Time resolved analysis of nanosecond pulsed laser processing of carbon fiber reinforced plastics

Abstract

Carbon fiber reinforced plastics (CFRP) are of high interest for lightweight construction within many industrial sectors. Although knowledge about the laser processing of CFRP is continuously increasing, deeper process knowledge is of importance for the development of automatable and controllable industrial processes. Furthermore, new laser sources are developed continuously, providing, for example, high pulse energies for short and ultrashort pulse laser processing. In order to develop a fast and high quality cutting process, the lifetime of potentially shielding plasmas and particles is a key factor. For this purpose, time resolved investigations of pulsed laser processing were conducted. A fiber guided nanosecond pulsed laser source with an average laser power of PL = 1.5 kW and a maximum pulse energy of EP = 80 mJ was used for cutting. The setup for the time resolved investigation consisted mainly of an industrial camera, enabling a minimum exposure time of tB = 13 μs and a light source with a pulse duration of tP = 500 ns in order to realize pictures of the process with high spatial and temporal resolutions. An optical setup with a light source and a camera on opposite sides of the processing zone was chosen for the investigation. In order to ensure distortion-free imaging of the processing zone, the camera was equipped with telecentric lenses. The time resolved analysis is realized by a laser signal as the master clock and a delay generator, enabling the activation of the camera and light source in a defined delay in relation to the laser pulse. With the described setup, cutting processes with different laser parameters were investigated. The focus of the analysis was the plasma and particle formation depending on two main factors for the observed process parameters. One factor is the time delay between the laser pulse and taking the picture, and the other factor is the cutting kerf depth. The investigation revealed the lifetime of the plasma as well as the changes in the plasma between two consecutive pulses. The results indicate a plasma lifetime in the range of t = 20 μs to t = 200 μs. Consequently, the optimum repetition rate for the chosen laser system is expected to be in the range of f = 5 kHz to f = 50 kHz. For deeper cutting kerfs and smaller ablation rates, the area of plasma plumes reduces and the lifetime of the plasma seems to reduce in an analogous manner.