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

在金属中，特别是在铝合金中，存在一定量的孔隙是不可避免的，这是因为它们的重要凝固收缩以及它们在液态时容易从水分中捕获氢的事实。溶液中的氢在凝固过程中释放，在微观结构中形成空腔。这些缺陷在铸造产品中很常见，但在锻造产品中经常被忽略，因为人们认为轧制或挤压后孔隙的数量和尺寸都会大大减少。随着三维原位成像技术的发展，一些研究者报道了氢微孔在导致轧制产品失效的损伤过程中的重要作用。他们表明，氢微孔，仍然存在轧制和热处理后，作为有效的微裂纹的成核点，导致试样的完全断裂。本研究的最终目标是找到新的解决方案，以减少铝合金中氢的负面影响。问题在于，氢溶解度数据存在于非常少的固相中，特别是在铝合金的凝固区间中，在该凝固区间中发生氢的排斥并产生孔隙。作为第一步，我们想要测量凝固区间中合金中的氢溶解度，因为所有先前的测量都是在合金为100%液体或100%固体的温度下进行的。这些测量将补充和改进文献中现有的数据。物种和浓度对系统的溶解度的影响将被调查和热力学模型将被开发，以了解氢溶解度的演变。作为第二步，将开发包括气相的固化模型。与气相成核相关的动力学方面将被建模，以及氢在原铝固相中的反向扩散。这项工作将更好地了解氢微孔和微裂纹的形成，防止热裂的发生。将在固相线以上进行蠕变试验，目的是将蠕变性能（尤其是失效应变）与合金中的氢含量联系起来。

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.