Showing papers on "Directional solidification published in 2022"

Parallel-GPU-accelerated adaptive mesh refinement for three-dimensional phase-field simulation of dendritic growth during solidification of binary alloy

Abstract: Abstract In the phase-field simulation of dendrite growth during the solidification of an alloy, the computational cost becomes extremely high when the diffusion length is significantly larger than the curvature radius of a dendrite tip. In such cases, the adaptive mesh refinement (AMR) method is effective for improving the computational performance. In this study, we perform a three-dimensional dendrite growth phase-field simulation in which AMR is implemented via parallel computing using multiple graphics processing units (GPUs), which provide high parallel computation performance. In the parallel GPU computation, we apply dynamic load balancing to parallel computing to equalize the computational cost per GPU. The accuracy of an AMR refinement condition is confirmed through the single-GPU computations of columnar dendrite growth during the directional solidification of a binary alloy. Next, we evaluate the efficiency of dynamic load balancing by performing multiple-GPU parallel computations for three different directional solidification simulations using a moving frame algorithm. Finally, weak scaling tests are performed to confirm the parallel efficiency of the developed code.

Processing of directionally cast nickel-base superalloys: solidification and heat treatments

Abstract: After presenting the state-of-the-art Bridgman casting process to obtain single crystalline components, as well as alternative directional solidification techniques, common processing defects (recrystallized grains, freckles, microsegregation, slivers, etc.) are reviewed and their formation mechanisms are discussed. The typical heat treatment sequence applied to Ni-base single crystal superalloy (when needed) is then detailed from both microstructure and mechanical properties points of view. Creep properties of these alloys are shown to be mainly dependent on the heat treatments following investment casting, especially the solution heat treatment. Conversely, fatigue properties are mainly controlled by the solidification processes, through the casting pore size and spatial distribution, when oxidation has no major contribution to the crack initiation mechanisms.

The role of incoming flow on crystallization of undercooled liquids with a two-phase layer

Abstract: Motivated by important applications of crystallization phenomena, we consider a directional solidification process for a binary melt with a two-phase (mushy) layer in the presence of weak melt flow. We consider the steady-state solidification scenario, so that the two-phase layer filled with solid and liquid material keeps its thickness. In addition, we consider that the melt flows onto the two-phase layer slowly in the opposite direction to directional crystallization and solidifies there. A complete analytical solution to non-linear two-phase layer equations is constructed in a parametric form, where the solid phase fraction represents a decision variable. The temperature and solute concentration distributions, mushy layer permeability and average interdendritic spacing as well as solidification velocity and mushy layer thickness are analytically determined. We show that incoming melt flow plays a decisive role on mushy layer parameters and internal structures. The solid phase fraction within the two-phase layer and its thickness essentially grow while the mushy layer permeability and average interdendritic spacing decrease with increasing intensity of incoming melt flow.

Data-Driven Optimization and Experimental Validation for the Lab-Scale Mono-Like Silicon Ingot Growth by Directional Solidification

Abstract: The casting mono-like silicon (Si) grown by directional solidification (DS) is promising for high-efficiency solar cells. However, high dislocation clusters around the top region are still the practical drawbacks, which limit its competitiveness to the monocrystalline Si. To optimize the DS-Si process, we applied the framework, which integrates the growing experiments, transient global simulations, artificial neuron network (ANN) training, and genetic algorithms (GAs). First, we grew the Si ingot by the original recipe and reproduced it with transient global modeling. Second, predictions of the Si ingot domain from different recipes were used to train the ANN, which acts as the instant predictor of ingot properties from specific recipes. Finally, the GA equipped with the predictor searched for the optimal recipe according to multi-objective combination, such as the lowest residual stress and dislocation density. We also implemented the optimal recipe in our mono-like DS-Si process for verification and comparison. According to the optimal recipe, we could reduce the dislocation density and smooth the growth rate during the Si ingot growing process. Comparisons of the growth interface and grain boundary evolutions showed the decrease of the interface concavity and the multi-crystallization in the top part of the ingot. The well-trained ANN combined with the GA could derive the optimal growth parameter combinations instantly and quantitatively for the multi-objective processes.

Ultrahigh-Strength Porous Ceramic Composites via a Simple Directional Solidification Process.

Abstract: Porous ceramics possess great application potential in various fields. However, the contradiction between their pores and their strength have significantly hampered their applications. In this study, we present a simple directional solidification process that relies on its in situ pore forming mechanism to fabricate Al2O3/Y3Al5O12/ZrO2 porous eutectic ceramic composites with a highly dense and nanostructured eutectic skeleton matrix and a lotus-type porous structure. The flexural strength of this porous ceramic composite with a porosity of 34% is 497 MPa at ambient temperature, which is a new record of the strength of all current porous ceramics. This strength can remain at 324 MPa when the temperature increases up to 1773 K because of its refined lamellar structure and strong bonding interface. We demonstrate an interesting application of the directional solidification in efficiently preparing the ultrahigh-strength porous ceramic with high purity. The findings will open a window to the strength of porous ceramics.

Microstructure transition of γ-TiAl alloys with abrupt cross-sections in yttria ceramic moulds during directional solidification by electromagnetic cold crucible

Abstract: Directional solidification of γ-TiAl alloys with abruptly varying cross-sections was performed in yttria molds using an electromagnetic cold crucible. The solid–liquid interface at different growth positions was revealed by quenching using a liquid metal coolant, and the temperature field was mapped with the assistance of a finite element simulation. On the basis of these findings, the process of grain growth was summarized. The columnar–equiaxed transition was found to occur during directional solidification, and a criterion for continuous growth was established, which demonstrates that increasing the withdrawing rate or decreasing the nucleation rate can promote continuous growth. The continuity of columnar grains is confirmed by analysis in orientation evolution which also reconstructs the orientation of α and β grains at elevated temperatures. The degree of directionality of microstructures at elevated temperatures is determined from the angle between the preferential growth direction and the actual growth direction of the β phase. It is found that the degree of directionality increases with increasing withdrawing rate and nucleation rate.

Effect of steel strip feeding on the columnar-equiaxed solidification in a large continuous casting round bloom

Abstract: The steel strip feeding process can effectively improve the macrosegregation, central porosity, shrinkage, cracking, and other defects of continuous casting (CC) round blooms. Based on the volume-averaged method, a three-phase mix columnar-equiaxed solidification model is established, which considers the nucleation, growth of the columnar and equiaxed crystals, deposition of equiaxed crystals, and transmission of solutes. The effect of the feeding strip on the temperature, velocity, phase distribution, and macrosegregation distribution in a large vertical CC round bloom was studied. The results show that the CC bloom shell is built by columnar crystals, and equiaxed crystals fabricate the central zone. The feeding of the steel strip enhanced the flow inside the liquid core, which changed the flow direction of the center and end of the liquid core. The liquid phase and the temperature field display a similar distribution, and the liquid core and V-shaped high-temperature zone will gradually shorten with the increasing feeding ratio. A highly consistent distribution of the phase and the macrosegregation was found, and the increase in feeding ratio promotes columnar-to-equiaxed transformation (CET) and improves macrosegregation. Compared with no feeding strip, the extreme values of the macrosegregation index of 1.0% feeding ratio decreased by 37.1%, and the CET point at the sectional radius increased by 17.39%.

Dynamics at crystal/melt interface during solidification of multicrystalline silicon

Abstract: Abstract A fundamental understanding of crystal growth dynamics during directional solidification of multicrystalline Si (mc-Si) is crucial for the development of crystal growth technology for mc-Si ingots for use in solar cells. In situ observation of the crystal/melt interface is a way to obtain direct evidence of phenomena that occur at a moving crystal/melt interface during growth. In this review, some of the phenomena occurring in the solidification processes of mc-Si are introduced based on our in situ observation experiments, after a brief introduction of the history of the development of crystal growth technologies to obtain mc-Si ingots for solar cells.