Abstract
In the last two decades, X-ray imaging techniques have been used increasingly to study metal solidification in real-time as, thanks to advances in X-ray sources (synchrotron and laboratory-based) and detector technology, images can now be obtained with spatio-temporal resolutions sufficient to record key phenomena and extract quantitative information, primarily relating to crystal growth. This paper presents an overview of the research conducted at the University of Oxford over the last 6 years as a partner in the UK’s Future Liquid Metal Engineering (LiME) Manufacturing Hub. The focus is on in situ X-ray radiography to investigate the solidification of Al alloys, including the formation of primary α-Al crystals, and the formation and growth of secondary intermetallic phases. Technologically, the thrust is to understand how to control as-cast phases, structures and element distributions, particularly elements associated with recycling, as a means to facilitate greater recirculation of aluminium alloys. We first present studies on refinement of primary α-Al, including extrinsic grain refinement using inoculation and intrinsic refinement based on dendrite fragmentation. Second, we describe studies on intermetallic phase formation and growth, because intermetallic fraction, morphology and distribution are frequently a limiting factor of alloy mechanical properties and recyclability. Then we present some of the latest progress in studying liquid flow during solidification and associated hot tear formation. Finally, future research directions are described.
Highlights
IntroductionThe mechanical properties of Al alloy components are frequently controlled by the microstructure that develops during solidification, because it is often impractical, impossible or not cost-effective to manipulate the as-cast microstructure (for example, by recrystallisation) via downstream, solid-state thermomechanical processing
Published: 24 February 2022The mechanical properties of Al alloy components are frequently controlled by the microstructure that develops during solidification, because it is often impractical, impossible or not cost-effective to manipulate the as-cast microstructure via downstream, solid-state thermomechanical processing
There are principally two approaches of grain refinement: an extrinsic approach using additions of insoluble inoculant particles that act as heterogeneous nucleation sites for grains/crystals; and an intrinsic approach that relies on fragmentation of existing columnar grains/crystals by applying external fields, known as grain multiplication
Summary
The mechanical properties of Al alloy components are frequently controlled by the microstructure that develops during solidification, because it is often impractical, impossible or not cost-effective to manipulate the as-cast microstructure (for example, by recrystallisation) via downstream, solid-state thermomechanical processing. Additions of a small amount of insoluble, micro-scale particles such as TiB2 and TiC in the form of Al-Ti-B and Al-Ti-C master alloys are used to effectively change coarse, elongated columnar α-Al grains to a finer, equiaxed grain morphology, which in turn promotes more isotropic mechanical properties [1,2]. If the underlying mechanisms of this grain refinement effect were more fully understood and controllable, strategies for more efficient grain refinement, or even refinement of secondary solidification phases, might be developed.
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