Microstructural evolution and mechanical properties of functionally graded austenitic-low-carbon steel produced via directed energy deposition

Abstract

In this study, the additive manufacturing of a functionally graded material (FGM) via directed energy deposition was investigated as an alternative to joining dissimilar metals. The metal powder composition of the FGM was gradually changed from fully low-carbon steel to austenite steel along the building direc-tion. A convolutional neural network model was employed to classify the austenite, martensite, and fer-rite phases in the FGM. The volume fraction of the phases was calculated using X-ray diffraction Rietveld refinement and compared with that predicted by the thermodynamic model and that determined from electron-backscattered-diffraction maps. The volume fraction of the bcc phase gradually increased, and the grain size decreased from top to bottom. Nanostructural investigations confirmed the absence of car-bide and twin structures due to the relatively low carbon concentration in the upper layers and the pres-ence of a hexagonal co-Fe phase with twin structures in the interlayers. Furthermore, electron channeling contrast images and kernel average misorientation maps revealed the activation of the deformation twin-ning and strain-induced transformation of the retained austenite to martensite, which increased the strain-hardening rate. This study can guide the selection of a tailored manufacturing strategy and process parameters to obtain the required material distribution.(c) 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).