Étude de la formation de la peau en coulée continue sans solidification au ménisque. II. Mécanismes de formation des marques

International audienceIn vertical continuous casting, the formation of linear marks observed all around the products – “the ripples” – is known to be due to the partial solidification of the meniscus of the free surface of the liquid metal. On the contrary, in the case of horizontal continuous casting or meniscusfree continuous casting, the presence of ripples or marks on the surface of the cast products still remains mysterious. In fact, the initial solidification against the cold mould occurs far from the free surface of the liquid metal. In order to better identify the solidification mechanisms that are responsible of the ripples appearance on the surface of the horizontal continuous casting products we have built two experimental devices. With the so-called «hot» device, it is possible to obtain spheroidal graphite cast iron (SG iron) samples that have been solidified in the same manner as the products in meniscus-free continuous casting (as far as the formation of the skin is concerned). The “cold” device allows the direct observation of the solidification of organic compounds that are transparent in the liquid state and opaque in the solid state.Pour identifier l'origine des marques de surface en coulée continue sans solidification au ménisque, un travail expérimental a été mené avec deux maquettes de simulation utilisant l'une des alliages métalliques, l'autre des alliages transparents. Après avoir exposé dans un premier article les résultats de l'observation de divers échantillons réalisés, nous présentons ici les résultats de l'observation directe des phénomènes. L'ensemble de ces données permet de proposer un premier modèle microscopique pour la formation des marques

Direct Simulation of a Solidification Benchmark Experiment

International audienceA solidification benchmark experiment is simulated using a three-dimensional cellular automaton-finite element solidification model. The experiment consists of a rectangular cavity containing a Sn-3 wt pct Pb alloy. The alloy is first melted and then solidified in the cavity. A dense array of thermocouples permits monitoring of temperatures in the cavity and in the heat exchangers surrounding the cavity. After solidification, the grain structure is revealed by metallography. X-ray radiography and inductively coupled plasma spectrometry are also conducted to access a distribution map of Pb, or macrosegregation map. The solidification model consists of solutions for heat, solute mass, and momentum conservations using the finite element method. It is coupled with a description of the development of grain structure using the cellular automaton method. A careful and direct comparison with experimental results is possible thanks to boundary conditions deduced from the temperature measurements, as well as a careful choice of the values of the material properties for simulation. Results show that the temperature maps and the macrosegregation map can only be approached with a three-dimensional simulation that includes the description of the grain structure

Experimental and Numerical Modeling of Segregation in Metallic Alloys

International audienceElectromagnetic levitation (EML) has been used as an experimental technique for investigating the effect of the nucleation and cooling rate on segregation and structure formation in metallic alloys. The technique has been applied to aluminum-copper alloys. For all samples, the primary phase nucleation has been triggered by the contact of the levitated droplet with an alumina plate at a given undercooling. Based on the recorded temperature curves, the heat extraction rate and the nucleation undercooling for the primary dendritic and the secondary eutectic structures have been determined. Metallurgical characterizations have consisted of composition measurements using a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometry and the analysis of SEM images. The distribution maps drawn for the composition, the volume fraction of the eutectic structure, and the dendrite arm spacing (DAS) reveal strong correlations. Analysis of the measurements with the help of a cellular-automaton (CA)-finite-element (FE) model is also proposed. The model involves a new coupling scheme between the CA and FE methods and a segregation model accounting for diffusion in the solid and liquid phases. Extensive validation of the model has been carried out on a typical equiaxed grain configuration, i.e., considering the free growth of a mushy zone in an undercooled melt. It demonstrates its capability of dealing with mass exchange inside and outside the envelope of a growing primary dendritic structure. The model has been applied to predict the temperature curve, the segregation, and the eutectic volume fraction obtained upon single-grain nucleation and growth from the south pole of a spherical domain with and without triggering of the nucleation of the primary solid phase, thus simulating the solidification of a levitated droplet. Predictions permit a direct interpretation of the measurements

Combined deformation and solidification-driven porosity formation in aluminum alloys

In die-casting processes, the high cooling rates and pressures affect the alloy solidification and deformation behavior, and thereby impact the final mechanical properties of cast components. In this study, isothermal semi-solid compression and subsequent cooling of aluminum die-cast alloy specimens were characterized using fast synchrotron tomography. This enabled the investigation and quantification of gas and shrinkage porosity evolution during deformation and solidification. The analysis of the 4D images (3D plus time) revealed two distinct mechanisms by which porosity formed; (i) deformation-induced growth due to the enrichment of local hydrogen content by the advective hydrogen transport, as well as a pressure drop in the dilatant shear bands, and (ii) diffusion-controlled growth during the solidification. The rates of pore growth were quantified throughout the process, and a Gaussian distribution function was found to represent the variation in the pore growth rate in both regimes. Using a one-dimensional diffusion model for hydrogen pore growth, the hydrogen flux required for driving pore growth during these regimes was estimated, providing a new insight into the role of advective transport associated with the deformation in the mushy region

Étude de l'effet du mouvement relatif cristal/liquide sur la croissance d'un cristal dendritique dans un liquide en surfusion

Ce travail expérimental vise à mesurer l'impact du mouvement relatif solide / liquide sur la croissance de grains équiaxes en chute libre. Pour cela, nous avons développé un dispositif rendant possible le suivi tridimensionnel et in situ de cristaux équiaxes sédimentant dans un liquide surfondu. Les expériences ont été réalisées avec des solutions transparentes de chlorure d'ammonium - eau (NH4Cl-H2O). Chaque cristal est photographié simultanément dans 2 directions orthogonales tout au long de sa trajectoire. Il est ainsi possible d'observer la morphologie du grain, de mesurer sa taille puis d'en déduire des vitesses de croissance. Les résultats de cette étude montrent que les pointes croissent d'autant plus rapidement qu'elles sont exposées à la convection et que la vitesse relative du cristal par rapport au fluide environnant est élevée

Interactions between solidification and fluid flow. Effects on cast structures and segregations

Natural or forced fluid flow of the bulk melt can induce several changes in the solidification history of a casting : it may change the heat transfer conditions, the as-cast grain distribution, and the segregations. On the other hand, solidification can result in intense fluid flow due to natural solutal convection and in severe local segregations. The two following practical cases illustrate the variety and complexity of interactions between the fluid flow and the solidification of alloys : - the effects of the stirring of the bulk liquid on the formation of the equiaxed zone during the continuous casting of steels, - the formation of freckles and related segregations during the directional solidification of nickel-base alloys. Informations are given about, either new experimental observations, or up-dated physical and numerical models related to each case chosen here as examples. In the case of the influence of stirring on the formation of the equiaxed zone, emphasis is put on the importance of the interaction of the fluid flow with the dendrites in the columnar zone and in the stirred region itself. In the case of the freckles, evidence is brought forward that the dendritic nature and structure of the mushy zone are not the only causes of the dependence of the freckling on the heat transfer conditions during directional solidification