Papers by Author: Jia Wei Mi

Abstract: The dendrite grain growth of a succinonitrile based transparent alloy, their fragmentation under an intense thermal shock and the subsequnet morphology evolution during solidification have been simulated using a two-dimensional binary alloy phase field model coupled with heat and solute transfer. The effect of a sudden, rapid change in the thermal environment (thermal shock) was implemented in the model and the resulting effect on the incipient dendritic grain morphology was studied. Thermal shock effectively promoted the fragmentation of the dendritic grains, providing a significant grain multiplication effect to refine the final solidification microstructure.

Abstract: This paper presents a combined experimental and modelling approach to understand dendrite fragmentation of atomised metal alloy droplets during deposition in spray forming, and to study quantitatively the relationship between this dendrite fragmentation behavior and subsequent microstructural evolution. A Gleeble 3500 physical simulator was used to create controlled thermal shock conditions in solid-liquid mixtures of Ni superalloy IN718 atomised powders, which simulated the environment of droplet deposition during the twin-atomiser spray forming of large diameter IN718 alloy billets at BIAM. The experiments were complemented by phase field modelling studies at Oxford. Experiment and modelling supported the hypothesis that the characteristic equiaxed spray formed microstructure depends critically upon the rapid remelting and thermal shock of fine-scale dendrites in solid particles in the spray to provide a high density of embryonic grains.

Abstract: 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.

Abstract: A numerical model has been developed to simulate the distribution of polygonal grain size in a sprayed microstructure formed from an alloy droplet spray containing a large number of solid, mushy and liquid droplets. The model takes into account the effects of: (1) the droplet size distribution; (2) its corresponding distribution of solid, mushy and liquid droplets at the instant of deposition; (3) the overall thermal condition of the spray formed preform during final solidification. The model has been validated against experiments of the spray forming of Ni superalloy rings, with modelled grain size distributions giving good agreement with measurements obtained by electron backscatter diffraction.

Abstract: 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.

Abstract: 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.

Abstract: A numerical finite difference model has been developed to describe the transient heat flow inside Ni superalloy IN718 billets manufactured by spray forming. This model described the progressive build-up and solidification of the billets, and accounted for the latent heat of solidification, convective and radiation heat loss, and heat removal through the substrate, coupled with transient heat transfer in the substrate. The model has been used to predict critical values of important process parameters: average droplet arrival temperature, deposition rate, and billet surface convective heat transfer coefficient, in terms of key microstructural features by correlation of quantitative predictions with qualitative microstructural investigations of the as-sprayed billets. On a micro scale, repeated re-heating/remelting of deposited layers because of subsequent deposition was predicted and has been suggested to be beneficial in reducing porosity and microsegregation, as well as in playing an important role in the formation of the characteristic equiaxed spray formed microstructure.