The influence of strontium on porosity formation in Al-Si alloys

Strontium modification is known to alter the amount, characteristics, and distribution of porosity in Al-Si castings. Although many theories have been proposed to account for these effects, most can be considered inadequate because of their failure to resolve contradictions and discrepancies in the literature. In an attempt to critically appraise some of these theories, the amount, distribution, and morphology of porosity were examined in sand-cast plates of Sr-free and Sr-containing pure Al, Al-l wt pet Si, and Al-9 wt pet Si alloys. Statistical significance testing was used to verify apparent trends in the porosity data. No apparent differences in the amount, distribution, and morphology of porosity were observed between Sr-free and Sr-containing alloys with no or very small eutectic volume fractions. However, Sr modification significantly changed the amount, distribution, and morphology of porosity in alloys with a significant volume fraction of eutectic. ne addition of Sr reduced porosity in the hot spot region of the casting, and the pores became well dispersed and rounded. This result can be explained by considering the combined effect of the casting design and the differences in the pattern of eutectic solidification between unmodified and Sr-modified alloys.

Eutectic grains in unmodified and strontium-modified hypoeutectic aluminum-silicon alloys

Additions of strontium to hypoeutectic aluminum-silicon alloys modify the morphology of the eutectic silicon phase from a coarse platelike structure to a fine fibrous structure. Thermal analysis, interrupted solidification, and microstructural examination of sand castings in this work revealed that, in addition to a change in silicon morphology, modification with strontium also causes an increase in the size of eutectic grains. The eutectic grain size increases because fewer grains nucleate, possibly due to poisoning of the phosphorus-based nucleants, that are active in the unmodified alloy. A simple growth model is developed to estimate the interface velocity during solidification of a eutectic grain. The model confirms, independent of microstructural observations, that the addition of 100 ppm strontium increases the eutectic grain size by at least an order of magnitude compared with the equivalent unmodified alloy. The model predicts that the growth velocity varies significantly during eutectic growth. At low strontium levels, these variations may be sufficient to cause transitions between flake and fibrous silicon morphologies depending on the casting conditions. The model can be used to rationally interpret the eutectic grain structure and silicon morphology of fully solidified aluminum-silicon castings and, when coupled with reliable thermal data, can be used to estimate the eutectic grain size.

The effect of silicon content on the microstructure and creep behavior in die-cast magnesium AS alloys

The effect of increasing levels of silicon on the microstructure and creep properties of high-pressure die-cast Mg-Al-Si (AS) alloys has been investigated. The morphology of the Mg2Si phase in die-cast AS alloys was found to be a function of the silicon content. The Mg2Si particles in castings with up to 1.14 wt pct Si have a Chinese script morphology. For AS21 alloys with silicon contents greater than 1.4 wt pet Si (greater than the alpha-Mg2Si binary eutectic point), some Mg2Si particles have a coarse blocky shape. Increasing the silicon content above the eutectic level results in an increase in the number of coarse faceted Mg2Si particles in the microstructure. Creep rates at 100 hours were found to decrease with increasing silicon content in AS-type alloys. The decrease in creep rate was most dramatic for silicon contents up to 1.1 wt pct. Further additions of silicon of up to 2.64 wt pct also resulted in significant decreases in creep rate.

Hall-Petch parameters in tension and compression in cast Mg-2Zn alloys

The flow stress in tension and compression has been measured as a function of plastic strain in Mg-2Zn castings with grain sizes ranging from 55 to 340 mum. Hall-Petch parameters have been calculated and are compared to those previously reported. In contrast to the behaviour of pure Mg grain refined with Zr and of hot-worked and recrystallised pure Mg and Mg-Zn alloys, the cast material shows little tension/compression asymmetry of the flow stress. The possible effects of texture and of twinning are noted. (C) 2004 Elsevier B.V. All rights reserved.

Morphological features of interfacial intermetallics and interfacial reaction rate in Al-11Si-2.5Cu-(0.15/0.60)Fe cast alloy/die steel couples

Soldering reactions are commonly observed during high pressure die casting of aluminium alloys, and involve the formation and growth of interfacial intermetallics between the die and the cast alloy. It is generally believed that close to 1% Fe is necessary in the aluminium alloy to reduce soldering. However, the role of iron in the interfacial reaction has not been studied in detail. In this investigation, reaction couples were formed between H13 tool steel substrates and an Al-11Si-2.5Cu melt containing either 0.15 or 0.60% Fe. Examination revealed distinctly different intermetallic layer morphology. The overall growth and chemistry of the reaction layer and the reaction rate measured by the consumption of the substrate were compared for the two alloy melts. It was demonstrated that a higher iron content reduces the rate of interfacial reaction, consistent with an observed thicker compact ( solid) intermetallic layer. Hence, the difference in reaction rate can be explained by a significant reduction in the diffusion flux due to a thicker compact layer. Finally, the mechanism of the growth of a thicker compact layer in the higher iron melt is proposed, based on the phase relations and diffusion both within and near the interfacial reaction zone. (C) 2004 Kluwer Academic Publishers.

Kinetics of reduction of lead smelting slags with solid carbon

Research techniques and a methodology have been developed that enable the reduction kinetics of molten lead smelting slags with solid carbon to be studied. The rates of reduction of PbO-FeO-Fe2O3-CaO-SiO2 slags with carbon have been measured for a range of slag compositions for PbO concentrations between 3 and 100 weight percent, and temperatures between 1423 and 1573 K. The reduction rates were determined for both graphite and coke. Within the range of process conditions examined, it has been shown that the reaction rates are almost independent of carbon reactivity, SiO2/CaO and SiO2/Fe ratio in the range of compositions investigated and are not influenced by the presence of sulphur in the slag.The apparent first order rate constants for oxygen removal increase with increasing PbO concentration and oxygen activity in the slag. The data indicate that the rate limiting reaction step for the reduction of lead slags with solid carbon is the chemical reaction at the gas/slag interface.

An analysis of the relationship between grain size, solute content, and the potency and number density of nucleant particles

To be able to determine the grain size obtained from the addition of a grain refining master alloy, the relationship between grain size (d), solute content (defined by the growth restriction factor Q), and the potency and number density of nucleant particles needs to be understood. A study was undertaken on aluminium alloys where additions of TiB2 and Ti were made to eight wrought aluminum alloys covering a range of alloying elements and compositions. It was found from analysis of the data that d = a/(3)root pct TiB2 + b/Q. From consideration of the experimental data and from further analysis of previously published data, it is shown that the coefficients a and b relate to characteristics of the nucleant particles added by a grain refiner. The term a is related to the maximum density of active TiB2 nucleant particles within the melt, while b is related to their potency. By using the analysis methodology presented in this article, the performance characteristics of different master alloys were defined and the effects of Zr and Si on the poisoning of grain refinement were illustrated.

Grain refinement efficiency and mechanism of aluminium carbide in Mg-Al alloys

Detailed microscopic examination using optical and electron microscopes suggests that Al4C3, often observed in the central regions of magnesium grains on polished sections, is a potent substrate for primary Mg. Calculations of the crystallographic relationships between magnesium and Al4C3 further support the experimental observations. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Corrosion behaviour of a pressure die cast magnesium alloy

High pressure die casting is the most important production method for casting magnesium alloy components, and uniformity of appearance is an important criterion for acceptance of a component by customers. This paper investigates the influence of uniformity in surface appearance of diecast AZ91D plates on their corrosion behaviour. Through immersion, hydrogen collection and weight loss measurements it was found that corrosion is more likely to occur on the areas of the plate that appear to be darker, leading to a non-uniformly corroded surface. Microstructural analysis showed that the non-uniformity in appearance is related to a difference in the morphology and distribution of porosity across the surface of a diecast AZ91D plate. The darker areas of the surface are high in porosity which breaks the continuity of the beta-phase network and provides shortcut paths for corrosion from the surface to the interior of the casting. The brighter shiny areas of the surface are much less porous, with isolated pores being confined by corrosion resistant beta-precipitates thus reducing the corrosion rate.

Manufacture of high pressure die-cast radio frequency filter bodies

The manufacture of a radio frequency filter box using high pressure die casting (HPDC) is compared to the traditional high speed machining route. This paper describes an industrial exercise that concluded HPDC to be an economical and appropriate method to produce larger volumes of thin-walled telecommunications components. Modifications to the component design were made to make the component suitable for the HPDC process. Development of the die design through simulation modelling is described. The wrought alloy was replaced by near-eutectic Al-Si die casting alloy that was found to give better temperature stability performance. Apart from the economic benefits, HPDC was found to give lower filter efficiency losses through better surface finish. The effects of HPDC process variables, such as intensification pressure and injection piston velocity, on component quality, particularly porosity levels, were investigated. The pressure was analysed in terms of HPDC machine set pressure and the pressure measured in the die cavity by pressure sensors. Porosity was found to decrease with increased pressure and slightly increase with higher casting velocities.