MD simulation of exit failure in nanometric cutting

Molecular dynamics (MD) simulations of nanometric cutting were conducted under different cutting conditions to investigate burr formation and exit failure in metals. The effect of relative ductility of the workmaterial (e.g. soft versus hard), tool rake angle (-30 degrees to +60 degrees), depth of cut (1.45-3.62 nm), and external constraint at workmaterial exit on the exit failure and burr formation in nanometric cutting was investigated. Two model FCC materials tone, to represent a ductile material, copper and the other, a somewhat more brittle material) were chosen by varying the appropriate Morse potential parameters in the MD simulation for the latter class. Although negative shear occurs with both cases (ductile and somewhat more brittle workmaterial), positive burr formation with no crack propagation into the workmaterial was observed with the more ductile metal. In contrast, negative burr formation with crack propagation into the workmaterial was observed with the not-so-ductile or somewhat brittle material. The burr dimensions (d and l) measured at pivot initiation point were observed to be higher than the final burr dimensions (t and h). This is attributed to the rotation of the foot and the subsequent change in the pivot point as cutting progresses. The variation of the ratio of final burr dimensions (t/h for both ductile and brittle material) was observed to match closely with the force ratio (F-t/F-c) variation with rake angle in nanometric cutting. This relationship is explained in terms of the cutting forces and the rotation of the resultant force vector with rake angle. The depth of cut was also observed to affect the mode of failure, resulting in positive or negative burr formation. As the undeformed chip thickness was increased, the resulting burr dimensions as well as the distance and depth of the initial pivoting point of the burr from the tool edge was observed to increase. The presence of external elastic constraint at exit resulted in the absence of burr formation. The simulation results were found to be in good agreement with the experimental results reported in the literature. (C) 2001 Elsevier Science B.V. All rights reserved.