Influence of pulse repetition rate and pulse energy on the heat accumulation between subsequent laser pulses during laser processing of CFRP with ps pulses
Influence of pulse repetition rate and pulse energy on the heat accumulation between subsequent laser pulses during laser processing of CFRP with ps pulses
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DOI：
10.1007/s00339-018-1891-z
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发表时间：
2018-07-01
影响因子：
2.7
通讯作者：
Graf, T.
中科院分区：
文献类型：
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作者：
Freitag, C.;Kononenko, T. V.;Graf, T.
后续激光脉冲 (HAP) 之间的热量积累是在皮秒 (ps) 激光脉冲激光加工碳纤维增强塑料 (CFRP) 过程中形成热影响区的主要原因之一。对于 CFRP，可以观察到所谓的基质蒸发区 (MEZ) 的形成。基于描述热积累效应的理论导出方程并假设一维热流，导出了 HAP 效应出现时的临界进给速率的表达式，作为脉冲重复率和脉冲能量的函数。该表达式提供了临界进给速率对脉冲能量和重复率的简单依赖性，这是工艺开发的有用工具。为了验证这种关系，我们使用专用的实验条件来确保一维热流，并用低于烧蚀阈值的注量照射材料，以确保吸收的能量完全保留在材料中，并且不会被烧蚀过程去除。通过这样做，我们能够证实理论模型的有效性。然而，在 CFRP 的实际切割过程中，当材料被去除时，当假设作为热量留在工件中的脉冲能量的一部分（因此不会与烧蚀的材料一起去除）恒定时，理论上预测的临界进给速率对脉冲能量和重复率的依赖性与实验结果存在偏差。因此，这种差异很可能归因于每次烧蚀过程后材料中剩余的热量部分（所谓的残余热量）取决于脉冲重复率和脉冲能量。在本研究中，首次报道了 CFRP 脉冲激光加工的这些依赖性。
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.