Determination of interface heat-transfer coefficients for permanent-mold casting of Ti-6Al-4V

Abstract

Interface heat-transfer coefficients (h ₀) for permanent-mold casting (PMC) of Ti-6Al-4V were established as a function of casting surface temperature using a calibration-curve technique. Because mold geometry has a strong effect on h ₀, values were determined for both of the two limiting interface types, “shrink-off” and “shrink-on.” For this purpose, casting experiments with instrumented molds were performed for cylinder- and pipe-shaped castings. The measured temperature transients were used in conjunction with two-dimensional (2-D) axisymmetric finite-element method (FEM) simulations to determine h ₀(T). For the shrink-off interface type, h ₀ was found to decrease linearly from 2000 to 1500 W/m² K between the liquidus and the solidus, from 1500 to 325 W/m² K between the solidus and the gap-formation temperature, and at a rate of 0.3 W/m² K/K thereafter. For the shrink-on interface type, h ₀ was found to increase linearly from 2000 to 2500 W/m² K between the liquidus and the solidus temperatures, from 2500 to 5000 W/m² K between the solidus and the gap-formation temperature, and to remain constant thereafter. The shrink-on values were up to 100 times the shrink-off values, indicating the importance of accounting for the interface geometry in FEM simulations of this process. The FEM-predicted casting and mold temperatures were found to be insensitive to certain changes in the h ₀ values and sensitive to others. A comparison to published h ₀ values for PMC of aluminum alloys showed some similarities and some differences.