Methode zur adaptiven thermischen Prozessführung beim Laser-Strahlschmelzverfahren

Abstract

In the laser beam melting process, one key aspect to achieve parts of high quality and dimensional accuracy is to ensure thermal stability throughout the build-up process. Compared to the consolidated material, the powder has thermally insulating properties. Thus, the desired quality can only be reached if the heat input takes into account the amount of powder in the vicinity of each point to be melted. This requirement is not properly considered by most of the available systems. They usually keep the laser power and scan rate constant or perform only basic adaptations. In overhanging regions, heat accumulates from track to track causing sintering effects, enlarged weld pools, and other overheating effects, making them particularly prone to defects. This leads to low dimensional accuracy, porosity, and also process abortion. Motivated by these problems, this work presents a method for an automatized and localized thermal management of the process with the aim to improve the dimensional accuracy of powder-sided surfaces. For this purpose, the local thermodynamics have been analyzed, and an adaptation theory developed regarding where, when, and how to deal with overheating through adjustment of the process parameters. These measures have been successfully demonstrated in finite element simulations. On the basis of these theoretical findings, a convergent and efficient algorithm has been developed that analyzes the local geometry and topology of the part. It identifies critical structures to create a enhanced pre-processing method with adapted exposure information. After having established an interface to the machines, the methods have been evaluated and validated experimentally. The experiments conducted with stainless steel 1.4404 as the powder material have shown significant improvement of the dimensional accuracy: the deviations from the desired shape have been reduced by up to 61% down to Standard melt pool depth without affecting the part density. The experimental studies in this work were focused on the adaptation of the energy input, but the pre-processing can be extended by further adaptation methods such as, for example, introduction of delay times between the scan tracks or adjustment of the scanning order. These are necessary to build up larger overhanging areas. However, a challenge that then arises is the susceptibility due to lower stiffness of the initial layers over the powder to thermally induced deformations. This needs to be overcome in parallel in order to extend the process limits.