Ph.D. Thesis Defense by Wesley L. Stone
Friday, March 28, 2003

(Dr. Tom Kurfess, advisor)

"Thermal Effects on Subsurface Damage during the Surface Grinding of Titanium Aluminide"

Abstract

The demands placed upon materials in mechanical applications have pushed engineers to develop new materials. For instance, the components on the hot gas path of a commercial aircraft engine see firing temperatures in excess of 1400°C. Pressures on the aircraft industry demand higher efficiency of its engines, which is achieved primarily through higher firing temperatures and weight reduction. One material that has been introduced as a possible alloy as a jet engine component is an intermetallic compound called titanium aluminide (TiAl). The specific alloy that has shown the greatest promise is called gamma titanium aluminide (g-TiAl), which is actually a mixture of g and a2 phases.

Titanium aluminide has enhanced high temperature strength, good oxidation and burn resistance, high elastic stiffness, and low density. The high strength to weight ratio makes it especially appealing; its density is approximately half that of the nickel alloys currently used as turbine airfoils. The challenge in machining TiAl is that it is very brittle at low temperatures, which is where most machining occurs. Grinding is a machining process used to achieve good surface finishes, maintain tight tolerances, generate contoured surfaces, and process difficult-to-machine materials. As a high-energy process, grinding can generate large temperature rises in the workpiece being machined. Previous research has developed a theoretical model to predict the depth of plastic deformation during the surface grinding of TiAl. This model did not account for the variation in material properties due to temperature changes.

This research has focused on improving the model for plastic deformation during surface grinding of TiAl. There has been a three-stage approach to achieving this objective: first, material properties were established as a function of temperature; second, temperatures during grinding were evaluated; and third, subsurface plastic deformation was investigated.

Using the facilities at Oak Ridge National Laboratory’s High Temperature Materials Lab, several material properties were established as a function of temperature, ranging from room temperature to 800°C: thermal diffusivity (a), specific heat (Cp), thermal expansion (TE), thermal conductivity (k), Vickers hardness (Hv), Young’s modulus (E), and Poisson’s ratio (v). Temperatures during grinding were established theoretically using J.C. Jaeger’s moving heat source theory; numerically via a Matlab code; and experimentally with an embedded thermocouple technique. The variable properties allowed for the modification of the theoretical subsurface damage model, which was validated using a technique called the bonded interface method. The completion of this research provides valuable information, useful in the introduction of titanium aluminide into new engineering applications.