Papers by Author: H.V. Atkinson

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Authors: H.V. Atkinson
Abstract: The numerical modeling of semi-solid processing requires data on the rheological properties of materials. This data is often obtained by rheometry but there are difficulties with characterizing high solid fractions, where the torque which can be exerted with the rheometer is insufficient. A number of other methods for measuring the flow parameters, including compression between platens, have been utilized. The various methods will be reviewed in this paper. Computational fluid dynamics modelers have often used data from steady state experiments but it is the behaviour during rapid transients which is more relevant to the actual semi-solid processing route.
Authors: F. Alshmri, H.V. Atkinson
Abstract: Abstract. Aluminum high silicon alloys have concerned many researchers due to their high wear resistance, lightness, high corrosion resistance and low thermal expansion. Casting of high silicon Al-Si alloys (i.e. Si content greater than 17 wt.%) will generate large degrees of segregation and coarse microstructures due to the low rates of solidification. The problems associated with ingot casting of hypereutectic Al-Si alloys (i.e. segregation, coarse microstructures and porosity) may be overcome by rapid solidification processing such as spray, weld, and chill methods (e.g. melt spinning). The alloys under consideration here contain Al, Si, Zr, Cu, Mg, Fe and Ni. These alloys were produced by rapid solidification i.e. melt spinning. The aim of this paper is to characterise the hardness of material produced by rapid solidification at various stages of production. Several alloy variants were examined and relate the hardness to the microstructure. Piston A390 made by casting was examined for comparison.
Authors: A.J. Smith, H.V. Atkinson, S.V. Hainsworth, H.B. Dong, R. Haghayeghi
Abstract: The ESEM (Environmental Scanning Electron Microscope) is an instrument that circumvents a limitation of conventional SEM, in that samples can be examined in a gaseous atmosphere rather than a vacuum. With a heating stage, dynamic processes can be observed in situ at high temperature. In this study, A201 aluminium alloy samples with globular structures have been examined in the semisolid region. In addition, a manipulator has been installed onto the heating stage to allow the probing of semi-solid surfaces. The paper shows the potential for manipulating semi-solid materials in order to better understand thixotropic phenomena.
Authors: G. Vaneetveld, Ahmed Rassili, H.V. Atkinson
Abstract: Thixoforging involves shaping alloys with a globular microstructure in the semi-solid state. To reach this kind of material, the Recrystallisation and Partial Melting (RAP) process can be used to obtain a globular microstructure from extruded material with liquid penetrating the recrystallised boundaries. Induction heating is used to apply the RAP process to slugs. One of the benefits of using this method of heating is the fast heating rate (20°C/s). This paper will help to improve heating parameters by showing their influence on 7075 aluminium alloy recrystallisation. These parameters are the heating rate; heating frequencies-power; presence or not of protective gas; position of the slug in the inductor; energy stored inside the slug; oxide layer on the slug side; chamfer of the slug upper corner.
Authors: P. Kapranos, H.V. Atkinson, David H. Kirkwood
Authors: H.J. Dai, H.B. Dong, H.V. Atkinson, Peter D. Lee
Abstract: A coupled cellular automaton-finite difference (CA-FD) model is used to simulate the detailed dendritic structure evolution of the columnar-to-equiaxed transition (CET) for Al-Cu alloys during solidification. The effects of material properties (nucleation undercooling, density of nuclei in bulk liquid and alloy solidification range) on the CET are investigated. Simulated results reveal that: (1) equiaxed grains form at an earlier stage with a smaller critical nucleation undercooling; (2) CET is promoted if the density of nuclei in bulk liquid is increased; (3) extending the alloy solidification range promotes the CET. Finally, CET maps corresponding to different alloy concentrations are constructed, illustrating the relationship between processing conditions and the resulting grain structures for alloys with different solidification ranges.
Authors: G. Vaneetveld, Ahmed Rassili, Jacqueline Lecomte-Beckers, H.V. Atkinson
Abstract: Thixoforging is a type of semi-solid metal processing at high solid fraction (0.5
Authors: H.V. Atkinson, P.J. Ward
Abstract: For hypereutectic Al/Si alloys, one of the advantages of thixoforming in comparison with casting routes is the relatively short processing times at high temperatures and hence limited coarsening of the Si phase. Coarse silicon particles give poor mechanical properties. Here two hypereutectic Al/Si alloys (magneto-hydro-dynamically (MHD) stirred A390 from Pechiney and an extruded A390 alloy from Showa in Japan) have been thixoformed to form pistons. Opening up the die entrance to the full width of the crown made the flow into the die more uniform and helped to reduce the tendency for large pores to form due to swirling of the slurry. Die heating reduced cold shuts in the skirt (thin section) of the piston. Placing inserts into the die to make holes for the piston pins (i.e. having an obstacle in the thicker regions) evened up the flow between the thick and the thin regions in the die. Massive pores experienced in earlier shots were then eliminated. Placing ceramic material in the die entrance considerably reduced the shrinkage porosity in the crown. The use of the Showa alloy, where the globular semisolid microstructure is achieved by a solid state deformation route rather than MHD, gave reduced shrinkage porosity and eliminated macrosegregation of the eutectic and the silicon. Computer modeling has aided optimization of the die.
Authors: H.V. Atkinson, P. Kapranos, D. Liu, S.A. Chayong, David H. Kirkwood
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