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Towards understanding grain nucleation under Additive Manufacturing solidification conditions

Prasad, A; Yuan, L; Lee, P; Patel, M; Qiu, D; Easton, M; StJohn, D; (2020) Towards understanding grain nucleation under Additive Manufacturing solidification conditions. Acta Materialia , 195 pp. 392-403. 10.1016/j.actamat.2020.05.012. Green open access

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Abstract

This paper provides insights into the effect of high thermal gradients and cooling rates on equiaxed grain nucleation and growth in conditions similar to those experienced during Additive Manufacturing (AM) processes. Bridgman type solidification is numerically simulated with columnar grains growing at a fixed pull rate under a user-imposed thermal gradient. Controlled inoculants of known nucleation undercooling were placed ahead of the growing columnar grains to allow quantitative analysis of nucleation events. At low thermal gradient and cooling rate only the inoculants with low nucleation undercooling were activated due to low melt undercooling driven by constitutional supercooling (CS). As the cooling rate is increased, for a given thermal gradient, a larger number of inoculants with higher nucleation undercoolings were activated. At higher cooling rates, thermal undercooling was generated by a lag in the growth rate of the solid-liquid (S-L) interface compared to the theoretical pull rate. Thus, thermal undercooling becomes dominant leading to the facilitation of nucleation on less potent substrates requiring higher undercooling. The results show a transition from solute-driven undercooling to cooling rate driven thermal undercooling which contributes to the undercooling that activates the nucleation events. Invoking the Interdependence model, it is also shown that the high cooling rate induced thermal undercooling reduces the size of the nucleation free zone substantially.

Type: Article
Title: Towards understanding grain nucleation under Additive Manufacturing solidification conditions
Open access status: An open access version is available from UCL Discovery
DOI: 10.1016/j.actamat.2020.05.012
Publisher version: https://doi.org/10.1016/j.actamat.2020.05.012
Language: English
Additional information: This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions.
Keywords: Additive Manufacturing, Columnar-to-Equiaxed Transition (CET), Cooling rate, Interdependence Model, Numerical Simulation
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Mechanical Engineering
URI: https://discovery.ucl.ac.uk/id/eprint/10098128
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