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Solidification mechanisms of unmodified and strontium-modified hypereutectic aluminium-silicon alloys

The effects of strontium on the solidi. cation mode of hypereutectic aluminium-silicon alloys have been studied. Samples were prepared from an aluminium-17wt% silicon-based alloy and strontium was added at several different concentrations. The development of the microstructure was investigated by cooling curve analysis, interrupted solidi. cation experiments and optical and scanning electron microscopy. It was found that nucleation of primary silicon is suppressed by additions of strontium. The suppressed nucleation results in supersaturation of the liquid prior to nucleation, and an increased growth rate after nucleation. As a result, the silicon crystals become less faceted and more dendritic with increasing strontium additions. Increasing the strontium concentration slightly refined the eutectic spacing and introduced a small amount of fibrous silicon. Electron back-scatter diffraction measurements were performed to determine the crystallographic relation between the primary and eutectic silicon phases. The eutectic silicon in the unmodified alloy does not have any crystallographic relationship with the primary silicon crystals. In contrast, the eutectic silicon crystals in the strontium-modified alloys often share an identical or twin relationship with nearby primary silicon crystals. The incidence of twinning within primary silicon crystals was relatively low and did not appear to increase with strontium additions.

Solidification and cooling of grey cast iron in shell moulds with particular reference to the sub-surface structure

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Improvement of the mechanical properties of 2124 Al/SiCp MMC plate by optimisation of the solution treatment

Static mechanical properties of 2124 Al/SiCp MMC have been measured as a function of solution temperature and time. An optimum solution treatment has been established which produces significant improvements in static mechanical properties and fatigue crack growth resistance over conventional solution treatments. Increasing the solution treatment parameters up to the optimum values improves the mechanical properties because of intermetallic dissolution, improved solute and GPB zone strengthening and increased matrix dislocation density. Increasing the solution treatment parameters beyond the optimum values results in a rapid reduction in mechanical properties due to the formation of gas porosity and surface blisters. The optimum solution treatment improves tensile properties in the transverse orientation to a greater extent than in the longitudinal orientation and this results in reduced anisotropy. © 1996 Elsevier Science Limited.

Multiphase modeling of melting / solidification with high density variations using XFEM

La modélisation de la cryolite, utilisée dans la fabrication de l’aluminium, implique plusieurs défis, notament la présence de discontinuités dans la solution et l’inclusion de la difference de densité entre les phases solide et liquide. Pour surmonter ces défis, plusieurs éléments novateurs ont été développés dans cette thèse. En premier lieu, le problème du changement de phase, communément appelé problème de Stefan, a été résolu en deux dimensions en utilisant la méthode des éléments finis étendue. Une formulation utilisant un multiplicateur de Lagrange stable spécialement développée et une interpolation enrichie a été utilisée pour imposer la température de fusion à l’interface. La vitesse de l’interface est déterminée par le saut dans le flux de chaleur à travers l’interface et a été calculée en utilisant la solution du multiplicateur de Lagrange. En second lieu, les effets convectifs ont été inclus par la résolution des équations de Stokes dans la phase liquide en utilisant la méthode des éléments finis étendue aussi. Troisièmement, le changement de densité entre les phases solide et liquide, généralement négligé dans la littérature, a été pris en compte par l’ajout d’une condition aux limites de vitesse non nulle à l’interface solide-liquide pour respecter la conservation de la masse dans le système. Des problèmes analytiques et numériques ont été résolus pour valider les divers composants du modèle et le système d’équations couplés. Les solutions aux problèmes numériques ont été comparées aux solutions obtenues avec l’algorithme de déplacement de maillage de Comsol. Ces comparaisons démontrent que le modèle par éléments finis étendue reproduit correctement le problème de changement phase avec densités variables.

Modelling and simulations of ductile iron solidification-induced variations in mechanical behaviour on component and microstructural level

The mechanical behaviour and performance of a ductile iron component is highly dependent on the local variations in solidification conditions during the casting process. Here we show a framework which combine a previously developed closed chain of simulations for cast components with a micro-scale Finite Element Method (FEM) simulation of the behaviour and performance of the microstructure. A casting process simulation, including modelling of solidification and mechanical material characterization, provides the basis for a macro-scale FEM analysis of the component. A critical region is identified to which the micro-scale FEM simulation of a representative microstructure, generated using X-ray tomography, is applied. The mechanical behaviour of the different microstructural phases are determined using a surrogate model based optimisation routine and experimental data. It is discussed that the approach enables a link between solidification- and microstructure-models and simulations of as well component as microstructural behaviour, and can contribute with new understanding regarding the behaviour and performance of different microstructural phases and morphologies in industrial ductile iron components in service.

Characterisation and investigation of local variations in mechanical behaviour in cast aluminium using gradient solidification, Digital Image Correlation and finite element simulation

Due to design and process-related factors, there are local variations in the microstructure and mechanical behaviour of cast components. This work establishes a Digital Image Correlation (DIC) based method for characterisation and investigation of the effects of such local variations on the behaviour of a high pressure, die cast (HPDC) aluminium alloy. Plastic behaviour is studied using gradient solidified samples and characterisation models for the parameters of the Hollomon equation are developed, based on microstructural refinement. Samples with controlled microstructural variations are produced and the observed DIC strain field is compared with Finite Element Method (FEM) simulation results. The results show that the DIC based method can be applied to characterise local mechanical behaviour with high accuracy. The microstructural variations are observed to cause a redistribution of strain during tensile loading. This redistribution of strain can be predicted in the FEM simulation by incorporating local mechanical behaviour using the developed characterization model. A homogeneous FEM simulation is unable to predict the observed behaviour. The results motivate the application of a previously proposed simulation strategy, which is able to predict and incorporate local variations in mechanical behaviour into FEM simulations already in the design process for cast components.

Assessing the impact of ammonia content in VFA-rich streams on the PHA-storing MMC acclimatization and the PHA production stage

The use of environmentally friendly products increased the interest in renewable resources as alternatives to petrochemical products. Polyhydroxyalkanoates (PHAs) are examples of such promising products, as they are biodegradable polymers with numerous potential applications. PHA production approach consists of using an open mixed microbial culture (MMC) and inexpensive feedstocks (waste or industry byproducts feedstock). The PHA process generally comprises three stages: (1) acidogenic fermentation (AF) stage (conversion of organic carbon into fermentation products); (2) culture selection stage (enrichment in PHA-storing organisms by applying Feast and Famine regime); and (3) PHA production stage (PHA accumulation up to the culture’s maximum capacity). AF of protein-rich residues results in ammonia-rich fermented streams, which can be presented as a challenge for the PHA production stage. The presence of ammonia during this stage may induce organisms to grow instead of producing PHAs. For this reason, the assessment of the effect of a high content of ammonia on PHA production it is the utmost importance. The main goal of the current project is to select a MMC enriched in PHA-accumulating organisms in conditions of high ammonia content and to evaluate the effects of ammonia presence during PHA accumulation. The culture was selected applying the Feast & Famine strategy, and fed, firstly, using a synthetic mixture of VFAs and later using a fermented stream obtained from the fermentation of protein-rich raw materials. The selected culture could accumulate up to 24% PHA per VSS with the synthetic mixture of VFAs and up to 29% for the real fermented stream. The PHA accumulation resulted in different production in the presence and absence of ammonia. Regarding to the synthetic feed, 59%wt. PHA (VSS basis) in the absence of ammonia, and 55%wt. (VSS basis) in the presence, were obtained. For the real feed, the PHA content was about 40%wt. (VSS basis) in both reactors.

Oral findings and dental care in a patient with myelomeningocele: case report of a 3-year-old child

Myelomeningocele (MMC) is a congenital malformation of the neural tube that occurs in the first weeks of pregnancy. This malformation refers to the caudal non-closure of the neural tube and neural tissue exposure, which lead to neurological problems, such as hydrocephalus, motor disability, genitourinary tract and skeletal abnormalities and mental retardation. Patients with MMC have an acknowledged predisposition to latex allergy and are usually at a high caries risk and activity due to poor oral hygiene, fermentable carbon hydrate-rich diet and prolonged use of sugar-containing medications. This paper addresses the common oral findings in pediatric patients with MMC, discusses the strategies and precautions to deal with these individuals and reports the dental care to a young child diagnosed with this condition.

Microstructural characterization of rapidly solidified and heat-treated Ti(92)B(8) eutectic alloy

In this work, Ti(92)B(8) alloy was processed via rapid solidification (splat-cooling) and then heat-treated at 700 degrees C and 1000 degrees C. A careful microstructural characterization indicated that, after rapid solidification, a very fine two-phase microstructure was produced with no significant saturation of B in alpha/beta Ti. There was no indication of amorphous formation in the rapidly solidified splats. Both alpha Ti and TiB were observed in the microstructures of the splats after heat-treatment at 700 degrees C and 1000 degrees C, confirming the stability of the alpha Ti+TiB two-phase region in this temperature range. (C) 2008 Elsevier Inc. All rights reserved.