Abstract
According to Hunt’s columnar-to-equiaxed transition (CET) criterion, which is generally accepted, a high-temperature gradient (G) in the solidification front is preferable to a low G for forming columnar grains. Here, we report the opposite tendency found in the solidification microstructure of stainless steels partially melted by scanning electron beam for powder bed fusion (PBF)-type additive manufacturing. Equiaxed grains were observed more frequently in the region of high G rather than in the region of low G, contrary to the trend of the CET criterion. Computational thermal-fluid dynamics (CtFD) simulation has revealed that the fluid velocity is significantly higher in the case of smaller melt regions. The G on the solidification front of a small melt pool tends to be high, but at the same, the temperature gradient along the melt pool surface also tends to be high. The high melt surface temperature gradient can enhance Marangoni flow, which can apparently reverse the trend of equiaxed grain formation.
Highlights
Additive manufacturing (AM) technologies are emerging rapidly and being applied to various materials
To reveal the relationships between microstructures and solidification conditions, we have investigated the solidification microstructures of 316L and 304 SSs induced by electron-beam irradiation
It is indicated that such regions were solidified under the condition with high flow velocities. These results suggest that equiaxed crystals are formed owing to a high velocity of fluid flow even in the powder-bed fusion (PBF)-additive manufacturing (AM) process, even though the G and the R are in the range for columnar grain formation
Summary
Additive manufacturing (AM) technologies are emerging rapidly and being applied to various materials. Materials are subsequently added to form parts with the desired shape in AM processes, and it enables us to build 3D parts with complicated geometry . AM technologies are proposed to be applied to fabricate SS parts as well [1,2,3,4,5,6]. Among the AM processes applied to metals, a powder-bed fusion (PBF)-type AM has been most commonly used in this decade for its accuracy and degree of freedom in the shape of objects fabricated. In the PBF-AM process, 3D parts are fabricated by repeating the selective melting of metal powders by laser or electron beams and subsequent solidification. AM-processed SS parts possessing a relative density higher than 99% can be fabricated with good reproducibility [2]
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