Scientists at the Fraunhofer Institute branch in Dresden have been researching the further development of additive manufacturing technology LPBF (Laser Powder Bed Fusion) for over 15 years. This process uses a focused laser beam that melts metallic powder specifically at the points where the component is to be built up layer by layer. In order to create a functional component from the powder, control over the entire physical and digital process chain is crucial. Optimized scanning strategies play a key role here: they define the sequence, length, alignment and spacing of the laser paths and thus enable a more precise, dimensionally accurate, filigree and homogeneous production of components with improved surface quality.
A special scanning strategy is required for grid-like, filigree structures, which are often needed in medical technology. Instead of the conventional contour-hatch scanning strategy, in which the laser first scans the outer contour and then fills the surface, the quasi-point scanning strategy is used here. Here, the laser only moves along very short, partially crossed scan paths. This procedure leads to more precise results, particularly with delicate components: The energy input remains evenly distributed, which prevents unwanted build-up and increases productivity as the laser has to cover shorter distances. In this way, for example, implants with delicate lattice structures such as stents can be produced, which are used in medicine to keep constricted blood vessels open. Superelastic shape memory alloys such as nickel-titanium (NiTi), which increase the load-bearing capacity of the structure, are ideal for particularly gentle applications.
Beyond medical technology, the potential of additive processing of NiTi can be seen in other areas of application. For example, an internal Fraunhofer project has developed a clamping and holding element specially designed for brittle materials such as ceramics. This component is characterized by local superelastic properties that enable a more even distribution of stresses and avoid critical loads.
Geometry-adapted scanning strategies for open-pored structures
A geometry-adapted scanning strategy is used for the production of fine, cellular structures, such as those required for bone implants. One example of this is the shoulder short-shaft implant developed at the Fraunhofer IWU, in which different areas of the component (such as bridges, cellular and fully solid zones) are identified using automated geometry recognition. These areas are given specific scanning strategies and laser parameters that are tailored to the respective topographical requirements. Compared to conventional processes, where only a uniform scanning strategy is used for the entire component, dimensional accuracy and distortion can be significantly improved. For the precise production of cellular structures, it is particularly important to optimize all geometry and process parameters as a whole.
Voronoi-based scanning strategies for demanding overhang areas
The LPBF process offers a high degree of freedom in the design of complex geometries, but places special demands on the production of overhang areas. These areas, which protrude beyond the underlying layers in a layered structure, are prone to defects, distortion and surface roughness and often require complex support structures in conventional processes. In collaboration with the TU Dresden (Chair of VPE), Fraunhofer IWU is developing a solution to ensure consistent production quality in overhang areas, which also requires no modifications to the production system and fewer support structures.
The innovative approach is based on a combination of automated geometry analysis, thermal simulation of the manufacturing process and a so-called Voronoi-based scanning strategy. The geometry analysis identifies critical areas where specific adjustments to the manufacturing parameters are required, for example by changing the scan path pattern, the size of the partial exposure areas or the scan vector alignment. Voronoi diagrams are used to divide the scan plane into cells that can continuously adapt to the shape of the part cross-section, keeping the size of the cells variable over the distance of the starting points (seed points).
In order to avoid manufacturing errors in overhang areas, the laser parameters are also adapted to the temperature curve determined by the geometry. The basis for this is a thermal simulation based on a new, faster approach. Although the simulation provides less precise results than detailed models, it allows the scanning strategies to be adapted quickly.
