Effects of electromagnetic force on melt flow and porosity prevention in pulsed laser keyhole welding

Porosity formation in pulsed laser keyhole welding was found to be affected by two competing factors: (1) the solidification rate of molten metal and (2) the back filling speed of molten metal during the keyhole collapse process. Porosity (pores/voids) was found in welds when the solidification rate of molten metal exceeds the back filling speed of molten metal. In this study, the use of electromagnetic force was proposed to control the back filling speed of molten metal, and a mathematical model was developed to investigate the effects of electromagnetic force on the transient melt flow, keyhole dynamics, and porosity formation. The results demonstrate that porosity in pulsed laser welding can be prevented by an applied electromagnetic force. Parametric studies to determine the desired strength of the electromagnetic force and its duration were also conducted to achieve quality welds.     

Role of metal foam in solidification performance for a latent heat storage unit

The current latent heat storage (LHS) units are usually poor in energy charging and discharging efficiency. Given this, a two dimensional (2D) numerical model of the energy discharging process is presented and comprehensively analyzed to predict the role of metal foam in the solidification performance of LHS units. In the model, the fractal geometry reconstructed by the fractal Brownian motion is utilized for the pore characterization of the metal foam. The proposed model is validated through a melting experiment in copper foams from the reference. The temperature dynamic response and the solidification front evolution in metal foam are analyzed and compared to those in a corresponding cavity. The roles of the fractal dimension and porosity in the solidification behaviors are quantitatively analyzed. The results report that the presence of metal foam enhances the solidification performance. For the main goal of maximizing the latent storage, the appropriate porosity of an LHS unit is dependent on the duration time for the heat discharging process in the real application of thermal energy storage. Even if the porosity is the same, the fractal dimension also affects the solidification performance. A decrease in the fractal dimension (lower degree of disorder for pore distribution) provides greater access to heat flow through the phase change material-foam composite and thus leads to improvement in the interstitial heat transfer, which in turn accelerates the rate of heat release. The fractal dimension is expected to be less than 1.5 for superior solidification performance.     

Transient natural convective heat transfer in porous medium with solidification of binary mixture

In this paper an enthalpy porosity method associated with finite control volume scheme and SIMPLE iteration was employed to solve Navier–Stokes equation coupled with energy equation through Ergun equation and Boussinesq approximation for studying the effect of two-dimensional transient natural convective heat transfer from a closed region of porous medium with the different porosity on solidification in carbon–iron system. As shown in the results, it is fund that the thickness of solidification layer is increased with time due to thermal coupled flow induced by natural convection; and the wall temperature is faster changed in porous medium with larger porosity, which corresponds to slow the growth of the solidification layer in binary system.     

Modeling and simulation of heat transfer phenomena during investment casting

Determining the heat transfer phenomena during casting processes is an important parameter for measuring the overall performance of process. It gives information about the properties of the metal being casted and its possible behavior in the mold during casting process. Improper determination of heat transfer phenomena and use of improper molding materials and casting conditions leads to defects such as misruns, cold shuts, shrinkage, pin holes, air holes and porosity in final product. A mathematical model was developed using standard transport equations incorporating all heat transfer coefficients to calculate the time for solidification of metal in casting and computer simulation of the model was carried out in C++ to validate the model. The metal used was pure iron casted in investment molds of silica sand with zircon coating. It was shown that airflow near the mold surfaces was partially restricted due to geometry of the molds and arrangement of the pieces around a tree. So, the changes in heat transfer coefficient also contribute towards time of solidification. The time calculated was found to be in good agreement with experimental values.     

FINITE-ELEMENT MODELING OF HEAT FLOW IN DEEP-PENETRATION LASER WELDS IN ALUMINUM ALLOYS

A two-dimensional finite-element nonlinear transient heat conduction model was developed and used to simulate deep-penetration keyhole laser welds in aluminum alloys. The weld thermal profiles were calculated in an arbitrary reference plane as the laser beam approached and passed the plane. From the calculated thermal profiles, three-dimensional quasi-steady-state shapes of the weld pools were determined. The predicted weld bead shape and dimensions were in good agreement with the experimental results. The experimental laser welds in aluminum alloys contained large amounts of porosity. The model predicted large mushy zones for aluminum laser welds during solidification, which in turn increase the probability of porosity formation by increased bubble entrapment.     

Simulation of Slag-Skin Formation in Electroslag Remelting Using a Volume-of-Fluid Method

A modified volume-of-fluid method is implemented in a fixed-grid, finite-volume model simulating transport phenomena, solidification, and electromagnetics. The VOF model agrees well with published results, and the complete model is used to investigate process variations in the electroslag remelting process, in which liquid metal is melted from a consumable electrode immersed in an electrically resistive slag. The molten metal sinks through the slag cap floating on the liquid metal pool while a slag skin freezes to the mold. Here a VOF tracks slag skin formation and its effects on melt rate with different current levels and ingot diameters.     

Numerical Simulation of Segregation Phenomena Coupled with Phase Change and Fluid Flow: Application to Metal Matrix Composites Processing

The injection of a liquid metal through a fibrous preform, located in an initially preheated mold, is one of the techniques used to manufacture metal matrix composites (MMCs). In order to reduce the chemical reactions between the fibers and the metal matrix, the fibrous reinforcement and the mold are commonly preheated up to initial temperatures much lower than the metal solidification temperature. Therefore, local metal solidification instantaneously occurs on fiber during liquid metal infiltration. When infiltrating metal alloy, unlike what happens when infiltrating a pure metal, both temperature and composition may vary within the matrix; this heterogeneity induces segregation within composites. A fiber scale numerical simulation was developed taking into account coupled physical phenomena which occur during the processing: flow of the liquid metal around the fibers, phase change phenomena, solute redistribution at the liquid/solid interface during alloy solidification, and species diffusion. This model predicts the segregation phenomena associated with fibrous preform infiltration by a binary alloy.     

高孔隙率泡沫金属相变材料储能、传热特性

以高孔隙率泡沫金属材料作为骨架制备而成的新型复合相变储能材料的导热系数将大大高于相变材料本身的导热系数,在储能过程中具有更好的传热效果。给出了较通用的高孔隙率泡沫金属材料等效导热系数的估算公式,并利用准稳态方法建立了复合相变材料在凝固过程的数值模型,对其凝固过程的传热特性进行了理论分析。以铝—石蜡和铜—石蜡复合材料作为研究对象。分析表明,采用复合储能材料可以使得其传热性能得到很大提高,但是也会使复合材料的储能能力有所降低。提出了一种平衡储能能力和传热性能的方法,当泡沫金属处于平衡孔隙率时,在传热性能得到极大提高的同时也使得其储能能力降低不多。同时,分析得到了外部换热环境对储能能力、传热性能以及平衡孔隙率的影响,即较大的对流换热时,若要取得适当的储能能力和传热性能,则需要较小的孔隙率。     

Analysis of Infiltration,Solidification, and Remelting of a Pure Metal in Subcooled Porous Preform

The parts fabricated by selective laser sintering of metal powders are usually not fully densified and have porous structure. Fully densified parts can be obtained by infiltrating liquid metal into the porous structure and solidifying the liquid metal. When the liquid metal is infiltrated into the subcooled porous structure, the liquid metal can be partially solidified. Remelting of the partially solidified metal can also take place and a second moving interface can be present. Infiltration, solidification, and remelting of metal in a subcooled porous preform obtained by laser sintering of metal powders are analytically investigated in this article. The governing equations are nondimensionalized and the problem is described using six dimensionless parameters. The temperature distributions in the remelting and uninfiltrated regions were obtained by an exact solution and an integral approximate solution, respectively. The effects of porosity, Stefan number, subcooling parameter, and dimensionless infiltration pressure are investigated.     

泡沫铜内石蜡凝固的孔隙尺度实验研究

对泡沫铜内石蜡凝固相变进行孔隙尺度实验研究。采用高分辨率相机与红外热像仪对凝固过程相场与温度场进行可视化,并通过热电偶测量石蜡与泡沫铜骨架局部温度以获得相变过程热响应及热非平衡特性。揭示了泡沫铜孔隙内凝固相变中包括固液相界面移动、液态石蜡流动及石蜡体积收缩等多个物理过程。研究表明:在多物理过程交互影响下,泡沫铜可高效扩展凝固相界面、提升样品热响应速率,采用孔隙率为0.974的泡沫铜可将石蜡凝固相变速率提升至2.8倍;泡沫铜能有效避免石蜡凝固过程由体积收缩引起的裂缝问题,消除由其引起的热阻;石蜡与泡沫铜骨架间存在局部热非平衡性,且在相变阶段尤为明显。     

Analytical solution for solidification of close-celled metal foams

This paper introduces an analytical model capable of predicting the location of solidification front as well as the full solidification time for heterogeneous materials such as close-celled metallic foams. Full numerical simulations with the method of finite difference are separately conducted to validate the analytical model. The model predicts that an increase in porosity causes significant retardation of full solidification as a result of decreased effective thermal conductivity and diffusivity of the porous medium. Effects of pore shape and cooling temperature on overall solidification behavior were also studied.     

A Numerical Study of 3D Turbulent Melt Flow and Solidification in a Direct Chill Slab Caster with a Porous Combo Bag Melt Distributor

A 3D turbulent melt flow and solidification of an aluminum alloy (AA-1050) for an industrial-sized direct chill slab casting process is modeled. The melt is delivered through a rectangular submerged nozzle and a non-deformable combo bag fitted with a bottom porous filter. The non-Darcian model, incorporating the Brinkman and Forchheimer extensions, is used to characterize the turbulent melt flow behavior passing through the porous filter. The casting speed and the effective heat transfer coefficient at the metal–mold contact region within the mold are varied. The above two parameters are found to have significant influence on the solidification process.     

Lattice Boltzmann Simulation of Incompressible Transport Phenomena in Macroscopic Solidification Processes

A lattice Boltzmann (LB) simulation strategy is proposed for the incompressible transport phenomena occurring during macroscopic solidification of pure substances. The proposed model is derived by coupling a passive scalar-based thermal LB model with the classical enthalpy–porosity technique for solid–liquid phase-transition problems. The underlying hydrodynamics are monitored by a conventional single-particle density distribution function (DF) through a kinetic equation, whereas the thermal field is obtained from another kinetic equation which is governed by a separate temperature DF. The phase-changing aspects are incorporated into the LB model by inserting appropriate source terms in the respective kinetic equations through the most formal technique following the extended Boltzmann equations along with an appropriate enthalpy updating scheme. The proposed model is validated extensively with one- and two-dimensional solidification problems for which analytical and numerical results are available in the literature, and finally, it is used for solving a benchmark problem, the Bridgman crystal growth in a square crucible.     

Modelling of impingement phenomena for molten metallic droplets with low to high velocities

Thermal spray coatings are formed by accelerating a stream of powder particles towards a targeted substrate surface where they impact, deform, and adhere. A fundamental understanding of the splat formation can pave the way for future developments in thermal spray technology through better understanding. Numerical modelling is applied in this investigation which simulates the detailed transient flow of a molten metal droplet impacting, deforming, and solidifying on a flat, solid substrate. The computations are carried out on a fixed Eularian structured mesh using a volume of fluid method to simulate the boundary between the metallic and atmospheric-gas phases. The results shed light on the break-up phenomena on impact and describe in detail how the solidification process varies with an increasing impact velocity.     

预处理对河道底泥固化效果的影响

为了通过预处理改变底泥的理化性质,增加底泥固化后抗压强度,实现底泥的资源化利用,比较了3种底泥预处理方法(Fenton氧化、热处理、淋洗)对底泥中有机质的质量分数、重金属总质量、重金属形态分布,特别是对底泥固化后抗压强度的影响。结果表明:固化后抗压强度与底泥中有机质的质量分数直接相关;可降低底泥中有机质的质量分数的预处理方法(Fenton氧化、热处理)对提高底泥固化后抗压强度效果较好,底泥固化后抗压强度高达2.7 MPa;鼠李糖脂淋洗虽然能够有效去除底泥中的重金属,但是淋洗剂中的有机物反而使底泥中有机质的质量分数增加,对固化不利。因此,Fenton氧化和热处理均可用作提高底泥固化强度的预处理方法,且固化后试样重金属浸出毒性低,对环境影响小。     

Thermo-fluidic characteristics of open cell metal foam as an anodes for DCFC,part I: Head loss coefficient of metal foam

A porous metal was suggested to be used for the anode of a DCFC. Thermo-fluidic characteristics of fuel-electrolyte mixture in the porous metal should be known in order for proper design of the anode. Previous researchers investigated pressure drop and heat transfer performance of fluids flowing through metal foams that have different pore densities and porosities. Various characteristic length scales were used for the Reynolds number and friction factor of metal foams in the previous works. In the present study, we propose a realistic definition of the characteristic length scale that is applicable to pressure drop evaluation in metal foams regardless of pore density and porosity. A series of experiments was conducted to obtain friction factor of metal foam. An equivalent diameter based on permeability and porosity appeared to best fit the experimental data produced using various metal foams. The relationship between the friction factor and Reynolds number through metal foams can be classified into three regimes. In Re < 20, ln f is inversely proportion to ln 16/Re. In Re > 2000, the value of friction factor approaches 0.17. The relationship between friction factor and Reynolds number of foam materials appeared to be similar trend to that of Moody chart.     

Chemical non-equilibrium modelling of columnar solidification

This paper deals with the macroscopic modeling and numerical simulation of columnar dendritic solidification of binary alloys. The macroscopic governing equations and associated effective transport properties were previously derived using a volume averaging technique with local closure. The macroscopic model takes into account the spatial variation of the pore-scale geometry within the mushy zone, which leads to additional terms involving porosity gradients. The second important feature concerns solute mass conservation, which is described by considering a macro-scale non-equilibrium accounting for chemical exchanges at the solid–liquid interface. A simplified version of the model is validated through a comparison of the numerical solution to three experiments available in the literature. Porosity extra terms are systematically estimated on the basis of these numerical simulations, and the influence on solidification of effective transport properties such as permeability and interfacial solute exchange coefficients is investigated.     

Fabrication of porous metal by selective laser melting as catalyst support for hydrogen production microreactor

To improve the hydrogen production performance of microreactors, the selective laser melting method was proposed to fabricate the porous metals as catalyst supports with different pore structures, porosities, and materials. The influence of the porous structures on the molecule distribution after passing through the porous metals was analyzed by molecular dynamics simulation. The developed porous metals were then used as catalyst supports in a methanol steam reforming microreactor for hydrogen production. Our results show that the porosity of the porous metal had significantly influence on the catalyst infiltration and the reaction process of hydrogen production. A lower degree of catalyst infiltration of the porous metal was obtained with lower porosity. A copper layer-coated stainless-steel porous metal with a staggered structure and gradient porosity of 80%–60% exhibited much larger methanol conversion and H₂ flow rate due to its better heat and mass transfer characteristic. Methanol conversion and H₂ flow rates could reach 97% and 0.62 mol/h, respectively. Finally, it was found that the experimental results were in good agreement with the simulation results.     

Discharge improvement of a phase change material-air-based thermal energy storage unit for space heating applications using metal foams in the air sides

This study examines the energy discharge of a phase-changing material (PCM)-based air heat exchanger using a metal foam inside the heat transfer fluid (HTF) channel. Such systems have various potential applications in the heating space and building ecosystem. Thermal energy storage (TES) often utilizes air as the HTF, which limits the heat transfer performance due to the low thermal conductivity. This paper aims to address this drawback via incorporating a metal foam into the HTF channel to enhance the thermal performance between the heat transfer fluid (air) and the PCM, which is considered as the novel side of this study. The combined system is mathematically modeled with an symmetrical, three-dimensional computational fluid dynamics method for various flow rates and inlet temperatures of the HTF with different geometric parameters of the metal foam. This study indicates the advantage of utilizing the porous medium in the air channel. The results show the HTF flow rate has a great influence on the discharging rate. The presence of the porous medium in the system improves the discharging process by 116% compared with a non-porous medium system at the same flow rate. The discharging time decreases by increasing the porosity, and the value of 90% is found as the best porosity value at the flow rate of 0.005 kg/s in this system. The solidification rate is proportional to the pore density because of the surface area impacts of the porous medium, also the pressure-drop and the pumping required are highly affected by the mentioned dependent parameters.     

Numerical Method for Calculation of Complete Casting Processes—Part I: Theory

This article presents a method for calculation of the complete casting process, including the pouring of the liquid metal into the mold, its solidification, the deformation of the solidified cast, the formation of airgaps between the cast and the mold and their influence on the heat transfer, and the residual stresses. An original phase-change procedure is developed, valid for an arbitrary number of pure metals and/or alloys. A collocated version of a segregated finite-volume method is used to calculate both the liquid metal flow and the deformations and stresses in solids.