Manufacturing Engineering 

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.