On the Role of Microcracks on Flow Instability in Low Speed Machining of CP Titanium

Flow instability is observed when machining titanium and its alloys and leads to the localization of plastic deformation into narrow bands. The shear strain at which this localization occurs serves as an important design parameter that measures the workability of a material and its suitability for various applications. In the current investigation, the phenomenon of microcracking during chip formation of grade 2 commercially pure (CP) titanium has been examined as a mechanism that promotes flow instability at low speeds. Based on this study, analytical and descriptive models that have been previously proposed for predicting the onset of flow instability have been revised to account for microcracking. Model predictions were experimentally verified for the orthogonal machining of CP titanium over a wide range of cutting speeds. Additionally, an investigation of plastic deformation across microcracks and localized shear zones was conducted in an effort to better understand strain hardening during chip formation.