New Grain Formation Mechanisms during Powder Bed Fusion

Tailoring the mechanical properties of parts by influencing the solidification conditions is a key topic of powder bed fusion. Depending on the application, single crystalline, columnar, or equiaxed microstructures are desirable. To produce single crystals or equiaxed microstructures, the control of nucleation is of outstanding importance. Either it should be avoided or provoked. There are also applications, such as turbine blades, where both microstructures at different locations are required. Here, we investigate nucleation at the melt-pool border during the remelting of CMSX-4® samples built using powder bed fusion. We studied the difference between remelting as-built and homogenized microstructures. We identified two new mechanisms that led to grain formation at the beginning of solidification. Both mechanisms involved a change in the solidification microstructure from the former remelted and newly forming material. For the as-built samples, a discrepancy between the former and new dendrite arm spacing led to increased interdentritic undercooling at the beginning of solidification. For the heat-treated samples, the collapse of a planar front led to new grains. To identify these mechanisms, we conducted experimental and numerical investigations. The identification of such mechanisms during powder bed fusion is a fundamental prerequisite to controlling the solidification conditions to produce single crystalline and equiaxed microstructures.

Toward a theory of the evolution of drop morphology and splintering by freezing

The drop freezing process is described by a phase-field model. Two cases are considered: when the freezing is triggered by central nucleation and when nucleation occurs on the drop surface. Depending on the environmental temperature and drop size, different morphological structures develop. Detailed dendritic growth was simulated at the first stage of drop freezing. Independent of the nucleation location, a decrease in temperature within the range from ~ −5 to −25°C led to an increase in the number of dendrites and a decrease in their width and the interdendritic space. At temperatures lower than about −25°C, a planar front developed following surface nucleation, while dendrites formed a granular-like structure with small interdendritic distances following bulk nucleation. An ice shell grew in at the same time (but slower) as dendrites following surface nucleation, while it started forming once the dendrites have reached the drop surface in the case of central nucleation. The formed ice morphology at the first freezing stage predefined the splintering probability. We assume that stresses needed to break the ice shell arose from freezing of the water in the interdendritic spaces. Under this assumption, the number of possible splinters/fragments was proportional to the number of dendrites, and the maximum rate of splintering/fragmentation occurred within a temperature range of about −10 °C to −20°C, in agreement with available laboratory and in situ measurements. At temperatures < −25°C, freezing did not lead to the formation of significant stresses, making splintering unlikely. The number of dendrites increased with drop size, causing a corresponding increase in the number of splinters. Examples of morphology that favors drop cracking are presented, and the duration of the freezing stages is evaluated. Sensitivity of the freezing process to the surface fluxes is discussed.

Влияние начальных условий на скорость фронта ламинарного пламени в газовых смесях

The scatter in laminar flame front speed caused by both an error in the composition of the combustible mixture and initial disturbances is reported. It's shown how the configuration of the initially planar front in laminar flame initial disturbances in a gas mixture of the same composition affects the scatter of speeds of expanding spherical flames. The experimental results previously obtained by the authors, demonstrating the scatter in the speed of the laminar flame front in an initially quiescent gas mixture of constant composition under the same conditions, are explained by integrating the Sivashinsky equation with various initial disturbances. The influence of combustible mixture composition errors on the parameters determining the speed of the flame front is analyzed. These parameters were recalculated for a possible scatter in the mixture composition, obtained based on data on the accuracy of the equipment used in previously published experiments.

Geological Carbon Sequestration by Reactive Infiltration Instability

We study carbon capture and sequestration (CCS) over time scales of 2000 years by implementing a numerical model of reactive infiltration instability caused by reactive porous flow. Our model focuses on the mineralization of CO2 dissolved in the pore water—the geological carbon sequestration phase of a CCS operation—starting 10–100 years after the injection of CO2 in the subsurface. We test the influence of three parameters: porosity, mass fraction of the Ca-rich feldspar mineral anorthite in the solid, and the chemical reaction rate, on the mode of fluid flow and efficiency of CaCO3 precipitation during geological carbon sequestration. We demonstrate that the mode of porous flow switches from propagation of a planar front at low porosities to propagation of channels at porosities exceeding 10%. The channels develop earlier for more porous aquifers. Both high anorthite mass fraction in the solid phase and high reaction rates aid greater amounts of carbonate precipitation, with the reaction rate exerting the stronger influence of the two. Our calculations indicate that an aquifer with dimensions 500 m × 2 km × 2 km can sequester over 350 Mt solid CaCO3 after 2000 years. To precipitate 50 Mt CaCO3 after 2000 years in this aquifer, we suggest selecting a target aquifer with more than 10 wt% of reactive minerals. We recommend that the aquifer porosity, abundance of reactive aluminosilicate minerals such as anorthite, and reaction rates are taken into consideration while selecting future CCS sites.

Viscous fingering and dendritic growth under an elastic membrane

We present an experimental investigation of interfacial fingering instabilities in a compliant channel, where the interface can adopt a planar front orthogonal to the direction of propagation over most of the channel width. Finite-length fingers develop on that front, similarly to the previously studied radial configuration with injection of air at constant flow rate (Pihler-Puzović et al., Phys. Rev. Lett., vol. 108 (7), 2012, 074502), but, unlike the radial case, the interface propagates steadily. This allows us to present the first quantification of the nonlinearly saturated fingering pattern and to demonstrate that the morphological features of the fingers are selected in a simple way by the local geometry of the compliant cell. In contrast, the local geometry itself is determined from a complex fluid–solid interaction. Furthermore, we find that changes to the geometry of the channel cross-section lead to a rich variety of possible interfacial patterns.