Bentonite bonded moulding sands: New insights in the interplay of heat and mass transport and mechanical properties

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 sands. 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 strengths of heat pre-treated and cyclically reused sands were related to the kinematics of water within the sand as measured by in-situ neutron radiography and neutron diffraction. Sands subjected to 22 cycles of drying (T = 120◦C) and remoistening (3 wt.%) revealed modifications in smectite lattice spacings and in water release behaviour from smectite interlayers. No significant change of tensile strength, however, was associated with these modifications. Contrarily, sands pre-treated with temperatures as low as 225◦C revealed a reduction of tensile strength which was correlated with irreversible modifications of the structural and compositional integrity of the smectites as manifested by substantially altered water kinematics. The results show that cyclic use of moulding sand has little impact on the sand quality as long as the temperature of the sand remains low. For the practice in foundry, the results imply that a rigorous separation of heat-exposed sand is advantageous. Sands bonded with sodiumand calcium-bentonite were studies in order to unravel the influence of the exchangeable cation on the binding properties. Changes in tensile strength as well as in heat and mass transport within the sand were investigated. Heat pre-treated Caand Na-bentonite bonded sand revealed differences in tensile strength. Although no major differences in the swelling behaviour could be detected, Na-bentonite bonded sands have a better mechanical performance than Ca-bentonite bonded sands both before and after the pre-treatment. The measured changes in tensile strength of both Caand Na-bentonite bonded sand correspond well with the irreversible alterations of the smectites.