Abstract
Synchrotron and laboratory-based X-ray imaging techniques have been increasingly used for in situ investigations of alloy solidification and other metal processes. Several reviews have been published in recent years that have focused on the development of in situ X-ray imaging techniques for metal solidification studies. Instead, this work provides a comprehensive review of knowledge provided by in situ X-ray imaging for improved understanding of solidification theories and emerging metal processing technologies. We first review insights related to crystal nucleation and growth mechanisms gained by in situ X-ray imaging, including solute suppressed nucleation theory of -Al and intermetallic compound crystals, dendritic growth of -Al and the twin plane re-entrant growth mechanism of faceted Fe-rich intermetallics. Second, we discuss the contribution of in situ X-ray studies in understanding microstructural instability, including dendrite fragmentation induced by solute-driven, dendrite root re-melting, instability of a planar solid/liquid interface, the cellular-to-dendritic transition and the columnar-to-equiaxed transition. Third, we review investigations of defect formation mechanisms during near-equilibrium solidification, including porosity and hot tear formation, and the associated liquid metal flow. Then, we discuss how X-ray imaging is being applied to the understanding and development of emerging metal processes that operate further from equilibrium, such as additive manufacturing. Finally, the outlook for future research opportunities and challenges is presented.
Highlights
Solidification is involved in the manufacture of a large variety of engineering alloy products, ranging from direct chill (DC) cast billets to additively manufactured structural components, during which the cooling rate spans a wide range from
Thanks to the high spatial and temporal 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 current review focuses principally on in situ X-ray imaging studies, it is worth noting that in situ X-ray diffraction is increasingly applied to solidification studies, including in tandem with imaging [65,66,67,68]
Summary
Solidification is involved in the manufacture of a large variety of engineering alloy products, ranging from direct chill (DC) cast billets to additively manufactured structural components, during which the cooling rate spans a wide range from
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