Thomas PERRIN – Ageing of precipitation-hardened aluminium alloys during service: microstructural evolution and associated mechanical properties

Jury

Mme Myriam DUMONT : Professeure des universités, Ecole nationale supérieure d'arts et métiers, Rapporteure.
Mr. Vladimir ESIN : Professeur des universités, Université de Lorraine, Rapporteur.
Mr. Joël DOUIN : Directeur de recherche, CNRS Occitanie Ouest, Examinateur.
Mr. Guilhem MARTIN : Maître de conférences, UGA, Examinateur.
Mr. Frédéric DE GEUSER : Directeur de recherche, CNRS Alpes, Directeur de thèse.
Mr. Pierre HEUGUE : Ingénieur docteur, Safran Transmission Systems, Invité.
 

Abstract

The housings and covers of turbojet engine transmission gearboxes are predominantly manufactured from precipitation-hardened aluminium alloys. A critical design requirement is ensuring their resistance to thermal ageing during operation due to the high temperatures of the oil contained in these components. The nanoprecipitates in these alloys undergo significant evolution during thermal exposure, which detrimentally affects mechanical properties. Understanding the microstructural evolution and its impact on mechanical performance over extended periods, up to 100,000 hours (approximately 10 years), is essential for accurate component dimensioning and service life prediction. However, experimental testing for thermal ageing times beyond 20,000 hours is impractical due to development schedule constraints, necessitating the development of reliable predictive models. A comprehensive experimental database is required to construct and validate such models. This thesis focused on investigating the evolution of mechanical properties, using extensive experimental data from samples aged under a temperature gradient for durations of up to 10,000 hours. The use of a  thermal gradient enabled the characterization of both mechanical and microstructural changes across a continuous spectrum of ageing temperatures (from 165°C to 245°C), employing highthroughput characterization techniques such as small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and hardness profiling. Additional isothermal ageing heat treatments allowed for the establishment of correlations between hardness changes and tensile properties (yield strength, ultimate tensile strength), allowing the prediction of tensile properties based on hardness data obtained from samples subjected to the temperature gradient. Localized analyses using transmission electron microscopy (TEM) and atom probe tomography (APT) were occasionally employed to provide deeper insight into the microstructural mechanisms occurring during overageing. This methodology was applied to two wrought aluminium alloys (2219-T851 and 2618A-T851) and a cast alloy (A357+1wt%Cu-T7), which are currently in use or considered for such  applications. The study revealed distinct behaviours in the alloys, involving complex phenomena in addition to precipitate growth and coarsening, such as phase transformations and metastable precipitate stabilization.  Nevertheless, kinetic modelling using time-temperature equivalence produced consistent results, enabling accurate predictions of the kinetic evolution of mechanical properties across the temperature range investigated.