Rheological behaviour of the mushy zone and its effect on the formation of casting defects during solidification

A new conceptual framework has been developed which explains the formation of shear-related casting defects such as porosity, segregation and tears. The theory relates defect formation to the mechanical behaviour of the partially solidified microstructure when shear stresses are developed during the filling of a casting and by the subsequent feeding processes during solidification. Two transition points, the dendrite coherency point and the maximum packing solid fraction, divide the mushy zone into three regions of different mechanical and feeding behaviours. The response of the mush to shear is related to the presence of these zones during solidification of a casting. The resulting defects are rationalized by considering the governing local shear stress and shear rate, local strength and time available for fluid flow. The design of the casting, the casting process used and the alloy composition all influence the relative importance of shearing on defect formation. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

Influence of Mg content on the microstructure and solid solution chemistry of Al-7%Si-Mg casting alloys during solution treatment

The solution treatment stage of the T6 heat-treatment of Al-7%Si-Mg foundry alloys influences microstructural features such as Mg2Si dissolution, and eutectic silicon spheroidisation and coarsening. Microstructural and microanalytical studies have been conducted across a range of Sr-modified Al-7%Si alloys, with an Fe content of 0.12% and Mg contents ranging from 0.3-0.7wt%. Qualitative and quantitative metallography have shown that, in addition to the above changes, solution treatment also results in changes to the relative proportions of iron-containing intermetallic particles and that these changes are composition-dependent. While solution treatment causes a substantial transformation of pi phase to beta phase in low Mg alloys (0.3-0.4%), this change is not readily apparent at higher Mg levels (0.6-0.7%). The pi to beta transformation is accompanied by a release of Mg into the aluminum matrix over and above that which arises from the rapid dissolution of Mg2Si. Since the level of matrix Mg retained after quenching controls an alloy's subsequent precipitation hardening response, a proper understanding of this phase transformation is crucial if tensile properties are to be maximised.

Semisolid casting of AlSi7Mg0.35 alloy produced by low-temperature pouring

AlSi7Mg0.35 alloy was cast into permanent moulds using different pouring temperatures (725 to 625degreesC). As the pouring temperature decreased, the as-cast microstructure changed from a coarse dendritic structure, through fine equiaxed grains to fine rosette-like grains. The as-cast materials were then partially remelted and isothermally held at 580degreesC prior to semisolid casting into a stepped die. The feedstock material cast from a high temperature filled only half the die, with severe segregation and other defects. The low-temperature-poured material completely filled the die with negligible porosity. The quality of semisolid castings is significantly affected by the microstructure of the semisolid feedstock material that arises from a combination of as-cast and subsequent thermal treatment conditions. The paper describes (a) the influence of pouring temperature on the microstructure of feedstock; (b) microstructure evolution through remelting and (c) the quality of semisolid castings produced with this material. For A17Si0.35Mg alloy, low temperature pouring in the range of 625-650degreesC followed by suitable isothermal holding treatment can result in good quality semisolid casting.

Improved VDC casting of aluminium alloys through an understanding of the surface properties of the molten metal

Vertical direct chill (VDC) casting of aluminium alloys is a mature process that has evolved over many decades through gradual change to both equipment design and casting practice. Today, air-pressurised, continuous lubrication, hot top mould systems with advanced station automation are selected as the process of choice for producing extrusion billet. Specific sets of operating parameters are employed on these stations for each alloy and size combination to produce optimal billet quality. The designs and parameters are largely derived from past experience and accumulated know-how. Recent experimental work at the University of Queensland has concentrated on understanding the way in which the surface properties of liquid aluminium alloys, e.g., surface tension, wetting angle and oxide skin strength, influence the size and shape of the naturally-stab le meniscus for a given alloy, temperature and atmosphere. The wide range of alloy-and condition-dependent values measured has led to the consideration of how these properties impact the stability of the enforced molten metal meniscus within the hot top mould cavity. The actual shape and position of the enforced meniscus is controlled by parameters such as the upstream conduction distance (UCD) from sub-mould cooling and the molten metal head. The degree of deviation of this actual meniscus from the predicted stable meniscus is considered to be a key driver in surface defect formation. This paper reports on liquid alloy property results and proposes how this knowledge might be used to better design VDC mould systems and casting practices.

Supersaturated solutions evaluated with an in vitro stratum corneum tape stripping technique

The flux of a compound across a membrane from any formulation, whether it contains penetration enhancers or not, is limited by its saturated solubility in the vehicle. Under such conditions the concentration of the permeant in the outer layers of the stratum corneum is also saturated. Consequently, when the permeation of a drug from a supersaturated solution leads to enhanced penetration, the concentration of the drug in the outer layers of the membrane is also supersaturated. Therefore, the stratum corneum may possess antinucleant properties which inhibit or retard the crystallisation process. In this study, the enhanced in vitro permeation of supersaturated solutions of piroxicam across human skin in diffusion cells was demonstrated. The amount of permeant in the stratum corneum was determined using a tape stripping technique. Supersaturated solutions up to four degrees of saturation were investigated which produced a linear relationship between the degree of saturation and the amount of piroxicam in the stratum corneum (R-2 = 0.970). Furthermore, the amount of piroxicam in the viable layers of the skin also increased with increasing degree of saturation. An analysis of the results suggested that enhanced penetration across human skin from supersaturated solutions of piroxicam may occur as a result of the antinucleating ability of the intercellular lipids of the stratum corneum. (C) 1997 Elsevier Science B.V.

Introduction: Casting new light on old stones

The purpose of this monograph is to take a new look at various aspects of stone artefact analysis that reveal important and exciting new information about the past. This invovles reorienting our methodological approach to stone artefacts as well as the questions asked of them. The papers making up this volume tackle a number of issues that have long been at the heart of archaeology’s problematic relationship with stone artefacts, including our understanding of the dynamic nature of past stoneworking practices, the utility of traditional classificatory schemes, and ways to unlock the vast amount of information about the strategic role of lithic technology that resides in stone artefact assemblages.

Eutectic solidification mode in sodium modified Al-7 mass %Si-3.5 mass %Cu-0.2 mass %Mg casting alloys

The influence of sodium (Na) on nucleation and growth of the Al-Si eutectic in a commercial hypoeutectic Al-Si-Cu-Mg foundry alloy has been investigated. The microstructural evolution during eutectic solidification was studied by a quenching technique. By comparing the orientation of the aluminium in the eutectic to that of the surrounding primary aluminium dendrites by EBSD, the eutectic solidification mode could be determined. The results show that the eutectic solidification starts near the mould wall and evolves with front growth opposite the thermal gradient on a macro-scale, and on a micro-scale with independent heterogeneous nucleation of eutectic grains in interdendritic spaces. Na-modified alloys therefore behave significantly differently from those modified by other elemental additions.

The effect of soft tissue deloading tape on thoracic spine pressure pain thresholds in asymptomatic subjects

The application of tape to deload soft tissue is used in the management of thoracic spine pain. A reported clinical feature of this treatment is reduced tenderness of the spine during postero-anterior mobilizations. A randomized, single blind, placebo controlled, repeated measures design study was employed to investigate the effects of deloading tape on pressure pain threshold measurements at the level of the T7 spinous process in an asymptomatic group of 24 subjects. Pressure pain thresholds were assessed prior to and following the application of deloading tape, placebo sham tape and no-tape control conditions. All subjects received all three conditions in a randomized order on three separate days. Differences between the pre- and post-measurements were used as indicators of change in a subject's pressure pain threshold. No significant change in pressure pain threshold measurements was found between conditions. In summary, this study demonstrated that deloading tape applied to the level of the T7 spinous process did not significantly change pressure pain threshold measurements in asymptomatic subjects, raising the possibility that any pain relieving effect may well be conditional upon pain being present. (C) 2002 Elsevier Science Ltd. All rights reserved.

An age hardening model for Al-7Si-Mg casting alloys

Yield strength (YS) ageing curves have been modelled for A356 and A357 aluminium casting alloys below the solvus temperature of the main hardening precipitate. Predictions are based on the Shercliff and Ashby methodology (Acta MetaH. Mater. 38 (1990) 1789) for wrought alloys. Differences between strengthening in wrought and cast Al-Si-Mg alloys are considered. A Brinell hardness to YS conversion incorporating strain hardening has been established to enable YS ageing curves to be predicted with reduced experimental effort. (C) 2002 Elsevier Science B.V. All rights reserved.