Computer Simulation Of Hydrogen In Aluminum

Hydrogen in aluminum or its alloys, probably well-known as a harmful impurity,induces porosity and limits fatigue life in cast aluminum alloys due to the largesolubility difference of H in liquid and solid aluminum. The main source of hydrogen inaluminum alloys is water vapor from the atmosphere surrounding the melting andholding furnaces where it reacts with molten aluminum forming monatomic hydrogen.Several processes of the interaction between aluminum melt and hydrogen have beenanalyzed theoretically, including the hydrogen absorption process and the hydrogenevolution process.The relationship between hydrogen resistance and hydrogen diffucivity has beendiscussed and some factors which influence hydrogen resisitance have been studied.The speed of hydrogen evolution is only related to the depth of melt bath and hasnothing to do with the superfacial area of the melt. It is found that at certain temperature,the deeper the melt bath, the slower the hydrogen evolution is. So when treated withtraditional techniques, the hydrogen can be hardly removed thoroughly.The hydrogen content in aluminum melts at different temperature was detected.The structure in aluminum melts was investigated by molecular dynamics simulation.The mechanism of hydrogen absorption has been discussed. The interdependencebetween melt structural properties and hydrogen absorption was obtained. The visicosiy,pair correlation function, first peak position, and coordination number was calculatedand differences in the structural properties were examined. It is considered thatarrangement of aluminum atoms produces a certain effect on the solution of hydrogen inAl melts. On one hand, as temperature grows, the distances between neighbor atomsdecrease, as well as the atomic coordination number, enlarging the internal free volumeof Al melts. However, at some certain temperature, the abrupt change of melt structurebrings the sudden fluctuation of free volume inside the melt, so the hydrogen contentchanges abnormally. One the other hand, the increasing atom motion leads to theweakened interaction between atoms. So in a certain temperature range, viscositydecreases with increasing temperature, but at particular temperature, the arrangement ofatoms within the atomic cluster has a sudden change in aluminum melts and aluminumatoms will be transported based on the new structure. The viscosity affects hydrogendiffusion in the melt, so altering of hydrogen content in molten aluminum is externalrepresentation of melt structural change. The effect of chlorine on improving hydrogen diffusion has been discussed. Bystudying the site preference of H in bulk aluminum, the mechanism of H embrittlementand vacancy binding effect has been discussed. The distrubition and concentration ofgas impurities in bulk aluminum has been studied and the diffusion process of hydrogenin aluminum melts was investigated by molecular dynamics simulation.Ab initio MD simulations were performed to study hydrogen diffusion in liquidaluminum at various temperature. The property of the structure and dynamics arepredicted in models of H in liquid aluminum. The coordination number of the system isreduced in presence of chlorine reduces and spontaneously interactions betweenaluminum and hydrogen is also weakened by chlorine. By increasing the atomicinterstices and reducing the atomic collision between Al and H, Hydrogen diffusion inliquid aluminum is accelerated by chlorine. Activation energy barrier of each system isalso obtained from the calculations, and relevant to diffusion, Eais much lower whenchlorine is in Al. Moreover, when oxidation is considered, Stability of oxide layer maybe destroyed by chlorine as the diffusicity of oxygen atoms changes.Diffusion path of hydrogen in solid aluminum and the impact of vacany onhydrogen diffusion are described by ab initio and ab initio molecular dynamics study.From the perspective of the dynamics vacancy binding effect is described specifically.Under normal circumstances, the tetrahedral interstitial in aluminum crystals is the moststable site for hydrogen. Due to vacancy binding effect, any direction that hydrogendiffuses away from the vacancy confronts with a large diffusion energy barrier. Whenthe hydrogen atoms diffuse away from the vacancy, the vacancy binding effect isgreatly weakened and has little influence on the hydrogen atom far away from it.Distortion energy accumulated by vacancies can only be released by trapping hydrogenatoms nearby. This is why it is extremely difficult to remove hydrogen completely atlow temperature. At the same time, the analysis of dynamic process is focused on anissue that the temperature increasing activates the diffusion of the hydrogen yetincreases the vacancy binding effect which weakens the hydrogen diffusion. Foraluminum processing, at low temperature, the most effective means to reduce thehydrogen content is to minimize vacancies and defects in the bulk metals.