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.     

Chip formation during microscale cutting of a medium carbon steel

Microscale orthogonal cutting tests were conducted on normalized AISI 1045 steel and the resulting chips examined using optical and Scanning Electron Microscopy (SEM). As the uncut chip thickness approaches the size of the smallest average grain type in the material, chip formation changes from continuous to a new type of chip called a quasi-shear-extrusion chip. Results indicate that the pearlite and softer ferrite grains play distinct roles in the plastic deformation process. A Finite Element (FE) model was developed to illustrate the behaviour of the chip formation process during microscale cutting of alternating hard and soft layers of material. The FE model is not an optimization tool, but simply an aid in understanding the mechanics of the microcutting process. The resulting chip morphology is compared to the FE model, and discussed. Stick–slip friction observed at the tool–chip interface is found to affect the transition between shearing and ploughing during the cutting process, chip curl, and the plastic deformation process throughout the chip. The ramifications of the results and model predictions are presented.     

Thermal and mechanical modeling analysis of laser-assisted micro-milling of difficult-to-machine alloys

This study is focused on numerical modeling analysis of laser-assisted micro-milling (LAMM) of difficult-to-machine alloys, such as Ti6Al4V, Inconel 718, and stainless steel AISI 422. Multiple LAMM tests are performed on these materials in side cutting of bulk and fin workpiece configurations with 100-300 μm diameter micro endmills. A 3D transient finite volume prismatic thermal model is used to quantitatively analyse the material temperature increase in the machined chamfer due to laser-assist during the LAMM process. Novel 2D finite element (FE) models are developed in ABAQUS to simulate the continuous chip formation with varying chip thickness with the strain gradient constitutive material models developed for the size effect in micro-milling. The steady-state workpiece and tool cutting temperatures after multiple milling cycles are analysed with a heat transfer model based on the chip formation analysis and the prismatic thermal model predictions. An empirical tool wear model is implemented in the finite element analysis to predict tool wear in the LAMM side cutting process. The FE model results are discussed in chip formation, flow stresses, temperatures and velocity fields to great details, which relate to the surface integrity analysis and built-up edge (BUE) formation in micro-milling.     

Modeling the effects of microstructure in metal cutting

Continuous chips from experimental orthogonal cutting of materials with a heterogeneous microstructure such as 1045 steel are better represented by finite element (FE) models that incorporate material microstructure into the model. A macroscale FE model that incorporated the material microstructure into the model was developed. This approach was found to be more accurate in reflecting the chip formation process than conventional homogeneous models. The heterogenous model showed a rippled chip free surface and defects on the machined surface. The plastic strain was much larger from the heterogeneous FE model versus the homogeneous model due to strain localization during chip formation.     

Milling error prediction and compensation in machining of low-rigidity parts

The paper reports on a new integrated methodology for modelling and prediction of surface errors caused by deflection during machining of low-rigidity components. The proposed approach is based on identifying and modelling key processing characteristics that influence part deflection, predicting the workpiece deflection through an adaptive flexible theoretical force-FEA deflection model and providing an input for downstream decision making on error compensation. A new analytical flexible force model suitable for static machining error prediction of low-rigidity components is proposed. The model is based on an extended perfect plastic layer model integrated with a FE model for prediction of part deflection. At each computational step, the flexible force is calculated by taking into account the changes of the immersion angles of the engaged teeth. The material removal process at any infinitesimal segment of the milling cutter teeth is considered as oblique cutting, for which the cutting force is calculated using an orthogonal–oblique transformation. This study aims to increase the understanding of the causes of poor geometric accuracy by considering the impact of the machining forces on the deflection of thin-wall structures. The reported work is a part of an ongoing research for developing an adaptive machining planning environment for surface error modelling and prediction and selection of process and tool path parameters for rapid machining of complex low-rigidity high-accuracy parts.     

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。     

An enhanced constitutive material model for machining of Ti–6Al–4V alloy

Material failure due to adiabatic shear banding is a characteristic feature of chip formation in machining of Ti–6Al–4V material. In this paper, an enhanced Zerilli–Armstrong (Z-A) based material flow stress model is developed by accounting for the effects of material failure mechanisms such as voids and micro-cracks on the material flow strength during shear band formation. These effects are captured via a multiplicative failure function in the constitutive material flow stress model. The strain and strain rate dependence of the material failure mechanism are explicitly modeled via the failure function. The five unknown constants of the failure function are calibrated using cutting force data and the entire model is verified using separate force, chip segmentation frequency and tool–chip contact length data from orthogonal cutting experiments reported by 0035 and 0040. Model predictions of these quantities based on the enhanced material model are shown to be in good agreement with experiments over a wide range of cutting conditions.     

An investigation into the forging of Bi-metal gears

This paper introduces a method for the production of bi-metal gears using the forging technique. To study the process, model materials of copper (tooth ring material) and lead (core material), were used for both experimentation and simulation. Firstly, experimental setup and test procedures are introduced and the bi-metal gears are forged with different thicknesses of the outer ring material. A simplified FE model is established based on the symmetry of a gear forging process, which enables the 3D FE analysis to be carried out in an efficient manner. The material flow and thickness distribution of the experimentally forged bi-metal gears are analysed and compared with FE predictions. The effect of friction on the axial lock caused by the material flow of the forged gears is also studied. Finally, simulations of different combinations of the inner core and outer ring materials, specifically steel (ring material), copper (ring and core material) and lead (core material) are performed. The numerical and experimental data showed that: thin rings can deform excessively, affecting the structure of the gear; and that both tooling friction and flow stress can significantly affect the relative material flow between the core and the ring.     

Experimental and theoretical investigation of fibre laser crack-free cutting of thick-section alumina

A major challenge in laser fusion cutting of thick-section ceramics is to overcome the thermal-stress induced cracking, which leads to catastrophic breakdown of the material integrity. In order to achieve crack-free cutting of ceramics, it is important to understand the mechanism of the transient temperature field and resulting stress distribution effect on crack formation. In this paper, both experimental and theoretical investigations are reported to understand crack formation characteristics in fibre laser cutting of thick-section Al₂O₃ ceramics. A three-dimensional (3D) finite element (FE) model for simulation of the transient temperature field and thermal-stress distribution together with material removal in laser cutting was developed. Crack formation characteristics were predicted by the model and validated by experiments. The effects of four process parameters i.e. laser peak power, pulse duration, pulse repetition rate and feed rate on temperature field, resulting stress distribution and potential crack formation were also investigated in this work. The study indicates that a transition from compressive to tensile stresses can be resulted in as the laser cutting parameters change, which is beneficial to resist the crack formation. Based on the experimental and numerical investigations, the process parameters were optimised and the fibre laser crack-free cutting of 6-mm-thick alumina was demonstrated for the first time.     

An FEM study on residual stresses induced by high-speed end-milling of hardened steel SKD11

Milling of Hardened steel SKD11 is usually a finishing process, therefore stable cutting process must be guaranteed at first. Residual stresses (RS) were studied in this paper with the help of finite element method (FEM) for its significant influence on the quality of machined part. A two-dimension (2D) fully thermo-mechanical coupled finite element (FE) model was employed to evaluate RS remaining in a machined component. The model was developed based on the effective rake angle and the variable undeformed chip layer. Johnson–Cook plasticity model was introduced to model the workpiece material. Coulomb friction was assumed at the tool–chip interface. Two same cutting tools were employed to model continuous feed milling process. RS profiles were obtained after the cutting and stress relaxation stages. The predicted RS profiles were in reasonable agreement with the experimental results.     

Numerical modeling of vibrothermography based on plastic deformation

This paper investigates vibrothermography for the detection of fatigue cracks in aluminum beams using combined experimental and finite element (FE) analyses. First, a FE modal analysis is carried out to predict the optimal excitation parameters to be employed in experimental investigations performed with an infrared camera. A coupled thermo-mechanical model involving plastic deformation heating is then built and used to simulate the thermographic inspection process. The model shows that the stress at the crack faces exceeds the material’s yield stress confirming that the heat generated during plastic deformation leads to crack detection. The model also predicts the detection of cracks as short as 1 mm that is confirmed experimentally with a maximum error of 0.46% on the temperature evolution. The Fourier transform applied on the numerical thermal response shows that the specimen’s temperature at the crack vicinity changes according to the excitation frequency and presents harmonics due to the nonlinearity induced by the crack.     

Arrhenius-Type Constitutive Model for High Temperature Flow Stress in a Nickel-Based Corrosion-Resistant Alloy

Hot deformation behavior of Nickel-based corrosion-resistant alloy (N08028) was studied in compression tests conducted in the temperature range of 1050-1200 °C and the strain rate range of 0.001-1 s^?1. The flow stress behavior and microstructural evolution were observed during the hot deformation process. The results show that the flow stress increases with deformation temperature decreasing and strain rate increasing, and that the deformation activation energy (Q) is not a constant but increases with strain rate increasing at a given strain, which is closely related with dislocation movement. On this basis, a revised strain-dependent hyperbolic sine constitutive model was established, which considered that the “material constants” in the original model vary as functions of the strain and strain rate. The flow curves of N08028 alloy predicted by the proposed model are in good agreement with the experimental results, which indicates that the revised constitutive model can estimate precisely the flow curves of N08028 alloy.     

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 new Thermo-viscoplastic Material Model for Finite-Element-Analysis of the Chip Formation Process

A new material model for describing the thermo-viscoplastic flow behavior of workpiece material in metal cutting is presented. In order to express the complex flow behavior which depends on the local strain, strain rate and temperature, a new methodology for sequential formulation is proposed. The material parameters which are achieved by using the flow stress data available at low strain rates are enhanced by matching the results of the experimental investigations and finite element simulations of the orthogonal cutting process. As a result, a material model which has a wide validity range of strain, strain rate and temperature is established.     

An extended shear angle model derived from in situ strain measurements during orthogonal cutting

There are numerous cutting models which describe the chip formation process. However, they are based on a number of simplifying assumptions. In order to verify these assumptions and to get a better understanding of the cutting process, the different stress states in the chip formation zone were determined by means of diffraction experiments with monochromatic high-energy synchrotron X-radiation during orthogonal, quasistatic cutting of the material C45E. The results from the experiments are compared with simulated stresses. The experimental data indicate that the assumption of a free chip flow according to the shear angle model of Opitz and Hucks is not valid. The model was therefore extended considering the normal stresses in direction of the chip flow.     

An integrated process for modelling of precipitation hardening and springback in creep age-forming

Creep age-forming (CAF) process has been developed and used to manufacture complex-shaped panel components in aerospace applications. CAF is based on the complex combination of stress relaxation, creep and age hardening. The aim of this paper is to introduce an integrated technique to model stress–relaxation, creep deformation, precipitate hardening and springback in a CAF process. Firstly, a new set of physically-based, unified creep-ageing constitutive equations is presented, which is based on the high temperature creep and ageing kinetics, and, is determined for a solution-treated and quenched AA7010. This new material model is then implemented in the commercial FE solver ABAQUS through a user defined subroutine. An integrated FE simulation process is introduced for the simulation of CAF and springback. In addition to the stress relaxation, creep-age precipitate growth and yield stress evolution during CAF are predicted.     

Microstructure-based finite element analysis of strain localization behavior in AA5754 aluminum sheet

A finite element (FE) analysis incorporating particles and grain structure is used to study the localization behavior of direct chill cast (DC) and strip cast (CC) AA5754 alloy sheet. A two-dimensional (2D) plane stress FE model is used to simulate deformation of a sample under uniaxial tension prior to necking. A 2D plane strain model is then used to simulate the post-necking behavior, up to fracture. The plane stress model shows that the strain required for the initiation of necking is similar in both materials, determined predominately by grain-level inhomogeneity, with constituent particles altering the localization path and localization strains, but only weakly. The plane strain model shows more through-thickness thinning during post-necking deformation of DC sheets compared with CC sheets. The CC material is also prone to shear-type failure, while the DC material exhibits a cup–cone-type failure. These differences arise from the microstructural difference between the two samples, where CC sheets contain more intermetallic particles in stringers compared with the DC sheets. This two-stage model is validated by experimental data which show similar limit strains in the DC and CC sheets but quite different fracture strains and fracture surface geometries.     

Grain Size and Orientation Effects When Microcutting AISI 1045 Steel

Microstructure has a significant effect on microscale cutting. This paper investigates the effect of grain size and orientation during microcutting of AISI 1045 steel. From experimental and finite element (FE) modeling observations, classification of the cutting scale is dependent upon the grain size of the workpiece material. Surface dimple size can be reduced provided there is a reduction in grain size and orientation of grain boundaries are not parallel to the shear plane during microcutting. Incorporating microstructures into a FE cutting model yields a more accurate reflection of the workpiece material's stress-strain behaviour in the primary shear zone.     

Microstructure-Mechanics Interactions in Modeling Chip Segmentation during Titanium Machining

Chip segmentation in machining of titanium alloys is strongly influenced by the microstructural state of the material. A numerical model is presented that incorporates material changes into the phenomenological behavior of the chip. It is calibrated by comparing results with experimental measurements at different cutting speeds and feeds. It predicts that at lower cutting speeds fracture propagates in the α-β phase towards the cutting tool face resulting in a discontinuous chip. At higher cutting speeds, temperature in the secondary shear zone reaches β transus increasing material ductility; the fracture propagates towards the outer surface resulting in a continuous but segmented chip.     

Characterization of the transition from ploughing to cutting in micro machining and evaluation of the minimum thickness of cut

When the machining process is miniaturized two process mechanisms, ploughing and chip formation, are essential and a critical cutting thickness needs to be exceeded so that not only ploughing will occur but chips will also be formed. The ploughing effect thereby influences the chip formation process, workpiece surface roughness, burr formation and residual stress state after processing and is therefore of great interest. In order to optimize the machining process a better understanding of the minimum thickness of cut is crucial.The changes in surface topography along the cutting track occurring during machining with a constant feed rate of the cutting tool were analyzed. The influence of the built-up edge phenomena on the micro machining process was investigated for normalized AISI 1045 using confocal white light microscopy and scanning electron microscopy. Furthermore the sin²ψ-method was applied in order to study the residual stress state in the workpiece surface induced by the machining process. Both surface layer properties investigated, surface roughness and residual stresses, show a characteristic transition indicating a change in the dominating process mechanisms. Based on these results a model is developed to determine the minimum thickness of cut. The minimum thickness of cut is found to significantly decrease with higher cutting velocities and to moderately increase with higher cutting edge radii. In addition a propagation of error for the values obtained with the model was performed, proving the quality of the model developed.