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.     

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.     

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.     

In situ strain measurement in the chip formation zone during orthogonal cutting

The strain and stress state in the chip formation zone determines the chip formation. However, it is difficult to obtain experimental data about the strain/stress fields during machining. For this reason, present chip formation models highly simplify the chip formation process. In order to extend the knowledge regarding the chip formation mechanisms, an experimental method for the in situ measurement of the elastic deformations within the chip formation zone during the cutting process has been developed. Using these deformations, the stress state can subsequently be calculated. The method is based on X-ray diffraction using high-energy synchrotron X-radiation during machining the workpiece in an orthogonal cutting process under quasistatic experimental conditions. The diffraction patterns are captured with a 2D detector. A comparison of the experimentally determined stresses at different measuring positions within the chip formation zone with results from a FEM cutting simulation shows a good qualitative and partially also quantitative consistency. Possibilities for the further performance increase of the method are identified so that the method can be used for the verification and extension of existing chip formation models in future.     

Modified material constitutive models for serrated chip formation simulations and experimental validation in machining of titanium alloy Ti–6Al–4V

Titanium alloys present superior properties such as high strength-to-weight ratio and resistance to corrosion but, possess poor machinability. In this study, influence of material constitutive models and elastic–viscoplastic finite element formulation on serrated chip formation for modeling of machining Ti–6Al–4V titanium alloy is investigated. Temperature-dependent flow softening based modified material models are proposed where flow softening phenomenon, strain hardening and thermal softening effects and their interactions are coupled. Orthogonal cutting experiments have been conducted with uncoated carbide (WC/Co) and TiAlN coated carbide cutting tools. Temperature-dependent flow softening parameters are validated on a set of experimental data by using measured cutting forces and chip morphology. Finite Element simulations are validated with experimental results at two different rake angles, three different undeformed chip thickness values and two different cutting speeds. The results reveal that material flow stress and finite element formulation greatly affects not only chip formation mechanism but also forces and temperatures predicted. Chip formation process for adiabatic shearing in machining Ti–6Al–4V alloy is successfully simulated using finite element models without implementing damage models.     

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 thermomechanical model of cutting applied to turning operations. Part I. Theory

In this paper, an analytical approach is used to model the thermomechanical process of chip formation in a turning operation. In order to study the effects of the cutting edge geometry, it is important to analyse its global and local effects such as the chip flow direction, the cutting forces and the temperature distribution at the rake face. To take into account the real cutting edge geometry, the engaged part in cutting of the rounded nose is decomposed into a set of cutting edge elements. Thus each elementary chip produced by a straight cutting edge element, is obtained from an oblique cutting process. The fact that the local chip flow is imposed by the global chip movement is accounted for by considering appropriate interactions between adjacent chip elements. Consequently, a modified version of the oblique cutting model of Moufki et al. [Int. J. Mech. Sci. 42 (2000) 1205; Int. J. Mach. Tools Manufact. 44 (9) (2004) 971] is developed and applied to each cutting edge element in order to obtain the cutting forces and the temperature distributions along the rake face. The material characteristics such as strain rate sensitivity, strain hardening and thermal softening, the thermomechanical coupling and the inertia effects are taken into account in the modelling. The model can be used to predict the cutting forces, the global chip flow direction, the surface contact between chip and tool and the temperature distribution at the rake face which affects strongly the tool wear. Part II of this work consists in a parametric study where the effects of cutting conditions, cutting edge geometry, and friction at the tool–chip interface are investigated. The tendencies predicted by the model are also compared qualitatively with the experimental trends founded in the literature.     

Mechanics of micro-milling with round edge tools

This paper presents analytical prediction of micro-milling forces from constitutive model of the material and friction coefficient. The chip formation process is predicted with a slip-line field model which considers the strain hardening, strain-rate and temperature effects on the flow stress of the material. The cutting force coefficients are identified from series of slip-line field simulations at a range of cutting edge radii and chip loads. The predicted cutting force coefficients are used to simulate micro-milling forces. The proposed chain of predictive micro-milling model is experimentally proven by conducting brass cutting tests with a 200 μm diameter helical end mill.     

Predictive cutting force model in complex-shaped end milling based on minimum cutting energy

A force model is presented to predict the cutting forces and the chip flow directions in cuttings with complex-shaped end mills such as ball end mills and roughing end mills. Three-dimensional chip flow in milling is interpreted as a piling up of the orthogonal cuttings in the planes containing the cutting velocities and the chip flow velocities. Because the cutting thickness changes with the rotation angle of the edge in the milling process, the surface profile machined by the previous edge inclines with respect to the cutting direction. The chip flow model is made using the orthogonal cutting data with taking into account the inclination of the pre-machined surface. The chip flow direction is determined so as to minimize the cutting energy, which is the sum of the shear energy on the shear plane and the friction energy on the rake face. Then, the cutting force is predicted for the chip flow model at the minimum cutting energy. The predicted chip flow direction changes not only with the local edge inclination but also with the cutting energy consumed in the shear plane cutting model. The cutting processes with a ball end mill and a roughing end mill are simulated to verify the predicted cutting forces in comparison with the measured cutting forces.     

An analytical finite element model for predicting three-dimensional tool forces and chip flow

A model of three-dimensional cutting is developed for predicting tool forces and the chip flow angle. The approach consists of coupling an orthogonal finite element cutting model with an analytical model of three-dimensional cutting. The finite element model is based on an Eulerian approach, which gives excellent agreement with measured tool forces and chip geometries. The analytical model was developed by Usui et al. [ASME J. Engng Indust. 100(1978) 222; 229], in which a minimum energy approach was used to determine the chip flow direction. The model developed by Usui required orthogonal cutting test data to determine the tool forces and chip flow angle. In this paper, a finite element model is used to supply the orthogonal cutting data for Usui's model. With this approach, a predictive model of three-dimensional cutting can be developed that does not require measured data as input. Cutting experiments are described in which good agreement was found between measured and predicted tool forces and chip flow angles for machining of AISI 1020 steel.     

Simulation of serrated chip formation in micro-milling of titanium alloy Ti-6Al-4V using 2D elasto-viscoplastic finite element modeling

Finite element simulations have been utilized in analyses of machining process for several decades. In mechanical micromachining, finite element simulation can also be used for predicting cutting forces, minimal chip thickness, temperatures, and tool wear. The accuracy of results and the computational cost are highly dependent upon the assumptions which govern that particular chip formation problem. This study presents a comparison of two different material assumptions in finite element simulation of micro-milling titanium alloy Ti-6Al-4V. The same simulation was conducted by using the elasto-viscoplastic and the viscoplastic material assumptions. The predicted results are compared against the experimental observations. The results have shown that the material assumption has a major effect on the mechanism of chip formation and heat generation but a minor effect on the cutting force and tool wear prediction. In terms of computational cost, it was found that the simulation with the viscoplastic material assumption can reduce simulation time up to eight times that of required for a simulation with elasto-viscoplastic assumption.     

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.     

A new thermomechanical model of cutting applied to turning operations. Part II. Parametric study

In Part I of this work, Molinari and Moufki [Int. J. Mach. Tools Manufact., this issue], an analytical model of three-dimensional cutting is developed for turning processes. To analyse the influences of cutting edge geometry on the chip formation process, global effects such as the chip flow direction and the cutting forces, and local effects such as the temperature distribution and the surface contact at the rake face have been investigated. In order to accede to local parameters, the engaged part in cutting of the rounded nose is decomposed into a set of cutting edge elements. Thus each elementary chip, produced by a straight cutting edge element, is obtained from an oblique cutting process defined by the corresponding undeformed chip section and the local cutting angles. The present approach takes into account the fact that for each cutting edge element the local chip flow is imposed by the global chip movement. The material characteristics such as strain rate sensitivity, strain hardening and thermal softening, the thermomechanical coupling and the inertia effects are considered in the modelling. A detailed parametric study is provided in this paper in order to analyse the effects of cutting speed, depth of cut, feed, nose radius and cutting angles on cutting forces, global chip flow direction and temperature distribution at the rake face. The influence of friction at the tool–chip interface is also discussed.     

航空铝合金7075-T651高速铣削锯齿形切屑的形成机理研究

目的分析航空铝合金高速铣削锯齿形切屑的形成过程及机理,为提高工件表面质量、延长刀具使用寿命提供理论依据。方法考虑航空铝合金在高速铣削过程中铣削厚度变化的特点,选用合理的本构模型及材料断裂准则,将三维铣削简化为二维变厚度的正交切削热力耦合有限元模型,对锯齿形切屑的形成过程进行有限元模拟,并经铣削试验验证有限元模型的准确性。结果在2～16 m/s的切削速度范围内,铣削力、切削温度、锯齿形切屑形貌均得到了准确的仿真。随着切削速度的增加,切屑厚度、切屑连续部分高度和剪切带间距都有减小的趋势,相反,剪切角随切削速度的增加而增大。切削速度为16m/s时,锯齿形切屑在切屑厚度较大的一侧出现,并随着切屑厚度减小而逐渐消失,变为均匀带状切屑,准确仿真了切削厚度变化下锯齿形切屑形貌。结论提出考虑剪切带宽度变化的三阶段锯齿形切屑形成模型,通过剪切带内外的应变、应变率和温度的变化分析了绝热剪切过程,并使用分割强度比参数量化锯齿形切屑应变程度,控制锯齿形切屑形态。     

Analytical and experimental study of shear localization in chip formation in orthogonal machining

A simplified theory of instability of plastic flow is applied to analyze the formation of shear localized chips in orthogonal machining. A flow localization parameter is expressed in terms of associated cutting conditions and properties of the workpiece material. The analysis, which indicates the important parameters in the cutting process, is used to investigate the effect of cutting conditions on the onset of shear localization and the formation of adiabatic shear banding in metal cutting. Comparisons are made between the analysis and experiments in which the flow localization parameter is obtained for several workpiece materials. The results of this investigation seem to support the analysis and its potential benefits in analyzing and/or remedying problems associated with chip formation and temperature generated in metal cutting. Presently at Advanced Technology Center, Valenite, Inc., Madison Heights,MI 48071, USA     

Orthogonal cutting mechanics of multi-directional carbon fiber reinforced polymer with interlaminar bonding effect

This paper determines the chip formation mechanism, fiber-matrix failure modes, and cutting forces in orthogonal cutting of multi-directional carbon fiber reinforced polymer (MD CFRP) with interlaminar bonding effect. The cutting experiments show that the varying chip formation angles with different fiber orientations in cutting unidirectional plies converge for MD CFRP. A new analytical mechanics model for cutting MD CFRP is developed to predict the chip formation angle and failure modes based on the minimum energy principle for all plies. The model with experimental validation reveals the different cutting mechanisms between UD and MD CFRPs.     

Thermomechanical modelling of oblique cutting and experimental validation

An analytical approach is used to model oblique cutting process. The material characteristics such as strain rate sensitivity, strain hardening and thermal softening are considered. The chip formation is supposed to occur mainly by shearing within a thin band called primary shear zone. The analysis is limited to stationary flow and the material flow within the primary shear zone is modelled by using a one-dimensional approach. Thermomechanical coupling and inertia effects are accounted for. The chip flow angle is determined by the assumption that the friction force on the tool face is collinear to the chip flow direction. At the chip–tool interface, the friction condition can be affected by the important heating induced by the large values of pressure and sliding velocity. In spite of the complexity of phenomena governing the friction law in machining, a reasonable assumption is to consider that the mean friction coefficient is primarily function of the average temperature at the tool–chip interface. Comparisons between model predictions and experimental results are performed for different values of cutting speed, undeformed chip thickness, normal cutting angle and inclination angle. A critical study is presented in order to show the influences of the input parameters of the model including the normal shear angle, the thickness of the primary shear zone and the pressure distribution at the tool–chip interface. The model permits to predict the cutting forces, the chip flow direction, the contact length between the chip and the tool and the temperature distribution at the tool–chip interface which has an important effect on tool wear.     

Prediction of chip flow direction during machining with self-propelled rotary tools

This paper presents a model for chip flow prediction during tube-end machining process using self-propelled rotary tools. The analysis performed is based on the transformation of the relative kinematic relationships in rotary cutting to that of the conventional cutting with large nose radius. Both relative and absolute chip flow angles are investigated. Tests were performed to measure the absolute chip flow angle, insert self propelled motion, and the deformed chip thickness during machining with self-propelled rotary tool under different cutting conditions. The predicted values of the absolute chip flow angle were in good agreement with that measured experimentally.     

Simulation of cutting process in peripheral milling by predictive cutting force model based on minimum cutting energy

The cutting force and the chip flow direction in peripheral milling are predicted by a predictive force model based on the minimum cutting energy. The chip flow model in milling is made by piling up the orthogonal cuttings in the planes containing the cutting velocities and the chip flow velocities. The cutting edges are divided into discrete segments and the shear plane cutting models are made on the segments in the chip flow model. In the peripheral milling, the shear plane in the cutting model cannot be completely made when the cutting point is near the workpiece surface. When the shear plane is restricted by the workpiece surface, the cutting energy is estimated taking into account the restricted length of the shear plane. The chip flow angle is determined so as to minimize the cutting energy. Then, the cutting force is predicted in the determined chip flow model corresponding to the workpiece shape. The cutting processes in the traverse and the contour millings are simulated as practical operations and the predicted cutting forces verified in comparison with the measured ones. Because the presented model determines the chip flow angle based on the cutting energy, the change in the chip flow angle can be predicted with the cutting model.     

Analysis on high-speed face-milling of 7075-T6 aluminum using carbide and diamond cutters

This research is concerned with the analytical and experimental study on the high-speed face milling of 7075-T6 aluminum alloys with a single insert fly-cutter. The results are analyzed in terms of cutting forces, chip morphology, and surface integrity of the workpiece machined with carbide and diamond inserts. It is shown that a high cutting speed leads to a high chip flow angle, very low thrust forces and a high shear angle, while producing a thinner chip. Chip morphology studies indicate that shear localization can occur at higher feeds even for 7075-T6, which is known to produce continuous chips. The resultant compressive residual stresses are shown for the variation of cutting parameters and cutting tool material. The analysis of the high-speed cutting process mechanics is presented, based on the calculation results using extended oblique machining theory and finite element simulation.