Tool-chip contact and chip formation in dry finish-machining of austempered ductile iron

Abstract

Straight dry finish-turning of a structural grade of austempered ductile iron (ADI) using cBN-TiC cutting tools was analysed. The influence of cutting speed on the characteristics of the tool-chip (TC) contact and chip-formation processes was the focus of this research. Experimental and analytical procedures were used in this respect. An approach to estimate the TC sticking contact length and TC stress distribution index was built to complement them. The feed rate, nominal rake angle, and depth-of-cut were kept at 50 μm rev−1, −26°, and 200 μm, respectively. The cutting speed was varied in the range of 50–800 m min−1. Strain-rate effects on the shear flow stress of structural ADI overpower their antagonist temperature impacts drastically in primary deformation zones (PDZs) and gradually in secondary deformation ones (SDZs). Accordingly, TC sliding-friction coefficient rises with the mean TC homologous temperature. The mean total energy brackets nearly match the range of specific cutting resistance for grinding of cast‑irons, related to the uncut-chip thickness of 14.23 μm. The levels of TC equivalent contact-stresses and mean TC contact temperature vividly reveal the vulnerability of cBN-TiC cutting-inserts edges for cutting speeds past 200 m min−1. The distribution index of TC contact stresses settled between 0.96 and 0.58. SDZPDZ softening degree decreased with cutting speed, landing between 76 and ~0%. Two bordering machining regimes, features of abrupt variations of local and global characteristics of the TC contact and chip-formation processes, emerged from the evolution of chip morphology with cutting speed: I: 50–150 m min−1, II: 200–800 m min−1. The pairing of these characteristics made it possible to blend them effectively. A better-informed procedure to select feed rate and cutting speed was then suggested.