In situ observation of the cryophile migration of hydrogen bubbles in Al-alloys during directional melting and the impact of surface tension

During metallurgical processing, gas bubbles nucleate and interact with the microstructure, which can result in severe defects. The research presented in this article focuses on in situ observation of bubble movement in semi-solid aluminum alloys (Al-Ge and Al-Cu) during a melting cycle. X-radiography is used to monitor the migration of H-bubbles in the semi-solid mush. For both alloys, H-bubbles are observed to burrow through the mushy zone towards the colder side of a horizontal gradient furnace leaving behind solute-enriched channels. This stands in contrast to the commonly observed bubble movement towards the hot side of the material caused by Marangoni flow-fields surrounding the gas pocket induced by gradients in the bubble’s surface tension. Different dependencies of the surface tension on temperature and concentration for the two alloys are presented as explanation for the observed atypical phenomenon. These results suggest different dominant effects acting on the bubbles, temperature for Al-Ge and concentration for Al-Cu, and explain the cryophile behavior observed.