Numerical modelling of acoustic streaming during the ultrasonic melt treatment of direct-chill (DC) casting

Acoustic streaming and its attendant effects in the sump of a direct-chill (DC) casting process are successfully predicted under ultrasonic treatment for the first time. The proposed numerical model couples acoustic cavitation, fluid flow, heat and species transfer, and solidification to predict the flow pattern, acoustic pressure, and temperature fields in the sump. The model is numerically stable with time steps of the order of 0.01 s and therefore computationally attractive for optimization studies necessitating simulation times of the order of a minute. The sump profile is altered by acoustic streaming, with the slurry region depressed along the centreline of the billet by a strong central jet. The temperature gradient in the transition zone is increased, potentially interfering with grain refinement. The cooling rate in the sump is also altered, thereby modifying the dendrite arm spacing of the as-cast billet. The relative position of the sonotrode affects the sump profile, with the sump depth decreased by around 5 mm when the sonotrode is moved above the graphite ring level by 100 mm. The acoustic streaming jet penetrates into the slurry zone and, as a result, the growth direction of dendritic grains in the off-centre position is altered

Effect of water temperature and induced acoustic pressure on cavitation erosion behaviour of aluminium alloys

Cavitation erosion is a major challenge for marine and fluid machinery systems. This study investigated the erosion performance of two as-cast aluminium alloys exposed to acoustic cavitation in water at temperatures of 10–50 °C and those were then compared with an extruded wrought alloy tested specifically at the temperature of maximum erosion. The results showed that the as-cast A380 alloy displayed exceptional resistance to cavitation erosion, with the lowest mass loss and surface roughness. This finding suggests that the as-cast A380 alloy is a suitable choice for lightweight, high-performance components in applications where cavitation resistance is critical

Effect of water temperature and induced acoustic pressure on cavitation erosion behaviour of aluminium alloys

Cavitation erosion is a major challenge for marine and fluid machinery systems. This study investigated the erosion performance of two as-cast aluminium alloys exposed to acoustic cavitation in water at temperatures of 10–50°C and those were then compared with an extruded wrought alloy tested specifically at the temperature of maximum erosion. The results showed that the as-cast A380 alloy displayed exceptional resistance to cavitation erosion, with the lowest mass loss and surface roughness. This finding suggests that the as-cast A380 alloy is a suitable choice for lightweight, high-performance components in applications where cavitation resistance is critical

Synthesis and characterization of (Al,Si)₃(Zr,Ti)-D0₂₂/D0₂₃ intermetallics: Understanding the stability of silicon substitution

ABSTRACT: (Al,Si)₃(Zr,Ti)-D0₂₂/D0₂₃ are phases that may form in aerospace and automotive aluminium alloys. The substitution of Zr/Ti in these solid solutions is widely reported in the literature; however, it remains relatively unexplored for Si. In this work, in situ precipitation of (Al,Si)₃(Zr,Ti)-D0₂₂/D0₂₃ intermetallics was performed using Al-Si-Zr-Ti alloys. The precipitation, sedimentation and concentration of numerous intermetallic particles were accomplished by filtrating the residual molten aluminium using a temperature/pressure-controlled vessel adapted with a PoDFA filter. A combination of SEM, TEM, XRD and EMP analysis allowed the identification of (Al,Si)₃(Zr,Ti)-D0₂₂/D0₂₃ intermetallics concentrated within α-FCC matrices of non-Si-doped (sample S2) and Si-doped (samples S4 and S6) alloys. EDS analysis confirmed that Zr and Ti substitute each other in the D0₂₂ and D0₂₃ phases, whereas Si substitutes in Al sites. Acceptance of Si inside the D0₂₃ phase was not expected according to FTlite (FactSage) and TCAL7 (Thermo-Calc) databases. Additionally, Si was found to enhance the formation of (Al,Si)₃(Zr,Ti)-D0₂₂ intermetallics with high Zr-content, contrary to FactSage 7.3 predictions. TEM results showed intermetallic/FCC crystal coherency for samples S2 and S6, implying that these intermetallics acted as nucleation sites for the Al-phase due to their small lattice mismatch. Furthermore, Si site occupancy was calculated for both (Al,Si)Ti-D0₂₂ and (Al,Si)₃Zr-D0₂₃ phases via DFT, showing that sites 2b and 4e are the most favorable for Si occupation, respectively. Finally, a thermodynamic model is derived to describe Si substitution upon solidification. Experimental and numerical examinations indicate that Si substitution preferentially occurs in the D0₂₂ intermetallics compared to the D0₂₃ phase

In situ investigation by X-ray radiography of Microstructure Evolution during Solidification of Binary Alloys

La radiographie X synchrotron ou avec une source microfocus a été appliquée pour étudier différents phénomènes dépendants du temps en relation avec la solidification directionnelle d'alliages Al-Cu. Les effets de la gravité ont été étudiés par comparaison d'expériences sur Terre et en microgravité dans le cadre du projet ESA-MAP XRMON. Les mouvements des fragments sont le sujet majeur de notre étude. Sur Terre, le mouvement des fragments est imposé par la poussée d'Archimède, avec une forte influence des effets de paroi et de la morphologie du fragment, alors qu'en microgravité, la force motrice pour le mouvement des fragments est l'écoulement du fluide interdendritique induit par la contraction du solide. L'effet d'un champ magnétique permanent sur la solidification des grains équiaxes dans un gradient de température a été également étudié. Nous avons montré qu'un couplage entre le gradient de température et le champ magnétique donne naissance à une force Thermo-électromagnétique qui agit sur les grains solides. Une bonne description a été obtenue en utilisant un modèle analytique pour une particule sphérique. Enfin, nous avons étudié l'évolution d'une zone pâteuse dans un gradient de température fixe. Trois régimes successifs ont été identifiés, suivant l'intensité de la diffusion du soluté dans la zone pâteuse et dans le bain fondu. L'analyse quantitative des radiographies par traitement d'image a clarifié le rôle de chaque phénomène de diffusion (TGZM, fermeture des canaux, murissement et diffusion du soluté dans le bain fondu).X-ray radiography with synchrotron and microfocus sources was applied to investigate various time-dependent phenomena related to directional solidification of Al-Cu alloys. Gravity effects were investigated by a comparative study of ground and microgravity experiments in the framework of ESA-MAP XRMON project. Fragment motion was the major subject of our investigation. On Earth, fragmentation motion was imposed by buoyancy, with a strong dependency on wall influence and fragment morphology, whereas in microgravity conditions, the driving force for fragment motion is the interdendritic fluid flow induced by the solid shrinkage. The effect of a permanent magnetic field on the solidification of equiaxed grains in a temperature gradient was also studied. We have shown that a coupling between the temperature gradient and the magnetic field gives birth to a Thermo-Electro-Magnetic force that acts on the solid grains. A good description was obtained by using an analytical model for a spherical particle. Finally we studied the evolution of the mushy zone in a fixed temperature gradient. Three successive regimes were identified, depending on the relative magnitude of solute diffusion in the mushy zone and in the bulk liquid. Quantitative analysis of radiographs by image processing enlightened the role of each diffusion phenomena (TGZM, channel closure, coarsening and solute diffusion in the bulk liquid)

Characterization of Motion of Dendrite Fragment by X-Ray Radiography on Earth and under Microgravity Environment

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