Influence of Rake Angle and Hardness Ratio on the Formation of Discontinuous Chip during Orthogonal Metal Cutting

In the present investigation, numerical models that accurately predict the chip formation and stress profiles in the work-piece during orthogonal metal cutting were developed using the explicit finite-element method (FEM). More specifically, a damage material model was utilized to capture the work-piece chip formation and simultaneous breakage of the chip into multiple fragments. In the simulation, a rigid steel cutter of different rake angles was moved at different velocities against a stationary work-piece with significantly different hardness values at constant friction for a cutting depth of 1 mm. The variation of cutting forces, stresses and chip morphology were analyzed. The simulation results indicated that the explicit FEM was a powerful tool for simulating metal cutting operations. The rake angle and hardness ratio were found to have significant effect on the chip morphology during metal cutting. The cutting forces were also influenced by rake angle and tool/work-piece hardness ratio. INTRODUCTION Understanding of chip formation mechanism is very important in machining process optimization and improving final part quality. Chip formation affects surface finish, cutting forces, temperature, tool life and dimensional tolerance. Understanding of chip formation mechanism for specific materials allows the determination of machining variables that make the cutting process more efficient and increase tool life during metal cutting. The use of finite element codes has proven to be an effective technique for analyzing material flow in cutting processes. Attempts to apply finite element techniques to machining have been made by many researchers [1, 2]. Most of these studies deal with a static situation (steady state solution) and not with the problem of chip formation and separation. Other models are able to consider the chip formation and separation process but require the use of non-commercial, specific finite element (FE) codes [3]. The model proposed in this study uses the commercial FE code LS-DYNA to simulate the metal cutting and discontinuous chip formation process. The simulations were performed for various tool rake angles at different cutter velocities against a stationary work-piece with significantly different hardness values. The computed cutting forces, deformations and chip morphology have also been compared with experimental data found in literature. MODELING DETAILS In this study, the metal cutting simulations were performed using the commercially available FE code, LS-DYNA. In the simulations, a cutting tool and work-piece (50 mm × 20 mm in dimensions) were considered for modeling. Two-dimensional quadrilateral elements were implemented for both the cutting tool and the work-piece. A mesh of 9550 nodes and 9201 elements were used. The mesh near the contact regions was refined in order to improve the accuracy of the numerical results. Displacement boundary conditions applied for the tool and work-piece were as follows: (a) the bottom nodes of the work-piece were fully constrained in XYZ direction; (b) the work-piece was also constrained in the Z direction and (c) the cutting tool was constrained in the YZ direction. The cutting tool had a rake angle, α, that was varied between +15, 0 and 15. The relief angle of the cutting tool was 15. For a given rake angle, the cutting tool was moved against the stationary work-piece at sliding velocities, v, of 1, 4, 10, 50 and 100 m/s in the X-direction. The depth of cut used in the simulation was 1 mm. The deformation of the cutting tool was assumed to be negligible compared to the work-piece. Material type Rigid_20 [4] was assigned to the cutting tool and the material type, MAT_105 Damage 2 [4] was used for the work-piece. The MAT_105 damage model is basically an elastic visco-plastic material model combined with the continuum damage mechanics (CDM) [4]. The damage constitutive law adopted in models allows defining advanced simulations of tool's penetration in the work-piece and chip formation. In the damage model, the damage parameters were defined such that the performance of the metal cutting simulations appeared reasonable and the mechanisms involved were very close to reality. The various materials and its properties such as Density (ρ), Young’s modulus (E) and Poisson’s ratio (ν) assigned in the simulation for the cutting tool and the work-piece are presented in Table 1. The Vickers hardness (HV) values for these materials are also presented in the same Table. The