Simulation of the influence of electromagnetic stirring during laser beam welding of thick-walled steel components with filler material

在本项目中，电磁搅拌对填充材料的分布在高功率激光束焊接钢厚度超过10毫米的影响将进行研究。因此，需要使用填充材料可能是由于改进的间隙桥接能力、热裂纹敏感性的降低或确保焊接的机械技术性能。用于改善填充材料混合的替代选择是多道焊接，这是耗时的，因此降低了工艺效率。该项目的主要重点是深入了解在高功率激光束焊接钢时通过振荡磁场进行电磁搅拌的原理。为此，将进行数值计算，解决基于温度依赖性材料特性的传热、流体动力学和电磁学的耦合问题。这允许根据工艺参数（激光功率、工艺速度、填充焊丝）和磁体系统的参数（磁场强度、振荡频率、焊接方向与磁场线之间的角度）来量化搅拌行为。仿真结果通过相应的实验进行了验证。物理机制是基于由于外部磁场的振荡而引起的涡电流的感应。不对称的设置以及在熔池前面的差距的电隔离导致熔体中涡流的集中。在外加磁场的作用下，形成了一个具有旋转分量的洛伦兹力分布，从而改善了熔体中的搅拌。该项目的主要目的是通过诱导不均匀的洛伦兹力分布，促进厚壁部件焊接过程中熔池深度的混合行为。最后，可获得数值模型，从而优化填充材料搅拌过程，并对物理效应和机制进行识别和量化。然后将没有铁磁特性的高合金奥氏体不锈钢的结果转移到具有明显磁滞的结构钢。

In this project, the influence of electromagnetic stirring on the distribution of filler material during high power laser beam welding of steel with thicknesses above 10 mm will be investigated. Hereby, the need for using filler material can be due to an improved gap-bridging capability, a reduction of the hot-cracking susceptibility or ensuring mechanical-technological properties of the weld. An alternative option for improving the filler material mixing is multi-pass welding which is time-consuming thus lowering the process efficiency. The main focus of the project is to gain insights into the principles of the electromagnetic stirring by means of oscillating magnetic fields in high power laser beam welding of steel. To that, numerical calculations will be conducted solving the coupled problem of heat transfer, fluid dynamics and electromagnetics based on temperature-depending material properties. This allows for a quantification of the stirring behavior depending on process parameters (laser power, process speed, filler wire) and parameters of the magnet system (magnetic field strength, oscillation frequency, angle between welding direction and magnetic field lines). The simulations are verified by accompanying experiments. The physical mechanism is based on the induction of eddy currents due to the oscillation of an external magnetic field. The asymmetric setup as well as the electric isolation of the gap in front of the melt pool lead to a concentration of the eddy currents in the melt. Together with the externally applied magnetic field, a Lorentz force distribution is formed which has a rotational component leading to an improved stirring in the melt.The main objective of the project is the promotion of the mixing behavior into the depth of the melt pool during welding of thick-walled components by induced inhomogeneous Lorentz force distributions. In the end, a numerical model is available allowing for a process optimization with regard to the stirring of the filler material and also for an identification and quantification of the physical effects and mechanisms. The results obtained for high-alloy austenitic stainless steels without ferromagnetic properties are then transferred to structural steels with a pronounced magnetic hysteresis.