Band-Like Distribution of Grains in Selective Laser Melting Track Under Keyhole Mode
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LASER is a powerful tool for materials processing. Recently, selective laser melting (SLM), a promising additive manufacturing technology, has gotten a lot of attention. Similar to that in laser welding, different melting modes have also been observed in SLM,[1] which are conduction mode, transition mode, and keyhole mode. The melting mode plays an important role in formability, microstructure, and mechanical properties in SLM.[2–4] In SLM under keyhole mode, more refined grains grew, and a new zone of equiaxed dendrites was found in the center of transverse cross section of melt pool.[4] The refined grains were believed to result from the fluctuations of flow field and multi remelting. On the other hand, Ting Qi et al.[4] treated grains in the new zone as equiaxed grains in three dimensions, which were attributed to the change of solidification conditions (ratio of thermal gradient and growth rate) in the deep melt pool. However, in laser welding, the similar zone near the centerline of the track is the interception of ‘‘axial columnar grains,’’ which are elongated grains in three dimensions.[5,6] Recently, grain structure in laser welding under keyhole mode was modeled in three dimensions,[6,7] where the weld pool was assumed to be
YAFEI WANG, LEILEI XING, KAILUN LI, CHENFAN YU, JING MA, and WEI LIU are with the School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, P.R. China. Contact e-mail: [email protected] ZHIJIAN SHEN is with the School of Materials Science and Engineering, Tsinghua University and also with the Department of Materials and Environment Chemistry, Arrhenius Laboratory, Stockholm University, 106 91, Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted August 24, 2018.
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at steady state, and grains were assumed to grow epitaxially along crystallographic h100i direction[6] or along the maximum heat flow direction.[7] However, the melt pool under keyhole mode in laser welding as well as in SLM is generally unsteady due to intense fluid flow, and the effect of fluid flow on grain growth can be underestimated. In consideration of fluid flow, the grain structure and grain growth mechanism in the unsteady melt pool under keyhole mode need more experimental research in laser welding and SLM. Actually, grain structure and grain growth mechanism have been widely studied in laser welding. Kou[8] has concluded that grains can either grow epitaxially from the fusion boundary and then compete with each other or nucleate on several kinds of nuclei. The nuclei can be dendrite fragments, detached grains, and heterogeneous nuclei. Fluid flow inside the melt pool plays a determined role in the formation of dendrites fragments and detached grains, which can refine the microstructure. Moreover, fluid flow affects the growth of columnar grain. Murakami et al.[9,10] observed that columnar grains in Al-Cu alloys deflected from their crystallographic h001i direction to the upstream direction. The flow velocity in their experiment ranged from 0.
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