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).;The 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).;For 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 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.;The 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.;Finally, the material models which were identified in the previous step were thereafter implemented in a developed analytical model for predicting cutting forces and temperatures (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.;Through 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 ?m) comparable to the cutting edge radius (5 and 25 ?m) 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. (Abstract shortened by UMI.).