Numerical study the flow stress in the machining process

In the machining process, the workpiece is under severe plastic deformation with large strain, high strain rate, and temperature. It is necessary to know the flow stress of workpiece material in such condition to better understand the mechanism of chip formation, tool wear and damage, etc. In this study, a Split Hopkinson Pressure Bar (SHPB) with synchronically assembled heating system was employed to study the flow stress similar to the deformation condition in the machining process. A phenomenological constitutive model was proposed by the regression analysis of the experimental results. Furthermore, orthogonal metal cutting processes were carried out by the finite element method (FEM). The cutting force predicted by the FEM showed good agreement with the experimental results, which confirmed that the proposed constitutive model can give an accurate estimate of the flow stress in the machining process.     

MACHINING OF CORTICAL BONE: SIMULATIONS OF CHIP FORMATION MECHANICS USING METAL MACHINING MODELS

This paper investigates chip formation in the machining of cortical bone and the application of isotropic elastic-plastic material models with a pressure dependent yield stress and a strain path dependent failure strain law to finite element calculations to predict observed behaviour. It is shown that a range of models can be created that result in segmented chip formations and a range of specific cutting forces similar to those observed experimentally. Results from the simulations provide an explanation for differences in the ratio of thrust to cutting forces observed between previous experimental studies, namely that the cutting tools used may have had different edge sharpness or degree of damage induced by the material removal process. Measurements of edge profiles from one of these studies support that explanation and emphasize the importance of tool toughness in maintaining efficient cutting of bone.     

Finite element modeling of 3D turning of titanium

The finite element modeling and experimental validation of 3D turning of grade two commercially pure titanium are presented. The Third Wave AdvantEdge machining simulation software is applied for the finite element modeling. Machining experiments are conducted. The measured cutting forces and chip thickness are compared to finite element modeling results with good agreement. The effects of cutting speed, a limiting factor for productivity in titanium machining, depth of cut, and tool cutting edge radius on the peak tool temperature are investigated. This study explores the use of 3D finite element modeling to study the chip curl. Reasonable agreement is observed under turning with small depth of cut. The chip segmentation with shear band formation during the Ti machining process is investigated. The spacing between shear bands in the Ti chip is comparable with experimental measurements. Results of this research help to guide the design of new cutting tool materials and coatings and the studies of chip formation to further advance the productivity of titanium machining.     

FINITE ELEMENT MODELING OF MACHINING: FROM PROOF-OF-CONCEPT TO ENGINEERING APPLICATIONS

For the past fifty years researchers have developed various machining models to improve cutting performance. Several approaches have been taken including analytical techniques, slipline field solutions, empirical approaches and finite element techniques. Of these, the finite element approach provides the most detailed information on chip formation and chip interaction with the cutting tool. Finite element models have been developed for calculating the stress, strain, strain-rate, and temperature distributions in both the chip and the workpiece. In addition, tool temperatures, machining forces and cutting power requirements can be determined. This information is extremely, useful for developing more fundamental understanding of complex machining problems. This paper presents a critique of finite element approaches used for simulating machining processes. Several applications of the finite element technique for simulating various machining problems are also reviewed. A new application for determining diffusion wear rates in cutting tools is described, and future directions for finite element modeling of machining processes are discussed.     

MANUFACTURING PROCESSES AND EQUIPMENT

ABSTRACT

For the past fifty years researchers have developed various machining models to improve cutting performance. Several approaches have been taken including analytical techniques, slipline field solutions, empirical approaches and finite element techniques. Of these, the finite element approach provides the most detailed information on chip formation and chip interaction with the cutting tool. Finite element models have been developed for calculating the stress, strain, strain-rate, and temperature distributions in both the chip and the workpiece. In addition, tool temperatures, machining forces and cutting power requirements can be determined. This information is extremely, useful for developing more fundamental understanding of complex machining problems. This paper presents a critique of finite element approaches used for simulating machining processes. Several applications of the finite element technique for simulating various machining problems are also reviewed. A new application for determining diffusion wear rates in cutting tools is described, and future directions for finite element modeling of machining processes are discussed.     

航空铝合金7075-T7451三维铣削过程有限元仿真

采用有限元分析软件AdvantEdge建立反映金属切削过程高温、大应变及高应变率状态的切削模型,模拟了航空铝合金7075-T7451铣削加工过程,获得了两个铣削周期内的铣削力变化曲线,并预测了工件及刀具上的温度分布;模拟了铣削过程中切屑的形成,获得了与实际切屑相似的切屑形状。通过铣削力实验获得了相同铣削条件下的铣削力值,与模拟铣削力值比较,发现两者具有较好的一致性,从而证明了有限元模型的正确性。     

Finite element simulation of chip flow in metal machining

Finite element studies of machining are becoming ever more sophisticated. A basic approach which removes the need, in an elastic–plastic analysis, to follow the development of chip formation from initial contact between work and tool, is the iterative convergence method (ICM). It develops a steady-state chip formation from an initial state of a fully formed chip loaded against a tool. It relies for its accuracy on the assumption that its simplified loading path coincides with the real developed flow at the end of the simulation. This paper examines the robustness of this assumption by studying the sensitivity of the simulation to changes of detail, within the ICM method, of how the flow develops; and it compares the simulated results with experiments. The experiment involves the turning of three free cutting steels, for which experimental flow stress variations with strain, strain rate and temperature, as well as information about the friction interaction between chip and tool, are available. The changes to the simulation method considered here are the structure of the finite element mesh, the measures of judging when the flow is fully developed, how the chip separates from the work at the cutting edge and the friction laws used during the approach to fully developed flow. It is shown that these do affect the outcomes of the simulation but within the ranges studied only to a minor extent and good agreement with experiment is achieved.     

A new reverse engineering method to combine FEM and CFD simulation three-dimensional insight into the chipping zone during the drilling of Inconel 718 with internal cooling

ABSTRACT

The use of cooling lubricants in metal machining increases both the tool life and the quality of workpieces and improves the overall sustainability of production systems. In addition to fulfilling these main functions, the focus of machining processes is also related to the reduction of environmental pollution. This can for example be achieved by an optimized arrangement of the cutting tool cooling channels. Therefore, the active cutting edges of the tool should be effectively supplied with a sufficient amount of cooling lubricant. An analysis of the tribological stress is rather difficult because the complex contact zone is inaccessible. Hence, optical investigations are often limited to only observing the chip formation or analyzing the process without considering the influence of the chips.

This article presents an innovative method, which enables a deeper three-dimensional insight into the chip formation zone during drilling with internal cooling channels, considering the cooling lubricant distribution and chip formation. The chip formation simulation based on the finite element method and the computational fluid dynamics flow simulation are combined. In this way, the differences between the different geometric models that do not allow any joint generation of numerical information due to missing interfaces are overcome.     

基于Deform-3D的细长杆切削加工仿真研究

为了更好地研究细长杆的切削过程,以有限元分析法为基础,借助Deform-3D软件的切削仿真模型和数值模拟分析环境,通过对金属切削过程中切屑的形成及流动进行模拟,获得切削力变化规律,切削热和应力应变的分布情况。仿真结果对实际加工过程中工艺效果的预测提供理论依据。     

金属切削过程的有限元法仿真研究

根据材料变形的弹塑性理论,建立了材料的应变硬化模型,采用有限元仿真技术,利用有限元软件ABAQUS对中碳合金钢40CrNiMo切削过程中剪切层及切屑的形成进行仿真,分析切削加工区域的应力、应变的分布。该方法比一般的试验法更省时省力,在研究金属切削理论、材料切削性能及开发刀具产品方面有着工程应用价值。     

NUMERICAL SIMULATION OF TITANIUM ALLOY DRY MACHINING WITH A STRAIN SOFTENING CONSTITUTIVE LAW

In this study, the commercial finite element software FORGE2005®, able to solve complex thermo-mechanical problems is used to model titanium alloy dry machining. One of the main machining characteristics of titanium alloys is to produce a special chip morphology named “saw-tooth chip” or serrated chip for a wide range of cutting speeds and feeds. The mechanism of saw-tooth chip formation is still not completely understood. Among the two theories about its formation, this study assumes that chip segmentation is only induced by adiabatic shear band formation and thus no material failure occurs in the primary shear zone. Based on the assumption of material strain softening, a new material law was developed. The aim of this study is to analyze the newly developed model's capacity to correctly simulate the machining process. The model validation is based on the comparison of experimental and simulated results, such as chip formation, global chip morphology, cutting forces and geometrical chip characteristics. A good correlation was found between the experimental and numerical results, especially for cutting speeds generating low tool wear.     

Numerical modeling of the influence of process parameters and workpiece hardness on white layer formation in AISI 52100 steel

White layer formation is considered to be one of the most important aspects to take into account in hard machining. Therefore, a large number of experimental investigations have been carried out in recent times on the formation mechanisms and properties of the white layer. However, up to now, only very few studies have been reported on modeling of the white layer formation. This paper presents a finite element model which predicts the white layer formation during machining of hardened AISI 52100 steel. This numerical model was properly calibrated by means of an iterative procedure based on the comparison between experimental and numerical data. The empirical model was also validated for a range of cutting speeds, uncut chip thickness, and material hardness values. This study provides excellent results concerning cutting force, temperature, chip morphology, and white layer. From this study, it was also possible to properly analyze the influence of process variables on the white layer formation.     

NUMERICAL AND EXPERIMENTAL INVESTIGATION OF LASER-ASSISTED MACHINING OF INCONEL 718

A numerical investigation of laser-assisted machining for Inconel 718 is presented. This study is based on a three-dimensional finite element model, which takes into account a new constitutive law of Inconel 718 as well as friction and heat transfer models at the tool-chip interface that are developed at the Aerospace Manufacturing Technology Centre (AMTC), of the National Research Council of Canada (NRC), Canada. The material flow stress is described as a function of the strain, the strain rate, and the temperature. The friction model accounts for the sticking and the sliding regions observed experimentally. The formulation of the heat transfer model is based on combining contact mechanics analysis with the solution of the thermal contact problem. The laser beam is modeled as a moving heat source, which is experimentally calibrated. To validate the three-dimensional finite element model, laser-assisted machining experiments were designed and carried out under different cutting conditions. The predicted cutting force and chip thickness are compared with the experimental results. The temperature, stress, strain, and strain rate fields in the primary deformation zone are investigated in order to reveal the plastic deformation process under laser-assisted machining operations.     

Study on mechanism of chip formation during high-speed milling of alloy cast iron

By adopting an equivalent geometry model of chip, a finite element model was developed to study the mechanism of chip formation during high-speed milling of alloy cast iron. Several key technologies such as material constitutive model, friction model, chip separation criteria, chip damage criteria, heat dissipation, and transfer were implemented to improve the accuracy of finite element simulation. Saw-tooth chip of alloy cast iron was observed. The chip shape and cutting force agreed well with experimental results. The simulation results show that the maximum cutting temperature produced with appearance of saw-tooth chip crack, and it is located on the chip-tool contact surface. The saw-tooth chip is caused by double actions of thermoplastic instability and plastic instability. The chip saw-tooth degree decreases when increasing the rotating speed, while it increases when increasing the feed speed. This work provides a useful understanding for chip formation process and helps to optimize machining parameters and process of high-speed milling of alloy cast iron.     

MODELING THE PHYSICS OF METAL CUTTING IN HIGH-SPEED MACHINING

Physical modeling of metal cutting was carried out to provide an understanding and prediction of machining process details. The models are based on finite element analysis (FEA), using a Lagrangian formulation with explicit dynamics. Requirements for material constitutive models are discussed in the context of high-speed machining. Model results address metal cutting characteristics such as segmented chip formation, dynamic cutting forces, unconstrained plastic flow of material during chip formation, and thermomechanical environments of the work-piece and the cutting tool. Examples are presented for aerospace aluminum and titanium alloys. The results are suited for analysis of key process issues of cutting tool performance, including tool geometry, tool sharpness, workpiece material buildup, and tool wear.     

Investigations on the effects of friction modeling in finite element simulation of machining

Accurately predicting the physical cutting process variables, e.g. temperature, velocity, strain and stress fields, plays a pivotal role for predictive process engineering for machining processes. These predicted field variables, however, are highly influenced by workpiece constitutive material model (i.e. flow stress), thermo-mechanical properties and contact friction law at the tool-chip-workpiece interfaces. This paper aims to investigate effects of friction modeling at the tool-chip-workpiece interfaces on chip formation process in predicting forces, temperatures and other field variables such as normal stress and shear stress on the tool by using advanced finite element (FE) simulation techniques.For this purpose, two distinct FE models with Arbitrary Lagrangian Eulerian (ALE) fully coupled thermal-stress analyses are employed to study not only the effects of FE modeling with different ALE techniques but also to investigate the influence of limiting shear stress at the tool-chip contact on frictional conditions, which was never done before. A detailed friction modeling at the tool-chip and tool-work interfaces is also carried by coupling sticking and sliding frictions. Experiments and simulations have been performed for machining of AISI 4340 steel using tungsten carbide tooling and the simulation results under increasing limit shear stress have been compared to experiments. The influence of limiting shear stress on the tool-chip contact friction was explored and validity of friction modeling approaches was examined. The results presented in this work not only provide a clear understanding of friction in FEM modeling of machining but also advance the process knowledge in machining.     

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.     

MODELING THE PHYSICS OF METAL CUTTING IN HIGH-SPEED MACHINING

ABSTRACT

Physical modeling of metal cutting was carried out to provide an understanding and prediction of machining process details. The models are based on finite element analysis (FEA), using a Lagrangian formulation with explicit dynamics. Requirements for material constitutive models are discussed in the context of high-speed machining. Model results address metal cutting characteristics such as segmented chip formation, dynamic cutting forces, unconstrained plastic flow of material during chip formation, and thermomechanical environments of the work-piece and the cutting tool. Examples are presented for aerospace aluminum and titanium alloys. The results are suited for analysis of key process issues of cutting tool performance, including tool geometry, tool sharpness, workpiece material buildup, and tool wear.     

Adiabatic shear in chip formation with negative rake angle

The mechanics of chip formation in grinding is investigated based on thermo-elastic-plastic finite element simulations of orthogonal cutting with a large negative rake abrasive-grits. The modeling is coupled with temperature and strain-rate-dependent flow stress characteristics of a work material SK-5 (0.93%C carbon steel). The shape of chip calculated is affected by the cutting speed and the undeformed chip thickness. In high-speed cutting, serrated chip formation caused by adiabatic shear, which is usually observed experimentally under the cutting conditions of grinding region, is obtained analytically without any consideration of crack propagation. Temperature and flow stress calculated in the primary shear zone vary periodically according to the segmentation of serrated chip. Then changes in temperature, flow stress, strain rate and strain at a material point fixed to and moving with chip is monitored in order to investigate the chip formation process. This clarifies the cutting mechanisms of different types of chip formation with negative rake.     

Finite element simulation of high-speed machining of titanium alloy (Ti�C6Al�C4V) based on ductile failure model

A Johnson?CCook material model with an energy-based ductile failure criterion is developed in titanium alloy (Ti?C6Al?C4V) high-speed machining finite element analysis (FEA). Furthermore, a simulation procedure is proposed to simulate different high-speed cutting processes with the same failure parameter (i.e., density of failure energy). With this finite element (FE) model, a series of FEAs for titanium alloy in extremely high-speed machining (HSM) is carried out to compare with experimental results, including chip morphology and cutting force. In addition, the chip morphology and cutting force variation trends under different cutting conditions are also analyzed. Using this FE model, the ductile failure parameter is modified for one time, afterword, the same failure parameter is applied to other conditions with a key modification. The predicted chip morphologies and cutting forces show good agreement with experimental results, proving that this ductile failure criterion is appropriate for titanium alloy in extremely HSM. Moreover, a series of relatively low cutting speed experiments (within the range of HSM) were carried out to further validate the FE model. The predicted chip morphology and cutting forces agree well with the experimental results. Moreover, the plastic flow trend along an adiabatic shear band is also analyzed.