Tool-work Material Interface Research Articles

Evaluation and quantification of diffusion wear between cutting chip and workpiece using forging press

The state of the interface between the workpiece and the cutting tool affects the cutting temperature and pressure on the tool surface during the cutting process. In particular, while cutting difficult-to-cut materials such as Ni-based alloy 718, the workpiece exhibits a high affinity for cutting tool materials and could easily adhere to them. Adhesion can, at times, adversely affect productivity. The diffusion between the cutting tool and the workpiece is a factor considered to contribute to the adhesion phenomenon during cutting. Addressing this issue involves choosing tool materials and coated materials with high resistance to diffusion and optimizing cutting conditions, particularly the cutting speed, which significantly impacts cutting temperature. However, because cutting tool wear comprises various forms, clarifying the effect of diffusion on tool wear remains open. In this study, to reproduce the diffusion phenomenon between cutting tool and workpiece, two pairs of test specimens were prepared: (1) cemented carbide-AISI 1045 and (2) cemented carbide-Alloy718, which could be held at high temperature under vacuum conditions by a forging press. The degree of diffusion phenomena was evaluated at each tool-work material interface, and the quantification of diffusion amount was performed by diffused element in each work material. Additionally, the theoretical analysis of the diffusion phenomenon using the thermodynamic and phase diagram calculation software Thermo-Calc was also performed.

A Methodology to Systematically Investigate the Diffusion Degradation of Cemented Carbide during Machining of a Titanium Alloy.

Using Ti6Al4V as a work material, a methodology to systematically investigate the diffusion degradation of cemented carbide during machining is proposed. The methodology includes surface characterization of as-tested worn inserts, wet etched worn inserts, metallographic cross-sectioned worn inserts as well as the back-side of the produced chips. Characterization techniques used include scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Auger electron spectroscopy (AES) and time of flight secondary ion mass spectroscopy (ToF-SIMS). The results show that the characterization of wet etched worn inserts gives quick and useful information regarding the diffusion degradation of cemented carbide, in the present work the formation of a fine crystalline W layer (carbon depleted WC layer) at the tool-work material interface. The present study also illuminates the potential of AES analysis when it comes to analyzing the degradation of cemented carbide in contact with the work material during machining. The high surface sensitivity in combination with high lateral resolution makes it possible to analyze the worn cemented carbide surface on a sub-µm level. Especially AES sputter depth profiling, resulting in detailed information of variations in chemical composition across interfaces, is a powerful tool when it comes to understanding diffusion wear. Finally, the present work illustrates the importance of analyzing not only the worn tool but also the produced chips. An accurate characterization of the back-side of the chips will give important information regarding the wear mechanisms taking place at the tool rake face–chip interface. Surface analysis techniques such as AES and ToF-SIMS are well suited for this type of surface characterization.

Toward a new tribological approach to predict cutting tool wear

This work aims at improving the numerical modelling of cutting tool wear in turning. The key improvement consists in identifying a fundamental wear model by means of a dedicated tribometer, able to simulate relevant tribological conditions encountered along the tool–workmaterial interface. Thanks to a design of experiments, the evolution of wear versus time can be assessed for various couples of contact pressure and sliding velocities (σn, Vs) leading to the identification of a new wear model. The latter is implemented in a numerical cutting model to locally simulate tool wear along the contact with regard to each local tribological loading.

Dry Machining Aeronautical Aluminum Alloy AA2024-T351: Analysis of Cutting Forces, Chip Segmentation and Built-Up Edge Formation

In this paper, machining aeronautical aluminum alloy AA2024-T351 in dry conditions was investigated. Cutting forces, chip segmentation, and built-up edge formation were analyzed. Machining tests revealed that the chip formation process depends on cutting conditions and tool geometry. So continuous and segmented chips are generated. Under some cutting conditions, built-up edge formation occurs. A predictive machining theory, based on a finite elements method (FEM), was applied to reproduce and explain these phenomena. Thermomechanical behaviors of the work material and the tool-work material interface were considered. Results of the proposed modelling were compared to experimental data for a wide range of cutting speed. It was shown that the feed force is well reproduced by the ALE-FE (arbitrary lagrangian-eulerian finite element) formulation and highly underestimated by the lagrangian finite element (LAG-FE) one. While, the periodic localized shear band, leading to a chip segmentation, is well reproduced with the Lagrangian FE formulation. It was found that the chip segmentation can be correlated to the cutting force evolution using the defined chip segmentation intensity parameter. For the built-up edge (BUE) phenomenon, it was shown that it depends on the contact/friction at the tool-chip interface, and this is possible to simulate by making the friction coefficient time-dependent.

Effect of the local friction and contact nature on the Built-Up Edge formation process in machining ductile metals

The BUE can affect the tool wear and surface integrity when machining ductile metals. The main goal of the present work is to investigate the close link between the BUE formation and the tribological behaviour at the tool–workmaterial interface when machining ductile metals. Machining AA2024-T351 aluminium alloy with cemented carbide tool WC–Co is considered as a case study. A new method based on the contact conditions variation (i.e. introduction of a time-dependent friction coefficient) at the tool–workmaterial interface is proposed to predict the BUE formation in the frame of a finite element (FE) modelling. A 2D ALE-FE model of orthogonal cutting has been developed for this purpose. Two cases have been considered, which correspond respectively to an abrupt change and a gradual evolution of the friction at the tool–workmaterial interface. Results are discussed based on the effects of the friction change on predicted thermomechanical fields at the cutting zone.

Characterization and Modelling of the Rough Turning Process of Large-scale Parts: Tribological Behaviour and Tool Wear Analyses

Machining large-scale parts in several industries (nuclear, naval, energy…) is a challenge for machine tools operators. Various problems are encountered, like respecting specified dimensions, deformation of the workpiece during machining, limit of the cutting speed, excessive tool wear, etc. All these difficulties are related to the large size of the machined part, which may have several meters and weigh several hundred kilograms. This study focuses on analysis of the rough turning process of a shell component, having few meters, as a part of steam generators of nuclear power plants. During the rough turning step, a high material removal rate (moderate cutting speed, but high depth-of-cut and feed rate) is necessary to achieve the workpiece in reasonable time. Experimental and theoretical analyses are conducted to highlight the intense thermomechanical loading at the tool–workmaterial interface. Revealed physical phenomena at the tool rake face, like adhesion and abrasive wear types, using various characterization techniques, are reproduced by a numerical model developed to simulate the cutting process. As an interest result, contact discontinuities at the tool–chip interface as well as where the wear is highly localized are well predicted as observed on scanning electron microscope. These contact discontinuities are attributed to the grooved rake face of the insert, designed with a chip breaker to reduce the tool–chip contact area and to promote the chip fragmentation. This study can be helpful for the design of rough turning inserts, by analysing the effectiveness of the rake face geometry (contact area, chip breaker…).

Thermomechanical modelling of the tool–workmaterial interface in machining and its implementation using the ABAQUS VUINTER subroutine

In this paper the complex thermomechanical behaviour at the tool – workmaterial interface in machining has been analysed. The contact behaviour is formulated in the frame of a Lagrangian Finite Element approach. The thermomechanical laws governing the tool – workmaterial interface in machining have been implemented via the VUINTER subroutine of ABAQUS/Explicit FE code. Using this interface, velocity dependent heat partition coefficient of the frictional heat has been implemented. Numerical results (calculated heat flux transmitted into the tool and cutting force) have been compared to experimental ones for different cutting conditions, which are in good agreement. It is shown that different couples of the heat partition and heat transfer coefficients can give the same heat flux transmitted in the tool. In addition, with the new developed VUINTER subroutine, it is possible to implement any mathematical model governing the friction and heat exchange for simulations of complex contact behaviours. • Metal cutting has been modelled and analysed focussing on the heat exchange at the tool–workmaterial interface. • The contact problem has been formulated in the frame of the finite element method. • Thermomechanical laws governing the tool–workmaterial have been implemented in a finite element code. • Rate-depend heat partition coefficient law has been implemented through the Abaqus VUINTER subroutine. • Comparisons between experimental and numerical heat fluxes and cutting forces have been performed.

Identification of friction and heat partition model at the tool-chip-workpiece interfaces in dry cutting of an inconel 718 alloy with cbn and coated carbide tools

Abstract This paper aims at characterizing the frictional behaviour at the cutting tool-workmaterial interface during the dry machining of a Inconel 718 in its aged state with various coated carbide tools and c-BN tools. A specially designed open tribometer has been used to characterize friction coefficient, heat partition coefficient under extreme conditions corresponding to the ones occurring in cutting. The tribometer provides the evolution of the apparent friction coefficient and of the heat partition coefficient for a large range of sliding velocity and contact pressure. It has been shown that friction coefficient as well as heat partition coefficient decrease with sliding velocity or contact pressure. A threshold effect of the contact pressure has been highlighted. On the contrary, any sensitivity to coatings deposited on carbide has been observed, whereas c-BN leads to very low friction coefficients.

Influence of Minimum Quantity Lubrication on Friction Coefficient and Work-Material Adhesion During Machining of Cast Aluminum With Various Cutting Tool Substrates Made of Polycrystalline Diamond, High Speed Steel, and Carbides

Due to the increasing emphasis on environmental constraints, industry works on how to limit the massive use of lubricants by using the micro-pulverization of oil in machining processes and, especially, in the machining of aluminum alloys for the automotive industry. The success of a machining operation is dependent on a friction coefficient and weak adhesion with the tool-work material interface. This paper aims at identifying the influence of cutting tool substrates (high speed steel (HSS), carbide, polycrystalline diamond (PCD)) and of minimum quantity lubrication (MQL) on the friction coefficient and on adhesion in tribological conditions corresponding to the ones observed in the cutting of aluminum alloys (sliding velocity: 20-1500 m/min). An open ball-on-cylinder tribometer, especially designed to simulate these tribological conditions through Hertz contact, has been used. It has been shown that HSS and carbide substrates lead to large friction coefficients (0.8–1) and substantial adhesion in dry conditions, whereas PCD substrates would lead to lower average friction coefficient values (0.4–0.5) and very limited adhesion, which proves the necessity of using PCD tools in the dry machining of aluminum. It has also been shown that the application of MQL leads to a large decrease of the friction coefficient (0.1–0.2) and eliminates almost all traces of adhesions on pins for any substrates, which shows that MQL is an interesting compromise between dry machining and flood cooling.

Characterisation of friction and heat partition coefficients at the tool-work material interface in cutting

The development of cutting simulation still requires an improvement in the understanding of the frictional phenomena at the tool-work material interface. This paper introduces a method for a fast identification of friction and heat partition models, based on a special tribometer able to simulate wide ranges of contact pressures and sliding velocities, similar to those occurring along the tool-work material interface in cutting. The method is applied for a wide spectrum of work materials and lubrication conditions. Combined with an analytical post-treatment, this set-up provides a modelling of the frictional behaviour that may improve significantly thermal aspects in cutting simulations.

Numerical Study of Residual Stress Induced by Multi-steps Orthogonal Cutting

Residual stresses induced by material removal have a major influence on the lifetime of machined pieces especially in its corrosion resistance and fatigue life. For that reason, numerical prediction of residual stress profile was the subject of many works. But most of these models are developed only for a single step. They does not consider the effect of hardening and thermal softening on the residual stresses induced by the material removal which is, in the reality, a multi-steps operation such as milling and grinding. The cutting tool is in contact with a part of material that was the finished piece in the previous step. In this paper a multi-steps model for orthogonal cutting has been developed in order to study the influence of the cumulated strain and temperature induced by the different steps on the residual stresses. The effect of tool edge radius and heat generated by flank friction on the predicted stress profile is modeled. Commercial finite element software ABAQUS with its Explicit and Implicit modules was used. Computed Numerical predicted stress fields are compared against measured residual stresses obtained by X-Ray diffraction. Moreover, in order to take into account all the physics in the tool-work material interface, spring back simulation was performed using ABAQUS Implicit.

Characterization of Friction and Heat Partition Coefficients during Machining of a TiAl6V4 Titanium Alloy and a Cemented Carbide

This article aims at characterizing the frictional behavior of a TiAl6V4 alloy and a carbide tool under extreme conditions corresponding to those occurring at the cutting tool–work material interface. A specially designed open tribometer was used to characterize the macroscopic friction coefficient, heat partition coefficient, and adhesion in the contact versus sliding velocity and contact pressure. It has been shown that titanium leads to intense adhesion, which seems to be even more intensive with high contact pressure and high sliding velocity, which limits the local sliding movement at the interface (stuck layer). However, the tribometer provides the evolution of an apparent friction coefficient and a macroscopic heat partition coefficient related to the shearing of titanium between the adhesive layer and the bulk material. An increase in sliding velocity or contact pressure induces a small decrease in the apparent friction coefficient as well as the heat partition coefficient. It has been shown that adhesion is thermally activated by a combination of contact pressure and sliding velocity, which leads to a threshold effect. Furthermore, the application of an emulsion showed a small decrease in the apparent friction coefficient associated to a decrease in adhesion. Finally, this work provides quantitative data on the apparent friction and heat partition coefficients versus sliding velocity and contact pressure that can support the development of macroscopic cutting models for titanium alloys.

3D modeling of residual stresses induced in finish turning of an AISI304L stainless steel

This paper addresses the development of a new methodology predicting residual stresses induced in finish turning of a AISI304L stainless steel. A hybrid approach combining experimental results and a numerical model is applied. The model simulates the residual stresses generation by applying equivalent thermo-mechanical loadings onto the machined surface without modeling the chip removal process, which enables rapid calculation. The shape and the intensity of equivalent thermo-mechanical loadings are identified through experimental measurements. Friction tests enable to model the thermal and mechanical loadings along the tool–workmaterial interface. Orthogonal cutting tests provide thermal and mechanical loadings below the primary and third shear zone. This model has already been presented in several papers, but only in a 2D configuration. The objective of this paper is to transfer this hybrid approach into a 3D configuration, which is closer to a concrete longitudinal turning operation. Based on this new model, the paper aims at investigating the interactions between each revolution. It is shown that around five revolutions are necessary to reach a steady state. Finally numerical results are compared with experimental measurements obtained by X-Ray diffraction. It is shown that residual stresses cannot be considered as homogeneous over the surface due to tool's feed. Additionally, the X-Ray beam is much too large to be able to quantify this heterogeneity. Based on average numerical values coherent with average values obtained by X-Ray diffraction, it is shown that the numerical model provides consistent results compared to experimental measurements for a large range of cutting speed and feed.

Effects of Lubrication Mode on Friction and Heat Partition Coefficients at the Tool–Work Material Interface in Machining

The quantification of friction coefficient along the tool–work material interface in machining remains an issue in tribology. This article aims at identifying the evolution of friction coefficient for a large range of sliding velocity during the machining of an AISI 4140 steel (290 HB) with a TiN-coated carbide tool. The influence of various lubricants (straight oil or emulsion) and lubrication modes (flow or mist) is investigated and compared to a dry sliding situation (dry machining). It has been shown that, in dry machining, the friction coefficient decreases with the sliding velocity until reaching a lower limit around 0.2. On the contrary, the presence of a straight oil significantly decreases friction coefficients to a value around 0.1. Emulsion enables a significant decrease of friction coefficient to around 0.2 for low sliding velocities, whereas its action is absent for higher sliding velocities. Oil mist exhibits an intermediate behavior. Finally, it has been shown that all kinds of lubrication lead to a large decrease of heat flux transmitted to cutting tools, even if heat flux is almost similar irrespective of the nature of oil and of its application mode. Straight oils decrease the heat flux transmitted to cutting tools due the decrease of friction coefficient, whereas they are not able to modify the heat partition coefficient at the interface. On the contrary, emulsions limit the transmission of heat flux to cutting tools due to their lower heat partition coefficient, even if their influence on friction is more limited.

Effects of a straight oil on friction at the tool–workmaterial interface in machining

The quantification of friction coefficient along the tool–workmaterial interface in machining remains an issue in tribology. This paper aims identifying the evolution of friction coefficient for a large range of sliding velocity during the machining of a AISI4140 steel (290 HB) with a TiN coated carbide tool. The influence of a straight oil is investigated compared to a dry sliding situation (dry machining). It has been shown that, in dry machining, friction coefficient decreases with the sliding velocity until reaching a downer limit around 0.2. On the contrary, the presence of a straight oil decreases significantly friction coefficients to a value around 0.1. Additionally, the friction coefficient becomes independent from the sliding velocity. It also been shown that a straight oil is able to penetrate the pin-workmaterial interface even if a very high contact pressure is present (∼3 GPa). The penetration duration is very fast, whereas its evacuation duration is much higher and strongly dependent on sliding speeds.

Phase Transformation and Its Effect on Flank Wear in Machining Steels

In machining, the main wear mechanism on the flank surface of a tool is commonly believed to be abrasive wear [1,9]. Accordingly, work materials with a higher concentration of hard inclusions are expected to develop higher flank wear rates. However, the previous turning experiment [9] with plain carbon steels containing varying amounts of cementite inclusions did not exhibit the expected flank wear behavior. Other imperative phenomenon must be occurring at the tool-work material interface during machining, which diminishes the abrasive action of the cementite inclusions. To investigate this behavior, a series of turning tests with AISI 1045, 1070, and 4340 steels have been conducted; and the newly generated surface layers are examined for phase identification using Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), and Transmission Electron Microscopy (TEM). At high cutting speeds (>200 m/min), flank wear is diminished as the cementite phase at the newly formed surface dissociates and diffuses into the matrix of the austenitic phase. Because the heated austenite phase is cooled extremely rapidly, martensitic and in some case even retained austenitic phases are formed. This is the evidence of phase transformation, which explains the flank wear data observed in [9]. In addition, phase transformation explains the scatter in flank wear data in the literature because the onset of phase transformation depends on the exact composition of the work materials as well as interfacial conditions such as temperature and pressure. This paper reports the experimental evidence of phase transformation and its consequence on flank wear in machining annealed steels.

Abstract

In this paper the complex thermomechanical behaviour at the tool–workmaterial interface in machining has been analysed. The contact behaviour is formulated in the frame of a Lagrangian Finite Element approach. The thermomechanical laws governing the tool–workmaterial interface in machining have been implemented via the VUINTER subroutine of ABAQUS/Explicit FE code. Using this interface, velocity dependent heat partition coefficient of the frictional heat has been implemented. Numerical results (calculated heat flux transmitted into the tool and cutting force) have been compared to experimental ones for different cutting conditions, which are in good agreement. It is shown that different couples of the heat partition and heat transfer coefficients can give the same heat flux transmitted in the tool. In addition, with the new developed VUINTER subroutine, it is possible to implement any mathematical model governing the friction and heat exchange for simulations of complex contact behaviours.