The space industry strives to combine high performance and reliability with reducing the weight of components. The best way to go is through additive manufacturing (AM). The additive manufacturing has become an important method in space exploration and its application due to its ability to enable near final shape products with only minor surface machining and reduced buy-to-fly ratio.
The paper provides a two way method for additive manufacturing process/ technologies. They include selective laser melting (SLM) and directed-energy deposition (DED).
- For the selective laser melting, it can produce parts with high dimensional accuracy and excellent mechanical properties, however it is time consuming and the chamber size of the SLM machine limits the size of the parts produced.
- In the case of the DED, is much quicker and does not have a maximum size limitation but at the expense of less dimensional accuracy of final parts and poorer mechanical properties.
The paper proposed a hybrid approach with the combination of the two techniques which could lead to a broad range of products aimed at maximizing the benefits offered by each technique. In this paper, Inconel 718 was proposed to be the material to be manufactured for space applications. This material contains a Ni-Cr based super alloy used in high temperature applications like in aviation and aerospace, because of its superior thermal and strength properties under extreme temperature and mechanical conditions.
The aim of the work was to apply the hybrid technology to the production of large and light part for space applications. An example that was manufactured or used in the paper was the production of strut end fittings with bionic lighter structure. The hybrid AM involves two steps first, the SLM then followed by the DED and thereafter it becomes a SLM/DED. The interface between this two additive manufacturing surfaces was inclined at an angle of 30 to provide enough space to apply the material by DED and achieved optimum bonding between the SLM and the DED parts without any defects. Further investigations were carried out on the hybrid part or samples. The sample was cut using an electrical discharge machining and then it was subjected to some testing. After the SLM/DED processing, the samples were cut off, the sample was heated at 1010⁰C for an hour to remove the lave phase and then after annealing, the samples were cooled to an aging temperature of 720⁰C with an increased flow of argon. The aging was done in two phase first in 720⁰C for 8hours and the second stage 620⁰C for 2hours. Then after aging it was cooled in water and in an increased flow of argon to room temperature.
The paper identified some tests that were carried out on the micro structures of this samples and they include: light microscopy, electron backscattering Diffraction (EBSD). For each test the DED part and the SLM part showed a different pattern.
- For the light microscopy: the image of the bond between the SLM and DED parts is shown. The left hand side was built first using the SLM process and has a finer microstructure while the right hand side which was built up using the DED process had a coarser microstructure. The side built up by the SLM had fewer defects whereas the DED part has more defects as a lack of fusion, gas porosity and shrinkage porosity.
- In the case of the EBSD, the SLM experiences a columnar grain appearance in the built up direction. The grains have a similar size and shape in the SLM sample, this indicates no crystallization during the AM process. Inside the grain, a misorientation was observed due to the stress that occurred during the SLM process, leading to slightly rotated dendrites inside the grain. As for the DED part, the microstructure was a bit different, columnar grains have zigzag shape inclined towards the scanning direction. A reduced color gradient was also visible inside the grains as the dendrites do not need to accommodate their growth with a slight rotation due to the lower solidification rate.
Conclusion: The result showed that with a proper geometry of the structure, a hybrid process is possible. Combining the two AM processes is a way to produce larger parts with geometrically complex structure. In such a way we can merge the advantages of both AM technologies. SLM allows accurate shapes to be created and unto that part we can build large parts more rapidly with DED technology.
Sources: M. Godec, S. Malej, D. Feizpour, C. Donik, M. Balazic, D. Klobcar, L. Pambaguian, M. Conradi, A. Kocijan. (2021). Hybrid additive manufacturing of Inconel 718 for future space applications. https://doi.org/10.1016/j.matchar.2020.110842