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

金属中存在一定数量的孔隙是不可避免的，特别是在铝合金中，因为它们具有重要的凝固收缩作用，而且当它们处于液态时，很容易从湿气中捕获氢。溶液中的氢在凝固过程中释放出来，在微观结构中形成空洞。这些缺陷在铸造产品中很常见，但在锻造产品中却经常被忽略，因为人们认为，在轧制或挤压后，气孔的数量和尺寸都大大减少了。随着三维原位成像技术的发展，一些研究人员报道了氢微孔在轧制产品破坏过程中导致失效的重要作用。结果表明，在轧制和热处理后，氢微孔仍然存在，是微裂纹的有效形核点，导致试样完全断裂。本研究的最终目标是找到新的解决方案，以减少氢在铝合金中的负面影响。问题是氢溶解度数据存在于很少的固相中，特别是在铝合金的凝固阶段，在那里氢发生排斥并产生孔隙。作为第一步，我们想要在凝固间隔内测量合金中的氢溶解度，因为之前所有的测量都是在合金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。研究了种类和浓度对体系溶解度的影响，并建立了热力学模型来理解氢溶解度的演变。作为第二步，包括气相在内的凝固模型将被开发。与气相成核有关的动力学方面将被建模，以及氢在原铝固相中的反扩散。这项工作将更好地了解氢微孔和微裂纹的形成，从而防止热撕裂的发生。在固相上进行蠕变试验，目的是将蠕变特性，特别是失效时的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.