Flux (metallurgy) | EPFL Graph Search

In metallurgy, a flux () is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining. Some of the earliest known fluxes were sodium carbonate, potash, charcoal, coke, borax, lime, lead sulfide and certain minerals containing phosphorus. Iron ore was also used as a flux in the smelting of copper. These agents served various functions, the simplest being a reducing agent, which prevented oxides from forming on the surface of the molten metal, while others absorbed impurities into slag, which could be scraped off molten metal. Fluxes are also used in foundries for removing impurities from molten nonferrous metals such as aluminium, or for adding desirable trace elements such as titanium. As cleaning agents, fluxes facilitate soldering, brazing, and welding by removing oxidation from the metals to be joined. In some applications molten flux also serves as a heat-tran

Etude du cycle biogéochimique du cuivre et du cadmium dans deux écosystèmes forestiers

Catherine Keller

The biogeochemical cycle of copper and cadmium has been studied in two forested ecosystems for two years. These two sites are: a) a Norway spruce forest (Picea abies) on an acid brown soil (FAQ: Dystric cambisol) and b) a mixed coniferous forest (Picea abies, Pinus cembra, Larix decidua and Abies alba) on a podzol. Both of them are located on low heavy metal polluted sites: the Swiss Plateau (850 m a.s.l.) for the first one and the oriental edge of the Mont-Blanc Massif (1650 m a.s.l.) for the second one. The Cu and Cd stocks in the vegetation and in the soil are evaluated. Heavy metal fluxes through the ecosystems are determined by analysis of concentrations and quantities of rain, throughfalls, stemflows, lysimetric soil solution and litterfall. In addition, the forms of Cu and Cd immobilization in soils are studied by means of sequential extraction, whereas immobilization forms in solution are studied by means of ion exchange chromatography and selective electrode. The results obtained allowed us to draw conclusions concerning the general pollution level in the experimental sites and the annual and/or detailed behaviour of Cu and Cd in the ecosystems. The conclusions are the following: As expected, the sites are not polluted since concentrations and fluxes are low. The chemical composition of solutions is modified during transition through the ecosystem, especially the copper concentrations. The acid brown soil humic layer is more efficient than the podzol humic layer in modifying this composition. There is no significant seasonal variation of concentrations and fluxes. However, the metal diffusion through the ecosystem is almost immediate. The soil plays an essential role in the immobilization of the heavy metals. The organic and organo-mineral layers are particularly active in this process. Exportations out of the ecosystems are limited. They are relatively more important for the podzol than for the acid brown soil and higher for Cd than for Cu. Transfer and stocking forms, and the ways of input to the soil depend on the metal considered: Copper input to the soil is mainly due to litterfall. In the soil, the metal has a preference to be associated with organic matter. In solution, it forms complexes with dissolved organic matter (mainly fulvic acid of MW < 1000 dalton). So, it is strongly retained within the soil and it cannot be easily drained out of the soil profile. At the opposite, cadmium is mostly transported by liquid means (throughfalls). In solution, it is present as free cadmium ions. In the soil, it is less strongly fixed than Cu since Cd is mostly associated with Fe and Mn oxides. For both of the metals, the free ion concentration in solution depends on pH and the saturation rate of organic matter. The latter has a heterogenous and polyfunctional character which explains a similar complexing capacity for all the samples studied. The differences observed in copper and cadmium behaviour can explain the variable diffusion of a polluting Cu and Cd flux through a given ecosystem and to the aquifer. The soil intervenes as a discriminating factor when the two experimental sites are compared.

EPFL1991

Characterization of capillary forces during liquid metal infiltration

Maryam Bahraini Hasani

This thesis addresses the link between capillarity parameters, i.e. surface tension and contact angle, and the infiltration of porous preforms with molten metal by pressure infiltration The method used is to conduct pressure infiltration experiments of selected preforms with molten aluminum or copper based alloys, and characterize the level and, where relevant, kinetics of metal ingress into the preforms. Several systems are investigated in turn, comprising chemically inert and reactive systems. Capillarity in the infiltration of alumina particulate preforms with copper are investigated by means of infiltration experiments conducted at 1200°C under argon with a low oxygen partial pressure, characteristic of equilibrium with carbon at that temperature. The experiments were run in a gas pressure driven liquid metal infiltration apparatus capable of melting copper under gas pressures up to 20 MPa. Capillarity parameters are extracted from drainage curves that plot the volume fraction of metal in the porous medium vs. the applied pressure. Mercury porosimetry, which is based on the same principle, is also used on similar preforms for comparison. The effect of volume fraction, particle geometry and capillary parameters on the drainage curve are studied and compared with the expression proposed by Brooks and Corey. The influence of the particle volume fraction and capillary parameters characterizing wetting in the two systems is discussed to derive an effective contact angle for wetting of alumina particles by molten metal. The value thus derived agrees with literature data from sessile drop experiments. A new technique is proposed for the direct measurement of capillary forces during infiltration in systems of relevance to the processing of metal matrix composites. A custom-designed device enables dynamic tracking of the molten metal surface while the infiltration apparatus is pressurized. It comprises an LVDT (Linear Variable Differential Transformer) that is fixed to the top of the infiltration machine and whose core is connected via an alumina tube to a graphite plunger that floats atop the liquid metal. Upon pressurization, the floater movement caused by flow of the metal into the preform is tracked by the LVDT. Capable of handling melt temperatures up to 1250°C and infiltration pressures up to 20 MPa, the method is essentially a high-temperature analogue of mercury porosimetry. Its accuracy is demonstrated by comparison with data obtained using other techniques and its use is illustrated with the infiltration of diamond particle preforms by Al and Al-Si alloys at 700°C. The present method for direct measurement of drainage curves during the infiltration process is then used for the characterization of wetting of SiC particle performs by molten aluminum and Al-12.2at%Si. The ceramic particle diameter is varied from 7 µm to 40 µm and infiltration was characterized at 750°C, varying the applied pressure from 0.1 to 10 MPa. From these data, the relevant work of immersion is calculated by comparison to the work exerted by the external gas pressure over the whole saturation range. The thus determined work of immersion is translated in an apparent contact angle during infiltration, which is compared with the values determined by other techniques reported in literature to find good agreement. Kinetics effects in reactive systems such as SiC/Al and graphite/Cu-Cr were studied by direct measurement of the drainage curves varying the pressurization rate and conducting measurements interrupted at a given pressure. In addition to the analysis of the capillarity parameters already used for the non-reactive systems, the data are analyzed by a simple model based on the Brooks-Corey relation allowing to link the triple-line velocity in liquid metal infiltration with the saturation given one characteristic distance. It is found that alloying elements in systems investigated drive infiltration at fixed pressure, but do not assist pressure infiltration. Quite to the contrary, they may even hamper efficient infiltration by blocking narrow channels via formation of reaction products with the preform material.

EPFL2007