Balkrishna C. Rao

The accuracy of prediction made by finite element modeling of machining processes significantly depends on the constitutive model used to describe the material behaviour at typical values of strain, strain rate and temperature encountered in cutting. In this paper a new constitutive relationship is established by modifying the Zerilli-Armstrong model based on the concept of dislocation mechanics. The efficacy of the proposed model has been demonstrated by implementing it as a constitutive law for Inconel 718, a nickel based super alloy. The constitutive data generated by using the distributed primary zone deformation model is utilised for calculating the material constants. The efficacy of the proposed model is validated by using a three-pronged evaluation process. The first evaluation procedure involved comparing flow stress predictions made by the proposed model with data generated by the distributed primary zone deformation model. The second evaluation process involved finite element simulations of the orthogonal machining process with the proposed material model as the constitutive relation. A user material subroutine is used for implementing the proposed constitutive relation into a finite element model of orthogonal machining process through ABAQUS/Explicit platform. Good agreement has been achieved between model predictions and experimental data in simulating chip formation with attendant cutting forces. In the third evaluation process, proposed constitutive law is further validated by comparing its flow stress predictions with the split Hopkinson pressure bar test data available in literature.

Large plastic deformation through cold-rolling refines microstructure through the accumulation of plastic strains over multiple stages. Machining achieves similar large plastic strains (1–10) in a single stage to produce ultra-fine-grained chips. Effect of cold-rolling and machining on microstructure and mechanical properties of Al6061 was investigated. As-received, solution heat-treated, and peak-aged plates were cold-rolled to 30, 50, and 70% thickness reductions. Ultra-fine-grained chips were produced from low-speed orthogonal-machining under plane-strain condition, using a restricted contact tool to minimize the chip curvature. Grains were equiaxed in as-received, solution heat-treated, and peak-aged bulk samples, while they were elongated in cold-rolled bulk. In chips, grains were elongated in one direction due to severe plastic flow. Hardness and ultimate tensile strength increased with thickness reduction. Chip hardness is 60% more than as-received material due to microstructure refinement. Metal cutting (single-stage process) and thickness reduction greater than 50% by cold-rolling (multi-stage) provide nearly the same enhancement in mechanical properties (40% more than bulk).

The results of the investigation on the influence of texture on chip formation during machining of Ti–6Al–4V alloy are reported in this work. Different textures are produced in this alloy by cold-rolling as-received and annealed material to 30–47% reductions. As-received and annealed samples exhibited basal and transverse textures while in cold-worked material, the basal texture disappeared and, a texture component with (0001) pole inclined at 70° from the Z direction was observed. Machining was done in these thermo-mechanically processed plates at a low cutting speed of 0.8 m/min to avoid temperature rise and the consequent microstructural modification. As-received and annealed material exhibited saw-tooth morphology of chips, while samples deformed to cold-work of 40% and above produced continuous chips with fine serrations at the free surface. Machined chips have retained the transverse texture in annealed material with reduced intensity, while the basal texture has disappeared in deformed material. The possible slip systems responsible for the development of these textures were identified using Schmid factors. The difference in morphology of chips between annealed and cold-worked samples is explained based on texture developed during machining, which has a bearing on the strain necessary for fracture initiation in the primary shear zone, thus affecting the spatial frequency of segmentation.