Kompakt grafitli dökme demirde delik delme işleminin mekanistik modellemesi ve kalinti gerilmelerin incelenmesi

Outstanding mechanical and thermal properties of Compacted Graphite Iron (CGI) have attracted the engine manufacturers attention recently. This is due to the superior characteristics of the microstructure of CGI, which makes it a promising material in automotive and locomotive industrıes. This unique microstructure is formed by the additional elements such as magnesium (Mg) and titanium (Ti) in specific amounts and decreasing the amount of sulfur (S) which promotes formation of spheroidal graphite phases. Addition of these elements prevents formation of spheroidal graphite phases and generates vermicular shape graphite phases. Vermicular graphite phases decrease the material's tendency for crack initiation and propagation, and hence increases the fatigue life significantly compared to gray cast iron, besides increasing the tensile strength. Moreover, CGI has better thermal conductivity in addition to its lighter weight compared to commonly used materials for engines such as gray cast iron. As a result, compacted graphite iron becomes a preferable material in the automotive industry especially in manufacturing of diesel engines. Drilling operation is a challenging operation in CGI engine blocks. Since the drilling operations are usually carried out at final steps of manufacturing cycle, any failure in tool will scrap the hole engine block. Therefore, mechanics of drilling and induced residuals by drilling operation in compacted graphite iron is the scope of the current study. For this purpose, primarily, geometry of the twist drill is modeled analytically. Cutting lips and chisel edges are responsible for material removal and cutting process in drilling. The most volume of material cutting is done by cutting lips, here the chisel edge acts as an indenting tool, which penetrates into material and ploughs the material without serious amount of cutting. Since the twist drill utilized in this study is split-line twist drill, the chisel edge has been considered as two different regions including orthogonal cutting zone and indentation zone. In order to model the cutting forces in cutting lips and orthogonal chisel edge, orthogonal tube cutting tests were carried out to obtain the orthogonal material parameters including shear angle, shear stress and friction angle for the workpiece-tool pairs. These parameters were found as a function of rake angle and cutting speed for several uncut chip thickness values using the regression methods. Cutting lips of the twist drill were divided into several elemental edges, whereas considering each of them as an oblique cutting edge. Using the evaluated material parameters from orthogonal tube cutting tests, orthogonal to oblique transformation was applied and the cutting forces for each elemental tool along the cutting edges of the drill were calculated. Elemental cutting forces, which have three-dimensional directions, were transformed into drill coordinates. Summing the transformed elemental cutting forces in the direction of the axis of the tool, the total thrust force for cutting lips were calculated. Similarly, summing the torque value of each element will results in total torque value of the cutting lips. The orthogonal cutting region of the chisel edge was also divided into several elements. Having the geometrical parameters of the chisel edge, the cutting forces were obtained and were transformed into the tool's coordinates. Summing the thrust forces values produced from chisel edge and cutting lips, the total cutting forces for the drilling process were obtained. Therefore, having geometrical parameters of twist drill and workpiece-tool pair parameters, drilling cutting forces can be predicted using the model. Finally the predicted values were verified by drilling experiments. The predicted and experimental results are in a good agreement. The predicted values will be used in cutting temperature predictions at other topics of the project. Drilled CGI parts are subjected to residual stress measurements in order to investigate the effect of mechanical and thermal loading during drilling operation. Drilled specimens were measured from two points in order to study the residual stress profile along the hole. The X-ray diffraction method was employed in order to measure the induced residual stresses at the machined surface. Afterwards, incremental hole-drilling technique and the electronic speckle pattern interferometry (ESPI) method were used to acquire the residual stress profile in depth. Combining the surface residual stress measure by the XRD method and subsurface residual stress results measured using incremental hole drilling method, leads to derive residual stress profile of component in depth from machined surface. Drilling operation induces compressive residual stress at bulk of the hole. It means that mechanical loadings are more dominant than thermal stresses during cutting operation. Mechanical behavior of material, high thermal conductivity of compacted graphite iron prevent the generation of high thermal stresses, which in turn causes tensile residual stresses. In addition, high rake angle values of twist drill at the periphery of tool where engaged with workpiece results it high shear angles and in turn reduction in cutting forces, which causes compressive residual stresses. Furthermore, the effects of cutting speed and feed rate on residual stress profiles were investigated. Increasing cutting speed alters the residual stress profiles to more tensile manner because of thermal gradients. However, increasing feed rate induces tensile residual stress in subsurface layers due to high elastic relaxation in subsurface layers.