Comprehension of chip formation in laser assisted machining

Laser Assisted Machining (LAM) improves the machinability of materials by locally heating the workpiece just prior to cutting. Experimental investigations have confirmed that the cutting force can be decreased, by as much as 40%, for various materials. In order to understand the effect of the laser on chip formation and on the temperature fields in the different deformation zones, thermo-mechanical simulations were undertaken. A thermo-mechanical model for chip formation was also undertaken. Experimental tests for the orthogonal cutting of 42CrMo4 steel were used to validate the simulation. The temperature fields allow us to explain the reduction in the cutting force and the resulting residual stress fields in the workpiece.Contrat Plan Etat Région (CPER) Pays de la Loir

Experimental characterization and numerical modeling of micromechanical damage under different stress states

The use of HSLA steels for the manufacture of automotive components is interesting from an engineering point of view. This family of steels, while possessing high strength, also has good formability and can be used in forming manufacturing processes. In some forming processes such as blanking, shear strain localization occurs, which causes damage and results in the final fracture of the material. This paper presents an experimental study based on in situ tests to understand and identify the physical mechanisms of ductile damage under two stress states: tension and shear. Different macroscopic tests were performed to calibrate a damage model based on a micromechanical approach. This damage model is based on the Gurson–Tvergaard–Needleman theory and presents recent improvements proposed by Nahshon and Hutchinson and by Nielsen and Tvergaard so as to better predict fracture under a wide range of stress states, especially with low levels of stress triaxiality. These extensions have made the identification of the material parameter more complicated. In this work an identification strategy has been proposed using tests on specimens with different shapes. The identified parameter values are validated and the fracture model show good predictive capability over a wide stress state range

Modelling, identification and application of phenomenological constitutive laws over a large strain rate and temperature range

A review of the different phenomenological thermo-viscoplastic constitutive models often applied to forging and machining processes is presented. Several of the most common models have been identified using a large experimental database (Hor et al., 2013). The latter consists of the tests were done in compression on cylindrical shaped specimens and in shear using hat-shaped specimens. The comparison between these different models is shown that the group of decoupled empirical constitutive models (e.g. the Johnson and Cook (1983) model), despite their simple identification procedures, are relatively limited, especially over a large range of strain rates and temperatures. Recent studies have led to the proposal of coupled empirical models. Three models in this class have also been studied. The Lurdos (2008) model shows the best accuracy but requires a large experimental database to identify its high number of parameters. After this comparison, a constitutive equation is proposed by modifying the TANH model (Calamaz et al., 2010). Coupling between the effects of strain rate and temperature is introduced. This model is easier to identify and does not require knowledge of the saturation stress. Compared to other models, it better reproduces the experimental results especially in the semi-hot and hot domains. In order to study real machining conditions, an orthogonal cutting tests is considered. The comparison between experimental test results and numerical simulations conducted using the previously identified constitutive models shows that the decoupled empirical models are not capable of reproducing the experimental observations. However, the coupled constitutive models, that take into account softening, improve the accuracy of these simulations

Teaching durability in automotive applications using a reliability approach

Fatigue phenomena, which appear generally below the yield stress, is the cause of more than 80 % of in-service mechanical failures. However, the optimization of the weight and cost when designing mechanical components or structures, linked to improved performance, leads to increasingly stressed components. Therefore a fatigue design approach must be done by the engineer. This paper shows the experience gained over five academic years of teaching fatigue the assessment of automotive components using a reliability approach to predict probability of failure, in the engineering school, Arts et Métiers ParisTech, in France. The choice was made to present a comprehensive fatigue assessment approach using a method, initially developed in the automotive industry and since extended to the aeronautical and mechanical industries. This method is known as the “Stress-Strength interference analysis”. The “Stress” represents the distribution of the driver severity, and the “Strength” represents the distribution of the fatigue strength of all the components. A suspension arm is used to illustrate the approach. The Dang Van multiaxial fatigue criterion is implemented in a Finite Elements Code and a danger coefficient is visualized on the meshed structure. The fatigue analysis is interpreted with respect to the target reliability sought by the car- manufacturer

Experimental and numerical analysis of micromechanical damage in the punching process for High-Strength Low-Alloy steels

Sequential sheet metal forming processes can result in the accumulation of work hardening and damage effects in the workpiece material. The mechanical strength of the final component depends on the “evolution” of these two characteristics in the different production steps. The punching process, which is usually in the beginning of the production chain, has an important impact on the stress, strain and damage states in the punched zones. It is essential that the influence of these mechanical fields be taken into account in the simulation of the forming sequence. In order to evaluate the evolution of each phenomenon, and in particular damage accumulation in the forming process, it is essential to characterize the punching process. The objective of this work is to understand and identify the physical damage mechanisms that occur during the punching operation and to establish relevant numerical models to predict the fracture location. The effect of the punch–die clearance on mechanical fields distribution is also discussed in this work

Numerical integration of an advanced Gurson model for shear loading: Application to the blanking process

A new extension of the Gurson damage model has been proposed recently to predict ductile fracture under shear dominated loads. The aim of this work is to verify the ability of this approach to simulate, in an accurate way, the damage evolution in shearing processes. An implicit stress integration algorithm is then developed to implement the new model in a finite element code. The numerical procedure is checked through simulations of shear and uniaxial tension tests on a single elements. The extended Gurson damage model is tested and applied to the punching process to compare its predictive ability with the original approach. The obtained numerical results are in good agreement with experimental results of the punching process, showing better ductile fracture predictions compared to the original Gurson model

Experimental and numerical study of laser-assisted machining of Ti6Al4V titanium alloy

Laser-assisted machining combines several experimental parameters such as cutting speed, feed rate, depth of cut, laser power and distance between tool rake face and the laser beam axis. The optimization of these parameters is necessary to ensure the efficiency of assistance and to increase productivity. This paper focuses on the understanding of the physical phenomena during laser-assisted machining, and on optimising this process. This contribution is based on an experimental and a numerical study. The experimentalpart highlights the effects of the laser power as well as the distance between the tool rake face and the axis of the laser beam. As for the numerical part, it was performed on the ABAQUS/Explicit software. The proposed model improves the understanding of the physical phenomena of chip formation and the cutting force reduction when machining with laser assistance. In addition, this model allows a better optimization of laser and cutting parameters.Numerical findings generally corroborate experimental results and can lead to some other information difficult to catch experimentally. The main contention in the paper is that the distance between the axis of the laser beam and the tool rake face is the most important parameter that controls the reduction of the cutting force. This cutting force reduction can exceed 50%

The influence of laser assistance on the machinability of the titanium alloy Ti555-3

The Ti533-3 alloy is a new titanium alloy which is starting to see increased use in the aeronautical domain to improve the durability of components and to optimize the weight/resistance ratio. This alloy is characterized by greater resistance compared to the more commonly used titanium alloys such as Ti6Al4V. However, a disadvantage of the Ti533-3 alloy is that it is very difficult to machine. In this work, the use of laser-assisted machining has been tested to improve chip formation by a thermal softening phenomenon and to improve the machining productivity of the alloy. A parametric investigation of laser assistance on the machinability of the Ti555-3 titanium alloy shows that: (1) the cutting forces can be greatly decreased if the surface temperature is high; (2) the thermal gradient induced by laser heating modifies the surface integrity in terms of strain hardening and residual stresses in the workpiece; and (3) the chip formation mechanisms are also changed, by increasing the sawteeth frequency when using laser assistanc

Experimental Study of tool Wear Mechanisms in Conventional and High Pressure Coolant Assisted Machining of Titanium Alloy Ti17

Titanium alloys are known for their excellent mechanical properties, especially at high temperature. But this specificity of titanium alloys can cause high cutting forces as well as a significant release of heat that may entail a rapid wear of the cutting tool. To cope with these problems, research has been taken in several directions. One of these is the development of assistances for machining. In this study, we investigate the high pressure coolant assisted machining of titanium alloy Ti17. High pressure coolant consists of projecting a jet of water between the rake face of the tool and the chip. The efficiency of the process depends on the choice of the operating parameters of machining and the parameters of the water jet such as its pressure and its diameter. The use of this type of assistance improves chip breaking and increases tool life. Indeed, the machining of titanium alloys is generally accompanied by rapid wear of cutting tools, especially in rough machining. The work done focuses on the wear of uncoated tungsten carbide tools during machining of Ti17. Rough and finish machining in conventional and in high pressure coolant assistance conditions were tested. Different techniques were used in order to explain the mechanisms of wear. These tests are accompanied by measurement of cutting forces, surface roughness and tool wear. The Energy-dispersive X-ray spectroscopy (EDS) analysis technique made it possible to draw the distribution maps of alloying elements on the tool rake face. An area of material deposition on the rake face, characterized by a high concentration of titanium, was noticed. The width of this area and the concentration of titanium decreases in proportion with the increasing pressure of the coolant. The study showed that the wear mechanisms with and without high pressure coolant assistance are different. In fact, in the condition of conventional machining, temperature in the cutting zone becomes very high and, with lack of lubrication, the cutting edge deforms plastically and eventually collapses quickly. By contrast, in high pressure coolant assisted machining, this problem disappears and flank wear (VB) is stabilized at high pressure. The sudden rupture of the cutting edge observed under these conditions is due to the propagation of a notch and to the crater wear that appears at high pressure. Moreover, in rough condition, high pressure assistance made it possible to increase tool life by up to 400%.Authors would like to thank « Région des pays de la loire » for funding of the project which is part of a PhD thesi

The effect of machining defects on the fatigue behaviour of the Al7050 alloy

During the High Speed Machining (HSM) of aircraft components, geometrical defects, such as mismatches or chatters, can be created. To obtain a high surface quality, an expensive manual grinding operation is systematically done to remove these defects. The aim of this study is to identify the impact of HSM defects on the fatigue behaviour of the aluminium alloy Al7050. After listing and reproducing the most frequently observed surface defects, fatigue tests are conducted under fully reversed plane bending loads. Investigations carried out in previous work showed that residual stresses and the strain hardening introduced by machining under these conditions can be neglected. Therefore, only the geometric aspect of the surface integrity is considered in this study. The results show that the fatigue strength decreases only when the surface roughness is significantly degraded. It is also pointed out that manual grinding allows the effect of the machining defects to be removed from the fatigue behaviour. In order to predict the influence of the surface condition on the fatigue behaviour, a numerical approach based on the real surface topology is also developed. Crack initiation sites that are numerically identified are in agreement with experimental results. Numerical simulation results are compared to the predictions of different fatigue criteria from the literature and discussed over a wide range of surface defects