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Aluminum alloys are the subject of increasing interest (in the automotive industry, as well as aircraft industry), aiming to reduce the weight of components and also allowing a profit in term of energy saving. Concerning the assembly, riveting has been widely used in the aircraft industry, whereas welding seems to be promising in the car industry in the case of aluminum alloys. Nevertheless, welding can generate defects, such as porosity or hot cracking, which could limit its development. One of the major problems associated with the welding of aluminum alloys is the formation of gas porosity. Aluminum alloy cleanliness remaining one of the aluminum industry's primary concerns, this project focuses on the formation and distribution of porosity in Al-Si welds. A literature review has been performed, to identify the mechanisms of porosity formation in welds and castings. Porosity distribution in welds has been investigated, based on three different welding techniques: hybrid Laser/MIG welding process, the electron beam welding process, and the MIG dual wire welding process. Porosity distribution results provide information on to the porosity formation mechanisms involved during welding. A complete microstructure, microhardness and EDX analysis have been carried out, to describe and quantify the solidification process within the welds.

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No description available.

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From a geomorphological point of view, and environments are characterised by complex process interactions and suites of landforms which can be sensitive to their controlling parameters. Relationships between sediment, moisture and soil crust characteristics are no exception. Field research and a programme of laboratory study were undertaken between 1993 and 1995 on the soils of the northern Badia of Jordan to advance knowledge on aspects of and zone soil dynamics, with a particular emphasis on crusting. The research focuses primarily on the effect soil crusts have upon the equilibrium of sediment dynamics at a hillslope scale and a ridge-furrow scale. The implications of the crust upon moisture storage within the surface layers of the soil are examined and the spatial characteristics which arise due to management practices and climate variables considered. A new, non-destructive dielectric technique to investigate moisture content in dryland soils has been developed and tested. Monitoring has taken place to examine the effects of irrigation upon the surface characteristics of the surrounding soil, with special reference to evaporation fluxes within a furrow and the associated precipitation of salts. The role of small-scale topography tends to be underestimated. Different types of crust have been studied from various topographic locations. Soil fabric and porosity have been studied, to increase understanding of micro-scale depositional and erosional processes. A new method of tracing' fine material through the upper soil profile has been developed. As crusts form, the tracer can be used to monitor the movement of fines, permitting a much clearer understanding of soil and water dynamics as a result of rainfall events.

Thermcapillary convection has been known as the dominant force in the flow and heat transfer during metals processings such as welding, drilling, cutting, and crystial growth, etc. Convection in the molten metal is typically vigorous and significant to the results of the process, in that it affects the size and shape of the pool, heat transfer, mixing of solutes, and ultimately microstructure of the finished product. In a melting or solidification process, thermocapillary effects may induce variations in local heat transfer, melting or solidification rates at the solid-liquid interface. thermocapillary flow originates at the hot wall and forms a surface layer along the free surface. The surface layer is driven by the thermocapillary force balanced by viscous stress. This region is followed by a region where the thermocapillary driving force has diminished due to a reduced temperature gradient. steady-state convection induced by thermocapillary and buoyant forces near a corner region of an enclosure having an inclined wall is numerically studied.Introducing an immobilizationt transformation, the shapes of the free surface subject to an incident flux are predicted by simultaneously solving mass, momentum and energy equations in the liquid and energy equation in the surrounding solid. The results provide a deep insight into local heat transfer, melting ¡Nsolidification rates at the solid-liquid interface and defects such as rippling, undercutting, humping, porosity, segregation, etc.