ANALYSIS: ADDITIVE MANUFACTURING
Taking powder bed fusion precision to the next level with in-situ laser ablation
Manuel Henn, Matthias Buser and Volkher Onuseit, of the University of Stuttgart’s IFSW, combine additive and subtractive laser processes to unlock new manufacturing possibilities
Laser-based powder bed fusion of metals (PBF-LB/M) is an additive manufacturing process used to create highly complex parts by melting multiple fused beads in consecutive layers of metal powder. The unrivalled freedom of design is its main selling point. Today PBF-LB/M is no longer found only in scientific labs, with several industrial applications ranging from prototype work and spare-part production to small quantity batch-produced components. The surface quality of PBF- LB/M parts is often compared to that of castings, and post processing including support structure removal, cleaning, deburring or sanding, which is mandatory to make parts ready to use. Subsequent machining operations are needed if additional features with an accuracy of a few micrometres are specified, such as threads, precision holes or bearing seats. This is necessary due to the inherent limitations in geometrical accuracy of PBF- LB/M in the lateral direction. The achievable precision is determined by process and
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system parameters, primarily by the minimum diameter of the laser beam on the surface of the printed part, which is typically 50 to 500µm. Additionally, feed rate and laser power determine the width of the melt pool and thus the minimum structure size. By leaving a gap between two adjacent scan lines, small structures such as internal slits with a width of about 100µm can be created1. However, the generated slits lack contour sharpness caused by the freeform solidification of the melt and the interactions of the incident laser beam, melt pool and powder2,3. This effect can be compensated to an extent using an even smaller focal diameter and finer powders4,5. But there are numerous industrial applications that require parts with internal micro-structures on the micrometre scale. Such applications cannot be realised with PBF-LB/M components if the structure is geometrically too sophisticated for common subtractive post-processes, or the structure is inaccessible for post-processing.
The aim Overcoming these limitations is the goal of a new research
Figure 1: Schematic illustration of the experimental setup for the combined additive and subtractive laser processes
project at the Institut für Strahlwerkzeuge (IFSW) at the University of Stuttgart. Together with its partners from the Laser Application Centre (LAZ) at Aalen University, and the Institute for Production Engineering (WBK) of the Karlsruhe Institute of Technology, the IFSW launched the research project ADDSUB as part of Innovation Campus Future Mobility, which is funded by the Ministry of Science, Research and Arts of Baden- Württemberg state. ADDSUB aims to combine
additive and subtractive laser processes in the same machine. An animation of such a process is on IFSW’s YouTube channel6. By laser ablating precise micro features in each layer, the team aims to increase achievable accuracy by at least an order of magnitude compared to the conventional PBF-LB/M process.
The application for this
technology within ADDSUB is printing top-performing soft magnetic components for electric motors. Such parts have been made from stacked sheet metal to combine the
superior flux-conducting properties of the base material with an eddy-current- prohibiting design, i.e. the high- resistance interface between the sheets. With PBF-LB/M, the design constraints by stacking sheets can be overcome, and the geometry of, for example, an optimised stator of a transverse flux machine, or an electric motor for wheel hub drives, can be produced as a single printed part. However, the thin slits required for eddy-current prevention pose a challenge to conventional PBF-LB/M. To exceed current limitations
on narrow slits in printed parts, ultrafast laser ablation of single lines within every newly built-up layer is going to be used to produce vertical slits across multiple layers, mimicking the effect of sheet stacking.
The experiments To the best of the authors’ knowledge, there is currently no laser beam source of combined continuous-wave and ultrafast capability with the needed high average power available on the market. Therefore, the experimental system
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Henn et al.
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