Viscosity of superalloy 718 by the oscillating vessel technique

Abstract

Superalloy 718 is a very widely used material with good mechanical properties at application temperatures exceeding those suitable for stainless steels. Investment casting suppliers are developing new casting techniques to enable the production of cleaner melts, thinner-walled, more complex components, and minimum costs. A key tool being used by foundry process engineers is computer simulation of the casting process. Computer simulations enable engineers to understand details of the heat and fluid flows that dominate microstructural development and the casting`s subsequent mechanical properties. Although measurements of the viscosities of molten metals have been reported using several techniques, the dominant technique at moderate to high temperatures is the oscillating vessel (or cup) technique. In this method, a molten metal is contained within a ceramic vessel suspended by a torsional pendulum. Torsional oscillations are then induced and the resulting motion is damped primarily by viscous dissipation within the molten metal under investigation. The logarithmic decrement of the amplitude of torsional oscillation required to calculate the viscosity must be due solely to the viscous damping of the molten metal. However, small amounts of additional damping arise due to internal friction in the suspension wire and viscous damping due to the cover gas. These extraneousmore » contributions are determined for each sample/crucible assembly by oscillation experiments after the sample freezes, and then simply subtracted from the decrements measured with the molten sample to yield the decrement solely due to the molten metal`s viscosity. The viscosity of the molten metal can then be determined by measuring the time period and decay of the oscillations.« less