Investigation and thermal modelling of compacted graphite iron drilling

The automotive industry has been trying to meet the demanding user expectations for a long time besides expanding environmental protection regulations. The most stringent regulation is the emission limitations. The engine designers spend huge energy to improve their technology in order to have a more effective combustion process in engines. However, this in not the only solution method for reducing emission, engine material change and creating same power with lighter engine weight is also another curative solution for this purpose. Nowadays, compacted graphite iron (CGI) is used instead of gray cast iron due to it's better mechanical properties in the diesel engine blocks, exhaust manifold and cylinder heads. The higher tensile strength, better corrosion and wear resistance of CGI providing a more efficient combustion environment than that of gray cast iron. By the help of alloying elements such as magnesium, titanium and chromium, CGI has improved mechanical properties. However, same alloys deteriorate the machinability significantly. The economical attainability and efficiency of the engine is the inevitable necessity of the present-day. An economic production of the engine can only be achieved with high machinability of the selected material. The knowing the cutting tool temperature distribution, selection of the appropriate cutting parameters and cooling/lubrication method are significantly crucial for understanding and improving the machinability characteristics of CGI. In this dissertation, CGI drilling and deep hole drilling were covered in seven chapters. First two chapters were refer to state of the art on CGI drilling process. Chapter 3 includes cutting mechanics of drilling process and force model development. The workpiece and tool temperature distribution were developed in Chapter 4 and chapter 5, respectively. The machinability analysis results were presented in Chapter 6. Finally, all results were summarized in Chapter 7. Solid twist drill bits consisting two main cutting regions: the cutting lips and the chisel edge. The chisel edge is located at the center position, which strategically provides the necessary center position for the workpiece. The chisel edge influences the provision of a smooth and stable cutting. In most instances, the chisel edge has a large rake angle influencing the plastic deformation through the integration of inward indentation and the consequent ploughing of the material. Cutting lips are identified as the primary cutting edges of the drill, vital for the cutting process. Oblique cutting mechanics are regarded as the cutting lips due to the variation of the rake and inclination angle. The identification of rake and inclination angles are vital in the calculation of cutting forces and torque along the cutting lip. Because of the different cutting behaviors, cutting forces were analyzed at the chisel and cutting lips separately. In chapter 3 section 3.3.1, the study incorporates an analysis of the thrust force and torque at the chisel edge through the division of the chisel edge into two parts. The indentation mechanism was integrated within the center of the chisel where the tool plows the material, and orthogonal cutting mechanics were incorporated within the cutting region between the center of the drill and the chisel edge-lip intersection. In chapter 3 section 3.3.2, using the discretizing method, thrust force and torque were evaluated for each element employing oblique cutting mechanics. The force and torque values for all elementary cutting tools were determined utilizing orthogonal-to-oblique transformation. The approach used in the force modeling considers the cutting force coefficient and the cutting velocity of each segment. The required parameters for calculating cutting force coefficients, shear angle, shear stress and friction angle were obtained from orthogonal tube cutting experiments. Orthogonal tests were performed using a range of rake angles and cutting speeds. The force model was experimentally verified for the chisel and cutting lip sections. The drilling process is one of the widely used machining processes to assemble manufactured parts. During the drilling process, temperature distribution in the workpiece is an important parameter as it adversely affects product quality including residual stress, dimensional error and the hardness of the machined surface. Therefore, thermal modeling is critical for the correct understanding of the process. In Chapter 4, the study utilized the transient heat transfer problem with a moving heat source model in the determination of the temperature distribution of the workpiece. The development of a direct solution to the problem was realized through a numerical process that integrates ABAQUS commercial software. The workpiece was developed as a 2D axis-symmetric model. The heat load variation was estimated for the chisel and cutting lip by using force and torque values that are calculated by a mechanistic model in Chapter 3. To obtain workpiece temperature distribution, calculated heat loads were employed in finite elemental analysis. Additionally, the heat load from the drill margin to the workpiece was calculated by an inverse heat transfer method, and it was integrated to the thermal model in dry drilling condition. The inverse heat transfer method has three main steps: Collecting experimental data, direct solution, and inverse solution. Experimental data were collected by embedded thermocouples. The direct problem was then solved via Abaqus, and the inverse problem was solved using MATLAB. Temperature measurement experiments were performed to confirm the thermal model of dry drilling. After that temperature distribution of KGDD workpiece was modeled under internal MQL condition. Heat fluxes at the margin, accumulated hot chips on the spiral drill flute, and the heat convection coefficient of the air–oil mixture were calculated using the inverse heat transfer method as well. The temperature distribution model of drill bit was covered in Chapter 5. The main reason of temperature rise on the tool is the friction between tool rake face and chip. The other reasons are the friction on the margin and the hot chip in the flute, which their effects on heat input were neglected in this dissertation. Accordingly, cooling and/or lubrication fluid can be applied in order to cool the drill and workpiece, and evacuate the hot chips. In this study, 6 bar air was used to cool the workpiece by heat convection during deep-hole drilling. Abaqus Standard Commercial Software was used to solve 3D transient heat transfer problems to predict temperature distribution of drill. In the finite element model, the contact surface between rake face and chip were divided by five partitions on the cutting lip region and one partition on the chisel region. Heat loads for six different partitions were estimated by Shaw's heat partition equation and heat flux was applied to divided surfaces. The heat transfer coefficient of air was set to 20 W/m2K and applied to free surfaces except hole surfaces. The heat transfer coefficient of air was calculated by using ANSYS CFX and applied to helical cooling hole surface and drill flank surfaces. The developed thermal model was verified by using embedded thermocouples for dry and using air, and good agreements were obtained between experimental and predicted temperature results. In chapter 6, the machinability of CGI was investigated for drilling and deep hole drilling cases. In section 6.1, the high-throughput and sustainable drilling of compacted graphite iron was studied. CGI drilling experiments were conducted using a 4 mm diameter coated carbide drill at 26.5 mm/s feed rate. In two repeated tests under three lubrication conditions: dry, dry with through-the-drill compressed air, and through-the-drill MQL, the drills were able to produce a maximum of 1740, 3150 and 2948 holes, respectively, before the breakage of the drill. The Joule-Thomson effect due to the expansion of high pressure air from through-the-drill holes at the drill tip, chip shape, chip size, and chip speed were also investigated. Results indicate that dry machining of CGI is technically feasible and chip evacuation and advanced tool cooling are important factors that affect drill life for high-throughput sustainable dry drilling of CGI. In section 6.2, the machinability studied for deep hole drilling case. Experiments were conducted using a 10 mm diameter uncoated carbide drill at different cutting parameters. The change of hardness, tool wear, microstructure and residual stress according to cutting parameters were observed.