Rhéologie d'un alliage coulé d'aluminium pour l'optimisation du procédé de forge d'une suspension automobile

Abstract

Aluminum alloys have seen their use increase drastically in recent years, especially in the automotive Industry, thanks to their good weight-mechanical properties. To optimize the use of such materials, it is necessary to know their mechanical properties. In forging, the behavior in hot compression becomes essential, on one side at the level of the stress but also to study the evolution of the microstructure. Various microstructural phenomena such as hardening, softening and recrystallization may occur during and after the deformation. These phenomena have an impact on the stress-strain behavior of the alloy. In order to optimize the forging process of a suspension arm, we will study the rheology of one aluminum alloy obtained by four different processes. It is, then, interesting to simulate the hot compression, thanks to the laws of behavior of the alloy. The present study goal is to know the evolution in stress-strain of the alloy of aluminum 6082, obtained by the four processes: the homogenized and extruded alloy 1, the not homogenized electromagnetically maintained cast alloy 2, the homogenized cast alloy 3 and the same cast alloy 3 but not homogenized. We will do uniaxial compression tests to these four types of process between 450°C and 540°C for strain rates between 0.01s-1 and 0.5s-1. The measurements of the compression force and the displacement give us the stresses and strain and allow us to obtain a relation of the type σ = f (ε). These tests enable us to understand the effect of the strain rate and the temperature on the stress curves during hot compression. The exploitation of these curves follows the hypothesis of uniform deformation within the sample. The stress-strain results consider the elevation of temperature due to the transformation of a part of the compression energy into heat and have been corrected of the friction. The latter has been studied according to different approaches and comparative calculations have been made to determine the influence of these corrections on the stress-strain curves. We can then model these curves using deformation laws, in particular the Arrhenius law, which transcribes microstructural phenomena of the alloy through the activation energy. This energy is calculated for each alloy by two ways: iteratively and numerically. The first method shows us that the activation energy is strongly influenced by the homogenization of the alloy. Then, we can use Hansel Spittel's law to simulate compression tests. The laws defined for the alloys of the study will be used to perform compression simulations identical to the experimental tests. During these simulations, we will then see that the assumption of uniform compression within the sample, is in fact not respected. We will study the effect of friction parameters on the geometry of the simulated sample in order to approximate the geometry of the experimental tests. We will then deduce the stress-strain curves of the simulation to compare them with the experimental curves. This will enable us to show the role played by the heterogeneities of deformations and strain rates within the sample and we will try to link them to the difference in stress obtained at a macroscopic level. In addition to this research on heterogeneity, the variation of the parameters of the behavior law (temperature, strain and strain rate) of the strain-stress curve will be studied, by suppressing or by modifying their effect during compression. We will then conclude on the effects of deformation heterogeneity, friction and the parameters of deformation laws.

Date	27 May 2019
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Philippe Bocher (Supervisor)