Rapid Solidification Behavior Research Articles

Numerical modelling and experimental validation on the discharging performance of paraffin/graphite matrix composite with various configurations of bulk density and wall temperature

Graphite material in the form of expanded has proven to be an effective material to remedy the low thermal conductivity of paraffin storage medium. The present work focuses on the solidification performance of carbon-based approach via graphite matrix composite with phase change to eliminate the bottleneck of low thermal conductivity issue of phase change materials for thermal energy storage applications beyond the current literature. Paraffin/graphite matrix composites with various bulk density values of 0 kg/m3 (Case 1), 23 kg/m3 (Case 2), 50 kg/m3 (Case 3), 100 kg/m3 (Case 4), and 143 kg/m3 (Case 5) under various operating conditions of wall temperature (Twall: 15 °C, 25 °C, 35 °C, and 45 °C), which mimic real applications' fluid temperature, are performed numerically in detail. The numerical model was validated with experimental outputs performed by the authors. Numerical results show that paraffin/graphite matrix composites have a great potential for the solidification/discharging process to recover the heat stored in the charging/melting period. Uniform and rapid solidification behaviour is achieved with the paraffin/graphite matrix composite in terms of conduction-dominated heat transfer via abundant thermal bridges compared to the pure paraffin case. Higher bulk density values and lower cold surface temperatures present superior performance in solidification rate and thermal energy recovery rate. However, the effect of the bulk density is slowed down with the beyond of certain value of bulk density (100 kg/m3). For 100 kg/m3 (Case 4) at Twall = 15 °C, the heat recovery rate is enhanced by 31.17 times and 3.84 times compared to the 0 kg/m3 and 23 kg/m3. The heat recovery rate of 100 kg/m3 is improved by 350.8 % for Twall = 15 °C in comparison with Twall = 45 °C. The total solidification rate (t = 17.5 min) increased by 90.94 % and 62.94 % for 100 kg/m3 at Twall = 35 °C compared to 0 kg/m3 and 23 kg/m3, respectively.

Rapid solidification of Al-Si alloys using differential fast scanning calorimetry

Tailoring microstructures and properties in metal alloys, e.g., in additive manufacturing, requires a deep understanding of its rapid solidification behavior. However, in many rapid solidification processes, in-situ investigations are difficult due to the lack of accurate measurements of cooling rates and nucleation undercooling. This study used differential fast scanning calorimetry (DFSC) to investigate the rapid solidification of micro-sized Al-Si alloy particles, with compositions including Al-Si1, Al-Si10, Al-Si12, and Al-Si20 (mass%), by employing controllable cooling rates ranging from 100 K/s to 95,000 K/s. Based on the analysis of nucleation undercooling and microstructure, the solidification sequence and different mechanisms of the α-Al phase formation were proposed. A modified model using classical nucleation theory (CNT), including surface heterogeneous nucleation, and interface/bulk heterogeneous nucleation for α-Al is employed. This study presents a practical approach to examining the rapid solidification behavior of metal particles, allowing the distinction between surface and interface/bulk nucleation.

Morphological stability of solid-liquid interfaces under additive manufacturing conditions

Understanding rapid solidification behavior at velocities relevant to additive manufacturing (AM) is critical to controlling microstructure selection. Although in-situ visualization of solidification dynamics is now possible, systematic studies under AM conditions with microstructural outcomes compared to solidification theory remain lacking. Here we measure solid-liquid interface velocities of Ni-Mo-Al alloy single crystals under AM conditions with synchrotron X-ray imaging, characterize the microstructures, and show discrepancies with classical theories regarding the onset velocity for absolute stability of a planar solid-liquid interface. Experimental observations reveal cellular/dendritic microstructures can persist at velocities larger than the expected absolute stability limit, where banded structure formation should theoretically appear. We show that theory and experimental observations can be reconciled by properly accounting for the effect of solute trapping and kinetic undercooling on the velocity-dependent solidus and liquidus temperatures of the alloy. Further theoretical developments and accurate assessments of key thermophysical parameters – like liquid diffusivities, solid-liquid interface excess free energies, and kinetic coefficients – remain needed to quantitatively investigate such discrepancies and pave the way for the prediction and control of microstructure selection under rapid solidification conditions.

Rapid solidification of hypoeutectic aluminum copper alloys using fast-scanning calorimetry

The rapid solidification behaviour of binary aluminum copper alloys in the hypoeutectic range of compositions has been studied in this work. Solidification at cooling rates up to 40,000 °Cs−1 can be achieved using a chip-based calorimeter, but present challenges in sample preparation and temperature correction, which are addressed in this work. Data from calorimetry has been used to produce fraction of solid curves which are important input to metal manufacturing processing models. Undercoolings range from around 50 °C at 100 °Cs−1 to around 100 °C at 40,000 °Cs−1. From these, a kinetic phase diagram was constructed for the hypoeutectic region of the Al-Cu system, for cooling rates up to 40,000 °Cs−1. The undercoolings measured in this experiment are considerably higher than those predicted theoretically for growth-based rapid solidification, but not as high as those predicted under homogeneous nucleation.

Directly fabricated Al2O3/GdAlO3 eutectic ceramic with large smooth surface by selective laser melting: Rapid solidification behavior and thermal field simulation

Selective laser melting (SLM), a novel approach for one-step melting and solidifying ceramic powder beds layer by layer without post-process of degreasing and sintering, has been developed to directly prepare highly dense (>95 %) Al2O3/GdAlO3(GAP) eutectic composite ceramics with large smooth surfaces. Compact net-shaped plates with the maximum size of 73 × 24 × 5 mm3 are obtained by different strategies of laser pre-heating and multi-tracks’ deposition without any binders. Combined with the finite element thermodynamic coupling simulation results, it is proved that the stress between the substrate and depositions during SLM can be greatly reduced by the step-up preheating, and thus effectively improving the ceramic forming quality. The macro-morphology, microstructure evolution, rapid solidification behavior and mechanical properties of the SLM-ed eutectic ceramics are systematically investigated at different laser processing parameters. The microstructure transforms from ultra-fine irregular eutectic to complex regular eutectic with the increase of the scanning rate. The average eutectic spacing, and solidification rate has an approximately linear relationship consistent with the Jackson-Hunt (JH) model. The microhardness and fracture toughness can reach 17.1 ± 0.2 GPa and 4.5 ± 0.1 MPa·m1/2, respectively. The results indicate that SLM method is a highly effective technique for fabricating high-performance net-shaped structural composite ceramics.

Performance Testing and Rapid Solidification Behavior of Stainless Steel Powders Prepared by Gas Atomization.

Gas atomization is a widely used method to produce the raw powder materials for additive manufacturing (AM) usage. After the metal alloy is melted to fusion, gas atomization involves two relatively independent processes: liquid breakup and droplet solidification. In this paper, the solidification behavior of powder during solidification is analyzed by testing the powder’s properties and observing microstructure of a martensitic stainless steel (FeCrNiBSiNb). The powder prepared by gas atomization has high sphericity and smooth surface, and the yield of qualified fine powder is 35%. The powder has typical rapid solidification structure. Collision between powders not only promotes nucleation, but also produces more satellite powder. The segregation of elements in powder is smaller as the result of high cooling rate which can reaches 4.2 × 105 K/s in average. Overall, the powder prepared by gas atomization is found to have good comprehensive properties, desired microstructure, and accurate chemical component, and it is suitable for various additive manufacturing techniques.

Nucleation Behavior of a Single Al-20Si Particle Rapidly Solidified in a Fast Scanning Calorimeter.

Understanding the rapid solidification behavior characteristics, nucleation undercooling, and nucleation mechanism is important for modifying the microstructures and properties of metal alloys. In order to investigate the rapid solidification behavior in-situ, accurate measurements of nucleation undercooling and cooling rate are required in most rapid solidification processes, e.g., in additive manufacturing (AM). In this study, differential fast scanning calorimetry (DFSC) was applied to investigate the nucleation kinetics in a single micro-sized Al-20Si (mass%) particle under a controlled cooling rate of 5000 K/s. The nucleation rates of primary Si and secondary α-Al phases were calculated by a statistical analysis of 300 identical melting/solidification experiments. Applying a model based on the classical nucleation theory (CNT) together with available thermodynamic data, two different heterogeneous nucleation mechanisms of primary Si and secondary α-Al were proposed, i.e., surface heterogeneous nucleation for primary Si and interface heterogenous nucleation for secondary α-Al. The present study introduces a practical method for a detailed investigation of rapid solidification behavior of metal particles to distinguish surface and interface nucleation.

Nucleation behaviour and microstructure of single Al-Si12 powder particles rapidly solidified in a fast scanning calorimeter

The understanding of rapid solidification behaviour, e.g. the undercooling versus growth velocity relationship, is crucial for tailoring microstructures and properties in metal alloys. In most rapid solidification processes, such as additive manufacturing (AM), in situ investigation of rapid solidification behaviour is missing because of the lack of accurate measurement of the cooling rate and nucleation undercooling. In the present study, rapid solidification of single micro-sized Al-Si12 (mass%) particles of various diameters has been investigated via differential fast scanning calorimetry employing controllable cooling rates from 100 to 90,000 K s−1 relevant for AM. Based on nucleation undercooling and on microstructure analysis of rapidly solidified single powder particles under controlled cooling rates, two different heterogeneous nucleation mechanisms of the primary α-Al phase are proposed. Surface heterogeneous nucleation dominates for particles with diameter smaller than 23 μm. For particles with diameter larger than 23 μm, the nucleation of the primary α-Al phase changes from surface to bulk heterogeneous nucleation with increasing cooling rate. The results indicate that at large undercoolings (> 95 K) and high cooling rates (> 10,000 K s−1), rapid solidification of single particle can yield a microstructure similar to that formed in AM. The present work not only proposes new insight into rapid solidification processes, but also provides a theoretical foundation for further understanding of microstructures and properties in additively manufactured materials.

A methodology to integrate melt pool convection with rapid solidification and undercooling kinetics in laser spot welding

In laser spot welding because of small melt pool and short process duration, the welding process takes place rapidly with high cooling rates. As a result the analysis of solidification in laser spot welding becomes challenging. The solidification takes place generally with non-equilibrium kinetics involving undercooling and recalescence. In this work, a methodology for handling the rapid solidification in laser spot welding is developed by solving the coupled transient conservation equations of mass, momentum and energy. Undercooling and recalescence are coupled with the melt pool convection and heat transfer using an interface tracking algorithm which predicts the rapidly evolving interface temperature and interface speed. The melt pool result is first validated with the published results. Thereafter, the effect of undercooling is investigated by presenting a detailed comparison of results obtained from the rapid (non-equilibrium) solidification model and the conventional (equilibrium) solidification model. The results conclude that undercooling and recalescence have profound effect on melt pool dimension, temperature, solidification time and melt pool convection. The rapid solidification behavior is delineated further with the help of solidification parameters, such as interface temperature, solidified thickness, temperature gradient (G) and solidification rate (R). The weld microstructure is estimated using the combined values of GR and G/R.

Rapid solidification growth mode transitions in Al-Si alloys by dynamic transmission electron microscopy

In situ dynamic transmission electron microscope (DTEM) imaging of Al-Si thin-film alloys was performed to investigate rapid solidification behavior. Solidification of alloys with compositions from 1 to 15 atomic percent Si was imaged during pulsed laser melting and subsequent solidification. Solely α-Al solidification was observed in Al-1Si and Al-3Si alloys, and solely kinetically modified eutectic growth was observed in Al-6Si and Al-9Si alloys. A transition in the solidification mode in eutectic and hypereutectic alloys (Al-12Si and Al-15Si) from nucleated α-Al dendrites at lower solidification velocities to planar eutectic growth at higher solidification velocities was observed, departing from trends previously seen in laser-track melting experiments. Comparisons of the growth modes and corresponding velocities are compared with previous solidification models, and implications regarding the models are discussed.

Erratum to: Microstructure Evolution and Rapid Solidification Behavior of Blended Nickel-Based Superalloy Powders Fabricated by Laser Powder Deposition

Laser powder deposition was performed on a substrate of Inconel 738 using blended powders of Mar M247 and Amdry DF3 with a ratio of 4:1 for repairing purposes. In the as-deposited condition, continuous secondary phases composed of γ-Ni3B eutectics and discrete (Cr, W)B borides were observed in inter-dendritic regions, and time-dependent nucleation simulation results confirmed that (Cr, W)B was the primary secondary phase formed during rapid solidification. Supersaturated solid solution of B was detected in the γ solid solution dendritic cores. The Kurz–Giovanola–Trivedi model was performed to predict the interfacial morphology and correlate the solidification front velocity (SFV) with dendrite tip radius. It was observed from high-resolution scanning electron microscopy that the dendrite tip radius of the upper region was in the range of 15 to 30 nm, which yielded a SFV of approx 30 cm/s. The continuous growth model for solute trapping behavior developed by Aziz and Kaplan was used to determine that the effective partition coefficient of B was approximately 0.025. Finally, the feasibility of the modeling results were rationalized with the Clyne–Kurz segregation simulation of B, where Clyne–Kurz prediction using a partition coefficient of 0.025 was in good agreement with the electron probe microanalysis results.

Microstructure Evolution and Rapid Solidification Behavior of Blended Nickel-Based Superalloy Powders Fabricated by Laser Powder Deposition

Microstructure Evolution and Rapid Solidification Behavior of Blended Nickel-Based Superalloy Powders Fabricated by Laser Powder Deposition

Rapid solidification and liquid-phase separation of undercooled CoCrCuFexNi high-entropy alloys

Fluxing and cyclic superheating technique was used to investigate the rapid solidification behavior of CoCrCuFexNi (x = 1.0, 1.5, 2.0, molar concentration) high-entropy alloys in this study. The microstructures of CoCrCuFexNi (x = 1.0, 1.5, 2.0) high-entropy alloys solidified at different undercoolings (ΔT) were investigated. Liquid-phase separation leading to Cu-rich and Cu-depleted regions, were obtained in the solidified microstructure from highly undercooled melt. This occurs when the melt undercooling exceeds a critical undercooling (ΔTcrit) of 160 K for CoCrCuFeNi, 190 K for CoCrCuFe1.5Ni and 293 K for CoCrCuFe2Ni alloy. However, typical dendrites and interdendritic regions were observed in rapid-solidified CoCrCuFexNi alloys prepared from melts with a small undercooling (ΔT < ΔTcrit). Conversely, a large amount of Cu-rich spheres and even egg-type structures were observed in alloys solidified from melts with large degree of undercooling, ΔT in excess of the critical value, ΔTcrit. A large amount of Cu-rich nano-phases were found in the matrix, possibly, due to the precipitation of Cu-rich phase from the supersaturated solid solution obtained during solidification. The positive enthalpies of mixing between Cu and the other elements in the multi-component alloys resulted in the occurrence of liquid-phase separation prior to the liquid–solid transformation starts. The sluggish diffusion effect of high-entropy alloys and rapid solidification play an important part in the precipitation of nanophase during the solid-state transformation in the Cu-based matrix. Similar to other immiscible alloys, liquid-phase separation occurred when a critical undercooling was exceeded. Differently, nanophases were found in the microstructures of multi-component CoCrCuFexNi (x = 1.0, 1.5, 2.0) alloys.

In Situ Synchrotron X-Ray Diffraction and Small Angle X-Ray Scattering Studies on Rapidly Heated and Cooled Ti-Al and Al-Cu-Mg Alloys Using Laser-Based Heating

Beam-based additive manufacturing (AM) typically involves high cooling rates in a range of 103–104 K/s. Therefore, new techniques are required to understand the non-equilibrium evolution of materials at appropriate time scales. Most technical alloys have not been optimized for such rapid solidification, and microstructural, phase, and elemental solubility behavior can be very different. In this work, the combination of complementary in situ synchrotron micro-x-ray diffraction (microXRD) and small angle x-ray scattering (SAXS) studies with laser-based heating and rapid cooling is presented as an approach to study alloy behavior under processing conditions similar to AM techniques. In rapidly solidified Ti-48Al, the full solidification and phase transformation sequences are observed using microXRD with high temporal resolution. The high cooling rates are achieved by fast heat extraction. Further, the temperature- and cooling rate-dependent precipitation of sub-nanometer clusters in an Al-Cu-Mg alloy can be studied by SAXS. The sensitivity of SAXS on the length scales of the newly formed phases allows their size and fraction to be determined. These techniques are unique tools to help provide a deeper understanding of underlying alloy behavior and its influence on resulting microstructures and properties after AM. Their availability to materials scientists is crucial for both in-depth investigations of novel alloys and also future production of high-quality parts using AM.

Molecular dynamics simulation of nanocrystal formation and deformation behavior of Ti3Al alloy

A molecular dynamics simulation study has been carried out to investigate nanocrystal formation of liquid Ti3Al alloy during rapid solidification and deformation behavior of the nanocrystalline Ti3Al alloy under tensile loading. The deformation mechanism of the nanocrystalline Ti3Al alloy during the tensile deformation processes has been discovered through the investigations of microstructural evolutions. The results emerging from the present analyses indicate that liquid Ti3Al alloy fully crystallizes at cooling rate of 1011K/s, and coherent twin boundaries are found in the nanocrystalline Ti3Al alloy during the rapid solidification. Moreover, as the crack source in the nanocrystalline Ti3Al alloy, the coherent twin boundary perpendicular to the stress direction induces and extends the crack during the plastic deformation process. These findings provide deeper insights into understanding the nanocrystal formation of liquid Ti3Al alloy and tensile deformation mechanism in the nanocrystalline Ti3Al alloy at nano-scale.

Nanocalorimetric Characterization of the Heterogeneous Nucleation of Rapidly Solidified Bismuth Droplets Embedded in a Zinc Matrix

In this paper, Zn-10Bi (wt.%) alloy with Bi droplets in size from micro- to nanometers embedded in the Zn matrix was prepared. Nanocalorimetry was used to investigate the melting and solidification behavior of the embedded Bi droplets. In the heating process, two endothermic peaks were observed attributing to the high sensitivity of the nanocalorimeter. One is close to the eutectic temperature of Zn-Bi alloy (527 K), while the other is close to the melting point of bulk Bi (544 K). In the cooling process, the Bi droplets solidify in a wide temperature range. The results demonstrate that the Bi droplets melt at the higher temperature but their solidification demand a larger undercooling. In addition, the nucleation kinetics of rapidly solidified nano-sized Bi droplets was studied based on the hemispherical cap model of heterogeneous nucleation, and the calculated results reveal the validity of classical heterogeneous nucleation theory in nano-scale at relative high cooling rates. In a word, due to the high sensitivity and large controllable cooling rate, nanocalorimetry will be a promising technique to investigate the rapid solidification behavior of embedded nano-sized droplets.

Rapid solidification behavior of nano-sized Sn droplets embedded in the Al matrix by nanocalorimetry

Al-10Sn (wt.%) melt-spun ribbons with nano-sized Sn droplets (20–400 nm in diameter) embedded in the Al matrix and bulk Sn distributed at Al grain boundaries were prepared. Differential fast scanning calorimetry (DFSC) based on nanocalorimetry and thin film technique was successfully applied to investigate the rapid solidification behavior of the embedded nano-sized Sn droplets at cooling rates ranging from 103 to 104 K s−1. Two broad exothermic peaks were observed in the DFSC curves. They were ascribed to the solidification of nano-sized Sn droplets with various catalytic activity factors f(θ). The cooling rate dependence of undercooling of nano-sized Sn droplets has been studied experimentally. The two series of undercooling which correspond to the two exothermic peaks increase slightly with the increases of cooling rate. Furthermore, a theoretical description of the experimental DFSC curves based on classical heterogeneous nucleation theory is developed. It is performed advancing a previously developed approach by assuming a smooth dependence of the droplet mass fraction on contact angle, m(θ), with a double Gaussian distribution during the nucleation process. This modified theoretical model is believed to be relevant also for other related rapid solidification processes.

An Al2O3/Y3Al5O12 eutectic nanocomposite rapidly solidified by a new method: Liquid–metal quenching

A liquid–metal quenching method is designed for studying the rapid solidification behavior of oxide eutectic composites. The lamellar spacing of an Al2O3/Y3Al5O12 eutectic nanocomposite processed using this method is only 50 nm, which is an order of magnitude smaller than that reported previously. Results from the microstructural characterization of the resulting materials are described. Using the classical eutectic growth model, the relative solidification rate in this method is calculated to be 1.88 × 10−2 m s−1.

Rapid solidification behavior of Cu–Co–Fe alloy

Abstract

Microstructural Evolution during Laser Resolidification of Fe-18 At. Pct Ge Alloy

The technique of laser resolidification has been used to study the rapid solidification behavior of concentrated Fe-18 at. pct Ge alloy. The microstructural evolution has been studied as a function of scanning rate of laser beam. Scanning electron microscopy (SEM) reveals the formation of a two-layer (designated as “A” and “B”) microstructure in the remelted pool. The A layer shows a band consisting of a network of interconnected channels and walls, quite similar to cell walls. The B layer shows dendritic growth. Transmission electron microscopic observations reveal the formation of bcc α-FeGe in the B layer. Laser melting has been found to play an important role in formation of the A layer. Microstructural evolution in B has been analyzed using the competitive growth criterion, and formation of bcc α-FeGe has been rationalized in the remelted layers.