Microstructure formation in selective laser melting of titanium alloys and influence on the machinability
Metallurgical, Thermal and Thermomechanical Treatment of Materials
Final Report Abstract
Within the scope of this project, various changes in the microstructure of a titanium alloy were successfully achieved using additive manufacturing by means of laser powder bed fusion (PBF-LB). These changes concerned the phase composition, grain sizes and densities. The applied titanium alloy was a near-β alloy with the chemical composition Ti-5Al-5V-5Mo-3Cr (in % by mass, abbreviated as Ti-5553). Due to its excellent mechanical properties, such as high tensile strength at low density (4.65 g/cm3), this alloy is typically used in the aerospace industry. The microstructure of Ti-5553 consists of two phases: a cubic β-phase, which forms the matrix, and a hexagonal α-phase, which can be precipitated in various modifications through targeted heat treatments. These modifications allow different properties such as ductility, hardness and toughness to be adjusted depending on the proportion of phases, making this alloy particularly suitable for structurally stressed components such as landing gear and frame areas in airplanes. As a rule, this material is processed conventionally using thermomechanical treatments. However, machining poses a major challenge due to its mechanical properties and poor thermal conductivity, which leads to high wear on the cutting tools and a significant reduction in tool life. By specifically adapting the process parameters in the PBF-LB process, dense (over 99% relative density), crack-free and distortion-free components could be produced. This included microstructures of pure β-structures with different grain sizes as well as components with proportions of α-phases produced directly in the PBF-LB process (as-built). Furthermore, the influence of the component orientation was investigated. After a comprehensive characterisation of the components concerning their microstructure, composition, hardness and mechanical properties, the machinability was evaluated. Tool wear, tool life and chip formation were analysed through milling tests. It was demonstrated that tool wear could be significantly reduced by specifically adjusting the microstructure in the PBF-LB process. Subsequently, industrial heat treatments (BASCA, STA) were conducted on the as-built specimens, resulting in increased mechanical properties. The effect on tool wear during machining persisted after heat treatment and longer tool life was achieved with the heat-treated additively manufactured specimens compared to conventionally and heat-treated specimens. Within the project, models were developed to predict the mechanical properties depending on the process parameters used in additive manufacturing. Further research is required with regard to the influencing variables in the PBF-LB process on machining and their possible transferability to other materials.
Publications
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Wirtschaftliche Zerspanung additiv gefertigter Titankomponenten. Zerspanungstechnik.de, 2022
Worpenberg, S.
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Einfluss des Fertigungsprozesses auf die Zerspanbarkeit von additiv gefertigten Ti-5553. Werkstatt + Betrieb 6/2023
Denkena, B.; Bergmann, B. & Worpenberg, S.
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Influence of LPBF-parameters and heat treatment on machinability and chip formation of additively manufactured Ti-5Al-5Mo-5V-3Cr alloy. Vortrag FEMS EU- ROMAT 2023, Frankfurt am Main, 07.09.2023
Denkena, B.; Bergmann, B.; Worpenberg, S.; Hufenbach, J.K.; Kühn, U. & Kunz, C.
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Tuning the materials properties of Ti-5553 by specific modification of the process parameters during LPBF. Vortrag LightMAT 2023, Trondheim, 22.06.2023
Kunz, C.; Worpenberg, S.; Kühn, U.; Bergmann, B.; Denkena, B. & Hufenbach, J.K.
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Zerspanbarkeit additiv gefertigter Titanbauteile. VDI-Z, 165(11-12), 18-20.
Denkena, Berend; Hufenbach, Julia; Bergmann, Benjamin; Kühn, Uta; Kunz, Clemens & Worpenberg, Sebastian
