Grain Structure Model Research Articles

Combining phase field modeling and deep learning for accurate modeling of grain structure in solidification

Additive manufacturing by wire deposition is a complex process as it generates overlapping and transient thermal fields, resulting in multiple cycles of solidification and remelting. Consequently, simulating the microstructure becomes challenging, making it difficult to optimize the process and predict the mechanical properties of an additive-manufactured part. In this work, a framework is developed, including several sequentially applied methods to obtain increasingly accurate information about the microstructure resulting from a wire deposition process. This framework is based on understanding experimental data and utilizing them as inputs to simplify the problem or accelerate numerical simulations. The experimental inputs consist of light optical images, SEM, and EBSD images. The methods developed here are initially geometric-based and then progress to shape recognition, unsupervised learning, and phase field modeling.

Sparse thermal data for cellular automata modeling of grain structure in additive manufacturing

Grain growth in the wake of the melt pool formed during alloy-based additive manufacturing (AM) is complex and multifaceted, depending on parameters governing heat transport, fluid flow, and solidification itself. Cellular automata (CA) models have proven effective in providing computationally efficient and physically sound predictions of grain structure for several AM problems, but their efficiency is tied to the performance of heat transport models. CA models use only a small portion of the problem’s temperature data (near the moving melt pool boundary), and much of the CA calculations do not affect the final result due to re-melting of material. Coupling of and communication between heat transport and solidification models, and eliminating operations irrelevant towards final grain structure prediction, will be necessary for using these methods for efficient simulation of large parts. We introduce a procedure of decoupling the CA from temperature field simulation, using files of relevant temperature data written by the heat transport model. This approach is validated against the standard coupling approach using data obtained through the computational fluid dynamics software OpenFOAM. Negligible differences are seen in grain size, volume, and texture distributions for multilayer simulation of test problems, while the quantity of temperature data for these test problems was reduced by four orders of magnitude (from 100s of GB to 10s of MB) and the code performance sped up by a factor of around 50. Variability in microstructure as a function of cell size, substrate, time step, and nucleation parameters is studied, and it is found that cell sizes less than or equal to 1.67 μm and sufficiently small time steps yield statistically equivalent microstructures. Finally, a potential use case for this CA approach—the layer-wise convergence in grain structure starting from extremes in initial grain size—is examined. This approach’s ability to simulate expected trends in nucleation and epitaxial grain growth for large regions of microstructure, simulated independently of heat transport models themselves, should prove useful for investigation of various microstructure uncertainties and prediction of part-scale experimental results.

Transient Modeling of Grain Structure and Macrosegregation during Direct Chill Casting of Al-Cu Alloy

Grain structure and macrosegregation are two important aspects to assess the quality of direct chill (DC) cast billets, and the phenomena responsible for their formation are strongly interacted. Transient modeling of grain structure and macrosegregation during DC casting is achieved with a cellular automaton (CA)–finite element (FE) model, by which the macroscopic transport is coupled with microscopic relations for grain growth. In the CAFE model, a two-dimensional (2D) axisymmetric description is used for cylindrical geometry, and a Lagrangian representation is employed for both FE and CA calculations. This model is applied to the DC casting of two industrial scale Al-6.0 wt % Cu round billets with and without grain refiner. The grain structure and macrosegregation under thermal and solutal convection are studied. It is shown that the grain structure is fully equiaxed in the grain-refined billet, while a fine columnar grain region and a coarse columnar grain region are formed in the non-grain-refined billet. With the increasing casting speed, grains become finer and grow in a direction more perpendicular to the axis, and the positive segregation near the centerline becomes more pronounced. The increasing casting temperature makes grains coarser and the negative segregation near the surface more pronounced.

High-Performance Copper Oxide Visible-Light Photodetector via Grain-Structure Model

Recently, copper oxide (CuO)-based visible-light photodetectors have attracted great interest due to their narrow bandgap (1.2 eV), low cost, and ease of fabrication. However, there has been insufficient theoretical analysis and study of CuO-based photodetectors, resulting in inferior performance in terms of responsivity, detectivity, and response speed. This work develops a method to enhance the performance of CuO photodetectors by engineering a grain structure based on a newly-developed theoretical model. In the developed theoretical grain-structure model, the grain size and the connections between grains are considered because they can strongly affect the optoelectronic characteristics of CuO photodetectors. Based upon the proposed model, the engineered CuO device achieves enhanced optoelectronic performance. The engineered device shows high responsivity of 15.3 A/W and detectivity of 1.08 × 1011 Jones, which are 18 and 50 times better than those of the unoptimized device, and also shows fast rising and decaying response speeds of 0.682 s and 1.77 s, respectively. In addition, the proposed method is suitable for the mass-production of performance-enhanced, reliable photodetectors. By using a conventional semiconductor fabrication process, a photodetector-array is demonstrated on a 4-inch wafer. The fabricated devices show uniform, high, and stable optoelectronic performance for a month.

Direct Macroscopic Modeling of Grain Structure and Macrosegregation with a Cellular Automaton–Finite Element Model

Grain structure and macrosegregation are two main factors determining mechanical properties of components and are strongly coupled during alloy solidification. A two-dimensional (2D) cellular automaton (CA)–finite element (FE) model is developed to achieve a direct macroscopic modeling of grain structure and macrosegregation during the solidification of binary alloys. With the conservation equations of mass, momentum, energy, and solute solved by a macroscopic FE model and the grain structure described by a microscopic CA model, a two-way coupling between the CA and FE models is applied. Furthermore, the effect of the fluid flow on the dendrite tip growth velocity is considered by modified dendrite tip growth kinetics. The CAFE model is applied to a quasi-2D benchmark solidification experiment of a Sn–3.0wt.%Pb alloy, and the grain structure and macrosegregation are predicted simultaneously. It is demonstrated that the model has a capacity to describe the undercooling ahead of the growth front. The growth directions of columnar grains, grain sizes, and columnar-to-equiaxed transition (CET) position are obviously modified by the fluid flow, and obvious segregated channels almost aligned with the orientations of the columnar grains are found. Qualitatively good agreement is obtained between the predicted segregation profiles and experimental measurements.

Numerical simulation of flexible micro-bending processes with consideration of grain structure

A finite element model of the flexible micro-bending process based on various grain sizes of pure copper is developed. The geometrical model of grain structure is established with Voronoi tessellation, which is employed to describe the polycrystalline aggregation. A model based on dislocation density is adopted to describe the flow stress of grain interior (GI) and grain boundary (GB) quantitatively. In this paper, silicon rubber is used as the flexible punch and four annealing conditions of pure copper as the workpieces, respectively. The influence of grain structure and grain size is discussed. It is observed that as the ratio of workpiece thickness (t) to grain size (d) decreases, the forming depth increases. The inhomogeneous deformation occurs in the coarse-grained micro-parts. Furthermore, the results indicate that the surface asperity increases with grain size. The numerical simulation results agree well with the tendency of experimental results. During the micro-bending process, the phenomenon of stress concentration occurs at the grain boundary of the micro-parts. The maximum von mises stress appears at the grain boundary located at the fillet position. The maximum von mises plastic strain primarily concentrates on the junction of the grain interior and grain boundary in the fine-grained parts, while it concentrates at the surface of the grain interior in the coarse-grained parts.

OS4-4 Image-base Modeling of Grain Structure in Aluminum Alloy Based on X-ray Microtomography(3D/4D image-based analyses and simulations 2,OS4 3D/4D image-based analyses and simulations,MEASUREMENT METHODS)

OS4-4 Image-base Modeling of Grain Structure in Aluminum Alloy Based on X-ray Microtomography(3D/4D image-based analyses and simulations 2,OS4 3D/4D image-based analyses and simulations,MEASUREMENT METHODS)

Modeling of Grain Structure and Heat-Affected Zone in Laser Surface Melting Process

A combination of phase-field and cellular automata methods is used to study the effect of initial grain size and laser power density on heat-affected zone (HAZ) formation during laser surface melting. Also, an analytical model is developed to estimate the depth of HAZ as a function of initial grain size and process parameters. Both analytical and numerical results indicate that the size of HAZ, as measured with respect to the changes in the grain structure, is inversely proportional to the initial grain size. They also show how increasing the laser power leads to an increase in the extent of HAZ. The proposed models thus provide a basis for the prediction and control of HAZ in laser surface melting.

Modeling of Dual-Phase Grain Structure in Ti-6Al-4V during Isothermal and Non-Isothermal Heat Treatment by Using Cellular Automata

In the titanium alloy Ti-6Al-4V the dual-phase grain structure, which forms during thermo-mechanical processing, is of high importance due to its effect on the mechanical properties. In general the most significant microstructural parameters are the amount of alpha and beta phase as well as their grain size. For this reason a new cellular automata method (CA) was developed to predict the evolving grain structure during isothermal and non-isothermal heat treatment. The probabilistic CA model is based on the diffusion controlled movement of grain and phase boundaries. During temperature changes an algorithm is adjusting alpha and beta phase fraction to maintain equilibrium phase values. Hence, the CA is capable to calculate grain coarsening as well as grain growth and shrinking in the two-phase area while heating and isothermal holding at forging temperature. The initial microstructure can be imported form virtual created microstructures, real micrographs and EBSD-images. The results are mean grain diameters, grain size distributions and virtually simulated microstructures which can be easily compared with real micrographs. The predicted microstructures are showing a good correlation to data in literature and experimental results.

Large cometary grains - their destruction and changes in the luminosity of comets

The catastrophic thermodynamic destruction of large cometary heterogeneous grains lying on the surface of a comet nucleus is examined. The core–mantle grain-structure model is assumed. Grain fragmentation as an explanation of sudden changes in cometary brightness is proposed. The approach presented to the problem of cometary outbursts is a development of a previous author's paper. The proposed mechanism is based on the idea of thermodynamical destruction of heterogeneous cometary grains. Numerical simulations have been carried out for a wide range of values of physical characteristics of cometary material. The results obtained are consistent with observational data. The main conclusion of this paper is that thermodynamical fragmentation of large grains can explain variations in brightness and also outbursts of comets.

Columnar-EquiaxedTransition inSolidification processing: The ESA-MAP CETSOL project

Many castings are the result of a competition between the growth of columnar and equiaxed grains. Indeed, microstructures are at the center of materials science and engineering, and solidification is the most important processing route for structural materials, especially metals and alloys. Presently, microstructure models remain mostly based on diffusive transport mechanisms so that there is a need of critical benchmark data to test fundamental theories of microstructure formation, which often necessitates to have recourse to solidification experiments in the reduced-gravity environment of space. Accordingly, the CETSOL (Columnar-Equiaxed Transition in SOLidification processing)-MAP project of ESA is gathering together European groups with complementary skills to carry out experiments and model the processes, in particular in view of the utilization of reduced-gravity environment that will be afforded by the International Space Station (ISS) to get benchmark data. The ultimate objective of the CETSOL research program is to significantly contribute to the improvement of integrated modeling of grain structure in industrially important castings. To reach this goal, the approach is devised to deepen the quantitative understanding of the basic physical principles that, from the microscopic to the macroscopic scales, govern microstructure formation in solidification processing under diffusive conditions and with fluid flow in the melt. Pending questions are attacked by well-defined model experiments on technical alloys and/or on model transparent systems, physical modeling at microstructure and mesoscopic scales (e.g. large columnar front or equiaxed crystals) and numerical simulation at all scales, up to the macroscopic scales of casting with integrated numerical models.

Device simulation and modeling of microcrystalline silicon solar cells

Abstract Device modeling for p–i–n junction basis thin film microcrystalline Si solar cells has been examined with a simple model of columnar grain structure utilizing two-dimensional device simulator. The simulation results of solar cell characteristics show that open-circuit voltage ( V oc ) and fill factor considerably depend on structural parameters such as grain size and acceptor doping in intrinsic layer, while short-circuit current density ( J sc ) is comparatively stable by built-in electric field in the i-layer. It is also found that conversion efficiency of more than 16% could be expected with 1 μm grain size and well-passivated condition with 10 μm thick i-layer and optical confinement.

2D-numerical analysis and optimum design of thin film silicon solar cells

Device modeling for p–i–n junction μc-Si basis thin film polycrystalline Si solar cells has been examined with a simple model of columnar grain structure and its boundary condition utilizing two-dimensional device simulator. As the simulation results of solar cell characteristics show, open-circuit voltage ( V oc) and curve fill factor (FF) considerably depend on those structural parameters, while short-circuit current density ( J sc) is comparatively stable by courtesy of homogeneous built-in electric field in the i layer. It has also been found that conversion efficiency over 12% could be expected with 1 μm grain size and well-passivated condition with 3 μm thick i-layer.

A stereological model of the degree of Grain Boundary-Pore Contact during Sintering

A simple geometric model of pore and grain structure has been used to explain the experimentally observed constancy of the degree of grain boundary-pore contact,R, throughout intermediate and final stage sintering. As modeled, constancy ofR in the final stage requires a balance among the number of pores per grain, the ratio of the pore and grain sizes, and the relative frequencies of pore locations at grain corners, edges, and surfaces. Experimentally, these location frequen- cies were found to remain constant for both pure and MgO-doped A12O3, over the observed range from 91 to 99.8 pct of full density. The number of pores per grain increased and the pore/grain size ratio decreased over this period for both materials. Employing experimental microstructural data for the above parameters in the model yielded the predicted linear plots, with intercepts givingR values very close to the values measured by direct stereological means. The model also yielded reasonable, constant values ofR for intermediate stage sintering and explains the constancy ofR throughout the transition from intermediate to final stage.

Model for the low-temperature transport of Bi-based high-temperature superconducting tapes.

A grain-structure model is used to obtain the low-temperature transport properties of polycrystalline Bi-based high-temperature superconducting tapes, including the magnetic-field-dependent critical current. The grain structure is regarded as resembling a brick wall, such that the net horizontal supercurrent passes from brick to brick chiefly through the horizontal junctions between bricks. A high-field critical-current plateau is predicted, assuming inhomogeneous Josephson junctions between highly anisotropic superconducting grains.