Prediction of solidification defects in relation with hydrogen contents in aluminium alloys

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The presence of a certain amount of porosity is unavoidable in metals, especially in aluminum alloys because of their important solidification shrinkage and the fact that they capture easily hydrogen from moisture when they are in the liquid state. The hydrogen in solution is released during solidification to form cavities in the microstructure. These defects are common in cast products but are frequently ignored in wrought products since it is assumed that porosities are greatly reduced in number and size after rolling or extrusion. With the developments of 3D in-situ imaging techniques, some researchers reported the important role of hydrogen micropores in the damaging process leading to failure in rolled products. They showed that the hydrogen micropores, still present after rolling and heat treating, act as efficient nucleation sites of microcracks leading to the complete fracture of the specimen. The ultimate goal of this research is to find new solutions to reduce the negative impacts of hydrogen in aluminum alloys. The problem is that hydrogen solubility data exists for very few solid phases, especially in the solidification interval of aluminum alloys, where the rejection of hydrogen occurs and generates porosity. As a first step, we want to measure the hydrogen solubility in alloys in the solidification interval since all previous measurements were made at temperature where the alloy was 100% liquid or 100% solid. These measurements will complement and improve the data existing in the literature. The influence of species and concentration on the solubility of the system will be investigated and thermodynamic models will be developed to understand the evolution of hydrogen solubility. As a second step, a solidification model including the gas phase will be developed. The kinetics aspects related to the nucleation of the gas phase will be modeled as well as the back-diffusion of hydrogen in the primary aluminum solid phase. This work will be done to better understand the formation of hydrogen micropores and microcracks precluding the onset of hot tearing. Creep tests performed above solidus will be performed with the objective to relate creep properties, especially strain at failure, with the hydrogen content in the alloy.