Contributions to understanding the high speed machining effects on aeronautic part surface integrity

摘要

To remain competitive, the aeronautic industry has increasing requirements for mechanical components and parts with high functional performance and longer in-service life. The improvement of the in-service life of components can be achieved by mastering and optimizing the surface integrity of the manufactured parts. Thus, the present study attempted to investigate, experimentally and theoretically, the tool/work material interactions on part surface integrity during the machining of aluminium alloys and hardened materials (low alloy steels) using orthogonal machining tests data. The studied materials are two aluminum alloys (6061-T6 and 7075-T651) and AISI 4340 steel. The AISI 4340 steel was machined after been induction heat treated to 58-60 HRC. These materials were selected in an attempt to provide a comprehensive study for the machining of metals with different behaviours (ductile and hard material).ududThe proposed approach is built on three steps. First, we proposed a design of experiment (DOE) to analyse, experimentally, the chip formation and the resulting surface integrity during the high speed machining under dry condition. The orthogonal cutting mode, adopted in these experiments, allowed to explore, theoretically, the effects of technological (cutting speed and feed) and physical (cutting forces, temperature, shear angle, friction angle, and length Contact tool/chip) parameters on the chip formation mechanisms and the machined surface characteristics (residual stress, plastic deformation, phase transformation, etc.). The cutting conditions were chosen while maintaining a central composite design (CCD) with two factors (cutting speed and feed per revolution).ududFor the aluminum 7075-T651, the results showed that the formation of BUE and the interaction between the tool edge and the iron-rich intermetallic particles are the main causes of the machined surface damage. The BUE formation increases with the cutting feed while the increase of the cutting speed reduces it and promotes the BUL formation on the rake face of the cutting tool. We demonstrated also that by controlling the cutting speed and feed, it is possible to generate a benchmark residual stress state and good surface finish which can improve the in-service life of structural parts made of AA7075-T651aluminum alloys. In this context, correlations have been established between the stress state and the cutting parameters such as cutting forces, shear angle and friction angle.ududWe also investigated the effects of cutting conditions on surface integrity of induction hardened AISI 4340 steel (58-60 HRC) using mixed ceramic inserts. This investigation was motivated by the fact that excessive induction hardening treatment resulted in deep hardened layers (2 mm) with related low compressive residual stresses which may affect the performance of the induction heat treated parts. A judicious selection of the finishing process that eventually follows the surface treatment may overcome this inconvenient. The results showed that the machining process induces significant compressive residual stresses at and below the machined surface. The residual stress distribution is affected by the cutting feed and the cutting speed. On one hand, surface residual stress tends to become tensile when the cutting speed is increased. On the other hand, an increase in cutting feed accentuates surface damage whilst it increases compressive surface residual stress. Microstructural analysis shows the formation of a thin white layer less than 2 μm and severe plastic déformations beneath the machined surface. These results attest that the dry hard machining using ceramic tools may be an alternative to grinding, considered expensive and time consuming, since an enhanced surface integrity in terms of residual stresses and microstructure conditions can be achieved.ududThe first step of this study (experimental study) showed that the surface integrity is closely related to the mechanisms of chip formation. These mechanisms, which are the origin of thermo-mechanical loads, can be quantified by two main parameters: the cutting forces and temperatures generated during machining. Therefore, any attempt to predict the characteristics of the machined surface integrity (residual stresses, transformation phase, etc.), should be, necessarily, involve the prediction of cutting forces and temperature generated during the machining. In this study, we opt out to develop a model for predicting cutting forces and temperatures based on a constitutive equation of the work material that takes into account the effect of strain, strain rate, and temperature. Therefore, the second step of this approach has focused on the identification of the Marusich constitutive equation in order to model the behavior of the materials in high-speed machining. To do so, we proposed an original methodology for identifying the coefficients of Marusich’s constitutive equation (MCE) which demonstrated a good capability for the simulation of the material behaviour in high speed machining. The proposed approach, which is based on an analytical inverse method together with dynamic tests, was applied to aluminums 6061-T6 and 7075-T651, and induction hardened AISI 4340 steel (60HRC). The analytical method consists of determining the material constants inversely using machining tests combined with the response surface models established in the part one of the present thesis. In this section, we investigated the sensitivity of the material constants to the selected temperature models used in the inverse method. Two sets of material coefficients, for each work material, were determined using two different temperature models (Oxley and Loewen-Shaw). The obtained constitutive equations were validated using dynamic tests and finite element (FE) simulation of high speed machining. A sensitivity analysis revealed that the selected temperature model used in the analytical inverse method affected significantly the identified material constants and thereafter predicted dynamic response and machining modeling. In general, material constants obtained using Oxley temperature model gave satisfactory results, compared to Loewen and Shaw model, in predicting the dynamic behaviour and also in predicting the cutting forces during the finite element simulation of the high speed machining of the tested materials.ududFinally, the material models which were identified in the previous step were thereafter implemented in a developed analytical model for predicting cutting forces and températures (the third step of the approach). We tested only the coefficients obtained by the Oxley temperature model, due to their better performance in predicting the cutting forces in FEM compared to those obtained by model Loewen and Shaw ones. This part of the study aimed to verify the coefficients determined for materials and also to generalize the Oxley machining theory for high speed machining of aluminum and hard steel alloy using semi-sharp and honed cutting tool edges. The predicted results were compared with experimental data from the present study and from the literature, covering a large range of cutting conditions (speed, feed, and rake angle). An encouraging good agreement has been found between predicted and measured cutting forces for all tested materials. The strain rate constants in the primary and secondary shear zone were found to be sensitive to the cutting conditions and their effects on the predicted data were discussed in detail. Thanks to the Marusich’s constitutive equation, the Oxley’s machining theory was extended to the high speed machining of aeronautic aluminum alloys and induction hardened steels. The proposed predictive model can be extended also to the prediction of the residual stresses whose their prediction using finite element method is complex and time consuming.ududThrough this experimental and theoretical study, we were able to emphasize the physical mechanisms that govern the chip formation and their effects on the machined surface integrity of two classes of metals (ductile and hard). The proposed approaches can be used in the optimization of the cutting conditions in order to control the surface integrity on the machined parts. Furthermore, the results of this study have been validated for feed rates (10 to 50 μm) comparable to the cutting edge radius (5 and 25 μm) used in the experiments. Thus, the developed models (analytical and finite element) can be extended for studying and modeling the conventional machining processes (turning, milling, and drilling) and nonconventional ones such as the micro-machining process.