Modeling the effect of the microstructure of compacted graphite iron on chip formation

Abstract

The unique mechanical and physical properties of compacted graphite iron (CGI) have awarded the material such desirable and increasing demands in both automotive and locomotive industries. The graphite round edges combined with irregular graphite boundaries highly enhance crack arrest resistance within the matrix and participate into the good adhesion of graphite–matrix interface, compared to gray cast iron. However, the praised mechanical performance of compacted graphite iron (CGI) compared to gray iron, and its superior thermal properties compared to nodular iron (ductile iron) have come with CGI's relative poor machinability. Finite element simulation of the microstructure of CGI will provide better understanding of the behavior of the metal during machining and will establish a good foundation of CGI machining optimization. Modeling of the microstructure of CGI chip considering the three main constituents of the metal; graphite, pearlite, and ferrite, was possible using the accumulated plastic strain fracture criterion. Although there is no distinctive boundary line between adiabatic shearing and surface crack initiation chip formation principles in real metal cutting, chip formation simulation showed that it was predominantly due to crack initiation and propagation. Cracks initiated at either the graphite particles or the graphite–matrix interface promoted by the fracture of surface graphite particles then progressed through the matrix. The characteristic segmental chip produced in CGI machining was mainly driven by the presence of graphite embedded in the matrix. Chip segments formation initiated at the graphite particles (or graphite–matrix interface) then progressed towards the chip-tool tip. Modeling of the graphite/matrix interface was based on the utilization of cohesive zone elements. Comparison between the modeled CGI microstructure and graphite-free modified microstructure highlights the significant role of graphite on chip characteristics. Comparison between the simulated segmental chip and real CGI chip validated the proposed simulation and proposes it as a valid foundation for further CGI machining optimization.