A chip formation mechanism taking into account microstructure evolution during the cutting process: Taking compacted graphite iron machining as an example

Abstract

Compacted graphite iron (CGI), a prototypical heterogeneous material, potentially demonstrates distinctive cutting deformation behaviours attributed to the random distribution of graphite and performance differences between graphite and the matrix, which have not yet received adequate attention. This study focuses on the influence of the microstructure characteristics of CGI on the formation of serrated chips. The morphology of the serrated chip segments during the orthogonal turning of CGI was observed in detail, the microstructures of the chip roots were characterised and analysed using various techniques, and a finite-element cutting simulation model considering the microstructural characteristics of CGI was developed. Results suggest that the formation of serrated chips in CGI is influenced by periodic and aperiodic brittle fractures, referred to as quasi-periodic brittle fractures, which are controlled by the distribution of graphite in CGI. This results in variations in the morphology and dimensions of the serrated chips in CGI. Plastic deformation is concentrated in a triangular deformation zone (TDZ) near the tool-chip interface, which is broader than the conventional secondary deformation zone. The experimental and simulation results revealed the reasons for the formation of the TDZ and emphasized the critical role of graphite in the formation of serrated chips in CGI. The graphite particles near the tool-chip interface promoted plastic deformation along the interface owing to the principle of minimum energy and restricted deformation perpendicular to the interface due to its structure, leading to the formation of the TDZ. The influence of graphite on material flow and the formation of the TDZ during the formation of serrated chips in CGI is a novel discovery. The microstructure evolution of the pearlite matrix in CGI caused by cutting deformation was analysed. The results demonstrate that the distinctive deformation behaviour of CGI contributes to the fragmentation of the pearlite structure, grain refinement, and increased dislocation density in the TDZ. Finally, the influence of the serrated chip formation mechanism on chip morphology and cutting force in CGI was discussed. These findings offer significant scientific insights and contribute to the fundamental understanding of the chip formation process in CGI.