Solidification Conditions Research Articles

Study on Mechanical Properties of Sandy Soil Solidified by Enzyme-Induced Calcium Carbonate Precipitation (EICP)

Earth–rock dams are widely distributed in China and play an important role in flood control, water storage, water-level regulation, and water quality improvement. As an emerging seepage control and reinforcement technology in the past few years, enzyme (urease)-induced calcium carbonate precipitation (EICP) has the qualities of durability, environmental friendliness, and great economic efficiency. For EICP-solidified standard sand, this study analyzes the effect of dry density, amount of cementation, standing time, perfusion method, and other factors on the permeability and strength characteristics of solidified sandy soil by conducting a permeability test and an unconfined compression test and then working out the optimal solidification conditions of EICP. Furthermore, a quantitative relationship is established between the permeability coefficient (PC), unconfined compressive strength (UCS), and CaCO3 generation (CG). The test findings indicate that the PC of the solidified sandy soil decreases and the UCS rises as the starting dry density, amount of cementation, and standing time rise. With the increase of CG, the PC of the solidified sandy soil decreases while the UCS increases, indicating a good correlation among PC, UCS, and CG. The optimal condition of solidification by EICP is achieved by the two-stage grouting method with an initial dry density of 1.65 g/cm3, cementation time of 6 d, and standing time of 5 d. Under such conditions, the permeability of the solidified sandy soil is 6.25 × 10−4 cm/s, and the UCS is 1646.94 kPa. The findings of this study are of great theoretical value and scientific significance for guiding the reinforcement of earth–rock dams.

Effect of Mold Material and Solidification Conditions on Microstructural and Mechanical Properties of Directionally Solidified Sn-0.7Cu Alloy Developed Using Microwave Energy

Effect of Mold Material and Solidification Conditions on Microstructural and Mechanical Properties of Directionally Solidified Sn-0.7Cu Alloy Developed Using Microwave Energy

Interpenetrating microstructure in laser powder-bed fusion parts using selective rescanning

In-situ microstructural control is desirable in additively manufactured metal parts due to limited post-processing options for net-shaped components. Here, we introduce a novel selective rescanning approach to control the local solidification conditions and the microstructure in metal parts produced by laser powder-bed fusion (LPBF). We show that the melt pool dimensions, grain size, and sub-grain cell structure can be selectively varied in three dimensions to engineer the mechanical response of LPBF parts. The lattice-based rescanning strategy enables the formation of an interpenetrating microstructure comprised of fine and coarse grains. The localized heating and cooling-induced thermal stresses increase the hardness and tensile strength of rescanned specimens. The study shows the potential of selective rescanning strategy as a promising avenue for achieving precise control of microstructure and properties in as-printed LPBF parts without subsequent processing.

On the origin of alkali feldspar megacrysts in granitoids. Part 2: evidence for nucleation and growth under magmatic conditions from crystal size distributions of the Cathedral Peak Granodiorite, California, USA

The mechanisms whereby alkali feldspar megacrysts form have been debated for several decades; yet, we do not understand well the processes that lead to their formation. We take advantage of glacially polished outcrop surfaces from the Cathedral Peak Granodiorite in the Tuolumne Intrusive Complex, CA to quantitatively characterize alkali feldspar textures, to provide better insight into their origin. On the glacially polished surfaces, we traced alkali feldspar crystals > 10 mm in the field. From the same localities, we also collected large slabs and stained them to reveal feldspar textures for crystals < 20 mm in size. We scaned the resulting field tracings and rock slabs to quantify CSDs using image processing techniques with the software ImageJ. The CSDs from glacially polished outcrop surfaces and complementary polished and stained rock slabs reveal two stages of crystallization. Crystals > 20 mm show log-linear CSDs with shallow slopes, suggesting magmatic nucleation and growth on timescales of thousands of years. Crystals < 20 mm define a second stage of crystallization, with much steeper slopes, suggesting a period of enhanced nucleation leading to formation of a groundmass during the final stages of solidification on timescales of decades to centuries. We do not find any evidence for CSDs affected by textural coarsening, or any effects of subsolidus processes. Our data suggest that these megacrysts form in large, slowly cooling magma, where low nucleation rates dominate. These crystals are not special in their magmatic formation—only in their size. A change in solidification conditions led to the formation of a groundmass, which warrants further study to better understand this crystallization stage in a plutonic environment.

A computational study of solidification kinetics in multicomponent alloys

Recent advances in materials processing technologies highlight the need to understand solidification kinetics in multicomponent alloys. Using a finite interface dissipation phase field model, we investigate planar front solidification rates in ternary alloys, used as a model system, as a function of various mass transport processes and nonequilibrium conditions. Simulation results reveal that the off-diagonal diffusion coefficients play an important role in controlling the solid–liquid (S–L) interface velocity and solute partition coefficients. Specifically, under various rapid solidification conditions negative values for the off-diagonal diffusion coefficients increase the solute partition coefficients due to the depletion of liquid phase concentrations ahead of the S–L interface. Given an undercooling, the S–L interface velocity increases with decreasing the off-diagonal diffusion coefficients. In broad terms, our work quantifies the role of coupled mass transport processes and nonequilibrium effects in solidification rates of multicomponent alloys.

Influence of ultrasonic excitation on the melt pool and microstructure characteristics of Ti-6Al-4V at powder bed fusion additive manufacturing solidification velocities

Cavitation and acoustic streaming created by in situ high-intensity ultrasound have been exploited for microstructural refinement during directed energy deposition (DED) additive manufacturing (AM) of alloy Ti‐6Al‐4V. Whether the same ultrasound-driven mechanisms are applicable to powder bed fusion (PBF) of Ti‐6Al‐4V remains uncertain. The primary factors that control the microstructure during deposition processing are the solidification velocity and temperature gradient, which are orders of magnitude higher for PBF than DED processes. This work examines the role of high-intensity ultrasound on the melt pool and resulting microstructure of Ti‐6Al‐4V under PBF solidification conditions. The effect of the laser scanning velocity and ultrasonic excitation on the melt pool and grain structure characteristics is interrogated through controlled line scans and area scans without powder. Temperature field simulations are used to estimate the solidification velocities and temperature gradients and to compare the processing conditions with previously characterized columnar-to-equiaxed transition diagrams. The acoustic pressures during processing are estimated using a coupled field acoustic-elastic finite element simulation and used in a Keller-Miksis model to assess the likelihood of cavitation vs. cavity size. The microstructure of the samples with or without ultrasonic excitation was characterized, allowing the equivalent grain diameters and aspect ratios to be compared. All samples, including those exposed to scanning velocities where cavitation did not occur, showed a reduction in grain aspect ratio when subjected to ultrasonic excitation, but the effect on equivalent grain diameter was inconclusive. A key finding is that the primary effects of ultrasonic excitation may be attributed to acoustic streaming, rather than cavitation, during PBF processing. Further development of the ultrasonic excitation technique explored may permit tailoring of the microstructure and texture characteristics in bulk AM parts.

GrainGNN: A dynamic graph neural network for predicting 3D grain microstructure

We propose GrainGNN, a surrogate model for the evolution of polycrystalline grain structure under rapid solidification conditions in metal additive manufacturing. High fidelity simulations of solidification microstructures are typically performed using multicomponent partial differential equations (PDEs) with moving interfaces. The inherent randomness of the PDE initial conditions (grain seeds) necessitates ensemble simulations to predict microstructure statistics, e.g., grain size, aspect ratio, and crystallographic orientation. Currently such ensemble simulations are prohibitively expensive and surrogates are necessary.In GrainGNN, we use a dynamic graph to represent interface motion and topological changes due to grain coarsening. We use a reduced representation of the microstructure using hand-crafted features; we combine pattern finding and altering graph algorithms with two neural networks, a classifier (for topological changes) and a regressor (for interface motion). Both networks have an encoder-decoder architecture; the encoder has a multi-layer transformer long-short-term-memory architecture; the decoder is a single layer perceptron.We evaluate GrainGNN by comparing it to high-fidelity phase field simulations for in-distribution and out-of-distribution grain configurations for solidification under laser power bed fusion conditions. GrainGNN results in 80%–90% pointwise accuracy; and nearly identical distributions of scalar quantities of interest (QoI) between phase field and GrainGNN simulations compared using Kolmogorov-Smirnov test. GrainGNN's inference speedup (PyTorch on single x86 CPU) over a high-fidelity phase field simulation (CUDA on a single NVIDIA A100 GPU) is 150×–2000× for 100-initial grain problem. Further, using GrainGNN, we model the formation of 11,600 grains in 220 seconds on a single CPU core.

Emergence of rapid solidification microstructure in additive manufacturing of a Magnesium alloy

Abstract Bioresorbable Mg-based alloys with low density, low elastic modulus, and excellent biocompatibility are outstanding candidates for temporary orthopedic implants. Coincidentally, metal additive manufacturing (AM) is disrupting the biomedical sector by providing fast access to patient-customized implants. Due to the high cooling rates associated with fusion-based AM techniques, they are often described as rapid solidification processes. However, conclusive observations or rapid solidification in metal AM — attested by drastic microstructural changes induced by solute trapping, kinetic undercooling, or morphological transitions of the solid-liquid interface — are scarce. Here we study the formation of banded microstructures during laser powder-bed fusion (LPBF) of a biomedical-grade Magnesium-rare earth alloy, combining advanced characterization and state-of-the-art thermal and phase-field modeling. Our experiments unambiguously identify microstructures as the result of an oscillatory banding instability known from other rapid solidification processes. Our simulations confirm that LPBF-relevant solidification conditions strongly promote the development of banded microstructures in a Mg-Nd alloy. Simulations also allow us to peer into the sub-micrometer nanosecond-scale details of the solid-liquid interface evolution giving rise to the distinctive banded patterns. Since rapidly solidified Mg alloys may exhibit significantly different mechanical and corrosion response compared to their cast counterparts, the ability to predict the emergence of rapid solidification microstructures (and to correlate them with local solidification conditions) may open new pathways for the design of bioresorbable orthopedic implants, not only fitted geometrically to each patient, but also optimized with locally-tuned mechanical and corrosion properties.

Towards Understanding Formation Mechanism of Cellular Structures in Laser Powder Bed Fused AlSi10Mg.

A new approach is proposed that identifies three different zones of the Si-rich network structure (the cellular structure) in laser powder bed fused (LPBF) AlSi10Mg alloy, based on the variation in morphology, grain growth transition, and melt pool solidification conditions. The three identified zones are denoted in the present work as the liquid solidification zone (LSZ), the mushy solidification zone (MSZ), and the heat affected zone (HAZ). The LSZ is the result of liquid-solid transformation, showing small planar growth at the boundary and large cellular growth in the center, while the MSZ is related to a semisolid reaction, and the HAZ arises from a short-time aging process. The boundary between the LSZ and MSZ is identified by the change of grain growth direction and the Si-rich network advancing direction. The boundary between MSZ and HAZ is identified by the start of the breakdown of the Si-rich network. In addition, it is found that the fracture is generated in and propagates along the HAZ during tensile tests.

Improving Microstructural Homogeneity of Investment‐Cast Single‐Crystalline Ni‐Base Superalloys by Using Fluidized Carbon Bed Cooling

One impeding factor for faster solidification in typical investment casting processes is the limited heat removal capability. This study compares the classical Bridgman technique and the fluidized carbon bed cooling (FCBC) casting method. In particular, the influence of the thermal design is investigated in terms of microstructural and compositional homogeneity in single‐crystalline CMSX‐4. These investment casting processes mainly differ in the baffle type used to insulate the hot and cold zones inside of the casting vessel. While the Bridgman process is based on radiation cooling, the FCBC casting process relies on a dynamic baffle made of expanded graphite that is buoyant on fluidized bed of glassy carbon beads. Such a dynamic insulation layer can adapt to the cross‐sectional variation of ceramic shell molds and provide a better thermal insulation, which results in more uniform solidification conditions. This enables a reduction in the primary dendrite arm spacing by up to 26%, an increase in the homogeneity of the microstructure and composition, as well as a reduction in overall porosity and mean pore size of 41%.

Controlling the Fe-intermetallic phases and mechanical properties of secondary Al-9Si-1Fe alloy with Cr and Mn additions

In secondary Al-Si based alloys, microalloying with Mn and Cr can modify harmful platelet-type AlxFeySiz intermetallic phases to less detrimental α-Alx(Fe, Mn, Cr)ySiz phase (script or polygonal morphologies). However, the α-Alx(Fe, Mn, Cr)ySiz phase morphology, phase composition and the addition of Fe-correcting elements can be influenced by solidification conditions. Therefore, this research is aimed to highlight the morphological evolution and mechanisms of α-Alx(Fe, Mn, Cr)ySiz phase in a Cr added Al-9 %Si-1 %Fe-0.2 %Cr (all weight percentage thereafter, unless otherwise stated) alloy with varying Mn concentrations (0.25 %, 0.5 %, and 0.8 %). Microstructure evolution of Fe intermetallic phases is investigated under different casting conditions using a wedge-shaped die, Cu-chill block and melt quenching experiments. Thermodynamic simulations have been performed using CALculation of PHAse Diagrams (CALPHAD) method and compared with the experimental results for phase composition and formation temperatures of α-Alx(Fe, Mn, Cr)ySiz phase. The results indicated that for 0.25Mn-0.2Cr addition to Al-9Si-1Fe alloy, compact morphology containing polygonal phases are formed in Cu-chill casting, while the wedge castings predominantly show a mixed structure with platelets and script type morphologies. Tensile tests revealed a higher elongation value of 6.6 % for mixed structure with platelet and script phases, which is decreased to 4.2 % for polygonal phases in Al-9Si-1Fe-0.2Cr-0.25Mn alloy. This study highlights the importance of solidification conditions on morphologies of Fe-intermetallic phases and the mechanical properties by comparing selected literature relevant to high pressure die-casting process.

Exceptional grain refinement induced by dispersed MgO particles in TIG-welded AZ31 alloy

Due to the low content of alloying elements and the lack of effective nucleation sites, the fusion zone (FZ) of tungsten inert gas (TIG) welded AZ31 alloy typically exhibits undesirable coarse columnar grains, which can result in solidification defects and reduced mechanical properties. In this work, a novel welding wire containing MgO particles has been developed to promote columnar-to-equiaxed transition (CET) in the FZ of TIG-welded AZ31 alloy. The results show the achievement of a fully equiaxed grain structure in the FZ, with a significant 71.9% reduction in grain size to 41 µm from the original coarse columnar dendrites. Furthermore, the combination of using MgO-containing welding wire and pulse current can further refine the grain size to 25.6 µm. Microstructural analyses reveal the homogeneous distribution of MgO particles in the FZ. The application of pulse current results in an increase in the number density of MgO (1-2 µm) from 5.16 × 104 m−3 to 6.18 × 104 m−3. The good crystallographic matching relationship between MgO and α-Mg matrix, characterized by the orientation relationship of [112‾0]α−Mg//[01‾1]MgO and (0002)α−Mg//(111)MgO, indicates that the MgO particles can act as effective nucleation sites for α-Mg to reduce nucleation undercooling. According to the Hunt criteria, the critical temperature gradient for CET is greatly enhanced due to the significantly increased number density of MgO nucleation sites. In addition, the correlation with the thermal simulation results reveals a transition in the solidification conditions within the welding pool from the columnar grain zone to the equiaxed grain zone in the CET map, leading to the realization of CET. The exceptional grain refinement has contributed to a simultaneous improvement in the strength and plasticity of welded joints. This study presents a novel strategy for controlling equiaxed microstructure and optimizing mechanical properties in fusion welding or wire and arc additive manufacturing of Mg alloy components.

Rapidly solidified Al-Cu-Co quasicrystalline alloy: microstructure and catalytic properties

This study presents the characterisation and evaluation of the catalytic potential of an alloy with a composition referring to the quasicrystal from the Al-Cu-Co system. The material was synthesised through a melt-spinning process, providing rapid solidification conditions and a favourable material form of thin ribbons. Microstructural and chemical analyses were performed using scanning electron microscopy, X-ray diffraction and transmission electron microscopy methods. The obtained results confirmed the presence of a major phase which was a decagonal quasicrystal in the form of globular grains and additional crystalline phases occurring along the grain boundaries. The catalytic potential of the materials was explored using phenylacetylene hydrogenation. The catalyst, obtained by pulverising the ribbons, demonstrated catalytic activity with over 40% phenylacetylene conversion and selectivity exceeding 60% for styrene production under mild reaction conditions. The recovered catalyst displayed a stable phase composition, as confirmed by X-ray diffraction, and unchanged morphology, indicating the possibility of catalyst reuse for subsequent reaction cycles. The presented results provide insight into the experimental verification of the catalytic properties of the developed environmentally friendly catalysts.

Solidification behavior and texture of 316L austenitic stainless steel by laser wire directed energy deposition

The solidification behavior and crystallographic texture of 316L austenitic stainless steel builds fabricated via laser-wire directed energy deposition additive manufacturing (AM) were investigated. Shielding gas set-up and build type (single-track vs. multi-track) were varied, which led to changes in the solidification conditions and the final build compositions. Primary δ-ferrite solidification is predicted based on equilibrium thermodynamic predictions of the bulk feedstock composition. However, the increased solidification velocity of the AM process promotes a solidification mode transition from primary δ-ferrite to primary austenite. Skeletal δ-ferrite and lathy δ-ferrite associated with primary δ-ferrite solidification and interdendritic δ-ferrite associated with primary austenite solidification were observed among the laser-wire directed energy deposition walls. Skeletal and interdendritic δ-ferrite exhibited a (001)δ // (001)γ parallel relationship with austenite and a {001} solidification texture aligned with the build direction for both austenite and δ-ferrite. Lathy δ-ferrite exhibited a Kurdjumov-Sachs orientation relationship, (101)δ//(11¯1)γ, with austenite due to austenite nucleation in the solid-state. It is shown that the solidification behavior of AM 316L is accurately represented by as-built composition data and a dendrite growth model, which combined, encompasses the compositional and solidification condition impacts on the solidification pathway.

New insights on dislocation forming mechanism of nickel-based superalloy fabricated by laser powder bed fusion

In this work, the forming mechanism of dislocation in laser powder bed fusion (LPBF) built IN738LC alloy was elucidated by studying the intrinsic relationships between the density and distribution of dislocation and the orientation and size of dendrite. The substructures with different dislocation densities and cell spacings were customized by controlling solidification conditions, and analyzed by multi-scale characterizing methods. The results show that an increase in geometrically necessary dislocation (GND) density from 1.48 × 1014 to 1.82 × 1014m−2 is accompanied by a decrease in cell spacing from 1046 nm to 640 nm as the solidification rate increases. However, the relationship between GND density and cell spacing doesn't follow Ashby's model. This indicates that GNDs are not entirely generated by thermal strain itself. On the other hand, the characterization results of cell structures show that a number of misfit dislocations are formed by the thermal strain induced deflection growth of the dendrites. These prove that the dislocations are comprehensively generated by thermal strain itself and dendritic deflection growth.

Laser remelting of a novel carbide-reinforced Ni-Co-Cr-Hf-C alloy with ultra-fine HfC dispersion

The dispersion of stable nano-particles is essential for improving the high temperature creep properties of Ni-base alloys. Additive manufacturing by laser powder bed fusion (LPBF) offers novel means to tailor particle distributions taking advantage of the fast and directional solidification inside travelling melt pools. To benefit of this opportunity, a novel in-situ carbide reinforced alloy Ni − 32Co − 28Cr − 3.5Hf − 0.25C (wt. %) was developed. In a first rapid approach laser remelting of a cast alloy block was used to mimic LPBF solidification conditions. After a subsequent annealing treatment at 1150 °C for 6 h, an ultra-fine and intragranular dispersion of nano-sized HfC particles was obtained. The HfC particles have an average size of 54 nm and a mean inter-particle distance of 326 nm, which is an exceptional refinement compared to the as-cast microstructure. The number density of HfC particles ranges around 1 × 1012/m2 in 2D and is estimated to be 2 × 1019/m3 in 3D.

Morphological diversity in directionally-solidified microstructures with varying anisotropy of solid-liquid interfacial free energy

Morphological diversity in directionally-solidified microstructures of fcc-based binary alloys was investigated using quantitative phase-field simulation. The growth morphology for each set of anisotropy parameters was identified from the degree of undercooling of the dendrite tip and a morphology map was constructed. We investigated the effects of solidification conditions and alloy systems on growth morphology and found that the pulling speed has a significant effect, while temperature gradient and partition coefficient have only small effects on the morphology selection. Furthermore, we examined the emergence of doublon and triplet dendrites under conditions of weak interfacial anisotropy.

Achieving a Columnar-to-Equiaxed Transition Through Dendrite Twinning in High Deposition Rate Additively Manufactured Titanium Alloys

AbstractThe coarse β-grain structures typically found in titanium alloys like Ti–6Al–4V (wt pct, Ti64) and Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242), produced by high deposition rate additive manufacturing (AM) processes, are detrimental to mechanical performance. Certain modified processing conditions have been shown to lead to a more refined grain structure, which has generally been attributed to a change in the solidification conditions with respect to the experimental Hunt diagram proposed by Semiatin and Kobryn. It is shown that with Wire Arc AM (WAAM) increasing the wire feed speed (WFS) is effective in promoting a columnar-equiaxed transition (CET). Conversely, estimates of the dendrite-tip undercooling using the KGT model suggest that this will be too small for free nucleation without the addition of artificial nucleants, due to the very low solute partitioning in Ti alloys. It is also shown that it is difficult to promote a CET with plasma transferred arc WAAM as computational fluid dynamics (CFD) melt-pool simulations indicate that the solidification parameters remain within the columnar region on the Semiatin-Kobryn Hunt map, within the constraints of a stable process. However, a high fraction of twin boundaries was observed in the refined β-grain structures seen at high WFS. This has been attributed to departure of $$\left\langle {001} \right\rangle _{\beta }$$ 001 β alignment from the direction of maximum thermal gradient, caused by the curvature of the fusion boundary, stimulating dendrite twinning during solidification. In addition, it is shown that increasing the WFS leads to a change in melt-pool geometry and a reduction of remelt depth, which promoted dendrite twinning and grain refinement.

An anisotropic lattice Boltzmann - phase field model for dendrite growth and movement in rapid solidification of binary alloys

A model coupling the lattice Boltzmann and the phase field methods with anisotropic effects is proposed, which is used to numerically describe the growth and movement of dendrites in rapid solidification of alloys. The model is quantitatively validated by the simulation of the continuous growth and the drafting-kissing-tumbling phenomenon of two falling particles, and then applied to investigate the effects of dendrite movement and interfacial non-equilibrium on evolution of dendritic patterns for Si-9.0at%As and the CET for Al-3.0wt%Cu alloys. Both the growth and remelt processes of isolated dendrites are studied, and the result reveals the remelting influences on dendrite growth and solute micro-segregation in the condition of directional solidification. This work demonstrates that the proposed model has a wide range of applicability and great potential to simulate the microstructure evolution with various solidification conditions.

Modeling of rapid solidification in Laser Powder Bed Fusion processes

This paper addresses a numerical framework for modeling site-specific microstructure evolution in Laser Powder Bed Fusion (LPBF), where rapid solidification significantly influences the microstructure and properties of manufactured components. The fine microstructure and rapid solidification can enhance mechanical properties in final parts, yet pose challenges in controlling microstructure and addressing potential defects. In this study, an in-house developed free surface thermo-fluidic Computational Fluid Dynamics (CFD) model is coupled with a binary alloy phase-field model characterized by rapid solidification kinetics. The CFD model considers complex fluid flows related to Marangoni and recoil pressure, laser-material interactions, and rapid melting and solidification of the melt pool. A convergence study for the phase-field model is conducted, comparing simulated partition coefficients with the Continuous Growth Model (CGM) theory. The study investigates the individual effects of temperature gradients and solidification rates on simulated microstructure. Results indicate that, for the same order of magnitude change, the solidification rate has a more pronounced effect on microstructure than temperature gradients. The paper concludes with the simulation of site-specific microstructure evolution within the melt pool, comparing the results with experiments conducted at varied scanning speeds. The predicted microstructure aligns well with experimental measurements, suggesting the potential utility of the developed phase-field model in quantitative microstructural modeling. Overall, this work contributes to understanding microstructure evolution under rapid solidification conditions, providing a fundamental basis for optimizing the LPBF process.