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

在金属中，特别是在铝合金中，由于其重要的凝固收缩，以及当它们处于液态时很容易从水分中捕获氢的事实，一定数量的气孔的存在是不可避免的。在凝固过程中，溶液中的氢被释放出来，在微观组织中形成空洞。这些缺陷在铸造产品中很常见，但在锻造产品中经常被忽略，因为人们假设在轧制或挤压后，气孔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，一些研究人员报道了氢微孔在导致轧件失效的破坏过程中的重要作用。结果表明，轧制和热处理后仍然存在的氢微孔是导致试件完全断裂的微裂纹的有效形核场所。本研究的最终目的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，只有很少的固相存在氢溶度数据，特别是在铝合金的凝固区间，在那里会发生氢的排斥并产生气孔。作为第一步，我们想要测量合金在凝固过程中的氢溶解度，因为所有以前的测量都是在合金100%液态或100%固态的温度下进行的。这些测量将补充和改进文献中现有的数据。将研究物种和浓度对体系溶解度的影响，并将建立热力学模型来了解氢的溶解度的演变。作为第二步，将建立包括气相的凝固模型。与气相形核和氢在原铝固相中的反向扩散有关的动力学方面将被模拟。这项工作是为了更好地理解氢微孔和微裂纹的形成，以防止热撕裂的开始。执行固相线以上蠕变试验的目的是将蠕变性能，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.