Invar processed by selective laser melting – Tailoring the thermal expansion coefficient by process-induced defects and residual stresses

• Applicant: Professor Dr.-Ing. Thomas Niendorf
• Subject Area: Materials in Sintering Processes and Generative Manufacturing Processes
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 456078747

The Fe-36%-Ni alloy Invar is known for its low coefficient of thermal expansion (CTE) and good mechanical properties in cryogenic environments. Due to its unique properties, Invar it is commonly used as a high precision and highly reliable material in components where superior dimensional stabilities are required in a wide temperature range. Since Invar is a material of relatively low strength, conventional machining of complex geometries is highly challenging and cost-intensive. As a near-net-shape manufacturing process, selective laser melting (SLM) represents a promising candidate to overcome the challenges related to conventional machining of the Invar alloy. An analysis of state-of-the-art clearly reveals that the general processability of Invar components by the SLM process has been affirmed. In addition, first results show that, based on the chosen set of process parameters, an influence on the thermal expansion coefficient is possible due to process-induces defects as well as residual stresses. However, systematic investigations of the interrelationships between defects, residual stresses, microstructure and CTE have not been addressed so far. Thus, the main objective of this research project is to tackle this research gap by gaining a deeper understanding of the relationship between process-induced defects, residual stresses and the thermal expansion behavior of SLM-manufactured Invar. By selecting apt processing parameters as well as analyzing the resulting defect- and microstructures (by scanning electron microscopy and computed tomography) and thermal expansion coefficients, Invar conditions with minimized CTE values are to be determined as a function of volume energy. Furthermore, the contribution of process-induced residual stresses for these conditions will be quantitatively determined and evaluated by a comparison between as-built and stress-relief annealed samples via X-ray diffraction. In order to analyze the influence of defects and residual stresses on the mechanical behavior, the CTE-minimized conditions are further subjected to comprehensive mechanical characterization. Since many components made of Invar are subjected to cyclic loading in practice, the fatigue behavior in the low-cycle-fatigue regime is investigated in addition to the behavior under monotonic load. Finally, fractography is performed allowing to rationalize the results from the fatigue tests in order to pave the way for a safe and reliable evaluation of the structural integrity of the CTE-minimized SLM Invar conditions.

• DFG Programme: Research Grants