| Description |
A fundamental understanding of machining-induced microstructural alterations and their correlation with topographical (surface roughness) and mechanical (microhardness and residual stresses) characteristics of surface integrity will aid in improved process planning for obtaining a desired surface quality. The final machined surface quality will govern the component's performance by influencing fatigue life, creep resistance, stress corrosion, and so forth. The selection of cutting conditions, cutting fluids, and cutting tools will eventually dictate the type of surface that is being produced in machining. The selection of these conditions is possible only by investigating and understanding their effects on the eventual state of the machined surface from both metallurgical and mechanical stand points. Hence, a multiscale surface integrity approach (macro, micro and submicro scale) has been adopted in this thesis to study the effects of cutting conditions, cutting tools, and varying cutting fluid applications on the final surface state. The major aim of this thesis is to establish a fundamental understanding among topographical, mechanical, and microstructural characteristics of machining-induced surface integrity in Ti-6Al-4V titanium alloy. Additionally, this thesis also investigates the performance of Targeted Minimum Quantity Fluid (TMQF) application which strategically targets the cutting fluid (300ml/h) on the rake and flank face of the cutting tool while machining. Two inherently different cutting fluids (vegetable oil and synthetic fluid) were tested with TMQF application. The results obtained from TMQF machining are subsequently compared to those from dry and conventional flood machining (6 l/min) for further evaluation. Two different cutting tools (an uncoated tungsten carbide tool and a PVD-coated tungsten carbide cutting tool) were used to evaluate their interaction with cutting fluid application as well as their effects on the final machined surface state. The microstructural characterization of the machined surfaces showed the presence of higher concentration of low angle grain boundaries (less than 5°) at the near machined surface for all cutting tool and cutting fluid applications under investigation. However, machining under the dry condition yielded a higher concentration of low angle grain boundaries at a larger depth (∼100μm) from the machined surface compared to the other three cutting fluid applications. The presence of low angle grain boundaries is an implication of severe plastic deformation. Furthermore, machining under the dry condition produced a tensile residual stress profile and a lower hardness profile unlike those obtained with the other three cutting fluid conditions. These results clearly indicate the correlation between microstructural and mechanical characteristics of machining-induced surface integrity. Flood machining with an uncoated tool and a PVD-coated tool produced contrasting residual stress profiles that signify the effect of the interactions between the cutting tool material and the cutting fluid application on the final surface state. Also, significantly different residual stress profiles were obtained for TMQF machining and flood machining which underline the importance of selecting the right cutting fluid application. The results from this thesis have the potential to close the knowledge gap between microstructural and mechanical characteristics of machining-induced surface integrity. Additionally, this thesis can aid in choosing the right combination of cutting fluid application and cutting tool for finish machining operation of Ti-6Al-4V titanium alloy to obtain a desired final superior surface integrity. |
