Formation of equiaxed grains in pure iron during laser powder bed fusion

Microstructural evolution of commercially pure (CP) iron (Fe) fabricated by Laser Powder Bed Fusion (LPBF) was studied to investigate the mechanisms responsible for the formation of equiaxed grains during additive manufacturing, a surprising phenomenon in the additive manufacturing of cubic metals, which normally produces columnar grains. Detailed electron microscopy was performed on the LPBF bulk, adjacent laser tracks, and single-track samples to examine solidification and phase transformation sequences with and without reheating from neighbouring tracks or subsequent layers. The microstructure throughout the LPBF bulk, including the top layer with no reheating from above, was dominated by equiaxed grains with weak texture. In contrast, a single laser track consisted of lath-shaped colonies within prior columnar grains, creating a Widmanstätten pattern. Reheating from neighbouring laser tracks was sufficient to gradually transform these lath-shaped structures into equiaxed grains via recrystallisation and grain growth. Electron backscatter diffraction (EBSD) analysis revealed a transformation path of Liquid→δ-BCC→γ-FCC→α-BCC, in contrast to previously proposed Liquid→δ-BCC→α-BCC where the absence of γ-FCC due to rapid cooling was assumed. This was evidenced by reconstructed columnar γ-FCC from α-BCC EBSD data assuming FCC and BCC orientation relationships (ORs), as well as misorientation relationships across a wide range of specimens. The misorientation distributions were also studied to shed light on the early stages of microstructural development, the potential inheritance of the initial γ microstructure by the resulting α, and the progress of recrystallisation and grain growth throughout the process.