Wet tensile strength testing in neutron radiography - cause-property relation studies in bentonite bonded molding sands

膨润土粘接型砂是最常用的铸造材料之一。被普遍接受的一个原因是它的可重用性。但在一定程度上降低了用型砂的粘结质量。因此，每次浇铸后都要添加5%的新模具材料。重复使用型砂的质量损失是由模具材料中粘土矿物的改变引起的，并导致湿抗拉强度降低以及其他影响。质量损失的一个后果是增加了对与水有关的铸造缺陷（如结痂）的敏感性。这些铸造缺陷是由破裂引起的，破裂的原因是，一方面是面向熔体的热区域压应力增加，另一方面是邻近较冷区域湿抗拉强度降低（即所谓的冷凝区，热区域的水分在其中重新凝结）。尽管凝结区对模具材料的物理性质具有重要意义，但在通过模具移动时，凝结区究竟发生了什么仍是未知的。例如，在破裂的情况下，模具材料中最薄弱位置的瞬时温度和水浓度是不够了解的。此外，破裂的位置和时刻对先前的温度和水梯度的依赖是未知的。因此，研究项目的主要目的是定量地调查这些关系。为此，湿拉伸强度测试将在中子射线照相中进行。通过这些实验，可以定量测量破裂位置和破裂时刻的水浓度和温度，以及相应的抗拉强度和先前的温度和水梯度。此外，还可以研究所有这些因素对最重要的工艺模具材料参数（如压实性和渗透性）的依赖关系。在新的定量数据的基础上，可以分析再利用模具材料的哪些性能是由哪些机制和过程引起的。这种对因果关系的科学定量认识远远超过了迄今为止存在的大多数经验知识。因此，这些见解可以从根本上有助于制定有效改善膨润土粘合型砂的再使用性的策略。

Bentonite bonded molding sand is among the most common mold materials used in founding. One reason for the common acceptance is its reusability. However, the binding quality of used molding sand is reduced to some degree. Therefore, 5% of new mold material is added after each cast. The quality loss of the re-used molding sand is caused by alterations of the clay minerals in the mold material and results in a reduced wet tensile strength among other effects. One consequence of the quality loss is the increased susceptibility to water-related casting flaws such as scabbing. These casting flaws are caused by ruptures which originate from the interaction of increased compressive stress in the hot area facing towards the melt on the one hand and reduced wet tensile strength in the neighboring cooler area on the other hand (the so called condensation zone, in which the moisture from the hot area re-condenses).In spite of the high significance of the condensation zone on the physical properties of the mold material it is still unknown what exactly takes place in the condensation zone while moving through the mold. For instance in the context of rupture, momentary temperatures and water concentrations at the weakest position in the mold material are insufficiently known. Also the dependence of the position and moment of rupture on precedent temperature and water gradients is unknown. A primary aim of the research project, therefore, is to investigate these relations quantitatively. For this purpose, wet tensile strength testing will be conducted in-situ in a neutron radiograph. By these experiments, water concentrations and temperature at the position and moment of rupture as well as the according tensile strengths and the precedent temperature and water gradients can be measured quantitatively. Moreover, the dependence of all these factors on the most important technical mold material parameters such as compactibility and permeability can be investigated.On the basis of the new quantitative data it is possible to analyze which properties in re-used mold material are caused by which mechanisms and processes. Such a scientific quantitative understanding of the cause-property relations by far exceeds the mostly empirical knowledge existing so far. The insights, therefore, can essentially contribute to a development of strategies for an effective improvement of the re-usability of bentonite bonded molding sands.