工艺参数对藕状多孔Mg合金结构影响的模拟仿真

在金属/气体共晶定向凝固工艺(Gasar)中,气体压力、过热度以及凝固速率是影响藕状多孔金属结构的最重要参数。建立了一个描述Gasar工艺过程的传热、传质、气孔形核、生长、合并及气孔与固相的协同生长的三维非稳态理论模型。并且采用有限差分的方式,实现了定向凝固铸造法Gasar工艺过程中藕状多孔结构形成及演变的计算机仿真,揭示了气体压力和凝固速率对藕状多孔金属最终结构参数的影响。基于Mg-H系的模拟与相应试验结果吻合较好,验证了所建仿真模型的合理性。     

<Emphasis Type="Italic">In Situ</Emphasis> Characterization of Porosity Evolution in A356 Alloy Directionally Solidified Under Different Solidification Velocities

The effect of solidification velocity on nucleation and growth of porosity in a directionally solidified A356 alloy was investigated in situ using microfocus x-ray imaging and directional solidification technology. Increasing solidification velocity produces high hydrogen solute segregation in the solidification front, leading to a considerable reduction of nucleation temperature for porosity formation. When the solidification velocity increases from 0.1 mm/s to 0.2 mm/s, the average nucleation temperature decreases from 634.0°C to 614.6°C. The increased solidification velocity also increases the rate and fluctuation of pore growth. As a result, the average pore size in the fast solidified specimen is smaller than that with a slow velocity.     

Hydrogen porosity in directionally solidified aluminium–copper alloys: a mathematical model

A combined continuum and stochastic model of diffusion-controlled growth was developed to simulate the formation of porosity during the solidification of aluminium alloys. The whole population of pores was tracked, rather than just the average values. A finite difference solution of the diffusion equations was used, combined with a stochastic model of nucleation. The growth of each individual pore was simulated, assuming the shape to be spherical until it impinged on the developing dendrites, at which point the growth was modelled as a hemispherically capped segmented cone, with the growth radius limited by the liquid space between the dendrites. A previously published model by one of the authors was used to predict the dendritic spacing as a function of the thermal conditions.The model was compared with in situ observations of the formation of porosity during the solidification of aluminium–copper alloys, where the size, distribution and morphological evolution of pores were measured as a function of temperature/time. The predicted development of the porosity, including the distribution in size and morphology, compared well with that observed experimentally. The qualitative agreement between the model predictions and experimental results supports the hypothesis that the effect of hydrogen and its diffusion must be incorporated into any accurate model of pore formation in aluminium alloys.     

On the Role of Bubbles in Metallic Splat Nanopores and Adhesion

Highly nanoporous surfaces were observed on the underside of Ni splats. Experiments varying process parameters and substrate treatments were performed to determine the mechanism of pore formation. A theory of impact-induced bubble nucleation and freezing into pores is presented, and calculations are compared with experimental results. Pore formation and morphology is strongly dependent on substrate (a) thermal properties as they affect time for bubble growth before solidification into pores and (b) roughness as submicron scratches enhance nucleation by providing heterogeneous sites and several micron grooves reduce the driving force for nucleation. Splat pull-off experiments are shown that suggest bubble nucleation and pore formation strongly affect adhesion, and represent a strong contribution to the effectiveness of surface roughening. Finally, this observation shows the potential for the manufacturing of high-surface area materials using thermal spray.     

Effect of Sr content on porosity formation in directionally solidified Al-12.3wt.%Si alloy

The influence of Sr addition on pore formation in directionally solidified Al-12.3wt.% alloy was investigated using X-ray detection, optical microscope, and SEM-EDX. Results indicate that addition of Sr significantly increases the number density and volume fraction of porosity. The considerable rise in volume fraction of porosity is attributed to the remarkable increase in the numbers of pores formed. It is found that Sr solute in liquid Al-Si al oy can diffuse into the oxide inclusions to form loose oxide aggregations which have more activity as the nucleation sites for porosity. Adding more Sr considerably increases the numbers of active nucleation sites. There is an obvious fluctuation of pore number density during steady state solidification, which is believed to be related to a fluctuation of local hydrogen supersaturation induced by the competition of pore nucleation and growth for hydrogen solute supplement.     

共晶反应定向凝固工艺制备多孔材料气孔形成和长大机理

本文采用热分析法研究几种金属一氢气共晶反应定向凝固制备藕状多孔材料气孔的形成和长大机理。结果表明,这种工艺气孔是借助熔融金属中的活性杂质非匀质形核的,而长大则是沿固液界面的法线方向向具有粗糙界面的气固相转移,最终形成气孔规则定向排列于金属基体中的多孔材料。     

Mechanism of nucleation and growth of hydrogen porosity in solidifying A356 aluminum alloy: an analytical solution

This study derives an analytical solution for the mechanism of nucleation and growth of hydrogen pore in the solidifying A356 aluminum alloy. A model of initial transient hydrogen redistribution in the growing dendritic grain is used to modify the lever rule for the mechanism of nucleation of pore. The model predicts the fraction of solid at nucleation, the temperature range of nucleation, the radius of hydrogen diffusion cell, and the supersaturation of hydrogen needed for nucleation. The role of solidus velocity in nucleation is explained. The parameters calculated from the model of nucleation are used for analyzing the mechanism of kinetic diffusion-controlled growth of pore, in which the mathematical transformations of variables are introduced. With the transformations, it is argued that the diffusion problem involving the liquid and solid phases during solidification could be treated as a classic problem of precipitation in the single-phase medium treated by Ham or Avrami. The analytical solution for the nucleation of pore is compared with the mechanism of macrosegregation. The predicted volume percent of porosity and radius of pore based on the mechanism of growth of pore is discussed with respect to the thermodynamic solution, the published experimental data, the numerical solutions, and the role of interdendritic fluid flow governed by Darcy’s law.     

Diffusion-controlled growth of hydrogen pores in aluminium–silicon castings: in situ observation and modelling

In situ observations were made of the nucleation and growth kinetics of hydrogen porosity during the directional solidification of aluminium–7 wt% silicon (Al7Si) with TiB₂ grain refiner added, using an X-ray temperature gradient stage (XTGS). The effect of altering the solidification velocity on the growth rate and morphology of the porosity formed was characterized by tracking individual pores with digital analysis of the micro-focal video images. It was found that increasing the solidification velocity caused the pore radius to decrease and pore density to increase. Insight gained from the experimental results was used to develop a computational model of the evolution of hydrogen pores during solidification of aluminium–silicon cast alloys. The model solves for the diffusion-limited growth of the pores in spherical coordinates, using a deterministic solution of the grain nucleation and growth as a sub-model to calculate the parameters that depend upon the fraction solid. Sensitivity analysis was carried out to assess the effects of equiaxed grain density, pore density, initial hydrogen content and cooling rate. The model agrees with the experimental results within the resolution limits of the XTGS experiments performed.     

Direct observation of the effect of strontium on porosity formation during the solidification of aluminium-silicon alloys

Strontium is added to many aluminium-silicon casting alloys at the level of a few hundred parts per million in order to modify the structure of the eutectic. However, the addition of strontium also alters the amount and morphology of the porosity formed during solidification. The effect of strontium on porosity formation is not well understood and there is no consensus upon the cause of the changes observed. This paper investigates the effect of strontium, and its interaction with titanium-boride grain refiner, upon pore nucleation and growth using an experimental technique that allows the in situ observation of pore formation. The technique is a combination of a traditional temperature gradient stage¹ and real time micro-focus radiography, termed an x-ray temperature gradient stage or XTGS.

The addition of strontium was found to depress the eutectic temperature causing an increased total time for pore growth; however, the initial growth rate of the pores was also reduced, producing little net change in final size. Strontium additions also altered the nucleation and stability of pores within the semi-solid region. Strontium in combination with titanium-boride grain refiner produced an even stronger change in the nucleation and stability of the pores, allowing pores that floated out of the semi-solid region to remain stable, rather than dissolving back into the liquid aluminium.     

Influence of melt cleanliness on pore formation in aluminium—silicon alloys

Porosity and inclusions are the most serious problems encountered in the production of aluminium castings. The formation of porosity during solidification can be affected by several factors such as melt hydrogen, cooling rate, alloying elements, modification and melt cleanliness. Of all these factors, melt cleanliness (i.e. inclusion concentration) is the most difficult to control and is often considered as a key factor influencing the pore nucleation. In the present paper, the effect of melt cleanliness on porosity formation in two aluminium foundry alloys (A356 and 319) was investigated systematically. Melts with different inclusion levels were prepared and tested by the reduced pressure test, and the melt cleanliness was quantified using a pressure filtration technique. In addition, the influence of filtration by ceramic foam filters on the porosity was studied.

Results indicate that, at the same hydrogen level and with increased inclusion concentration, the density of reduced pressure samples decreases, and the amount of porosity and the number of pores increase. This trend is confirmed in both 356 and 319 alloys over a wide range of hydrogen levels. These results are discussed in the context of the nucleating effects of inclusions on pore formation during solidification. The practical application of the reduced pressure test for evaluation of both the melt cleanliness and the dissolved hydrogen level is outlined.     

半固态处理中球晶形成与演化的直接观察

采用丁二腈水透明模型合金 ,通过实时观察技术对半固态处理过程中的组织形成及演化机理进行了研究。结果表明 ,球晶是由液相形核长大产生的 ,而非由传统形成机制———断裂枝晶钝化控制 ;并且半固态处理过程中剪切速率显著影响球晶的形成和分布。随着搅动速率提高 ,球化率逐步降低 ,当搅动速率足够高时 ,无球化发生 ;同时 ,球化率对搅动速率的突变非常敏感 ,搅动速率的突变容易造成球晶的大量形核生长。     

A comprehensive model of ordered porosity formation

The model presented in this paper considers heat transfer, gas diffusion and simultaneous growth of solid and gas fractions in an ordered porosity ingot. Special attention is paid to the gas pore nucleation. The diffusion boundary layer ahead of the solid/liquid interface is analysed with respect to the diffusion process and structure formation, both controlled by external gas pressure, solidification velocity and gas fluxes between solid, liquid and pores. The relationships between processing parameters and structure characteristics are described. How the porosity structure is affected by the size of gas nuclei and the number of nucleation sites on the solid/melt interface is analysed.     

Simulation of the three-dimensional morphology of solidification porosity in an aluminium–silicon alloy

A novel extension of the cellular automata technique for microstructural modelling is presented, allowing simulation of the evolution of the complex three-dimensional morphology of porosity during the solidification of an aluminium–silicon alloy. The complex morphology arises due to the restriction of the growth of the pores by the developing solid phase. The model predicts the average properties of the porosity formed, together with the distribution in size and morphology.The model is used to determine the influence of a variety of applied conditions (e.g. thermal history, pressure, hydrogen content) and material properties (nucleation behaviour, alloy composition) upon the pore morphology, as characterized by the average and extreme dimensions. The relative magnitude of the effect of each parameter and the interactions between parameters upon the porosity are statistically analysed. The simulated pore size shows the largest sensitivity to applied pressure, hydrogen content and solidification time, together with interactions between solidification time and pressure. These results are in good agreement with previously reported experimental behaviour.     

Two-dimensional cellular automaton model for simulating structural evolution of binary alloys during solidification

1Introduction During solidification of cast metallic materials,simulations of microstructural evolution,which track kinetics in a local fashion,are of interest for two reasons.First,from a fundamental point of view,it is desirable to better understand the…     

Numerical modeling of solidification morphologies and segregation patterns in cast dendritic alloys

A comprehensive stochastic model for simulating the evolution of dendritic crystals during the solidification of binary alloys was developed. The model includes time-dependent calculations for temperature distribution, solute redistribution in the liquid and solid phases, curvature, and growth anisotropy without further assumptions on the nucleation and growth of dendritic crystals. Stochastic procedures previously developed by Nastac and Stefanescu (Modell. Simul. Mater. Sci. Engng, 1997, 5(4), 391) for simulating dendritic grains were used to control the nucleation and growth of dendrites. A numerical algorithm based on an Eulerian–Lagrangian approach was developed to explicitly track the sharp solid/liquid (S/L) interface on a fixed Cartesian grid. Two-dimensional mesoscopic calculations (i.e. at the dendrite tip length scale) were performed to simulate the evolution of columnar and equiaxed dendritic morphologies including the formation of the columnar-to-equiaxed transition. The effects of solutal and curvature undercoolings on the evolution of both the dendrite morphology and segregation patterns during the solidification of binary alloys were analyzed in detail.     

Al—Cu合金等轴枝晶组织形成的模拟及计算机可视化

通过解用热焓法处理结晶潜热的二维不稳定热传导方程模拟凝固温度场,利用连续形核模型和枝晶尖端生长的动力学模型,即KGT模型模拟Al-4%Cu合金等轴晶的形成,将凝固温度场的模拟和晶粒组织形成的模拟相耦合,计算了该合金的凝固等轴晶组织特征值,包括晶粒密度和平均晶粒尺寸,并在计算机上实现了晶粒形成过程的可视化;与实验结果进行比较表明,模拟结果与实验结果基本吻合。     

Solidification behaviour of AZ91D alloy under intensive forced convection in the RDC process

Rheo-diecasting (RDC) is a new semisolid processing technology for production of near net shape components. In this work, the solidification behaviour of AZ91D alloy under intensive forced convection in the RDC process was investigated experimentally to understand the effects of the intensity of forced convection, shearing time and shearing temperature on the nucleation and growth behaviour. It was found that under intensive forced convection, heterogeneous nucleation occurred continuously throughout the entire volume of the solidifying melt. All the nuclei could survive due to the uniform temperature and composition fields created by the forced convection. This has been named as continuous effective nucleation. It is also found that the nuclei grow spherically with an extremely fast growth rate. This makes the primary solidification essentially a coarsening process, in which Ostwald ripening takes place by dissolution of the smaller particles. Secondary solidification of the intensively sheared semisolid slurry takes place also through effective nucleation, but with dendritic growth. Increasing the intensity of forced convection enhances nucleation and promotes the formation of the primary phase during the secondary solidification in the shot sleeve. The final solidification microstructure is strongly dependent on the presence of turbulence rather than the shear rate.     

金属凝固微观组织形成的数值模拟

依据金属凝固的基本理论,在Oldfield的连续形核和晶粒长大模型基础上,引入了时间因子,运用随机方法,建立了连续冷却条件下形核和生长的随机模型,使形核和生长模型函数与时间直接相关,反映了连续冷却条件下的金属凝固微观组织的演化过程。基于上述模型,编制了计算模拟程序,模拟了Zn-5Al共晶合金微观组织的凝固过程,并用试验结果对模拟结果进行了验证对比,模拟结果与试验结果基本相符。     

藕状多孔金属Mg的Gasar工艺制备

金属／气体共晶定向凝固(Gasar)是一种制备规则多孔金属的新工艺．本文利用自行开发的Gasar装置，成功制备了具有规则气孔分布的藕状多孔金属Mg，并研究了气体压力对气泡形核、气孔率、气孔大小和分布的影响。     

Gasar工艺下藕状多孔银的气孔结构研究

金属-气体共晶定向凝固（Gasar)是制备藕状多孔金属的新工艺,利用自行研制的Gasar装置,成功地制备了不同纯氧分压下的藕状多孔银试样,研究了氧气分压对藕状多孔银气孔形貌（气孔率、气孔尺寸和分布、气泡形核）的影响。结果表明：氧气分压对气孔形貌影响十分显著。随着氧气压力的增加,气孔率增大而平均气孔直径减小。