Effect of edge hone radius on chip formation and its microstructural characterization in hard milling of AISI H13 steel

Abstract

Chip formation as a key factor during the machining process can affect cutting stability and machined surface integrity, specifically the formation of saw-tooth chip. The saw-tooth chip formation with respect to edge hone radius during hard milling of AISI H13 steel (50 ± 1 HRC) was investigated in this paper involving chip morphology, metallurgical and mechanical property evolution, chemical composition distribution, and the mechanism of saw-tooth chip formation. The chip microstructural characterization was determined by means of optical microscopy, scanning electronic microscopy (SEM), a nano-tester, and X-ray diffraction (XRD). First, the effect of edge hone radius on chip morphology and evolution of chip color has been investigated. The results highlight that saw-tooth pitch decreases while the degree of segmentation is opposite with edge hone radius and the chip back surface color evolves gradually from purple to blue. Secondly, larger edge hone radius induces a thicker white layer generated on chip back surface and a higher value of nano-hardness identified inside an adiabatic shear band (ASB) as well as on chip back surface. In addition, the occurrence of phase transformation which occurs on chip back surface is observed which can be semi-quantified by the chip back surface color. Furthermore, the detected carbon content almost keep unvaried both in adiabatic shear band and on chip back surface, showing no impact on nano-hardness variation. Finally, it is indicated that the mechanism of saw-tooth chip formation in hard milling of AISI H13 steel is mainly attributed to adiabatic shearing accompanying with cyclic crack formation which facilitates the degree of segmentation. This research is a basic study for tool edge geometry preparation as well as controlling hard milling process.