Microstructure Control Research Articles

In-situ Selective Laser Heat Treatment for microstructural control of additively manufactured Ti-6Al-4V

As-built Laser Powder Bed Fusion (LPBF) Ti-6Al-4V typically exhibits a fully acicular α′-martensite microstructure, and requires post-process heat treatment in order to decompose the martensite and achieve sufficient ductility. In the present study, we demonstrate a simple concept based on in-situ Selective Laser Heat Treatment (SLHT) that can effectively alter the microstructure and activate the decomposition of the α′-martensite into a lamellar α+β microstructure within a short time scale (~30s). SLHT consists of multiple rescanning of the printed part, with low energy density, triggering solid-state phase transformations. Operando X-ray diffraction has been performed on cuboid and thin wall geometries, and was augmented by thermal finite element simulations. Upon SLHT, a gradual formation of the β phase as well as an α/α′peak narrowing trend have been evidenced through X-ray diffraction, as an indication of the diffusional nature of α′-martensite decomposition. Moreover, through fine tuning of the process parameters at the final stage of SLHT, a controlled temperature evolution during cooling was achieved, leading to preservation of the β phase, a product of the decomposition, down to room temperature. Complementary microstructural characterizations via EBSD, SEM, and TEM confirm the presence of a lamellar α+β microstructure after SLHT. Our results evidence, for the first time, the fast kinetics of α′-martensite decomposition under in-situ SLHT. The approach is meant to be implemented at selected locations during the LPBF process, avoiding time-consuming post processing steps, and leading to composite-like, architected microstructures.

Recrystallization behavior of a hot-rolled TiBw/TA15 composite under electropulsing heat treatment

In the present work, low-current-density electropulsing was applied to study the recrystallization behavior of a hot-rolled TiBw/TA15 composite. To highlight the athermal effect of electropulsing, traditional heat treatment experiments on composite materials were conducted under the same conditions. Test results indicated that recrystallization behavior can be improved compared with that in traditional heat treatment. The potential mechanism by which electropulsing promotes recrystallization was investigated, and the influence of the reinforcement phase on recrystallization was discussed. This work will provide freedom for the composite's process design and microstructure control.

Simulation of local metastable microstructural states in large tools: construction and validation of the model

This study presents the development and experimental verification of a simulation model for estimating the local microstructure of a tool geometry after heat treatment. The experiment involved subjecting a metallic block of dimensions 40 × 50 × 50 mm, made of the ledeburitic cold work steel DIN EN 1.2379 (X153CrMoV12; AISI D2), to a heat treatment in a laboratory furnace at 1000 °C for 20 min. Thermocouples were strategically placed to record time-temperature profiles at different locations within the block. Following the heat treatment, the local microstructure was determined through quantitative image analysis, and the local hardness was measured as a function of the distance from the block’s edge to its core. These measurements were then correlated with the corresponding time-temperature curves obtained from the thermocouples. To replicate the local time-temperature profiles, the thermophysical properties of the steel were experimentally determined and incorporated into a finite element analysis heat transfer simulation using Abaqus FEA® software. This simulation approach, combined with the MatCalc software, facilitated the calculation of various local microstructural characteristics such as carbide content, carbide type, carbide distribution, and chemical composition of the matrix. Furthermore, the content fractions of the microconstituents of the matrix, including martensite and retained austenite, were determined based on the simulated martensite start temperature, employing an optimized function fitted to experimental data. The developed simulation model offers potential applications in two important areas. Firstly, it can be used to adapt heat treatment processes for tools of different sizes in production, optimizing their mechanical properties. Secondly, it enables efficient optimization of heat treatment routes by considering changing initial states, leading to high process quality in terms of mechanical properties. Overall, this study provides valuable insights into the estimation and control of local microstructure in tool geometries through the use of a validated simulation model.

Modeling and simulation of the flash sintering: An approach to study the influence of process attributes on the flash phenomenon and microstructure

Field-assisted sintering still has gaps in explaining and modeling the flash phenomenon, as well as in enabling control over the sintering process to achieve desirable densification and microstructure control in ceramic materials.This study investigates methods for constructing computational models based on machine learning for predicting the onset temperature of the flash event (Tonset) and bulk density (BD) from a combination of various process attributes: powder composition, green sample geometry, electric field configuration, and heating. Samples of 3 mol% and 8 mol% yttria-stabilized zirconia were shaped into cylindrical geometry and were sintered using flash sintering and multi-step flash sintering processes, employing the following variations: current density; DC and AC electric field; and holding time. The experimental data were used for constructing and evaluating prediction models for Tonset and BD using k-fold cross-validation and the following algorithms: Random Forest, Support Vector Machine, Artificial Neural Network, K-Nearest Neighbors, and Multivariate Linear Regression Analysis. The model performance was assessed by comparing the measured values with the respective predicted values based on the Pearson correlation coefficient (r), mean absolute error (MAE), and root mean square error (RMSE). The models based on the Artificial Neural Network algorithm showed the best performance for predicting Tonset (r = 0.98; MAE = 11.88; RMSE = 14.88) and BD (r = 0.91; MAE = 1.86; RMSE = 2.30). The best models were used in conjunction with the Wrapper method for selecting attributes that significantly impact the sintering process, allowing the identification of 6 main attributes that impact Tonset and 9 attributes that impact BD in the flash-sintered samples.

Partially melted powder in laser based directed energy deposition: Formation mechanism and its influence on microstructure

The powder–melt pool interaction behavior is crucial in laser-based directed energy deposition (LDED). Partially melted particles, which are formed as a result of this interaction, significantly influence on the microstructure and mechanical performance of multi-material and metal-matrix composites fabricated via LDED. However, the presence of partially melted particles is a contentious issue that has been overlooked in single-material LDED studies. Furthermore, the investigation of partially melted particles is hindered by the difficulties in direct observation. To overcome this obstacle, this study was conducted using a single-bead Ti–6Al–4V printing experiment with a relatively high oxygen content to distinguish partially melted particles directly. The formation mechanism of the partially melted particles was revealed through experimental studies combined with numerical analysis using a self-established model. Additionally, the influence of partially melted particles on the grain structure of LDED–fabricated parts was investigated in a low–oxygen environment. The partially melted particles tend to survive close to the surface of the deposited layer. As the penetration depth increased, the particle size decreased and the aspect ratio increased. The formation of partially melted particles collectively depends on the laser power, scanning velocity, powder size and powder feed speed, differing from the common conclusion that an insufficient input energy results in poor powder melting behavior. Furthermore, a Ti–6Al–4V sample with high–fraction equiaxed grains was fabricated using optimized processing conditions. The partially melted particles significantly affected the solidification behavior. In addition to the heterogeneous nucleation mechanism caused by the partially melted particles, a novel seed crystal mechanism was proposed to support the abnormal formation of equiaxed grains. This study highlights the importance of partially melted particles in LDED, and provides useful insights into in-situ microstructural control in LDED.

Modification of grain boundary microstructure by controlling dissolution behavior of θ particles in Cr-containing hypereutectoid steel

To overcome the brittleness of high‑carbon steels, it is necessary to develop the strategy to remove the carbides on the grain boundaries and strengthen grain boundaries. In this paper, we achieved them by simply controlling the heat treatment conditions and the microstructure before heat treatment. It was found the dissolution behavior of the θ-cementite particles is changed greatly with and without Cr in the high‑carbon steels. In the Cr-free hypereutectoid steel, dissolution of θ particles at high temperatures is remarkably fast, but the dissolution behavior is greatly affected by the microstructure in the Cr-containing hypereutectoid steel. In the Cr-containing hypereutectoid steel, the dissolution of θ particles into the γ-austenite phase was rapid in steel with smaller θ particles in fine α grains, but slow in steel with larger θ particles in coarse α grains. The θ particles on the grain boundaries preferentially dissolved and rapidly disappeared in the initial stage of the dissolution treatment owing to the contribution of grain boundary diffusion, since the grain boundary diffusion of Cr is fast as comparable to C. However, the θ particles inside the grains slowly dissolved and then remained until the later stage, owing to slow dissolution via the partition local equilibrium (PLE) mode controlled by the lattice diffusion of Cr. Owing to the preferential dissolution of θ particles on the grain boundaries in the initial stage, large amounts of Cr atoms dissociated from the θ particles diffused along the grain boundaries with grain boundary diffusion, forming Cr-enriched grain boundaries. Such grain boundary microstructure control is expected to lead to the suppression of intergranular fracture, resulting in the improvement of fracture toughness.

On the Control of Elemental Composition, Macro-, and Microstructure of Directionally Solidified Additive Products from Nickel-Based Alloy

The present work establishes the influence of heat input and methods of heat removal at the wire-feed electron beam additive manufacturing (EBAM) process on the structure of an additive product made of a nickel-based alloy. The following printing approaches are considered: changes in heat input, 3D printing strategy, and heat removal conditions due to (1) heating of the substrate, (2) partial suppression of radiative heat dissipation, and (3) thermal insulation of the substrate. It is shown that epitaxial growth of dendrites occurs in each case. However, in the case of an increase in speed and a change in the 3D printing strategy, the directed dendritic growth is interrupted. Preheating of the substrate and subsequent maintenance of the temperature reached during the EBAM process, as well as partial suppression of the radiative component of heat removal, allow to obtain the most uniform directional structure.

Microstructure Control of LiCoO2‐Li10GeP2S12 Composite Cathodes by Adjusting the Particle Size Distribution for the Enhancement of All‐Solid‐State Batteries

AbstractMicrostructure control of composite electrodes comprising active materials and solid electrolytes is imperative to achieve sufficient ion‐ and electron‐conductive pathways for the development of all‐solid‐state Li ion batteries. Here, we synthesized Li10GeP2S12 solid electrolytes with various particle size distributions by milling and filtering. Impedance spectroscopy revealed that the ionic conductivities in the bulk were hardly changed by the synthetic processes. This enabled us to investigate only the microstructure effects on the electrochemical properties of the composite electrodes. Microscopic and electrochemical tests of the LiCoO2‐Li10GeP2S12 composite cathodes clarified that the size distributions of the Li10GeP2S12 drastically affected the microstructures in the composite cathodes, such as contacts at interfaces and voids between particles. The size distributions also contributed to the appropriate ratio of LiCoO2 to Li10GeP2S12 for superior charge/discharge properties. The (de)intercalation reversibly proceeded in the composite cathode using the filtered Li10GeP2S12 even though the ratio of Li10GeP2S12 decreased from 50 % to 30 % in volume. This study demonstrated the possibility of high‐energy‐density composite cathodes for all‐solid‐state batteries by control of microstructures in composite electrodes.

Microstructure evolution and mechanical properties of a high Nb–TiAl alloy via HIP and heat treatment

Hot isostatic pressing (HIP) and heat treatment (HT) are useful methods for adjustment and control of microstructure and mechanical properties of TiAl alloys, especially for casting components. Due to the vast varieties in chemical composition, phase constitution and microstructure of TiAl alloys, the evolution of microstructure and precise control of mechanical properties are vague and uncertain. In this study, the microstructure, phase content and mechanical properties of Ti–46Al–6Nb-0.1C (at%) alloy are systematically investigated during the whole processing state. HIP and HT can change the microstructure, phase content, and lamellae thickness within the lamellar colonies through phase transformation, which in turn significantly affect the tensile properties. Small lamellar spacing and high α2 phase content can improve the strength of the alloy, while equiaxed γ grains and thick γ lamellae can improve its plasticity at room temperature. HIP at 1270 °C can improve its plasticity but reduce the ultimate tensile strength. Subsequent with HT at 1350 °C, microstructure can be modulated with thin lamellae, resulting in higher uniformity and well-balanced mechanical properties.

Local control of microstructure and mechanical properties of high-strength steel in electric arc-based additive manufacturing

Additive manufacturing offers a significant potential for producing metallic parts with distinctly localised microstructures and mechanical properties, commonly known as functional grading. While functional grading is generally accomplished through compositional variations or in-situ thermo-mechanical treatments, variation of process parameters during additive manufacturing can offer a promising alternative approach. Focusing on the electric arc-based additive manufacturing process, this work focuses on the functional grading of high-strength steel (S690 grade) by adjusting the travel speed and inter-pass temperature. Through a combination of thermal simulations and experimental measurements on single bead-on-plate depositions, it is shown that the microstructure and the mechanical properties of parts can be controlled through the rational adjustment of process parameters. A rectangular block was fabricated to demonstrate functional grading using a constant wire feed rate and varying travel speed. The rectangular block consisted of a low heat input (LHI) region deposited between high heat input (HHI) zones. A graded microstructure was obtained with the HHI zones composed of a mixture of polygonal ferrite, acicular ferrite, and bainite, while the LHI region was primarily composed of martensite. The hardness and profilometry-based indentation plastometry measurements indicated that the LHI region exhibited higher hardness (32%) and strength (50%), but lower uniform elongation (80%), compared to the HHI zones. The present study demonstrates the potential to achieve functional grading by adjusting process parameters in electric arc-based additive manufacturing, providing opportunities for tailor-made properties in parts.

Microstructural Control and Alloy Design for Improving the Resistance to Delayed Fracture of Ultrahigh-Strength Automotive Steel Sheets

The demand for higher-strength automotive steel sheets has increased significantly for lightweight and safe body concepts. However, the increment of the steel strength is often limited by the potential occurrence of delayed fracture. This paper discusses proper microstructure control and alloy design to improve the resistance against the delayed fracture of ultrahigh-strength automotive steel sheets in order to increase the usable upper limit of their strength and provides basic data serving as a practical guide for solving the problem of delayed fracture in ultrahigh-strength automotive steel sheets. It is confirmed that grain refinement, the appropriate dual-phase structure of martensite with ferrite or retained austenite, and surface decarburization, increase the resistance to delayed fracture. In terms of alloy design, the effects of Nb, Mo, and B on the delayed fracture resistance of hot-stamped steels have been investigated. The results suggest that there are other reasons for Nb to improve delayed fracture resistance in addition to grain refinement and the ability to trap hydrogen by its precipitates, as has been conventionally believed. Regarding Mo, it was clearly demonstrated that the segregation of this element at the grain boundary plays a main role in improving the delayed fracture resistance.

Microstructural Control by Cooling Rate in β-type and Sintered Ti-3.6Fe-5Zr-0.2B (Mass%) Alloy Fabricated by Spark Plasma Sintering and Heat Treatment

The β-type and sintered Ti-3.6Fe-5Zr-0.2B (mass%) alloy has been consolidated by spark plasma sintering, followed by a β solution treatment (ST). In order to obtain a high-strength ductile balance, water quenching or air cooling is used after ST. Modification of sintering conditions, which leads to 100% of the relative density, improves the tensile ductility. The Fe addition causes a large local lattice and compressive strain to the bcc Ti lattice; in the water-quenched sample, α” martensite phases appear in the β matrix. When air cooling is applied after the ST, bimodal α lath phases are instead precipitated during the cooling in nanoscale, and the formation of α” martensite phases is suppressed. This results in high strength and better ductility when compared with those in the water-quenched sample, particularly in tensile properties. The air-cooled sample reveals attractive mechanical properties in both tension and compression modes.

Machine learning-assisted constitutive modeling of a novel powder metallurgy superalloy

The flow behavior and microstructure evolution during thermomechanical processing (TMP) of powder metallurgy superalloys are very important for their service behavior. In this contribution, a machine learning-assisted physical model was proposed to predict the flow stress, dynamic recrystallization (DRX) kinetic, and grain size evolution of a novel FGH4113A superalloy during the TMP. The work hardening and dynamic softening stages were modeled separately to predict the flow stress. The novel aspect of the proposed model is that the strong coupled effects of deformation temperature, strain rate, true stain, and primary γ' precipitates on the DRX kinetic were quantitatively described by the genetic algorithm-based artificial neural network (GA-ANN). Besides, the grain size modeling considered the pinning effect by primary γ' precipitates and the deformation-induced DRXed grain growth. Comparing the experimental and predicted flow curves, DRX fraction, and average grain size reveals that the proposed model has a preferable prediction accuracy than the conventional model. Then the specific DRX kinetic and grain size evolution characteristics were revealed. The promoting effect of strain rate on the DRX degree was gradually increased with the deformation temperature. Finally, the 3D processing map based on the developed model was established to guide TMP's optimized design and microstructure control. Three domains were divided and discussed: grain refinement, constant, and growth. The optimal processing window is defined as 1000–1050 °C/0.001–0.006 s−1 and 1075–1100 °C/0.008–0.01 s−1.

Microstructure control of porous Si3N4 whisker scaffolds by in-situ carbothermal reduction and nitridation reaction

Si3N4 whisker scaffolds have great potentials owing to the superior properties of Si3N4 and their unique porous structures. Obviously, the properties of the scaffolds are closely related to the morphology and distribution of phases, and the bonding between whiskers. This study proves that the above-mentioned microstructural features can be tailored to a great extent by the in-situ growth of secondary Si3N4 products via carbothermal reduction and nitridation (CRN) of SiO2 inside a β-Si3N4 whisker network. Specifically, the yield and morphology (grain size, aspect ratio, etc.) of the CRN product are found to be strongly dependent on the composition and amount of the reactants (SiO2 and C/SiO2 molar ratio) and reaction temperature. When reacted at 1300 ℃, a distinctive microstructure is formed in which nano sized and CRN-derived Si3N4 whiskers are uniformly distributed in the original micron sized whisker network. The whisker scaffolds reveal improved flexural strength if compared with that without CRN-derived Si3N4. This finding indicates the formation of a strong bonding between the β-Si3N4 whiskers by the growth of the CRN product. The measured low dielectric constant (2.6 ∼ 3.1) and low dielectric loss (<1.1 ×10−2) of the scaffolds makes them suitable for wave transmission application.

Development of metal AM technology for gas turbine components

Mitsubishi Heavy Industries, Ltd. (MHI) Group has been developing additive manufacturing (AM) as a method that can manufacture parts with complex shapes and considering its application to manufacturing processes. In combustor components, application of AM process to rapid prototyping and multi-cluster nozzles for hydrogen or ammonia gas fuel is being considered. In turbine parts, with the aim of improving performance by reducing the amount of cooling air, the adoption of a complex internal cooling structure, which cannot be made with conventional manufacturing methods but can only be made by AM, is being considered. This paper describes design for AM technology for gas turbine components and metal AM process technology such as building simulation based high stiffness support design and pre-set distortion, microstructure control by laser scanning conditions, quality control through in-process monitoring tools and application of AM technology to gas Turbine Components.

Understanding and control of Zener pinning via phase field and ensemble learning

Zener pinning refers to the dispersion of fine particles which influences grain size distribution via movement of grain boundaries in a polycrystalline material. Grain size distribution in polycrystals has a significant impact on their properties including physical, chemical, mechanical, and optical to name a few. We explore the use of Phase-field modeling and machine-learning techniques to understand and improve the control of grain size distribution via Zener pinning in polycrystalline materials. We develop a machine learning model that determines the relative importance of various parameters to exercise microstructure control via Zener pinning. Our workflow combines high-throughput phase-field simulations and machine learning to address the computational bottlenecks associated with large-scale simulations as well as identify features necessary for microstructure control in polycrystals. A random forest (RF) regression model was developed to predict grain sizes based on five Phase-field model parameters, achieving an average prediction error of 0.72 nm for the training data and 1.44 nm for the test data. The importance of the input parameters is analyzed using the SHapley Additive exPlanations (SHAP) approach which reveals that diffusivity, volume fraction, and particle diameter are the most important parameters in determining the final grain size. These findings will allow us to select the best second-phase particles, optimize grain size distributions and thus design microstructures with the desired properties. The developed method is a highly versatile and generalizable approach that can be used to assess the combined effects of individual features in the presence of multiple variables.

Magnetic Responsive On‐Site Switching of the Directional Droplet Self‐Transport in Shape Memory Slippery Tube

AbstractThe phenomenon of controlled droplet transport has promising application prospects in various fields. Active droplet transport mode is controllable through continuous external stimuli. By contrast, self‐transport is a more environmentally friendly and energy‐efficient passive transport mode but lacks controllability. In this study, controlled self‐transport is achieved by constructing a shape memory polymer (SMP) tube with a lubricated magnetic‐responsive gel inner surface. The asymmetrical shape of the tube, combined with the lubricated inner surface, enables directional self‐transport of droplets without external stimuli. Furthermore, the resistance on the inner gel surface can be altered by regulating the magnetic field to achieve effective active control during the self‐transport process. Thus, smart in situ control of droplet transport can be achieved by integrating the macroscale shape variation of the tube with the dynamic control of the inner surface microstructure. Owing to the fast magnetic responsivity and in situ controllability of the self‐transport process, the SMP lubricated tube demonstrates the ability to transport a variety of liquids and can be designed as a micro‐reactor for step‐by‐step droplet detection. The findings of this study may provide guidance for the development of intelligent interface materials and microfluidic devices.

The technical potential of a sous-vide processing method for developing high-moisture textured soy protein

This study explored the potential of sous-vide processing as a novel technique for transforming low-moisture textured soy protein (TSP) into a product with high moisture content and texture comparable to meat. We hypothesized that the sous-vide treatment would enable precise control of the TSP microstructure. In the ensuing process, the TSP maintained the moisture content at approximately 70% and changed color towards darker tones. Additionally, the porous microstructure changed, transitioning from a large to a smaller air layer. As the treatment continued, both the hardness and texturization index of the TSP were reduced. Furthermore, the secondary structure of the protein exhibited an increase in β-sheet and α-helix structures, indicating enhanced hydrogen bonds, hydrophobic interactions, and disulfide bonds. Optimally, a sample with 24 h at 90℃ displayed textural characteristics similar to chicken breast. The investigation underlines the sous-vide method as a revolutionary technique yielding high-moisture content and improved texture for TSP.

One-Step Hydrothermal Preparation of a Corncob-Derived Porous Adsorbent with High Adsorption Capacity for Urea in Wastewater: Sorption Experiments and Kinetics Study.

With rapid industrial development, the massive generation of nitrogenous wastewater poses a serious threat to both human beings and the ecosystem. Bio-based adsorbents are considered promising adsorption materials for many applications. However, their complex preparation procedures, large energy consumption, and difficulty of microstructure control hinder their practical applications. In this study, a new corncob-derived porous adsorbent (CPA) with excellent urea adsorption capacity in wastewater was prepared by the one-step hydrothermal process. The effects of the hydrothermal process conditions on the urea adsorption capacity of the CPA were evaluated and optimized using the response surface methodology, and a kinetic analysis of the CPA was also carried out. Our findings showed that the adsorption process of urea by the adsorbent followed the Langmuir isotherm and pseudo-second-order kinetic models. The high adsorption capacity for urea was attributed to the abundant porous structure and the hydrogen bonds formed between the adsorbent and the amine group in urea, which made it more conducive to the adsorption of urea. Therefore, we believe that CPA could be a promising adsorbent for urea removal in wastewater.

Controlling polyethylene branching via surface confinement of Ni complexes

The heterogeneous surface support can play a key role in determining polymer microstructure, as we show for a novel variant of Ni-catalyst from the family of late-transition metal complexes; this extends the toolbox for novel catalytic solutions in industrial processes. Novel variants of single-atom catalysts (Ni-FO-Al@SiO2, Ni-FO-Si@SiO2, Ni-O-Al@SiO2, and Ni-O-Si@SiO2) were prepared in the form of unsymmetrical a-diimine Ni complexes (Ni-OH and Ni-FOH) and then characterized by inductively coupled plasma - optical emission spectrometry (ICP-OES) and X-ray photoelectron spectroscopy (XPS) analysis. Ethylene slurry-phase polymerization was performed both via self-supporting and covalent-tethering strategies to systematically study the surface confinement effects. High catalytic activity was maintained under the slurry-phase polymerization (as high as 3.9 × 106 g of PE (mol of Ni)−1 h−1). The crucial features of high molecular weight (>106 g mol−1) and high branching density (as high as 180.1BD/1000C) were found among the PE samples produced via heterogeneous polymerization. A detailed investigation suggested that surface functional groups, such as OH and Cl, coordinate with the active Ni species via their lone pairs and terminate the ethylene polymerization. Microstructure analysis of the PE confirm that the supporting substrate provides the chance to modulate the chain-walking behavior of these Ni catalysts. Systematic high-temperature 1H and 13C NMR analysis indicated that the PE branching density could significantly decrease by surface confinement from the solid substrate. Until now, such microstructure control has been mainly realized via the laborious synthesis of bulky a-diimine ligands.