The DC casting process used for the production of cast aluminium alloy products intended for processing by rolling, extrusion or forging is an economically important process with approximately 10 million tonnes of DC cast product being produced annually world wide [1]. Process productivity, particularly with respect to elimination of casting defects and hence process scrap is an important factor to DC cast product producers. The literature reporting the DC casting process, particularly with respect to the formation of defects on the cast surface, is reviewed and the mechanisms for the formation of such defects examined. A universally understood and accepted explanation was found for only one of the normal surface defects encountered in practice. A number of samples of commercially cast DC products were subject to detailed cast surface examination, particularly surface microstructures. The results of this examination and the literature survey identified molten metal surface tension as a possible contributing factor affecting the molten metal meniscus stability within the DC casting mould. Meniscus instability is linked with the formation of surface defects. The literature on surface tension of aluminium alloys provided only limited information hence the surface tension of a range of binary and ternary alloys, including commercial alloys was determined, producing data hitherto not available. Of the common alloying elements used in commercial aluminium alloys, iron and magnesium were found to significantly reduce the surface tension. Surface fracture also resulted in a reduction in surface tension for the majority of alloys tested. The surface tension data is combined with mould thermal and physical model calculations to propose a mechanism for the formation of the cast surface defects. The model proposes a maximum stable size for the meniscus according to the alloy and mould conditions. Conditions outside these limits result in meniscus instability and the formation of cast surface defects. The model suggests possible operating changes that may reduce the incidence of surface defect formation. The work also identifies a number of areas requiring further investigation before major practical process changes aimed at cast surface defect elimination, may be formulated.

The DC casting process used for the production of cast aluminium alloy products intended for processing by rolling, extrusion or forging is an economically important process with approximately 10 million tonnes of DC cast product being produced annually world wide [1]. Process productivity, particularly with respect to elimination of casting defects and hence process scrap is an important factor to DC cast product producers. The literature reporting the DC casting process, particularly with respect to the formation of defects on the cast surface, is reviewed and the mechanisms for the formation of such defects examined. A universally understood and accepted explanation was found for only one of the normal surface defects encountered in practice. A number of samples of commercially cast DC products were subject to detailed cast surface examination, particularly surface microstructures. The results of this examination and the literature survey identified molten metal surface tension as a possible contributing factor affecting the molten metal meniscus stability within the DC casting mould. Meniscus instability is linked with the formation of surface defects. The literature on surface tension of aluminium alloys provided only limited information hence the surface tension of a range of binary and ternary alloys, including commercial alloys was determined, producing data hitherto not available. Of the common alloying elements used in commercial aluminium alloys, iron and magnesium were found to significantly reduce the surface tension. Surface fracture also resulted in a reduction in surface tension for the majority of alloys tested. The surface tension data is combined with mould thermal and physical model calculations to propose a mechanism for the formation of the cast surface defects. The model proposes a maximum stable size for the meniscus according to the alloy and mould conditions. Conditions outside these limits result in meniscus instability and the formation of cast surface defects. The model suggests possible operating changes that may reduce the incidence of surface defect formation. The work also identifies a number of areas requiring further investigation before major practical process changes aimed at cast surface defect elimination, may be formulated.

The continued interest in the use of magnesium alloys for new applications demand the successful production of high quality wrought alloys. Magnesium Elektron seek to reliably produce high quality alloy billets by the DC casting method combined with ultrasonic inspection. The main objectives of this study are to characterize the defects which are currently found in the material and to understand the ability of the ultrasonic inspection technique currently employed to detect the defects. This study began by locating defects using the ultrasonic inspection method which were then characterised using X-ray Computed Tomography (XCT) 3D imaging technique. Attempts were then made to understand and simulate the mechanisms by which the defects form during the casting process. The simulations were used to investigate the flow patterns during casting and the growth kinetics of the intermetallic phase. The initial phase of this research established that the defects found comprised of an entrained oxide film entangled with an abundance of intermetallic phase particles. These defects were found to be present in the size range of 0.5 – 5 mm, and were deleterious to the materials mechanical properties. Greater understanding of the ultrasonic inspection process was achieved and informed improvements to assisting the production of high quality feedstock. Simulation of the formation of the defects indicated that there was a region in which the oxide films could form and be free to enter into the final cast product. Simulation of the growth of the intermetallic particles demonstrated that precipitation from the liquid occurs in the mould during which particles are carried by the melt flow and experiences a complex thermal history. The combination of the two phases was established to be due to entanglement of the oxide and particles which when combined will settle out of the melt as a single defect. Improved filtering and melt handling methods were recommended to eliminate the defects and reliably produce high quality alloys.

The direct chill (DC) casting technique to produce billets for extrusion and ingots for rollingwas developed in the 1930s. The principle, which is still valid, is a two-stage cooling with a primary cooling at a mould surface followed by water spraying directly on the surface. Improvements of this technique have mainly focused on changes to the primary cooling, where a water-cooled metal mould has been replaced by different techniques to minimize cooling at this stage. The drive for development comes from the extrusion industry, which can increase the productivity and quality of extruded profiles by improving the billet surface appearance and structure. Hot top casting supported by airflow against the casting surface during the primary cooling is currently the standard procedure to achieve acceptable billet surfaces. The goal is to minimize the depth of the surface segregation zone, which is the governing factor for the appearance of different phases in the surface region. Billet surface quality is evaluated by quantifying surface appearance, segregation zone thickness, and  occurrence of large Mg2Si and β-particles near the surface. The β-Al5FeSi intermetallic phase and coarse Mg2Si particles have negative effects on extrudability and workability of 6xxx Al alloys billets. To achieve extruded products with a high surface quality the as-cast billets are  heat-treated before extrusion. During heat treatment the undesired intermetallic particles, i.e., β-AlFeSi platelets are transformed to rounded α-Al(FeMn)Si intermetallic phases. In this  research the formation of the surface segregation for smooth defect-free surfaces in both as-cast and homogenized billets was studied. In addition, the surfaces with defects such as wavy, spot and vertical drag defects were investigated and possible mechanisms for initiation of those defects were explained. Moreover, for a better understanding of the homogenization process in-situ studies of the heat treatment of 6082, 6005, 6060 and 6063 Al alloys were carried out by using a transmission electron microscope (TEM). Based on the observations, an explanation of the probable mechanisms taking place during transformation from β-to α-phase was presented. / <p>Vid tidpunkten för disputationen var följande delarbeten opublicerade: delarbete 5 manuskript, delarbete 6 inskickat och delarbete 7 inskickat.</p><p>At the time of the doctoral defence the following papers were unpublished: paper 5 manuscript, paper 6 submitted, paper 7 submitted.</p>

Cette thèse s’inscrit dans le projet PRINCIPIA (PRocédés INdustriels de Coulée Innovants Pour l'Industrie Aéronautique) de l’ANR MATETPRO (Matériaux et Procédés pour des Produits Performants). L'objectif de ce projet est la promotion de nouveaux alliages aluminium-cuivre-lithium à destination de l'industrie aéronautique afin d'apporter une alternative aux composites. Cependant, ces alliages sont sujet à une importante porosité pour deux raisons : une forte solubilité à l'hydrogène et une facilité d'oxydation. Dans ce projet, le but de la thèse était d'établir un modèle de prédiction de la porosité à l'échelle du produit. La porosité se forme lors de la solidification de l’alliage à cause d'une plus faible solubilité de l'hydrogène dans le solide. La teneur en hydrogène dans la phase liquide va augmenter par ségrégation et provoquer la nucléation des pores. Il est donc nécessaire de prendre en compte la solidification dans la modélisation de la porosité. De plus, la composition locales modifie la cinétique de croissance des pores et la microstructure exerce une contrainte mécanique sur les pores qui modifie leur équilibre chimique. Après une première partie consacrée à améliorer les connaissances sur les phénomène de transport dans la coulée semi-continue d'aluminium, nous avons modélisé la formation de porosité en se basant sur les modèles disponibles. Le modèle a reproduit l'inhomogénéité de la porosité observée expérimentalement sur une plaque d'alliage aluminium-magnésium. L'analyse nous a montré que la limitation de la croissance par le temps de diffusion de l'hydrogène était responsable de ce profil particulier. La densité volumique des pores est critique pour la limitation de la croissance par la diffusion de l’hydrogène. En fonction de la densité, la croissance passe d'une croissance limitée à une croissance non limitée / This thesis is part of the project PRINCIPIA (PRocédés INdustriels de Coulée Innovants Pour l'Industrie Aéronautique) of the ANR MATEPRO (MATériaux Et PROcédés pour des produits performants). The goal of this project is the promotion of new aluminum-copper-lithium alloys for the aeronautic industry in order to propose an alternative to composite materials. Unfortunately, these alloys are highly sensitive to the appearance of porosity during the alloy creation process. It is due to a high hydrogen solubility and oxidation. Inside this project, my work was to establish a porosity model at the scale of the ingot. Porosity starts to develop during the solidification process due to a lower solubility of hydrogen in the solid phase. Hydrogen content in liquid phase increases by segregation and leads to pores' nucleation. Thus, it is necessary to take into account solidification for porosity-modeling purposes. It is even more important because the alloys' local composition alters the pores' growth and the microstructure modifies the chemical equilibrium by pinching effect.After a first part dedicated to general improvement of knowledge about transport phenomena in DC casting, the porosity formation model was developed based on model found in literature. The model was able to reproduce the inhomogeneity experimentally observed in an aluminum-magnesium ingot. This profile is explained by the hydrogen diffusion time which limits the pore growth. The pore density is critical for the growth limitation by hydrogen diffusion. Depending on the density, the growth switch from a non limited to a limited growth.