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

本课题将研究电磁搅拌对10 mm以上钢材在高功率激光焊接过程中填充材料分布的影响。因此，使用填充材料的需要可以是由于改善了缝隙桥接能力、降低了热裂敏感性或确保了焊缝的机械工艺性能。改善填充材料混合的另一种选择是多道焊接，这很耗时，从而降低了工艺效率。该项目的主要重点是深入了解在高功率激光束焊接中利用振荡磁场进行电磁搅拌的原理。在此基础上，根据材料的温变特性，进行了数值计算，解决了传热学、流体力学和电磁学的耦合问题。这允许根据工艺参数(激光功率、加工速度、填充丝)和磁体系统的参数(磁场强度、振荡频率、焊接方向和磁力线之间的夹角)来量化搅拌行为。通过配套的实验对模拟结果进行了验证。其物理机制是基于外部磁场振荡引起的涡流感应。熔池前面间隙的不对称设置和电隔离导致了熔体中涡流的集中。在外加磁场的共同作用下，形成了具有旋转分量的洛伦兹力分布，从而改善了熔体的搅拌。该项目的主要目标是通过诱导不均匀的洛伦兹力分布，促进厚壁构件焊接过程中的混合行为进入熔池深度。最后，可以得到一个数值模型，以便对填料的搅拌过程进行优化，并对物理效应和机理进行识别和量化。对于没有铁磁性能的高合金奥氏体不锈钢，所获得的结果随后被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.