Chip Morphology and Chip Formation Mechanisms During Machining of ECAE-Processed Titanium

Abstract

Severe plastic deformation (SPD) processing such as equal channel angular extrusion (ECAE) has been pioneered to produce ultrafine grained (UFG) metals for improved mechanical and physical properties. However, understanding the machining of SPD-processed metals is still limited. This study aims to investigate the differences in chip morphology when machining ECAE-processed UFG and coarse-grained (CG) titanium (Ti) and understand the chip formation mechanism using metallographic analysis, digital imaging correlation (DIC), and nano-indentation. The chip morphology is classified as aperiodic saw-tooth, continuous, or periodic saw-tooth, and changes with the cutting speed. The chip formation mechanism of the ECAE-processed Ti transitions from cyclic shear localization within the low cutting speed regime (such as 0.1 m/s or higher) to uniform shear localization within the moderately high cutting speed regime (such as from 0.5 to 1.0 m/s) and to cyclic shear localization (1.0 m/s). The shear band spacing increases with the cutting speed and is always lower than that of the CG counterpart. If the shear strain rate distribution contains a shift in the chip flow direction, the chip morphology appears saw-tooth, and cyclic shear localization is the chip formation mechanism. If no such shift occurs, the chip formation is considered continuous, and uniform shear localization is the chip formation mechanism. Hardness measurements show that cyclic shear localization is the chip formation mechanism when localized hardness peaks occur, whereas uniform shear localization is operative when the hardness is relatively constant.

Study of the Shear Strain and Shear Strain Rate Progression During Titanium Machining

“…Figures 4(a)-4(e) show the saw-tooth chip morphology using a cutting speed of 0.1 m/s, which has been commonly reported during machining of titanium and its alloys using a wide range of cutting speeds [2,[30][31][32][33][34]. Its chip morphology is considered aperiodic saw-tooth at a cutting speed of 0.1 m/s based on the chip surface topography and the chip formation images [35] even that the chip free surface looks like folds. However, as the cutting speed increases to 0.5 m/s, the saw-tooth features at the free surface are suppressed and a continuous chip forms as shown in Figs.…”

Dynamics of chip formation during the cutting process using imaging techniques: A review

“…They noticed that the built‐up edge structure obtained at the highest cutting speed was thinner than the lowest one. The differences in chip morphology during machining were investigated by Davis et al 84 They used the DIC technique to determine the shear strain rate during chip formation. They found that if the shear strain rate distribution contains a shift in the chip flow direction, the chip morphology has a saw‐tooth pattern; otherwise, the chip formation is continuous.…”

“…(C) Saw-tooth segment and (D) continuous chip. Reproduced with permission 84. Copyright 2018, ASME, formed during the cutting process F I G U R E 13 (A) Tool-chip friction schematic.…”

Digital image correlation for sensing kinematic fields in manufacturing processes: a review

“…To date, the research of the cutting process by using the DIC technique is mainly focused on two conditions: studying low-speed cutting with a high-speed camera and high-speed cutting observation with a double-frame camera. For instance, Davis et al investigated the chip-formation mechanism using the DIC technique with an in situ high-speed camera in which the chip morphology is characterized according to its cross-sectional geometry using different cutting speeds (Davis et al, 2018a). Also, Davis et al further studied the strain and strain rate progression with the primary shear zone during chip formation based on the accurate measurement provided by the DIC technique with a high-speed camera.…”