Primary Shear Zone Research Articles

Investigation of temperature distribution in orthogonal cutting through dual-zone contact model on the rake face

A two-dimensional analytical model to calculate the temperature distribution in orthogonal cutting with dual-zone contact on the rake face is presented. The study considers heat generation in the primary shear zone and on the rake face. The material behavior in the primary shear zone is represented by Johnson-Cook constitutive equation while the contact on the rake face is modeled by sticking and sliding friction zones. This new temperature distribution model is used to determine the maximum temperature on the rake face and two-dimensional temperature distribution in the chip and on the tool surface. The dual-zone contact model on the rake face and convection boundary condition on the flank face are the important contributions of this work. The simulation results of the developed model are compared with experimental results where a good agreement is demonstrated.

Modelling of Oblique Cutting Process using Coulombs Friction Law for Ti-6Al-4V Alloy

For the machining of the materials, the temperature aspect plays an important role in the forces evolved during the process. This includes the temperature evolved in the primary shear zone as well as the tool-chip interface. To reduce the forces thus one of the priorities in machining process to get a better surface finish and longer tool life. High speed machining process is being widely used in the automobile and aeronautical industries these days where superfine surface finish is the basic requirement. For such a process, a force modelling is done that considers the friction law which is the function of the tool-chip interface temperature which is in turn used to calculate different parameters and ultimately the forces evolved in machining process. These can be further extended to the Ball End Mill where the flute can be divided into infinitesimal elements such that each element can be considered as an oblique cutting edge

Experimental investigation of the shear angle variation during orthogonal cutting

In this paper, we investigate the variation of the shear angle during orthogonal cutting processes. In orthogonal cutting, the material in front of the cutting edge is sheared over a primary shear zone that is approximated by the shear plane. The so-called shear angle corresponding to the shear plane has significant role in theoretical cutting force models and in the dynamics of cutting. This paper is dedicated to determining the shear angle experimentally for stationary and non-stationary cutting based on high-speed camera recordings and cutting force measurements. Furthermore, the relationship between simple theoretical shear angle models and the measured shear angle variation is also provided.

3D milling modeling: mechanical actions, strains, strain rates and temperature calculations in the three cutting zones

Analytical and semi-analytical modelling of manufacturing processes involving material removal are of great interest to scientists and industrialists. With this type of modelling, we are able to identify optimal cutting parameters based on geometric and thermomechanical quantities, without having to carry out experimental trials or costly simulations (thus saving time and reducing costs). Compared with other machining techniques, milling involves additional complexities arising from the variation in geometric parameters in the machining configuration and in kinematic parameters when operational. This paper presents a new 3D modelling analysis applied to milling, which takes into account phenomena generated by the three-dimensional kinematics of the process. To complete this thermomechanical approach to cutting, improvements have been made to a basic model configuration. The model that has been developed can now map strains, strain rates, stresses and temperatures along the cutting edge, in the primary, secondary and tertiary shear zones. Forces and cutting moments at the theoretical tool tip are estimated at a local then a global scale, and compared with experimental results from previous work.

The influence of cutting parameters on the defect structure of subsurface in orthogonal cutting of titanium alloy

Abstract

Improved analytical prediction of chip formation in orthogonal cutting of titanium alloy Ti6Al4V

The aim of this paper is to propose an analytical model of chip formation for precise prediction of orthogonal cutting of Ti6Al4V. This alloy is used broadly in aerospace components; hence, prediction of thermomechanical parameters of its orthogonal cutting is crucial for various industrial applications. The suggested analytical model needs only cutting parameters and tool geometry as input; it can predict not only cutting forces but also main features of a primary shear zone and a tool-chip interface. A non-equidistant shear zone model is employed to calculate shear strains and a shear strain rate in the primary shear zone, and a simplified heat-transfer equation is used for temperature. A Calamaz-modified Johnson–Cook material model that accounting for flow softening at high strains and temperature-dependent flow softening is applied to assess shear stresses in the primary shear zone. In addition, a shear-angle solution is modified for Ti6Al4V. At the tool-chip interface, a contact length, equivalent strain and an average temperature rise are defined. Besides, the effect of sliding and apparent friction coefficients is investigated. For a sawtooth chip produced in the cutting process of Ti6Al4V, the segmented-chip formation is analysed. A chip-segmentation frequency and other parameters of the sawtooth chip are also obtained. The predicted results are compared with experimental data with the cutting forces, tool-chip contact length, shear angle and chip-segmentation frequency calculated with the developed analytical model showing a good agreement with the experiments. Thus, this analytical model can elucidate the mechanism of the orthogonal cutting process of Ti6Al4V including predictive capability of continuous and segmented chip formation.

Subsurface Deformation Generated by Orthogonal Cutting: Analytical Modeling and Experimental Verification

Subsurface deformation of the cutting process has attracted a great deal of attention due to its tight relationship with subsurface hardening, microstructure alteration, grain refinement, and white layer formation. To predict the subsurface deformation of the machined components, an analytical model is proposed in this paper. The mechanical and thermal loads exerted on the workpiece by the primary and tertiary shear zones are predicted by a combination of Oxley's predictive model and Fang's slip line field. The stress field and temperature field are calculated based on contact mechanics and the moving heat source theory, respectively. However, the elastic–plastic regime induced by the material yielding hinders the direct derivation of subsurface plastic deformation from the stress field and the work material constitutive model. To tackle this problem, a blending function of the increment of elastic strain is developed to derive the plastic strain. In addition, a sophisticated material constitutive model considering strain hardening, strain rate sensitivity, and thermal softening effects of work material is incorporated into this analytical model. To validate this model, finite element simulations of the subsurface deformation during orthogonal cutting of AISI 52100 steel are conducted. Experimental verification of the subsurface deformation is carried out through a novel subsurface deformation measurement technique based on digital image correlation (DIC) technique. To demonstrate applications of the subsurface deformation prediction, the subsurface microhardness of the machined component is experimentally tested and compared against the predicted values based on the proposed method.

Microstructure Analysis of Segmented Chips of Super Duplex Stainless Steel Sandvik SAF 2507™ Using Electron Microscopy Techniques

Abstract Machined chips of Sandvik SAF 2507™ which is a super duplex stainless steel (SDSS), are known to exhibit segmentation. These segmented chips are known to contain primary shear zones (PSZs) and secondary shear zone (SSZs) zones. PSZ regions exhibit adiabatic shear bands (ASBs). To analyze such ASBs, electron backscatter diffraction (EBSD) is an excellent technique. However, due to severe deformation and grains in the range of a few nanometers, the information obtained is limited when using conventional EBSD (c-EBSD) and instead, Transmission Kikuchi Diffraction (TKD)/transmission EBSD (t-EBSD) can be used with great advantage. In this work, TKD and c-EBSD techniques were used to analyze PSZs/ASBs and SSZs regions, respectively. The backscattered electron (BSE) images of PSZs/ASBs clearly showed alternate bands of austenite and ferrite with very fine elongated grains along the shear direction. On the other hand, the SSZs exhibited alternate regions of equiaxed and non-equiaxed or deformed grains. Furthermore, c-EBSD showed that ferrite was significantly recrystallized and austenite was deformed to a great extent. Similar observations were also noticed in the PSZs/ASBs using TKD: that ferrite recrystallizes to a greater extent than austenite. Another interesting observation was that in the regions farther away from the SSZs, the grains adjacent to PSZs/ASBs exhibit higher numbers of low angle grain boundaries (LAGBs) than the grains in the middle of the segments. This could be attributed to higher extent of deformation in the neighboring regions of severely deformed PSZs/ASBs.

A unified analytical cutting force model for variable helix end mills

The paper proposes a unified analytical cutting force model based on a predictive machining theory for variable helix end mill considering cutter runout. The variable helix end mill is divided into a set of differential oblique elements along the axial direction. The cutting process of oblique element is based on the non-equidistant shear zone model and the equivalent plane method. The cutting forces of oblique element are modeled by shearing force components due to shearing at the shear zone and edge force components due to rubbing in the tertiary zone. In the primary shear zone, a modified Johnson-Cook model is introduced to account for the material size effect affected by varying instantaneous uncut chip thickness (IUCT) during milling process. In the tertiary zone, edge radius and the partial effective rake angle are included in the analytical model in order to take into account the rubbing effect precisely. The total instantaneous cutting forces are obtained by summing up the cutting forces acting oblique elements on all flutes. The unified analytical cutting force model is verified by experimental data using four different types of end mills, and a good agreement of the predicted and measured cutting forces shows that the proposed model is valid for variable helix end mills.

Predictive analytical modeling of cutting forces generated by high-speed machining of ductile and hard metals

ABSTRACTThis paper proposes an analytical cutting forces model based on an extension of the Oxley's machining theory (OMT) to high-speed machining of ductile and hard metals. In this model, the materials' behavior was modeled using the Marusich's constitutive equation (MCE). Furthermore, The OMT was modified to be able to capture the effects of the cutting tool edge radius and the burnishing phenomenon by implementing a variable rake angle equation and the Briks criterion, respectively. The predictive model was validated using experimental data obtained during the orthogonal machining of two aluminum alloys (AA6061-T6 and AA7075-T651) and induction-hardened AISI4340 steel (58-60 HRC). The results showed that the predicted and experimental cutting forces were in reasonable agreement for all tested materials. The strain rate constant in the primary shear zone (C0) was found to be significantly sensitive to the cutting conditions and work material, and its effect on the predicted data was highlighted and discussed in depth. On one hand, it was found that AA6061-T6 is less sensitive to the strain rate compared to the AA7075-T651. On the other hand, all tested materials were found to be more sensitive to the strain rate at low cutting speeds and/or cutting feeds.

On the instability of chip flow in high-speed machining

High cutting speed usually gives rise to a breakdown of steady chip flow and results in a serrated flow pattern, which is one of the most fundamental and challenging problems in metal cutting. Here, we systematically analysed the experimental results of high-speed cutting on various typical metallic materials over wide ranges of cutting speeds. With considering the coupling effects of inertial, tool-chip compression and material convection, the critical condition for the onset of serrated chip flow is determined based on a stability analysis of the deformation inside primary shear zone. It is found that the emergence of the serrated chip flow is dominated by a dimensionless number which characterized the competition among the effects of inertia, thermal softening, strain hardening, elastic unloading, viscous diffusion and thermal diffusion. More interestingly, a power law between the serration frequency and the Reynolds thermal number Pe is clearly revealed.

Surface feature formation mechanism during finish milling of gray cast iron

Gray cast iron (GCI) is highly anisotropic at the microscale consisting of stochastically distributed and orientated graphite flakes within a ferric matrix. The anisotropy of the microstructure endows gray cast iron with favorable damping characteristics which makes it a common material for machine tool structural components. However, the microstructure inhibits the formation of a finished surface of sufficient quality for use as a slideway when milled with a defined cutting edge. The mechanisms of irregular surface formation during the machining of GCI were investigated using both a 3-D finite element cutting simulation and milling experiments. Investigation of simulation and machining tests indicates that the interaction of the primary shear zone in front of the cutting edge and graphite flakes is the cause of microcavity formation on the machined surface.

Impact of work hardening, tool wear and geometry response on machinability during turning AL‐6XN super austenitic stainless steel: A work hardening and wear studies on AL‐6XN alloy

This work aimed to identify the work‐hardening regions in shear zones when the AL‐6XN super austenitic stainless steel alloy was machined using a turning machine. A quick‐stop approach was used to generate frozen chip roots. Two cutting speeds of 94 m/min and 65 m/min, a feed rate of 0.2 mm/rev and depth of cut of 1 mm were applied in this study. Microhardness measurements were executed in the hardening region (workpiece–shear zone–chip zone). Results showed that the hardness values were significantly increased in the primary shear zone and the formed chip. The wear of the cutting tool used to machine the alloy was investigated using scanning electron microscope. Excessive notch wear was located on the cutting edge when 65 m/min cutting speed was utilized while at 94 m/min cutting speed, the cutting edge was completely damaged as a flake of the edge was removed due to hard cutting processes. The profile measurement of the cutting tool was executed using an optical profilometer to reveal the changes in the cutting edges locations and profiles and their implication on machining process.

Investigation on size effect of specific cutting energy in mechanical micro-cutting

Mechanical micro-cutting is one of advanced processes to manufacture the miniature parts. The uncut chip thickness is one of the two important parameters in mechanical cutting. The uncut chip thickness and the cutting edge radius are at the same length scale, and the cutting tool extrudes and shears the workpiece with the round cutting edge during micro-cutting process. Size effect of specific cutting energy in micro-cutting is significant in comparison with that in macro cutting. A slip-field model is proposed in this paper to analyze the deformation process of workpiece material in micro-cutting. Johnson–Cook constitutive model, which includes shear strain hardening, shear strain rate hardening, shear temperature softening, and plasticity strain gradient (PSG) effects, is applied to the calculation of the shear flow stress at the primary shear zone (PSZ). Predicted cutting force using the shear flow stress is validated by the micro-orthogonal cutting experiments. The specific cutting energy is also calculated. It shows that the size effect on specific cutting energy in micro-cutting is influenced greatly by the shear strain hardening, shear strain rate hardening, shear temperature softening, and relative cutting length due to the ratio of uncut chip thickness to cutting edge radius. The plasticity strain gradient has effect on the specific cutting energy when the uncut chip thickness is smaller than 25 μm for micro-orthogonal cutting AISI 1045 steel.

Infrared measurement of the temperature at the tool–chip interface while machining Ti–6Al–4V

The challenges associated with machining titanium alloys (e.g., Ti–6Al–4V) are directly related to high cutting tool temperatures due to the low thermal conductivity of these alloys and the heat generated in the primary shear zone and at the tool–chip interface. Transparent yittrium aluminum garnet (YAG) tools are used in the current study to orthogonally machine a Ti–6Al–4V disk. Although YAG tools are not industrially relevant, they permit the temperature on the tool–chip interface to be measured. These measurements are relevant because they can be used to validate cutting models, which are in-turn used by industry to improve cutting processes. An infrared camera, using a high frame rate (700Hz) and a large field of view (20mm2), observes the tool–chip interface through these tools and measures the temperature distribution and records the chip curl and breakage while cutting with a feed rate of 50μm/rev and cutting speeds between 20m/min and 100m/min. In addition to the temperature measurements, cutting forces are recorded and the chip formation is documented using a high-speed (3kHz) visible-light camera. Results show that radiant temperature increases with speed while the cutting and thrust forces show no significant trend. Analysis of the temperature distribution from one edge of the chip to the other reveals differences from 6 % to 21 %, indicating that caution must be used when performing thermographic measurements of chip temperatures from the side of the cutting zone. Finally, post process measurements are performed using a scanning white-light interferometer to investigate any correlation between the tool condition and cutting temperature. Although the qualitative analysis of some cases appears to reveal a correlation between the condition of the YAG tool and the measured temperature distribution, further work work is required to understand this relationship.

Experimental and finite element analysis of machinability of AL-6XN super austenitic stainless steel

This paper presents a finite element cutting model based on physical microstructure to investigate the thermo-mechanical behaviour of AL-6XN Super Austenitic Stainless Steel in the primary shear zone. Frozen chip root samples were created under dry turning operation to observe the plasticity behaviour occurring in the shear zones to compare with the model for analysis. Chip samples were generated under cutting velocities at 65 and 94 m/min, feed rate at 0.2 mm/rev and depth of cut at 1 mm. Temperature on the cutting zone was recorded by infrared thermal camera. Secondary and backscatter electron detectors were used to investigate the deformed microstructure and to calculate the plastic strain. Experimental results showed the formation of microcracks (build-up edge triggers) at the chip root stagnation zone of both samples. The austenite phase patterns were evident against the cutting tool tip in the stagnation zone of the chip root fabricated at 65 m/min. The movement of these patterns caused the formation of the slip lines within the grains. The backscatter diffraction maps showed the formation of special grain boundaries within the slip lines, work-hardening layer and in the chip region. Strain measurements in the microstructures of the chip roots fabricated at 94 and 65 m/min showed high values of 6.5 and 5.7 (mm/mm) respectively. The finite element model was used to measure the stress, strain, temperature and chip morphology. Numerical results were compared to the outcomes of the experimental work to validate the finite element model. The model validating process showed good agreement between the experimental and numerical results, and the error values were calculated. For a 94- and 65-m/min cutting speeds, 7.5 and 5.2% were the errors in the strain, 3 and 2.5% were the error in the temperature and 4.7 and 6.8% were the error in the shear plane angles.

Research on the nanometric machining of a single crystal nickel via molecular dynamics simulation

Molecular dynamics simulations are employed to study the nanometric machining process of single crystal nickel. Atoms from different machining zones had different atomic crystal structures owing to the differences in the actions of the cutting tool. The stacking fault tetrahedral was formed by a series of dislocation reactions, and it maintained the stable structure after the dislocation reactions. In addition, evidence of crystal transition and recovery was found by analyzing the number variations in different types of atoms in the primary shear zone, amorphous region, and crystalline region. The effects of machining speed on the cutting force, chip and subsurface defects, and temperature of the contact zone between the tool and workpiece were investigated. The results suggest that higher the machining speed, larger is the cutting force. The degree of amorphousness of chip atoms and the depth and extent of subsurface defects increase with the machining speed. The average friction coefficient first decreases and then increases with the machining speed because of the temperature difference between the chip and machining surface.

Interaction between the local tribological conditions at the tool–chip interface and the thermomechanical process in the primary shear zone when dry machining the aluminum alloy AA2024–T351

In this work, a predictive machining theory, based on a finite element model, were applied to dry orthogonal cutting of aluminum alloy AA2024–T351. In order to analyze the effects of chip formation process and local friction coefficient on the thermomechanical load along the tool rake face and the round cutting edge, an Arbitrary Lagrangian Eulerian (ALE) model was developed. To validate the present model, the FE results have been compared to the experimental data for a wide range of cutting speed. This shows a good agreement in both trends and values for cutting forces and tool–chip contact length. The ALE approach appears as an appropriate formulation to reproduce: (i) the tribological conditions corresponding to large values of friction coefficient such as in dry machining of aluminum alloys, and (ii) the material flow process around the round cutting edge. It was observed that a transition from a sliding contact to a sticking–sliding contact occurs when the local friction coefficient and the thermal softening are large enough. Predicted results show also that the decrease of the cutting forces as the cutting speed increases is mainly due to the variation of the tool–chip contact length in terms of cutting velocity. The effect of material flow around the round cutting edge on the distributions of frictional stress and pressure has also been analyzed.

Pulsed laser-assisted machining of Inconel 718 superalloy

Nickel-based superalloys including Inconel 718(IN718) are widely used in aerospace industries due to their superior high temperature strength, toughness, and corrosion resistance. These alloys are difficult to machine mainly because of their low thermal conductivity and high work hardening rate, which cause steep temperature gradient and high cutting forces at the tool edge. The application of laser assisted machining is the subject of many new researches since shear forces; surface coarsening and tool wear are reduced. The aim of this investigation was to evaluate laser assisted machining behavior of a 718 Inconel superalloy from the view point of machining specific energy, surface roughness, tool wear and chip appearance. Experimental apparatuses used included optical and scanning electron microscopy, spark emission spectroscopy, and EDS analysis. The results indicated that increasing the temperature to about 540°C just ahead of primary shear zone, can result in 35% reduction of machining specific energy, in comparison with conventional machining. Furthermore, surface coarsening and tool wear were reduced by 22% and 23% respectively. Flank wear was the main deteriorating factor on cutting tools during laser assisted machining. SEM micrographs indicated that increase in temperature has no noticeable effect on finished workpiece surface. Analysis of variance obtained from regression analysis indicated that frequency of laser beam has the most influential effect on temperature. The optimum conditions for laser assisted machining of 718 superalloy is suggested as follows: 80Hz frequency, 400W power, 24m/min cutting speed, and 0.052mm/rev feed rate along with 540°C temperature, 2.51J/mm2 machining specific energy and 130N cutting force.

Enhancing surface integrity by high-speed extrusion machining

High-speed machining (HSM) is an advanced machining technology to form components. However, the poor surface integrity tends to appear due to chip flow instability in HSM. It is found that the surface integrity results from the competition of shear deformation instability between in primary shear zone (PSZ) and in separating shear zone (SSZ). To improve the surface integrity of machined components, the systematic high-speed extrusion machining (HSEM) experiments of magnesium alloy AZ31B with different constraint extrusion factors (CEFs) were carried out. The instability of shear deformation in PSZ is suppressed, and the microwaves on machined surface disappear when CEF is equal to or larger than a certain value. The measurements of the machined surface show that an improvement of surface integrity is achieved if CEF exceeds a certain value. The theoretical model for HSEM was established to elucidate the critical CEF. The underlying physics of surface integrity in HSEM is further revealed. The experimental results verify the validity of the theoretical model.