Chip Formation Mechanism of Inconel 718: A Review of Models and Approaches

Inconel 718,a nickel,chrome and iron alloy,has special advantages,such as high-temperature strength,thermal resistance and corrosion resistance,which facilitate wide usage in the aerospace industry,especially in the hot sections of gas turbine engines.However,machining this alloy is correlated closely with the material’s inherent properties such as excellent combination of strength,hardness and toughness,low thermal conductivity and the tendency to adhere to cutting tools.This nickel alloy also contains inclusions of hard abrasive carbide particles that lead to work-hardening of the workpiece material and thus abrasive wear of the cutting tool.That is,the machining of Inconel 718 is always influenced by high mechanical and thermal loads.This article reviews the chip formation mechanism of Inconel 718.One of the main characteristics in machining of Inconel 718 is that it will produce serrated or segmented chips in a wide range of cutting speeds and feeds.Existing studies show that the chip serration or segmentation by shear localization affects the machined surface integrity,and also contributes to the chip’s evacuation and the automation of machining operations.Thus,research conclusion indicates that the serrated or segmented chip phenomenon is desirable in reducing the level of cutting force,and detailed analysis of models and approaches to understand the chip formation mechanism of Inconel 718 is vital for machining this alloy effectively and efficiently.Therefore,this article presents some summaries on the models and approaches on the chip formation in machining of Inconel 718.

ANALYTICAL CHIP FORMATION MODEL OF MICRO-END-MILLING

A new analytical chip formation model is proposed for micro-end-milling operations. The model calculates an instantaneous uncut chip thickness by considering the combination of exact trochoidal trajectory of the tool tip and tool run-out, while the simplified circular trajectory and the neglected run-out create negligible change in conventional-scale chip formation models. Newton-Raphson iterative method is employed during the calculation to obtain quadratic convergence. The proposed approach allows the calculation of instantaneous uncut chip thickness to be done accurately and rapidly, and the prediction accuracy of this model is also verified by comparing the simulation results to experimental cutting forces.

FINITE ELEMENT ANALYSIS FOR CHIP FORMATION IN HIGH SPEED TURNING OPERATIONS BY ARBITRARY LAGRANGIAN EULERIAN METHOD

A two-dimensional finite element （FE） model for the high speed turning operations when orthogonally machining AISI H13 tool steel at 49HRC using poly crystalline cubic boron nitride （PCBN） is described. An arbitrary Lagrangian Eulerian （ALE） method has been adopted which does not need any chip separation criteria as opposed to the traditional Lagrangian approach. Through FE simulations temperature and stresses distributions are presented that could be helpful in predicting tool life and improving process parameters. The results show that high temperatures are generated along the tool rake face as compared to the shear zone temperatures due to high thermal conductivity of PCBN tools.

Experimental investigation of tool wear and chip formation in cryogenic machining of titanium alloys

Titanium alloys are one of the most important design materials for the aircraft industry. The high strength-to-density-ratio and the compatibility with carbon fibre reinforced plastic are the reasons for a raising application in this field. The outstanding properties lead to challenging machining processes. High strength and low heat conductivity affect high mechanical and thermal loads for the cutting edge. Thus, the machining process is characterized by a rapid development of tool wear even at low cutting parameter. To reach a sufficient productivity it is necessary to dissipate the resulting heat from the cutting edge by a coolant. Therefore the cryogenic machining of two different titanium alloys is investigated in this work. The results point out the different behavior of the machining processes under cryogenic conditions because of the reduced thermal load for the cutting tool. According to this investigation, the cryogenic cooling with COa enables an increase of the tool life in comparison to emulsion based cooling principles when machining the α＋β-titanium alloy Ti-6Al-4V. The machining process of the high strength titanium alloy Ti-6Al-2Sn-4Zr-6Mo requires an additional lubrication realized by a minimum quantity lubrication （MQL） with oil. This combined cool- ing leads to a smoother chip underside and to slender shear bands between the different chip segments.

Finite element analysis of chip formation and residual stresses induced by sequential cutting in side milling with microns to sub- micron uncut chip thickness and finite cutting edge radius

In this paper, the effect of four sequential cuts in side milling of Ti6Al4V on chip formation and residual stresses （RS） are investigated using finite element method （FEM）. While the open literature is limited mainly to the studies of orthogonal sequential cutting with the constant uncut chip thickness greater than 0.01 mm, it is suggested herein to investigate not only the variable uncut chip thickness which characterises the down milling process, but also the uncut chip thickness in the sub-micron range using a finite cutting edge radius. For the resulting ductile machining regime, the characteristics of the chip mor- phology, the force profiles, the plastic deformation and temperature distributions have been analyzed. Furthermore, this study revealed that the RS should be extracted toward the area where the insert exits the workpiece in the FE simulation of the down-milling process. The simulation of a number of sequential cuts due to the consecutive engagements of the insert is required in order to capture the gradual accumulation of the RS before reaching an asymptotic convergence of the RS profile. The predicted RS are in reasonable agreement with the experimental results.

Numerical study via total Lagrangian smoothed particle hydrodynamics on chip formation in micro cutting

Numerical simulation is an effective approach in studying cutting mechanism.The widely used methods for cutting simulation include finite element analysis and molecular dynamics.However,there exist some intrinsic shortcomings when using a mesh-based formulation,and the capable scale of molecular dynamics is extremely small.In contrast,smoothed particle hydrodynamics(SPH)is a candidate to combine the advantages of them.It is a particle method which is suitable for simulating the large deformation process,and is formulated based on continuum mechanics so that large scale problems can be handled in principle.As a result,SPH has also become a main way for the cutting simulation.Since some issues arise while using conventional SPH to handle solid materials,the total Lagrangian smoothed particle hydrodynamics(TLSPH)is developed.But instabilities would still occur during the cutting,which is a critical issue to resolve.This paper studies the effects of TLSPH settings and cutting model parameters on the numerical instability,as well as the chip formation process.Plastic deformation,stress field and cutting forces are analyzed as well.It shows that the hourglass coefficient,critical pairwise deformation and time step are three important parameters to control the stability of the simulation,and a strategy on how to adjust them is provided.

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

Imaging techniques have been widely implemented to study the dynamics of chip formation.They can offer a direct method and a full field measurement of the cutting process,providing kinematic information of the chip formation process.In this article,the state of the art of the imaging techniques reported in the literature has been summarized and analyzed.The imaging techniques have been applied to study the chip formation mechanism,friction behavior,strain/strain rate,and stress fields.Furthermore,the study of surface integrity has been advanced by deriving the thermo‐mechanical loading,subsurface deformation,and material constitutive model from the imaging technique.Finally,achievements in the area of imaging techniques have been summarized,followed by future directions for their application in the study of surface integrity.

Stability of turning processes for periodic chip formation

The prediction of chatter vibration is influencedby many known complex phenomena and is uncertain. Wepresent a new effect that can significantly change the sta-bility properties of cutting processes. It is shown that themicroscopic environment of chip formation can have alarge effect on its macroscopic properties. In this work, acombined model of the surface regeneration effect and chipformation is used to predict the stability of turning pro-cesses. In a chip segmentation sub-model, the primaryshear zone is described with a corresponding materialmodel along layers together with the thermodynamicbehavior. The surface regeneration is modeled by the time-delayed differential equation. Numerical simulations showthat the time scale of a chip segmentation model is sig-nificantly smaller than the time scale of the turning process;therefore, averaging methods can be used. Chip segmen-tation can decrease the average shear force leading todecreased cutting coefficients because of the non-lineareffects. A proper linearization of the equation of motionleads to an improved description of the cutting coefficients.It is shown that chip segmentation may significantlyincrease the stable domains in the stability charts; fur-thermore, by selecting proper parameters, unbounded sta-bility domains can be reached.

Experimental Investigation of Material Removal in Elliptical Vibration Cutting of Cortical Bone

To benefit tissue removal and postoperative rehabilitation,increased efficiency and accuracy and reduced operating force are strongly required in the osteotomy.A novel elliptical vibration cutting(EVC)has been introduced for bone cutting compared with conventional cutting(CC)in this paper.With the assistance of high-speed microscope imaging and the dynamometer,the material removals of cortical bone and their cutting forces from two cutting regimes were recorded and analysed comprehensively,which clearly demonstrated the chip morphology improvement and the average cutting force reduction in the EVC process.It also revealed that the elliptical vibration of the cutting tool could promote fracture propagation along the shear direction.These new findings will be of important theoretical and practical values to apply the innovative EVC process to the surgical procedures of the osteotomy.

3D finite element prediction of chip flow, burr formation, and cutting forces in micro end-milling of aluminum 6061-T6

Abstract Predictive models for machining operations have been significantly improved through numerous methods in recent decades. This study proposed a 3D finite element modeling （3D FEM） approach for the micro end-milling orAl6061-T6. Finite element （FE） simulations were performed under different cutting conditions to obtain realistic numerical predictions of chip flow, burr formation, and cutting forces. FE modeling displayed notable advantages, such as capability to easily handle any type of tool geometry and any side effect on chip formation, including thermal aspect and material property changes. The proposed 3D FE model considers the effects ofmiU helix angle and cutting edge radius on the chip. The prediction capability of the FE model was validated by comparing numerical model and experimental test results. Burr dimension trends were correlated with force profile shapes. However, the FE predictions overestimated the real force magnitude. This overestimation indicates that the model requires further development.

Cutting Behavior of Cortical Bone in Different Bone Osteon Cutting Angles and Depths of Cut

Cortical bone is semi-brittle and anisotropic,that brings a challenge to suppress vibration and avoid undesired fracture in precise cutting process in surgeries.In this paper,a novel analytical model is proposed to represent cortical bone cutting processes.The model is utilized to predict the chip formations,material removal behavior and cracks propagation under varying bone osteon cutting angles and depths.Series of orthogonal cutting experiments were conducted on cortical bone to investigate the impact of bone osteon cutting angle and depth of cut on cutting force,crack initialization and propagation.The observed chip morphology highly agreed with the prediction of chip formation based on the analytical model.The curly,serrated,grainy and powdery chips formed when the cutting angle was set as 0°,60°,90°,and 120°,respectively.Cortical bone were removed dominantly by shearing at a small depth of cut from 10 to 50μm,and by a mixture of pealing,shearing,fracture and crushing at a large depth of cut over 100μm at different bone osteon angles.Moreover,its fracture toughness was calculated based on measured cutting force.It is found that the fluctuation of cutting force is suppressed and the bone material becomes easy to remove,which attributes to lower fracture toughness at bone osteon cutting angle 0°.When the cutting direction develops a certain angle to bone osteon,the fracture toughness increases then the crack propagation is inhibited to some extent and the fluctuation of cutting force comparatively decreases.There is a theoretical and practical significance for tools design and operational parameters choice in surgeries.

Microstructure evaluation of shear bands of microcutting chips in AA6061 alloy under the mechanochemical effect

The mechanochemical effect on the microcutting of AA6061 alloy is studied through characterization on the chip. A pronounced reduction of machining forces and chip thickness was observed with mechanochemical effect during microcutting. Furthermore, electron backscattered diffraction(EBSD) and transmission electron microscopy(TEM) observations were performed on the chips and shear bands. The result reveals much coarser grains(24.6 μm in size) in the surfactant-affected chip than that in the surfactant-free chip(13.5 μm). Different grain orientations are induced by microcutting. {100}<001> and{110}<112> grain orientations are majority for surfactant-free chip, and {110}<001> and {100}<110>dominate most for all grain orientations for surfactant-affected chip. Additionally, since less localized shear strain and lower temperature are generated inside the shear band with mechanochemical effect,almost no recrystallization phenomena can be observed in this region. TEM analysis shows that fewer subgrains and dislocations could be observed inside the shear band of the surfactant-affected chip in comparison with the surfactant-free chip. Based on the high-resolution transmission electron microscopic(HRTEM) observations, dislocations were observed at the atomic scale. The results show that the main dislocation motion mode in shear bands of surfactant-free and surfactant-affected chips are dislocation climb and dislocation glide, respectively.

Feasibility study of creep feed grinding of 300M steel with zirconium corundum wheel

The ultrahigh strength 300M steel has been commonly used in the manufacture of aircraft landing gear and rotor shaft parts due to its excellent mechanical properties.Creep feed grinding is one of the essential operations during the whole component manufacturing processes.In this work,the feasibility of creep feed grinding of 300M steel by using the hard zirconium corundum wheel was theoretically and experimentally evaluated.A variety of responses including grinding forces,temperature fields,specific grinding energy,surface integrity and chip modes were carefully recorded.Besides,the mechanism of ground surface profile generation and the spatial frequency spectrum of the surface profile were tentatively analyzed.It was found that the wheel speed has a relative influence on the grinding forces and temperatures of which the work hardening effect dominates the material removal with lower wheel speed while the thermal softening becomes crucial as the wheel speed exceeds the critical value for the studied 300M steel.Furthermore,a scattered spatial frequency spectrum for the generated surface profile was noticed with lower wheel speed while the spectrum gathers towards the lower frequency values with higher amplitude as the wheel speed increases.The shearing chip and flowing chip dominates the main chip type,indicating the excellent abrasive sharpness during the grinding process.In general,the used zirconium corundum wheel presents feasibility for the creep feed grinding of 300M steel because of the high material removal rate,absence of surface burn,low wheel wear and higher compressive residual stresses.

Molecular dynamics modeling of a single diamond abrasive grain in grinding

In this paper the nano-metric simulation of grinding of copper with diamond abrasive grains, using the molecular dynamics （MD） method, is considered. An MD model of nano-scale grinding, where a single diamond abrasive grain performs cutting of a copper workpiece, is presented. The Morse potential function is used to simulate the interactions between the atoms involved in the procedure. In the proposed model, the abrasive grain follows a curved path with decreasing depth of cut within the workpiece to simulate the actual material removal process. Three different initial depths of cut, namely 4 A, 8 A and 12 A, are tested, and the influence of the depth of cut on chip formation, cutting forces and workpiece tempera- tures are thoroughly investigated. The simulation results indicate that with the increase of the initial depth of cut, average cutting forces also increase and therefore the temperatures on the machined surface and within the workpiece increase as well. Furthermore, the effects of the different values of the simulation variables on the chip formation mechanism are studied and discussed. With the appropriate modifications, the proposed model can be used for the simulation of various nano-machining processes and operations, in which continuum mechanics cannot be applied or experimental techniques are subjected to limitations.