The influence of friction models on finite element simulations of machining

In the analysis of orthogonal cutting process using finite element (FE) simulations, predictions are greatly influenced by two major factors; a) flow stress characteristics of work material at cutting regimes and b) friction characteristics mainly at the tool-chip interface. The uncertainty of work material flow stress upon FE simulations may be low when there is a constitutive model for work material that is obtained empirically from high-strain rate and temperature deformation tests. However, the difficulty arises when one needs to implement accurate friction models for cutting simulations using a particular FE formulation. In this study, an updated Lagrangian finite element formulation is used to simulate continuous chip formation process in orthogonal cutting of low carbon free-cutting steel. Experimentally measured stress distributions on the tool rake face are utilized in developing several different friction models. The effects of tool-chip interfacial friction models on the FE simulations are investigated. The comparison results depict that the friction modeling at the tool-chip interface has significant influence on the FE simulations of machining. Specifically, variable friction models that are developed from the experimentally measured normal and frictional stresses at the tool rake face resulted in most favorable predictions. Predictions presented in this work also justify that the FE simulation technique used for orthogonal cutting process can be an accurate and viable analysis as long as flow stress behavior of the work material is valid at the machining regimes and the friction characteristics at the tool-chip interface is modeled properly.     

Investigations on the effects of multi-layered coated inserts in machining Ti-6Al-4V alloy with experiments and finite element simulations

This paper presents investigations on turning Ti-6Al-4V alloy with multi-layer coated inserts. Turning of Ti-6Al-4V using uncoated, TiAlN coated, and TiAlN + cBN coated single and multi-layer coated tungsten carbide inserts is conducted, forces and tool wear are measured. 3D finite element modelling is utilized to predict chip formation, forces, temperatures and tool wear on these inserts. Modified material models with strain softening effect are developed to simulate chip formation with finite element analysis and investigate temperature fields for coated inserts. Predicted forces and tool wear contours are compared with experiments. The temperature distributions and tool wear contours demonstrate some advantages of coated insert designs.     

Optimization of dry machining parameters for high-purity graphite in end milling process via design of experiments methods

This investigation applied the designs of experiments (DOE) approach to optimize parameters of a computer numerical control (CNC) in end milling for high-purity graphite under dry machining. The groove difference (i.e., dimensional accuracy of groove width) and the roughness average at the bottom plane of the inside groove (i.e., the plane of end milling) were studied. Planning of experiment was based on a Taguchi orthogonal array table. The analysis of variance (ANOVA) was adapted to identify the most influential factors on the CNC end milling process. Simultaneously, applying regression analysis a mathematical predictive model for predictions of the groove difference and the roughness average has been developed in terms of cutting speed, feed rate, and depth of cut. The feed rate is found to be the most significant factor affecting the groove difference and the roughness average in end milling process for high-purity graphite. Additionally, the tool worn surfaces after machining were examined by the optical zoom scope (OZS).     

A finite element analysis of residual stresses in stretch turning

An analytical method has been proposed in this paper to determine the residual stresses in stretch turning. This is done using the thermal elasto-plastic finite element method to determine the transient temperature distribution, followed by thermal elasto-plastic stress analysis. An attempt to explain the mechanism of stretch turning is made, and the effects of stretch level on residual stresses are also discussed.     

Thermo-mechanical modeling of orthogonal machining process by finite element analysis

A plane strain finite element method is used with a new material constitutive equation for 1020 steel to simulate orthogonal machining with continuous chip formation. Deformation of the workpiece material is treated as elastic–viscoplastic with isotropic strain hardening, and the numerical solution accounts for coupling between plastic deformation and the temperature field, including treatment of temperature-dependent material properties. To avoid numerical errors associated with large deformation of elements, automatic remeshing is used, with at least 15 rezonings required to achieve a satisfactory solution. Effects of the uncertainty in the constitutive model on the distributions of strain, stress and temperature around the shear zone are presented, and the model is validated by comparing average values of the predicted stress, strain, strain rate and temperature at the shear zone with experimental results. Parametric effects associated with cutting speed and initial work temperature are considered in the simulations.     

3D finite element analysis of tool wear in machining

The paper is focused on the 3D numerical prediction of tool wear in metal cutting operations. In particular, an analytical model, able to take into account the diffusive wear mechanism, was implemented through a specific subroutine. Furthermore, an advanced approach to model heat transfer phenomena at the tool-chip interface was included in the numerical simulation. The adopted simulation strategy gave the possibility to properly evaluate the tool wear. The 3D FEM results were compared with some experimental data obtained turning AISI 1045 steel using uncoated WC tool; a good agreement was found out.     

有限元模拟预测AA6111合金半连续铸件的热裂敏感性（英文）

为了预测凝固过程中的热裂敏感性(HTS)并提高铝合金铸件的质量,采用直接有限元(FE)方法建立AA6111的本构方程。通过有限元模型和热撕裂准则的耦合,建立用于工业AA6111合金半连续铸造的热撕裂模型。将此模型应用于实际制造过程,表征铸造速度、底部冷却、二次冷却以及几何形状变化对HTS的影响。结果表明,铸坯的HTS随着速度和铸坯半径的增大而增加,而随着底部界面传热系数或二次水冷却速率的增大而降低。该模型展示出在模拟热裂引发时结合最大孔隙率的能力,这将对优化铸造条件和化学成分以达到HTS最小化从而控制铸造质量产生重大影响。     

带缺口的焊丝拉拔试验及有限元分析

对带缺口的活性无镀铜焊丝的变形趋势进行试验和有限元分析,解决金属在拉拔过程中自接触的算法问题。得出带有纵向U型缺口焊丝添加活性剂的效果最好,纵向V型缺口次之,横向U型缺口添加活性剂的效果最差。并对带缺口的焊丝进行拉拔试验,验证有限元分析的正确性,同时对焊丝截面进行扫描电镜能谱分析,得出闭合后的凹槽内部确实含有拉拔前涂覆的活性剂中的Se元素。     

Adhesion analysis and dry machining performance of CVD diamond coatings deposited on surface modified WC-Co turning inserts

This paper investigates the effects of different surface pretreatments on the adhesion and performance of CVD diamond coated WC-Co turning inserts for the dry machining of high silicon aluminum alloys. Different interfacial characteristics between the diamond coatings and the modified WC-Co substrate were obtained by the use of two different chemical etchings and a CrN/Cr interlayer, with the aim to produce an adherent diamond coating by increasing the interlocking effect of the diamond film, and halting the catalytic effect of the cobalt present on the cemented carbide tool. A systematic study is analyzed in terms of the initial cutting tool surface modifications, the deposition and characterization of microcrystalline diamond coatings deposited by HFCVD synthesis, the estimation of the resulting diamond adhesion by Rockwell indentations and Raman spectroscopy, and finally, the evaluation of the dry machining performance of the diamond coated tools on A390 aluminum alloys. The experiments show that chemical etching methods exceed the effect of the CrN/Cr interlayer in increasing the diamond coating adhesion under dry cutting operations. This work provided new insights about optimizing the surface characteristics of cemented carbides to produce adherent diamond coatings in the dry cutting manufacturing chain of high silicon aluminum alloys.     

Determination of tool friction in presence of flank wear and stress distribution based validation using finite element simulations in machining of titanium and nickel based alloys

Tool friction plays a very important role in machining titanium and nickel-based alloys and is an important parameter in Finite Element based machining simulations. It is the source for the high amount of heat generation, and as a result, the excessive flank wear during machining these materials. The worn tool is known to create poor surface qualities with high tensile surface residual stresses, machining induced surface hardening, and undesirable surface roughness. It is essential to develop a methodology to determine how and to what extent the friction is built up on the tool. This study facilitates a determination methodology to estimate the stress distributions on the rake and flank surfaces of the tool and resultant friction coefficients between the tool and the chip on tool rake face, and the tool and the workpiece on tool flank face. The methodology is applied to various tool edge radii and also utilized in solving stagnation point location on the tool edge. Predicted friction results are further validated with comparison of predicted stress distributions from FE simulations for machining of titanium alloy Ti-6Al-4V and the nickel-based alloy IN-100. It was found that tool stresses and friction are mainly influenced by tool rake angle, edge radius, and tool flank wear and are slightly affected by the cutting conditions in the ranges that were considered in this study.     

Electro-thermal measurements and finite element method simulations of a spark plasma sintering device

Current, voltage and temperature measurements were performed at different points of the system to identify the controlling parameters of the spark plasma sintering (SPS) process. The very low inductance effects despite the high intensity current circulating through the SPS column justifies the use of Joule heating to characterize the phenomenon. The measurements also enabled the improvement and validation of an earlier electro-thermal numerical model developed using the finite element method (FEM). It has been shown that the electrical resistivity and the thermal conductivity of each of the elements are crucial parameters for the simulations. These parameters strongly modify the current modeled, thereby affecting the temperature distribution throughout the SPS column.     

Finite element modelling of the thermo-mechanical behavior of coatings under extreme contact loading in dry machining

During metal cutting processes, intensive friction and high temperature generated at the tool chip interface affect the cutting zone of the tool, by inducing damage and wear. To improve the cutting tool's life, thin hard coatings, synthesized by physical or chemical vapor deposition (PVD or CVD) techniques, are often used as protective layers. In this work, numerical/theoretical analysis of dry machining has been performed to study the impact of different coating layers on the machining process. Four cases are considered: an uncoated tool made of tungsten carbide (WC-Co) and coated tungsten carbides in three different configurations. The first one is made of one layer namely TiN, the second one (hypothetical carbide insert) is composed of two layers (Al₂O₃ and TiN), and the last one has three layers (TiCN, Al₂O₃ and TiN). The workpiece material is an AISI 316L stainless steel. All cutting conditions are fixed in order to highlight the effect of coatings independently from others influencing parameters. The analysis has shown the impact of the different configurations of coatings on the temperature level inside the tool and on its surface, on the pressure and also on the cutting and feed forces.     

THS6350卧式加工中心转台蜗杆的有限元模态分析

采用有限元法对一种新型蜗杆进行了自由模态分析和研究.计算出蜗轮蜗杆的固有频率和相应的振型,并用试验模态法对测出的试验结果进行了验证,以保证有限元模型的正确性.所得结论反映了蜗杆的动力学特性,司时也为蜗杆传动系统的动态响应分析及结构设计提供了理论依据.     

Crystal plasticity finite element simulations of pyramidal indentation in copper single crystals

Vickers and Berkovich indentation experiments in single crystals are examined via three-dimensional finite element simulations. Annealed and strain-hardened copper are used as model materials to assay the physical consistency of the continuum crystal plasticity theory used in the simulations. Agreement between experiments and simulations is predicated in terms of hardness, instrumented indentation applied load (P)–penetration depth (h_s) curves, and material pileup and sinking-in development at the contact boundary. It is shown that while hardness is essentially insensitive to loading direction, some degree of anisotropy is present in the P–h_s curves. The influence of crystal slip anisotropy and of prior straining on the dislocation density patterns underneath sharp indenters is examined. A similarity is found between contact responses obtained with the J₂ associated-flow theory (describing elasto-plastic deformation in isotropic polycrystalline metals) and those from present crystal plasticity simulations. The implications for mechanical property extractions are discussed.     

A new approach for the friction identification during machining through the use of finite element modeling

Finite Element Modeling (FEM) of chip formation has proved great sensitivity to tool/chip friction coefficient. This parameter cannot be adequately identified through conventional tests, because thermal and mechanical loadings during these tests are far from those encountered during machining. In this study, the inadequacy of using constant Coulomb's friction coefficient in FEM is showed. Although a good agreement is found for cutting force and chip thickness variables, significant differences can be found for feed force and tool–chip contact length. Differences of more than 50% are observed in some cases for those variables when FEM results are compared with experimental ones. A new approach to identify a friction model after experimental tests will be detailed. This new approach involves application of a variable friction coefficient at the tool–chip interface, which allows obtaining a better agreement between numerical results (differences close to 10%) regarding the feed force.     

Increased accuracy by using dynamic finite element method in the constrain-surface stereolithography system

The E-DARTS apparatus is a commercial rapid prototyping (RP) system that uses the constrain-surface stereolithography (SL) technology. The advantages of the type SL system are saving resin and apparatus fees, although its accuracy needs to be increased. To find the best scanning path planning is one solution to improve the accuracy of the built part. In this paper, the diagnostic H-4 parts have been fabricated and simulated according to different path plans. The critical dimensions of the built H-4 diagnostic parts are also measured and compared with the analytical results, showing that the best path plan for building the H-4 diagnostic part is the contour-out path. In addition, it has been shown that using the skip skill in raster fill is also effective for improving the full filling of the big or long area.     

Analysis of residual stresses induced by dry turning of difficult-to-machine materials

Critical issues in machining of difficult-to-cut materials are often associated with short tool-life and poor surface integrity, where the resulting tensile residual stresses on the machined surface significantly affect the component's fatigue life. This study presents the influence of cutting process parameters on machining performance and surface integrity generated during dry turning of Inconel 718 and austenitic stainless steel AISI 316L with coated and uncoated carbide tools. A three-dimensional Finite Element Model was also developed and the predicted results were compared with those measured.     

Analysis of ultrasonic assisted machining (UAM) on regenerative chatter in turning

Ultrasonic assisted machining (UAM) is an advanced technology for improving the machining process, especially for hard materials. This paper presents an experimental and theoretical study toward the effect of UAM on chatter. Theoretical explanation of the effect of UAM on chatter is not fully presented in the available literature yet. In this paper, considering fixed tool geometry, theoretical dynamic equations for UAM are represented. The approach is demonstrated by deriving dynamic formulation of UAM in turning, considering both turning equation and Merchants ultrasonic machining equations. A time domain analysis is fulfilled on each machining condition to verify whether it has a stable vibration or an unstable chatter vibration. Subsequently, an experimental setup is designed and manufactured to investigate UAM effect on regenerative chatter. Special conical shape for workpiece is designed to experimentally generate different points of stability lobe. The generated oscillation by a piezoelectric actuator is transferred, amplified, and concentrated on the tip of the tool by appropriate design of a cutting tool, which is vibrated in its bending mode. The obtained results are encouraging, and indicating good agreement between experimental and theoretical results.     

Considering the influence of heating rate,complex hardening and dynamic strain aging in AISI 1045 machining: experiments and simulations

In the modelling of machining operations, constitutive models must consider the material behavior subject to high plastic strains, high strain rates, high temperatures and high heating rates. A new material model for AISI 1045, which captures time-dependent plastic response associated with interrupted austenite transformation under short (sub-second) heating times, is deployed to simulate orthogonal cutting experiments. High speed video and digital image correlation measurements are used to capture chip behavior. The new model, which also includes complex strain hardening and dynamic strain aging effects, show better agreement with experiments at high cutting speeds compared with a basic Johnson-Cook material model from the literature.