Dendritic growth in Al-Si alloys during brazing. Part 2: Computational modeling

The computational heat and mass transfer modeling approach presented in this paper emphasizes the influence of undercoolings on dendrite structure formations of the alpha phase crystals inherent to advanced phases of an aluminum brazing netshape manufacturing sequence. In the first segment of this work, the empirical evidence involving the outcome of the solidification process and its kinetics was presented. In this paper, simulation of the alpha phase crystal pattern formation is corroborated with empirical findings obtained by utilizing an AA4343/AA3003 brazing sheet exposed to controlled atmosphere brazing (CAB) in ultra-high purity nitrogen.     

Brazing manufacturing technology of plate-fin heat exchanger for solid oxide fuel cells

The plate-fin heat exchanger (PFHE) is the core equipment used to achieve the high efficiency of solid oxide fuel cell (SOFC), however, the issues of high-performance brazed joints in the manufacturing of the PFHE have been a challenge due to the poor mechanical properties. This study proposes a brazing manufacturing technology of isothermal solidification with the optimized post bonding heat treatment strategy, to synergistically improve the strength-ductility property and homogenize microstructure of brazed joints, aiming at guaranteeing the high energy efficiency of SOFC. The results show that brazing at 1065 °C for 25 min achieves the complete isothermal solidification and an intermetallic-free joint centerline with numerous borides generating in the diffusion-affected zone. Solution treatment then dissolves large quantities of acicular and blocky borides. The uniformity of grain size and kernel average misorientation distribution is also improved due to recrystallization, which becomes more pronounced after solution aging treatment. In addition, solution aging treatment results in an improvement in the ultimate tensile strength of the brazed joint, which is more prominent than that after solution treatment. However, the increase in elongation after solution aging treatment is smaller than after solution treatment, while still much higher than the as-brazed joint due to the dissolution of boride precipitates and growth of twin boundaries. The results demonstrate that the proposed brazing manufacturing technology not only homogenizes microstructure, but also significantly improves strength and ductility, further promoting the long-life operation of SOFC.     

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.     

Computational Modeling of GMAW Process for Joining Dissimilar Aluminum Alloys

A three-dimensional transient model is developed to solve for heat transfer, fluid flow, and species distribution during a continuous gas metal arc welding (GMAW) process for joining dissimilar aluminum alloys. The phase-change process during melting and solidification is modeled using a fixed-grid enthalpy-porositytechnique, and Scheil's model is used to determine coupling among composition, temperature, and the liquid fraction. The effect of molten droplet addition to the weld pool is simulated using a “cavity” model, in which the droplet heat and species addition to the molten pool are considered as volumetric heat and species sources, respectively, distributed in an imaginary cylindrical cavity within the molten pool. To establish the model for joining dissimilar alloys, results for joining two pieces of a similar alloy are also presented. The dissimilar welding model is demonstrated using a case study in which a plate of wrought aluminum alloy (with approximately 0.5 wt% Si) is butt-welded to an aluminum cast alloy plate (with approximately 10 wt% Si) of equal thickness using a GMAW process. Macrosegregation, along with the associated heat transfer and fluid flow phenomena and their role in the weld pool development, are discussed. The model is able to capture some of the key features of the process, such as differential heating of the two alloys, asymmetric weld pool development, mixing of the molten alloys, and the final composition after solidification.     

Growth of multi-crystalline silicon ingot by improved directional solidification process based on numerical simulation

A multi-crystalline silicon (Si) ingot was simulated and grown using the improved directional solidification (DS) process. Numerical simulations were performed with two different cooling paths and two different coolant flow rates in order to demonstrate the thermal characteristics in the improved DS furnace during the crystal growth. The temperature distributions in the furnace and locally (at the silicon ingot) were predicted as a function of time. From these result, a multi-crystalline Si ingot weighing 300 kg was grown within 40 h using the improved DS process. The Si ingot had a grain size that was larger than 5 mm, and the structure of the ingot was in the form of vertical columns. From the analysis results, the Si ingot exhibited a resistivity below 2 Ω cm and a life time above 3 μs.     

Heat, mass and momentum transport behaviors in directionally solidifying blade-like castings in different electromagnetic fields described using a continuum model

A previous continuum model proposed and recently modified by the authors for describing the heat, mass and momentum transport phenomena in dendrite solidification process of alloy castings was further extended to the solidification cases in an arbitrary electromagnetic (EM)-fields. The extended continuum model and a FEM/FDM joint solution technique were successfully applied to the numerical simulations of directional solidification transport processes in blade-like castings of Pseudo-binary In718 base-4.85 wt.%Nb and Al-4.5 wt.%Cu alloys under a static or harmonic EM-field of different strengths/frequencies. The computational results demonstrate the availability of the present continuum modeling to treat an EM-STP problem, and also reveal that the volume-contraction-driven liquid feeding flow is much more difficult to be suppressed than the buoyancy-induced by means of applying a static magnetic field.     

Wetting and Spreading Of Liquid Metals Through Open Microgrooves and Surface Alterations

Surface-tension-driven flows (isothermal or non-isothermal) of microlayers of complex liquids over substrates under reactive wetting conditions at elevated temperatures are greatly influenced by interface interactions and topography of surface alterations. For example, understanding of the spreading of molten metals over metal substrates with complex topography may be interpreted as spreading over multiple connected networks of open microchannels. Hence, understanding of the kinetics of wetting and spreading of such reactive systems through microchannels is of a key interest. This article provides an overview of wetting/spreading phenomena related to migration of molten metal microlayer over smooth, rough, and/or well-organized-topography surfaces, such as microchannels. Systems involving a liquid metal medium temperature range (at ～850–873 K, Al-Si over Al), and a low temperature range (～420–520 K, Pb-Sn and Ag-Sn over Cu and Cu-Sn) have been considered. Kinetics data involving triple-line movement and its modeling were supported by real-time in situ visualizations. Targeted applications of these fundamental studies involve the art of brazing of compact aluminum heat exchangers for HVAC&R (heating, ventilation, air conditioning, and refrigeration), thermal management for aerospace, and soldering processes (in particular lead-free) for electronics industries.     

Anodic behavior of Al current collector in 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] amide ionic liquid electrolytes

The anodic behaviors of aluminum current collector for lithium ion batteries were investigated in a series of 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] amide room temperature ionic liquids (RTILs) and EC + DMC electrolytes. It was found that the aluminum corrosion, which occurred in EC + DMC electrolytes containing LiTFSI, was not observed in the RTIL electrolytes. Further research showed that a passive film with amide compounds as main components formed firmly on aluminum surface during the anodic polarization in the RTIL electrolytes, which inhabited the aluminum corrosion. In addition, the additives generally used in the batteries, such as ethylene carbonate, ethylene sulfite and vinyl carbonate, as well as temperature did not obviously affect the aluminum passive film, the oxidation of the RTILs increased at the elevated temperature, which only resulted in the corrosion potential of aluminum in the RTIL electrolytes shifted to more negative potential, a passive film still firmly formed on the aluminum surface to surpassed the further oxidation of the aluminum current collector. Those results lead to a potential for the practical use of LiTFSI salt in the room temperature ionic liquid electrolytes for lithium ion batteries.     

Modeling on transient microstructure evolution of solid-air solidification process under continuous cooling in liquid hydrogen

The safety hazard brought by the oxygen-rich solid formed by the solidification of air in liquid hydrogen cannot be ignored. A numerical model describing the evolution of dendrite during solidification of nitrogen-oxygen binary solutions in liquid hydrogen is developed. By introducing the reduction factor, the growth anisotropy of the six-fold symmetric dendrite simulated by the Cartesian grid is effectively reduced. The reliability of the model is verified by comparing the tip growth rate of dendrites with six-fold symmetry with analytical models. On this basis, the microstructure evolution and solute segregation of solid-air dendrites are investigated. The results show that with the increase of the cooling rate, the dendrite growth rate accelerates while aggravating the solute segregation, and the outer edge of the dendrite is more likely to reach the oxygen-rich state. The improvement in solute diffusibility enables dendrites to reach an oxygen-rich state at larger sizes, but also accelerates dendrite growth. The initial composition has little effect on the microstructure evolution and growth rate of the solid-air dendrite. However, in the presence of forced convection, the solid-air dendrite morphology loses its symmetry and makes the upstream dendrite arms reach an oxygen-rich state at a smaller size. This study helps to understand the air solidification process in liquid hydrogen and provides theoretical guidance for the safety research of liquid hydrogen system.     

电渣重熔过程中传热及凝固组织的数值模拟

在凝固传热数学模型基础上,采用正态分布形核模型和二维偏心生长模型,模拟了钢锭重新凝固过程温度场及钢锭凝固组织的生长情况.数值结果表明：电渣重熔钢锭以倒“V”形的柱状晶为主,中心和底部为等轴晶,模拟结果与实验结果符合良好.随着渣温的提高,熔池变得深且宽;随着侧壁换热系数的增大,熔池深度变浅;随着重熔速度的减小,熔池深度也逐渐变浅;较低的渣池温度、较大的对流换热系数有利于等轴晶形成,而重熔速度对凝固组织的影响不大.     

The effects of current density and amount of discharge on dendrite formation in the lithium powder anode electrode

A compacted lithium powder anode was used to improve the demerits of dendrite formation of lithium metal. Dendrite formation of lithium metal was restrained to use compacted lithium powder anode under a specific amount of discharge and the current density. In this study, the amount of discharge and the current density which suppress dendrite formation at the surface of a lithium powder electrode were investigated on an experimental basis. Discharge/charge reactions were accomplished on various values of the amount of discharge and current density by using beaker cells. It was analyzed by SEM images whether dendrite was formed or not on the surface of lithium powder electrode. From the various experiments, the relationship between current density and total amount of discharge was deduced as a simple mathematical model. From the model, the critical condition of total amount of discharge for dendrite formation in Li-powder electrode was increased from 0.1 mA cm⁻² to 1 mA cm⁻² current density. However, the critical condition of total amount of discharge was decreased over 1 mA cm⁻². Using the model, the condition whether dendrite formed or not on the Li-powder anode could be estimated.     

Effect of wire temperature on the microstructural evolution of Si films in hot-wire chemical vapor deposition for digital display and photovoltaic devices

The formation of crystalline phase in Si by hot-wire chemical vapor deposition (HWCVD) was investigated, focusing on the microstructural evolution as a function of hot-wire temperature. The microstructure of films deposited on a Si wafer was compared between hot-wire temperatures of 1590, 1670, and 1800 °C. A heavily twinned structure was observed at the wire temperature of 1670 °C, which resulted in the apparent intensity peak of (1 1 1) hexagonal-closed packed (HCP) crystalline Si from a typical face-centered cubic (FCC) crystalline Si structure in the X-ray diffraction analysis. The twin-related HCP crystalline phase was markedly diminished at 1800 °C and hardly observed at 1590 °C. The observed deposition behavior was approached by the effect of the wire temperature on the size of charged nanoparticles formed in the gas phase in the HWCVD process.     

Effect of nano-silica filler in polymer electrolyte on Li dendrite formation in Li/poly(ethylene oxide)-Li(CF3SO2)2N/Li

Lithium dendrite growth in Li/poly(ethylene oxide) (PEO)-Li(CF₃SO₂)₂N (LiTFSI)-nano-SiO₂/Li was examined using direct in situ observation under galvanostatic conditions at 60 °C. Both the onset time of dendrite formation and the short-circuit time of the cells were extended by the addition of nano-SiO₂ filler into the polymer electrolyte, of which an acid-modified nano-SiO₂ filler was the most effective. The onset time was dependent on the current density in the range from 0.1 to 1.0 mA cm⁻². Li dendrite growth in Li/PEO₁₈LiTFSI/Li at 60 °C for current densities of 0.1 and 0.5 mA cm⁻² started at 125 and 15 h, respectively. PEO₁₈ LiTFSI with addition of 10 wt% acid-modified 50 nm SiO₂ showed extended dendrite formation onset times of 250 h at 0.1 mA cm⁻² and 32 h at 0.5 mA cm⁻². The suppression of dendrite formation at the Li/PEO₁₈ LiTFSI interface could be explained by enhancement of the conductivity and suppression of the interface resistance between lithium and the polymer electrolyte by addition of the nano-SiO₂ filler. The electrical conductivity of 4.1 × 10⁻⁴ S cm⁻¹ and interface resistance of 405 Ω cm² for PEO₁₈ LiTFSI at 60 °C were respectively increased to 7.2 × 10⁻⁴ S cm⁻¹ and decreased to 77 Ω cm² by the addition of 10 wt% acid-modified nano-SiO₂.     

Effect of co-doping nano-silica filler and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide into polymer electrolyte on Li dendrite formation in Li/poly(ethylene oxide)-Li(CF3SO2)2N/Li

Lithium metal dendrite growth in Li/poly (ethylene oxide)-lithium bis (trifluoromethanesulfonyl) imide (PEO₁₈LiTFSI), nano-silica, and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide (PP13TFSI) composite solid polymer electrolyte/Li was investigated by direct in situ observation. The dendrite onset time decreased with increasing current density and deviated from Sand's law in the current density range of 0.1-0.5 mA cm⁻² at 60 °C. Lithium dendrite formation was not observed until 46 h of polarization at 0.5 mA cm⁻² and 60 °C, which is a significant improvement compared to that observed in Li/(PEO₁₈LiTFSI)/Li, where the dendrite formation was observed after 15 h polarization at 0.5 mA cm⁻² and 60 °C. The suppression of dendrite formation could be explained by the electrical conductivity enhancement and decrease of the interface resistance between Li and the polymer electrolyte by the introduction of both nano-SiO₂ and PP13TFSI into PEO₁₈LiTFSI. The electrical conductivity of 4.96 × 10⁻⁴ S cm⁻¹ at 60 °C was enhanced to 7.6 × 10⁻⁴ S cm⁻¹, and the interface resistance of Li/PEO₁₈LiTFSI/Li of 248 Ω cm² was decreased to 74 Ω cm² by the addition of both nano-SiO₂ and PP13TFSI into PEO₁₈LiTFSI.     

Solidification and melting behaviors and characteristics of molten salt in cold filling pipe

The solidification and melting phenomena and performances of molten salt during cold filling process in a straight pipe are numerically investigated using volume of fluid model. As the molten salt is filled into a cold pipe, the molten salt adjacent to the cold wall is rapidly cooled, and the solidification phenomena appears. After the whole pipe is filled, the solidification layer begins to melt by high temperature fluid heating. Because of the solidification layer, the flow section obviously shrinks, and the pressure loss remarkably increases. During the solidification and melting processes, the fluid temperature in the region with phase change only varies near the freezing point, and it quickly rises after the melting process. Because of the absorption or release of latent heat, the boundary heat flux of molten salt is increased in the solidification region, while it will be decreased in the melting region. As the inlet temperature rises, the pressure loss apparently decreases with the thickness of solidification layer decreasing. However, when the inlet flow velocity increases, the thickness of solidification layer decreases, but the flow resistance without phase change increases, so the pressure loss has a maximum at moderate flow velocity.     

Research on charge and discharge performance of a new molten salt heat storage material

针对高效率太阳能热发电传热储热的需求,本工作对新型熔盐（LiNO₃：NaNO₃：KNO₃物质的量比例为2：3：5）进行了充放热传热性能研究。通过Fluent模拟结果表明最后熔化的区域是位于罐体底部的边角位置,为了在工程应用中防止此区域出现熔化死角,应增加熔化装置。通过充放热实验验证了模拟结果,实验研究结果表明罐体内的熔盐在加热过程中,是自上而下逐渐熔化的,且熔盐始终存在温度分层,而熔盐冷却凝固的过程并没有分层现象。     

Experimental determination of the interfacial heat transfer during cooling and solidification of molten metal droplets impacting on a metallic substrate: effect of roughness and superheat

The interfacial heat transfer between a solidifying molten metal and a metallic substrate is critical in many processes such as strip casting and spray deposition. As the molten metal cools down and solidifies, the interface undergoes a change from the initial liquid/solid contact to a solid/solid contact, leading to very dynamic variations in the rate of interfacial heat transfer. This article presents the results of an experimental study of the contact heat transfer when molten nickel or copper droplets are dropped on an inclined metallic substrate. The interfacial heat transfer coefficient, h, between the melt and the substrate is evaluated by matching model calculations with the top splat surface temperature history measured by a fast-response pyrometer. The results suggest that a high value of the interfacial heat transfer coefficient h (10⁴ to 3×10⁵ W/m² K) is achieved when the molten splat is in contact with the substrate, followed by a smaller value (<10⁴ W/m² K) during the later stages of solidification and the solid cooling phase. A parametric study was performed to investigate the effect on h of the metal/substrate materials combination, the melt superheat, and the substrate surface roughness, and some of the results are also presented.     

Preparation of size-controlled fine Al particles for application to rear electrode of Si solar cells

Micron-sized aluminum (Al) particles are widely used in the fabrication of rear electrodes of Si solar cells. Moreover, the rear electrode of Si solar cells can be fabricated at relatively low electrode firing temperature using submicron Al particles whose sizes can be easily controlled, but there have not been any clear methods to obtain submicron Al particles yet. In this study, we first successfully prepared size-controlled Al submicron particles via a wet chemical process using dibutyl ether solvent and then to fabricate rear electrodes of Si solar cells; to our knowledge, this is the first such application of submicron Al particles. The geometric mean diameter (D_p) of the Al particles could be controlled from 139 to 614 nm by adjusting the reaction temperature, and the prepared Al particles showed geometric standard deviations (σ_g) of 1.25-1.30. The Al particle size was reduced to ∼35 nm by adding an organic surfactant to the precursor solution for Al particle preparation. Rear electrodes were fabricated by firing the screen printed Al paste films comprising Al particles with geometric mean diameters of ∼379 nm from 600 to 900° C. The electrode fired at 750 °C showed the minimum electrical sheet resistivity of 77.9 mΩ/□ (specific resistivity: 97.3 μΩ cm) and contained a BSF (back surface field) with a thickness of ∼3 μm. Our results indicate that the reported method can be used to minimize the thermal defects in the rear electrode of Si solar cells by lowering the electrode firing temperature.     

合金枝晶生长及微细界面热质传递模拟

利用元胞自动机法(CA)耦合对流和热质传递模型模拟了Al-Cu二元合金微观组织在多种影响因素下的二维生长过程,分析了枝晶凝固、微界面热质传递以及微流动等微细现象之间的相互作用,得到了单枝晶以及多个枝晶在微流作用下的生长规律。模拟结果表明:(1)枝晶凝固过程中溶质富集于生长前沿。随着枝晶生长,凝固前沿远离冷源,枝晶尖端温度逐渐增大,而浓度逐渐变小;(2)流动对于枝晶的生长有着重要影响。流动破坏了枝晶生长的对称性,下游溶质浓度大于上游,枝晶在上游方向优先生长,而在下游方向有所抑止;(3)多个枝晶生长时,枝晶彼此间有阻碍生长的作用,二次枝晶臂的形成相对减少,枝晶间几乎不存在微流动。     

Formation of weld crater in GMAW of aluminum alloys

Both mathematical modeling and experiments have been conducted on the formation of the crater formed in a GMAW of aluminum alloy 6005-T4. Transient weld pool shape and the distributions of temperature and velocity were calculated by a three-dimensional numerical model. The final weld bead shape and dimensions were obtained. Corresponding experiments were conducted and in good agreement with modeling predictions. Metallurgical characterizations were also performed on the experimental samples. It was found that due to the fast solidification of the weld pool after the termination of the welding arc, there is no time for the molten metal to flow back towards the weld pool center and close up the crater. Thus a crater was formed at the end of the weld bead. Solidification cracking was formed at the center of weld crater. A “back-up” technique was proposed to allow extra molten metal to flow back to the crater and fill it up. The crater was successfully filled and the crater cracking was eliminated.