One of the major reasons for the formation of a heat-affected zone during laser processing of carbon fiber-reinforced plastics (CFRP) with repetitive picosecond (ps) laser pulses is heat accumulation. A fraction of every laser pulse is left as what we termed residual heat in the material also after the completed ablation process and leads to a gradual temperature increase in the processed workpiece. If the time between two consecutive pulses is too short to allow for a sufficient cooling of the material in the interaction zone, the resulting temperature can finally exceed a critical temperature and lead to the formation of a heat-affected zone. This accumulation effect depends on the amount of energy per laser pulse that is left in the material as residual heat. Which fraction of the incident pulse energy is left as residual heat in the workpiece depends on the laser and process parameters, the material properties, and the geometry of the interaction zone, but the influence of the individual quantities at the present state of knowledge is not known precisely due to the lack of comprehensive theoretical models. With the present study, we, therefore, experimentally determined the amount of residual heat by means of calorimetry. We investigated the dependence of the residual heat on the fluence, the pulse overlap, and the depth of laser-generated grooves in CRFP. As expected, the residual heat was found to increase with increasing groove depth. This increase occurs due to an indirect heating of the kerf walls by the ablation plasma and the change in the absorbed laser fluence caused by the altered geometry of the generated structures.
This work was supported by the German Research Foundation (DFG) within the project ‘Entrance’ (Funder Id: 10.13039/501100001659, grant GR 3172/17-1) and the Russian Foundation of Basic Research (grant 15-02-91347).
 A. Wolynski, H. Haloui, P. Mucha, A. Gleiter, P. French, et al., in ‘Proceedings of International Congress on Applications of Lasers & Electro-Optics ICALEO 2010’, (Anaheim, USA, 26 September–30 September 2010).Search in Google Scholar
 M. Fujita, H. Ohkawa, M. Otsuka, T. Somekawa, Y. Maeda, et al., in ‘Proceedings of International Congress on Applications of Lasers & Electro-Optics ICALEO 2014’, (San Diego, USA, 19 October–23 October 2014).Search in Google Scholar
 B. T. Kelly, in ‘Physics of Graphite’, (Applied Science Publishers, London and New Jersey, 1981).Search in Google Scholar
 C. Freitag, L. Alter, R. Weber and T. Graf, in ‘Proc. of Lasers in Manufacturing 2015’, (München, Germany, 2015).Search in Google Scholar
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