Grain refinement leads, in general, to a decreased tendency to hot tearing, a more dispersed and refined porosity distribution, and improved directional feeding characteristics during solidification. Reduced as-cast grain size can also lead to improved mechanical properties and wrought processing by reducing the recrystallized grain size and achieving a fully recrystallized microstructure. It is now well established that the two key factors controlling grain refinement are the nucleant particles including their potency, size distribution and particle number density, and the rate of development of growth restriction, Q, generated by the alloy chemistry which establishes the undercooling needed to trigger nucleation events and facilitates their survival. The theories underpinning our current understanding of nucleation and grain formation are presented. The application of the latest theories to the light alloys of Al, Mg and Ti is explored as well as their applicability to a range of casting and solidification environments. In addition, processing by the application of physical processes such as external fields and additive manufacturing is discussed. To conclude, the current challenges for the development of reliable grain refining technologies for difficult to refine alloy systems are presented.

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Recent advances in grain refinement of light metals and alloys

Insight into Si poisoning on grain refinement of Al-Si/Al-5Ti-B system

Grain refinement of hypoeutectic Al-Si alloys with B

The columnar to equiaxed transition (CET) of grain structures associated with processing conditions has been observed during metallic additive manufacturing (AM). However, the formation mechanisms of these grain structures have not been well understood under rapid solidification conditions, especially for AM of superalloys. This paper aims to uncover the underlying mechanisms that govern the CET of AM metals, using a well-tested multiscale phase-field model where heterogeneous nucleation, grain selection and grain epitaxial growth are considered. Using In718 as an example, the simulated results show that the CET is critically controlled by the undercooling, involving constitutional supercooling, thermal and curvature undercoolings in the melt pool, which dictates the extent of heterogeneous nucleation with respect to the grain epitaxial growth during rapid solidification.

Insight into the mechanisms of columnar to equiaxed grain transition during metallic additive manufacturing

In the current study, cylindrical samples of AlSi10Mg alloy were fabricated using direct metal laser sintering (DMLS) technique in vertical and horizontal directions. The microstructure of the samples was analyzed using scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. It was observed that, by changing the building direction from vertical to horizontal, columnar to equiaxed transition (CET) occurred in the alloy. While 75% of the grains in the vertical sample were columnar, by changing the direction to horizontal, 49% of the grains evolved with columnar shape and 51% of them were equiaxed. Moreover, the texture of DMLS-AlSi10Mg alloy changed due to CET. While {001} fiber texture evolved in the vertical sample, the direction tilted away from the building direction in the horizontal one. Using the fundamentals of solidification and constitutional undercooling, the solidification behavior of AlSi10Mg alloy during DMLS process was modeled. It was observed that, the determinant parameter in CET during DMLS of AlSi10Mg alloy is the angle between the nominal growth rate and h k l > direction of the growing dendrite, which is controlled by the geometry and building direction of the sample. Further TEM studies confirmed that, CET alters the shape and coherency of Si precipitates and dislocation density inside the α-Al dendrites in DMLS-AlSi10Mg alloy.

Columnar to equiaxed transition during direct metal laser sintering of AlSi10Mg alloy: Effect of building direction

A quantitative study of solute diffusion field effects on heterogeneous nucleation and the grain size of alloys

High-intensity ultrasound was applied during the preparation of an Al-5Ti-1B master alloy when reacting fluoride salts with molten Al. The reaction rate was significantly increased with TiB2 particles of much reduced mean diameter and narrow width spread produced in ~4 minutes. The improved grain refining performance of the TiB2 in a commercial purity Al was studied and modelled using free growth model. The increased number and reduced size of TiB2 particles provided an enhanced grain refining capability.

Refinement of TiB2 in Al-Ti-B Grain Refiner Alloys by Ultrasound and the Effect on Al Grain Size

A finite element model has been developed to simulate the coupled effects of deposit shape evolution and heat flow inside spray formed ring-shaped deposits. The shape model was developed using Matlab and included the features of: (1) atomiser scanning; (2) substrate movement relative to the atomiser; and (3) sticking efficiency. Atomiser scan and various substrate horizontal travel speeds were studied to optimise the ring shape in terms of useful materials suitable for downstream processing. The heat flow model was developed using the commercial finite element code Femlab. A data mapping technique was developed to transfer thermal data between different domains when the computational domains are subject to changing geometry and therefore the coupled effects of shape evolution and heat flow were addressed. Spray forming of ring deposits was performed on the large spray forming unit at Oxford University. In-situ temperature measurements were carried put for acquisition of boundary conditions and validation of the heat flow model. Heat flow modelling revealed that edge effects had a strong influence on the ring thermal history and the porosity distribution inside the deposits is closely related to the local solidification time.

Modelling Shape Evolution and Heat Flow of Spray-Formed Ring Preforms

For high quality premium metallic alloys in critical applications e.g. aerospace structural components and electronic devices, non-metallic particles (inclusions) present in the materials often cause serious problems during services, such as stress concentration, impairment in strength and ductility, toughness degradation, and poor surface finish. Therefore, it is always important to separate and remove such non-metallic particles from the matrix materials especially during the liquid state prior to cast into a final product. Electromagnetic separation—a method of separating non-metallic inclusions from liquid metal by imposing an electromagnetic field to the channel or pipe (termed separator hereafter) through which the liquid metal flows—has been identified as a generic technology for the production of ultra-clean metals because this process can be easily implemented by controlling the magnitude and direction of an imposed electromagnetic force. Laboratory-scale experiments have demonstrated successfully the separation of (i) non-metallic inclusions (Al2O3 4) or SiC) from Al melt, (ii) Fe-rich phase from hypoeutectic Al–Si alloys, primary Si particles from hypereutectic Al–Si alloys, and Fe–Al–Zn dross from galvanizing Zn melt, where a single straightpass circular or square separator with a uniform cross-section are commonly used. For industrial applications, the feasibility of using a bundle of small channels for inclusion removal has also been investigated. However, for an alternating magnetic field, weak force density in the central region of a bulk melt is unavoidable due to the inherent skin effect of high frequency magnetic field, making it difficult for the particles initially inside the central region of a separator or channel to be moved towards the wall. Thus the efficiency of separating inclusions from liquid metal flowing through a single straight-pass separator is limited, especially in the case of high liquid metal flow rate. This paper presents a novel method to increase the efficiency of separating inclusions by using a multistage multichannel separator i.e. dividing a single channel to sub-channels in the flow direction. A numerical method has been developed to simulate the flow field in a single-square and a two-stage multi-square separator, and the particle concentration and separation efficiency of dross particles from zinc melt flowing through both separator arrangements. The main reason for improvement in separation efficiency by the multi-square separator is discussed and comparison with the corresponding experiments is also presented.

Modelling the Electromagnetic Separation of Non-metallic Particles from Liquid Metal Flowing through a Two-stage Multichannel

The optimisation of spray forming IN718 alloy rings for aeroengine applications was investigated using both modelling and experimental approaches. A multiphysics numerical model has been developed and implemented to assist in the optimisation of the spray forming process. IN718 alloy ring preforms were spray formed at University of Oxford (UK) and The University of Bremen (Germany). A variety of on-line monitoring facilities were integrated onto spray forming units to (1) investigate the dynamics of alloy melt atomisation and droplet deposition at a sprayed surface; and (2) acquire ring preform thermal history and various thermal boundary conditions for the numerical model. Modelling and experiments were performed iteratively to investigate the effects of key spray forming parameters including gas metal flow ratio, atomiser scan, substrate heating schemes on the resulting ring preform shape, internal heat flow and solidification. It was found that preform top surface temperature and alloy liquid fraction inside the preform during spray forming were critical factors in governing the formation of macro/microporosity and the grain size of as-sprayed preforms. In the optimised conditions, IN718 alloy ring preforms were characterised by a microporosity of less than 1.5% and randomly oriented equaxied grains of 20-50 μm.

Jia Wei Mi

Papers

A quantitative study of solute diffusion field effects on heterogeneous nucleation and the grain size of alloys

Refinement of TiB2 in Al-Ti-B Grain Refiner Alloys by Ultrasound and the Effect on Al Grain Size

Modelling Shape Evolution and Heat Flow of Spray-Formed Ring Preforms

Modelling the Electromagnetic Separation of Non-metallic Particles from Liquid Metal Flowing through a Two-stage Multichannel

Optimisation of Spray Forming Ni Superalloys via Process Modelling and On-Line Monitoring