[en] Highlights: ► We model a bubble train flow in a long microchannel. ► Bubbles are growing due to pressure drop along a channel and mass transfer. ► We investigate how flow is close to phase equilibrium. ► The smaller the channel diameter and the larger its length, the closer the multiphase system to phase equilibrium. - Abstract: A model of a bubble train flow accompanied with mass transfer in a long capillary tube is developed. In contrast to models presented in literature, our modeling approach accounts for expansion of gas bubbles and flow velocity increase along the channel due to the pressure drop caused by friction losses. The model performance is illustrated by a number of computational examples. The distributions of the bubble velocity and the volumetric mass transfer coefficient along the channels of different diameters are presented. The deviation of the dissolved gas concentration from the saturation concentration along the channel is used as a characteristic of closeness of a fluid system to the phase equilibrium. The calculations explicitly demonstrate that the deviations of gas–liquid mixture flows from equilibrium in long capillary channels of small diameters are small. The effect of the channel diameter, the channel length, and the bubble nucleation frequency on the deviation of the system from equilibrium is also studied.

[en] Recent research and a simulation of heat transfer and solidification during acoustically generated convection showed that the location of the coolest liquid, and thus the place where the first grains are expected to form, is under the sonotrode. Further, the generated vigorous convection produces a very flat temperature gradient in the bulk of the melt facilitating the formation of a refined equiaxed structure throughout the casting. This study validates these findings through a series of experiments on an Al-2 wt pct Cu alloy, which evaluate grain formation under the sonotrode over time and relate this to the formation of the macrostructure of a cast ingot. Analysis of the results confirms the predictions of the simulation and shows that, for the conditions applied, most grains nucleated in the cavitation zone are swept into the melt by acoustically generated convection and, over a period of 70 seconds, the number of grains increase and they grow with spherical and globular morphology gradually filling the casting with refined equiaxed grains. It was found that the macrostructure of each casting is made up of three microstructural zones. A fine grained equiaxed zone forms from the bottom of the casting due to settling of grains during and after termination of ultrasonic treatment (UST), which increases in size with the increasing duration of UST. Above this zone, a coarse-grained structure is formed due to depletion of UST-generated grains on termination of UST. At the top of the casting, a zone of columnar grains growing from the top surface of the melt is formed. The latter two zones decrease in size with the increasing UST duration until 80 seconds, when the macrostructure consists entirely of the equiaxed zone.

[en] Highlights: • Mass transfer from CO₂ to brine is studied mathematically. • CO₂ dissolution is a function of process parameters. • Eliminating the CO₂ leakage risks for geological sequestration. Carbon dioxide ($C{O}_2$) sequestration is considered to be one of the most effective technologies of mitigating greenhouse gas emissions. In this technology, single phase supercritical $C{O}_2$ is injected into an underground geological formation such as a deep saline aquifer. Existing sequestration projects demonstrate that successful implementations are possible; however, significant uncertainties associated with the risks of leakage remain an obstacle for broader use of this technology. The security of underground disposal could be considerably increased by dissolving the $C{O}_2$ in a brine produced from the aquifer, then re-injecting the mixture underground. The dissolution process occurs before the mixture reaches the aquifer; this significantly reduces or completely eliminates the risks of $C{O}_2$ leakage. This technique can drastically extend the amount of worldwide aquifers available for carbon sequestration. As was previously shown, complete dissolution could be achieved in a surface pipeline operating under the pressure of a target aquifer, where $C{O}_2$ is injected. In this paper, a comprehensive model of $C{O}_2$ droplet dissolution in a vertical injection well is presented. The model accounts for droplet breakup, coalescence, and dissolution processes as well as temperature and pressure variations over well depth. Feasibility and results are discussed and compared with surface dissolution options.

[en] Highlights: • Average grain size of 154 μm has been obtained in the Al-V alloys. • Equilibrium primary Al₁₀V particles form in-situ with cooling rate around 3.5 K/s. • HRTEM images show good lattice matching between Al₁₀V particles and Al grains. • Al₁₀V particles have two 3D morphologies: plate and octahedron. • Al₁₀V particles are responsible for the grain refinement via enhanced nucleation. Grain refinement of cast commercial purity aluminium by vanadium and the underlying mechanism have been investigated. Addition of 0.3 wt% and 0.4 wt% vanadium leads to columnar to equiaxed transition and the average grain sizes are refined to around 196 μm and 154 μm, respectively. Pro-peritectic equilibrium Al₁₀V particles are identified near the grain centres. These Al₁₀V particles have either octahedron or plate morphology with the bound planes belonging to {111} crystallographic planes. Three orientation relationships are also determined between the Al₁₀V particles and aluminium grains. Crystallographic analysis based on the experimental orientation relationships indicates that the Al₁₀V particles have relatively high nucleation potency for solid aluminium. Calculation of free growth undercooling based on the size distribution of the Al₁₀V particles reveals that the relatively large size of Al₁₀V particles facilitates the grain initiation of aluminium grains on these particles. Moreover, it is found that the level of vanadium added provides sufficient growth restriction effect in the aluminium melt as quantified by its growth restriction factor. All the three factors, i.e., sufficient potency of Al₁₀V particles, relatively large size of the Al₁₀V particles and adequate growth restriction effect by solute vanadium work in concert to achieve the grain refinement observed in Al-V alloys.

[en] Abstract Using synchrotron X-ray high speed radiography, the fragmentation and refinement of pre-existing primary Al₂Cu intermetallic dendrites induced by ultrasonic melt processing in a hypereutectic Al-35% Cu alloy were studied in-situ and in real time. The alloy was melted, contained and processed in a quartz tube crucible with a middle section of approximately 300 μm-thick channel where the observations were made. Direct observation of intermetallic fragmentation and detachment unambiguously confirms that the acoustic cavitation and streaming flow play a crucial role in fragmentation of the intermetallic dendrites. Furthermore, the remelting effect due to transport of hot liquid via acoustic streaming flow and the stress against the intermetallic dendrites caused by acoustic streaming flow are found to be the dominant fragmentation mechanism in the present experiments. It is also suggested that cavitation bubbles or bubble clouds contribute to fragmentation not only by mechanically fracturing the dendrites but also by facilitating the effect of acoustic streaming flow on dendrites. At last, clear observation of equiaxed intermetallic dendrites growing from small fragments after ultrasonic melt processing provides the first conclusive evidence of the refinement mechanism, i.e. the acoustic cavitation and acoustic streaming flow progressively break the intermetallic dendrites into small fragments. Most of these small fragments are able to survive and then act as nuclei for the subsequent solidification of intermetallic phases, consequently leading to intermetallic refinement in the solidified microstructure.

[en] Billets from an AA4032 alloy are usually produced by direct-chill (DC) casting to subsequently manufacture piston components by hot forging process. This work presents the effect of combined copper (Cu) and erbium (Er) addition on microstructure, mechanical properties and thermal expansion of an AA4032 alloy at room and elevated temperatures. Metallographic examination of samples was carried out to characterize the eutectic refinement, primary Si particles and second phase formation at different levels of Cu and Er additions. The results revealed that the amount of primary Si particles increased with increasing Cu addition from 1% to 3.5%. This indicated that Cu addition shifted the eutectic point in the alloy system. However, the Er addition resulted in complete elimination of primary Si particles and refinement of eutectic silicon phases. These results were supported by Thermo-Calc calculations. With increasing Cu and Er concentration to 3.5% and 0.4%, respectively, the hardness increased from 116 HB to 144 HB. The ultimate tensile strength (UTS), yield strength (YS) and elongation (El) were investigated at both room and elevated temperatures. At room temperature, the UTS of these alloys were enhanced with Cu and Er alloying from 279 MPa to 312 MPa. At 350 °C, the UTS was improved to 117 MPa while El was maintained at about 22%. This indicated that Er addition was effective in optimizing the high-temperature properties. The tensile fracture surfaces of the specimens showed that the main failure mechanism was predominantly due to the cracking of primary Si particles in the Al matrix, resulting in the brittle fracture of the alloys without Er. The fracture surface of the samples with Er addition displayed the path through the refined eutectic phase in the ductile fracture mode. The coefficient of linear thermal expansion (CTE) decreased to about 18.3 × 10⁻⁶ K⁻¹ at high operating temperature (100–350 °C) for the Er containing alloy. Therefore, this study suggests that the combination of Cu and Er additions in an AA4032 alloy controls the beneficial microstructure in terms of primary Si particles, the refined eutectic Si phase and secondary phases in the Al matrix, which improves mechanical and thermal performances. The low coefficient of linear thermal expansion (CTE) of this alloy makes it suitable for elevated temperature applications.

[en] Ultrasound processing of metal alloys is an environmental friendly and promising green technology for liquid metal degassing and microstructural refinement. However many fundamental issues in this field are still not fully understood, because of the difficulties in direct observation of the dynamic behaviours caused by ultrasound inside liquid metal and semisolid metals during the solidification processes. In this paper, we report a systematic study using the ultrafast synchrotron X-ray imaging (up to 271,554 frame per second) technique available at the Advanced Photon Source, USA and Diamond Light Source, UK to investigate the dynamic interactions between the ultrasonic bubbles/acoustic flow and the solidifying phases in a Bi-8%Zn alloy. The experimental results were complimented by numerical modelling. The chaotic bubble implosion and dynamic bubble oscillations were revealed in-situ for the first time in liquid metal and semisolid metal. The fragmentation of the solidifying Zn phases and breaking up of the liquid-solid interface by ultrasonic bubbles and enhanced acoustic flow were clearly demonstrated and agreed very well with the theoretical calculations. The research provides unambiguous experimental evidence and robust theoretical interpretation in elucidating the dominant mechanisms of microstructure fragmentation and refinement in solidification under ultrasound.

[en] The mechanism underlying the considerable refinement of primary Al_3Ti intermetallic particles induced by ultrasonic treatment (UST) in an Al-0.4 wt% Ti alloy in the fully liquid state was investigated. Scanning electron microscopy, energy dispersive X-ray spectroscopy, focused ion beam 3D tomography and transmission electron microscopy were used to clearly identify that α-Al_2O_3 particles were located at or near the centres of primary Al_3Ti particles in the samples solidified with and without UST. Crystallographic evaluation using the edge-to-edge matching model and experimental determination of orientation relationships between the α-Al_2O_3 and primary Al_3Ti particles using the convergent beam Kikuchi line diffraction patterns confirmed the high potency of α-Al_2O_3 particles as nucleation sites for the Al_3Ti phase. Based on the experimental results, the refining mechanism is discussed in terms of proposed hypotheses in the literature. It is suggested that the significant refinement of primary Al_3Ti particles upon UST is due to the cavitation-induced deagglomeration and distribution of the α-Al_2O_3 particles and the cavitation-enhanced wetting of the α-Al_2O_3 particles by liquid aluminium.