Inverse Columnar-Equiaxed Transition (CET) in 304 and 316L Stainless Steels Melt by Electron Beam for Additive Manufacturing (AM)

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.

Digital Twin Science of Metal Powder Bed Fusion Additive Manufacturing: A Selective Review of Simulations for Integrated Computational Materials Engineering and Science

“…( In a recent analysis of temperature distribution variations, which was performed using CtFD instead of the FEM process, the flow velocity caused by the Marangoni effect was found to affect the solidification microstructure strongly (55,99) .…”

“…The effect can give rise to the convex melt-pool surface even in a vacuum where the recoil pressure is negligible. Figure 12 shows the snapshots of meltpool simulated by CtFD for the case of EB-irradiation on a bulk of 316L stainless steel (99) .…”

“…(a) Bird's eye view, (b) side view on the vertical cross-section along the center of the scanning line. The color indicates the temperature (after Y. Miyata et al(99) , Reproduced under the CC-BY license. )…”

Effect of Rapid Heating and Cooling Conditions on Microstructure Formation in Powder Bed Fusion of Al-Si Hypoeutectic Alloy: A Phase-Field Study

“…Generally, the solidification microstructures in metals can be determined by the solidification conditions, such as the temperature gradient, G, at the liquid/solid interface and the solidification rate (i.e., the velocity of the solid/liquid interface), R , based on Hunt’s columnar–equiaxed transition (CET) criteria [ 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ]. According to the CET criteria, because the melt-pool boundary where the solidification begins has a large G and a small R , it can be predicted with a high degree of confidence that columnar crystals form more easily in its vicinity, whereas equiaxed grains form more easily at the center of the melt pool.…”

“…As shown in Figure 1 , equiaxed crystalline grains and epitaxially grown columnar crystals are observed. In the PBF AM process, the defined solidification conditions can vary within a single melt-pool [ 29 ]. Thus, different microstructures can form under diverse solidification, remelting, and resolidification conditions because of the varied overlapping of melt-pools.…”