material to be joined is subjected to mechanical vibrations of high frequency, causing the interface between the materials to melt from friction. The interface resolidifies, resulting in a spot weld between two materials. The high strength to weight ratio of TPMCs make them highly desirable in the automotive and aerospace fields. However, USSW characteristics are hard to predict, largely due to unpredictable temperature spikes that occur during the welding process. These temperature spikes can cause the matrix to overheat and degrade, decreasing the strength of the resultant weld.

In order to better characterize – and eventually control – ultrasonic spot welds, this work will investigate the driving factors behind the temperature evolution within the welding process and seek to identify factors contributing to temperature spikes that are detrimental to the matrix material within the weld. To accomplish this, an analytical model will be created that encompasses key aspects of joint design such as weld amplitude, pressure, and energy that impact the temperature profile during welding. The analytical model will be verified utilizing a thermal camera capable of capturing the surface temperature of the TPC during USSW. An in-depth understanding of what causes temperature spikes in USSW will reveal how to better control the temperature evolution over the duration of the welding process. Controlling welding temperature may also allow for predictions of the viscous flow of the matrix material that is responsible for the final bond area of the weld, which can also be correlated to joint strength. The predictability of spot welding is an important area that is currently lacking from industry. By pairing experimental temperature measurements with a newly developed analytical model, weld predictability may be achieved.