Simulated Microstructures Research Articles

Integrated fatigue life predictions of aluminium castings using simulated local microstructure and defects

In this work, an integrated simulation approach previously developed for static FE analyses is extended to microstructure- and defect-based fatigue life assessments of castings. The approach, the closed chain of simulations for cast components, combines casting process simulation with microstructure modelling and local material characterisation to generate heterogeneous material data for FE analysis and fatigue life assessment. The method is demonstrated on a High-Pressure Die Cast aluminium component. Areas with a high risk of defects are identified based on the simulated solidification conditions, and heterogeneous material data for the fatigue life analysis is generated. Fatigue testing has been performed with different levels of porosities to quantify the effect of defects on the element-specific Wöhler curves. Pore characteristics are assessed using 2D X-ray, fracture surface analysis and Kitagawa diagram. The results highlight the importance of taking the risk of defect formation into consideration when designing industrial aluminium castings subjected to fatigue loads.

A phase field methodology for simulating the microstructure evolution during laser powder bed fusion in-situ alloying process

A recently-developed [1] multi-component phase field model has been utilized to investigate microstructure evolution during in-situ alloying of a blended elemental Ti-1Al-8V-5Fe alloy powder via the Laser Powder Bed Fusion process. The process of in-situ alloying, where elemental powder is used instead of pre-alloyed powder, was studied by performing two simulations having: (1) a uniform initial composition, and (2) a spatially varying initial composition to represent different powder particles. Specifically, the grain morphology, solute distribution, competitive growth and nucleation under the two different scenarios were simulated and compared. To assist the microstructure simulations, a macro-scale finite element model was developed to simulate the heat transfer during LPBF process. The thermal history data calculated by the finite element model was provided to the phase field model in order to simulate transient dendritic growth behaviour. The results show that a set of evenly-spaced columnar dendrites form in the uniform initial composition case, whereas when the initial composition is spatially varying, non-uniform dendrites having elongated shape can develop. It is also shown that competitive growth between dendrites is influenced by nucleation. For the spatially varying initial composition case, the results indicate that full alloying is difficult to achieve during the LPBF printing process; this incomplete alloying greatly influences the dendrite morphology and solute distribution.

A 3D Multiscale Model for Prediction of the Microstructure Evolution in Ti6Al4V Components Produced by Laser Powder Bed Fusion

A transient 3D multiscale model has been created to simulate the evolution of solidification microstructures rapidly and effectively in Ti6Al4V parts produced through Laser Powder Bed Fusion (LPBF). The microstructure simulation tool has been enhanced to account for rapid solidification conditions in Ti6Al4V alloys during processing of multi-layer multi-track LPBF parts. The simulation tool can evaluate the impact of part geometry, lase power input, laser speed, and laser beam shape on the formation of the microstructure in LPBF-processed Ti6Al4V alloy. The multiscale model considers several factors, including preferential crystallographic growth direction, isomorphism, epitaxy, melt pool motion, and temperature gradients to generate the observed texture and morphology of the microstructure in Ti6Al4V components. The model can evaluate various microstructure characteristics, such as grain size and texture. Consequently, it could aid in controlling the formation of solidification microstructures in Additive Manufacturing (AM) processed parts. The simulation tool has been previously validated by using IN625 laser remelting experiments made by National Institute of Standards and Technology. The 3D simulation tool can also be utilized to predict the microstructure formation in Ti6Al4V components produced by the Electron Beam AM processes.

Microstructure simulation of AA2219 alloy in hot/warm forming and heat treatment using cellular automata methods

Microstructure simulation of AA2219 alloy in hot/warm forming and heat treatment using cellular automata methods

A viscoplastic-coupled phase field modelling for mechanical behaviors of thermal barrier coating system with randomly microporous structures

In this study, we have introduced a novel viscoplastic-coupled phase field approach to simulate the deformation and failure progression of thermal barrier coating systems (TBCs) with disordered microporous structures. The theory has been incorporated in the finite element method, for the first time, to characterize crack initiation and propagation in TBCs, considering both time-dependent deformations and tension–compression asymmetry mechanical behaviors. Furthermore, we have proposed a numerical framework for failure modeling of porous solids, which includes the quantitative characterization of microstructures, the parametric finite element modeling, and the visualization of calculation outcomes. We have performed several simulations of different microstructures, which have been compared against experimental data, demonstrating the predictive capabilities of our method in intricate structures. Our findings have revealed that cracks tend to emerge from nearby micropores, and an increase in porosity can lead to earlier crack initiation and a decrease in TBCs strength. Furthermore, micropores in the same horizontal line tend to cause cracks to propagate more easily, resulting in a substantial reduction in strength. For TBCs exposed to cyclic loading and thermal growth oxidation (TGO) growth stress, interfacial cracks are primarily caused by the deflection of cracks in the top coat (TC) layer. The method presented in this study offers a fresh perspective on the failure mechanism of solids with complex microstructures and contributes to the mechanical improvement of TBCs based on structural considerations.

Formula omitted]-CP: Open source dislocation density based crystal plasticity framework for simulating temperature- and strain rate-dependent deformation

This work presents an open source, dislocation density based crystal plasticity modeling framework, ρ-CP. A Kocks-type thermally activated flow is used for accounting for the temperature and strain rate effects on the crystallographic shearing rate. Slip system-level mobile and immobile dislocation densities, as well slip system-level backstress, are used as internal state variables for representing the substructure evolution during plastic deformation. A fully implicit numerical integration scheme is presented for the time integration of the finite deformation plasticity model. The framework is implemented and integrated with the open source finite element solver, Multiphysics Object-Oriented Simulation Environment (MOOSE). Example applications of the model are demonstrated for predicting the anisotropic mechanical response of single and polycrystalline hcp magnesium, strain rate effects and cyclic deformation of polycrystalline fcc OFHC copper, and temperature and strain rate effects on the deformation of polycrystalline bcc tantalum. Simulations of realistic Voronoi-tessellated microstructures as well as Electron Back Scatter Diffraction (EBSD) microstructures are demonstrated to highlight the model’s ability to predict large deformation and misorientation development during plastic deformation.

Phase field theory for pressure-dependent strength in brittle solids with dissipative kinetics

A phase field theory of fracture mechanics is formulated to address dependence of material strength on stress state, noting that brittle solids are usually more fracture resistant when mean stress is compressive. Fracture is addressed via a dissipative kinetic law which instills rate dependence and viscous resistance to propagation. A dependence of kinetic viscosity on triaxiality for locally compressive stress states is proposed. Solutions are derived to one-dimensional problems for an isotropic material, linear or nonlinear elastic, under different loading protocols: uniaxial stress extension, uniaxial stress compression, and uniaxial strain compression. Analytical results enable determination of parameters and verification of finite element predictions. Three-dimensional simulations of polycrystalline microstructures are enacted under similar loading protocols. Results provide insight into effects of crystalline microstructure and anisotropy for different stress states. With increasing confinement, the ratio of macroscopic pressure to shear stress increases, and influences of microstructure and anisotropy on average stress and ductility are reduced.

Multiphysics Electrochemical Impedance Simulations of Complex Multiphase Graphite Electrodes

Electrochemical impedance spectroscopy (EIS) is a widely used technique to measure the properties of materials in electrochemical systems. However, the obtained quantities are difficult to directly connect to the microstructure-level phenomena. In this work, detailed electrochemical microstructure simulations were performed to investigate the EIS behavior of the phase-separating graphite electrodes. The Cahn–Hilliard phase-field equation was employed to model Li transport in the graphite particles. In graphite electrodes that were in single-phase stages, the obtained charge-transfer resistance reflected the total active surface areas. The effect of pore tortuosity, which dominates an electrode’s high-rate performance, cannot be reflected in the EIS behavior. In two-phase coexistence graphite electrodes, when phase boundaries were present on the particle surfaces, the simulations exhibited an inductive loop on the simulated EIS curve. In the core–shell phase-morphology cases, the EIS measurements reflected only the properties of the shells. The resulting EIS curves are indistinguishable from those in the single-phase cases. While Fick’s law of diffusion has been mistakenly employed to model Li transport in phase-separating graphite electrodes, our simulations showed that the EIS curves obtained using the Fickian diffusion model are very similar to those obtained using the Cahn–Hilliard phase-field model. This presented tool provides unprecedented detailed simulations to connect the intrinsic material properties, the electrochemical processes in the microstructures, and the resulting EIS behavior.

Microstructural and finite element simulation analysis of the crack formed during heat treatment in the carburized 22CrMoH steel gear shaft

This study analyzed the crack formed during heat treatment in the carburized 22CrMoH gear shaft. Qualitative analysis of composition, observation of microstructure and morphology analysis of crack section were carried out by Optical Microscope (OM) and Scanning Electron Microscope (SEM) with an Energy Dispersive Spectrometer (EDS). The results showed that a large size abnormal inclusion with a length of 2.4 mm and a width of 0.2 mm existed in the crack initiation region 10 mm from the surface of the gear shaft. The inclusion was composed of Ca, Al, Mg and Si oxides. The finite element simulation results showed that a significant stress concentration existed near the inclusion, and the maximum value exceeded the yield strength of the 22CrMoH gear shaft during quenching process and led to the generation of cracks. Long strip MnS inclusions were observed in the crack propagation direction and promoted the crack propagation. In addition, the finite element method was used to simulate the stress concentration of inclusions with different length and positions in the gear shaft during quenching. The results showed that the stress concentration increased with the decrease of the distance between the inclusion and the surface or the increase of the inclusion size. In order to avoid the recurrence of cracks, the steel plant was advised to strictly control the size and positions of inclusions. After improvement, cracks did not appear again.

Anisotropic microstructure, properties and molecular dynamics simulation of CoCrNi medium entropy alloy fabricated by laser directed energy deposition

CoCrNi medium entropy alloy additive part was fabricated by laser directed energy deposition process. Based on an optimized additive manufacturing process parameters, the microstructure, tensile properties and corrosion performance in the XOY and XOZ planes were compared to learn the anisotropic properties of the additive part. The majority of the additive parts consisted of equiaxed grains, a small proportion consisted of elongated needle-like grains. The equiaxed and elongated needle-like grains in the XOY plane were finer compared to that in the XOZ plane. The corrosion electrochemical activity is uniform in the XOY plane, but varies at different heights in the XOZ plane base on scanning vibrating electrode technique results. Electron backscatter diffraction results near the fracture indicated a larger and relatively more homogeneous residual stress peak in the XOY plane. The tensile and yield strengths in the XOZ plane were lower than those in the XOY plane by approximately 98.5 MPa and 19.1 MPa. While the elongation in the XOZ plane was more than twice as high as that in the XOY plane. The reason for the anisotropic tensile behavior was the strong texture in the XOY plane and the relatively random grain orientation in the XOZ plane. Single crystal and polycrystalline molecular dynamics were used to simulate the anisotropic tensile properties in the XOZ and XOY planes. Single crystal simulation results showed that the maximum stress was 19.0 GPa and elongation was 13.0% when the stretching was along the [110] crystal orientation. The minimum stress was 17.1 GPa and elongation is 11.1% elongation when the stretching was along the [11 1‾] crystal orientation. Anisotropy existed for CrCoNi alloys in terms of tensile properties along different orientations. Polycrystalline simulation in the XOY plane results showed that the main cause of anisotropy between them was the tangling of the 1/6<112> dislocations and the 1/6<521> dislocations, which caused obstacles to dislocation motion and thus improved the strength of CoCrNi additive parts. High tensile strength in the XOY plane, not the XOZ plane, was attributed to 1/6<112> and 1/6<521> dislocation tangles at the (1 1‾ 0)/(115) planes.

On rate dependence and anisotropy in phase field modeling of polycrystalline fracture

A phase field theory of fracture incorporating rate dependence (i.e., kinetic viscosity) and anisotropic elasticity is implemented to study strength and failure modes in polycrystalline materials. After presentation of the constitutive framework, a new analytical solution is derived for one-dimensional (1D) homogeneous loading at constant strain rate, wherein crack viscosity is enabled. The three-dimensional (3D) theory is implemented in a finite element (FE) context, with static and dynamic solutions verified up to peak loading for homogeneous microstructures. Extensive simulations of heterogeneous polycrystalline microstructures follow, where baseline material properties correspond to either boron carbide (B4C) or titanium diboride (TiB2). Results quantify effects of loading rate, elastic anisotropy, cleavage energy anisotropy, and grain boundary surface energy on tensile strength. Notable are effects of strong basal texture, which influences average strength via both elastic anisotropy and basal cleavage plane orientation. Effects of loading rate are naturally more prominent for materials that fail mostly intergranularly rather than transgranularly, in agreement with experimental observations.

Enhancing the Predictive Capabilities of Microstructure Simulations of PBF-LB/M by an Evaluation of Nucleation Theories

Powder Bed Fusion of Metals using a Laser Beam (PBF-LB/M) has proven to be a competitive manufacturing technology to produce customized parts with a high geometric complexity. Due to process-specific characteristics, such as high cooling rates, the microstructural features can be tailored. This offers the possibility to locally control the mechanical properties. Therefore, the grain structure has to be reliably predicted at first. The starting point of the grain formation and the growth process is characterized by the nucleation. Over the course of this study, various nucleation theories were applied to the PBF-LB/M process and their suitability was evaluated. The two Sc-modified aluminum alloys Scalmalloy® and Scancromal® were processed with a novel experimental PBF-LB/M setup. By performing melt pool simulations based on the Finite Element Method (FEM), the input data for the nucleation models were obtained. The simulatively predicted nucleation zones based on the different theories were compared to real metallographic images and to literature results. It was found that the phenomenological approach should be used whenever no first-time-right prediction of the simulation is necessary. The physically based models with the heterogeneous nucleation should be applied if a first-time-right prediction is striven for. For applications in PBF-LB/M, the nucleation models should be extended in terms of the influence of precipitates and the high cooling rates during the manufacturing process. The presented approach may be used to further assess grain nucleation models for various additive manufacturing processes.

Calibration of Cellular Automaton Model for Microstructure Prediction in Additive Manufacturing Using Dissimilarity Score

Abstract Additive manufacturing (AM) simulations offer an alternative to expensive AM experiments to study the effects of processing conditions on granular microstructures. Existing AM simulations lack support from reliable validation techniques. The stochastic nature and spatial heterogeneity of microstructures make it difficult to validate the simulated microstructures against experimentally obtained images through statistical measures such as average grain size. Another challenge is the lack of reliable and automated methods to calibrate the model parameters, which are unknown and difficult to measure directly from experiments. To overcome these two challenges, we first present a novel metric to quantify the difference between granular microstructures. Then, using this metric in conjunction with Bayesian optimization, we present a framework that can be used to reliably and efficiently calibrate the model parameters. We employ this framework to first calibrate the substrate microstructure simulation and then the laser scan microstructure simulation for Inconel 625. Results show that the framework allows successful calibration of the model parameters in just a small number of simulations.

Investigation of Electromechanical Reliability and Radio Frequency Performance of a Highly Stretchable Liquid Metal Conductor for Wearable Electronics

Abstract Liquid metal-based gallium conductors exhibit unique physical and electromechanical properties, which make them excellent candidates for the next generation of wearable electronics. In this study, a novel fluid phase-based gallium conductor was stencil printed on thermoplastic polyurethane (TPU) to fabricate a stretchable conductor as well as a stretchable radio frequency (RF) transmission line. The electromechanical reliability of the conductor during high elongation as well as cyclic tension and bend fatigue was evaluated and compared with commercially available stretchable silver-filled polymer paste. The microstructure of the liquid metal conductor and the silver paste was investigated via scanning electron microscopy (SEM) before and after the samples were subjected to high elongation (&amp;gt;100%). Unlike the silver paste, the liquid metal conductor maintained its microstructural integrity while its resistance showed a linear response to changes in length. A cyclic tension fatigue test confirmed the fatigue-free performance of the liquid metal conductor during 8000 stretching cycles at a strain amplitude of 30%. The electromagnetic structure of the RF transmission line was simulated and then compared to the measured data. The measurements for insertion loss showed that U-bending, 90 deg twisting, and 1000 stretching cycles at a strain amplitude of 100% did not have a significant impact on the RF performance. Details of the DC tests and RF measurements, including the microstructural analysis and simulation results, will be discussed in this article.

Impact-induced bonding process of copper at low velocity and room temperature

The impact-induced bonding can be foundation of fast solid-state additive manufacturing. The mechanisms behind impact-induced bonding have not been widely acknowledged due to complexity caused by too transient process and extreme loading conditions. We report a novel impact-induced bonding process of copper flakes. This bonding process possesses attractive features of longer duration, macroscopic characteristic length and low ambient temperature (typically at room temperature). Meanwhile, finite element simulation based on actual grain configurations and crystal plasticity reveals that the bonding strength can reach the strength of bulk material. Three types of bonding modes are observed and corresponding mechanisms are extracted. It is found that the embedding between microstructures and the recrystallization of grains are significant factors of impact-induced bonding. Both microstructure characterization and simulation with material point method support recrystallization but not melting.

Overlapping Features and Microstructures of Coarse Grain Heat-Affected Zones in Swing Arc Narrow Gap GMA Welded EH40 Grade Steels

Swing arc narrow gap GMA welding experiments were carried out with a Box–Behnken response surface design. Weld metal and heat-affected sizes were measured from the joints obtained, and an ANOVA was performed to obtain well-fitting models for definition of the heat-affected length. Overlapping patterns and microstructures were analyzed and observed in zones within the heat-affected length through the thickness direction. In addition, thermal processes in typical zones of HAZs were calculated by FEM and analyzed to explain the patterns in the typical coarse grain heat-affected zones (CG-HAZs) with thermal simulated microstructures attached. It was realized that a single pass could only be confused with an austenitized process by two passes. The coarse grain heat-affected zone of a single pass could be divided into an unaltered coarse grain heat-affected zone (UACG-HAZ), a supercritically reheated coarse grain heat-affected zone (SCRCG-HAZ) and an intercritically reheated heat-affected zone (IRCG-HAZ). It is likely that there would be an intercritically reheated UACG-HAZ upon the UACG-HAZ. The microstructures in the CG-HAZs and the UACG-HAZ were mainly lath bainite and a little acicular ferrite; the microstructures in the SCRCG-HAZ were short lath bainite, granular bainite and acicular ferrite and the microstructures in the IRCG-HAZ were massive textures and secondary austenite decomposition products. The cooling times in the typical bainite transformation procedures were similar to one another in a secondary austenitized process and significantly longer than those in a single austenitized thermal cycle, which caused similar patterns in reheated CG-HAZs and an increase in acicular ferrite compared to CG-HAZs. The prior austenite grain sizes caused differences among the reheated CG-HAZs.

Numerical simulation of the nickel alloy microstructure formed in the process of hot fogging

The paper presents a comprehensive analysis of the deformation and thermal states of the Waspaloy alloy billet heated to different initial temperatures of 1100°C and 1150°C and subjected to free upsetting to an average diameter of ~1060 mm at the deformation rate of 100 mm/s. The thermodynamic forces acting on the billet trigger the process of dynamic recrystallization, which is associated with the appearance and growth of low-defect nuclei of new grains instead of the deformed ones. To describe the material microstructure evolution, the phenomenological approach implemented in the DEFORM-2D/3D software package was applied. The simulation was based on the modified Johnson - Mehl - Avrami - Kolmogorov (JMAK) model, whose equations allow calculating the volume fraction of recrystallized material and describing the grain structure transformation of metal alloys. The results of solution of the non-stationary temperature problem are used to construct the temperature fields in the Waspaloy alloy billet during its transportation through air from the furnace to the deforming equipment within 45 seconds and during the subsequent upsetting process. For the latter, the force and strain characteristics, including the force required to complete this process, are determined in the framework of the plastic flow theory, and the characteristics of the grain structure of the nickel alloy, such as the average size of recrystallized grains and their volume fraction, are determined in the framework of the JMAK model. The results obtained by numerical simulation make it possible to substantiate an optimal selection of parameters of billet deformation ensuring the formation of the required material structure.

Linking simulated polycrystalline thin film microstructures to physical vapor deposition conditions

We present a generalized multi-phase-field model to predict the growth of polycrystalline thin films fabricated by physical vapor deposition. The model accounts for the explicit transport of atomic species to the substrate and the competing diffusion processes on the surface and in the bulk of the film leading to the formation of films with specific microstructures. We used magnetron sputtering conditions (pressure, voltage, working distance, substrate orientation) to calculate the energy and direction of the arriving atoms at the substrate using Monte Carlo simulations with the SiMTRA code. Our simulation results capture the dependence of the microstructure on deposition conditions, and delineate the relationship between process parameters and the formation of columnar microstructures and surface roughness characteristic of thin films. These simulation predictions are in agreement with transmission electron microscopy characterization of sputtered films. Through our systematic investigation of competing growth mechanisms, we provide insights into the complex relationships between deposition conditions and bulk and surface morphologies.

Thermophysical properties in the Al-Cu-Ag system: A combined CALPHAD and first-principles study

Thermophysical properties play an essential role in the microstructure simulation during precipitation and solidification processes in materials science and engineering. In this work, a comprehensive investigation on the molar volume and surface tension of the Al-Cu-Ag ternary system was performed by the CALPHAD (CALculation of PHAse Diagram) approach and the first-principles calculation. Based on the critical review of experimental and theoretical data, the molar volume description of pure Ag in the liquid phase, and subsequently the mole fraction dependent thermophysical databases of molar volume and surface tension for the Al-Cu-Ag system including metastable phases were constructed via the CALPHAD approach. Then considering the excess molar volume, the surface tension of solution alloys could be predicted by the Butler equation using a home-made code with the input of thermodynamic and thermophysical parameters. With the increment of temperature, molar volume of the alloys increases while the surface tension decreases. All the calculated results using the present parameters are in satisfactory agreement with the corresponding experimental data, which could be subsequently applied to simulate the microstructure evolution like precipitation kinetics and phase-filed simulation.

Image-based microstructural simulation of thermal conductivity for highly porous wood fiber insulation boards

The thermal conductivity of wood fiber insulation boards is significantly influenced by the microstructure of the fiber network and in general, the efficiency of wood fiber insulation boards increases with porosity. For higher raw densities, the raw density is a good predictor for the thermal conductivity. For lower raw densities however, this simple relation does not hold anymore. Here, structural information gained from 3D computed tomography images at several scales, modeling of the microstructure, and numerical simulation of the thermal conductivity are combined to get deeper insight into which and how microstructural features influence the thermal conductivity. The model-based simulation as described here shows that the presence and orientation of wood fiber clusters impact the thermal conductivity significantly.