Residual stresses in machined titanium (Ti-6A1-4V) alloys.

摘要

Surface integrity plays a very important role in the life and functionality of machined surfaces used in a variety of engineering applications. Manufacturing processes and their sequence, along with the selection of cutting conditions and cutting tools, eventually dictate the type of surface that is being produced. Surface integrity is subdivided into several components, of which, some important components are surface roughness, residual stresses and subsurface microstructures. Enhanced understanding of all these factors and their interactions will potentially lead to advanced knowledge driven machining process planning.;This thesis focuses on an experimental investigation of the effects of cutting tool coatings and cutting fluid application on surface integrity (residual stress, surface roughness and subsurface microstructure) in machined Ti6Al4V titanium alloy. For measuring residual stresses, the hole drilling method was used in this thesis research. The tools selected for the machining were uncoated flat-faced, uncoated grooved, multilayered (TiCN/Al2O 3/TiN) CVD coated grooved, and single-layered (TiAlN) PVD coated grooved tools with tungsten carbide substrates. Three different cutting fluid application conditions were used namely: dry, minimal quantity lubrication (MQL) and flood.;To illustrate the significance of this thesis, it was observed that a grooved multilayered CVD coated cutting tool under the influence of MQL condition, induced the highest near-surface residual stresses; on the other hand, the same tool, when machined under dry condition, produced the lowest residual stresses. Thus, it can be seen that a specific cutting tool material and/or geometry produce significantly different surface integrity when it is combined with different cutting fluid application conditions. Moreover, the microstructural analysis performed on these machined workpieces revealed significant changes in the subsurface microstructure with respect to the type of cutting tool-cutting fluid application combination used and correlated strongly with the measured residual stress profiles.;The combined effect of the type of cutting tool along with the type of cutting fluid application condition on surface integrity is extremely significant. The results and findings of this thesis have the potential to aid in choosing the combination of the cutting tool and the cutting fluid application that are best suited for machining. Apart from that, this thesis also provides several recommendations for future research on the fundamentals of the interactions between machining parameters such as tool coatings, tool geometry and cutting fluid applications and their significant effects on the generated surface integrity and the life of the component there after.