Weber Rudolf, Graf Thomas, Freitag Christian, Feuer Anne, Kononenko Taras, Konov Vitaly I
Opt Express. 2017 Feb 20;25(4):3966-3979. doi: 10.1364/OE.25.003966.
In any pulsed and repetitive laser process a part of the absorbed laser energy is thermalized and stays in the material as residual heat. This residual heat is accumulating from pulse to pulse, continuously increasing the temperature, if the time between two pulses does not allow the material to sufficiently cool down. Controlling this so-called heat accumulation is one of the major challenges for materials processing with high average power pulsed lasers and repetitive processing. Heat accumulation caused by subsequent pulses (HAP) on the same spot and heat accumulation caused by subsequent scans (HAS) over the same spot can significantly reduce process quality, e.g., when the temperature increase caused by heat accumulation exceeds the melting temperature. In both cases, HAS and HAP, it is of particular interest to know the limiting number of pulses or scans after which the heat accumulation temperature exceeds a critical temperature and a pause has to be introduced. Approximation formulas for the case, where the duration of the heat input is short compared to the time between two subsequent heat inputs are derived in this paper, providing analytical scaling laws for the heat accumulation as a function of the processing parameters. The validity of these approximations is confirmed for HAP with an example of surface ablation of CrNi-steel and for HAS with multi-scan cutting of carbon fiber reinforced plastics (CFRP), both with a picosecond laser at an average power of up to 1.1 kW. It is shown that for the important case of 1-dimensional heat flow the limiting number of heat inputs decreases with the inverse of the square of the average laser power.
在任何脉冲式和重复式激光加工过程中，一部分吸收的激光能量会被热化，并作为残余热量留在材料中。如果两个脉冲之间的时间不足以使材料充分冷却，这种残余热量会逐脉冲累积，持续升高温度。控制这种所谓的热量累积是高平均功率脉冲激光材料加工和重复加工的主要挑战之一。同一位置后续脉冲引起的热量累积（HAP）以及同一位置后续扫描引起的热量累积（HAS）会显著降低加工质量，例如当热量累积导致的温度升高超过熔化温度时。在HAS和HAP这两种情况下，特别值得关注的是知道脉冲数或扫描数的极限，超过这个极限热量累积温度会超过临界温度，就必须引入停顿。本文推导了热输入持续时间相较于两个后续热输入之间的时间较短这种情况下的近似公式，给出了作为加工参数函数的热量累积的解析比例定律。通过使用皮秒激光、平均功率高达1.1 kW对CrNi钢进行表面烧蚀的HAP示例以及对碳纤维增强塑料（CFRP）进行多扫描切割的HAS示例，证实了这些近似的有效性。结果表明，对于一维热流这种重要情况，热输入的极限数量随平均激光功率平方的倒数而减少。
