Microstructure Evolution of EBM Gamma Titanium Alumiide (TNM-B1)

• Applicant: Professor Dr.-Ing. Christoph Leyens
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 406109547

Over the past decade additive manufacturing has become of great scientific and industrial interest. This enables tailored component manufacturing for a wide range of applications, for example in aerospace and medicine industry. These industries often rely on difficult to process materials. Powder-bed based methods like the electron beam melting process (EBM) emerged as an excellent choice for such use cases due to high process temperatures and slow cooling rates.Titan aluminides are known to be a potential candidate for material substitution in aerospace industry due to their excellent high temperature behavior as well as low density. However these materials exhibit a ductile-to-brittle transition at around 700 °C. Thus casting or processing by selective laser melting (SLM), a generative process as well, are susceptible to thermal induced cracks. The EBM process could establish as an alternative processing route for near net shape manufacturing of complex parts consisting of those materials.This project focusses on the experimental investigation of the microstructural evolution of EBM-manufactured TNM-B1 parts – a further development of TNM-alloy – along a common EBM manufacturing chain. TNM alloys are titanium aluminides with improved workability due to decreased segregation and texture susceptibility compared to conventional gamma-TiAl alloys. The microstructure of TNM-alloys significantly influences the component properties and is controlled by a thermal treatment subsequent to the EBM-processing. Thus a key point of this project is to investigate the influence of those two processes on the microstructural evolution of TNM alloys. Thereby two main aspects shall be addressed. First a possible evaporation of elements which lead to changes of the chemical and phase composition and second a possible microstructural change in comparison to casted TNM-B1 components. Former researches already highlighted the evaporation of aluminum during EBM-processing for other titanium aluminides. For the TNM system this could lead to an increased amount of brittle alpha2-TiAl-phase and thus influence microstructure and residual stress state. An increased risk of thermal induced cracking especially during quenching steps (e.g. subsequent to the solution annealing heat treatment) might be the consequence. Taking this into account the evolution as well as the stability of residual stresses along the EBM-manufacturing chain shall be investigated. The emphasis of the residual stress examinations will be on the multi-step heat treatment subsequent to the EBM-process.Based on all results the applicability of the EBM-process for component manufacturing from TNM-B1 or TNM-alloys will be evaluated. There the outcomes of the microstructural and residual stress state investigations will also be used to identify relevant and critical processes and process parameters along the EBM manufacturing chain.

• DFG Programme: Research Grants