Reduction of the mesh size influence on the results of a lagrangian finite element machining mode

Mesh dependence of the results of a finite element model are well known in many fields such as in structural design. This problem is however not much addressed in the literature for machining modelling although it is crucial for the quality of the results and the predictive aspect of the model. In this work, an orthogonal cutting model of the titanium alloy Ti6Al4V is exploited. The model formulation is Lagrangian and a damage criterion with eroding elements is used. A strong sensitivity of the results to the size of the elements is observed and the results do not converge when the size of the mesh decreases. To address this issue, a non-local damage criterion that reduces the mesh dependence of the results is introduced. The results show a strong decrease of their dependence to the size of the mesh. The recommendation is to use elements length that is not too far from the size of the grains of the material to avoid a dramatic increase of the computing time for very small elements and the absence of converged results for too large elements

Chip formation mechanism using finite element simulation

Prediction of chip form produced during machining process is an important work when considering workpiece surface creation and possible damage caused by chips generated during machining. The paper presents a set of new results of cutting chip formation from the latest FEM model development. Generally three types of chips, namely, continuous, serrated and discontinuous chips, are generated during metal machining. The formation of these three types of chips is investigated in relation to various influential factors, such as rake angles and depth of cuts. Progressive damage model with damage evolution criterion is employed into the FEM model to reduce mesh dependency. It has been demonstrated that finite element simulation is a good tool for evaluation of chip formation in relation to operational parameters, tool settings as well as material properties

Numerical modeling and analysis of Ti6Al4V alloy chip for biomedical applications

The influence of cutting forces during the machining of titanium alloys has attained prime attention in selecting the optimal cutting conditions to improve the surface integrity of medical implants and biomedical devices. So far, it has not been easy to explain the chip morphology of Ti6Al4V and the thermo-mechanical interactions involved during the cutting process. This paper investigates the chip configuration of the Ti6Al4V alloy under dry milling conditions at a macro and micro scale by employing the Johnson-Cook material damage model. 2D modeling, numerical milling simulations, and post-processing were conducted using the Abaqus/Explicit commercial software. The uncut chip geometry was modeled with variable thicknesses to accomplish the macro to micro-scale cutting by adapting a trochoidal path. Numerical results, predicted for the cutting reaction forces and shearing zone temperatures, were found in close approximation to experimental ones with minor deviations. Further analyses evaluated the influence of cutting speeds and contact friction coefficients over the chip flow stress, equivalent plastic strain, and chip morphology. The methodology developed can be implemented in resolving the industrial problems in the biomedical sector for predicting the chip morphology of the Ti6Al4V alloy, fracture mechanisms of hard-to-cut materials, and the effects of different cutting parameters on workpiece integrity

The chip-flow behaviors and formation mechanisms in the orthogonal cutting process of Ti6Al4V alloy

This work involves experimental and analytical investigations of chip flow stability in metal cutting process. First, in cutting experiments of Ti6Al4V alloy, the transformation of chip morphology from continuous to serrated and later to discontinuous was observed as the cutting speed increased. Scanning electron microscopic (SEM) observation of the shear fracture surface demonstrated shear-localized instability and intergranular failure behaviors. Then we used the improved orthogonal cutting model (OCM) to analyze the plastic flow process of work materials in a plane strain state. A corresponding governing equation system was set up, the dimensionless governing parameters were determined by dimensional analysis, and an instability criterion was established by linear perturbation analysis. Analytical results showed that the plastic instability of chip flow could take place in a continuous chip, which is different from the shear-localized instability in a serrated chip. Finally, in terms of the balance conditions between the kinetic energy and the surface energy, the sawtooth growth behavior in serrated chips and the formation mechanism of discontinuous chips were studied

Finite Element Simulation of Chip Segmentation in Machining a Ti 6al-4v Alloy

Ti 6Al-4V, a titanium alloy introduced in 1954, is considered the workhorse amongst the titanium alloys and is available in all product forms. It is extensively used in aerospace industry because of its excellent strength-to-weight ratio maintained at elevated temperatures, fracture resistance characteristics and exceptional corrosion resistance. The machinability of this alloy is generally considered to be poor owing to its several inherent properties. It is very reactive chemically and therefore has a tendency to weld to the cutting tool during machining. Its low thermal conductivity increases the temperature at the tool/workpiece interface, which affects the tool life adversely. Additionally, its high strength maintained at elevated temperature, low modulus of elasticity further impairs its machinability. In order to overcome the machinability issues associated with machining Ti 6Al-4V, an attempt has been made in this study to observe the effect of machining conditions on the chip formation, rake face and shear zone temperatures and cutting forces. To simulate orthogonal metal cutting of Ti 6Al-4V a commercial, general-purpose FE code (AdvantEdge) has been used. AdvantEdge has the facility to incorporate user-defined material subroutine (UMAT). Using this, Johnson-Cook material model and Recht's catastrophic shear failure criterion are incorporated into the UMAT subroutine code. Finite element simulations are conducted for a range of cutting speeds from 10 m/min to 100 m/min using two different depths of cut of 0.25 and 0.5 mm, for different rake angles from -15 to 45 using depth of cut of 0.5 mm and cutting speed of 30 m/min and for different values of coefficient of friction ranging from 0.3 to 0.9 using depth of cut of 0.5 mm and cutting speed of 30 m/min. Results of the simulations are compared with the experimental data and are found to be in close agreement. Mechanism of chip formation studied from simulations closely matched with that proposed in the literature. Effect of cutting speed, depth of cut, rake angles and coefficient of friction on cutting forces, temperature, strains and chip morphology is studied. Finally, cutting speed for the onset on chip segmentation is found for two different depths of cut.Mechanical & Aerospace Engineerin

A 2D finite element analysis of the effect of numerical parameters on the reliability of Ti6Al4V machining modeling

The numerical analysis, based on the finite element modeling (FEM), presents nowadays an efficient computational tool. It allows a better understanding of several thermo-mechanical phenomena involved during the machining process. However, its reliability heavily depends on the accurate definition of the numerical model. In this regard, a FE analysis focused on the 2D modeling of the Ti6Al4V dry orthogonal machining was carried out in this study. The relevance of different numerical meshing approaches and finite elements topologies was studied. The effect of the friction coefficient on the numerical chip morphology, its geometry, the cutting and the feed forces was investigated. The adequacy of several compared adaptive meshing approaches, in terms of the modeling of severe contact conditions taking place around the cutting-edge radius, was underlined in the current study. However, numerical serrated chips, closer to the experimental ones, were only predicted when the pure Lagrangian formulation was adopted and a proper determination of the failure energy was carried out. The definition of different mesh topologies highlighted the efficiency of the 4-node quadrangular mesh, with a suitable edge length, in increasing the agreement with the experimental data, while reducing the computing times

Ti6Al4V metal cutting chip formation experiments and modelling over a wide range of cutting speeds

Measured forces, chip geometry and tool temperatures from machining a mill annealed Ti6Al4V at cutting speeds mainly from 1 to 100 m/min, but in some cases down to 0.1 m/min, are reported, as well as mechanical testing of the material. Finite element simulations with inputs the measured flow stress, and subsequently a small high temperature strain hardening recovery correction, and a failure model calibrated from the cutting tests at speeds from 1 to 10 m/min, give satisfactory agreement with the higher speed tests once surface strain hardening and damage from the previous pass of the tool are taken into account. This paper’s originality is firstly to show that more complicated flow stress models involving large strain softening are not needed provided shear failure is included; and secondly its failure model: this proposes a non-zero failed shear stress depending on local pressure and temperature. The simulations provide relations between tool mechanical and thermal loading and cutting conditions to aid process improvement

Advancements in material removal mechanism and surface integrity of high speed metal cutting : a review

The research and application of high speed metal cutting (HSMC) is aimed at achieving higher productivity and improved surface quality. This paper reviews the advancements in HSMC with a focus on the material removal mechanism and machined surface integrity without considering the effect of cutting dynamics on the machining process. In addition, the variation of cutting force and cutting temperature as well as the tool wear behavior during HSMC are summarized. Through comparing with conventional machining (or called as normal speed machining), the advantages of HSMC are elaborated from the aspects of high material removal rate, good finished surface quality (except surface residual stress), low cutting force, and low cutting temperature. Meanwhile, the shortcomings of HSMC are presented from the aspects of high tool wear rate and tensile residual stress on finished surface. The variation of material dynamic properties at high cutting speeds is the underlying mechanism responsible for the transition of chip morphology and material removal mechanism. Less surface defects and lower surface roughness can be obtained at a specific range of high cutting speeds, which depends on the workpiece material and cutting conditions. The thorough review on pros and cons of HSMC can help to effectively utilize its advantages and circumvent its shortcomings. Furthermore, the challenges for advancing and future research directions of HSMC are highlighted. Particularly, to reveal the relationships among inherent attributes of workpiece materials, processing parameters during HSMC, and evolution of machined surface properties will be a potential breakthrough direction. Although the influence of cutting speed on the material removal mechanism and surface integrity has been studied extensively, it still requires more detailed investigations in the future with continuous increase in cutting speed and emergence of new engineering materials in industries

Numerical analysis of constitutive coefficients effects on FE simulation of the 2D orthogonal cutting process: application to the Ti6Al4V

In this paper, a deep study of constitutive parameters definition effect is done in order to guarantee sufficient reliability of the finite element machining modeling. The case of a particular biphasic titanium alloy Ti6Al4V known by its low machinability is investigated. The Johnson-Cook (JC) elasto-thermo-visco-plastic-damage model combined with the energy-based ductile fracture criteria is used. Segmentation frequency, chip curvature radius, shear band spacing, chip serration sensitivity and intensity, accumulated plastic strain in the formed chip segments, and cutting forces levels are determined where their dependency to every constitutive coefficient is examined and highlighted. It is demonstrated from the separate variation of every plastic and damage parameters that an interesting finite element modeling (FEM) relevance is reached with the adjustment of JC strain hardening coefficients term, thermal softening parameter, exponent fracture factor, and damage evolution energy. Moderate and high cutting speeds are applied to the cutting tool in the aim to test their impact on shear localization, chip segmentation, and numerical forces levels as well as to approve previous highlighted findings related to constitutive parameters definition. In general, this study focuses on a prominent decrease in identification process cost with the previous knowledge of the most affecting constitutive coefficients while keeping an interesting agreement between numerical and experimental results

Determination of the Deformation State of a Ti-6Al-4V Alloy Subjected to Orthogonal Cutting Using Experimental and Numerical Methods

Orthogonal cutting of Ti-6Al-4V alloy was studied. Surface roughness, chip thickness and shear band frequency increased with the feed rate and cutting speed. Serrated chips were formed due to shear band. Strain and flow stress distributions in the material ahead of the tool tip were estimated from shear angle measurements and microhardness measurements respectively. The stress-strain data obtained in this way was used in numerical models. Two numerical models were developed by using two-dimensional Lagrangian element formulation and Smoothed-particle hydrodynamics formulations employing the Johnson-Cook constitutive relationship that utilised the experimental data generated from the machined material with the damage criteria. The Lagrangian element formulation predicted the strain and temperature generated in the material ahead of the tool tip as 1.65 and 1222 K respectively, which were in agreement with the experimental strain (1.65) and temperature (1217 K). The predicted results using Lagrangian element formulation correlated well with the experimental findings