Chip Segmentation Research Articles

Decomposition of cutting forces with respect to chip segmentation and white layer thickness when hard turning 100Cr6

This paper deals with hard turning of steel 100Cr6 with a hardness of C. Surface integrity expressed in terms of white layer thickness is studied as a function of tool wear and corresponding thermo-mechanical load of the machined surface. Total force is decomposed into components associated with the chip separation Fγ and the component Fɑ, which is associated mostly with the flank wear land. The results of experiments show that normal and shear components of the force Fɑ, increase progressively with the developed flank wear VB and correlate significantly with the white layer thickness. Furthermore, tool wear is also strongly dependent on the insert rake geometry, which contributes to the cutting force component of Fγ associated with the chip separation. For this reason, the chip formation depends on the progress of the tool wear.

New Phenomenological Material Constitutive Models for the Description of the Ti6Al4V Titanium Alloy Behavior Under Static and Dynamic Loadings

The analysis and optimization of rapid or severe material forming process, where high gradients of plastic deformations, strain rates and temperatures are reached during the material flow, remains today a major challenge. Several light metallic alloys, as the titanium ones, are widely used in many industrial applications. Despite their wide spread adoption, particular phenomena are encountered during their machining: high plastic strains gradients, heavy strain rate localization, chip segmentation, accelerated tool wear, etc. Although the recent advances in the experimental devices, it is still difficult experimentally to investigate the instantaneous mesoscopic phenomena taking place during severe forming processes. Therefore, the use of numerical simulations, in addition to specific experimental measurements, presents an efficient alternative for a better understanding of machining processes. Given thought the significant sensitivity of the modelling to the reliable definition of the thermo-visco-plastic workpiece material behavior and referring to the physically based mesoscopic constitutive model proposed in previous works of Gavrus, this study focuses on formulating and identifying phenomenological rheological laws. Their ability to accurately reproduce the isotropic plastic behavior of the Ti6Al4V titanium alloy for both static and dynamic loadings states, as well as in a wide range of plastic strains, plastic strain rates and temperatures, is checked. A specific iterative non-linear regression method is used for the identification of all corresponding material parameters. Their adequacy is discussed and comparisons with experimental results of the literature are set up. A specific user material subroutine VUHARD© is implemented into the commercial code Abaqus®/Explicit. Numerical simulations of experimental compression tests are performed to valid the rheological models’ identification and the material flow prediction. A 2D Finite Element Modelling of the Ti6Al4V orthogonal cutting is performed and the accuracy of proposed constitutive models is examined.

A three-dimensional finite element-approach to investigate the optimum cutting parameters in machining AA2024

This research work presents a numerical study of the orthogonal cutting process employing a finite element approach to optimize dry machining of aluminium alloy 2024. The main objective of the research work is to perform three-dimensional finite element simulations for a better understanding of temperature distribution and residual stresses development in the workpiece and tool regions along depth of cut direction. While, two-dimensional models don't predict true picture of aforesaid parameters along cutting depth due to material's out of plane flow and deformation. In the present study, effects of tool rake angles (7°, 14°, 21°) and cutting speeds (200, 400, 800 m/min) upon variations in chip geometry at various sections along workpiece width (depth of cut) have been discussed at large. Furthermore, cutting forces and tool-workpiece temperature profiles are also in depth analysed. The findings will lead the manufacturers to better decide post machining processes like heat treatment, deburring, surface treatments, etc. The results showed that a combination of a rake angle of 14° at cutting velocity of 800 m/min produces serrated chip segments with relatively moderate cutting forces in comparison to other parametric combinations. The efficacy of the presented finite element model is verified by comparing the numerically obtained results with experimental ones.

Evaluation on advantages of low frequency assisted drilling (LFAD) aluminium alloy Al7075

Drilling operation of dissimilar stack materials is a crucial assembly operation in airframe manufacturing; this is due to both the number of holes drilled in an aircraft structure and the fact that this is an operation performed in the finishing phase of the manufacturing chain. This paper aims to evaluate an alternative strategy for conventional drilling and peck drilling, which are now the most widespread solutions used for drilling aluminium alloys. The alternative approach proposed by this paper consists of low frequency assisted drilling (LFAD) performed into frequencies vibrations (between 50 to 100 Hz). In this paper, the chip formation process of drilling assisted by low-frequency vibrations of FC/Al stack material has been analytically modelled and compared with the conventional drilling of aluminium. Results show chip segmentation during the drilling operation resulting in less temperature increasing, avoiding problems in the final geometrical quality of the hole, and burr formation.

Sensitivity analysis of the input parameters of a physical based ductile failure model of Ti-6Al-4V for the prediction of surface integrity

Abstract In machining of Ti-6Al-4V, it is commonly reported the appearance of segmented chip produced by adiabatic shearing (at high cutting speeds) and lack of ductility (at low cutting speeds). Moreover, machining is a manufacturing process that is based on applying external energy to the workpiece to produce a separation of a material layer. Thus, to analyze the physics involved in the new surface generation and in the chip segmentation process, it is necessary to apply ductile failure models. However, the characterization of fracture models in machining conditions (temperature, strain rate, stress triaxiality, Lode angle etc.) is an arduous task. Therefore, to define a ductile failure model applicable to machining it is almost inevitable to apply inverse simulations strategies to obtain reliable results in the not tested conditions. Nevertheless, there is few information about the influence of the input parameters of ductile failure model in fundamental outputs and even less in surface integrity aspects. The aim of this research was to conduct a sensitivity analysis of the influence of the input parameters of a physical based ductile failure model not only in fundamental variables (forces, temperatures and chip morphology) but also on surface integrity (surface drag). To this end, a subroutine was developed for the ductile failure model and it was implemented in the Finite Element Method (FEM) software AdvantEdge. Subsequently, using a statistical software and the Design of Experiments (DOE) technique the influence of the input parameters of the failure model on the outputs was analyzed.

Analysis of machining performance of AISI 420 stainless steel using conventional and ultrasonic assisted turning

Conventional Turning (CT) is being used traditionally for machining various materials, but CT lacks productivity, as it results in rapid tool wear and poor surface finish. To overcome these problems, recently, Ultrasonic Assisted Turning (UAT) process has emerged as a potential solution. Recent studies done on the UAT process have shown noticeable enhancement in tool life, chip morphology, and surface finish. These outcomes have attracted the research community towards further development of the UAT process. This study presents a comparison between the UAT and the CT processes for the turning of the AISI 420 stainless steel. In this context, the full factorial methodology is used for designing the experiment. Factors chosen for the experiment are feed and spindle rotation speed having three levels of each. The three responses selected for this study are namely surface roughness parameters (Ra, Rz and, Rt), power consumption, and chip morphology. An approximated reduction of 43% in the surface roughness value Ra and a marginal reduction of 3.13% in power consumption is observed for the UAT process. Discontinuous and segmented chip formation is observed for the UAT process, while the continuous chip is observed for the CT process.

Simulation of segmental chip formation when high-speed cutting steels and titanium alloys

Simulation of segmental chip formation when high-speed cutting steels and titanium alloys

Analysis of flat-end milling forces considering chip formation process in high-speed cutting of Ti6Al4V titanium alloy

This paper proposes a unified numerical and analytical approach to predict flat-end milling forces considering the chip morphology and cutting force in high-speed cutting of titanium alloy (Ti6Al4V). A two-dimensional finite element (FE) model of the orthogonal cutting process is developed by applying a displacement-based ductile failure criterion. With this FE model, the segmented chip formation is analyzed. The mesh dimension is investigated as an effective factor in the chip segmentation. The numerical results demonstrate that the chip morphology is significantly affected from the mesh dimension while the average cutting force varies slightly with the mesh dimension. The mesh dependency of the chip morphology can be decreased by applying the non-local progressive damage model involving the intrinsic material length. An attempt is also made for modeling and prediction of cutting forces in high-speed flat-end milling. The milling force constants which are generally derived from experimental calibrations are required to predict the milling forces by using the unified mechanics of cutting approach. Here, the numerical FE simulations are carried out to characterize the milling force constants. The milling forces predicted analytically are validated by comparing with those obtained from the experimental study. Finally, the behavior of the milling forces can be effectively analyzed through the proposed approach based on the chip formation process.

Machinability Assessment in High Speed Turning of High Strength Temperature Resistant Superalloys

The paper discusses the effect of cutting parameters and cutting tool material on chip compression ratio, cutting forces and surface roughness in turning of high strength temperature resistant superalloys (HSTR). The experiments were performed in dry cutting environment on precision CNC lathe with fixed depth of cut of 0.5[Formula: see text]mm. Analytical model is developed to determine chip segmentation frequency, shear angle and shear strain and it is correlated with the machining parameters. The machinability of the selected superalloys is assessed in terms of cutting force, chip compression ratio and surface roughness. It is found from the experimental analysis cutting force magnitude is less at higher cutting speed for all the superalloys. Chip compression ratio is found maximum in case of Inconel 718 due to precipitation hardening of alloy and followed by Inconel 600 and Inconel 800. The chip segmentation frequency is high at lower cutting speed for Inconel 600 due significant strain hardening. Serrated chips are produced during machining of three selected superalloys and it is found that serrated tooth spacing decreases with cutting speed. Shear plane angle increases on cutting speed increases which effect tool workpiece contact length during machining resulted thin, short and snarled chips was produced. From analytical modeling it shows that shear strain decreases with cutting speed which indicate that at higher cutting speed material deformed elastically than plastically. The effect of cutting tool material is observed on the surface roughness. The better surface finish is obtained with coated carbide inserts as compared to ceramic inserts for all the selected superalloys. However, Inconel 800 shows higher surface roughness due to combination of (Ni–Cr–Fe) alloying element which is responsible for carburization of surface layer during machining.

Effects of Tool Edge Geometry on Chip Segmentation and Exit Burr: A Finite Element Approach

The effects of different tool edge geometries (hone and chamfer (T-land)) on quantitative measurement of end (exit) burr and chip segmentation (frequency and degree) in machining of AA2024-T351 are presented in this work. The finite element (FE) approach is adopted to perform cutting simulations for various combinations of cutting speed, feed, and tool edge geometries. Results show an increasing trend in degree of chip segmentation and end burr as hone edge tool radius or chamfer tool geometry macro parameters concerning chamfer length and chamfer angle increase. Conversely, the least effects for chip segmentation frequency have been figured out. Statistical optimization techniques, such as response surface methodology, Taguchi’s design of experiment, and analysis of variance (ANOVA), are applied to present predictive models, figure out optimum cutting parameters, and their significance and relative contributions to results of end burr and chip segmentation. Various numerical findings are successfully compared with experimental data. The ultimate goal is to help optimize tool edge design and select optimum cutting parameters for improved productivity.

Research on machining compacted graphite iron under oil-on-water cooling and lubrication conditions based on modified material model

Oil-on-water (OoW) techniques, including external OoW sprayed to both the rake face and flank face (EOoWrf), as well as cryogenic air mixed with OoW (CAOoW), improve the machinability of compacted graphite iron (CGI). This paper proposes a modified material model for RuT400 CGI cutting, based on flow softening and weak thermal softening effect, as observed during split-Hopkinson pressure bar tests. Employing a modified material model and Cock-Latham damage model, a thermo-mechanical coupled finite element (FE) model, for RuT400 cutting, is presented. The simulations, considering dry cutting, EOoWrf, and CAOoW, are conducted for the purpose of revealing the causes for the RuT400 difficult-to-cut property and for establishing how the mechanism of OoW can improve cutting performance. The results show that the effective stress, required to form chip segments in RuT400 machining, is much lower than in hardened steel, so RuT400 is more likely to form a serrated chip. In dry cutting, the chip bottom surface is subject to high strain and temperature, leading to the work-material phase transformation, followed by aggravating adhesive and abrasive wears. Although EOoWrf has little influence on chip morphology and effective stress, it reduces the temperature and strain on chip bottom surface by reducing friction, thus suppressing the occurrence of phase transformation. Due to low friction and high heat exchange, CAOoW produces the lowest tool-chip interface temperature and minimal adhesive wear. For high-speed cutting of RuT400, compared to dry cutting, EOoWrf and CAOoW can effectively reduce cutting forces and maintain the tool temperature in a lower range, where low friction on tool-chip interface plays a key role. The proposed FE model can be used in the future to improve CGI’s machinability, by changing cooling parameters, tool geometry/material, and other machining variables.

Effects of Ultrasonic Vibration Assistance on Chip Formation Mechanism in Cutting of Ti–6Al–4V

AbstractThis paper presents the effects of ultrasonic vibration assistance on shear band formation and chip segmentation mechanism in orthogonal cutting of Ti–6Al–4V. Experimental observations of chip microstructure show that the shear bands disappear when vibration assistance is applied along tangential direction at certain cutting speeds. A plastic chip flow model is developed to predict the stress and temperature variations in the primary shear zone at cutting, chip elastic recovery, and tool-chip separation periods. The simulation results show that the temperature in the primary shear zone in vibration-assisted cutting is much lower when compared with conventional cutting, therefore suppresses the generation of shear bands. The simulations of average cutting forces and pitch lengths of chip segments are compared with the experimental results. A finite element model is further developed to prove the temperature reduction when ultrasonic vibration assistance is applied.

Mechanics of Grains Size and Orientation on the Machining of AISI S-7 Steel Grade Using Numerical Technique

Two-dimensional models namely homogeneous, uniformly distributed grains and non-uniformly distributed grains were developed for AISI S-7 steel grade metal, to investigate the effect of the grains size and orientation on the chip morphology and temperature rise during the orthogonal machining process. A Johnson-Cook model was used for the simulation study. The machining was done at a medium speed of 60 ms-1 and the depth of cut was maintained at 5 mm in all the above cases. Compared to other models, results showed that nonuniformly distributed grains develop alternate low and high shear bands and generate maximum temperature in the high shear band zone, which was found to be detrimental during the machining process compared to other models. Intergranular chip segmentation was observed in the case of uniformly distributed grains. For the same material, three different chip morphologies and temperature profiles were observed during the orthogonal machining process. Detailed discussion on the mechanics of machining of the above-said models was done that may be advantageous for material and tool design scientist.

Investigation on chip deformation behaviors of Zr-based bulk metallic glass during machining

Abstract Machining is an essential process for manufacturing high precision bulk metallic glass (BMG) components. It requires a fundamental understanding of the BMG chip deformation behaviors and their underlying mechanisms. In this study, chip deformation in orthogonal cutting of zirconium (Zr)-based BMG was investigated. A series of cuts was obtained to analyze morphological characteristics of chip and its related cyclic force. Further, the shear yielding stress, temperature, and strain rate during cutting were evaluated. By analyzing the separation of chip-workpiece and the fracture of chip, the formation mechanism for BMG chip was revealed. Based on the improved Atkins’ model, the energy consumption in chip formation was calculated, and its relations to chip morphology were discussed. The results showed that BMG cutting produced serrated chip. Individual chip segment deformation involved the formation of the primary shear zone (PSZ) and the secondary shear bands (SSBs). The fracture of the PSZ was found to be responsible for chip serration, and the SSBs were involved in the enhancement of the material plasticity. During low-speed cutting, the PSZ experienced high temperature (>995 K) and high strain rate (>2400 s−1). An increase in cutting speed or depth of cut led to the increase in the temperature and strain rate; however, reduced the shear yielding stress. The serrated chip formation is attributed to the fact that two cracks originated from the chip root and the free surface of the chip, which propagated face to face along the PSZ, eventually leading to the fracture of the PSZ. The generated serration energy accounted for the maximum cutting power during chip formation, while the elastic deformation energy within the PSZ accounted for the least. Increasing depth of cut led to an increase in generated serration energy. Increasing cutting speed resulted in significant increase in friction work at tool-chip contact interface, exacerbating chip material deformation.

Investigation of machining characteristics of hard-to-machine Ti-6Al-4V-ELI alloy for biomedical applications

Dry machining of Ti-6Al-4V alloy was investigated using SNMA120408 grade inserts. The material studied is designed for orthopedic applications. The effects of main cutting speed (VC), at constant feed rate (F) and depth of cut (DC) on machining characteristics (Feed force (Ff), radial force (Rf), Tangential Force (Tf)) and surface integrity (i.e., tool-chip contact length, chip segmentation, surface roughness, and tool wear) were examined. Experimental data indicate the cutting speed as the major parameter with direct impact on the machining characteristics. Increasing of the cutting speed promotes higher tangential forces that allow a decrease of the chip contact length; a smaller contact length results in a lower surface roughness and flank wear rate, respectively. To gain further insight from the simulated turning process an advanced Finite Element (FE) model was developed. The numerical model was built on the DEFORM-3D commercial software by incorporating the experimental cutting parameters. The numerical simulations results agree very well with experimental outputs in terms of cutting forces (FCS), tool-chip (T-C) contact length. Therefore, it was possible to estimate with accuracy the effective stress (σE) and the cutting temperature (TC). Further, due to its high robustness, the numerical model developed can be implemented in solving the industrial challenge (i.e., biomedical field) for predicting formations of serrated chip segment, chip thickness, potential types of chips, types of fracture mechanism and tool wear mechanism/rate generated during machining process.

Elucidation on the Microstructural and Mechanical Properties of Tailored VAL12 Hybrid Composites with ZrO<sub>2</sub> Dispersoids Fabricated by Squeeze Casting Technique

The effect of ZrO2 (2, 4, 6 and 8 wt%) dispersoids with 1 wt% graphite on the microstructural, mechanical properties on squeeze cast VAL12 matrix hybrid composite is investigated in the present study. The hybrid composites were characterized using advanced characterization techniques to reveal its microstructural and physical properties. The microscopic examinations using optical and SEM technique reveal that the addition of dispersoids accelerates the nucleation kinetics, thus attaining fine, equiaxial grains in hybrid composites. The squeeze cast composites show almost nil porosity, defects and owing to it, the actual density of the composites are found to be more than 95% as that of the theoretical density values. The hardness values and tensile values increase with respect to the increase in percentage addition of ZrO2. The tensile results show that there is an appreciable increase in the UTS values of composites without much loss in its ductility as the addition of graphite improves the self-lubricating property and provides wettability during the casting. Fractographic studies on tensile tested specimen reveal that the crack occurs in both matrix and particles showing the good interface between matrix and dispersoids. Machinability studies reveal the formation of continuous chips in hybrid composites with a lower percentage of reinforcement (up to 4% ZrO2 + 1%Gr) and segmented chips in case of the composite with 8% ZrO2 + 1%Gr, as the increase in the percentage of dispersoids improve the chip breakability of the composites. On an overall, the hybrid aluminium matrix composites with 1%Gr and 6 % ZrO2 unveiled better optimal results.

Finite element simulation study on pre-stress multi-step cutting of Ti-6Al-4V titanium alloy

The residual stress on the machined surface has a significant impact on the service life and fatigue strength of machined parts. In the former studies, it has been confirmed that both pre-stress cutting process and multi-step cutting process can significantly affect the residual stress distribution of the finishing machined surface, which gives expression to the increasing trend of compressive residual stress. In this paper, a finite element software Abaqus was used to establish a two-dimensional finite element simulation model for titanium alloy cutting. The finite element model was validated by the chip shape characteristics and cutting forces obtained by orthogonal cutting experiments. Based on the validated model, the simulations of single-step cutting, multi-step cutting, pre-stress single-step cutting, and pre-stress multi-step cutting processes were conducted. The chip shape characteristics, cutting forces, and residual stress distribution of the machined surface under four machining processing conditions were comparably investigated. The influence mechanism and coupling effect of multi-step machining and pre-stress machining on residual stress distribution on machined surface was explained. Investigation results indicate that both multi-step cutting and pre-stress cutting processes can increase the compressive residual stress, while multi-step cutting will change the position of minimum residual stress along the depth direction and increase the chip segmentation degree. Among the four cutting processing conditions, the pre-stress multi-step cutting will effectively increase compressive residual stress value to the utmost extent, which should be attributed to the coupled effect of pre-stress cutting and multi-step cutting processes.

Tool wear and chip analysis after the hard turning of AISI D6 steel assisted by LN2

Sustainability is a concept which is widely considered nowadays, including in factories where machining operations are present. The search for methods able to improve the performance of industrial processes without damaging the environment or the worker’s health has been the main goal of several investigations. In this context, cryogenic machining is a technique that has been studied as an alternative to the use of mineral oil-based cutting fluids, mainly in the machining of titanium and nickel alloys. Investigations on the cryogenic machining of hard tool steels are still scarce in the literature. This article presents results from a series of turning trials under dry and cryogenic conditions using a hardened AISI D6 tool steel bar (57 HRC) as the workpiece. For the cryogenic machining tests, liquid nitrogen was delivered to the flank face, rake face and on both faces of PCBN inserts. The main cutting parameters (cutting speed, feed rate, and depth of cut) were kept constant during the trials. Tool wear and chip morphology were the output variables studied. The results show that the liquid nitrogen was able to reduce the cutting tool wear, providing a tool lifetime around 50% longer compared with the dry process. Moreover, the frequency of chip segmentation was diminished under cryogenic conditions in comparison with the dry process.

Machining of particulate-reinforced metal matrix composites: An investigation into the chip formation and subsurface damage

This paper develops a microstructure-based model for investigating the mechanisms of chip formation and subsurface damage in the machining of particulate-reinforced metal matrix composites (PRMMCs). The morphology and distribution of the particles, the debonding of the particle-matrix interface and the fracture of particles and the matrix were comprehensively integrated into the modelling. The numerical analysis was also verified by relevant cutting experiments. It was confirmed that the established model can accurately predict the machining process, including chip morphology, subsurface deformation and cutting forces. It was found that the depth of cut significantly influences the machined surface integrity and cutting forces. Particle fracture mainly occurs along the primary shear zone in the cutting path. High strain and stress concentration induced by the extrusion of the cutting edge or large particles on the machined surface can also cause particle fracture beneath the cutting path. The brittle fracture of the particles located in the cutting path can form large cavities; while the deformed matrix material can partially cover these cavities to form a smooth surface. The matrix deformation occurs primarily along the shear plane, and the chip segments are separated easily along the plane with clustered particles. Serrated chips are formed in cutting the A359/SiC composite.

Experimental investigation of dynamic chip formation in orthogonal cutting

Orthogonal planing experiments are conducted with high-speed camera recordings and simultaneous cutting force measurements in order to analyze chip formation during machine tool vibrations. By means of carefully designed periodic forcing of the cutting tool, non-stationary cutting experiments are performed including wave generation, wave removal and wave-on-wave cutting, which are then compared to stationary cutting tests. The experiments are performed to address how the cutting force is affected by the chip thickness variation, the surface waviness and the fluctuation of the cutting direction. The results are used to assess some theoretical models that involve shear angle variation and chip segmentation.