Continuous ultrasonic welding (CUW) of thermoplastic composites is a novel joining technique able to produce long welded seams at high welding speeds. Our state-of-the-art welding setup entails an industrial robot on a track and an in-house developed end effector. Currently, no quality inspection methodology has been implemented; therefore, it is not known how the weld quality can be correlated to sensor and generator data. Such a method would enable the prediction of the weld quality after the process and be a stepping stone for closed-loop controlled welding minimizing defects. The goal of the current study is to use two off-the-shelf sensors for in-situ monitoring of the welding process and correlating the sensor data to the weld quality.

A laser line scanner is introduced behind the consolidator, measuring the step height between the top surfaces of the overlapping adherends. A forward-looking infrared (FLIR) camera is introduced, looking at the region of the top adherend behind the consolidator. This camera captures the residual heat present in the top adherend. The main findings from this study are that it is possible to use these two sensors to monitor the welding process and that the data acquired from them can be used to make predictions of the weld quality along its length. The thickness of the welded interface, which can be measured from the laser line scanner data, can give an indication of whether the interface is unwelded, welded, or overheated.

Regarding the FLIR camera, variations in the temperature at the top surface do correspond to variations in the quality of the weld interface, and furthermore, the rate at which the measured temperature changes is positively correlated to the magnitude of the power consumed by the ultrasonic generator. The FLIR camera can therefore be a powerful sensor to determine weld quality and its consistency throughout the length of the weld.

Ultrasonic wood welding is an ecofriendly method for rapidly joining wooden components in less than 2 s. However, this dynamic process results in low mechanical performance and poor durability under wet conditions. Inspired by natural wood's robust interlocking cellular structure, which leverages lignin fusion to enhance structural integrity, lignin fusion at wood interfaces is optimized, significantly improving lap shear strength and wet durability. These results demonstrate that enhanced lignin fusion at interfaces is crucial for obtaining strong wood joints by positioning lignin as a sustainable energy concentrator, promoting greener manufacturing of sustainable structures into complex shapes. The joints exhibit lap shear strengths and wet durability comparable to those achieved with water-based wood and epoxy adhesives, while also demonstrating conductivity which could be leveraged for multifunctional features such as strain sensing. The approach can be extended to other manufacturing methods, such as hot-pressing and continuous robotic manufacturing, emphasizing its potential for scalability and broad industrial adoption.