Bending Strength of Salt Core Comprised of KCl-NaCl-Na₂CO₃-K₂CO₃ Systems

Abstract

  Expendable cores with high strength and removability are required to manufacture undercut shaped products by high pressure die casting. This paper highlights bending strength of a water soluble salt core, which is one of the most attractive candidates for the expendable core. In this paper, the strength of salt cores comprised of NaCl-KCl-Na₂CO₃-K₂CO₃ multi component system were investigated by considering thermodynamics of the salt mixture and experimentally by the four point bending test. Phase diagrams and thermodynamic functions calculated from the thermodynamic data of the multi component system show that there are four composition areas where high strength is expected. A strength map of the multi component system determined by the bending test shows that high strength over 20MPa was obtained in three composition areas. This experimental result is in good agreement with theoretical prediction by thermodynamic data. Salt cores made of such high strength salt mixture should be usable for high pressure die casting. Also, the liquidus temperature of some compositions in these areas was between 873K and 973K, and was low enough to fabricate salt cores from molten salt. In another area, high strength over 15MPa was obtained but composition with high strength was very limited. SEM-EDX analysis shows that sodium content in the primary chloride phase is different among specimens. The strength of the primary phase also depends on the sodium content. Sodium content therefore plays a very important role in the control of the strength of specimens. In the decomposition region of the chloride phase, strength was found to be relatively low if the primary phase was chloride.