Analysis of Machining a Sinter-Hardened Powder Metallurgy Steel: Significance of Localized Densification of Uncut Chip Material during Chip Formation

It is well known that machining chips provide significant information about the material's machinability. Nevertheless, chips resulting from machining porous metals have not been adequately explored earlier despite the rising demand for porous metals in engineering applications. The objective of the present study is to fill this research gap. In this context, orthogonal machining (groove cutting) of FLC-4608 sinter-hardenable steel compacts having 90% relative density was performed at several cutting conditions, i.e., combinations of cutting velocities and feed rates, and the chips were analysed for machinability assessment. The cross-sectional microstructure and geometry of chips were examined, and mathematical equations were formulated to evaluate chip formation characteristics: efficiency and process stability. These characteristics were contemplated as machinability criteria in the present study. It was found that machining typically results in shear localized chip segments exhibiting plastic deformation and near-full densification of uncut chip segments. Both these phenomena were found to have a combined influence on the chip formation characteristics and, in turn, machinability. Increased cutting velocity and feed rate could improve the chip formation efficiency, as determined in the study. It was also determined that decreased cutting velocity could reduce the chip segmentation frequency and improve the process stability. Additionally, since shear angle is a well-known indicator of chip formation efficiency, a modification in the nomograph was proposed to identify shear angle in machining porous metals. The present study potentially helps PM practitioners comprehend chip formation in machining porous metals and attain precision in longer machining runs.