Abstract: A microstructure-level model for simulation of machining of cast irons using the finite element method is presented. The model explicitly combines ferritic and pearlitic grains with graphite nodules to produce the ductile iron structure. The behaviors of pearlite, ferrite, and graphite are captured individually using an internal state variable model for the material model. The behavior of each phase is dependent on strain, strain rate, temperature, and amount of damage. Extensive experimentation was conducted to characterize material strain rate and temperature dependency of both ferrite and pearlite. The model is applied to orthogonal machining of ductile iron. The simulation results demonstrate the feasibility of successfully capturing the influence of microstructure on machinability and part performance. The stress, strain, temperature, and damage results obtained from the model are found to correlate well with experimental results found in the literature. Furthermore, the model is capable of handling various microstructures in other heterogeneous materials such as steels.

Abstract: Observations are made regarding the influence of cutting speed and friction on cutting force by way of finite element modeling. Simulations are validated by comparison of cutting forces and chip morphologies for the Al6061-T6. Analysis of cutting forces over a wide range of cutting conditions suggests an important role of the secondary shear zone in the decrease of cutting force as a function of speed, even well into what is considered to be the adiabatic machining regime. The proposition is supported by a decrease in chip thickness and significant increase in temperature at the tool-chip interface as the speed is increased. Temperatures in the primary shear zone rise only modestly and cannot account for the change in cutting force. Furthermore, the effect contributes to the nonlinear increase of forces with respect to feed as opposed to a plowing force by the cutting edge radius.

Abstract: It has been suggested recently [1–4] that an increase in the hardness value often observed when the indentation size is reduced (indentation size effect) in metals is a consequence of the dependence of the flow stress of the metal on the strain gradient. Here, we show based on an analysis of the strain gradient in machining, that a similar size effect should be observable when machining at small values of the undeformed chip thickness. Such a size effect will manifest itself as a continuous increase in the specific cutting force as the chip thickness is reduced. Furthermore, the size effect in machining is likely to be much more pronounced than in indentation, because of the more intensive strain gradient prevailing in the deformation field in machining. The dependence of the flow stress on strain gradient is not an artificial construct but has a well-established basis in the dislocation theory of hardening [5]. Our analysis suggests that an effective test of plasticity constitutive laws that incorporate the strain gradient as a parameter can be achieved using a simple 2-D cutting experiment with a sharp tool carried out at very low speeds. Such a test may be carried out with ductile pure metals as workpieces wherein this size effect is expected to be most significant.

Abstract: A microstructure-level simulation model was recently developed to characterize machining behavior of heterogeneous materials. During machining of heterogeneous materials such as cast iron, the material around the machining-affected zone undergoes reverse loading, which manifests itself in permanent material softening. In addition, cracks are formed below and ahead of the tool. To accurately simulate machining of heterogeneous materials the microstructure-level model has to reproduce the effect of material softening on reverse loading (MSRL effect) and material damage. This paper describes procedures used to calculate the material behavior parameters for the aforementioned phenomena. To calculate the parameters associated with the MSRL effect, uniaxial reverse loading experiments and simulations were conducted using individual constituents of ductile iron. The material model was validated with reverse loading experiments of ductile iron specimens. To determine the parameters associated with fracture of each constituent, experiments and simulation of notched specimens are performed. During the validation stage, response of simulated ductile iron was in good agreement with the experimental data.

Abstract: The choice of a measurement strategy in multi-station machining systems is crucial for subsequent successful identification of the root causes of machining errors. The measurement strategy selection is currently not a systematic process and it involves human and expert intervention. Recent advances in the linear state-space modeling of multi-station machining processes facilitate a more formal characterization of measurements in machining and a more systematic approach in the selection of the features that need to be measured. In this paper, we employ the Stream of Variation methodology to evaluate various measurement schemes in multi-station machining systems. Based on this evaluation, measurement synthesis procedures are presented, such that the resulting measurement scheme achieves a tradeoff between measurement uncertainty and the cost of measurements.

Abstract: Tailor-welded blanks (TWB) are widely used for stamped auto body panels because of their great benefits in weight and cost reduction. However, the weld line in a tailor-welded blank causes serious concerns in formability because of material discontinuity and additional inhomogeneous stress/strain distribution. This paper proposes a blank holding force (BHF) control strategy to control the weld line movement, distribute the deformation more uniformly and thereby improve TWB formability. The control methodology is developed based on a simplified 2-D sectional analytical model that estimates the stress/strain distribution and the BHFs required for each side of the flange with dissimilar materials. The model can be further extended to 3-D analysis by superimposing the 2-D sectional analysis results around the entire binder ring and thus determining the required BHF for the whole panel. Finite element simulations are performed to study the effects of forming parameters on the weld line movement. Experiments have been conducted to verify the analytical model and partial finite element simulations. Both analysis and experiments demonstrated that a lower BHF should be applied on the thicker blank side to allow more metal to flow-in for obtaining more uniform strain distribution. The proposed BHF control is proven to be a good approach to enhancing TWB formability.

Abstract: By considering the effects of drill grinding errors and drill deflections, dynamic cutting chip thickness models were developed which in combination with workpiece surface inclination effects allowed the formulation of expressions for the dynamic cutting chip cross-sectional area. By using the dynamic chip thickness and dynamic cutting chip area to replace their static counterparts, static drilling force models were extended to facilitate the prediction of dynamic cutting forces in micro-drilling processes. The geometric characteristics of micro-drills were considered in these mechanistic force models. Separate thrust, torque and radial force models for the major cutting edges, secondary cutting edge and for the indentation zone were developed. The effects of drill installation errors on the radial cutting forces acting on the chisel edge and the major cutting edges were also included.

Abstract: Modeling of machining operations requires the use of constitutive relations which could represent as close as possible the material behavior in the primary and secondary zones. The knowledge of these behavior laws involves the use of different types of sophisticated mechanical tests which should provide with sufficient accuracy the material behavior for the relevant conditions of machining. In this paper, first, the flow stress of 100Cr6 (AISI 52100) bearing steel in its HV730 hardness state has been identified in order to assess the machinability in case of hard turning. With this, the dependence of the flow stress on strain, strain rate and temperature, which poses significant difficulty, is presented. Second, the material machinability is evaluated with a shear instability criterion, enabling the prediction of chip formation with or without the shear localization. Quick-stop tests have been carried out on the bearing steel treated at different hardness values showing the chip formation variation. Micro-hardness tests performed on these quick-stop test samples show the effects of cutting temperature. A greater understanding of applied machinability is gained through this precise study of work material physical properties and behavior.

Abstract: The guiding principles adopted for teaching the Comprehensive Mechanical Design course in the Department of Mechanical Engineering of Auburn University are presented. These were introduced to better serve the need for well-trained engineering personnel, where technical skills must be combined with good communication and management skills.

Abstract: Hydroforming is a process used to manufacture tubular sections from circular tubes. Two systems for hydroforming are mainly used, high pressure hydroforming (HPH), and pressure sequence hydroforming (PSH). In this paper, an energy approach is used to calculate the compressive forces developed in the tube wall during deformation in PSH. The pressure required to prevent buckling of the tube wall is estimated based on a two dimensional buckling model. The differences in the mechanics of deformations between HPH and PSH are explained. The advantages and limitations of both systems are also summarized.

Abstract: Drilling is the most frequently employed operation of secondary machining for fiber-reinforced materials owing to the need for structure joining. Delamination is one of the serious concerns during drilling. Practical experience proves the advantage of using such special drills as saw drill and candle stick drill. Comprehensive delamination models for twist drill, saw drill and candle stick drill in drilling of composite materials are constructed in the present study. Of the three, the saw drill offers the highest critical thrust force followed by the candle stick drill, while the traditional twist drill allows for the lowest thrust force. A guide for drill design can be developed based on the proposed models.

Abstract: Problems of excessive coil deformation after winding will present difficulties in later material handling. Sheet material with a lower stiffness is more likely to deform than solid sheet coil. In this paper, numerical and analytical methods are used to study the factors causing this problem. The results can be used to estimate the requirement of the supporting core material to prevent this excessive coil deformation.

Abstract: Laser forming of steel is a hot forming process with high heating and cooling rate, during which strain hardening, dynamic recrystallization, and phase transformation take place. Numerical models considering strain rate and temperature effects only usually give unsatisfactory results when applied to multiscan laser forming operations. This is mainly due to the inadequate constitutive models employed to describe the hot flow behavior. In this work, this limitation is overcome by considering the effects of microstructure change on the flow stress in laser forming processes of low carbon steel. The incorporation of such flow stress models with thermal mechanical FEM simulation increases numerical model accuracy in predicting geometry change and mechanical properties.

Abstract: Residual stress relief by pulsed magnetic treatment is attractive since the process is carried out at room temperature and magnetic fields that are easy to produce and control can be used. The extent of residual stress reduction due to a particular pulsed magnetic treatment was determined. Initially curved strip specimens were held flat to produce a known residual stress and then subjected to a pulsed magnetic treatment. Curvature measurements and microhardness measurements before and after treatment and a mechanical model relating stress and indentation size were used to specify the change in residual stress due to pulsed magnetic treatment. Changes of stress near the surface of the specimens due to treatment were obtained. Reductions of residual stress of 4–7% were obtained for the particular pulsed magnetic treatment used on lower initial stress level specimens. For a higher initial stress level stress reductions of 8–13% were measured.