In-situ observation of hydrogen micropore evolution during directional solidification of Al-Cu-Li alloy

Abstract

This study used X-ray microtomography to conduct in-situ observations of hydrogen micropore nucleation and growth during the directional solidification of an Al-Cu-Li alloy in a vacuum temperature gradient furnace. The kinetics of hydrogen microporosity evolution were traced and elucidated, and the micropore migration mechanism during directional solidification was revealed. The observed evolution of hydrogen microporosity can be divided into two basic stages: the first stage was characterized by micropore nucleation manifested as a sharp increase in the number density of micropores, with an average micropore nucleation temperature of (607 ± 9.6) °C and the maximum formation rate of 1.6 micropores/s; the second stage (560–595 °C) was dominated by micropore growth, as indicated by a significant increase in equivalent diameter at the maximum growth rate of 4.05 μm/s. The micropore growth kinetics were effectively described by an exponential function based on these observations. Notably, the Marangoni flow field induced by the differences in surface tension around the hydrogen micropores in the mushy zone caused them to migrate towards the cooler side of the temperature gradient, in contrast to the commonly observed migration of micropores towards the warmer side of directionally cooled metals; this atypical phenomenon was caused by the different effects of temperature and alloy component concentrations (Cu and Li elements) on surface tension. The cryophilic migration behavior observed in this study was primarily influenced by the Cu concentration, i.e., the Mukai-Lin-Laplace effect.

Keywords

Al-Cu-Li alloy / Hydrogen micropore / In-situ X-ray radiography / Growth kinetics / Cryophilic migration

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Funding

Key Technologies Research and Development Program(2024YFB4607203)

National Natural Science Foundation of China(52073030)

National Natural Science Foundation of China-Guangxi Joint Fund(U20A20276)

RIGHTS & PERMISSIONS

Shanghai University and Periodicals Agency of Shanghai University and Springer-Verlag GmbH Germany, part of Springer Nature