Discontinuous Chip Formation Research Articles

Diamond machining of additively manufactured Ti6Al4V ELI: Newer mode of material removal challenging the current simulation tools

Single point diamond machining (SPDM) produces smooth machined surfaces that other production methods cannot match. While the mechanics of machining of cast alloys with SPDM is well-explored, the realm of SPDM for additively manufactured parts remains largely uncharted. This work reveals new insights into the surface generation process of an additively manufactured titanium alloy, specifically, a Ti6Al4V Extra Low Interstitials (ELI) alloy workpiece. Our examination of the chip morphology unveiled a distinct mode of chip removal, previously unrecorded in existing literature. During SPDM of additively made Ti6Al4V ELI workpiece, identification of numerous pores and discontinuities in the chips flowing on the tool rake face, indicating periodic intermittent cracking during the material's plastic flow was seen. To examine this phenomenon, a finite element analysis (FEA) model was developed. While the FEA model can well explain the machining mechanics and chip morphology of SPDM of cast Ti6Al4V ELI reported in the literature, it failed to describe the chip morphology that are obtained during machining of additively made workpiece in this work. This disparity underscores the need for innovative simulation approaches tailored for additively manufactured components. The experimental observations in this study highlight a unique form of chip formation in contrast to conventional Ti6Al4V alloy machining processes. At lower feeds, there was a presence of short, discontinuous chip formation with tearing at the outer periphery. Conversely, at higher feeds, a long, continuous ribbon-like chip formation was observed. In addition, some typical additive manufacturing defects appear on the machined surface and chips. Through optimisation of the SPDT parameters, a surface roughness (Ra) value of about 11.8 nm was achieved on additively manufactured Ti6Al4V ELI workpiece. This work provides a fresh perspective on the mechanics of SPDM for additively manufactured components, offering a stepping stone for subsequent studies.

Chip Morphology Prediction in Inconel 718 Milling through Machine Learning to Control Surface Integrity

A nickel-based aerospace superalloy, Inconel 718 presents machining challenges because of its hardness and strength. Monitoring and predicting chip morphology during milling is essential for early defect detection and process optimisation. This study examines the correlation between sensor signals with surface roughness and chip morphology in milling Inconel 718 using machine learning (ML). Due to progressive tool wear and heat generation, the surface roughness varies in addition to the chip exhibiting different morphologies, such as continuous, discontinuous, and oxidised chips. AE signals were analysed in the time and frequency domains to identify chip morphology transitions. An accelerometer captured cutting vibration signals that showed higher instability during discontinuous chip formation. Chip colour due to oxidization varies with milling forces as a result of tool wear. Based on multiple sensor data fusion, a random forest model predicts better chip morphology from different machining parameters. The integrated ML system enables real-time monitoring of chip morphology mechanisms through diverse signals. This permits early diagnosis of surface integrity and chip morphologies indicating imminent tool wear. The approach enhances process stability and tool life when milling difficult-to-machine alloys. It demonstrates the viability of relating sensor signals to fundamental mechanisms through AI for intelligent machining.

Investigation on tool wear mechanisms and machining tribology of hardened DC53 steel through modified CBN tooling geometry in hard turning

DC 53 steel has emerged as a possible replacement of AISI D2 steel possessing competitive hardness and better toughness. In the current work, turning of DC 53 steel was conducted via Xcel modified inserts by varying workpiece hardness levels (40 and 60 HRC), cutting speed (130 and 160 m/min), feed rate (0.07 and 0.112 mm/rev), and depth of cut (0.07 and 0.17 mm). A two-level 4-factor full factorial design was employed entailing 16 runs. An analysis of variance (ANOVA) was conducted to statistically analyze the effect and contributions of input parameters on response variables namely tool life, surface roughness, volume of material removed, power consumption, and machining zone temperature. Results show that the tool life, surface roughness, volume of material removed, and machining zone temperature are primarily affected by the hardness of DC53 with PCRs of ~ 96%, ~ 25%, ~ 62%, and ~ 25%, respectively. At a 40 HRC hardness value, true crater wear was observed due to continuous chips sliding at the rake face while for the workpiece having a 60 HRC, discontinuous chip formation produced less prominent crater wear. SEM images revealed complete delamination of the coating from the tool surface with adhesion and attrition wear identified as the main wear mechanisms. The formation of a groove pattern was also noticed on the flank face. The minimum surface roughness was 0.90 µm-Ra for the workpiece having a 40 HRC hardness level, and the same value was obtained for 60 HRC as well. The threshold value of the feed rate for the excellent performance of these inserts was less than 0.20 mm/rev. Additionally, the turning process proved to be productive for this material along with a lower surface roughness value in comparison to the wire EDM process.

On the function of lead (Pb) in machining brass alloys

Lead has traditionally been added to brass alloys to achieve high machinability, but the exact mechanisms at work are still debated. Lead-free brass alternatives could be developed if these mechanisms were better understood. Accordingly, machinability characteristics were investigated for two brass alloys with similar mechanical properties and phase composition, but with very different machining characteristics because one has 3 wt.% lead (CuZn38Pb3) while the other has only 0.1 wt.% (CuZn42). The effect of the lead was investigated using infrared temperature measurement, electron microscopy, secondary ion mass spectroscopy, quick-stop methods, and high-speed filming. Neither melting of lead nor its deposition on the tool rake surface takes place during machining thus confirming its limited lubrication and tribological effects. Instead, the main role of lead is to promote discontinuous chip formation. Lead deforms to flake-like shapes that act as crack initiation points when the workpiece material passes through the primary deformation zone. This effect prevents the development of stable tool–chip contact, thus lowering cutting forces, friction, and process temperature.

High-Speed Cutting of Synthetic Trabecular Bone—A Combined Experimental–Computational Investigation

Orthopaedic surgical cutting instruments are required to generate sufficient forces to penetrate bone tissue while minimising the risk of thermal and mechanical damage to the surrounding environment. This study presents a combined experimental–computational approach to determine relationships between key cutting parameters and overall cutting performance of a polyurethane-based synthetic trabecular bone analogue under orthogonal cutting conditions. An experimental model of orthogonal cutting was developed, whereby an adaptable cutting tool fixture driven by a servo-hydraulic uniaxial test machine was used to carry out cutting tests on Sawbone® trabecular bone analogues. A computational model of the orthogonal cutting process was developed using Abaqus/Explicit, whereby an Isotropic Hardening Crushable Foam elastic-plastic model was used to capture the complex post-yield behaviour of the synthetic trabecular bone. It was found that lower tool rake angles resulted in the formation of larger discontinuous chips and higher cutting forces, while higher rake angles tended to lead to more continuous chip formation and lower cutting forces. The computational modelling framework provided captured features of both chip formation and axial cutting forces over a wide range of cutting parameters when compared with experimental observations. This experimentally based computational modelling framework for orthogonal cutting of trabecular bone analogues has the potential to be applied to more complex three-dimensional cutting processes in the future.

Surface prediction and measurement for modulated tool path (MTP) turning

This paper describes a time-domain simulation for predicting surface finish in modulated tool path (MTP) turning, which uses sinusoidal axis motions in the feed direction to produce discontinuous chips for ductile workpiece materials. The simulation includes: the low frequency and low amplitude tool oscillation in the feed direction; the time-varying chip thickness, cutting force, and tool displacement; and the plastic side flow effect used to calibrate the effective tool nose radius. Comparisons between predicted and measured surface profiles are presented for turning a 6061-T6 aluminum cylinder as a function of the MTP oscillation frequency and amplitude with discontinuous chip formation.

Observations of Machining Performance during Turning AA6351-B4C Composite

Abstract This study investigates the machining characteristics of stir-cast AA6351-boron carbide composite under dry condition. The influences of input parameters on output responses, such as temperature rise, cutting force, tool wear, and surface roughness, are investigated. The monolayer titanium nitride–coated solid carbide insert tool CNMG120412 (80° diamond shaped with a nose radius of 1.2 mm) has been used to perform the experiments. The temperature and cutting forces are measured by using a FLIR infrared thermal image camera and piezoelectric dynamometer. The reduction in temperature is noted at a low spindle speed (Vc), which is due to the discontinuous chip formation. The increase in feed rate (f) has reduced the coefficient of friction (μ) as a result of a decrease in the friction angle. The lowest value of μ, 0.5567, is obtained at a low Vc and a high f. The increase in temperature at a high Vc reduces the specific cutting pressure and results in a low thrust force. The tool wear is examined by using a scanning electron microscope to analyze the wear mechanisms. At a high Vc, an increase in temperature reduces mechanical load and protects the tool from abrasion wear; it results in good surface finish. A low surface roughness value of 1.08 μm is achieved at a high Vc and ensures the surface quality.

A new method for defining the ploughing forces in processing of brittle materials with the formation of discontinuous chips

When monitoring cutter wear during microcutting, it is necessary to consider the ploughing force of the tool flank face (PFs). The method of measurement differs based on the wear of the cutting tools and cannot be applied to determine PFs. When we cut brittle materials using a tool with wear there are significant fluctuations in the cutting forces due to the unstable chip formation, so the cutting forces cannot be measured. A new method for defining PFs when cutting brittle materials with the formation of discontinuous chips has been proposed in this study. The reliability of the new method has been confirmed by comparing PFs obtained with the help of different methods of cutting materials with the formation of continuous chips.

A new method for defining the ploughing forces in processing of brittle materials with the formation of discontinuous chips

When monitoring cutter wear during microcutting, it is necessary to consider the ploughing force of the tool flank face (PFs). The method of measurement differs based on the wear of the cutting tools and cannot be applied to determine PFs. When we cut brittle materials using a tool with wear there are significant fluctuations in the cutting forces due to the unstable chip formation, so the cutting forces cannot be measured. A new method for defining PFs when cutting brittle materials with the formation of discontinuous chips has been proposed in this study. The reliability of the new method has been confirmed by comparing PFs obtained with the help of different methods of cutting materials with the formation of continuous chips.

Mechanical, tribological and electrical properties of ZrB2 reinforced Cu processed via milling and high-pressure hot pressing

The present work investigates the effect of (0–10 wt%) ZrB2 reinforcement on densification, mechanical, tribological and electrical properties of Cu. The consolidation of Cu–ZrB2 samples was carried out using a hot press (temperature: 500 °C, pressure: 500 MPa, time: 30 min, vacuum pressure: 1.3 × 10-2 mbar). The bulk density of the hot-pressed Cu composites decreased from 8.84 g/cc to 8.16 g/cc and the relative density of samples lowered from 98.6% to 92.1% with the addition of ZrB2. The incorporation of hard ZrB2 (up to 10 wt%) improved the hardness of Cu (1.32–2.55 GPa). However, the yield strength and compressive strength of Cu composites increased up to 5 wt% ZrB2, and further addition of ZrB2 lowered its strength. The yield strength of Cu samples varied from 602 to 672 MPa and the compressive strength between ~834 and 971 MPa. On the other hand, the coefficient of friction (COF) (from 0.49 to 0.18) and wear rate (from 49.3 × 10-3 mm3/Nm to 9.1 × 10-3 mm3/Nm) of Cu–ZrB2 samples considerably decreased with the addition of ZrB2. Significantly low wear was observed with Cu-10 wt% ZrB2 (Cu-10Z) samples, which is 5.41 times less than pure Cu. As far as the wear mechanisms are concerned, in pure Cu, continuous chips (wear debris) were formed during sliding wear by plowing. Whereas the major amount of material loss was occurred due to the plowing mechanism with discontinuous and short chip formation for Cu–ZrB2 composites. The electrical conductivity of Cu–ZrB2 samples decreased from 75.7% IACS to 44.1% IACS. In particular, Cu with ZrB2 (up to 3 wt%) could retain the conductivity of 66.8% IACS. This study reveals that the addition of ZrB2 (up to 3 wt%) is advantageous to have a good combination of properties for Cu.

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.

A semi-empirical analytical model to predict the axial cutting force of AZ31B magnesium extrusions

In several industries where weight reduction is a crucial design goal, especially transportation, magnesium alloys are becoming a favourable option to further innovate since magnesium is the lightest structural metal available. Cylindrical, seamless extrusions composed of AZ31B magnesium alloy were subjected to quasi-static and dynamic axial cutting to assess the potential for magnesium alloys to be exploited for crashworthiness applications. Geometrically similar aluminum extrusions composed of AA6061-T6 and AA6082-T6 alloys, which are commonly utilised for energy absorbers, were also tested under similar loading conditions to provide data for comparison. A semi-empirical model to predict the cutting forces in magnesium extrusions was derived using the experimental findings from this study. This revised model accounts for the discontinuous chip formation mechanism and frictional effects more appropriately than previously developed models. The extrusions possessed 1.5 mm wall thicknesses, outer diameters of 57 mm or 62 mm, and free lengths of 180 mm. Quasi-static tests were conducted on a Tinius-Olsen compression machine and dynamic tests on a drop tower with a 57 kg falling mass at an impact velocity of 7 m/s. Under the observed cutting deformation mode, all specimens produced evenly sized, petalled sidewalls. However, for the aluminum extrusions long, continuous chips formed ahead of the cutter while short, discontinuous chips formed for the magnesium extrusions. The corresponding energy dissipation ranged from 1.23 kJ to 3.57 kJ for the aluminum specimens and 0.63–0.72 kJ for the magnesium specimens.

Al6061 surface roughness and optical reflectance when machined by single point diamond turning at a low feed rate.

Ultra-precision face turning of Al6061 mirrors using single point diamond turning (SPDT) was undertaken to investigate the correlation between the surface roughness and reflectance. By reducing the feed rate, the optimal feed rate when the chip formation became unstable was studied. Most importantly, the impact on the roughness and reflectance was examined when the chip formation ceased to be continuous. It was determined that for a feed rate below 3 mm/min, the surface roughness always improved as the feed rate decreased, at the cost of a reduction of the normalized reflectance. The reduction of reflectance was determined to be the result of the lower material removal rates that led to a discontinuous chip formation.

Characterization of Chip and Burr Formation at High Speed Machining of Nitronic 33 Steel Alloy

The energy consumption and machinability index of metallic alloys are very important in determining the economic and environmental value of manufacturing process. Various machinability problems with Nitronic 33 steel alloy have been reported in literature. These problems have been attributed to the work hardening of the material during machining operation and hence greatly influences and contributes to the green house gas emission. In this work, the chip and burr formation during the machining of Nitronic 33 steel alloy was investigated in other to optimize the cutting parameters and provide a knowledge base for machinists when machining austenitic stainless steels. The result shows that although continuous chips were formed throughout the machining tests, an evidence of continuous chip with built-up edges was also observed. This phenomenon tends to initiate the formation of discontinuous chips especially at high pressure coolant flow of 7 and 9.7 MPa. It is concluded that conventional cutting environment at 90 m/min cutting velocity is the optimum process parameter most suitable for machining Nitronic 33 steel alloy. The research outcome will address some of the problems encountered during high speed machining of Nitronic 33 steel alloy and the general understanding of the machinability of this alloy.

A numerical cutting model for brittle materials using smooth particle hydrodynamics

Numerical studies on the cutting process of brittle materials have not previously been conducted, due to the difficulties involved, which stem from the differences to their fracture mechanisms compared to those of ductile materials. The purpose of this study is to propose and implement a numerical cutting model that is able to represent the removal mechanism in brittle material. For this purpose, cutting behavior was simulated using the smooth particle hydrodynamics (SPH) method and the Johnson-Holmquist material model. The numerical cutting model of brittle materials proposed here was able to realize the propagation of cracks and the formation of discontinuous chips that are cutting characteristics in brittle materials. Location data was analyzed to judge the status of surface roughness after the numerical cutting process was analyzed. Numerical analysis was conducted to find optimal cutting conditions by investigating the surface roughnesses resulting from changes to various factors (cutting speed, cutting depth, rake angle). Finally, using the optimal cutting conditions found (high cutting speed, low cutting depth, and a rake angle close to 0°) was shown to contribute to improved surface roughness.

Investigation on the Elemental Chip Formation Process in Hard-to-Machine Material Cutting

The mechanism of discontinuous chip formation has been studied less than the mechanism of continuous chip formation. However, when most modern materials having specific physical and mechanical properties are subject to machining, such processes are featured by discontinuous chip formation. The paper describes the basic dependencies of discontinuous chip parameters on machining modes. This is a trial undertaken to introduce an explanation of how the basic factors of the cutting process influence over parameters of chip formation.

An explicit finite element model to study the influence of rake angle and friction during orthogonal metal cutting

A fundamental understanding of the tribology aspects of machining processes is essential for increasing the dimensional accuracy and surface integrity of finished products. To this end, the present investigation simulates an orthogonal metal cutting using an explicit finite element code, LS-DYNA. In the simulations, a rigid cutting tool of variable rake angle was moved at different velocities against an aluminum workpiece. A damage material model was utilized for the workpiece to capture the chip separation behavior and the simultaneous breakage of the chip into multiple fragments. The friction factor at the cutting tool–workpiece interface was varied through a contact model to predict cutting forces and dynamic chip formation. Overall, the results showed that the explicit finite element is a powerful tool for simulating metal cutting and discontinuous chip formation. The separation of the chip from the workpiece was accurately predicted. Numerical results found that rake angle and friction factor have a significantly influence on the discontinuous chip formation process, chip morphology, chip size, and cutting forces when compared to the cutting velocity during metal cutting. The model was validated against the experimental and numerical results obtained in the literature, and a good agreement with the current numerical results was found.

Studies on the formation of discontinuous chips during rock cutting using an explicit finite element model

Predicting the fragmentation process during rock cutting poses significant technical challenges. In this respect, previous research related to rock cutting using various numerical methods was reviewed in detail. A method for simulating the fragmentation process during the mechanical cutting of rock was then introduced using the explicit finite element code LS-DYNA. In the numerical simulations, the base rock material properties were defined using a damage constitutive model. This model simulates the separation of rock chips from the base rock material and the subsequent breakage of the chips into multiple fragments. In the simulations, a rigid steel cutting tool was translated at various sliding velocities (1, 4, 10, 50, and 100 mm/s) against a stationary rock material. For a given sliding velocity, simulations were conducted for various cutting depths (1, 2, 3, and 4 mm). The variation of stresses and the amount of chip formation at different depths of cut and velocities were analyzed. The simulations indicated that the cutting forces and chip morphology were significantly influenced by sliding velocities and cutting depths. Overall, the results indicate that the explicit finite element method was a powerful tool for simulating rock cutting and the chip fragmentation processes, as it was able to predict chip separation behavior from the base rock at different depths of cut accurately.

High Performance Turning of Austempered Ductile Iron (ADI) with adapted Cutting Inserts

Heat treated Ductile Iron with austenitic-ferritic matrix (ADI) has a high potential for the substitution of forged steel and conventional Ductile Iron. Advantages of ADI compared to steel are good castability and lower density. In comparison to conventional Ductile Iron, the increased ratio of tensile strength/ductility with higher resistance against abrasive wear and durability are of advantage. In the industrial applications, the material sided advantages are counterbalanced by the higher costs of the demanding machining, especially in rough turning operations. In this presentation, the development of cutting inserts for high performance turning operations, which are adapted on the specific requirements of the machining of ADI 900, are described. In the first step, the particular machinability, which can be traced back to the metallic matrix, was specified. The characteristic, discontinuous chip formation in combination with the strong tendency of hardening of the austenitic− ferritic matrix was identified as a major wear mechanism, resulting in extreme peaks of cutting forces and high specific mechanical load on the insert edge. For an enhanced evaluation of chip formation and the resultant thermomechanical tool load, a 3D FEM simulation model was developed. By using this 3D simulation model of the longitudinal external turning process, the maximum cutting forces were specified in addition to the influence of the chip segment formation frequency. The obtained results from the simulation have been used to develop an optimised insert geometry. By using the optimised tool, an increased tool life of 70% could be observed. The shown results are part of the PhD Thesis authors Dissertation “Werkzeug− und Prozessauslegung zur Drehbearbeitung von austenitisch-ferritischem Gusseisen mit Kugelgraphit”

Development of an experimentally validated abrasive-grain cutting model using a hybrid Euler–Lagrange finite element formulation

This work describes the creation of an experimentally validated abrasive-grain cutting model using a hybrid Euler–Lagrange finite element formulation. This model was developed to better understand the micro-mechanical cutting that occurs during the grinding manufacturing operation. The model was developed in stages using the commercially available package LS-DYNA® with each stage undergoing experimental validation. Two cutting tool geometries were utilized to represent idealized abrasive grains that form a grinding wheel. A round-nosed tool was used to approximate the general size of abrasive grains and a flat-nosed tool was used to approximate the size and active cutting edge shape of abrasive grains. The development began with a simple spherical indentation model to test the feasibility of the numerical formulation, to determine essential contact parameters, and to verify an elastoplastic material model. The indentation model was then expanded to include relative motion between a round-nosed cutting tool and the workpiece using the same parameters that were identified with the indentation model. The round-nosed cutting model was used to determine the coefficient of friction between the tool and the workpiece. Analysis of the results from the round-nosed cutting model revealed that a plowed lip of plastically deformed material forms in front of the tool while a ridge of material forms on the sides of the residual groove as a result of flow around the tool. Subsurface stresses were shown to be more affected by depth of cut as compared to cutting speed. A case study showed that smaller abrasive grains produced lower overall forces and reduced the size of the subsurface stress fields; however, peak stresses were shown to increase slightly with decreasing grain size. Finally, a flat-nosed cutting tool was introduced in–place of the round-nosed tool to create a three-dimensional model of abrasive-grain cutting with chip formation. This finalized model incorporated all of the identified numerical parameters from the previous two development stages. The results revealed markedly different deformation and subsurface stresses. The workpiece material was shown to flow up the sides and around the cutting tool producing a continuous chip. Material thinning of the chip as it flowed around the tool demonstrated likely areas of crack formation, which may lead to discontinuous chip formation. The subsurface stresses were shown to be concentrated along a thin shear band at the root of the chip in accordance with metal cutting theory. Stresses below the flat-nosed cutting tool were shown to be below the yield stress of the workpiece material.