Modeling and Simulation of CGI Machining on Microstructure Level

Sammanfattning: incorporation of new design materials – e.g. in order to promote lightweight design – often leading to significant changes in manufacturing conditions, which can be assessed in an efficient way by simulation rather than more expensive testing. The current contribution is concerned with the constitutive modeling of Compacted Graphite Iron (CGI) and its application to simulate orthogonal machining thereof. Although CGI consists in general of pearlite, graphite and ferrite, focus is put on the constitutive modeling of the pearlitic phase since it is the dominating constituent with respect to machinability. The continuum hardening response is modeled with the Johnson-Cook (JC) plasticity model and the ductile fracture response with the Johnson-Cook dynamic failure criterion, both involving effects of large strains, high strain rates and high temperatures. Furthermore, the model has been calibrated against experimental data found in the literature for a pearlitic rail steel considered as a good representative for the pearlitic phase in the CGI-material. This assumption is strengthened by the good correlation obtained between simulated chip formation and cutting forces in the current work and the experimentally obtained chip formation and measured cutting forces in a related project. The proper representation of the finite deformation inelasticity problem is also discussed. The traditional way to represent the material in metal cutting applications via hypoelastic-inelastic material models, incorporating the objective corotational Jaumann stress rate can be shown to give an inaccurate response for particular loading situations, e.g. in simple shear where a spurious softening response is obtained. Therefore, we instead propose to use the alternative Green-Naghdi corotational stress rate to formulate the hypoelasticinelastic response which does not show this type of unphysical behavior. 2D orthogonal cutting simulations have been conducted in Abaqus/Explicit using both types of models (based on the Jaumann and the Green-Naghdi stress rate respectively) and the results thereof are compared and discussed. The results of these models are also compared with the results of a thermodynamically consistent hyperelastic-inelastic material model using the same type of JC-hardening.