Laser-assisted machining has been considered as an alternative for difficult-to-machine materials such as metallic alloys and ceramics. Machining of some materials such as high chromium alloys and high strength steels is still a delicate and challenging task. Conventional machines or computer numerical control (CNC) machines and cutting tools cannot adapt easily to such materials and induce very high costs for operations of rough machining or finishing. If laser-assisted machining can be implemented successfully for such materials, it will offer several advantages over the traditional methods including longer tool life, shorter machining time and reduced overall costs. This thesis presents the results of the research conducted on laser assisted machining of hard to wear materials used in making heavy duty mineral processing equipment for the mining industry. Experimental set up using a high power Nd:YAG laser beam attached to a lathe has been developed to machine these materials using cubic boron nitride (CBN) based cutting tools. The laser beam was positioned so that it was heating a point on the surface of the workpiece directly before it passed under the cutting tool. Cutting forces were measured during laser assisted machining and were compared to those measured during conventional machining. Results from the experiments show that with the right cutting parameters and laser beam position, laser assisted machining results in a reduction in cutting forces compared to conventional machining. A mathematical thermal model was used to predict temperatures within the workpiece at depths under the laser beam spot. The model was used to determine the effect of various cutting and laser parameters on the temperature profile within the workpiece. This study shows that laser assisted machining of hard to wear materials such as high chromium white cast iron shows potential as a possible economical alternative to conventional machining methods. Further research is needed before it can be introduced in industry as an alternative to conventional machining.

There is growing demand for micro and meso scale devices with applications in the field of optics, semiconductor and bio-medical fields. In response to this demand, mechanical micro-cutting (e.g. micro-milling) is emerging as a viable alternative to lithography based micromachining techniques. Mechanical micromachining methods are capable of generating three-dimensional free-form surfaces to sub-micron level precision and micron level accuracies in a wide range of materials including common engineering alloys. However, certain factors limit the types of workpiece materials that can be processed using mechanical micromachining methods. For difficult-to-machine materials such as tool and die steels, limited machine-tool system stiffness and low tool flexural strength are major impediments to the use of mechanical micromachining methods. This thesis presents the design, fabrication and analysis of a novel Laser-assisted Mechanical Micromachining (LAMM) process that has the potential to overcome these limitations. The basic concept involves creating localized thermal softening of the hard material by focusing a solid-state continuous wave laser beam of diameter ranging from 70-120 microns directly in front of a miniature (300 microns-1 mm wide) cutting tool. By suitably controlling the laser power, spot size and speed, it is possible to produce a sufficiently large decrease in flow stress of the work material and, consequently, the cutting forces. This in turn will reduce machine/tool deflection and chances of catastrophic tool failure. The reduced machine/tool deflection yields improved accuracy in the machined feature. In order to use this process effectively, adequate thermal softening needs to be produced while keeping the heat affected zone in the machined surface to a minimum. This has been accomplished in the thesis via a detailed process characterization, modeling of process mechanics and optimization of process variables.

This thesis presents an investigation of novel laser assisted micromachining processes that addresses the limitations of micromachining of hard-to-machine materials. Two different laser assisted approaches are used to machine hard metals and high strength ceramics. For hard metals, the basic approach involves localized thermal softening of the workpiece material by focusing a solid-state continuous wave near infra-red laser beam in front of the micro milling tool (end mills of 0.1 to 0.5 mm diameter). By suitably controlling the laser power, spot size and scan speed, it is possible to produce a sufficiently large reduction in the flow strength of the work material and consequently the cutting forces and tool deflections. A force model is developed to predict the cutting forces in Laser Assisted Micro Milling (LAMM) of hard metals. For high strength ceramics, the approach involves use of a two step process. In the first step, thermal cracks are generated in a confined volume by the steep thermal gradients generated by laser irradiation of the workpiece. In the second step, the weakened region is removed by a micro grinding tool. The characterization and modeling of the process serve as bases for users of the two approaches to select optimal process parameters.

Doctor of Philosophy / Department of Industrial & Manufacturing Systems Engineering / Shuting Lei / Laser-assisted machining (LAM) is a promising non-conventional machining technique for advanced ceramics. However, the fundamental machining mechanism which governs the LAM process is not well understood so far. Hence, the main objective of this study is to explore the machining mechanism and provide guidance for future LAM operations. In this study, laser-assisted milling (LAMill) of silicon nitride ceramics is focused. Experimental experience reveals that workpiece temperature in LAM of silicon nitride ceramics determines the surface quality of the machined workpiece. Thus, in order to know the thermal features of the workpiece in LAM, the laser-silicon nitride interaction mechanism is investigated via heating experiments. The trends of temperature affected by the key parameters (laser power, laser beam diameter, feed rate, and preheat time) are obtained through a parametric study. Experimental results show that high operating temperature leads to low cutting force, good surface finish, small edge chipping, and low residual stress. The temperature range for brittle-to-ductile transition should be avoided due to the rapid increase of fracture toughness. In order to know the temperature distribution at the cutting zone in the workpiece, a transient three-dimensional thermal model is developed using finite element analysis (FEA) and validated through experiments. Heat generation associated with machining is considered and demonstrated to have little impact on LAM. The model indicates that laser power is one critical parameter for successful operation of LAM. Feed and cutting speed can indirectly affect the operating temperatures. Furthermore, a machining model is established with the distinct element method (or discrete element method, DEM) to simulate the dynamic process of LAM. In the microstructural modeling of a β-type silicon nitride ceramic, clusters are used to simulate the rod-like grains of the silicon nitride ceramic and parallel bonds act as the intergranular glass phase between grains. The resulting temperature-dependent synthetic materials for LAM are calibrated through the numerical compression, bending and fracture toughness tests. The machining model is also validated through experiments in terms of cutting forces, chip size and depth of subsurface damage.

Master of Science / Industrial & Manufacturing Systems Engineering / Shuting Lei / Transparent brittle materials such as glass and silicon dioxide have begun to replace the conventional materials due to the advantageous properties including high strength and hardness, resistance to corrosion, wear, chemicals and heat, high electrical isolation, low optical absorption, large optical transmission range and biocompatibility. However because these materials are extremely hard and brittle, development of an ideal machining process has been a challenge for researchers. Non-traditional machining processes such as abrasive jet and ultrasonic machining have improved machining quality but these processes typically results with issues of poor surface integrity, high tool wear and low productivity. Therefore a machining technique that overcomes the disadvantages of existing methods must be developed. This study focused primarily on improving the machinability and attaining crack-free machined surfaces on transparent brittle materials by inducing micro cracks or seed damages on the subsurface of the materials. The hypothesis was that micro-cracks induced by femtosecond laser would synergistically assist the material removal process by a cutting tool by weakening or softening the material, followed by conventional machining process. Laser induced damages due to varying laser intensities and at different depths in bulk BK7 glass was studied in order to select the optimal laser machining conditions for the experiments. Dimensional and structural profiles of laser cracks are observed using an optical microscope. A comparative study of machined untreated BK7 samples and damage induced BK7 samples was conducted. Due to its simple process kinematics and tool geometry, orthogonal machining is used for the study. Results showed that machining laser-treated samples caused an average 75% force reduction on comparison to machining of untreated samples. Laser treated machined samples were produced without subsurface damages, and reduced tool wear was noted. Overall improved machinability of BK7 glass samples was achieved.

Cost-effective machining of hardened steel (>60 HRC) components such as a large wind turbine bearing poses a significant challenge. This thesis investigates a new laser tempering based hybrid turning approach to machine hardened AISI 52100 steel parts more efficiently and cost effectively. The approach consists of a two step process involving laser tempering of the hardened workpiece surface followed by conventional machining at higher material removal rates using lower cost ceramic tooling to efficiently cut the laser tempered material. The specific objectives of this work are to: (a) study the characteristics of laser tempering of hyper-eutectoid 52100 hardened steel, (b) model the laser tempering process to determine the resulting hardness, and (c) conduct machining experiments to evaluate the performance of the laser tempering based hybrid turning process in terms of forces, tools wear and surface finish. First, the microstructure alterations and phase content in the surface and subsurface layers are analyzed using metallography and x-ray diffraction (XRD) respectively. Laser tempering produces distinct regions consisting of - a tempered white layer and a dark layer- in the heat affected subsurface region of the workpiece. The depth of the tempered region is dependent on the laser scanning conditions. Larger overlap of laser scans and smaller scan speeds produce a thicker tempered region. Furthermore, the tempered region is composed of ferrite and martensite and weak traces of retained austenite (~ 1 %). Second, a laser tempering model consisting of a three dimensional analytical model to predict the temperature field generated by laser scanning of 52100 hardened steel and a phase change based hardness model to predict the hardness of the tempered region are developed. The thermal model is used to evaluate the temperature field induced in the subsurface region due to the thermal cycles produced by the laser scanning step. The computed temperature histories are then fed to the phase change model to predict the surface and subsurface hardness. The laser tempering model is used to select the laser scanning conditions that yield the desired hardness reduction at the maximum depth. This model is verified through laser scanning experiments wherein the hardness changes are compared with model predictions. The model is shown to yield predictions that are within 20 % of the measured hardness of the tempered region. Using the laser scanning parameters determined from the laser tempering model, cutting experiments using Cubic Boron Nitride (CBN) tools and low cost alumina ceramic tools are conducted to compare the performance of laser tempering based hybrid turning with the conventional hard turning process. The machining experiments demonstrate the possibility of higher material removal rates, lower cutting forces, improved tool wear behavior, and consequently improved tool life in the laser tempering based process. In addition, the laser tempered based hybrid turning process produce is shown to yield lower peak-to-valley surface roughness height than the conventional hard turning process. Furthermore, it is found that lower cost ceramic tools can be used in place of CBN tools without compromising the material removal rate.