ALE Simulation of Orthogonal Cutting: a New Approach to Model Heat Transfer Phenomena at the Tool-Chip Interface

This paper presents a new procedure to evaluate the global heat transfer coefficient in orthogonal cutting. The knowledge of the actual heat transfer conditions is a fundamental issue as far as the life, tool wear and tool substitution interval are regarded. More in detail, an Arbitrary Lagrangian-Eulerian approach was utilised to model orthogonal cutting process and the numerical simulations were validated by making experimental tests for identifying cutting forces and internal tool temperatures. A mild steel was cut utilising both an uncoated (WC) and a coated (TiN) tool. On the basis of both experimental and simulative data, a consistent model of the global heat transfer coefficient as function of the local pressure and temperature at the tool-workpiece interface was developed.     

Modelling of orthogonal cutting processes with the method of smoothed particle hydrodynamics

In the last decades, mesh-free methods for simulating various cutting processes have been used very widely as they can eliminate numerical problems in the simulation of material failure and large plastic deformations. This paper deals with the results from modelling the orthogonal cutting of AISI 1045 steel using smoothed particle hydrodynamics (SPH) method. Moreover, it is determined how the parameters of the SPH solver such as initial smoothing length, initial particle density and coefficient for the timestep increase affect the prediction error for the values of cutting force and chip compression ratio as well as computing time. The optimum values of the SPH solver parameters are determined by minimising an objective function. The best balance between the prediction error of machining variables and computing time is achieved for an initial particle density of 40 μm and a coefficient for the timestep increase of 0.4.     

FE-simulation of machining processes with a new material model

In the field of materials mechanics the influence of the state of stress on the plastic deformation behavior of metals is known since decades. However, the state-of-stress influences are usually not considered in structural or processing simulations. Nevertheless, its application in the numerical investigation of manufacturing processes seems very promising since, for example, machining processes are characterized by complex states of stress. Consequently, its incorporation in the computation of the workmaterial's flow stress may increase the physical conformity and accuracy of cutting FE-analysis.This paper presents the creation and experimental validation of a 3D-FEM model of the longitudinal turning process with an extended modified Bai–Wierzbicki material model (extended MBW model). This newly developed material model evaluates the influence of state of stress as well as damage on the strain hardening behavior. In addition, it takes temperature and strain rate effects into consideration, whose influences are both typically higher in cutting processes than in structural–mechanical problems.For the validation of the proposed material model, longitudinal turning experiments were conducted on AISI 1045 steel. Four different cutting tools and process conditions were investigated, which cover a broad range from finishing to roughing. A high speed camera was used to film the chip formation and chip flow in order to compare it to the simulation results. The three cutting forces components were also collected. Measured chip temperatures were taken from the literature. The validation showed that the implementation of the selected material model results in a close agreement between experimentally obtained and predicted chip geometries, cutting forces and chip temperatures.     

A simple inverse procedure to determine heat flux on the tool in orthogonal cutting

The applications of numerical simulation to machining processes have been more and more increasing in the last decade: today, a quite effective predictive capability has been reached, at least as far as global cutting variables (for instance cutting forces) are concerned. On the other hand, the capability to predict local cutting variables (i.e. stresses acting on the tool, temperature distribution, residual stresses in the machined surface) has to be furtherly improved, as well as effective experimental procedures to validate numerical results have to be developed.The aim of this paper is the proposition of an innovative approach, based on an simple inverse procedure, in order to identify both the heat flux flowing into the tool through the rake face and the heat transfer coefficient between the tool and the environment during a typical orthogonal cutting process. It is worth pointing out that the effective determination of such quantities is necessary in order to carry out a reliable prediction of the temperature distribution in the tool during the process.The procedure is based on the integration of numerical simulations, inverse approach algorithms and experimental tests.     

Thermal modelling of end milling

Determination of the temperatures during machining is one of the most important challenges for accurate milling simulations. Coupled with excessive shearing, plastic deformation and friction in a small region of cutting, the temperatures in milling may have very significant impact on parts and tools such as dimensional errors, residual stresses and tool wear. Temperature exhibits a non-linear complex-modelling problem in milling process. In this article, for the first time, a novel thermal modelling is introduced for fast and accurate prediction of temperatures in end milling processes. A theoretical modelling approach and experimental validations are presented for various cutting conditions.     

Determination of temperature distribution in the cutting zone using hybrid analytical-FEM technique

In this study, the temperature distribution in the cutting zone was determined by integrating thermal analytical and simulation models of orthogonal cutting process with uncoated and coated carbide tools. Primarily, 2D FEM simulations were run to provide numerical solutions of temperatures occurring at different points through the chip/tool contact region and the coating/substrate boundary under defined cutting conditions. In addition, an analytical model for heat transfer in the cutting tool and its partitioning, proposed in References [W. Grzesik, P. Nieslony, Physics based modelling of interface temperatures in machining with multilayer coated tools at moderate cutting speeds, Int. J. Mach. Tools Manufact. 44 (2004) 889–901; W. Grzesik, P. Nieslony, A computational approach to evaluate temperature and heat partition in machining with multilayer coated tools, Int. J. Mach. Tools Manufact. 43 (2003) 1311–1317], was employed to generate the input data to computations of the tool–chip interface temperature. The changes of the temperature distribution fields resulting from varying heat flux transfer conditions are the main findings of the FEM simulations. Finally, the analytically and numerically predicted average temperatures were validated against the tool-work thermocouple-based measurements and discussed in terms of relevant literature data.     

Increasing tool life by adjusting the milling cutting conditions according to PVD films’ properties

The coated tools cutting performance in up and down milling depends significantly on the PVD film material properties. The related wear mechanisms at various cutting speeds can be sufficiently explained considering the developed tool loads and the non-linear coating impact resistance versus temperature. Various PVD coated cemented carbide inserts were tested at different cutting conditions. The corresponding cutting loads and temperatures were determined by FEM simulations and the films’ impact resistance by impact tests. A correlation between the impact resistance and the cutting performance at corresponding temperatures contributed to the optimum adjustment of the cutting parameters to the film properties.     

Application of ultra-thin fluorine-content lubricating films to reduce tool/workpiece adhesive interaction during thread-cutting operations

This paper considers the application of lubricious fluorine-organic-based surface-active substances such as perfluorpolyether (PFPE) films for thread-cutting operations. The coefficient of friction of samples prepared both with and without the lubricating film was studied over a range of temperatures from 20 to 550 °C, to test the potential for this film to reduce tool/workpiece adhesive interaction. An 18% reduction in the coefficient of friction was observed at temperatures as high as 400–450 °C for this film. This temperature is within the typical range of thread-tapping operations. To explore this potential application further, the PFPE films were applied on the surface of thread-cutting taps made of high-speed steel. Cutting tool life was also investigated for regular, spiral point, and spiral tap designs, both with and without the PFPE films. The surface morphology of the worn-cutting tools was studied using an SEM and the shape as well as the micro-structure of the chips was also investigated in detail. Overall it was shown that the application of the PFPE film improved the cutting tool life by a factor of two, decreased the average cutting torque by 8% and produced tightly curled chips.     

Friction model for friction stir welding process simulation: Calibrations from welding experiments

The accurate 3D finite element simulation of the Friction Stir Welding (FSW) process requires a proper knowledge of both material and interface behaviors, but friction, the key phenomenon of this process, is quite difficult to model and identify. According to the extreme encountered conditions and the highly coupled nature of the material flow, simple tribological tests are not representative enough, so the welding process itself has been utilized in most analyses of the literature, although its complexity has led to use simplified numerical models and approaches. The recent development of more accurate 3D simulation software, which allows modeling the entire complexity of the FSW process, makes it possible to follow a much more rigorous inverse analysis (or calibration) approach. FSW trials are conducted on an Al 6061 aluminum plate with an unthreaded concave tool. Forces and tool temperatures are accurately recorded at steady welding state, for different welding speeds. The numerical simulations are based on an Arbitrary Lagrangian Eulerian (ALE) formulation that has been implemented in the Forge3^® F.E. software. The main feature of the numerical approach is to accurately compute the contact and frictional surface between the plate and the tool. A first study using Norton's friction model show the great sensitivity of welding forces and tool temperatures to friction coefficients, the need to take into account the changes brought to the contact surface by slight friction variations (thanks to the ALE formulation), the possibility to get very accurate calibrations on forces, and the impossibility to properly render the tool temperature profile. On the other hand, the use of Coulomb's friction model allows obtaining realistic temperature profiles and so calibrating a friction coefficient that offers an excellent agreement with experiments, on forces as much as on tool temperatures, for various welding speeds.     

A new experimental methodology to analyse the friction behaviour at the tool-chip interface in metal cutting

This paper investigates a new test to analyse the friction behaviour of the tool-chip interface under conditions that usually appear in metal cutting. The developed test is basically an orthogonal cutting process, that was modified to a high speed forming and friction process by using an extreme negative rake angle and a very high feed. The negative rake angle suppresses chip formation and results in plastic metal flow on the tool rake face. Through the modified kinematics and in combination with a feed velocity that is five to ten times higher than in conventional metal cutting, the shear and normal stresses are only acting in a simple inclined plane, allowing to calculate the mean friction coefficient analytically. In addition, the test setup allows to obtain the coefficient of friction for different temperatures, forces and sliding velocities. Experiments showed, that the coefficient of friction is strongly dependent on the sliding velocity for the example workpiece/tool material combination of C45E+N (AISI 1045) and uncoated cemented carbide.     

Tools with built-in thin film thermocouple sensors for monitoring cutting temperature

The ability to monitor in real time, the thermal activation and thermal impact to cutting tools has been very appealing to the manufacturing industries. Such responses can be measured with appropriate sensors such as thin film thermocouples (TFTs) built in cutting tools. The challenges have been to instrument the tool, equipment and sensors, which can withstand high stress and temperature in machining process. In this work, the sequence of fabricating the built-in TFTs and experimental setups are proposed and demonstrated. The cutting experiments are carried out under different cutting conditions for A6061-T6 aluminum alloy and finally cutting temperature is measured at very high cutting speeds up to 16 m/s.     

Process simulation using finite element method — prediction of cutting forces, tool stresses and temperatures in high-speed flat end milling

End milling of die/mold steels is a highly demanding operation because of the temperatures and stresses generated on the cutting tool due to high workpiece hardness. Modeling and simulation of cutting processes have the potential for improving cutting tool designs and selecting optimum conditions, especially in advanced applications such as high-speed milling. The main objective of this study was to develop a methodology for simulating the cutting process in flat end milling operation and predicting chip flow, cutting forces, tool stresses and temperatures using finite element analysis (FEA). As an application, machining of P-20 mold steel at 30 HRC hardness using uncoated carbide tooling was investigated. Using the commercially available software DEFORM-2D™, previously developed flow stress data of the workpiece material and friction at the chip–tool contact at high deformation rates and temperatures were used. A modular representation of undeformed chip geometry was used by utilizing plane strain and axisymmetric workpiece deformation models in order to predict chip formation at the primary and secondary cutting edges of the flat end milling insert. Dry machining experiments for slot milling were conducted using single insert flat end mills with a straight cutting edge (i.e. null helix angle). Comparisons of predicted cutting forces with the measured forces showed reasonable agreement and indicate that the tool stresses and temperatures are also predicted with acceptable accuracy. The highest tool temperatures were predicted at the primary cutting edge of the flat end mill insert regardless of cutting conditions. These temperatures increase wear development at the primary cutting edge. However, the highest tool stresses were predicted at the secondary (around corner radius) cutting edge.     

A critical analysis on the friction modelling in orthogonal machining

Despite the development of high performance finite element-based codes, the simulation of machining still represents a very hard task due to the geometric complexity of the real chip-tool systems and the high cutting speed that requires very long simulation times. For these reasons, many aspects related to machining are not very clear and so easy to simulate. In this paper a rigorous investigation on the role played by the implemented friction model within a 2D simulation of orthogonal cutting was carried out, taking into account different models proposed by the researchers in the last years. The main simulation results were compared with experimental measurements in order to verify if it is possible to identify the best model. Once the comparison with mechanical variables was completed, a subsequent study on temperature predictions utilizing the above friction models was executed as well. The results of this integrated numerical and experimental work are carefully reported in the paper.     

硬质合金刀具铣削高硅铝合金CE11的摩擦特性研究

对高硅铝合金这一难加工材料的研究现状进行了分析,并确定采用铣削加工摩擦实验方法进行硬质合金刀具的摩擦学行为研究。建立试验平台及试验方案,并进行铣削力和铣削温度的试验数据采集,求解刀-屑摩擦因数;根据摩擦因数分析主轴转速、摩擦时间和摩擦接触区温度等因素对刀具摩擦特性的影响。研究结果对改善高硅铝合金材料的切削加工性能具有重要指导意义,并为后续各种涂层刀具的摩擦磨损特性研究提供了依据。     

钛合金TC4切削过程流动应力模型研究

运用有限元技术对切削过程进行仿真可以预测切削力、切削温度、应力分布,优化刀具参数和切削条件。建立适合于切削条件中大应变、高应变率条件下材料的流动应力模型,是切削过程有限元仿真的关键技术。文章通过正交切削实验和有限元迭代的方法,修正了难加工材料TC4在大应变、高应变率条件下的J-C流动应力模型,使修正模型能够适应切削仿真中的大应变、高应变率要求。计算结果表明,采用新的J-C流动应力模型进行计算,所得主切削力值与实验测量值的平均误差从36.28%降为12.06%,进给力的平均误差由原来的61.03%降为现在的25.57%。该修正的流动应力模型比用霍普金森实验所得到的流动应力模型更适合于切削过程的有限元仿真,可以提高切削仿真的计算精度。     

Analysis of the manufacturing chain of CVD diamond coated shaft type cutting tools

Hypereutectic aluminium silicon alloys, e.g. casted AlSi17Cu4Mg, are commonly used in the automotive and aeronautical industries. These alloys consist of hard, abrasive silicon particles in a soft aluminium matrix and thus place high mechanical loads on the tool during machining processes. Polycrystalline Diamond or CVD (chemical vapour deposition) diamond based cutting tools can be used for the high speed machining of these alloys due to their high hardness and wear resistance. Diamond thin film coatings of different film morphologies are commonly applied on cemented carbide tools using Hot Filament CVD. The distinguishing characteristic to other coatings is utmost hardness resulting in high resistance to abrasion, low tendency to adhesion and low friction coefficient. The manufacturing of CVD diamond coated shaft type cutting tools is challenging due to the complex design of the cutting edges and the demanding stress behaviour during tool application. The influencing parameters of substrate type, chemical and mechanical substrate pre-treatment as well as diamond film modification on the tool cutting performance are discussed. The manufacturing route of CVD diamond coated thread milling drills is analysed with the use of material and tribological tests. The complex thread manufacturing tools are then applied in the machining of AlSi17Cu4Mg, whereby the tool performance is characterised with respect to their wear behaviour, the process forces and temperatures as well as the workpiece quality.     

Heat generation and temperature prediction in metal cutting: A review and implications for high speed machining

Determination of the maximum temperature and temperature distribution along the rake face of the cutting tool is of particular importance because of its controlling influence on tool life, as well as, the quality of the machined part. Numerous attempts have been made to approach the problem with different methods including experimental, analytical and numerical analysis. Although considerable research effort has been made on the thermal problem in metal cutting, there is hardly a consensus on the basics principles. The unique tribological contact phenomenon, which occur in metal cutting is highly localized and non-linear, and occurs at high temperatures, high pressures and high strains. This has made it extremely difficult to predict in a precise manner or even assess the performance of various models developed for modelling the machining process. Accurate and repeatable heat and temperature prediction remains challenging due to the complexity of the contact phenomena in the cutting process. In this paper, previous research on heat generation and heat dissipation in the orthogonal machining process is critically reviewed. In addition, temperature measurement techniques applied in metal cutting are briefly reviewed. The emphasis is on the comparability of test results, as well as, the relevance of temperature measurement method to high speed cutting. New temperature measurement results obtained by a thermal imaging camera in high speed cutting of high strength alloys are also presented. Finally, the latest work on estimation of heat generation, heat partition and temperature distribution in metal machining is reviewed. This includes an exploration of the different simplifying assumptions related to the geometry of the process components, material properties, boundary conditions and heat partition. The paper then proposes some modelling requirements for computer simulation of high speed machining processes.     

Prediction of shear angle variation in orthogonal wave-removing processes

An analytical model is presented for predicting the dynamic response of the shear plane during an orthogonal wave-removing process. The model is derived based on the work-hardening slip-line field theory in cutting mechanics. It takes into consideration the variations of the hydrostatic stress along the shear plane and the mean frictional coefficient on the tool-chip interface in response to the change in the cutting configuration. The results from a series of simulation studies show that the predictions from the model are in very good agreement with the existing experimental evidence for a wide range of cutting conditions.     

细长轴双刀车削加工让刀量误差优化的研究与应用

通过建立细长轴对称式双刀车削模型,推导出刀具加工点理论让刀量公式。对细长轴双刀车削进行静力学仿真分析,得到刀具在加工点让刀量数据。考虑到刀具径向力与背吃刀量的关系,以及引起刀具让刀量误差的其他因素,在刀具加工点理论让刀量公式中引入修正系数k,对仿真曲线和预测理论曲线进行对比分析,确定修正系数k的大小。最后进行数控车削试验,结果表明:将优化后的理论让刀量公式作为补偿函数车削细长轴能显著提高其加工精度。     

Improving low carbon steel chip breakability by cryogenic chip cooling

This paper presents a cryogenic machining approach to improve the chip breakability for cutting AISI1008 low carbon steel, a typical ductile material. A cryogen delivery system supplies liquid nitrogen (LN2) coolant specifically to chip faces in a properly scaled jet instead of flooding the general cutting zone. The concept for this cooling approach is to locally and selectively bring the chip temperature down to the embrittlement temperature of the workpiece material while minimizing the cooling effect on the primary shear plane, which can otherwise cause increased cutting resistance. The embrittlement temperature was determined to be between −50 and −120°C from standard material tensile and impact tests. A finite element (FE) simulation was performed to predict the chip temperature under various cutting conditions. The numerical results and cutting tests performed on turning operations are used to explain the varied chip breaking improvements for a given range of feeds and cutting speeds.