P. K. Sung

Estimating densities of liquid transition-metals and Ni-base superalloys

Abstract To estimate the densities of liquid Ni-base superalloys, the densities and temperature coefficients of density (dρ/dT) of the liquid transition-metals, which are used as alloy elements in Ni-base superalloys, were gathered, reviewed, and applied to a simple correlation. The correlation is particularly useful to estimate dρ/dT of many transition-metals for which there are no data available. To demonstrate how the results can be applied, the densities of liquid Ni-rich Ni-Al-Cr-Ta alloys were considered and regressed to yield the liquid density as a function of temperature and composition. As a verification of this approach, the liquid densities of five Ni-base superalloys were also estimated. By including a regression estimate of the molar mixing volume, the estimates of the liquid densities agreed with the measured values to ±2.5%.

Simulations of microporosity in IN718 equiaxed investment castings

A finite element model for simulating dendritic solidification of multicomponent-alloy castings has been enhanced to include the calculation of pressure and redistribution of gas-forming elements during solidification and cooling. The model solves the conservation equations of mass, momentum, energy and alloy components, and the gas-forming elements, hydrogen and nitrogen. By solving the transport of gas solutes and comparing their Sieverts pressure with the local pressure, the model can predict regions of possible formation of intergranular porosity. Calculations were performed on equiaxed Ni-base superalloy (IN718) plate castings. The potential to form microporosity was analyzed with different variables including the mass transfer of hydrogen and nitrogen from the casting to the casting/mold gap, the final grain size, a grain-shape parameter and the thickness of the plate casting. The most important factor was found to be the mass transfer coefficient. The results were also affected by the final grain size and grain-shape parameter.

Estimation of densities and coefficients of thermal expansion of solid Ni-base superalloys

Abstract To calculate the densities of solid Ni-base superalloys as functions of temperature and composition, lattice parameters of 18 alloys at 20°C and coefficients of thermal expansion (CTEs) of 17 alloys were estimated by combining available data. To estimate the lattice parameters of the alloys at 20°C, the changes in the lattice parameter of Ni caused by additions of the elements were simply summed to get the lattice parameters of alloys. This procedure results in calculated densities that are

Directional solidification and convection in small diameter crucibles

Abstract Pb–2.2 wt.% Sb alloy was directionally solidified in 1, 2, 3 and 7 mm diameter crucibles. Pb–Sb alloy presents a solutally unstable case. Under plane–front conditions, the resulting macrosegregation along the solidified length indicates that convection persists even in the 1 mm diameter crucible. Al–2 wt.% Cu alloy was directionally solidified because this alloy was expected to be stable with respect to convection. Nevertheless, the resulting macrosegregation pattern and the microstructure in solidified examples indicated the presence of convection. Simulations performed for both alloys show that convection persists for crucibles as small as 0.6 mm of diameter. For the solutally stable alloy, Al–2 wt.% Cu, the simulations indicate that the convection arises from a lateral temperature gradient.

Continuum model for predicting microporosity in steel castings

Using a finite element model for simulating dendritic solidification of multicomponent-alloy castings, the pressure and redistribution of gas-forming elements during solidification and cooling in AISI 8620 steel casting alloy were calculated. The model solves the conservation equations of mass, momentum, energy, each alloy component and gas-forming elements (i.e. hydrogen and nitrogen). By solving for the concentrations of hydrogen and nitrogen in the intergranular liquid and comparing the sum of their Sieverts pressures with the local pressure within the mushy zone of the alloy, the model predicts regions of possible formation of porosity. The thermal boundary conditions on test-bar castings were deduced from a thermal calculation performed with a commercial code, ProCAST™. With these realistic thermal boundary conditions, our porosity-simulations were carried out for many cases with combinations of different initial contents of the gas-forming elements: hydrogen in the range of 3–7 ppm and nitrogen in the range of 0–100 ppm. The calculated results are summarized in a plot that separates castings expected to have porosity from those with no porosity. The effect of adding titanium to form TiN inclusions and inhibit the development of porosity during solidification was also investigated.

Transport properties and transport phenomena in casting nickel superalloys

Nickel superalloys that are used in the high-temperature regions of gas-turbine engines are cast by directional solidification (DS). In the DS processes, the castings are cooled from below, and three zones exist during solidification: (1) an all-solid zone at the bottom, (2) a “mushy zone” that is comprised of solid and liquid material, and (3) an overlying all-liquid zone. Computer simulations can be useful in predicting the complex transport phenomena that occur during solidification, but realistic simulations require accurate values of the transport properties. In addition to transport properties, the thermodynamic equilibria between the solid and liquid during solidification must also be known with reasonable accuracy. The importance of using reasonably accurate estimations of the transport properties is illustrated by two-dimensional simulations of the convection during solidification and the coincidental macrosegregation in the DS castings of multicomponent Ni-base alloys. In these simulations, we examine the sensitivity of the calculated results to measured partition ratios, thermal expansion coefficients, and viscosities that are estimated by regression analyses and correlations of existing property data.