Enrico Filippi

A Lagrangian FEM Model to Produce Saw-Toothed Macro-Chip and to Study the Depth of Cut Influence on its Formation in Orthogonal Cutting of Ti6Al4V

The foundations of micro-milling are similar to macro-milling but the phenomena it involves are not a simple scaling-down of macro-cutting. The importance of the minimum chip thickness is one of the significant differences between the two processes. The lagrangian FEM model presented in this paper aims to study the depth of cut influence on chip formation of Ti6Al4V in orthogonal cutting. It is firstly used to compare the modelled saw-toothed macro-chip morphology and cutting forces to experimental cutting results from literature. Then a minimum chip thickness prediction is performed by decreasing the depth of cut. Finally this study is the opportunity to highlight the specific features of micro-cutting reported in literature, such as the effective negative rake angle of the tool or the size effect. The model presented brings therefore a numerical contribution to the comprehension of these phenomena.

On the introduction of adaptive mass scaling in a finite element model of Ti6Al4V orthogonal cutting

Abstract Machining by chips removal operations depend on a large set of parameters, which leads to a time consuming and expensive experimental optimization of this process. Numerical finite element modelling of orthogonal cutting is its most attractive alternative at this time. Apart from the difficulties caused by the complexity of the phenomena involved, the industrial application of this method comes up against unacceptable CPU computing time. The mass scaling is a numerical technique allowing to artificially speed up these calculations. This paper presents its application to a Lagrangian orthogonal cutting finite element model of the most commonly machined titanium alloy, Ti6Al4V, which has not previously been performed. Once the adaptive mass scaling is enabled, the CPU computing time is reduced by about 70% for a typical computation. This improvement should not be performed at the expense of the quality of the results by comparison to the experimental reference (chip morphology, formation mechanism, cutting forces, teeth formation frequency, etc.), nor impact significantly the numerical computation (total mass increase of the model, for example). This study shows that, when used carefully, the adaptive mass scaling constitutes an efficient method to reduce the CPU computation time. It should therefore be considered for the development of future models.

Dynamic Simulation of the Micro-Milling Process Including Minimum Chip Thickness and Size Effect

Micro-milling with a cutting tool is a manufacturing technique that allows production of parts ranging from several millimeters to several micrometers. The technique is based on a downscaling of macroscopic milling process. Micro-milling is one of the most effective process to produce complex three-dimensional micro-parts, including sharp edges and with a good surface quality. Reducing the dimensions of the cutter and the cutting conditions requires taking into account physical phenomena that can be neglected in macro-milling. These phenomena include a size effect (nonlinear rising of specific cutting force when chip thickness decreases), the minimum chip thickness (under a given dimension, no chip can be machined) and the heterogeneity of the material (the size of the grains composing the material is significant as compared to the dimension of the chip). The aim of this paper is to introduce some phenomena, appearing in micromilling, in the mechanistic dynamic simulation software ‘dystamill’ developed for macro-milling. The software is able to simulate the cutting forces, the dynamic behavior of the tool and the workpiece and the kinematic surface finish in 2D1/2 milling operation (slotting, face milling, shoulder milling,…). It can be used to predict chatter-free cutting condition for example. The mechanistic model of the cutting forces is deduced from the local FEM simulation of orthogonal cutting. This FEM model uses the commercial software ABAQUS and is able to simulate chip formation and cutting forces in an orthogonal cutting test. This model is able to reproduce physical phenomena in macro cutting conditions (including segmented chip) as well as specific phenomena in micro cutting conditions (minimum chip thickness and size effect). The minimum chip thickness is also taken into account by the global model. The results of simulation for the machining of titanium alloy Ti6Al4V under macro and micro milling condition with the mechanistic model are presented discussed. This approach connects together local machining simulation and global models.

Comparison of Several Behaviour Laws Intended to Produce a Realistic Ti6Al4V Chip by Finite Elements Modelling

The final aim of finite elements modelling is to help in the choice of the cutting parameters and in the comprehension of the involved phenomena. Representing correctly the behaviour of the machined material is hard due to the extreme conditions encountered, although this is a key parameter to develop a realistic model. Four laws are used in this paper to represent the Ti6Al4V. They are all based on the Johnson-Cook law. This study shows that the influence of the behaviour law is high on the chip morphology and on the forces and that the strain softening phenomenon should be taken into account. For the cutting conditions adopted, it is however necessary to add damage properties in the chip to obtain a morphology and a cutting force evolution close to the experimental reference.

Uncertainty Management of Cutting Forces Parameters and its Effects on Machining Stability

Virtual manufacturing is a field of research which numerically simulate all the manufacturing processes seen by a mechanical part during its production (for example casting, forging, machining, heat treatment,…). Its use is rising on various industries to reduce production costs and improve quality of manufactured parts. One of the most challenging component of the whole simulation chain is the simulation of machining operations due to some of its specificities (need of material law at high strain, strain rates and temperature, heterogeneities of machined material, influence of residual stresses,…).In order to circumvent these difficulties, macroscopic models of machining process have been developed in order to compute more global information (cutting forces, stability of the process, tolerance or roughness for example). For this approach, the cutting forces computation is done by using simple analytical law based on mechanistic approach. The parameters of the models have no clear physical meaning (or at least cannot be linked to intrinsic properties of the material to be machined) and are therefore considered constants for a given set of simulations.The aim of this paper is to take into account the uncertainty on the variability of the cutting force signal during machining operation used as input for mechanistic model identification. The variability of the response during a test on fixed conditions (cutting tool, machined material and cutting parameters) is taken into account to develop a model where parameters of the model can evolve during a given operation.The proposed model is then used as an input of a milling operation simulation in order to study its influence on machining stability as compared to a classical approach.

Experimental investigation of parameter‐dependent analytical cutting force models for the simulation of the milling process

Optimisation of the milling process is often based on simplified models of the cutting forces generated during the machining operations. Most of the simulation algorithms use simple linear relationship between cutting forces and the undeformed section of the chip. The coefficients of those models (often called specific pressure) are considered to be constant with respect to the other parameters. The variation of those coefficients as a function of the cutting speed or the chip thickness has been experimentally observed but most of the authors neglect this effect for simplicity sake. This paper proposes an experimental method to take the variation of these cutting coefficients with respect to cutting parameters into account. Analysis of variance is performed to identify the significant parameters. A design of experiment approach is followed to select the adequate experimental points to ensure maximal precision with the minimal number of machining tests.

Technologies for Dependability and Maintenance

Maintenance is the science that studies the compendium of technical and management actions, from the point of view of planning as well as management, which enable an organisation to perform economic, safe, efficient and environmentally effective management, according to established quality standards, of any type of asset or group of assets (Fedele et al., 2004). Considering the general applicability to any merchandise, technical and productive sector, as well as the strategic scope of this science for the continuance of human livelihood, maintenance is one of the systems theory sciences, to which the general principles of systems engineering apply.

Influence on surface characteristics of electron beam melting process (EBM) by varying the process parameters

The use of additive manufacturing processes keeps growing in aerospace and biomedical industry. Among the numerous existing technologies, the Electron Beam Melting process has advantages (good dimensional accuracy, fully dense parts) and disadvantages (powder handling, support structure, high surface roughness). Analyzes of the surface characteristics are interesting to get a better understanding of the EBM operations. But that kind of analyzes is not often found in the literature. The main goal of this study is to determine if it is possible to improve the surface roughness by modifying some parameters of the process (scan speed function, number of contours, order of contours, etc.) on samples with different thicknesses. The experimental work on the surface roughness leads to a statistical analysis of 586 measures of EBM simple geometry parts.

Effect of HIPping (Hot Isostatic Pressing) on electron beam melting Ti6Al4V parts after machining

The fast growing of Additive Manufacturing (AM) leads us to study the functionality of parts built by these processes. Recently, the Electron Beam Melting process and the Direct Melting Laser Sintering process are used to produce parts in the biomedical and aeronautical fields. The Ti6Al4V is largely used in these fields. This paper present an experimental study of machining Ti6Al4V alloy produced by Electron Beam Melting (EBM) before and after HIPping (Hot Isostatic Pressing). The results shows that the hipping has no significant influence on specific cutting pressure.

Numerical Simulation of Machining Operations on Flexible Parts

Airspace industry components frequently need high added value part including some featuredifficult to manufacture. One of the best example is the thin walls of parts (airplanes frames orthe turbine blades) that have a very low stiffness. The finishing operations for high height to thicknessratio parts lead to chatter vibrations, unacceptable dimensional errors or poor surface finish. The optimalmachining strategy determination is often based on trial and error and may not be cost effective(acceptable conditions can be far from the optimum). Simulation of the milling process is a powerfulmean to accelerate the search for better cutting parameters. Cutting forces, vibrations, geometricerrors or roughness can be predicted before the production of the first parts. The classical mechanisticapproach is even though limited while machining flexible parts because the dynamic response ofthe workpiece changes with the position of the cutter. The objective of this paper is to demonstratethe adaptation of numerical simulation of milling operation for the machining of thin-walled plates.Three complementary approaches are developed: location-dependent stability lobes, quasi static approachand full dynamic simulation. Location dependent stability lobes extend the classical theoryto take into account the variation of dynamic response along the workpiece. Quasi static approach isintended to deal with form error during chatter-free machining operations. Full dynamic simulation isa more complex approach intended to simulate the behavior of the complete tool/machine/workpiecesystem. The numerical approach is compared to experimental tests performed on thin plate of titaniumalloys.