(Dr. Shreyes N. Melkote, advisor)
"Prediction of Process Induced Microstructural Changes and Residual Stresses in Orthogonal Hard Machining"
From a machining standpoint, materials with a hardness greater than 45 Rc are classified as “hard”. Such components are usually finish machined by grinding. The advent of Polycrystalline Cubic Boron Nitride (PCBN) cutting tools has however opened up several new avenues in finish machining such as turning and milling. Parts produced by hard machining have been found to be comparable in quality to ground parts. This, coupled with higher material removal rates and environmental acceptability of hard machining make it a more attractive option than grinding.
The main stumbling block that hard turning faces before industry acceptance is the presence of undesirable surface artifacts known as white layers found after the machining operation. It is the accepted belief that white layers are detrimental to fatigue life of the component. This is so because white layers from grinding have been traditionally associated with tensile residual stresses. However, investigators in hard turning have reported tensile as well as compressive states of stress for similar materials being machined under similar conditions. To date, there has been no effort to systematically assimilate the effects of workpiece and tool material properties and cutting variables to predict temperature, stresses, resulting microstructures and therefore residual stresses in one work.
The focus of this work is to enable prediction of microstructural changes and residual stresses produced in orthogonal machining of hardened steels. In order to do this accurately, the finite element method will be used with some analytical modeling to achieve the desired results.
The aspect of this work that is unique to modeling of hard machining
processes is the prediction of resulting microstructures and residual stresses
as a function of both metallurgical evolution and thermo-mechanical behavior
of the workpiece material. This will serve to provide a better physical
understanding of material and cutting parameter interactions in hard machining.
Moreover, this would aid in optimizing machining conditions in terms of
surface artifacts generated. Lastly, it will serve as a platform upon which
three-dimensional formulations of hard turning and milling may be built.