In the research project "Microstructure formation during selective laser beam melting of titanium alloys and influence on machinability" funded by the German Research Foundation (DFG), the Institute of Production Engineering and Machine Tools (IFW) at Leibniz University Hannover, together with the Leibniz Institute for Solid State and Materials Research in Dresden, worked out the fundamentals of the influence of the process parameters of the PBF-LB process on the microstructural properties and the resulting machinability of the titanium alloy Ti-5Al-5V-5Mo-3Cr (Ti5553). As part of this research project, the IFW investigated the machinability of various microstructural modifications that resulted from different process parameters. Using a model-based methodology, a link was established between the input variables of the laser process, the mechanical properties, the resulting microstructure and the resulting machinability. Titanium alloys are considered difficult-to-machine materials due to their thermomechanical properties. "The results generated in the project contribute to further increasing the economical and resource-saving production of components for the aerospace industry," explains project researcher Sebastian Worpenberg.
The titanium alloy analysed is a near-β alloy. Due to the large temperature difference between the heat source (melt pool) and the heat sink (already solidified material), high cooling rates can be achieved. In this way, it is possible to achieve a pure β-structure using the PBF-LB process. In general, however, the microstructure is characterised by a large anisotropy of the microstructural properties and by elongated grains along the build-up direction. In addition, the grain size is significantly influenced by the selected parameters of the laser process.
In the first stage of the project, the influence of the process parameters of the PBF-LB process on the resulting microstructure of the Ti5553 alloy was investigated. In addition, some samples were subjected to different heat treatments. All samples were characterised with regard to their microstructure and mechanical properties. In the subsequent step, the knowledge gained was summarised in a model for mapping the above-mentioned properties as a function of the process parameters. Parallel to the determination of the microstructure and the mechanical properties, the machinability of the respective samples was evaluated in the form of milling tests. In addition to knowledge of the tool wear behaviour, the aim was also to gain knowledge of the influence of the machining process on the formation of the component surface area. Within these investigations it could be shown that the tool wear behaviour is significantly influenced by the process parameters of the PBF-LB process. This influence is also present after the usual industrial heat treatments.
With the help of the results generated in the project, it is possible to select model-based process parameters that contribute to resource-saving machining without endangering the safety of the component in later use due to inadequate mechanical properties.
Contact:
For further information, please contact Sebastian Worpenberg from the Institute of Production Engineering and Machine Tools at Leibniz University Hannover on +49 511 762 2537 or by e-mail (worpenberg@ifw.uni-hannover.de).
