Papers by Author: H.V. Atkinson

Abstract: One-phase modelling is widely used as an optimization and design tool for semi-solid casting process. By this approach, semi-solid alloy is taken as a homogeneous fluid and flow behaviours are represented by using the rheological properties of the mixture. A single set of conservation equations is solved to simulate the mould filling, without considering the different motilities between the liquid and the solid phase. Therefore, defects due to phase segregation cannot be predicted and particle tracking cannot be carried out. This study is focused on multiphase (i.e. a liquid-particle-air system) modelling during the semi-solid filling process of a thin-walled component. By using this approach, the solid phase is treated as a power law fluid and the phase interactions among the phases are introduced for the semi-solid alloy system. Practical semi-solid die casting is carried out for verification of phase segregation. The final distribution of air is calculated and proved by practical blistering examination. The simulation results are verified to be accurate in a reasonable range, indicating an approach for modelling semi-solid filling including the formation (and avoidance) of blisters.

Abstract: The fraction liquid present during semi-solid processing has a critical effect. Conventionally the process window has been defined by inspecting the liquid fraction versus temperature curve (derived from thermodynamic prediction using a thermodynamic prediction software package for example, or derived from differential scanning calorimetry results). It has been assumed that a freezing range with temperature is required for semi-solid processing to be possible. However, recently a South African group (Curle, Moller and Wilkins) has shown that it is possible to rheo-process both high-purity aluminium and a binary Al-Si eutectic alloy i.e. materials with no freezing range. This behaviour highlights the fact that it takes time for liquid to form i.e. the kinetics of melting are important. Here the liquid fraction vs time for high purity aluminium is derived from experimental results to identify the process window in terms of time rather than temperature. The time sensitivity in thixoforming or rheocasting depends on the sample mass, the heat flux and the phase transformation temperature. It is also important in determining the vulnerability to defects such as hot tears, which tend to occur particularly with the alloys which are conventionally wrought rather than cast such as the 2000 series aluminium alloys.

Abstract: The processing window is important for the semisolid processability of alloys. This study focusses on the kinetics of diffusion. It compares prediction of fraction liquid versus temperature taking into account both thermodynamic and kinetics, with experimental results from Differential Scanning Calorimetry (DSC) and Single Pan Scanning Calorimetry (SPSC). SPSC is a novel technique with an order of magnitude higher accuracy than DSC. A range of Al-Si binary alloys has been investigated. The studies reveal that the simulation results predicted by DICTRA (DIffusion-Controlled TRAnsformations) show the same pattern with experimental results in the relationship of fraction liquid-temperature. However, the SPSC results are closer to the prediction results than DSC curves even with the relatively large sample size associated with SPSC. This is potentially a significant result as conventionally one of the difficulties is predicting the liquid fraction versus temperature for the heating of a billet for semi-solid processing. DSC results are known to be unrepresentative because the heating rates which can be achieved in DSC are much lower than those in induction heating. In addition, the DSC results are dependent on sample size and heating rate. The long term aim is to gain confidence in prediction with software packages which will reduce trial and error.

Abstract: The main alloys which have been semi-solid processed commercially are based on aluminium (particularly the cast compositions) and magnesium. There is a strong drive to broaden the range of alloys to the wrought compositions for aluminium, more creep-resistant magnesium recipes and to higher temperature alloys such as those based on copper, steels, stellites and cast irons. This paper will summarise the issues with such development including the scientific and practical issues for alloy design and the thermodynamic prediction of alloys suitable for semi-solid processing. After an initial introduction to semi-solid processing routes, the most important alloy systems for semi-solid processing from a development point of view (aluminium, magnesium, steels and composites- including nanocomposites) will be discussed. The key issues of alloy design specifically for semi-solid processing will be drawn out through the text.

Abstract: Metal matrix nanocomposites (MMNCs) are promising materials to produce engineering components for the automotive and aerospace industry. This study aims to determine the feasibility of Al/TiB2 nanocomposite fabrication by the combination of the ultrasonic method and flux-assisted particle incorporation for the production of thixoforming feedstock material. Flux assistance has been invoked to attempt to overcome challenges with the presence of oxide on the surface of the foil in the aluminium foil capsulate method. A356 alloy has been reinforced with 0.25 wt.% TiB2 nanoparticles using different methods; 1. Flux-assisted casting, 2. Flux-assisted casting with ultrasonic cavitation, and 3. The Al foil capsulate method for particle feeding with ultrasonic cavitation. The composite fabricated by the ultrasonic method with the use of flux agent provided a non-dendritic microstructure which is the requirement for thixoforming. It was found that the flux-assisted casting method is not appropriate for achieving nanoparticle entry into the melt in MMNC fabrication due to the buoyancy forces of nanoparticles, unlike micron-sized particles.

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.

Abstract: Metal matrix nanocomposites (MMNCs) could be strong candidates for use in the automotive and aerospace industry, where the mechanical performance is a crucial factor. This study aims to determine the feasibility of Al/SiC nanocomposite fabrication by the ultrasonic technique for the production of thixoforming feedstock material. Aluminium (A356) matrix composites were fabricated with the addition of 0.2 wt.% SiC nanoparticles using an ultrasonic method. Two different particle feeding mechanisms; the Al foil double capsulate method and the crucible placement approach, were employed. Better results were obtained for the double capsulate method. Also, Ti and Nb probe tips were tried to provide ultrasonic vibration in the melt. It has been shown that Nb is a promising ultrasonic probe tip material to produce MMNCs above 650 °C as it is thermodynamically stable in comparison with Ti.

Abstract: Aluminium silicon alloys are the most used raw material for automotive applications. One of the main limitations on using aluminium high silicon alloys is the formation of coarse brittle phases under conventional solidification conditions. However, rapid solidification processing (RS) (for example, through melt spinning) is very effective in limiting the coarsening of primary silicon due to the high cooling rate. In the present work, characterisation of the material at the first stage of production as melt-spun ribbon and flake has been carried out. The microstructures show typical characteristics of a ‘featureless zone’ on the wheel-side and coarser microstructures on the air-side, with clusters of silicon particles evident. At high magnification, on the wheel-side, TEM and FEGSEM reveal local variations in the silicon and aluminium content (although on average there is no macrosegregation from the wheel-side to the air-side during solidification). In FEGSEM, the ‘rosette-structure’ also displays local variations in Al, Si, Fe, Cu and Ni over a scale of a few microns.

Abstract: High temperature operation of power plant increases their efficiency and reduces emissions. Low gamma-prime fraction nickel-based superalloys, such as IN617, are becoming increasingly important in substituting for conventional steel components due to their better performance at higher temperatures. The aim of this work is to model the microstructural evolution in this alloy under typical operating conditions and relate this to its creep response. Although classified as solid solution strengthened alloys, they contain a range of different precipitates which contribute to the creep resistance, the most influential of these being gamma prime. The model is developed in three stages. Firstly, a precipitate-level variational model is derived to describe the nucleation and growth of individual particles and their interactions. The results are then passed up to a grain-level simulation where the response of a statistically significant particle ensemble is simulated via the evolution of particle distributions. The resulting model incorporates the effects of heat treatment and stress to give the simultaneous size and number evolution of the different phases in the material. This information is then used to construct plastic strain-temperature-time diagrams in order to estimate the creep rupture life of the material.

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.