Hydrogen In Aluminum Research Articles

Impeller design assisted by physical modeling and pilot plant trials

A physical model of a batch aluminum degassing reactor equipped with the rotor-injector technique was used to measure deoxidation kinetics of water, assuming that this kinetics is similar to dehydrogenization of aluminum. Performances of three different impeller designs were tested with the model, two of them available commercially, while the third one is a design proposed in this work, which shows a better performance than the two commercial designs reducing the degassing time between 14% and 34%, the gas consumption between 14% and 32%, and an increment in gas efficiency between 22% and 49% compared with the commercial designs. Performance of the impellers in aluminum was tested in a pilot degassing unit, and again, the impeller design proposed showed a better performance by reducing the amount of hydrogen in liquid aluminum after 10min of degassing 1/2 respect to the commercial design A and 2/3 respect to the design B.

РАЗРАБОТКА МЕТОДИКИ ОПРЕДЕЛЕНИЯ МАССОВОЙ ДОЛИ ПОВЕРХНОСТНОГО И РАСТВОРЕННОГО ВОДОРОДА В АЛЮМИНИИ И НИЗКОЛЕГИРОВАННЫХ АЛЮМИНИЕВЫХ СПЛАВАХ МЕТОДОМ НАГРЕВА В ТОКЕ ИНЕРТНОГО ГАЗА АРГОНА

РАЗРАБОТКА МЕТОДИКИ ОПРЕДЕЛЕНИЯ МАССОВОЙ ДОЛИ ПОВЕРХНОСТНОГО И РАСТВОРЕННОГО ВОДОРОДА В АЛЮМИНИИ И НИЗКОЛЕГИРОВАННЫХ АЛЮМИНИЕВЫХ СПЛАВАХ МЕТОДОМ НАГРЕВА В ТОКЕ ИНЕРТНОГО ГАЗА АРГОНА

Hydrogen diffusion in liquid aluminum fromab initiomolecular dynamics

Ab initio molecular dynamics simulations are used to describe the diffusion of hydrogen in liquid aluminum at different temperatures. Quasi-instantaneous jumps separating periods of localized vibrations around a mean position are found to characterize the hydrogen motion at the microscopic scale. The hydrogen motion is furthermore analyzed using the van Hove function. We highlight a non-Fickian behavior for the hydrogen diffusion due to a large spatial distribution of hydrogen jumps. We show that a generalized continuous time random walk (CTRW) model describes the experimental diffusion coefficients in a satisfactory manner. Finally, the impact of impurities and alloying elements on hydrogen diffusion in aluminum is discussed.

Ab initio based understanding of diffusion mechanisms of hydrogen in liquid aluminum

Ab initio molecular dynamics simulations are used to describe the diffusion of hydrogen in liquid aluminum at different temperatures. We show that the hydrogen motion does not follow a Brownian motion caused by a broad distribution of spatial jumps that can exceed 15 times the interatomic AlH distance. This breakdown is also evidenced in the calculation of the self-part of the van Hove distribution function that is not the Gaussian expected for a Fickian process. We show that the hydrogen motion can be described well by a generalized continuous time random walk model leading to computed self-diffusion coefficients of H in liquid aluminum in good agreement with experimental ones. Finally, the impact of impurities and alloying elements is discussed.

States of hydrogen and deuterium in chemically charged high purity aluminum

In the present study, the deuterium distribution through the specimen thickness during charging and aging at various temperatures was characterized using the secondary ion mass spectrometry (SIMS) method. The advantages of this method are that the actual concentration–depth profiles are obtained from hydrogen (H) introduced during the growth, storage and melting environments (in single crystal or polycrystalline) of aluminum. Separately, the deuterium (D) concentration–depth profiles introduced from chemical processes or interactions with water during the experimental test processes, obtained by charging was characterized. Analysis of the pores, voids and bubbles related to hydrogen and deuterium was performed by small and wide X-ray scattering (SAXS and WAXS) and by high magnification transmission and scanning electron microscopy (TEM and SEM). Large (some micrometers in diameter) to very small (nanometers in diameter) sizes in the distribution and variability in the density of the voids/bubbles were found. The internal and surface bubbles and dense distributions of voids near dislocations related to the hydrogen distribution were studied. Moreover, the hydrogen–vacancy interactions related to microstructure changes must be taken into account in the process of characterizing the state of hydrogen in aluminum. An interpretation of the two dimensional anisotropic iso-intensities obtained by SAXS for octants voids was coded to obtain iso-intensity in the [1 1 0] plane in reciprocal space.

The True Origin of Ductile Fracture in Aluminum Alloys

It has generally been assumed that metals usually fail as a result of microvoid nucleation induced by particle fracture. Here, we concentrate on high-density micropores filled with hydrogen in aluminum, existence of which has been largely overlooked until quite recently. These micropores exhibit premature growth under external loading, thereby inducing ductile fracture, whereas the particle fracture mechanism operates only incidentally. Conclusive evidence of a micropore mechanism is provided by the observation of an instantaneous release of gas at failure. We can therefore conclude that the growth of micropores dominates ductile fracture. Since the material we used has a standard pore density, we can assume that an identical fracture mechanism operates in other aluminum alloys. This finding suggests that intense heat treatment, which is generally believed to enhance the mechanical properties through homogenization, may have entirely the opposite effect. This revelation will have a major impact on the engineering design of metals.

Hydrogen in aluminum

Hydrogen in aluminum

Review of the Quantitative and Rapid Measurement Technology of Hydrogen in Aluminum and Magnesium Alloy Melt

Pinhole and micro-shrinkage are the ordinary defects founded in the aluminum and magnesium alloy castings,the existence of hydrogen is the main reason for these defaults. So the measurement of hydrogen in aluminum and magnesium alloy melt is an important means to test the degassing effect and to ensure the quality of the melt,and it is significant to improve the casting's quality.With the increasing application of aluminum and magnesium alloy,the development of quantitative and rapid measurement of hydrogen in aluminum and magnesium alloy melt gets more attractive to the metallurgists and foundrymen. The principles,instruments and features of several popular measurement techniques,such as inert gas circulation method,electrochemical method,pressure evaluation method based on Sieviet's law and the method based on acoustic emission,are analyzed and evaluate respectly. It is indicated that those measurement techniques taking high testing speed and low testing cost as main characteristics have a great prospect.

Solid State Electrochemical Sensors in Metal Processing

Measurement and control of impurity elements in the processing and refining of metals is critical in order to achieve optimum properties of the metals concerned. Solid electrolyte sensors for monitoring oxygen are well established and have made a major impact in the metal industry. This paper presents a review of developments in measuring elements other than oxygen with reference to real industrial trials and applications. Recently, a new sensor for the determination of dissolved hydrogen in molten aluminium has been developed and successfully validated in industry. This sensor relies on a proton conducting perovskite as the solid electrolyte and a sealed solid mixture of either titanium and hydrogen or zirconium and hydrogen as the reference electrode. Furthermore, a new sulphur sensor for molten iron has been advanced and tested. This sensor utilises strontium β-alumina as the solid electrolyte, which has been optimised for thermal shock resistance and toughness by the incorporation of partially stabilised zirconia. An auxiliary layer of strontium sulphide forming in situ during sensor operation serves as the sensing electrode, and a sealed mixture of molybdenum and molybdenum sulphide is used as the reference electrode. The aim of these developments is to optimise and automate process control in order to minimise the amount of out-of-specification products.

Solid state electrochemical hydrogen sensor for aluminium and aluminium alloy melts

The presence of dissolved hydrogen in molten aluminium and its alloys has a critical impact on the quality of cast aluminium components. The quantitative analysis of hydrogen in these melts is therefore of major importance in the aluminium industry. Research work conducted at the University of Cambridge and subsequent development work performed in conjunction with an industrial partner, have resulted in a novel, and now commercialised, electrochemical hydrogen sensor for aluminium melts. The sensor operates in the potentiostatic mode and relies on a proton-conducting solid electrolyte and a metal/hydrogen-based solid reference electrode. This article summarises the main steps of the underlying research and development programme, covering the actual gas sensor, test measurements in gas phases under laboratory conditions, the probe for molten metal application, and test measurements in aluminium melts under industrial conditions.

Hydrogen diffusion in aluminum melts: An ab initio molecular dynamics study

The diffusion process of hydrogen in aluminum melts was investigated by molecular dynamics simulation. The pair correlation function, first peak position, and coordination number was calculated and differences in the structural properties among Al-H, Cl-H, and Al-Cl pair were examined. The mechanism of chlorine on improving hydrogen diffusion was discussed. From an ab initio molecular dynamics calculations, the diffusivity of hydrogen in liquid aluminum as D(T)=(0.118×10−4m2/s)exp(−0.316 eV/kT) is obtained, which is in good agreement with the experimental data. Correspondingly the diffusivity with presence of chlorine is promoted as D(T)=(0.09×10−4m2/s)exp(−0.251 eV/kT). It can be concluded that the diffusion of hydrogen in aluminum melts can be enhanced in the presence of chlorine.

Investigation of Inner Vacuum Sucking method for degassing of molten aluminum

Hydrogen is a harmful gas element that is appreciably soluble in aluminum and its alloys. Removal of hydrogen from molten aluminum has been one of the most important tasks in aluminum melt processing. In this paper, a patented degassing process, which is based on principle of vacuum metallurgy, is proposed. A porous head that connects a vacuum system is immersed in the molten aluminum. The vacuum is created within the porous head and the dissolved hydrogen will diffuse unidirectionally towards the porous head according to Sievert's law. In this way, the hydrogen in the molten aluminum can be removed. The Fick's diffusion equation is used to explain hydrogen transfer in the molten aluminum. RPT experiments are carried out to evaluate the effectiveness of the new degassing process. The experiments indicate that the hydrogen content can be dramatically reduced by use of this process.

A Novel Technology of Hydrogen Measurement Based on Directional Solidification in Molten Aluminum Alloy

Hydrogen is a harmful element in aluminum. How to accurately measure hydrogen content in molten aluminum alloy has been significant for improving the metallurgical qualities of aluminum products. Currently, Reduced Pressure Test (RPT) based on the famous Sievert’ law is the most commonly used method to measure the hydrogen in aluminum. However, due to the changes in solidification morphology of alloys under different solidification conditions, hydrogen will be closed more or less in the sample by the developed dendrites, which results in the dispersion of test results. According to the principle of solute redistribution, an improved reduced pressure technology under directional solidification was proposed in this paper. The solidification interface was pushed on along the single direction by controlling the temperature gradient of solid-liquid interface. Hydrogen is released in front of the solid-liquid interface through the solute redistribution. The result shows that the hydrogen content increases and the mean variance decreases with the increasing of the temperature gradients. The method was proved to be of higher accuracy.

Analysis on Behavior of Hydrogen in Ion-Plated Pure Aluminum

An increasing interest has been being taken in hydrogen as a clean energy for solving the global environmental problems. In order to use the hydrogen in safety, investigation on the hydrogen behavior is required. Although hydrogen microprint technique (HMPT) has been known to be effective to investigate the hydrogen behavior, the low detection efficiency for hydrogen was reported. Ion-plating (IP) was reported to increase the detection efficiency in HMPT emitted from the specimen by plastic deformation. On the other hand, no such increase was found for hydrogen permeating through the specimen ion-plated with substrate heating in the previous study by the authors. In the present study, the sheet samples of pure aluminum with 99.99% purity were dehydrogenated and subjected to (a) holding in the IP chamber, (b) bombardment with Ar ions, (c) substrate heating after the bombardment and (d) holding in air. Hydrogen behavior in these samples has been investigated by means of thermal desorption spectroscopy (TDS). The amount of desorbed hydrogen was evidently larger in the conditions of (a) and (b) than in (d). However, the amount of desorbed hydrogen was decreased by the substrate heating (c) to the same level as in (d).

First Principle Study on Hydrogen Diffusion in Degassing

The effects of chlorine on improving hydrogen diffusion have been studied by first principle calculations and we obtain the diffusivity of hydrogen in liquid aluminum which is in agreement with the experimental data. It can be concluded that the diffusion of hydrogen in aluminum melts can be enhanced in the presence of chlorine and mobility of aluminum is depressed by forming alumina layer which can impede the diffusion of hydrogen.

Effects of Chlorine on Aluminum Refining: A Review and an <i>Ab Initio</i> Molecular Dynamics Study

A comprehensive introduction of current aluminum refining technology is reviewed in this work especially the effects of chlorine on refining process is discussed. The mechanism of chlorine on improving hydrogen diffusion has been studied by ab initio molecular dynamics calculations and we obtain the diffusivity of hydrogen in liquid aluminum which is in agreement with the experimental data. It can be concluded that the diffusion of hydrogen in aluminum melts can be enhanced on the presence of chlorine.

Effect of temperature of humid air on susceptibility of hydrogen embrittlement and initiation criterion of intergranular cracking in 7075-T6 aluminum alloy

The effect of test temperature on the susceptibility to hydrogen embrittlement (HE) of a high-strength aluminum alloy 7075 with T6 (peak-aged) temper was examined using the slow strain rate technique (SSRT) under a strain rate 1.39×10−6 [1/s]in 65% relative humidity air at temperatures between 10°C and 80°C. On the basis of understanding the condition among stress, strain and time leading to intergranular cracking (IGC) and its temperature dependence, the controlling criterion of IGC initiation by HE was studied. The HE susceptibility increased with increasing temperature and the IGC extension (initiation and propagation) was more promoted. Both the unstressed pre-exposure time in 90% relative humidity air at 30°C and the elastic deformation time in SSRT tests had little effect on the embrittlement behavior. The time parameter defining the IGC initiation was recognized as the plastic deformation time ti (p) until reaching the maximum load. The activation energy evaluated from Arrhenius plots of 1/ti (p) was 16.4 [kJ/mol], nearly equal to the reported value for lattice diffusion of hydrogen in a pure aluminum or alloy 7050-T6. It is supported from these results and analysis that a high hydrogen concentration attained at the site near surface by lattice diffusion controls the IGC initiation.

Visualization of Diffusive Hydrogen

In recent years, the use of hydrogen as a clean energy has been paid attention to in terms of the prevention of global warming. Tanks composed solely of steel and cylinders consisting of aluminum liner reinforced with C-FRP in the surrounding are used to store high-pressure hydrogen gas in hydrogen stations and in fuel cell vehicles, respectively. On the other hand, hydrogen embrittlement has been known to occur in some metallic materials under several certain conditions. Also, it has been generally known that the environmental hydrogen that invades the material during services plays major role in hydrogen embrittlement. For this reason, investigations on the behavior of environmental hydrogen in metallic materials are needed. In this study, the behavior of environmental hydrogen in pure aluminum, 6061 aluminum alloy and 7075 aluminum alloy has been investigated by means of tritium autoradiography.

Hydrogen in aluminum during alkaline corrosion

The thermodynamic state of hydrogen in aluminum during alkaline corrosion was investigated, using a two-compartment hydrogen permeation cell with an Al/Pd bilayer membrane. The open-circuit potential of the Pd layer in a pH 7.0 buffer solution was monitored to sense the hydrogen chemical potential, μ H . At pH 12.5–13.5, the measurements established a minimum μ H of 0.55 eV relative to the ideal gas reference, equivalent to a H 2 gas pressure of 5.7 GPa. Statistical mechanics calculations show that vacancy-hydrogen defects are stable in Al at this condition. A dissolution mechanism was proposed in which H at very high μ H is produced by oxidation of interfacial aluminum hydride. The mechanism explains the observed rapid accumulation of H in the metal by extensive formation of vacancy-hydrogen defects.

Degassing, hydrogen and porosity phenomena in A356

The decrease in the solubility of hydrogen in liquid aluminium with temperature has been believed to be a major source of porosity. Therefore, degassing treatment is carried out in foundries to decrease porosity in castings. However, it was shown that it is difficult to nucleate hydrogen porosity in the absence of bifilms where bifilms simply aid the growth of pores. For this purpose, several casting experiments were carried out with commercial A356 alloy. In the first series of testing, the melt was degassed and then upgassed to three different levels. In the second series, the melt was first upgassed and then degassed gradually. In another test, hydrogen level was kept constant and sample collection was carried out. In all the trials, 10 bars were cast into sand mould to produce cylindrical samples for tensile testing. A step mould was used to investigate the porosity distribution and tensile samples were also collected from the same casting. For each casting experiment, a reduced pressure test sample was taken to check metal quality by using bifilm index. It was found that the turbulence and vortex (i.e. increase in bifilm population) during rotary degassing has a more significant effect on mechanical properties and porosity than the hydrogen content.