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To investigate the unique hook-shaped residual stress profile generated from hard turning process, an improved orthogonal (2-D) Finite Element (FE) model is established to include the ploughing effect of cutting edge. The model is further decomposed into two FE sub-models (sub-model 1 and sub-model 2) to determine the thermal and mechanical effects on the residual stress profiles by saw-tooth chip formation process and honed-edge ploughing process respectively. The two FE sub-models are sequentially adopted to evaluate the compression effect induced by chip formation process and ploughing effect resulted from honed-edge cutting tool on residual stress profile. Their separated and integrated effects on residual stress hook-shape profile are addressed by comparing the predicted residual stresses by sub-model 1, sub-model 2, the two sub-models’ superposition, and the whole improved FE model. The results show that chip formation effect on residual stress profile happens earlier than the ploughing effect. Chip formation effect provides a foundation for the finalized residual stress profile by determining the maximum depth and magnitude of the compressive residual stress. Ploughing process generates much more thermal load to produce the tensile residual stress in hard turned surface and sequentially drives the final resultant residual stress into an obvious hook-shaped by modifying the previous compressive residual stress profile. The location with the maximum compressive residual stress is identified as the critical position to separate the mechanical load and thermal load generated from ploughing effect. The decomposition methodology on mechanical and thermal effects is proposed and thoroughly discussed in the paper.Copyright © 2012 by ASME

To explore the effects of cutting speed, feed rate and rake angle on chip morphology transition, a thermomechanical coupled orthogonal (2-D) finite element (FE) model is developed, and to determine the effects of tool nose radius and lead angle on hard turning process, an oblique (3-D) FE model is further proposed. Three one-factor simulations are conducted to determine the evolution of chip morphology with feed rate, rake angle, and cutting speed, respectively. The chip morphology evolution from continuous to saw-tooth chip is described by means of the variations of chip dimensional values, saw-tooth chip segmental degree and frequency. The results suggest that chip morphology transits from continuous to saw-tooth chip with increasing feed rate and cutting speed, and changing a tool’s positive rake angle to negative rake angle. There exists a critical cutting speed at which the chip morphology transfers from continuous to saw-tooth chip. The saw-tooth chip segmental frequency decreases as the feed rate and the tool negative rake angle value increases; however, it increases almost linearly with the cutting speed. The larger negative rake angle, the larger feed rate and higher cutting speed dominate saw-tooth chip morphology while positive rake angle, small feed rate and low cutting speed combine to determine continuous chip morphology. The 3-D FE model considers tool nose radii of 0.4 mm and 0.8 mm, respectively, with tool lead angels of 0 deg and 7 deg. The model successfully simulates 3-D saw-tooth chip morphology generated by periodic adiabatic shear and demonstrates the continuous and saw-tooth chip morphology, chip characteristic line and the material flow direction between chip-tool interfaces. The predicted chip morphology, cutting temperature, plastic strain distribution, and cutting forces agree well with the experimental data. The oblique cutting process simulation reveals that a bigger lead angle results in a severer chip deformation, the maximum temperature on the chip-tool interface reaches 1289 deg, close to the measured average temperature of 1100 deg; the predicted average tangential force is 150N, with 7% difference from the experimental data. When the cutting tool nose radius increases to 0.8 mm, the chip’s temperature and strain becomes relatively higher, and average tangential force increases 10N. This paper also discusses reasons for discrepancies between the experimental measured cutting force and that predicted by finite element simulation.

To evaluate the residual stress distribution along cutting direction in hard turning process, an explicit dynamic thermo-mechanical orthogonal Finite Element Model (FEM) is developed to consider the correlation between residual stress distribution and chip morphology and plough effect by cutting edge. The FEM adopts Johnson-Cook (J-C) model to describe work material property, the critical equivalent plastic strain criterion to simulate chip separation behavior, and the revised coulomb’s law to capture the friction pattern between the tool and chip interface. The FEM is validated by comparing the predicted and experimental chip morphology and residual stress distribution. The residual stress distribution in hard machined surface along cutting direction is accurately captured by using sharp and honed cutting edge tools. The residual stresses by sharp tool demonstrate a periodical characteristic, the fluctuation amplitudes are determined in the surface and subsurface along the cutting direction, and the fluctuation frequency corresponds to that of the saw-tooth chip. However, the residual stresses by honed cutting edge tool demonstrate an indistinct periodic characteristic, the fluctuation frequency in surface and subsurface is larger than that of the saw-tooth chip. Saw-tooth chip formation process by sharp tool is identified to analyze the residual stress scatter periodic mechanism, which associates with the fluctuation of cutting force and temperature. The plough process by honed cutting edge tool is identified to explain the equilibrium effect on the amplitude and frequency of residual stress scatter in hard turned surface and subsurface. The periodical fluctuation characteristics of residual stress in hard turned surface and subsurface is revealed and verified by determining its amplitude and frequency corresponding to that of the saw-tooth chip. The analysis will enhance the fatigue life prediction accuracy by incorporating the effect of residual stresses periodical fluctuation on the crack initiation and propagation life in hard turned surface and subsurface.Copyright © 2011 by ASME

This paper proposes a thermo-mechanical orthogonal cutting finite element model (FEM) to investigate the variation of chip morphology from continuous chip to small and large saw-tooth chip. The corresponding experiments of hard turning AISI 52100 steel are conducted to validate the proposed FE model. Three one-factor simulation experiments are conducted to determine the evolution of chip morphology along feed rate, rake angle and cutting speed respectively. The chip morphology evolution is described by the variations of dimensional values, saw-tooth degree and chip segmental frequency. The research suggests that chip morphology transit from continuous to sawtooth chip with increasing the feed rate and cutting speed, and changing a positive rake angle to a negative rake angle. There exists a critical cutting speed at which the chip morphology transfers from continuous to saw-tooth chips. The saw-tooth chip segmental frequency decreases as the feed rate and negative rake angle value increase, but increases almost linearly with the cutting speed. The larger negative rake angle, the larger feed rate and high cutting speed dominate the sawtooth chip morphology while positive rake angle, small feed rate and low cutting speed determine continuous chip morphology.

To analyze AISI 52100 steel response in hard turning under pre-stressed conditions, an explicit dynamic thermo-mechanical finite element model (FEM) has been developed. The FEM adopts Johnson-Cook constitutive model to describe the workpiece material property; and Johnson-Cook failure model as chip separation criterion; a modified coulomb’s friction law determines the friction behavior at the tool/chip interface. 500MPa of tensile and compressive pre-stress are imposed on the workpiece to simulate the pre-tension-stress and pre-compression-stress conditions respectively in hard turning. The effect of pre-stress on hard turning is determined by analyzing and comparing the simulation results under the three different pr-stress conditions, in terms of saw-tooth chip morphology, cutting forces, plastic strain distribution and temperature distribution on chip, plastic strain distribution and temperature distribution in machined surface, and especially the residual stress in machined surface. It identifies hard cutting under pretension-stress condition is an effective approach to generate a beneficial compressive residual stress profile in hard turning characterized by a bigger value of compressive residual stress and a deeper penetration depth into the hard turned surface which are helpful to enhance the fatigue life of machined components.Copyright © 2010 by ASME