Résumé: Immiscible Al–Sn–Cu alloys may offer attractive properties, attaining superior tribological and mechanical properties when Sn-rich soft particles are homogeneously distributed in the reinforced Al–Cu matrix. In this paper, the solidifications of both Al-10 wt.% Cu-10 wt.% Sn and Al-10 wt.% Cu-20 wt.% Sn alloys were investigated to analyze the successive stages that occur during the controlled cooling of these alloys, from the initial formation of the α-Al dendritic array to the final eutectic reaction. In particular, we focus on the liquid-phase demixing occurring during the solidification path, which leads to the formation of Sn droplets in the melt through a nucleation-growth process. Horizontal directional solidifications were performed on thin samples in a Bridgman-type furnace, with in situ and real-time observations using X-ray radioscopy. Two different behaviors have been found concerning liquid separation: for the low-Sn-content alloy, liquid demixing occurs in one single step, whereas for the high-Sn-content alloy, it is a two-step process, with first the nucleation of a few small Sn droplets followed by a sudden formation of a large amount of wide Sn droplets. The possible causes of these different behaviors are discussed in relation to the literature, namely, either a switch from immiscible to miscible liquids or a transition from the binodal region to the spinodal region.

Résumé: The increasing importance of Al-Cu-Sn alloys as materials for producing self-lubricating bearing materials in automotive industries requires the development of uniform microstructures with improved performance, which could be achieved by a precise control of solidification processes. Adding Sn to Al-Cu binary alloy could dramatically alters the solidification path, generating liquid phase separation and monotectic reaction. In this paper, Al-10 wt% Cu-X wt% Sn (with X = 0; 5; 10 and 20) alloys have their solidification paths investigated by three different complementary approaches. Firstly, the solidification paths were calculated by using the CALPHAD method through Thermo-Calc software revealing the sequence of transformations that could occur during the solidification of these alloys, from the formation of the initial α-Al dendrites to the final eutectic reaction. Secondly, experimental thermal analysis was carried out by DSC (Differential Scanning Calorimetry), which reveals the successive events that occur during the controlled melting and cooling of these alloys. Thirdly, directional solidification was performed on all alloys, with in situ and real-time observations achieved through the utilization of X-radiography. The comparison between the various approaches showed a suitable correspondence between the history of the alloy solidifications computed by Thermo-Calc and those obtained experimentally (DSC and directional solidification experiments). Additional information about the dynamics of each reaction was obtained during the directional solidification experiments in terms of microstructures, segregation, and the genesis of the liquid phase separation that occurred for high Sn-content alloys.

Résumé: Three series of directional solidification experiments on refined Al-20wt.%Cu alloys have been carried out with different temperature gradients, and for each of them a wide range of cooling rates were applied. The experiments were performed in horizontal configuration to minimize the impact of gravity-driven phenomena and characterized in situ and in real-time by using the X-radiography technique. The influence of the temperature gradient on the microstructure formation (impact of Temperature Gradient Zone Melting: TGZM), the nucleation distance, the average grain size and morphology (elongation factor and grain orientation) have been analysed quantitatively. The experimental results are discussed with current theoretical models and similar experimental works.

Résumé: Gravity effects such as natural convection in the liquid phase and buoyancy forces acting on the solid phase have a strong influence on the grain structure and microstructure formation dynamics during the solidi fication of metal alloys. It is thus very useful to undertake experimental studies that will provide benchmark data for a deeper understanding of the role of such gravity effects. In this paper, we study the formation of the equiaxed grain structure during refined Al-20wt.%Cu solidification in a temperature gradient for three different configurations: horizontal, vertical upward and vertical downward solidification. The key grain characteristics, namely grain size, grain elongation and grain growth orientation, were determined for all experiments. A comparative analysis was performed to identify the dominant effects of gravity using the experiment in horizontal configuration as reference case. The impact of buoyancy on the grain structure formation was highlighted for the experiment in vertical upward configuration, inducing a final grain structure with a wider grain size distribution. For the experiment in vertical downward configuration, the final grain structure is composed of thinner and longer grains. The origin of these differences was linked to the impact of grain flotation and solute flows on the equiaxed microstructure development.

Résumé: Solidification experiments on Al–20 wt.%Cu alloys with grain refiner were performed in a Bridgman-type furnace to investigate the impact of gravity-driven phenomena on the Columnar-to-Equiaxed Transition (CET) during directional solidification (i) in microgravity, on board the sounding rocket MASER-14 and (ii) on Earth in three growth directions, namely horizontal, vertical upward (counter-gravity direction) and vertical downward (in-gravity direction). The CET was provoked by a step increase of the cooling rate and was visualised in situ and in real-time using X-radiography. This paper reports direct observations of dendritic columnar growth, CET and the subsequent equiaxed regime, and quantitative characterisation of the microstructures. The increase in constitutional undercooling that induces the nucleation of the first grains ahead of the columnar front can be related to the rapid thermal change following the increase in cooling rate. The nucleation distance of equiaxed grains ahead of the columnar front is larger in microgravity than on Earth and possible origins for this difference are discussed. Both mechanical and solutal blocking mechanisms were observed at CET. Mechanical blocking is attributed to the occurrence of solidification-induced shrinkage flow. The relative importance of each blocking mechanism is detailed for each growth configuration. In particular, mechanical blocking is shown to be the principal type of impingement in the microgravity experiment. The final equiaxed grain structure was characterised, and the results confirm that gravity effects become less significant at high growth rates.

Résumé: The paper presents detailed analyses of solidification experiments performed on a refined Al-20 wt.%Cu alloy using the SFINX (Solidification Furnace with IN situ X-radiography) laboratory facility. Directional solidifications of a sheet-like sample were carried out in a horizontal configuration, with the main surface of the sample parallel to the ground. The sample was solidified for a wide range of cooling rates to obtain various grain structures, from columnar to elongated and equiaxed. The formation of the grain structure was observed in-situ and in real-time by X-radiography, which allows the dynamic of solidification phenomena to be thoroughly analyzed. Based on the radiographs, quantitative measurements were performed to accurately describe the solidified grain structure, namely the nucleation position, nucleation rate, grain size, grain elongation factor and growth orientation. The experiments showed that increasing the growth velocity leads to a decrease of both the grain size and grain elongation factor, resulting in a more homogeneous and isotropic grain distribution. The grain characteristic parameters were also sensitive to variations of the temperature gradient in the Field-of-View. The results were discussed by analyzing the impact of the solidification parameters on the constitutionally undercooled liquid zone ahead of the solidification front.

Résumé: On earth, gravity-related phenomena are unavoidable, such as thermo-solutal convection caused by density gradients in the melt and buoyancy when the liquid phase is denser than the solid phase. Such phenomena can drastically affect both the grain density and their morphology during equiaxed solidification processes. For these reasons, fundamental studies comparing the influence of solidification parameters with and without gravity effects are important to obtain benchmark data, which are useful to understand and then control the final structure of materials in industrial processes. In the present work, the impact of the solidification parameters on the dendritic grain structure formation and on the final grain size and shape was investigated in situ by using X-radiography for different growth orientations with respect to gravity. In a first step, experiments were carried out with various solidification parameters and with the furnace in horizontal position, with the main surface of the sample being perpendicular to gravity to limit gravity-related phenomena. In a second step, experiments were carried out with identical solidification parameters but with the furnace in a vertical position, and for two solidification directions (upward and downward). A comparative study between horizontal and vertical experiments was carried out. Phenomena related to gravity have been highlighted and their respective impact on the solidification front propagation was analysed.

Résumé: The performances of structural metallic materials are largely associated to the solidification microstructures, which are strongly dependent on gravity effects, namely natural convection in the melt and buoyancy forces acting on solid elements (grains or dendrite fragments) surrounded by the liquid phase. For this reason, fundamental studies investigating the influence of gravity phenomena on solidification process are required to obtain benchmark data, which are mandatory to understand and then control the final structure of materials in industrial processes. This contribution presents a summary of experimental results obtained for directional solidification of Al-20wt.%Cu alloys within the framework of the ESA-MAP entitled XRMON (in situ X-Ray monitoring of Advanced metallurgical processes under microgravity and terrestrial conditions). The impact of gravity on the dynamics of the solidification process was investigated in situ by using Xradiography for different growth orientations with respect to gravity (horizontal, upward and downward solidifications). A comparative study between the different growth orientation experiments was carried out, that enabled us to highlight the gravity effects on the grain structure formation during the solidification.

Résumé: Mechanical properties of materials are deeply related to the solidification microstructure, which results from the solidification conditions such as the thermal history and gravity-related phenomena. The understanding of the relationship between the final grain structure and the processing conditions is thus crucial to improve industrial products. In this work, we will present a study of the gravity effects on the dynamic of the microstructure formation. It is well known that gravity-related phenomena such as thermo-solutal convection caused by density gradients in the melt, and buoyancy when the liquid phase is denser than the solid phase or vice-versa, are unavoidable and can affect drastically both the grain number and their morphology. The aim of our study is to investigate directional solidification experiments of refined Al-20wt.%Cu alloys, using in-situ and real time X-radiography. Experiments were carried out on sheet-like samples (thickness ≈ 250 μm) in the laboratory device SFINX (Solidification Furnace with IN situ X-radiography), for two different furnace orientations with respect to the gravity direction: - Horizontal orientation where the main surface of the sample is perpendicular to the gravity direction. For this configuration the gravity related phenomena are limited in the thickness of the sample. - Vertical orientation where the main surface is parallel to the gravity direction. For this position, the temperature gradient can be either parallel to gravity (upward solidification) or antiparallel to gravity (downward solidification). The gravity effects are significant in the liquid phase of the sample for this configuration. The comparison of the grain formation mechanisms during the experiments performed with the same solidification parameters (growth velocity and temperature gradient), but with the two different configurations (horizontal and vertical) allows us to reveal the different effects related to the presence of gravity and that affect the grain structure. The main phenomena highlighted from this comparison and that affect drastically the grains structure formation are the grain floatation during the upward solidification due to the buoyancy force, and the Cu-enriched plume formation during the downward solidification.

Résumé: To study the impact of growth rate on the grain structure, an in-situ study of solidification of refined Al-20wt.%Cu alloy at a fixed temperature gradient and under a wide range of cooling rates has been carried out using SFINX (Solidification Furnace with in situ X-radiography) laboratory facility. A special attention has been paid to the variation of the grain number density and the dendritic grain morphology. It is revealed that, as the growth rate increases, the nucleation rate augments and the grain morphology evolve from elongated grain towards isotropic equiaxed grain.

Résumé: The properties of materials are related to the solidification microstructure, which results from the solidification conditions such as the thermal conditions and gravity-related phenomena. The understanding of the relationship between the grain structure and the processing conditions is crucial to improve industrial products. In this work, gravity effects (thermo-solutal convection and buoyancy) on the dynamic of the microstructure formation will be presented. Gravity effects are unavoidable and can affect both the grain number and its morphology. This study investigates directional solidification experiments of refined Al-20wt.%Cu alloy, using in-situ and real-time X-radiography. Experiments were performed on sheet-like samples of 250 in thickness, in SFINX (Solidification Furnace with IN situ X-radiography) laboratory device, for two different furnace orientations with respect to the gravity direction: 1) Horizontal orientation: where the main surface of the sample is perpendicular to the gravity direction. For this configuration, the gravity-related phenomena are confined to the sample thickness. 2) Vertical orientation: where the main surface is parallel to the gravity direction. This orientation can take two configurations depending on the temperature gradient direction (upward or downward solidification). For those configurations, gravity effects are significant in the sample liquid phase. The comparisons of the grain formation mechanisms during the experiments performed with the same solidification parameters and in different configurations reveal the gravity-related effect that affect the grain structure. The highlighted phenomena are the grain floatation during the upward solidification due to the buoyancy force, and the Cu-enriched plume formation during the downward solidification.