Densification and anticorrosion performances of vacuum evaporated aluminium coatings on NdFeB magnets

Al coated NdFeB magnets obtained by vacuum evaporation technique were densified by high energy ball milling method.The surface morphology,metal composition and micro structure of the coatings were characterized by scanning electron microscopy,X-ray diffraction and X-ray photoelectron spectroscopy,respectively.The anticorrosive properties were investigated by potentiodynamic polarization curves and neutral salt spray test.The pores in the Al coatings of columnar crystals(Al) induced by the evaporation technique,were apparently filled in the following ball milling process,leading to the densification of Al coatings and the evident improvement of the anticorrosive performances.When treated with ball milling for 30 min,the sample achieves the best anticorrosive performances with the self-corrosion potential of-0.87 V,self-corrosion current density of 1.65 μA/cm~2 and the neutral salt spray(NSS) time of 144 h(red rust).The improvement of the anticorro sive performances of vacuum evaporated Al coating mainly lies in the densification effect of the coating,which depends on different loading conditions of ball milling process.     

Effect of substituting Na2O for SiO2 on the non-isothermal crystallization behavior of CaO-BaO-Al2O3 based mold fluxes for casting high Al steels

The non-isothermal crystallization behaviors of CaO-BaO-Al₂O₃-based mold fluxes in which SiO₂ had been replaced by Na₂O were investigated, using an infrared furnace combined with digital microscopy as well as X-ray diffraction. The initial crystallization temperatures, crystalline phases generated at different cooling rates, crystallization rates, Ozawa indices N_O (as determined using Mo's equation) and effective crystallization activation energies (as determined using the Friedman equation) were evaluated. The initial crystallization temperature increased along with the Na₂O content in the flux. In addition, the precipitation of CaF₂ crystals was inhibited while the growth of CaAl₂O₄ crystals was enhanced at Na₂O concentrations over 4?wt%. Increasing the cooling rate inhibited the precipitation of MgAl₆O₁₀, and of CaSiO₃, MgAl₂SiO₆ and NaAlSi₃O₈, respectively, in fluxes containing 0, 4 and 8?wt% Na₂O (NS1, NS2 and NS3 samples). The peak instantaneous and average crystallization rates in these specimens were increased as the Na₂O level was raised, as a direct result of changes in their N_O values. The NS1 flux consistently had the lowest N_O value, while the NS2 had the largest value in the primary stage of crystallization but the central value in the secondary stage, and the N_O values of the NS3 flux exhibited the opposite trend. The effective crystallization activation energies decreased with increasing Na₂O levels, and this was evidently an important factor affecting the initial crystallization temperature.     

Rapid degradation of metamitron and highly complex mixture of pollutants using MIL-53(Al) integrated combustion synthesized TiO2

In the present work, we report a new approach for the synthesis of TiO₂-MIL-53(Al) (MIL: Matériaux de l′Institut Lavoisier) nano-composite. The integration of microwave synthesized metal organic framework (MOF) with combustion synthesized metal oxide is reported for the first time. The synthesized materials were characterized using an X-ray diffractometer, Fourier transform infrared spectrometer, scanning electron microscope, energy dispersive X-ray spectroscope, energy dispersive X-ray fluorescence, porosimeter, and UV–visible diffuse reflectance spectroscope. The diffraction pattern of the composite indicated the presence of TiO₂ as well as MIL-53(Al). Furthermore, the presence of anatase–rutile mixed phase of TiO₂ in the synthesized material was observed. The Fourier transform infrared spectrum was in support of XRD observations. A significantly low surface area and porosity of the composite material, compared to its parent components, was observed. The surface micrograph image of the composite depicted its nano-sheet type morphology. The energy dispersive X-ray analysis confirmed the presence of Ti, Al, C and O in the composite material. The composite material exhibited a narrow band-gap compared to the pristine MIL and TiO₂.As an application, the photocatalytic activity of synthesized material was determined by the degradation of metamitron. The application part is also one of the novelties of the present work. The effect of catalyst loading, initial concentration of metamitron and presence of inorganic ions on the photocatalytic degradation was investigated. The reusability of the composite material was evaluated. A plausible mechanism of photocatalytic degradation is proposed based on radical scavenging experiments. To the best of our knowledge, to date, there is no study on the application of TiO₂-MOF composite for the photocatalytic degradation of herbicide. Further, the synthesized material was successfully used for the photocatalytic degradation of a complex mixture of pollutants comprising metamitron, methylene blue, rhodamine B and 2, 4-dichlorophenol. This is a first of its kind study to demonstrate the application of TiO₂-MOF composite for photocatalytic degradation of a highly complex mixed pollutant.     

A new synthetic metamodel methodology for liquid‐propellant engine's cooling system optimization

The present paper strives for optimization of the cooling system of a liquid‐propellant engine (LPE). To this end, the new synthetic metamodel methodology utilizing the design of experiment method and the response surface method was developed and implemented as two effective means of designing, analyzing, and optimizing. The input variables, constraints, objective functions, and their surfaces were identified. Hence, the design and development strategy of combustion chamber and nozzle was clarified, and 64 different experiments were carried out on the RD‐161 propulsion system, of which 47 experiments were approved and compatible with the problem constraints. This engine used all three modes of cooling: the radiation cooling, the regenerative cooling, and the film cooling. The response surface curves were drawn and the related objective function equations were obtained. The analysis of variance results indicate that the developed synthetic model is capable to predict the responses adequately within the limits of input parameters. The three‐dimensional response surface curves and contour plots have been developed to find out the combined effects of input parameters on responses. In addition, the precision of the models was assessed and the output was interpreted and analyzed, which showed high accuracy. Therefore, the desirability function analysis has been applied to LPE's cooling system for multiobjective optimization to maximize the total heat transfer and minimize the cooling system pressure loss simultaneously. Finally, confirmatory tests have been conducted with the optimum parametric conditions to validate the optimization techniques. In conclusion, this methodology optimizes the LPE's cooling system, a 2% increase in the total heat transfer, and a 38% decrease in the pressure loss of the cooling system. These values are considerably large for the LPE design.     

Solidification microstructure-dependent hydrogen generation behavior of Al–Sn and Al–Fe alloys in alkaline medium

This work deals with the development of quantitative correlations of hydrogen evolution performance with solidification microstructural and thermal parameters in Al–1Sn, Al–2Sn, Al–1Fe, and Al-1.5Fe [wt.%] alloys. The cellular growth as a function of growth and cooling rates is evaluated using power type experimental laws, which allow determining representative intervals of microstructure length scale for comparison purposes with the results of immersion tests in 5 wt%NaOH solution. For both Al alloys systems, hydrogen evolution becomes slower as the alloy solute content increased. However, for a given alloy composition, whereas a more homogeneous distribution of Sn-rich particles promotes faster hydrogen generation using Al–Sn alloys, coarsening of Al₆Fe IMCs (intermetallic compounds) fibers favors hydrogen production using Al–Fe alloys. When solidification conditions that result in a range of cellular spacings within 16 and 19 μm are considered, the specific hydrogen production of the Al-1wt.%Fe alloy is higher than that of the other studied alloys.     

Optimisation of the spark plasma sintering process for high volume fraction SiCp/Al composites by orthogonal experimental design

Based on orthogonal experimental design (OED), the effects of the sintering pressure, sintering temperature and holding time on the mechanical properties of 50 vol% silicon carbide particle (SiCp)/2024Al composites prepared by spark plasma sintering (SPS) were investigated. The sintering pressure had the greatest effect on the density and bending strength of the material among these three factors, followed by sintering temperature and holding time. The optimised process conditions for producing the 50 vol% SiCp/2024Al were sintering at 550 °C for 5 min under 40 MPa, which resulted in a composite material with a density of 99.7% and good interface bonding with a comparatively high bending strength of 766.65 MPa. This work provides a promising method to produce high volume fraction composites that can meet high strength requirements.     

Interconnected SiC–Si network reinforced Al–20Si composites fabricated by high pressure solidification

The microstructures and mechanical properties of the interconnected SiC–Si network reinforced Al–20Si composites solidified under high pressures were investigated. The results demonstrate that the complete interconnected SiC–Si network can be obtained by high pressure solidification, and the connected micron-sized pores are uniformly distributed in the interconnected SiC–Si network. The compressive strength and microhardness of the SiC/Al–20Si composites solidified under 3 GPa were 723 MPa and 229 HV_0.05, respectively. Furthermore, the fracture process of SiC/Al–20Si composites was studied by in situ TEM tensile testing. The result shows that the crack first initiated and propagated at the Al/Si interface under an external load, and the SiC particles in the interconnected SiC–Si network can effectively hinder the crack propagation, thus enhancing the strength.     

First-principles investigation on stability and electronic structure of Sc-doped θ′/Al interface in Al−Cu alloys

The properties of Sc-doped θ′ (Al₂Cu)/Al interface in Al−Cu alloys were investigated by first-principles calculations. Sc-doped semi-coherent and coherent θ′ (Al₂Cu)/Al interfaces (Sc doped in Al slab (S1 site), Sc doped in θ′ slab (S2 site)) were modeled based on calculated results and reported experiments. Through the analysis of interfacial bonding strength, it is revealed that the doping of Sc at S1 site can significantly decrease the interface energy and increase the work of adhesion. In particular, the doped coherent interface with Sc at S1 site which is occupied by interstitial Cu atoms has very good bonding strength. The electronic structure shows the strong Al—Cu bonds at the interfaces with Sc at S1 site, and the Al—Al bonds at the interfaces with Sc at S2 site are formed. The formation of strong Al—Cu and Al—Al bonds plays an important role in the enhancement of doped interface strength.     

多级深度还原法制备Ti6Al4V合金粉体的基础研究

目前Ti6Al4V合金粉体的生产方法主要有雾化法、机械合金法和氢化脱氢法,它们都以Kroll法生产海绵钛为基础。使用钛的氧化物作为原料的熔盐电解法和金属热还原法仍处于研究阶段。本文依据变价金属Ti和V的氧化物在还原过程中逐级还原的特性,提出使用金属氧化物(TiO₂, V₂O₅)作为原料的多级深度还原法制备Ti6Al4V合金粉体的新思路。首先计算了TiO₂-V₂O₅-Mg-Ca体系的吉布斯自由能变,结果表明先进行镁热自蔓延反应,后进行钙热深度还原反应制备Ti6Al4V合金粉体在热力学上具备可行性。然后通过实验进行了的验证。镁热自蔓延反应产物酸浸后除去MgO可得到氧含量为15.84%的多孔Ti-Al-V-O前驱体。配入金属Ca后进行深度脱氧可得到低氧Ti6Al4V合金粉体(Al: 6.2wt.%, V: 3.64wt.%, O: 0.24wt.%, Mg: 0.01wt.%, Ca: < 0.01wt.%)。     

Three-dimensional visualization and quantification of microporosity in aluminum castings by X-ray micro-computed tomography

Porosity is a major issue in solidification processing of metallic materials.In this work,wedge die casting experiments were designed to investigate the effect of cooling rate on microporosity in an aluminum alloy A356.Microstructure information including dendrites and porosity were measured and observed by optical microscopy and X-ray micro-computed tomography(XMCT).The effects of cooling rate on secondary dendrite arm spacing(SDAS)and porosity were discussed.The relationship between SDAS and cooling rate was established and validated using a mathematical model.Three-dimensional(3-D)porosity information,including porosity percentage,pore volume,and pore number,was determined by XMCT.With the cooling rate decreasing from a lower to a higher position of the wedge die,the observed pore number decreases,the porosity percentage increases,and the equivalent pore radius increases.Sphericity of the pores was discussed as an empirical criterion to distinguish the types of porosity.For different cooling rates,the larger the equivalent pore radius is,the lower the sphericity of the pores.This research suggests that XMCT is a useful tool to provide critical 3-D porosity information for integrated computational materials engineering(ICME)design and process optimization of solidification products.