In situ observation of the impact of hydrogen bubbles in Al–Cu melt on directional dendritic solidification

Abstract

Much research has already been focused on the solid-bubble interaction in the interdendritic space for solidifying materials. However, commonly, bubble nucleation is not limited to the mushy zone but also occurs in the liquid melt. In the present research on an Al-$$10 \, \%\mathrm {wt. \,}$$ 10 % wt . Cu alloy, the interaction between these bubbles and the approaching solidification front becomes apparent under in situ X-radiography and allows for new insights into the influence of bubbles on the solidifying microstructure. The observed effects comprise bulging of the solidification front toward the bubble, bending of dendrites in front of the bubble, coronal outgrowths surrounding the bubbles, as well as bubble growth, bubble pushing, and bubble eruption. It is found that for the present Al–Cu alloy, the local variation in the solidification speed can be attributed to the bubbles’ insulating properties. The range of this effect was observed to be up to $$900 \,\upmu \text {m}$$ 900 μ m , depending on the bubble diameter, locally increasing solidification speed by up to $$350 \, \%$$ 350 % . The influences of Marangoni vortices and coronal nucleation of misoriented dendrites around bubbles on the homogeneity of the microstructure are discussed. A comparison with experiments on model alloys and simulations from various other studies highlights the similarities and differences to this metallic alloy system.

In situ investigation of the Columnar-to-Equiaxed Transition during directional solidification of Al–20 wt.%Cu alloys on Earth and in microgravity

“…Bubbles that nucleated in the mushy zone caused small displacement of the dendrites while developing and left spherical liquid areas after collapsing. Similar behaviors have been recently reported and studied in details for instance by Murphy et al [36] and Werner et al [37]. In vertical configuration, the bubbles floated due to buoyancy and/or thermal Marangoni effect [38].…”

Locked-lamellar eutectic growth in thin Al-Al2Cu samples: In situ directional solidification and crystal orientation analysis

“…More practically, our observations show that, due to the drifting dynamics of tiltedlamellar patterns, the appearance of a micron-sized disoriented crystal caused by a localized, accidental deformation can have a large impact on the eutectic-grain structure on the scale of the ingot. Similar, yet more dramatic crystal misorientation effects have been ob-served in irregular-eutectic [47] and dendritic samples [48]. Such processes are likely to take place in the vicinity of the container walls in bulk samples as well.…”

X-ray Imaging of Alloy Solidification: Crystal Formation, Growth, Instability and Defects

“…These solidification defects can significantly downgrade the final mechanical properties such as tensile ductility and fatigue life. Since the 1990s, in situ X-ray imaging techniques have been used to understand better the formation of solidification defects [43,44,46,[126][127][128][129] and their interaction with other microstructural features [41,45,99,[130][131][132][133].…”

“…(e-g) A radiograph sequence showing the growth of a pore in the inter-dendritic region that induced dendrite fragmentation, in a directionally solidified Al-12wt%Ge alloy [134]. (h-j) Radiographs showing a pore growing ahead of a planar α-Al solidification front, causing the front to bulge during directional, horizontal solidification of an Al-10wt%Cu alloy [45].…”

“…Unlike the aforementioned works that largely focused on X-ray imaging techniques, the current work instead presents a comprehensive review of knowledge provided by in situ X-ray imaging, for better understanding of solidification theories. Thanks to the high spatial (sub-micron) and temporal (microseconds) resolution offered by third-generation synchrotron sources, significant progress has been made in the understanding, validation and development of theories and models for near-equilibrium solidification, including solute suppressed nucleation (SSN) of both primary solid-solution α-Al [9][10][11][12][13][14] and secondary ordered intermetallics [15,16], dendritic growth of α-Al [17][18][19][20][21][22] and faceted, twin plane re-entrant (TPRE) growth of Fe-rich intermetallics [23][24][25][26], crystal fragmentation [27][28][29][30][31][32], morphological transition [33][34][35][36][37][38][39][40] and defect formation [41][42][43][44][45][46][47]. The focus of solidification research on Al alloys derives from the relatively easy-to-achieve melting temperatures of around 660 °C, and excellent absorption contrast between Al and typical alloying elements such as Cu and Zn.…”