Temperature history influence on laser beam absorption at laser material processing

了解激光-物质相互作用过程中耦合到材料中的热能比例是规划基于激光或激光辅助材料加工的重要因素。拟议研究项目的主要目标是在与工业应用相关的选定钢的动态加工中的(散焦)激光能量的耦合，特别是关于温度历史和相关的表面改性。为此，应进行激光辐照实验，以改变温度周期，同时与温度同时瞬时记录光谱反射率。每个样品的表面状况都要在加工之前和之后进行表征。基于反射率和表面条件的结果，建立了吸收行为依赖于温度历史的唯象模型。必须在保护气体气氛下和在正常空气条件下研究其吸收行为。这样就有可能考虑到氧化的影响。此外，还研究了表面处理(即抛光、研磨、不处理)对能量耦合动力学的影响。所需的结果被认为是数值模拟中更精确的吸收函数的基础，数值模拟确实取决于初始表面条件和温度历史，而不是标准的吸收系数是过程分离值。这种吸收模型有很大的应用领域，例如热成形、工艺和过程链中的变形补偿、激光辅助机械成形和冲压、热传导焊接、硬化等等。

The knowledge of the ratio of thermal energy coupled into the material during laser-matter-interaction is an important factor for planning of laser based or laser assisted material processing. The main goal of the proposed research project is the coupling of (defocused) laser beam energy at dynamic processing of selected steels being relevant to industrial applications with the particular regard to temperature history and thereby related surface modification. For this purpose laser irradiation experiments are to be carried out regarding varying temperature cycles while transient recording of spectral reflectivity in simultaneity with temperature. Each specimens surface condition is to be characterized prior to and after the processing. Based on reflectivity and surface condition results a phenomenological model on absorption behavior depending on temperature history is to be constructed. The absorption behavior has to be investigated under shielding gas atmosphere as well as in normal air conditions. This way it is possible to take effects of oxidation into account. Additionally, influences of surface treatment (i.e. polishing, milling, no treatment) on the dynamics of energy coupling are to be investigated. The desired results are supposed to be a basis for more accurate absorption functions for numerical simulations, which do depend on initial surface condition and temperature history instead of the standard approach of an absorption coefficient being a process detached value. There is a large area of applications for such an absorption model, for instance thermal forming, distortion compensation in processes and process chains, laser assisted mechanical forming and punching, heat conduction welding, hardening and many more.