High speed machining induced residual stresses in Grade 5 titanium alloy

The surface and near surface residual stress state induced by machining may have a significant effect on the structural integrity of some important mechanical components. This article describes an experimental investigation of this stress state during high speed machining of Grade 5 titanium alloy (Ti6Al4V). The size and depth of the stress field is evaluated by non-contact probing measurement techniques as a function of cutting speed (70-200 m/min) and depth of cut (0.25-1.0 mm). X-ray diffraction and synchrotron energy dispersive diffraction are used to evaluate the surface (X-ray diffraction) and sub-surface (energy dispersive diffraction) residual stress fields. The results indicate that both techniques may be successfully employed to evaluated machining induced residual stress fields and that the energy dispersive diffraction method is eminently suitable for probe depths up to 100 mm in Grade 5. The results clearly indicated that significant residual stresses are introduced during machining both at conventional and high cutting speeds. The stresses are largely compressive and aligned with the main cutting direction for both rough and finish cuts. The results also show that the residual stress level is a strong function of the cutting speed. The overall stress level effectively becomes more tensile but also decreases with increasing cutting speed.