Solidification Of Melt Research Articles

Rational design and multi-stage formation mechanisms of FeCrNiAl medium-entropy alloy strengthened by multi-scaled dual phases

Constructing multimodal microstructure in high/medium-entropy alloys (H/MEAs) is expected to further improve the performance via introducing heterostructure strengthening mechanism. In this study, FeCrNiAl-based MEAs were rationally designed to form a special microstructure with trimodal B2 nanoparticles and rod-shaped Laves phase into body-centered-cubic (BCC) structured matrix, via the CALPHAD method and composition adjustment. To systematically exhibit the microstructure evolution, phase formation and stabilization mechanisms, we reported the representative Fe61.93Al15.62Cr13.88Ni6.25Nb0.97Mo1.16Ta0.19 (at.%) alloy. It is found that the multi-scaled B2 particles with distinct mean radii of 225 nm, 15 nm, 1–5 nm precipitate in the melt solidification, aging and cooling processes in turn, and the Laves phase also forms in the aging stage. The primary and secondary B2 particles precipitate in the classical nucleation and growth pathway, while the ultrafine B2 particles follow a spinodal decomposed mechanism. By co-alloying Mo, Nb, and Ta elements, the thermal stability of B2 and Laves phase is significantly improved, resulting in a much lower coarsening rate (k = 8 × 10−27 m3/s) than that in common B2-strengthened BCC alloys at 800 °C. This ultra-stable and heterostructured alloy should have good application potential at elevated temperature, and should be a paradigm for future development of multi-modal precipitate strengthened alloys.

Calibrating uncertain parameters in melt pool simulations of additive manufacturing

Melt pool scale numerical modeling of additive manufacturing (AM) processes can provide predictive capabilities and theoretical insight into the process-property-structure-performance relationships for AM parts. Despite capabilities of numerical models to solve complex multi-physics problems, it is often important to consider a tradeoff between detailed physics and computational cost. Therefore, sources of uncertainty in both experimental conditions and the parameters needed for modeling require models to be validated against empirical evidence. Here, a method is proposed to calibrate uncertain parameters used in continuum-scale melt pool models for powder bed fusion (PBF) AM. Both a simplified heat transfer model and a heat transfer and fluid flow model were investigated. A surrogate model and Markov chain-based optimization algorithm calibrated melt pool geometry for models within experimental variation of the target melt pool width and depth from the NIST AM-Bench 2018-02 dataset. The melt pool temperature distributions, solidification parameters, and simulated multi-layer solidification microstructures were compared between the two models. Similar results from both models indicate that calibrated, lower fidelity numerical models may be used in place of higher fidelity models to generate melt pool solidification data. These calibrated models therefore enable lower computational cost melt pool simulations without a noticeable decrease in simulation accuracy for grain-scale microstructure simulations.

Gas-kinetic scheme based phase-field method for numerical simulations of dendrite growth

In this work, a numerical model coupling the gas-kinetic scheme (GKS) and phase-field (PF) method is established to simulate the equiaxed dendrites growth during pure melt solidification in two dimensions. In the proposed model, the GKS and the PF method are used to simulate the thermal flow field and the dendrite growth, respectively, and coupled through the source term of the temperature field. To validate the proposed formulation, the natural convection case in a closed square cavity and the growth case of equiaxed dendrites are adopted to verify the accuracy of the GKS and the GKS-PF coupled model, respectively. After that, the coupled model is applied to simulate the equiaxed dendrites growth during pure Ni solidification under diffusion and natural convection conditions. The numerical results show that the proposed model is an effective and reliable method for the numerical simulation of equiaxed dendrites growth of the pure metal solidification process.

Microstructure and Properties of Multilayer Niobium-Aluminum Composites Fabricated by Explosive Welding

In this study, a layered composite material consisting of alternating aluminum and niobium layers and cladded on both sides with titanium plates was obtained by explosive welding. Microstructure of the composite was thoroughly studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as well as by energy dispersive X-ray spectroscopy (EDX) and electron backscattered diffraction (EBSD). Microhardness measurements, tensile test, and impact strength test were carried out to evaluate the mechanical properties of the composite. Formation of mixing zones observed near all interfaces was explained by local melting and subsequent rapid solidification. Mixing zones at Nb/Al interfaces consisted of metastable amorphous and ultrafine crystalline phases, as well as NbAl3 and Nb2Al equilibrium phases. Niobium grains near the interface were significantly elongated, while aluminum grains were almost equiaxed. Crystalline grains inside the mixing zones did not have a distinct crystallographic texture. Microhardness of Al/Nb mixing zones was in the range 546–668 HV, which significantly exceeds the microhardness of initial materials. Tensile strength and impact strength of the composite were 535 MPa and 82 J/cm2, respectively. These results confirm the high bonding strength between the layers.

Morphology and microstructure of crystalline YAG-Al2O3 composites grown by the horizontal directional crystallization

The peculiarities of the horizontal directional crystallization of the melt (Y3Al5O12, 90 wt% and Al2O3 10 wt%) were studied. The evolution of both the morphology of the ingot and the microstructure of the inclusions as a function of the rate of melt solidification was established. It was shown that the crystallization of the ingot begins with the growth of the YAG single crystal, which turns into the YAG cellular growth, with cells surrounded by the YAG-Al2O3 eutectic. Cellular growth is replaced by dendritic YAG growth followed by spontaneous crystallization. The inclusions consist of eutectic YAG-Al2O3, which usually presented pores caused by the solidification of the eutectic in the confined volume. The morphological stability of the crystallization front is partially kept until dendritic growth occurs. Parts of the ingot with a dendritic structure can be an effective light-scattering medium that can be used as a host for luminescent ions in converters of solid-state light sources.

Cellular breakdown and carrier lifetimes in gold-hyperdoped silicon

Ion implantation of transition metals into Si, followed by pulsed laser melting and rapid solidification, shows promise for making Si devices with sub-band gap optoelectronic response. We study Si implanted with Au at doses ranging from 1015–1016 at cm−2, with all but the lowest dose exhibiting interface breakdown during solidification, resulting in heavily defected layers. Terahertz photocarrier lifetime measurements confirm that layers with breakdown show recombination lifetimes of about 100 ps, compared to 800 ps for a layer with no breakdown. Device measurements, however, show more photoresponse at 1550 nm in a layer with breakdown than in a layer without. The results suggest that avoiding breakdown may be desirable but might not necessarily be imperative for making a useful device.

Acceleration of RBF-FD meshless phase-field modelling of dendritic solidification by space-time adaptive approach

• A novel meshless approach for phase-field modelling of dendritic growth. • Reduction of discretisation-induced anisotropy by the scattered nodes. • Increase in computational efficiency by a space-time adaptive approach. A novel adaptive numerical approach is developed for an accurate and computationally efficient phase-field modelling of dendritic solidification. The adaptivity is based on the dynamic quadtree domain decomposition. A quadtree decomposes the computational domain into rectangular sub-domains of different sizes. Each sub-domain is extended to ensure overlap communication between neighbouring sub-domains. In each sub-domain, uniform distribution of computational nodes is generated. The product between the node density and the sub-domain area is fixed to ensure the h-adaptivity. The adaptive approach provides the highest density of computational nodes at the solid-liquid interface and the lowest density in the bulk of the phases. The meshless radial basis function generated finite difference (RBF-FD) method is applied for the spatial discretisation of the partial differential equations which arise from the phase-field model. The RBF-FD method is especially appealing since it allows straightforward spatial discretisation of partial differential equations on scattered node distributions. The use of scattered node distribution reduces the discretisation-induced anisotropy in the phase-field modelling of dendritic growth. The forward Euler scheme is used for temporal discretisation. The adaptive time-stepping is employed to speed up the calculations further. The performance of the novel numerical approach is tested for dendritic solidification of supercooled pure melts and supersaturated dilute binary alloys at arbitrary preferential growth directions. The impact of the numerical parameters on the accuracy and computational efficiency is thoroughly analysed. It is shown that the RBF-FD method, defined on scattered node distribution, together with the space-time adaptive approach, represents an accurate and efficient technique for solving the phase-field models of dendritic solidification.

Towards 3-D texture control in a β titanium alloy via laser powder bed fusion and its implications on mechanical properties

Fusion-based additive manufacturing (AM) offers a new opportunity to design metallic materials with complex, direction-dependent mechanical properties by controlling the orientation distribution of the constituent grains—also known as texture. Texture control may be achieved site-specifically by varying the processing variables and tailoring the local melt pool geometry and solidification kinetics. However, the type and direction of textures currently achievable in AM alloys are limited by the melt pool geometry itself. In this work, we advance the capabilities of controlling crystallographic texture in laser powder bed fusion (LPBF) by producing specimens of β titanium-niobium alloy with textures aligned along three different directions: the laser scan direction (SD), the build direction (BD), and—for the first time—the direction perpendicular to both BD and SD (i.e., PD). We achieve this three-dimensional (3-D) texture control by fine tuning the keyhole melt pool geometry and amount of overlap throughout the build. We test the tensile properties of the three different specimens along their respective texture axes and elucidate the relationships between crystallographic orientation, mesostructure, and mechanical behavior. We find that the novel PD texture exhibits the best combination of strength, strain hardenability, and ductility. We ascribe these results to the unique mesostructure of this specimen. This work opens new opportunities for designing novel materials with directional properties by achieving three-dimensional (3-D) texture control during fusion-based AM.

Effect of beryllium on the morphology of initial ingots and the structural phase state of fast-quenched STEMET 1108 brazing alloy ribbons

The STEMET 1108 grade (copper – tin – indium – nickel) of filler metal is currently used to braze bronze to tungsten plated with pure copper in divertors of the International Thermonuclear Experimental Reactor (or, ITER). Such filler metals are obtained as a result of rapid solidification of melt on a rapidly spinning copper wheel (melt spinning). When ingots are cast from which brazing ribbons are further produced, pores occur in them, through which indium evaporates. All this may affect the quality of the final product. The authors propose to alloy ingots with beryllium to stop the pore formation. This paper looks at the effect of beryllium on the quality of filler metal ingots and ribbons. The paper describes the results of a study that looked at the structural phase state of filler metals using electron microscopy and X-ray diffraction techniques, as well as a synchrotron radiation source. Both ingots and ribbons were found to have the same phase composition that consists of copper-based FCCsolid solutions and contains phosphide Cu3P. Beryllium containing ribbons are thinner than beryllium-free ribbons. In both cases, a dendritic structure is formed across the entire ribbon thickness. It is demonstrated that beryllium alloying in the range of 0.05 to 0.1 wt. % helps to significantly reduce the porosity of the initial ingots without compromising their structural phase state. In addition, it helps prevent the evaporation of indium. Hence, the difference in the structural phase state between beryllium alloyed and non-alloyed ribbons is insignificant and only concerns their dendritic structure, while there is no noticeable difference in their phase composition.

Understanding the effects of temporal waveform modulation of laser emission power in laser powder bed fusion: Part II - Experimental investigation

The laser powder bed fusion (LPBF) process has historically been operated with high-brilliance fibre laser sources with continuous wave (CW) emission. Nonetheless, temporal waveform modulation of the laser emission power at high-frequency levels can provide a means to enhance the deposition process by modifying the melt dynamics and solidification mechanisms. In order to disclose the effect of different waveform shapes and their parameters, an experimental study using an open LPBF system was conducted. This paper is the second part of an investigation on this topic, which aims to validate the analytical model proposed in the first part of the work. The LPBF system that was developed enabled the power emission profiles to be programmed during single-track depositions. Four different waveform shapes were tested (namely square wave, ramp up, ramp down and triangle wave) at different levels of waveform amplitude (ΔP= 200–400 W) and different frequencies (fw = 2–4–6–8 kHz) during the single-track deposition of stainless steel AISI316L. High-speed imaging acquisitions allowed the melt dynamics to be disclosed and the melt-oscillation frequency to be identified. Larger waveform amplitudes and waveforms with sudden variations of emission power generated melt ejections and process instabilities. Stable conditions could be identified when employing ramp up and triangle waveforms with ΔP = 200. Melt-surface oscillation frequency corresponded to the values imposed via the modulation of the laser emission power, thus validating the analytical model of Part I, which correlated the melt-surface temperature to the recoil pressure induced over the molten pool. Optical microscopy images and metallographic cross-sections confirmed the high-speed video observations. Three-dimensional reconstructions of the depositions via focus variation microscopy allowed the build rates and roughness of the single tracks to be determined. Build rates obtained in stable deposition conditions with waveform modulation are analogous to values obtainable with CW emission, and beneficial effects over the roughness were reported.

A Lagrangian approach to ex-vessel corium spreading over ceramic and concrete substrates using moving particle hydrodynamics

Understanding the spreading of molten core materials (corium) is vital for achieving post-accident heat removal and maintaining the integrity of the reactor containment during a severe nuclear accident. The spreading of highly viscous corium over sacrificial concrete and ablation-resistant ceramic substrates, consistent with recent VE-U9 experiments at the VULCANO facility, is investigated using a Lagrangian moving particle hydrodynamics (MPH) method. The spreading dynamics are coupled with a heat transfer model to account for the increase in melt viscosity due to solidification. Three alternative boundary conditions are considered at the melt-substrate interface to mimic the influence of low viscosity concrete ablation products on the spreading dynamics. Simulation of spreading under no-slip conditions closely resembled the spreading behavior observed over the inert ceramic substrate, advancing with a crawling motion with a steep profile in the proximity of the melt front. Spreading terminated due to a combination of thermal radiation from the free surface and the heat transfer to the substrate, leading to the formation of a continuous crust. Introduction of slip boundary models, to mimic lubrication of the interface by a film of low-viscosity concrete decomposition products, enabled the prediction of the gliding flow observed over the sacrificial concrete substrate, characterized by a shallow melt topology near the spreading front. The simulation results demonstrate excellent potential for Lagrangian models such as MPH to predict complex spreading dynamics in the presence of both melt solidification and lubrication at the substrate.

Thermo-fluid flow behavior of the IN718 molten pool in the laser directed energy deposition process under magnetic field

PurposeThis paper aims to understand the magnetohydrodynamics (MHD) mechanism in the molten pool under different modes of magnetic field. The comparison focuses on the Lorenz force excitation and its effect on the melt flow and solidification parameters, intending to obtain practical references for the design of magnetic field-assisted laser directed energy deposition (L-DED) equipment.Design/methodology/approachA three-dimensional transient multi-physical model, coupled with MHD and thermodynamic, was established. The dimension and microstructure of the molten pool under a 0T magnetic field was used as a benchmark for accuracy verification. The interaction between the melt flow and the Lorenz force is compared under a static magnetic field in the X-, Y- and Z-directions, and also an oscillating and alternating magnetic field.FindingsThe numerical results indicate that the chaotic fluctuation of melt flow trends to stable under the magnetostatic field, while a periodically oscillating melt flow could be obtained by applying a nonstatic magnetic field. The Y and Z directional applied magnetostatic field shows the effective damping effect, while the two nonstatic magnetic fields discussed in this paper have almost the same effect on melt flow. Since the heat transfer inside the molten pool is dominated by convection, the application of a magnetic field has a limited effect on the temperature gradient and solidification rate at the solidification interface due to the convection mode of melt flow is still Marangoni convection.Originality/valueThis work provided a deeper understanding of the interaction mechanism between the magnetic field and melt flow inside the molten pool, and provided practical references for magnetic field-assisted L-DED equipment design.

The grain refinement of Mg alloy subjected to dual-frequency ultrasonic melt treatment: A physical and numerical simulation

Ultrasonic melt processing of magnesium (Mg) alloys has received widespread attention. However, the cavitation behavior and microstructure evolution are difficult to be directly observed in high temperature melts. In this work, single-frequency ultrasonic field (SUF) and dual-frequency ultrasonic field (DUF) were introduced into succinonitrile (SCN) melt and real-time images of dendrite growth and evolution were captured to explore the regulation mechanism of DUF on melt solidification structure. Numerical simulation and corresponding experiments were performed to investigate the acoustic pressure distribution and cavitation area of DUF in Mg alloy melt. Ultrasonic treatment increased the SCN dendrite growth rate and refined the solidification microstructure, and a higher efficiency was achieved by DUF when compared to SUF when the total electric power was the same. DUF decreased sound pressure attenuation and enlarged cavitation area. A result of this improvement was that its grain refinement efficiency was 13.8% and 25.6% higher than SUF at 15 kHz and 20 kHz, respectively. The input power ratio plays a crucial part in improving the grain refinement efficiency of DUF. While ensuring the symmetrical distribution of cavitation area, the grain refinement efficiency can be significantly optimized by appropriately increasing the power share of 15 kHz ultrasound is optimal at a power ratio of 2:1.

Melting solidification of oily sludge with coal fly ash: Fusion characteristics, phases transformation and metals leaching behavior

Melting solidification is a promising method to achieve the harmless treatment of oily sludge (OS). In this study, the coal fly ash (CFA) was used as an additive to co-treat with OS. The effects of CFA addition on the fusion characteristics and phases transformation of OS were studied, and the leaching behavior of heavy metals after melting solidification was also evaluated. The results indicated that the addition of CFA had a synergistic effect on the thermal characteristics of OS. SiO2 and Al2O3 in CFA reacted with CaO in OS to form gehlenite (Ca2Al2SiO7), which reduced the ash fusion temperatures (AFTs) of OS. But the AFTs of mixed samples increased with the addition of CFA from 10 wt% to 50 wt%, and this might be due to more anorthite (CaAl2Si2O8) formation based on the gehlenite. As the increase of CFA into OS, the microstructures of co-treated mixtures were gradually loose. In addition, the leaching concentrations of heavy metals in the co-melting samples were reduced by 13.22–76.70 mg/L, which were lower than the standard limit, and the products presented low ecological risk indices. The obtained results indicated that the CFA effectively regulated the melting behaviors of OS and promoted the solidification of heavy metals, which provided a safe and stable disposal approach for OS treatment.

Influence of electropulsing treatment on crystallization and structure of calcium silicate-based melt

Electropulsing treatment (EPT) is a promising technology for controlling the phase transition during the solidification of melts owing to its electric and thermal effects. In this study, the influence of EPT on the crystallization and melt structure of a calcium silicate-based mold flux was investigated. The results showed that the morphology of crystals that precipitated in the mold flux changed from elongated columnar to block shape, and the equivalent grain diameter of the crystals increased with increasing voltage from 0 to 20 V. The mass fraction of Ca4Si2O7F2 precipitated in the mold flux decreased with increasing impulse voltage, whereas that of Ca2Mg0.75Al0.5Si1.75O7 increased. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses suggest that the network structure of both silicate and aluminate was simplified by electropulsing because the simpler structural units of Q0, Q1, [AlO6]9+, etc., increased with increasing impulse voltage, whereas the complex structural units of Q2, Q3, and [AlO4]5+ decreased. The extra electric field force is the repulsion force between two oppositely charged ions, which was the root of the network structure simplification and crystallization promotion. The results obtained in this study provide an innovative method for locally controlling the crystallization behavior of mold flux in a mold.

Modeling of the Bi2O3·SiO2 Melt Cooling Process and the Products of Melt Solidification under Various Conditions

Modeling of the Bi2O3·SiO2 Melt Cooling Process and the Products of Melt Solidification under Various Conditions

A Review on Solid-State-Based Additive Friction Stir Deposition

Additive manufacturing (AM) is an important technology in Industry 4.0. In recent years, solid-state-based additive friction stir deposition (AFSD) has attracted much attention, as it can avoid the inherent defect of melting and rapid solidification in electron beam-based or laser-based AM technologies. The macro and micro laws, finite element simulation, and engineering application technology of the AFSD process are still in their early stages. This paper mainly reviews the equipment, mechanism, the effect of process parameters on macro/micro characters, and the engineering applications of the AFSD process. Further, based on the complex loading conditions during the AFSD process, some perspectives are proposed, including the characterization method, unified constitutive model, novel composite manufacturing technology, and systematic study of the AFSD process.

Increasing density and mechanical performance of binder jetting processing through bimodal particle size distribution

Binder jetting is an additive manufacturing (AM) technology that has gained popularity and attention in recent years for production applications in tooling, biomedical, energy, and defense sectors. When compared to other powder bed fusion-based AM methods, binder jetting processes powder feedstock without the need of an energy source during printing. This avoids defects associated with melting, residual stresses, and rapid solidification within the parts. However, one of the challenges of this process is the relatively lower densities which impacts part density, and subsequently, sintering and mechanical properties. In this study, we investigated the influence of bimodal powder size distributions (a mixture of coarse to fine particles) as a method for increasing part density and mechanical strength, and used stainless steel (SS) 316L bimodal mixtures in this case. Four unimodal and two bimodal groups were evaluated under similar AM processing conditions for sintered density measurements and flexural strengths. Our results demonstrated that bimodal size distributions showed a statistically significant increase in density by 20% and ultimate flexural strength by 170% when compared to the highest performing unimodal group. In addition to experimental findings, reactive molecular dynamics simulations showed that the presence of finer powders along with coarser particles in the bimodal particle mixture contribute to additional bonds that are stronger across the particle interfaces. Findings from this study can be used to design bimodal particle size distributions to achieve higher density and better mechanical properties in binder jetting AM process.

A CFD Modeling Coupled with VOF Method and Solidification Model for Molten Jet Breakup at Low Velocity

In a reactor severe accident, molten jet breakup and solidification are important behaviors after large pours of molten material fall into the coolant in-vessel or ex-vessel. However, heat and mass transfer processes inside melt during jet breakup have not been studied sufficiently. Existing research on jet fragmentation is relatively macroscopic, and the micro interface condensation details are not well studied. In this paper, a two-dimensional multiphase computational fluid dynamics (CFD) code with the Volume of Fluid (VOF) method and solidification model is applied to simulate molten jet breakup with surface solidification. The VOF model is used to capture the interface, study the details, and add the influence of solidification. Solidification and instability can be seen at the interface. In order to simulate melt solidification, an energy equation is modeled using an enthalpy-based formulation, and viscosity variation during phase change is taken into account. The comparative results between the CFD code and jet breakup experiments show that melt jet front position histories, breakup length, and breakup time are in good agreement with the experiments. The simulation results show that crust formation of the jet surface suppresses surface instability and jet breakup behavior. As the interfacial temperature decreases, the droplet cumulative mass fraction decreases, and the solidified metal proportion increases. The simulation results by the CFD code with the solidification model are valuable and important for understanding the molten jet breakup mechanism.

Study of the effect of electromagnetic stirring on the quality of melt solidification in continuous casting

Study of the effect of electromagnetic stirring on the quality of melt solidification in continuous casting