Papers by Author: Plato Kapranos

Abstract: This research investigates the effects of particle size and weight percent on the hardness of A356 matrix composite reinforced with SiC particulates produced by semi-solid stir casting. The particle sizes of the reinforcing phase were in the form of 37 and 100 μm SiC powders with loading content of 5, 10, 15 and 20 percent by weight. A356 alloy was melted and SiC powder was then gradually added into the continuously stirred slurry to promote uniform distribution of the particulates. It has been found that in the as cast condition, the average hardness values of A356 reinforced with different amounts of 37μm SiC, increase as expected with increasing amounts of particulates. A356 reinforced with 100 μm 10 wt% SiC, followed by T6 treatment, exhibited the highest percentage of hardness increment.

Abstract: Thixoforming or Semi-Solid Metal (SSM) forming is a manufacturing route that relies on a non-dendritic microstructure, a structure that is retained after processing resulting in components of fine and uniform microstructures with enhanced mechanical properties as compared to conventional cast products. In this project, an investigation is conducted on the microstructural development of Aluminium A201 alloy in the semi-solid range, through a combination of variable time and temperature conditions. Using equilibrium and non-equilibrium (Scheil) curves, the solidus and the liquidus temperatures as well as the melting behaviour of this alloy are estimated. The temperature range for the thixoforming process typically lies within the 40-60% fraction liquid and this is found to be between 620oC to 637oC for this particular alloy. The aim is to heat treat specimens of this alloy within the semi-solid range (the process window) and observe the resulting microstructures to establish if they comprise of near-spheroidal solid phase particles surrounded by a liquid phase. This microstructure gives the material its thixotropic properties; i.e. the material flows like a liquid when sheared but thickens and behaves like a solid when it is allowed to stand. Three different routes for obtaining the non-dendritic microstructure necessary for thixoforming are investigated in this work: Magneto-Hydrodynamic (MHD) stirring, Strain Induced Melt Activated (SIMA) and Semi-Solid Rheocasting via Cooling Slope (SSR+CS) route. These routes are compared and their potential for future thixoforming applications studied.

Abstract: Thixoforming as a Semi-Solid Metal Processing (SSM) route is a near net shape forming of metals in the semi-solid state, i.e. within the freezing or melting range between the fully solid and fully liquid states. Aluminum A201 is a copper containing casting alloy with additional small quantities of magnesium, silicon and silver. Although this alloy is difficult to cast, it has a particularly high response to age-hardening and therefore offers mechanical properties close to the wrought 2014 alloy. Alloy development experiments carried out at the University of Sheffield on A201 alloy have shown that feedstock of this alloy can be produced having uniform non-dendritic microstructures amenable to thixoforming, exhibits thixotropic properties under these conditions, can be shaped into complex near net-shape parts and develops impressive mechanical properties after appropriate heat treatment. A201 aluminum alloy billets when thixoformed in the semi-solid state and heat treated with a T7 followed by T6 treatment before being mechanically tested, have exhibited properties approaching those of wrought 2000 series aluminum alloys. Here follows a review of work done to date on shaping the A201 aluminum alloy in the semi-solid state, describing the hopes, expectations and fulfilment of researchers in the field.

Abstract: There has been a great debate whether grain refinement is more effective when shearing above or below the liquidus. In this work, examination of the microstructural evolution in AA7075 alloy has been performed, after stirring above & below the liquidus. The results suggest that shearing above the liquidus is more effective. The outcomes indicate that at temperatures below the liquidus, the convective forces produced by electromagnetic forces cannot break the dendrites due to low velocities, whilst at shearing above the liquidus through a cavitation-enhanced nucleation mechanism grain refinement is promoted. It appears that by applying magnetic forces above the liquidus temperature, shearing effects result in finer grain sizes and consequent enhancement of properties. Keywords: Electromagnetic Casting, Grain Refinement, Cavitation Mechanism

Abstract: The main goal of this work is the analysis of rheological properties of steel alloy at high temperatures, just below the solidus point, and in the semi-solid state with low liquid phase content. Data obtained from the analysis can form the basis of numerical simulation for designing and optimizing the thixoforming processes. The rheological properties should be known over a wide temperature range so that the simulations could also predict defects such as incomplete die filling. The analysis concerned M2 tool steel alloy. The paper also discusses development of globular microstructure in partially melted steel.

Abstract: Extrusion is a well established technology for the production of complex sections of aluminium alloys. Thixo-extrusion in comparison to traditional hot-extrusion offers several advantages such as lower extrusion and friction forces, higher material fluidity, longer tool life etc. Aluminium alloy 5182 is an important commercial alloy characterized by high strength and ductility, high corrosion resistance and good formability; it is commonly used for the production of wrought automotive components and it is also suitable for semi-solid applications thanks to its wide solidification range. The aim of this paper is to attempt the shaping of 5182 Al-Mg alloy through the thixo-extrusion process using a ceramic tool and evaluating the effect of different routes of making the feedstock on the semisolid microstructure. Particularly, two different methods were investigated: Near-solidus casting and Roll-casting using a cooling slope. All the samples produced were characterized by metallographic analysis in order to measure globule size and shape factor, as the main criteria used for assessing thixo-formability.

Abstract: The birth of Semi-Solid Metal Forming (SSM) or as it has now come to be widely known, Thixoforming, is a typical case of development of a technological innovation. Serendipity, stroke of luck, call it what you may, the beginning of SSM is based on an almost accidental discovery by a student carrying out a series of meticulous experiments. On the one hand, some technological failures have contributed to the lack of success across the board for SSM technologies. On the other hand, the ‘long childhood’ of the resulting technology or the process of moving from ‘Innovative Idea to Market' has been largely the result of difficult and in hindsight sometimes wrong managerial decisions, occasional personality clashes, patent rights and at times unavoidable all out business 'warfare'. Of course, hindsight is beautiful but unfortunately it always comes after the event. However, if one looks carefully at some of the notable successes of SSM forming one can discern that the problems were more on the human scale; people failures rather than technology failures. This paper aims to bring out some of these points by outlining the historical development of Thixoforming.

Abstract: Thixoforming involves shaping metal components in the semi-solid state. Work on the thixoforming of high temperature materials, such as steel, is still at its initial stage; this is mainly due to the high processing temperatures involved and the potential for oxidation. For thixoforming to be possible, it is preferable for an alloy to have an appreciable melting range and before forming the microstructure must ideally consist of solid metal spheroids in a liquid matrix. This paper discusses the thixoforming load versus displacement curves of HP9/4/30 steel semi-solid slugs under compression. The data from the corresponding load-displacement curves is converted into viscosity against shear rate adopting Stefan’s equation for flow between two parallel planes. The viscosity at processing conditions, i.e. at processing temperatures in the range of 1470 to 1480°C and zero to two minutes soak times, showed a rapid decrease initially, which corresponds to a rapid breakdown in the structure, followed by a steady decrease to a near constant value with increasing rate. The work shows pseudoplasticity (or shear-thinning) behaviour of the HP9/4/30 semi-solid slurries. This data would be required for modeling the die fill with these slurries.

Abstract: The main purpose of work was to develop a methodology of physical and numerical simulation of the thixocasting processes. For the purpose of the studies an experiment was conducted using a GLEEBLE 3800 simulator. In this work, the GLEEBLE 3800 system was adapted for physical simulation of the processes of light metal alloy forming in the solid-liquid state. The physical simulations of thixoforming processes and characterization of thixotropic materials were supported by computer simulations using ADINA software. The numerical model of thixoforming processes was developed in order to estimate values of the rheological model parameters.