Bentonite bonded moulding sands [thesis]

Korbinian Schiebel
Sand moulded casting is still one of the most efficient and cheapest methods to produce metal parts of complex shapes. Due to their cyclic reusability and low emission of particles and organic compounds during casting, bentonite bonded sands have an excellent environmental performance. Moreover, sands discarded after a maximum of cycles can be reused as raw material for in various applications. Wet tensile strength testing is a common method to assess the quality of bentonite bonded moulding
more » ... ds. For wet tensile strength testing, a specimen is first heated from above in order to simulate heat-driven moisture transport induced by the casting process. Then, stress is applied until the specimen ruptures. In the first part of this study, neutron radiography imaging was applied to moulding sands in-situ during heating and wet tensile strength testing in order to investigate the effects of water kinematics on the tensile strength. Neutron radiography allowed the localization of the rupture plane and the quantitative determination of the local water content with sub-mm resolution. Quantification of the temperature of the rupture plane and of the heat kinematics within the specimen was accomplished by temperature measurements both in-situ and ex-situ. In this way, experimental data correlating the wet tensile strength with the specific conditions of moulding sands at the rupture plane were obtained for the first time. Series of experiments with different initial sand moisture contents were conducted. The results show that the weakest location within a sand profile can be pinpointed at the interface between evaporation and condensation zone (i.e., at the 100°C isotherm), where water vaporisation starts and the water bridges connecting the sand grains collapse. During casting, however, the moulding sand undergoes irreversible alterations, which deteriorate its properties. Aim of this work, therefore, was to gain accurate insights into the relation between sand alteration and property change. To this end, the wet tensile [...]
doi:10.5282/edoc.22659 fatcat:eyqzadfqifa6paa53ky5mg2c34