In this work, a novel phenomenological model is proposed to study the liquid-to-solid phase change of eutectic and hypoeutectic alloy compositions. The objective is to enhance the prediction capabilities of the solidification models based on a-priori definition of the solid fraction as a function of the temperature field. However, the use of models defined at the metallurgical level is avoided to minimize the number of material parameters required. This is of great industrial interest because, on the one hand, the classical models are not able to predict recalescence and undercooling phenomena, and, on the other hand, the complexity as well as the experimental campaign necessary to feed most of the microstructure models available in the literature make their calibration difficult and very dependent on the chemical composition and the treatment of the melt. Contrarily, the proposed model allows for an easy calibration by means of few parameters. These parameters can be easily extracted from the temperature curves recorded at the hot spot of the quick cup test, typically used in the differential thermal analysis (DTA) for the quality control of the melt just before pouring. The accuracy of the numerical results is assessed by matching the temperature curves obtained via DTA of eutectic and hypoeutectic alloys. Moreover, the model is validated in more complex casting experiments where the temperature is measured at different thermocouple locations and the metallurgical features such as grain size and nucleation density are obtained from an exhaustive micrography campaign. The remarkable agreement with the experimental evidence validates the predicting capabilities of the proposed model.

Numerical simulations of metal alloy solidification are used to gain insight into physical phenomena that cannot be observed experimentally. These models produce results that are used to draw conclusions about a process or alloy and often compared to experimental results. However, uncertainty in model inputs cause uncertainty in model results, which have the potential to significantly affect conclusions drawn from their predictions. As a step toward understanding the effect of uncertain inputs on solidification modeling, uncertainty quantification (UQ) and sensitivity analysis are performed on a transient model of solidification of Al–4.5 wt % Cu in a rectangular cavity. The binary alloy considered has columnar solidification morphology, and this model solves equations for momentum, temperature, and species conservation. UQ and sensitivity analysis are performed for the degree of macrosegregation and solidification time. A Smolyak sparse grid algorithm is used to select input values to construct a polynomial response surface fit to model outputs. This polynomial is then used as a surrogate for the complete solidification model to determine the sensitivities and probability density functions (PDFs) of the model outputs. Uncertain model inputs of interest include the secondary dendrite arm spacing (SDAS), heat transfer coefficient, and material properties. The most influential input parameter for predicting the macrosegregation level is the dendrite arm spacing, which also strongly depends on the choice of permeability model. Additionally, the degree of uncertainty required to produce accurate predictions depends on the outputs of interest from the model.

This paper investigates avenues for controlled initiation and augmentation of the mechanical and thermal energetic output of shock-triggered vapor explosions (VEs) with Al–GaInSn alloys; furthermore, enabling a means for impulsive hydrogen gas generation within milliseconds. Using a submerged electronic bridgewire detonator or rifle primer caps as the shock trigger for VE initiation, experiments were conducted with 10 g melt drops at initial temperature between 930 K and 1100 K, aluminum mass contents between 0.3 wt.% and 20 wt.%, and water temperatures between 293 K and 313 K. It was found that combined thermal–chemical Al–GaInSn–H 2 O explosive interactions can readily be controllably induced via shocks and are of greater intensity than the pure (spontaneous) thermally driven explosions observed with unalloyed Sn and GaInSn. Shock pressures up to 5 MPa were recorded about 10 cm from the explosion zone; a factor of 5 higher than the ∼1 MPa over pressures generated from spontaneous GaInSn–H 2 O explosions reported in our previous study. Al–GaInSn–H 2 O explosive interactions also exhibited rapid enhancements to the “impulse” H 2 production rate. Hydrogen/vapor bubble volumes up to 460 ml were observed approximately 4 ms after the explosion, equating to a mechanical work and instantaneous power output of 47 J and 11.75 kW, respectively. In comparison with available, analogous, triggered-explosion studies with Al melt drops, our Al–GaInSn alloy melt at 1073 K generated up to 18 times (∼2000%) more hydrogen per gram of aluminum when compared with experiments with molten Al at a much higher melt temperature of 1243 K.

A high thermal conductivity thermal paste can be developed by mixing the oxidized liquid metal alloy (OLMA) with graphene. Four kinds of graphene-OLMA pastes were synthesized at graphene concentrations of 0.25 wt%, 0.75 wt%, 1.5 wt%, and 2.0 wt%, respectively. The paste structures were characterized by MicroXCT-400, which can be used to readily measure the air pocket size, and their thermal conductivities measured by a laser flash analysis method. It is found that the OLMA structure is very different from the liquid metal alloy (LMA), and a small amount of air pockets were formed in the OLMA. The air pocket size significantly affected the thermal conductivity of the graphene-OLMA paste. When the graphene concentration increased, as shown in Fig. 1(c)-(e), the paste's thermal conductivity increased. However, more air pockets were formed around the graphene. In particular, when the graphene concentration increased to 2.0 wt%, clusters of graphene, as shown in Fig. 1(f), were formed resulting in the formation of big air pockets in the thermal paste, which directly affected the thermal conductivity as shown in Fig. 1(g). We thought that when the graphene concentration increases, the thermal conductivity should increases. But the results show that it was not and then we used MicroCT to see the internal structure of the thermal paste and found that the air pockets were formed and significantly affects the thermal performance.

Vertically aligned carbon nanotube (VACNT) films with high thermal conductance and mechanical compliance offer an attractive combination of properties for thermal interface applications. In current work, VACNT films synthesized by the chemical vapor deposition method were used as thermal interface material (TIM) and investigated experimentally. The liquid metal alloy (LMA) with melting point of 59 °C was used as bonding material to attach VACNT films onto copper plates. In order to enhance the contact area of LMA with the contact surface, the wettability of the contact surface was modified by plasma treatment. The thermal diffusivity, thermal conductivity, and thermal resistance of the synthesized samples were measured and calculated by the laser flash analysis (LFA) method. Results showed that: (1) VACNT films can be used as TIM to enhance the heat transfer performance of the contact surface; (2) the LMA can be used as bonding material, and its performance is dependent on the LMA wettability on the contact surface. (3) When applying VACNT film as the TIM, LMA is used as the bonding material. After plasma treatment, comparison of VACNT films with the dry contact between copper and silicon showed that thermal diffusivity can be increased by about 160%, the thermal conductivity can be increased by about 100%, and the thermal resistance can be decreased by about 31%. This study shows the advantages of using VACNT films as TIMs in microelectronic packaging.

Amorphous fiber, ribbon, or film is produced through melt spinning. In this manufacturing process, a continuous delivery of amorphous material is simultaneously dependent on the wheel spinning rate, metallic liquid viscosity, surface tension force, heat transfer inside the melt pool and along the substrate, and other parameters. An analysis of a free-jet melt spinning for fiber manufacture has been performed to relate the process control parameters with amorphous formation. We present a numerical simulation of transient impingement of a free melt jet with a rapidly rotating wheel, along with theoretical estimates of melt ribbon thickness, to investigate dynamical characteristics of the flow in melt pool. The nucleation temperature and the critical cooling rate are predicted in the paper for alloy Fe75–Si10–B15 (at. %). Thermal conduction is found to dominate undercooling in melt spinning by comparing the temperature and velocity measurements with our numerical simulation and the analytical solutions.

Many random substitutional solid solutions (alloys) will display a tendency to atomically order given the appropriate kinetic and thermodynamic conditions. Such order–disorder transitions will result in major crystallographic reconfigurations, where the atomic basis, symmetry, and periodicity of the alloy change dramatically. Consequently, phonon behavior in these alloys will vary greatly depending on the type and degree of ordering achieved. To investigate these phenomena, the role of the order–disorder transition on phononic transport properties of Lennard–Jones type binary alloys is explored via nonequilibrium molecular dynamics simulations. Particular attention is paid to regimes in which the alloy is only partially ordered. It is shown that by varying the degree of ordering, the thermal conductivity of a binary alloy of fixed composition can be tuned across an order of magnitude at 10% of the melt temperature, and by a factor of three at 40% of the melt temperature.

The paper explores the potential of a recently developed special front tracking method in the identification of the interface between columnar and equiaxed structures formed during a binary alloy solidification, driven by thermosolutal convection. The method, based on theoretical and experimental dendrite tip kinetics, is capable of directly distinguishing between the columnar mush and the undercooled liquid/equiaxed region developing ahead of the dendrite tip curve. A new numerical model and its computational algorithm are proposed, where the classical Eulerian volume averaged description of the transport processes is coupled with the Lagrangian front tracking method on a fixed control-volume grid. Having thus distinguished zones of different dendrite structures, distinct simulation models are used within each of the zones, e.g., the Darcy’s porous medium model in the stationary dendrite region, and a model of slurry with floating dendrites in the equiaxed part of the mush. The calculated temperature and solute concentration fields are compared with the relevant results of the classical enthalpy-porosity model, for an example problem of Pb-48 wt % Sn alloy solidification driven by diffusion and thermosolutal convection. And a good match with both solutions is exhibited. A preliminary validation study is also presented by comparing the available experimental data with the model predictions. Possible reasons for some observed discrepancies between the calculations and the experimental findings are discussed. Finally, the proposed front tracking approach is used to address the question of the impact of dendrites floating in the liquid on the flow pattern and macrosegregation in the solidifying alloy.

During the casting process of green sand mold, air gaps will form between the metal and sand mold. The air gaps will make it difficult to analyze the heat transfer at the mold/metal interface. Generally, an interfacial heat transfer coefficient is employed to evaluate the heat flux transferred across the air gaps. Though the interfacial heat transfer coefficient is highly important, its value is not easily obtained by using the direct experimental or theoretical method. With temperature-measured data, some inverse methods can be used to predict the coefficient. However, the latent heat released and undercooling during the solidification of the molten metal and the moisture of the green sand mold complicate the associated temperature calculations. To overcome this difficulty, a lump capacitance method is proposed in this study to calculate the interfacial heat transfer coefficient for the casting process in green sand mold. Thermalcouples are utilized to measure the temperatures of sand mold and metal. The geometry of casting is cylindrical and the castings are A356 alloy and Sn-20 wt. % Pb alloy. With the predicted interfacial coefficients, the temperature field of the metal was solved numerically. Based on the solidification time, the numerical results are in good agreement with the experimental ones. This verified the feasibility of the proposed method and it can be applied in the future study or design of a casting process.

Lightweight yet precise, temperature control protocols are critical in a variety of applications. This is especially true in space where weight and volume are at a premium and reliability is paramount. In space, complex processes to manage the heat fluxes generated from within and absorbed from space by the spacecraft are usually implemented. Surfaces having different heat fluxes might need to be controlled separately and maintained at different temperatures. The work presented in this paper evaluates a novel laser surface modification process to form micro-column arrays (MCA) on any material for use as highly adaptive radiators. The MCA-structured surfaces have experimentally been shown to have excellent emissive properties. Finite element methods have been used to simulate the temperature profiles for surfaces with and without MCA compared to pin fin structures as a function of input heat flux density. In the case of Ti, our models show that pin fin arrays are better heat radiating surfaces than equivalent MCA structures with cone-like profiles. Such structures, however, are difficult to modify and usually require complicated and expensive fabrication processes. Overall, MCA structures are shown to allow good control over base surface temperature for varying heat fluxes and different MCA aspect ratios. For Ti, under steady state conditions, an aspect ratio of 12 has been shown to be optimal for surface heat reduction. Preliminary experimental results show that the temperature drop is inline with that theoretically predicted.

The laser cladding process inherently includes multiscale, highly nonlinear, and non-equilibrium transport phenomena due to nonuniform and rapid heat flow caused by the laser and the material interaction. In this work, a process model of solidification micro-structure evolution for the laser cladding process has been studied by utilizing a phase-field method. The phase-field method has become a widely used computational tool for the modeling of solidification micro-structure evolution with the advantage of avoiding tracking the interface explicitly and satisfying interfacial boundary conditions. In the present work, the numerical solutions of a phase-field model have been analyzed. The linking of the macroscale process and solidification microstructure evolution was examined by considering the relationship of macro- and micro-parameters. The effects of melt undercooling and anisotropy on the solidification micro-structure have also been studied. The predicted results with different undercoolings were compared with the microsolvability theory and a good agreement was found. Different solidification morphologies of different locations in the melt-pool are also investigated. To quantitatively study the effect of heat flux on the dendritic growth, the dendrite tip analysis was carried out. It was observed that the dendrite tip that grows in the same direction with the heat flux shows a much higher velocity than a tip that grows in the opposite direction of the heat flux.

An apparatus for the measurement of the total hemispherical emittance and specific heat of metals has been developed. The measurement principle is based on the calorimetric technique: the sample, heated by Joule effect and placed in a vacuum chamber, exchanges radiative heat transfer with the walls of the container, kept at a relatively low temperature. Emittance is deduced from the radiative heat transfer laws at the steady state. When the heating power is switched off, the specific heat of the sample can be recovered from the time history of the sample temperature during the cooling transient. Measurements have been performed on samples of aluminum Anticorodal alloy and Inconel 718 alloy under different surface conditions in the 350 - 635 K range.

Low-dimensional nanostructured materials are promising candidates for high efficiency solid-state cooling devices based on the Peltier effect. Thermal transport in these low-dimensional materials is a key factor for device performance since the thermoelectric figure of merit is inversely proportional to thermal conductivity. Therefore, understanding thermal transport in nanostructured materials is crucial for engineering high performance devices. Thermal transport in semiconductors is dominated by lattice vibrations called phonons, and phonon transport is often markedly different in nanostructures than it is in bulk materials for a number of reasons. First, as the size of a structure decreases, its surface area to volume ratio increases, thereby increasing the importance of boundaries and interfaces. Additionally, at the nanoscale the characteristic length of the structure approaches the phonon wavelength, and other interesting phenomena such as dispersion relation modification and quantum confinement may arise and further alter the thermal transport. In this paper we discuss phonon transport in semiconductor superlattices and nanowires with regards to applications in solid-state cooling devices. Systematic studies on periodic multilayers called superlattices disclose the relative importance of acoustic impedance mismatch, alloy scattering, and crystalline imperfections at the interfaces. Thermal conductivity measurements of mono-crystalline silicon nanowires of different diameters reveal the strong effects of phonon-boundary scattering. Experimental results for Si/SiGe superlattice nanowires indicate that different phonon scattering mechanisms may disrupt phonon transport at different frequencies. These experimental studies provide insight regarding the dominant mechanisms for phonon transport in nanostructures. Finally, we also briefly discuss Peltier coolers made from nanostructured materials that have shown promising cooling performance.

The solidification of a dilute alloy (bismuth-tin) under Bridgman crystal growth conditions is investigated. Computations are performed in two dimensions with a uniform grid. The simulation includes the species concentration, temperature and flow fields, as well as conduction in the ampoule. Fully transient simulations have been performed, with no simplifying steady state approximations. Results are obtained under microgravity conditions for pure bismuth, and for Bi-0.1 at.% Sn and Bi-1.0 at.% Sn alloys, and compared with experimental results obtained from crystals grown in the microgravity environment of space. For the Bi-1.0 at.% Sn case the results indicate that a secondary convective cell, driven by solutal gradients, forms near the interface. The magnitude of the velocities in this cell increases with time, causing increasing solute segregation at the solid/liquid interface. Finally, a comparison between model predictions and results obtained from a space experiment is reported. The concentration-dependence of the alloy melting temperature is incorporated in the model for this case. Satisfactory correspondence is obtained between the predicted and experimental results in terms of solute concentrations in the solidified crystal.

An experimental study has been conducted to investigate the effects of grain transport on the columnar to equiaxed transition (CET) in dendritic alloy solidification. Using the aqueous ammonium chloride solution as a transparent model alloy, experiments were performed in a vertical test cell with cooling from the top, resulting in unidirectional columnar crystals growing downwards. Ahead of the columnar front, equiaxed nuclei were observed to originate mostly by fragmentation of the columnar dendrites in the presence of a thermally driven flow in the melt beneath the columnar mushy zone. Being heavier than the liquid, these fragments fall into the bulk melt where they may grow or remelt. The survived equiaxed crystals finally settle towards the floor and pile up to form an equiaxed bed. The CET occurs when the bottom equiaxed packed bed rises and eventually obstructs the columnar mushy zone growing from the upper surface. Therefore, the CET in the present configuration was predominantly controlled by the sedimentation of equiaxed crystals. A parametric study by varying initial concentration, cooling rate, and superheat was performed.

Experiments were performed with binary metal alloys to validate a continuum mixture model for alloy solidification. Ingots of two compositions, Pb-20%Sn and Pb-40%Sn, were cast in a permanent mold, and the solidification process was simulated. Temperature histories were measured during casting, and composition profiles were found in the solidified ingot. Dendritic arm spacings were found from optical micrographs of the alloy microstructure and used to determine a constant in the Blake-Kozeny submodel for the mushy zone permeability in the liquid-solid interaction term of the momentum equations. Scaling analysis from a previous work and a large uncertainty in the permeability constant suggested that predictions of the composition are extremely sensitive to the choice of a permeability model. Three simulations of each casting were performed using the permeability constant as a parameter, and measured temperatures and compositions were compared with predictions based on different model constants. In the region of the liquidus interface, where all of the significant advection of solute takes place, the results suggest that the Blake-Kozeny model based on measured dendritic arm spacings significantly underpredicts the resistance of the dendritic array to fluid flow.

A model was developed for surface melting and resolidification of both pure metal and binary alloy substrates. Nonequilibrium kinetics are introduced in the model to account for the departure from thermodynamic equilibrium at the solid/liquid interface. The modeled problem involves a moving boundary with both heat and solute diffusion and is solved by an implicit control volume integral method with solid/liquid interface immobilization by coordinate transformation. To illustrate the model capabilities, we have analyzed laser-induced surface melting of pure metals (Al, Cu, Ni, Ti) and dilute Al–Cu alloys, and some typical results are presented. The computation results show some large solid overheating and melt undercooling effects, which result from the high heat flux and the slow kinetics. Large interface velocity variations are also seen during the process, depending on the substrate material and laser flux. Complex interface velocity variations during the earlier stages of resolidification were also predicted for the alloys, and result from interactions between the several physical mechanisms involved. Results on interface temperatures, solute concentrations, and nonequilibrium partition coefficients are also presented.