Solidification System Research Articles

Cell/dendrite transition and electrochemical corrosion of Pb–Sb alloys for lead-acid battery applications

The aim of this article is focused on a comparative experimental study of the electrochemical feature of as-cast Pb–2.2wt.% Sb alloy with cellular/dendritic transition for applications in the manufacturing of lead-acid battery parts. A water-cooled unidirectional solidification system is used to obtain the alloy samples. Electrochemical impedance spectroscopy (EIS) plots, potentiodynamic polarization curves and equivalent circuit analysis are used to evaluate corrosion resistance in a 0.5M H2SO4 solution at 25°C. The cellular Pb–2.2wt.% Sb alloy is found to have a current density which is of about 3 times lower than that of the dendritic Pb–2.2wt.% Sb alloy. The Pb–2.2wt.% Sb alloy has lower current density than both the Pb–1wt.% Sb and the Pb–6.6wt.% Sb alloys evidencing its potential for application as positive grid material in lead-acid batteries. It is also verified that a conventional casting with low cooling rate of about 0.6°Cs−1 produces coarser cellular spacings which is more appropriate for the manufacturing of the Pb–2.2wt.% Sb alloys grids due to its corresponding electrochemical behavior.

Corrosion resistance of directionally solidified Al–6Cu–1Si and Al–8Cu–3Si alloys castings

The aim of this article is to compare the electrochemical corrosion resistance of two as-cast Al–6 wt.% Cu–1 wt.% Si and Al–8 wt.% Cu–3 wt.% Si alloys considering both the solutes macrosegregation profiles and the scale of the microstructure dendritic arrays. A water-cooled unidirectional solidification system was used to obtain the as-cast samples. Electrochemical impedance spectroscopy (EIS) and potentiodynamic anodic polarization techniques were used to analyze the corrosion resistance in a 0.5 M NaCl solution at 25 °C. It was found that the Al–8Cu–3Si alloy has better electrochemical corrosion resistance than the Al–6Cu–1Si alloy for any position along the casting length. At the castings regions where the Cu inverse profile prevailed (up to about 10 mm from the castings surface) the corrosion current density decreased up to 2.5 times with the decrease in the secondary dendrite arm spacing.

Modeling of macrosegregation in steel ingot: Influence of mold shape and melt superheat

A continuum model for the transport phenomena in solidification systems is used to investigate the formation mechanism of macrosegregation in a 3.3 t steel ingot. Numerical scheme with explicit time stepping in solidification problems is developed for solving coupled temperature and concentration fields, and equations of momentum. Experimental measurements and numerical results in the literature are used as indications of the validity of present prediction. Influences of the mold shape and the melt superheat upon macrosegregation are investigated. Results show that for ingots with the same weight, reducing the size of hot top favors a pronounced positive-negative-positive concentration distribution along the centerline. A-segregates and positive-negative-positive concentration distribution are not found in the ingot with a modified hot top that has a more uniform section area. Higher superheat reduces the height of bottom negative segregation cone. For cases with superheat larger than zero, positively segregated patches are observed at the ingot bottom.

Growth and characterization of multicrystalline silicon ingots by directional solidification for solar cell applications

This study is focussed on the growth of multicrystalline silicon ingots with large grains by controlling the silicon melt cooling rate to initiate dendritic nucleation in the initial stage of the solidification. Two ingots were grown with different undercooling rates and compared with a reference ingot grown by standard cooling conditions. All ingots were grown in a lab scale directional solidification system. The wafers cut from all three ingots have been characterized for resistivity, minority carrier lifetime and dislocation density measurements by four point probe, quasi steady state photo conductance and PV Scan, respectively. The wafers were converted into solar cells and their electrical parameters have been measured. The cells fabricated from ingot 2 show slightly higher efficiencies in comparison with ingot 1 and the reference one. The present cooling rate was not enough to initiate the dendrite nucleation in the beginning of the solidification. Hence, there is no significant difference was observed in the crystal quality of the grown ingots 1 and 2.

Mechanism and modeling of silicon carbide formation and engulfment in industrial silicon directional solidification growth

This paper proposes a mechanism of carbon species formation and transport in the gas phase and silicon carbide particle formation and engulfment in the liquid phase. A numerical model considering various forces acting on silicon carbide particles is developed to quantify particle transport and particle engulfment. Numerical simulations are conducted to study fluid flow and temperature distribution in an industrial directional solidification system and particle distribution in the solidified silicon. Strategies to reduce carbon contamination and improve ingot quality are proposed.

Numerical simulation of macrosegregation in large multiconcentration poured steel ingot

A continuum model for the transport phenomena in solidification systems is used to investigate the formation of macrosegregation in a 360 t multiconcentration poured steel ingot. A numerical scheme with explicit time stepping in solidification problems is employed to solve coupled temperature and concentration fields, and equations of momentum. The proposed scheme is tested against an experimental concentration field reported in the literature. The influences of the concentration in each ladle and the ladle pouring time upon macrosegregation are studied. The simulation results show that, with a shorter time for the last ladle, macrosegregation is not significantly reduced with MCP compared with conventional single concentration pouring. A longer ladle time produced a marked reduction in the extent of positive segregation at the top of the ingot. Any positive segregation in excess of the industrial requirement limit is shifted to the riser. If the last ladle is poured after a longer time, growth of channel segregates is initiated by local remelting.

Effects of the furnace pressure on oxygen and silicon oxide distributions during the growth of multicrystalline silicon ingots by the directional solidification process

Oxygen impurities can reduce the carrier lifetime of mc-Si solar cells. In this study, simulations of the transient temperature, velocity and concentrations of oxygen and silicon oxide are carried out in order to clarify the transport mechanism of oxygen impurities in the silicon melt and silicon oxide through argon gas, in a directional solidification system (DSS) furnace. As the solidification fraction enlarges, the oxygen concentration in the melt diminishes, because of the reduction in the amount of crucible surface immersed below the silicon melt. When the solidification fraction is small, two pairs of vortices appear in the melt. Oxygen originating from the crucible is carried towards the free surface by the upper vortex. Oxygen concentration is higher with a higher furnace pressure rather than with a lower one due to the low SiO evaporation at the free surface. When the solidification fraction increases, the upper vortex gradually disappears. The lower vortex occupies almost the whole of the melt, with the exception of a small upper central region where a small vortex forms because of the cooling effect of the argon gas. Oxygen impurities carried by the lower vortex along the crystallization front towards the central region are obstructed by this small vortex. The size of the small vortex increases as the solidification fraction increases. Since the small vortex is stronger when the furnace pressure is higher, the concentration is lower around the central region. This means that the oxygen concentration is smaller when the furnace pressure is higher rather than lower. The simulation results agree well with the experimental results.

Nucleation and bulk growth control for high efficiency silicon ingot casting

Directional solidification and casting systems used in silicon production are reviewed first. Common and distinguished features are summarized, and the problems appearing at nucleation and bulk growth stages are discussed. The relationship between growth rate and temperature in the heat exchanger block for an industrial growth system is developed and the predicted results are compared with experimental measurements. A commercial industrial system is simulated to study fluid flow and heat transfer in the growth system, and hot zone design is optimized. Suggestions are made for further improvement of industrial silicon casting systems.

Thermal system design and optimization of an industrial silicon directional solidification system

This paper reports numerical investigation of multicrystalline silicon (mc-Si) ingot production using an industrial-scale directional solidification furnace capable of producing 500kg silicon ingot. Specific examination is made on thermal distribution, interface shape and velocity field. Evaluation is performed for the applicability of thermal system design to reduce carbon and oxygen impurity, improve crystal quality and enhance energy efficiency. The effects of gas flow, heater position and geometric configuration on temperature distribution are discussed as well to provide the essential knowledge for system optimization.

In situ study on dendrite growth of metallic alloy by a synchrotron radiation imaging technology

This study was trying to observe the real-time dendrite growth of Sn-Bi and Sn-Pb binary alloys by a synchrotron radiation imaging technology. The imaging system includes an intense and high brightness synchrotron radiation source, a high-resolution and fast-readout charge coupled device camera, an alloy sample and a Bridgman solidification system. The imaging experiments were done at Beijing Synchrotron Radiation Facility with an updated synchrotron radiation imaging technique, diffraction-enhanced imaging, which was firstly used to study the dendrite growth of metallic alloy. A series of growth behavior and morphology evolution of dendrite have been in situ observed, such as columnar-to-equiaxed transition, dendrite competition, dendrite fragmentation and floating, etc., which can offer the direct proofs to verify or improve the solidification theories of metallic alloy. This research opens a novel window for the study of alloy solidification and enables the unambiguous understanding of solidification processes in optically opaque, metallic alloys.

MODELAGEM MATEMÁTICA DA TRANSFERÊNCIA DE CALOR NA INTERFACE METAL/MOLDE

MODELAGEM MATEMÁTICA DA TRANSFERÊNCIA DE CALOR NA INTERFACE METAL/MOLDE

Mechanical properties of Sn–Zn lead-free solder alloys based on the microstructure array

The aim of this study is to develop a comparative experimental study interrelating mechanical properties, solidification thermal parameters and microstructure characteristics of a hypoeutectic Sn–4 wt.% Zn, a hypereutectic Sn–12 wt.% Zn and a eutectic Sn–9 wt.% Zn solder alloys. A water-cooled vertical upward unidirectional solidification system was used to obtain the samples. It was found that a more homogeneous distribution of the eutectic mixture, which occurs for smaller dendritic spacings in hypoeutectic and hypereutectic alloys, increases the ultimate tensile strength. The resulting microstructure of the eutectic Sn-9 wt.% Zn alloy has induced higher mechanical strength than those of the Sn–4 wt.% Zn and Sn–12 wt.% Zn alloys. It was found that the eutectic alloy experiences a microstructural transition from globular-to-needle-like Zn-rich morphologies which depend on the solidification growth rate. It is also shown that a globular-like Zn-rich morphology provides higher ultimate tensile strength than a needle-like Zn-rich eutectic morphology.

Comparison of electrochemical performance of as-cast Pb–1 wt.% Sn and Pb–1 wt.% Sb alloys for lead-acid battery components

A comparative experimental study of the electrochemical features of as-cast Pb–1 wt.% Sn and Pb–1 wt.% Sb alloys is carried out with a view to applications in the manufacture of lead-acid battery components. The as-cast samples are obtained using a water-cooled unidirectional solidification system. Pb–Sn and Pb–Sb alloy samples having similar coarse cell arrays are subjected to corrosion tests in order to assess the effect of Sn or Sb segregation in the cell boundary on the electrochemical performance. Electrochemical impedance spectroscopy (EIS) diagrams, potentiodynamic polarization curves and an equivalent circuit analysis are used to evaluate the electrochemical parameters in a 0.5 M H 2SO 4 solution at 25 °C. Both the experimental and simulated EIS parameters evidence different kinetics of corrosion. The Pb–1 wt.% Sn alloy is found to have a current density which is of about three times lower than that of the Pb–1 wt.% Sb alloy which indicates that dilute Pb–Sn alloys have higher potential for application as positive grid material in maintenance-free Pb-acid batteries.

Correlação entre propriedades mecânicas e arranjo dendrítico de ligas Sn-Zn utilizadas em solda sem presença de chumbo

O desenvolvimento de solda sem chumbo tem sido uma tarefa árdua para cientistas de materiais pelas preocupações com a saúde e com o meio ambiente devido ao conteúdo de chumbo das soldas convencionais. O objetivo do presente trabalho é investigar a influência dos parâmetros da microestrutura dendrítica nas propriedades mecânicas das ligas sem chumbo: Sn-4 e 12%p Zn. Amostras das ligas em estudo foram obtidas por experimentos de solidificação conduzidos em um aparato que proporciona uma solidificação unidirecional vertical ascendente refrigerado à água (25 °C ±2°C), o qual garante a extração de calor apenas pela base do lingote em regime transitório de extração de calor. Os resultados experimentais permitiram estabelecer correlações entre parâmetros térmicos e espaçamentos dendríticos secundários com a resistência à tração e alongamento. Conclui-se dos resultados experimentais que o alongamento específico mostra-se significantemente melhorado (em torno de 10%) com o refino microestrutural para ambas as ligas. Encontrou-se também que em taxas de resfriamento na faixa entre 0,5 e 10 ºC/s, a liga Sn-4%p Zn é aquela que apresenta a menor variação entre limites de resistência à tração (31 a 32 MPa) e alongamento que a liga Sn-12%p Zn (29 a 33 MPa).

Electrochemical corrosion of Pb–1wt% Sn and Pb–2.5wt% Sn alloys for lead-acid battery applications

The aim of this study was to compare the electrochemical corrosion behavior of as-cast Pb–1 wt% Sn and Pb–2.5 wt% Sn alloy samples in a 0.5 M H 2 SO 4 solution at 25 °C. A water-cooled unidirectional solidification system was used to obtain the as-cast samples. Electrochemical impedance spectroscopy (EIS) diagrams, potentiodynamic polarization curves and an equivalent circuit analysis were used to evaluate the electrochemical corrosion response. It was found that a coarse cellular array has a better electrochemical corrosion resistance than fine cells. The pre-programming of microstructure cell size of Pb–Sn alloys can be used as an alternative way to produce as-cast components of lead-acid batteries with higher corrosion resistance associated with environmental and economical aspects.

Modeling and improvement of silicon ingot directional solidification for industrial production systems

This paper presents the numerical simulation of heat transfer and solidification process in an industrial silicon directional solidification system. The energy efficiency in the production system is analyzed. The effects of the heating power and the position of the side insulation layer on the solidification interface position/shape are investigated. The numerical results indicate that the directional solidification of the silicon can be controlled by adjusting the heating power and moving the side insulation layer. The knowledge gained from the thermal analysis can be used to improve the energy efficiency in the current silicon production system. An easy method to detect the solidification interface shape was proposed and has been conceptually proven.

Effects of Microstructural Arrangement on Hot Corrosion Resistance of a Pb-Sb Alloy for Battery Grids

It is well known that the solution characteristics, such as pH and temperature may affect the corrosion mechanism and the corrosion behavior. Lead acid batteries manufacturers have provided modifications into the grid project in order to decrease battery grid weight as well as to reduce the production costs, and to increase the battery life-time cycle and the corrosion-resistance. The performance of lead-acid batteries in automotive applications can significantly be affected by temperature variation. The aim of this study was to evaluate the effects of the microstructural morphology of a Pb-6.6wt%Sb alloy under conditions of hot corrosion. A water-cooled unidirectional solidification system was used to obtain coarse and fine dendritic microstructures. Electrochemical impedance spectroscopy (EIS) diagrams, potentiodynamic polarization curves and an equivalent circuit analysis were used to evaluate the corrosion behavior of fine and coarse dendritic samples in a 0.5M H2SO4 solution in three different working temperatures. It was found that independently of the working temperature, samples with finer dendritic microstructures provide better corrosion resistance than coarser ones, which is an indication that the former microstructural pattern may provide a higher battery life-time in severe temperatures than a coarser one.

Hot corrosion resistance of a Pb–Sb alloy for lead acid battery grids

The aim of this study was to evaluate the effects of the microstructural morphologies of a Pb–6.6 wt%Sb alloy on the resulting corrosion resistance in a 0.5 M H 2SO 4 solution at different temperatures: environment temperature, 50 °C and 70 °C. A water-cooled unidirectional solidification system was employed permitting a wide range of microstructures to be analyzed. Electrochemical impedance spectroscopy (EIS) diagrams, potentiodynamic polarization curves and an equivalent circuit analysis were used to evaluate the corrosion behavior of the Pb–Sb alloy samples. It was found that with increasing temperatures the general corrosion resistance of Pb–Sb dendritic alloys decreases, and that independently of the working temperature finer dendritic spacings exhibit better corrosion resistance than coarser ones.

Gravity-driven inverse segregation during transient upward directional solidification of Sn–Pb hypoeutectic alloys

Upward unidirectional solidification experiments were performed with Sn–4.0 wt.%Pb and Sn–12.5 wt.%Pb alloys. Transitory conditions of heat flow extraction were sustained during these solidification experiments, which mean that all solidification variables (cooling rate, tip growth rate, thermal gradient, and metal/mold heat transfer coefficient) have varied freely along solidification. In this work, the macrosegregation phenomenon is experimentally and theoretically evaluated considering both solidification shrinkage and gravity induced flows. The numerical model is based on a one-dimensional solution consisting of an implicit/explicit time integration scheme to couple thermal and solutal fields, which is supported by a finite volume numerical modeling technique to be solved. The applied solidification system permitted columnar growth to prevail along the castings. Since the Pb-rich melt tends to flow downward (in favor of the gravity vector), the bottom of the castings exhibited positive Pb content, mainly induced by gravity. The numerically simulated and experimental temperature and concentration profiles were compared and a very good agreement has been observed.

Influences of solute content, melt superheat and growth direction on the transient metal/mold interfacial heat transfer coefficient during solidification of Sn–Pb alloys

Several factors such as alloy composition, melt superheat, mold material, roughness of inner mold surface, mold coating layer, etc., can affect the transient metal/mold heat transfer coefficient, h i . An accurate casting solidification model should be able to unequivocally consider these effects on h i determination. After this previous knowledge on interfacial heat transfer, such models might be used to control the process based on thermal and operational parameters and to predict microstructure which affects casting final properties. In the present work, three different directional solidification systems were designed in such a way that thermal data could be monitored no matter what configuration was tested with respect to the gravity vector: vertical upward and downward or horizontal. Experiments were carried-out with Sn–Pb hypoeutectic alloys (5 wt.% Pb, 10 wt.% Pb, 15 wt.% Pb and 30 wt.% Pb) for investigating the influence of solute content, growth direction and melt superheat on h i values. The experimentally obtained temperatures were used by a numerical technique in order to determine time-varying h i values. It was found that h i rises with decreasing lead content of the alloy, and that h i profiles can be affected by the initial melt temperature distribution.