Analysis of melting and resolidification in a two-component metal powder bed subjected to temporal Gaussian heat flux

Melting and resolidification of a subcooled, two-component metal powder bed subjected to temporal Gaussian heat flux is investigated analytically in this paper. The integral approximate solutions for preheating, melting with shrinkage, and resolidification are obtained. An increase in heat source intensity or powder bed porosity will result in an increase of the melt pool depth, melt pool temperature, and the overall processing time. The melt pool becomes shallower with increasing subcooling because more heat flux is needed to melt the powder. A change in the liquid phase porosity will increase the process time only slightly and have a negligible effect on the amount of melted material produced.     

Analysis of Partial Melting in Metal Powder Bed with Constant Heat Flux

Rapid melting of a subcooled single-component metal powder bed in Selective Laser Sintering (SLS) is analyzed in this paper. Under irradiation of a pulse laser beam, the surface of the powder particle is molten first while the core of the particle remains solid. The temperature of the liquid layer is higher than the melting point, while the temperature of the solid core is below the melting point. Therefore, the mean temperature of the partially molten particle is within a range of temperature adjacent to the melting point. In addition, the powder bed experiences a significant density change during melting because the interstitial gas initially in the pore space is driven out as melting progresses. Melting in SLS of single-component metal powder can therefore be modeled as that occurring in a range of temperature with significant density change. The temperature distributions in the solid, liquid, and mushy zones and locations of the various interfaces are obtained by using an integral approximate method. The effects of initial porosity, dimensionless initial temperature, and dimensionless thermal conductivity of the interstitial gas on the surface temperature and locations of the interfaces are investigated.     

双级双向热底冷盖熔铝炉

介绍一种新型燃油熔铝炉。该炉是将大型熔铝炉分解成两组双级、双向炉,采用冷炉盖、热炉底连体结构以及预热炉料的烟气强迫循环系统,与４台普通单炉相比,可明显减少散热面和能量消耗。垂直炉料安装的整体烧嘴结构,能在整个熔炼周期内保持恒定均匀的供热强度,能将多品种、小批量合金组织在同一套熔炼设备中集中开炉,连续作业,是中小铝厂熔炼设备改造的理想炉型。     

Finite element modeling of the porosity formation in castings

The aim of the present paper was to contribute to understanding the origin and effects of porosity in aluminum die-castings by characterizing the distribution and geometry of the porosity. It also seeks to develop a predictive model for microporosity formation during solidification through an analysis of the alloy solidification path, the presence of gas dissolved in the molten metal and the flow of liquid metal through the mushy zone. The finite element method was used for solving porosity formation problem jointly with the problem for heat and mass transfer.     

GRAVITY- AND SOLIDIFICATION-SHRINKAGE-INDUCED LIQUID FLOW IN A HORIZONTALLY SOLIDIFIED ALLOY INGOT

A basic-variable, explicit finite difference scheme is used to solve the unsteady and strongly pressure-coupled velocity problems of liquid flows induced both by thermosolutal buoyancies and solidification shrinkage in a binary alloy solidification process. A sample calculation, performed on an IBM personal computer, for a horizontally solidified Al-4.5%Cu alloy in a high-H/L ratio cavity with a top riser shows that the shrinkage established pressure gradient in the mushy region can be several hundred, even several thousand, times larger than that in the bulk liquid region.     

Stress effect on the swelling/shrinking behavior of an AB2 alloy during hydrogenation cycles

The stress influence on a Ti_0.85Zr_0.15Mn_1.33V_0.3 alloy during hydrogenation cycles has been studied in a special design test cell for mechanical studies during hydrogenation. The alloy was synthesized in an induction melting furnace with an inert Ar atmosphere. The full basic characterization of the material is presented, from a structural and morphological point of view by X-ray powder diffraction (XRPD), Scanning Electron Microscopy (SEM), Energy Dispersion X-ray spectroscopy (EDX) and laser grain size analysis, and, from a thermodynamic point of view by the measure of the Pressure composition Isotherms (P-c-I) at 23 and 80 °C. Besides, in order to measure the mechanical properties of the material, volume variations measurements during hydrogenation cycles were performed. The samples were milled up to sizes beneath 1.25 mm, and afterwards placed into a cylindrical die under three different levels of axial stress, 0.0325 MPa, 0.12 MPa and 0.84 MPa, for more than 100 cycles. For higher stress levels the powder bed shrinks as cycles continue, while for lower stresses the powder bed swells. A natural tendency for the powder to decrepitate and agglomerate was observed, as the size distribution of the particles reduces and the shape become uneven, generating an increase of the porosity and volume in the case of the lower stress sample. As the stress increases, the grains rearrange easily and a polydisperse distribution of particles facilitates the accommodation of smaller grains between bigger ones, producing a reduction in the porosity and volume of the hydride bed after several cycles.     

The accuracy of a solid packing fraction model in recognizing zones of different dendritic structures

A front tracking method on a fixed control-volume grid, based on assumed dendrite tip kinetics, is applied to discuss the accuracy of a numerical model where the coherency solid fraction is used in the identification of diverse dendritic regions developing within a mushy zone during binary alloy solidification driven by diffusion and thermo-solutal buoyancy forces. It is shown that this critical value of a solid volume fraction is not constant but changes with time and along the border separating the regions.     

Solidification of a ternary alloy liquid layer on a substrate subject to self-consistent melting

Micro-electro-mechanical (MEMS), plasma or powder spray deposition, surface coating, semiconductor technology, splat cooling, single and twin-roller melt-spinning, strip and slab casting, melt-extraction, etc. are usually characterized by solidification of a thin liquid layer on a cold substrate. A one-dimensional model of enthalpy formation of the energy and species conservation equations with thermodynamic relationships from ternary equilibrium diagram are solved to study the solidification processes for ternary alloys molten liquid layer on the ternary eutectic solid substrate. The solidification path of the liquid layer may pass through the primary, cotectic and eutectic solidification regions. The melting and re-solidification of the substrate happens at the ternary eutectic point. The thermal physical properties of the splat and substrate are identical and imperfect contact of contact surface between the splat and substrate is considered. The temperature functions as compositions are assumed as linear along the liquidus surface and cotectic curves. The temperature distributions of the solidified splat and the melted, re-solidified substrate, the thicknesses of the different mushy layers of splat and melting of substrate subject to different process parameters and thermal physical properties are quantitatively and extensively investigated. The initiation times for primary, cotectic mushy and eutectic solid fronts of splat and the complete re-solidification times of the substrate are affected by different parameters, these are also investigated. Results of this study are compared with experimental data provided by Aitta et al. The growth rates of the cotectic and eutectic fronts are found to agree well with experimental data. The effects of initial solute concentrations of liquid layer, solute concentrations and temperatures at the binary and ternary eutectic points on the thicknesses of different mushy layers are important and presented.     

A partial shrinkage model for selective laser sintering of a two-component metal powder layer

A partial shrinkage model for selective laser sintering of a metal powder mixture that contains two kinds of metal powders with significantly different melting points is developed. Laser-induced melting accompanied by partial shrinkage, liquid metal flow driven by capillary and gravitational forces, and resolidification of the metal powder layer are modeled using a temperature transforming model. The effect of volume fraction of the gas in the sintered region on the sintering process is investigated.     

Melting in a vertical cylindrical tube: Numerical investigation and comparison with experiments

The present work numerically investigates melting of a phase-change material (PCM) in a vertical cylindrical tube. The analysis aims at an investigation of local flow and thermal phenomena, by means of a numerical simulation which is compared to the previous experimental results .The numerical analysis is realized using an enthalpy–porosity formulation. The effect of various parameters of the numerical solution on the results is examined: in particular, the term describing the mushy zone in the momentum equation and the influence of the pressure–velocity coupling and pressure discretization schemes. PISO vs. SIMPLE and PRESTO! vs. Body-Force-Weighted schemes are examined. No difference is detected between the first two. However, considerable differences appear with regard to the last two, due to the mushy zone role.Image processing of experimental results from the previous studies is performed, yielding quantitative information about the local melt fractions and heat transfer rates. Based on the good agreement between simulations and experiments, the work compares the heat transfer rates from the experiments with those from the numerical analysis, providing a deeper understanding of the heat transfer mechanisms. The results show quantitatively that at the beginning of the process, the heat transfer is by conduction from the tube wall to the solid phase through a relatively thin liquid layer. As the melting progresses, natural convection in the liquid becomes dominant, changing the solid shape to a conical one, which shrinks in size from the top to the bottom.     

MODEL OF THE EFFECT OF ALLOY CONTENT ON SHELL STRENGTH DURING SOLIDIFICATION OF BINARY ALLOYS

A thermomechanical model of unidirectional solidification of binary alloy systems is presented. The goal of the model is to begin to explore the effect of alloy content on the mechanical behavior of the solidifying shell by first examining the effect on lateral strength. The shell solidifies onto a semi-infinite mold proceeding behind a mushy zone that grows into an initially quiescent fluid. Deformation of the shell is modeled with a thermohypoelasioviscous constitutive law that allows for examination of the idealized case of elastic deformation of the casting as well as the case where strain rate relaxation due to viscous creep predominates. Any effects of alloy content on the coefficients in the constitutive model are ignored so that the calculated effects on strength arise entirely from the size of the mushy zone. Aluminum-magnesium alloys solidifying onto a copper mold are considered as specific examples using a linearized portion of the Al-Mg phase diagram. The material with the smallest alloy content exhibits the greatest shell strength for the same cooling histories. That material with the widest freezing range has the lowest strength. For the elastic model, the average strength always increases with time, whereas for the elastoviscous case it can decrease with time to the point where the alloy content has virtually no effect on strength.     

CFD modelling of bed shrinkage and channelling in fixed-bed combustion

Combustion of fixed fuel beds in grate furnaces is common within production of heat and power from solid fuels. Available theoretical and experimental experience provides a solid base of knowledge on how a conversion model of a fuel bed, using Computational Fluid Dynamics (CFD), needs to be structured and solved. Most existing models, however, handle the conversion in one single dimension of constant bed properties; when observing a burning fuel bed in a grate furnace it becomes apparent that the fuel bed is neither homogeneous nor uni-dimensional. In this study, a two-dimensional model of the combustion of fixed fuel beds has been developed for the purpose of studying the influence of heterogeneous fuel-bed properties on the conversion. In the model, the available experience from fuel-bed modelling by means of the sub-models for fixed-bed conversion was structured into a fluid-flow scale and into a fuel-particle scale, in which new formulations describing the shrinkage of the fuel bed on a multi-particle scale was introduced. Both available and new sub-models were introduced into a pre-existing CFD-platform, in which the framework for simulating fluid flow in porous media was used to solve also the conversion related processes acting within the particle scales as well as within the multi-particle scales. The complete model was validated with good correspondence between available measurements of temperature and species concentration in a wood-char combustor. In addition, the modelled shrinkage was found to well describe the observed shrinkage of the fuel bed in a combustion experiment. Results of model simulations by using heterogeneous bed porosity show that a porous passage through the bed risks causing channelling in the fuel bed – a phenomenon common in modern grate furnaces and suspected to cause increased emissions of nitric oxides and unburned carbon compounds. The channelling tendency could, however, to a large extent be reduced by grates of higher flow resistance. The natural porosity increase attributable to the packing of particles onto a wall was shown to concentrate combustion disturbances close to the surface of the grate. Thus, larger changes in the porosity than caused by natural fuel packing against a wall are needed to give rise to channels that emerge through the fuel bed.     

连铸复合式水口内层用镁铝尖晶石质耐火材料的合成研究

以镁铝尖晶石为骨料、等摩尔的氧化镁粉和氧化铝微粉为基质料,通过一步固相烧结法合成了水口内层用镁铝尖晶石质耐火材料,考察了原料组成、骨料粒度和成型压力对一步煅烧法制备镁铝尖晶石烧结性能的影响。研究结果表明,随着骨料粒度增大和基质料含量的增加,试样的烧结性变差,烧成试样的烧后线收缩率变小,显气孔率逐渐增大;随着成型压力的增加,烧成试样的显气孔率和烧后线收缩都明显减小。以80目尖晶石为骨料,按尖晶石：氧化镁粉：氧化铝微粉=20：40：40配比(mol％),并以200MPa的压力成型,可以获得结构致密、烧后线收缩率适中的尖晶石质水口内层材料。     

Laser consecutive pulse heating and phase change: Influence of spatial distribution of laser pulse intensity on melting

Laser consecutive pulse heating of solid surface and the influence of the laser pulse parameter on the melting and mushy zone formation in the irradiated region are investigated. The laser pulse parameter (β) defines the spatial distribution of the laser pulse power at the irradiated surface; in which case, β = 0 represents the Gaussian profile while β = 1 corresponds to the ring type of laser power distribution with the peak intensity away from the center (symmetry axis). β is set in such a way that the energy content in each pulse with different β values becomes the same. The control volume approach is used when modeling the heating and phase change processes. The laser pulse parameter is selected to alter the laser power intensity distribution across the irradiated surface while modifying the location and magnitude of the laser peak power intensity at the irradiated surface. It is found that the laser pulse parameter alters the sizes of the melting and mushy zones in the surface region.     

Optimal operation of alloy material in solidification processes with inverse heat transfer

A transient nonlinear inverse heat transfer problem arising from alloy solidification processes is considered. In practice, the solidus and liquidus interface motions and thus the mushy zone thicknesses are pre-given to control the material quality. To achieve the desired front motions, the required time-dependent boundary conditions have to be predicted on both mold sides simultaneously. In this study, the enthalpy method is used for the derivation of governing equations. Hence, the inverse problem will be solved only in a single spatial and temporal domain. The conjugate gradient method with adjoint equation is applied for the resulting minimization problem. The method is applied as comparison for pure material with other previous studies. Then, alloy material with different front velocities is set up to investigate the solidification process. The obtained results show a close agreement between the desired and computed front motions and mushy zone thickness.     

MICROSOLIDIFICATION PROCESS IN MULTICOMPONENT SYSTEM

In conventional solidification of multicomponent mixtures, a mushy zone appears between the pure solid and liquid regions and promotes stable solidification by accepting the rejected solute regionally. From the standpoint that the fineness of inhomogeneity influences the mechanical properties in material processing, the linking of macro heat transfer and microsolidification in the mushy zone was studied. First, the crystal growth and its accompanying concentration field near the advancing front of the mushy zone were observed precisely by using the light absorption method. It was clarified that the mushy zone consisted of the leading front in which the frame structure formed with an accompanying concentration boundary layer and a growing region where the solidification proceeds by fattening of the crystals. Second, the mechanism of side-branch evolution was studied in conjunction with interfacial instability due to constitutional supercooling and curvature supercooling around the primary arm surface. Summarizing these results, the microsolidification process is discussed quantitatively in relation to macro heat transfer.     

Hydrogen impact on the shrinkage behaviors of wustite packed beds above 900 °C

The steel industry is facing increasing pressure and challenges from the environment in recent years. The urgent utilization of clean energy not only reduces greenhouse gas emissions, but also promotes future innovations in blast furnace iron making technology. Hydrogen (H₂) energy is considered to be one of the most promising alternatives to carbon-based fossil energy for the reduction of iron oxides. Therefore, the gaseous reduction of iron oxides by H₂ has been intensively studied for decades. However, the impact of H₂ on the shrinkage behavior of iron oxide packed beds above 900 °C has rarely been studied, and the interaction between H₂ and carbon monoxide (CO) in the shrinking process is not fully understood. Therefore, this study uses H₂, CO and H₂+CO mixture gas for the well-designed high temperature experiments of wustite (FeO) packed beds. The results show that H₂ protects the coke from further damage in the packed bed at 900–1000 °C, and the corresponding shrinkage rate (SR) decreases from 0.31%/°C for CO case to 0.16%/°C. Meanwhile, when the temperature exceeds 1350 °C, the packed bed under the CO atmosphere accelerates shrinkage due to the melting and dripping of the metallic iron after carbonization. By contrast, when CO is replaced by H₂, the carbonization process is controlled by the solid state diffusion of coke carbon rather than the reverse Boudouard reaction. As a result, the lower carbonization efficiency not only increases the transition temperature by up to 100 °C, but also reduces the weight of the melted hot metal by one third, which significantly improves the bed permeability.     

In-flight oxidation of composite powder particles during thermal spraying

This paper presents a mathematical model of the in-flight thermal behavior and oxidation of powder particles during thermal spray deposition. In particular, stainless steel and Cr₃C₂–NiCr particles are investigated. The in-flight model accounts for the acceleration and deceleration of the particles during flight under variable fluid velocity, while the thermal model takes into account heating, melting, cooling and possible solidification as the particle travels towards the substrate. A finite difference method is used to solve the thermal energy conservation equation of the particles. The model includes non-equilibrium calculations of the phase change phenomena in the liquid–solid (mushy) zone and dissolution of the ceramic reinforcement in the composite particles. The growth of the oxide layer at the particle surface is represented by a modified boundary condition, which includes finite-rate oxidation. The results obtained give the interrelations between various process parameters and the oxidation phenomenon.     

Mushy zone equilibrium solidification of a semitransparent layer subject to radiative and convective cooling

Equilibrium solidification in a semitransparent planar layer is studied using an isothermal mushy zone model. The layer is made up of a pure material being emitting, absorbing and isotropically scattering and is subject to radiative and convective cooling. The model involves solving simultaneously the transient energy equation and the radiation transport equation. An implicit finite volume scheme is employed to solve the energy equation, with the discrete ordinate method being used to deal with the radiation transport. A systematical parametric study is performed and the effects of various materials optical properties and processing conditions are investigated. It is found that decreasing the optical thickness and increasing the scattering albedo both lead to a wider mushy zone and a slower rate of solidification.