Yan-xiang Li

Influence of solidification mode on pore structure of directionally solidified porous Cu-Mn alloy

By the directional solidification of metal-gas eutectic method (GASAR), porous Cu-Mn alloy with oriented pores was fabricated successfully. The variation of pore structure was studied by experiments. The results show that the pore structure is primarily dependent on the solidification mode (planar, columnar cellular, columnar dendritic, equiaxed dendritic), which is controlled by the solidification process. By numerical simulation, it is noted that along with solidification, the solidification mode of the alloy transforms from cellular to columnar dendritic and finally to equiaxed dendritic. Through increasing melt temperature and mold preheating, the range of equiaxed dendrite could be decreased, which helps to extend the region of oriented pore structure.

Calculation of hydrogen solubility in molten alloys

Abstract A thermodynamic model was developed to calculate the hydrogen solubility in molten alloys based on the hydrogen solubility in constitutional pure liquid metals and their interaction parameters. The calculated results have a good agreement with the documented experimental results. The closer the molten alloy to an ideal liquid is, the more accurate the calculated results are. The compound forming ability and molar mixing heat of the constitutional elements take important roles in influencing the hydrogen solubility in molten alloys.

Three-point bending performance of a new aluminum foam composite structure

Abstract A new composite structure based on aluminum foam sandwich and fiber metal laminate was proposed. A layer of glass fiber was provided at the interface between the metal panel and the aluminum foam core in this composite structure, using adhesive technology to bond the materials together by organic glue in the sequence of metal panel, glass fiber, aluminum foam core, glass fiber and metal panel. The experimental results show that the new composite structure has an improved comprehensive performance compared with the traditional aluminum foam sandwiches. The optimized parameters for the fabrication of the new aluminum foam composite structure with best bending strength were obtained. The epoxy resin and low porosity aluminum foams are preferred, the thickness of aluminum sheets should be at least 1.5 mm, and the type of glass fiber has little effect on the bending strength. The main failure modes of the new composite structures with two types of glues were discussed.

Evolution of porous structure with dealloying corrosion on Gasar Cu–Mn alloy

Abstract The evolution of nanoporous structure with dealloying condition was investigated, thus, the mechanism of porous structure evolution was uncovered. The Gasar Cu–Mn alloy was dealloyed by room and elevated temperature chemical corrosion, low and high current level electrochemical corrosion, four types of porous structures, including uneven corrosion pits, hybrid porous, haystack type and bicontinuous model were prepared by chemically and electrochemically dealloying the porous Cu–34.6%Mn alloy made by the Gasar process. Then, the surface diffusion coefficient ( D S ) and the diffusion frequency ( k D ) of Cu atom, as well as the dissolution frequency ( k E ) of Mn atom were calculated with dealloying condition. The dealloyed morphologies for room temperature chemical corrosion and low current level electrochemical corrosion were similar due to the same D S . While the dealloyed structures changed from bulk hybrid porous structure to bicontinuous porous film with decreasing k D / k E .

Effects of cell wall property on compressive performance of aluminum foams

Abstract The effects of cell wall property on the compressive performance of high porosity, closed-cell aluminum foams prepared by gas injection method were investigated. The research was conducted both experimentally and numerically. Foam specimens prepared from conditioned melt were tested under uniaxial compressive loading condition. The cell wall microstructure and fracture were observed through optical microscope (OM) and scanning electron microscope (SEM), which indicates that the cell wall property is impaired by the defects in cell walls and oxide films on the cell wall surface. Subsequently, finite element (FE) models based on three-dimensional thin shell Kelvin tetrakaidecahedron were developed based on the mechanical properties of the raw material and solid material that are determined by using experimental measurements. The simulation results show that the plateau stress of the nominal stress–strain curve exhibits a linear relationship with the yield strength of the cell wall material. The simulation plateau stress is higher than the experimental data, partly owing to the substitution of solid material for cell wall material in the process of the establishment of FE models.

Effect of melt superheat on structural uniformity of lotus-type porous metals prepared by unidirectional solidification

Abstract Structural uniformity is an important parameter influencing physical and mechanical properties of lotus-type porous metals prepared by directional solidification of metal-gas eutectic (Gasar). The effect of superheat on structural uniformity as well as average porosity, pore morphology of porous metals was studied. The experimental results show that, when the superheat is higher than a critical value (Δ T c ), the bubbling or boiling phenomenon will occur and the gas bubbles will form in the melt and float out of the melt. As a result, the final porosity will decrease. In addition, a higher superheat will simultaneously cause a non-uniform porous structure due to the pores coalescence and bubbling phenomenon. Finally, a theoretical model was developed to predict the critical superheat for the hydrogen to escape from the melt and the corresponding escapement ratio of hydrogen content. Considering the escapement of hydrogen, the predicted porosities are in good agreement with the experimental results.

Pore structure of unidirectional solidified lotus-type porous silicon

Abstract Lotus-type porous silicon with elongated pores was fabricated by unidirectional solidification under pressurized hydrogen. Porosity, pore diameter, and pore length can be adjusted by changing solidification speed and hydrogen pressure. The porosity of the ingot is nearly constant under different solidification speeds, but decreases with the increase of hydrogen pressure. The overall porosities of ingots fabricated at different hydrogen pressures were evaluated through a theoretical model. Findings are in good agreement with experimental values. The average pore diameter and pore length increase simultaneously while the average pore aspect ratio changes slightly with the decreases of solidification speed and hydrogen pressure. The average pore length is raised from 7 to 24 mm and the pore aspect ratio is raised from 8 to 20 respectively with the average pore diameter promoted by about 0.3 mm through improving the superheat degree of the melt from 200 to 300 K.

Oxidation kinetics of oxide film on bubble surface of aluminum foams produced by gas injection foaming process

Abstract In the range of 620–710 °C, air was blown into A356 aluminum alloy melt to produce aluminum foams. In order to study the influence of temperature on the thickness of oxide film on bubble surface, Auger electron spectroscopy (AES) was used. Based on the knowledge of corrosion science and hydrodynamics, two oxidation kinetics models of oxide film on bubble surface were established. The thicknesses of oxide films produced at different temperatures were predicted through those two models. Furthermore, the theoretical values were compared with the experimental values. The results indicate that in the range of 620–710 °C, the theoretical values of the thickness of oxide film predicted by the model including the rising process are higher than the experimental values. While, the theoretical values predicted by the model without the rising process are in good agreement with the experimental values, which shows this model objectively describes the oxidation process of oxide film on bubble surface. This work suggests that the oxidation kinetics of oxide film on bubble surface of aluminum foams produced by gas injection foaming process follows the Arrhenius equation.

Oxide film on bubble surface of aluminum foams produced by gas injection foaming process

Abstract Based on A356 aluminum alloy, aluminum foams were prepared by gas injection foaming process with pure nitrogen, air and some gas mixtures. The oxygen volume fraction of these gas mixtures varied from 0.2% to 8.0%. Optical microscopy, scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) were used to analyze the influence of oxygen content on cell structure, relative density, macro and micro morphology of cell walls, coverage area fraction of oxide film, thickness of oxide film and other aspects. Results indicate that the coverage area fraction of oxide film on bubble surface increases with the increase of oxygen content when the oxygen volume is less than 1.2%. While when the oxygen volume fraction is larger than 1.6%, an oxide film covers the entire bubble surface and aluminum foams with good cell structure can be produced. The thicknesses of oxide films of aluminum foams produced by gas mixtures containing 1.6%–21% oxygen are almost the same. The reasons why the thickness of oxide film nearly does not change with the variation of oxygen content and the amount of oxygen needed to achieve 100% coverage of oxide film are both discussed. In addition, the role of oxide film on bubble surface in foam stability is also analyzed.

Hydrogen diffusion coefficient in liquid metals evaluated by solid–gas eutectic unidirectional solidification

Abstract Based on the solid–gas eutectic unidirectional solidification technique and the principle of unidirectional solidification of single-phase alloy, a new method for evaluating the diffusion coefficient of hydrogen in liquid metals was proposed. Taking Cu–H2 system for example, the influences of argon partial pressure and superheat degree of melt on the diffusion coefficient of hydrogen in liquid metal were studied and the predicted values were similar to each other. The obtained temperature-dependent equation for diffusion coefficient of hydrogen in liquid copper is comparable with experimental data in literature, which validates the effectiveness of this method. The temperature-dependent equations for diffusion coefficient of hydrogen in liquid Mg, Si and Cu–34.6%Mn alloy were also evaluated by this method, along with the values at the melting point of each metal and alloy.