Mesoscopic Deformation Behaviour of Inconel 718 Fabricated By The Laser Powder Bed Fusion Process

Afshin, Ehsan (2023). Mesoscopic Deformation Behaviour of Inconel 718 Fabricated By The Laser Powder Bed Fusion Process. PhD thesis The Open University.



The advantages of Inconel 718 (IN718) components fabricated by the Selective Laser Melting (SLM) process have been well documented, and these components can be used in critical applications, such as aerospace industries. However, further studies are necessary to gain a deeper understanding of their deformation behaviour. As a result of characteristics of the SLM process, the IN718 samples fabricated by this process have a microstructure that is characterized by crystallographic texture and columnar grain structure. The main objective of this thesis is to investigate the effects of grain morphology and crystal orientation on the deformation behaviour of IN718 fabricated by SLM process. Furthermore, since forging is a reliable manufacturing process that is widely used in many industries, a comparative study of IN718 fabricated by both SLM and forging processes were conducted. To achieve these aims, both samples were subjected to two heat treatments at 1010 °C and 1100 °C to mitigate some defects in the SLM fabricated sample. These heat treatments provided similar conditions in terms of phase formation in both SLM and forged samples to have a better comparative study. Heat-treating both samples at 1010 °C and 1100 °C led to transformation of the Laves and carbides to strengthening phases (γ′ and γ″) in SLM fabricated sample, and similar phases in both SLM and forged samples have been nucleated. In fact, during the dissolving of the Laves and carbide, the Nb will be released, which will lead to the formation of γ′ and γ″ phases. Although the crystal orientation of both SLM and forged heat-treated samples at 1010 °C did not change, recrystallisation occurred at 1100 °C for both SLM and forged samples. The Digital Image Correlation (DIC) technique has been used here to study the deformation behaviour at the mesoscopic scale for IN718 sample fabricated by both SLM and forging processes. The strain in grains during deformation has been analysed by combining DIC and electron backscattered diffraction (EBSD) data. It has been found that due to the different morphology, and size of the grains in the melting pool area, there is a specific strain distribution pattern in the SLM fabricated sample, while in the forged fabricated sample the strain distributes randomly because of the homogenous grain structure. The columnar grains growing across the melting pool in the SLM fabricated sample were subjected to lower strain in compression than the grains in the melting pool. In addition, the columnar grains can vary in shape and size throughout the microstructure of SLM fabricated samples, and the more columnar grains there are, the closer their strain is to the average strain. Moreover, it has been concluded that the crystal orientation of the grains does not affect the strain distribution in both SLM and forged samples. Finally, the effects of heat treatments on the deformation behaviour at the mesoscopic scale have been studied. Moreover, deviation which is the difference between the highest and lowest strain values in various areas of a DIC strain map has been calculated. All heat-treated SLM samples and forged samples showed a decrease in deviation as the average strain increased in the elastic area, whereas deviation increased for SLM as-fabricated samples and forged as-received samples. Heat-treating both SLM and forged samples at 1010 °C resulted in decreasing the mean deviation in the elastic and beginning of the plastic area.

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