Characterization and quantification of mechanisms influencing the process reliability of Laser Beam Melting by experimental and numerical investigations

拟议研究项目的目的是提高增材制造工艺的知识，该工艺采用激光束熔化金属粉末。计划的调查将考虑熔融金属的冷却和凝固过程以及液态金属的蒸发，这影响工艺特性和产品质量，特别是关于气孔的出现。同时，对热区附近的热致残余应力进行了分析。拟议项目的重点将是分析所选择的工艺参数（如激光功率、扫描速度）、所采用的材料（导热性、相变焓、粉末粒度和形状）以及熔池中的流体和热力学效应（对流、马兰戈尼效应、蒸发、液体表面拓扑结构和激光束反射等）之间的相互作用。获得的见解将允许更稳定的束熔化过程，也有助于降低生产中的废品率，减少新零件设计的测试运行次数，并允许在确定新材料时缩短测试程序。首先进行了实验工艺分析，以估计主要工艺参数对工艺稳定性的影响程度。在下面的相互作用的机制进行了更详细的研究，并采用实验和数值模拟量化。本文首次将光滑粒子流体力学方法应用于激光熔化过程的模拟。对于SPH流体和固体，可以用拉格朗日粒子来描述，它们具有各自相的性质。局部宏观值，如温度，通过使用所考虑位置附近的所有粒子的平均操作来恢复。根据今天的技术状况和我们的初步工作，SPH方法将允许高效处理多相变化和具有复杂几何形状的粉末层。这将使模拟激光束熔化考虑到所有相关的物理机制。将进行实验研究以验证数值方法并拓宽对熔化过程的认识。为了充分捕捉快速瞬态过程，将评估并采用合适的测量技术来解决问题。为了研究熔池动力学，以及由于保护气体的应用和熔体蒸发引起的粉末颗粒从加热区吹走的效应，将使用高速摄像机。

The aim of the proposed research project is an advancement of the knowledge on additive manufacturing processes, which employ lasers beams for melting metal powders. The planned investigations will consider the process of cooling and solidification of the molten metal as well as the evaporation of liquid metal, which influences process characteristics and product quality, especially with respect to the occurrence of pores. Also thermally induced residual stresses in vicinity of the heating zone will be analyzed. The focus of the proposed project will be the analysis of the interactions between on one side the selected process parameters (e.g. laser power, scanning speed), the materials employed (heat conductivity, enthalpies of phase changes, powder particle size and shape) and on the other side the fluid- and thermodynamic effects in the melt pool (convection, Marangoni-effect, evaporation, liquid surface topology and laser beam reflections etc.). The obtained insights will allow for more stable beam melting processes and also help to reduce reject rates in production, reduce the number of test runs for new part designs and also allow to shorten the testing program when qualifying new materials.Initially an experimental process analysis is conducted in order to estimate the extent of influence of major process parameters on the process stability. In the following the mechanisms of interaction are investigated in greater detail and are also quantified employing experiments as well as numerical simulations. For the first time the method of smoothed particle hydrodynamics (SPH) will be applied for the simulation of laser beam melting. With SPH fluids and solids are described by Lagrangian particles, which carry properties of the respective phase. Local macroscopic values, such as temperature, are recovered by averaging operations using all particles in vicinity of the position considered. Judging by todays state of technology and our preliminary work, SPH methods will allow for a highly efficient handling of multiple phase changes and of powder layers featuring complex geometries. This will enable simulations of laser beam melting taking into account all relevant physical mechanisms. Experimental investigations will be conducted to validate the numerical method and broaden the knowledge on the melting process. In order to adequately capture rapid transient processes, suitable measurement techniques will be evaluated and adopted to the problem. For investigations of melt pool dynamics and also of the blow-away effect of powder particles from the heating zone due to application of shielding gases and also due to evaporation of the melt, high-speed cameras will be employed.