Papers by Keyword: Dendrite Fragmentation

Abstract: A summary is given of the history of our understanding of dendrite coarsening, including particularly fragmentation. Much is now understood about this process as it takes place in directional solidification of a quiescent melt. Much less is understood about it in the rapidly cooled, turbulent environment of semi-solid casting. The importance of dendrite fragmentation in semi-solid processing is that it is key to obtaining fine final grain size, grain spheroidicity and rapid production rate. I have chosen in this keynote paper to talk about the fundamentals of an important part of the semisolid casting process ... that of “dendrite fragmentation.” The paper is written with an eye to its possible practical usefulness to researchers in process innovation. If we understood the dendrite fragmentation mechanism better, could we achieve finer, more numerous, grains than we do now Could fully non dendritic structures be obtained industrially in short processing times

Abstract: The effect of a Pulsed Electro Magnetic Field (PEMF) on the solidification of an Al‑15 wt%Cu alloy was studied in situ by synchrotron X-ray radiography. Samples were solidified with and without the presence of the PEMF while recording radiographs, enabling observation and quantification of dendrite fragmentation by image analysis. Fragmentation increased with a PEMF and was attributed to induced inter-dendritic flow.

Abstract: High speed imaging, including the ultrafast synchrotron X-ray imaging facility at the beamline 32-ID-B of the Advanced Photon Source (APS), was used to study in-situ (1) the dynamics of ultrasonic bubbles inside a water suspension with an acoustic field of varied pressure; and (2) the interaction of a pulsing bubble at a primary dendrite arm tip inside a succinonitrile-1wt% camphor organic transparent alloy. A simple finite element based model was developed to simulate the stress distribution inside the dendrite due to the pulsing of the ultrasonic bubble, providing more evidence for understanding quantitatively the ultrasonic wave induced dendrite fragmentation phenomenon.

Abstract: In the fragmentation theory of T. Campanella et al, the local remelting of dendrite arms is induced by the solute-rich fluid flow. Based on this theory, the effects of linear EMS intensity and solute content on CET of steel are investigated. The conclusions are as follows: The criterion for dendrite fragmentation under linear EMS is derived based on dendrite fragmentation theory by T. Campanella et al. And the criterion is verified with steel experiments. It is valid for steel under the Linear EMS. Investigation is carried out on relation between critical volume fraction of solid and solute content at the time of dendrite fragmentation (CET occurrence). It is concluded that critical volume fraction of solid is small with low EMS intensity and it decreases with the increase of solute content (C, Mn). The reason is that it causes that flow in the mushy zone becomes small which leads to CET occurrence difficult.

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: In-situ synchrotron X-ray radiography has been used to study columnar and equiaxed dendritic growth in directional solidification of Al-Cu alloys employing a Bridgman furnace. Nominal spatial and temporal resolutions of 1.5 µm and 150 ms, respectively, were obtained with a 1.3 %1.3 mm2 field of view, and a signal-to-noise above 99.5 %. Dedicated processing software has been developed to allow for quantitative extraction of data such as solid-liquid interface morphology, local propagation velocities and constitutional gradients from the images. The data collected also contain unprecedented in-situ observations on dendrite fragmentation. The limited field of view together with a slight sample position dependency in the heat transfer coefficients made it necessary to impose thermal gradients, G > 10 K/mm, to have reproducible solidification processes. Non-vanishing horizontal G-components contributed to convection that at some occasions resulted in observable effects on growing crystals or on liquid segregates.