H13 tool steel is widely used in the mold and die industry. Due to tighter geometric tolerances and higher quality expectations, the use of hard machining has increased over the years. Hard machining refers to the machining of materials in their hardened state. The challenges with hard machining are rapid tool wear and maintaining a high surface integrity of the machined surface. Surface integrity is measured in terms of surface roughness, residual stresses, presence of surface and subsurface cracks, and the quality of the developed microstructure. In order to minimize wear and improve product quality, researchers are working on the development of different tool coatings. Some of the recent tool coatings function by adapting to their environment using heat to form thin layers of oxides, referred to as ―tribo-films‖, on the surface of the tool. If engineered properly, these tribofilms can prolong tool life and improve the surface integrity of a hard machined surface. A titanium based nano multi-layered coating (TiAlCrSiYN/TiAlCrN) has been developed by researchers at the MMRI. The tribological performance of two different coatings TiAlCrSiYN/TiAlCrN and TiAlCrN were tested in a hard machining metal cutting process. The impact of these coatings on tool wear, Cutting process (Chips) and Surface Integrity (Quality of machined surface) was assessed. This research involves characterizing the coating to understand how the formation of different oxide films (tribofilms) effect tool wear and surface integrity. The generation of these tribofilms is sensitive to coating composition and cutting condition (temperature/pressure). Next, an in-depth characterization of the chips produced during machining was carried out as part of studying the effect of different tribological conditions between the tool and workpiece. The chip's hardness, oxidation, chip formation mechanism and topography as the chip slid against the cutting tool surface was studied. Also, the Surface integrity of the machined part was investigated, considering its microstructure, residual stresses and surface roughness. Lastly, tests were performed in an attempt to accelerate the generation of beneficial tribofilms. Results indicate significant improvement in wear life and surface integrity of the machined surface due to the generation of tribo-films in this machining application. / Thesis / Master of Applied Science (MASc)

<p> High-end optical grade applications would benefit greatly from the unique mechanical and chemical properties of titanium alloys. However, the standard process of manufacturing optical components has not been explored in depth for titanium alloys. </p> <p> Thus the focus of this work was to extend ultra precision machining technology to produce optical grade surfaces on titanium components. An optical surface is characterized by surface roughness less than 10nm R_rms which are typically produced with single crystal diamond tools having a cutting edge radius on the order of 50-100 nm. A cutting speed of 60m/min, feed rate of 1.5 µm/ rev and depth of cut of 2 µm, was identified to achieve the surface finish target, but the practical limitation of this process was still with tool life and the rapid degradation of surface finish over time. </p> <p> This was attributed to the adhesion of titanium material on the tool that resulted in material pull out and side-flow during machining. Results obtained from the characterization of the tool and workpiece led to the identification of graphitization as the initial wear mechanism. As the cutting edge rounds-off due to graphitization, the rate of adhesion of the workpiece material onto the tool increased. For this reason solutions were explored that would reduce the graphitization process and delay the onset of intense adhension. </p> <p> Thus a coating technology involving Perfluoro Polyether (PFPE) was chosen. Tribometer analysis under a load of 500N and temperature of 450ºC between the uncoated and PFPE coated diamond tools and titanium pins showed a remarkable reduction in COF from 0.275 to 0.05. A significant enhancement in tool life and surface quality was also achieved in single point diamond turning (SPDT) of titanium alloys using PFPE coated diamond tools. Tool life was based on an assessment of the cutting length achieved before the surface roughness exceeded the targeted value of 10 nm R_rms and it improved from 1.25 km and 5.1 km with PFPE coated tools. </p> / Thesis / Doctor of Philosophy (PhD)

The high energy lasers are emerging as an innovative material processing tool to effectively fabricate complex shapes on the hard and brittle structural ceramics, which previously had been near impossible to be machined effectively using various conventional machining techniques. In addition, the in-situ measurement of the thermo-physical properties in the severe laser machining conditions (high temperature, short time duration, and small interaction volume) is an extremely difficult task. As a consequence, it is extremely challenging to investigate the evolution of surface topography through experimental analyses. To address this issue, an integrated experimental and computational (multistep and multiphysics based finite-element modeling) approach was employed to understand the influence of laser processing parameters to effectively control the various thermo-physical effects (recoil pressure, Marangoni convection, and surface tension) during transient physical processes (melting, vaporization) for controlled surface topography (surface finish). The results indicated that the material lost due to evaporation causes an increase in crater depth of machined cavity, whereas liquid expulsion created by the recoil pressure increases the material pileup height around the lip of machined cavity, the major attributes of surface topography (roughness). Also, it was found that the surface roughness increased with increase in laser energy density and pulse rate (from 10 to 50Hz), and with the decrease in distance between two pulses (from 0.6 to 0.1mm) or the increase in lateral and transverse overlap (0, 17, 33, 50, 67, and 83%). The results of the computational model are also validated by experimental observations with reasonably close agreement.

The arrival on the market of a new kind of CNC machines which can both add and remove material to an object paved the way to a new approach to the problem of repairing damaged components. The additive operation is performed by a Direct Laser Deposition (DLD) tool, while the subtractive one is a machining task. Up to now, repair operations have been carried out manually and for this reason they are errors prone, costly and time consuming. Refurbishment can extend the life of a component, saving raw materials and resources. For these reasons, using a precise and repeatable CNC machine to repair valuable objects is therefore very attractive for the sake of reliability and repeatability, but also from an economical and environmental point of view. One of the biggest obstacles to the automation of the repairing process is represented by the fact that the CAM software requires a solid CAD model of the damage to create the toolpaths needed to perform additive operations. Using a 3D scanner the geometry of the damaged component can be reconstructed without major difficulties, but figuring out the damage location is rather difficult. The present work proposes the use of octrees to automatically detect the damaged spot, starting from the 3D scan of the damaged object. A software named DUOADD has been developed to convert this information into a CAD model suitable to be used by the CAM software. DUOADD performs an automatic comparison between the 3D scanned model and the original CAD model to detect the damaged area. The detected volume is then exported as a STEP file suitable to be used directly by the CAM. The new workflow designed to perform a complete repair operation is described placing the focus on the coding part. DUOADD allows to approach the repairing problem from a new point of view which allows savings of time and financial resources. The successful application of the entire process to repair a damaged die for injection molding is reported as a case study. In the last part of this work the strategies used to apply new material on the worn area are described and discussed. This work also highlights the importance of using optimal parameters for the deposition of the new material. The procedures to find those optimal parameters are reported, underlying the pros and cons. Although the DLD process is very energy efficient, some issues as thermal stresses and deformations are also reported and investigated, in an attempt to minimize their effects.

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