IN-situ observation of hydrogen bubble formation and interaction with its surroundings during solidification and melting of Al-alloys

Abstract

During metallurgical processing, gas bubbles nucleate and interact with the microstructure, which can result in severe defects. Pores, their agglomeration and networks formed by them are lowering e. g. fatigue strength in aluminium alloys [1,2]. Hence, research on nucleation, growth and interaction of bubbles with the microstructure forming during solidification in metallic alloys is a topic of great interest both from a scientific point of view and for the metallurgical industry. Here, we focus on bubble formation and interaction with the surrounding dendrite network in the mushy zone of Al-Cu and Al-Ge alloys. Experiments were carried out in a X-ray transparent Bridgman-Stockbarger furnace and in-situ monitored by X-radiography. The samples were pre-conditioned with a well-defined amounts of hydrogen using a gas loading and casting furnace [3]. From the cast blocks thin rectangular alloy samples (5x50x0.15mm) were prepared and subsequently processed in horizontal configuration thus minimizing buoyancy effects. Solidification experiments showed bulging of the solidification front in the vicinity of a bubble, bending of dendrites approaching a bubble, and coronal outgrowth [4]. Further, we focused on the thermo- and the soluto-capillary effects [5]. To obtain a clean view on both effects bubble formation and interaction with the surrounding matrix is observed during melting of the alloy. We show that an accurate knowledge of thermophysical properties, in particular, surface tension as a function of temperature and composition is required to understand the observed cryophile migration of bubbles, namely Marangoni-motion towards the cold side and not as commonly observed and expected to the hot side. This behavior is explained by taking into account the behavior of surface tension as otained from EML experiments. [1] Y.X. Gao, J.Z. Yi, P.D. Lee, T.C. Lindley, Fatigue Fract. Eng. Mater. Struct. 27 (2004), 559. [2] P. Osmond, V. Le, F. Morel, D. Bellett, N. Saintier, Procedia. Eng. 213 (2018), 630. [3] T. Werner, P. Lehmann, J. Baumann, F. Kargl, and B. Tyburska-Pueschel, Rev. Sci. Instrum. 91 (2020), 043901. [4] T. Werner, M. Becker, J. Baumann, C. Pickmann, L. Stuzt, and F. Kargl, J. of Materials Science 56 (2021), 8225. [5] T. Werner, M. Becker, J. Baumann, X. Xiao, C. Pickmann, L. Sturt, J. Brillo, and F. Kargl, Acta Mater. 224 (2022), 117503.