Papers by Author: Mark A. Easton

Abstract: This paper investigates the use of ultrasonic treatment on the grain refinement of Mg-Al alloys in castings similar to commercial gravity castings. It shows that it is a very effective form of grain refinement but it is limited spatially and by the advancing solidification front if partially-solidified product is washed into the die. It was found that the best way to obtain a homogeneous fine grain size through-out the casting was to combine a grain refining addition, SiC, with ultrasonic treatment during the initial stages of solidification.

Abstract: A numerical model that describes solidification of primary aluminium grains and nucleation and growth of eutectic cells is used to analyse the solidification of an Al-12.5wt% Si alloy. Nucleation of eutectic cells is modelled using an Oldfield-type nucleation model where the number of nuclei in the melt is determined by the amount of active nuclei and the local undercooling from the surface to the centre of a plate casting. Eutectic grains are modelled as spheres growing between the dendrites. The growth velocity of the eutectic cells is a function of undercooling. Experimentally determined growth parameters from the literature that depend on the type of modification (unmodified, Na-modified or Sr-modified) are used to describe differences in growth of the alloys. Modelling results are compared with solidification experiments where an Al-12.5wt%Si alloy was cast in unmodified, Na modified and Sr modified forms. The model confirms experimental observations of how modification and alloy composition influence nucleation, growth and finally the size of eutectic cells in the alloys. Modelling results are used to explain how cooling conditions in the casting act together with the nuclei density in the liquid and the growth velocity of the eutectic cells to determine the size and distribution of eutectic cells in the solidified material.

Abstract: A study of the influence of minor additions of copper or manganese on hot tear susceptibility in Al-Mg-Si alloys has been conducted. Testing was carried out using a laboratory scale hot tearing rig and the results were validated using an analysis of cast house cracking scrap data for 6060 and 6063 extrusion billet alloys. Mn content was found to have a strong influence on hot tearing rates.

Abstract: The Interdependence Theory is a theoretical description of grain formation that links heterogeneous nucleation to grain growth early in the initial transient of a previously nucleated grain. Thus nucleation is the result of a repeating cycle of growth and nucleation events moving towards the thermal centre of a casting. The principles of this theory are introduced and then the Interdependence equation that embodies the Interdependence Theory, is applied to the prediction of experimental grain size data for aluminium, magnesium and titanium-based alloy systems.

Abstract: This paper reports on the results of the castability of three MRI alloys (153A, 153M and 230D). MRI153A was found to cast best, with castings produced rated with a quality approaching AZ91. MRI230D produced the next best castings, whilst MRI153M showed the worst castability across a range of conditions. However, these alloys showed a tendency to build-up oxide in the melt transfer tube leading to melt transfer problems. This was particularly severe in MRI230D.

Abstract: In this study effects of heat treatments on the creep resistance at 177°C/90MPa of a high-pressure die-cast Mg-2.70La-1.50Y (wt.%) alloy were examined. It was found that ageing at 160°C for 24 h (T5) or a solution treatment at 520°C for 1 h (T4) improved creep resistance and caused no blistering on the surface or dimensional changes to the die-cast specimens. TEM was used to characterize the microstructures of heat-treated samples. Improvements to creep resistance might be attributed to the pinning or otherwise retarding of dislocation motion by precipitates and/or solute atoms during creep.