Influence of pulse repetition rate and pulse energy on the heat accumulation between subsequent laser pulses during laser processing of CFRP with ps pulses

Heat accumulation between subsequent laser pulses (HAP) is one of the major reasons for the formation of a heat-affected zone during laser processing of carbon fiber-reinforced plastics (CFRP) with picosecond (ps) laser pulses. In the case of CFRP, the formation of a so-called matrix evaporation zone (MEZ) can be observed. Based on a theoretically derived equation that describes the heat accumulation effect and presuming one-dimensional heat flow, an expression for the critical feed rate at which the HAP effect sets in was derived as a function of the pulse repetition rate and the pulse energy. This expression provides a simple dependency of the critical feed rate on the pulse energy and the repetition rate that is a useful tool for process development. To verify this relation, we used dedicated experimental conditions that ensured one-dimensional heat flow and irradiated the material with fluences below the ablation threshold to ensure that the absorbed energy completely remains in the material and is not removed by an ablation process. By doing so, we were able to confirm the validity of the theoretical model. During the actual cutting of CFRP, however, hence, when material is removed, the theoretically predicted dependency of the critical feed rate on the pulse energy and the repetition rate deviates from experimental results when one assumes the fraction of pulse energy that is left in the workpiece as heat (hence not removed with the ablated material) to be constant. The difference, therefore, is most likely attributed to the fact that the fraction of heat remaining in the material after each ablation processes, the so-called residual heat, depends on both the pulse repetition rate and the pulse energy. These dependencies are reported for the first time for pulsed laser processing of CFRP within the presented study.