On the Catastrophic Shear Instability in High-Speed Machining of an AISI 4340 Steel

Abstract

An AISI 4340 Steel (325 BHN) was machined at various speeds up to 2500 m/min (8000 SFPM). Longitudinal midsections of the chips were examined metallurgically to delineate the differences in the chip formation characteristics at various speeds. Chips were found to be continuous at 30 to 60 m/min (100 to 200 SFPM) but discontinuous below this speed. Instabilities in the cutting process, leading to different types of cyclic chip formations, were observed at cutting speeds above 60 m/min (200 SFPM). Fully developed catastrophic shear bands separated by large areas (segments) of relatively less deformed material, similar to that when machining titanium alloys, were observed in the chips at cutting speeds above 275 m/min (800 SFPM). The intense shear bands between the segments appeared to have formed subsequent to the localized intense deformation of the segment in the primary shear zone. As the cutting speed increases, the extent of contact between the segments is found to decrease rapidly. At speeds of 1000 m/min (3200 SFPM) and above, due to rapid intense, localized shear between the segments, these segments were found to separate completely as isolated segments instead of being held intact as a long chip. The speed at which this decohesion occurs was found to depend upon the metallurgical state of the steel machined and its hardness. As in the case of machining titanium alloys, the deformation of the chip as it slides on the tool face, i.e., “secondary shear zone,” appeared to be negligible when machining this AISI 4340 steel at high speed. Based on the metallurgical study of the chip and the similarities of machining this material at high speed and that of titanium alloys at normal speed, a cyclic phenomenon in the primary shear zone is identified as the source of instability responsible for the large-scale heterogeneity and a mechanism of chip formation when machining AISI 4340 steel at high speed is proposed.

In situ analysis of flow dynamics and deformation fields in cutting and sliding of metals

“…One form of shear localization is caused by thermal softening, and commonly occurs when cutting metals of low thermal diffusivity (e.g. Ti-6Al-4V, AISI 4340 steel) and at high cutting speeds (adiabatic conditions) [20,21]. Owing to this localization, the chip morphology can change from continuous to discontinuous form upon only increasing the cutting speed.…”

“…discontinuous chip, segmented chip) and roughness patterns that occur on chip surfaces. These features have been attributed variously to adiabatic shear localization [20,21], ductile fracture [13,22], shearing by 'stack-of-cards' type slip [1], inhomogeneous micro-scale plastic flow [14,23], machine stiffness [8] and bifurcative instability [24], all with common underlying features of unsteady flow and non-uniform deformation. The diverse chip morphologies are not unique to metals but have been observed also in cutting of materials such as glass [25], metallic glasses, polymers [26] and paraffin wax [27].…”

Experimental performance analysis of an inverse dynamics CNC compensation scheme for high-speed execution of curved toolpaths

“…Moreover, we focus only on the intrinsic machine dynamics, and do not consider cutting forces, external disturbances, etc. Dynamic forces are often dominant in high-speed machining (HSM) of complicated shapes, incurring large rates of acceleration/deceleration [2,14,16,17]. It should also be emphasized that the scheme requires the toolpath geometry to be communicated to the real-time interpolator as a sequence of extended analytic curve segments -it is not compatible with part programs employing numerous short linear/circular G code segments.…”

Modelling of the Relationship of Adiabatic Shear and Cutting Conditions for Higher Cutting Speed Ranges

“…Although some scholars ascribed the occurrence of serrated chips to a periodic crack initiated in the free surface of the workpiece ahead of the tool [4] and [5]. Other researchers, including Recht [6], Komandui [7], Davies [8], Molinari [9] and Ma [10], attributed the formation of serrated chips to a repeated thermo-plastic instability occurring in the primary shear zone, particularly for ductile materials [2] and [11]. On one side, the occurrence of adiabatic shear in high speed cutting will increase the tool wear and influence the quality of the finished surface.…”