Microstructural processes occurring during creep of friction stir welded AA2024-T3 alloy

Abstract

The poor weldability of AA2024 aluminum alloy limits its use for industrial applications. Being a nonfusion welding process, Friction Stir Welding (FSW) seems to be a promising solution for welding this alloy. FSW was applied in the current study in order to butt weld AA2024T3 aluminum alloy plates and to study the creep behavior of the weld. Creep tests were conducted at 250°C and 315°C, both on the parent material and on the friction stir welded specimens. A comprehensive Transmission Electron Microscopy (TEM) study together with a High Resolution Scanning Electron Microscopy (HRSEM) study and Energy Dispersive X-ray Spectroscopy (EDS) analysis was conducted in order to investigate the microstructural processes. The parent material seems to contain two kinds of Cu-rich precipitates: coarse precipitates having the size of a few microns each and uniformly dispersed fine nanosized precipitates. However, this microstructure was found to be unstable at the temperature range of 250-315°C; secondary precipitation was found to take place. This secondary precipitation is responsible for grain boundary decoration and the appearance of secondary rod-shaped precipitates and for some degree of coarsening of the nanosized precipitates inside the grains. TEM study yielded that the material undergoes dynamic recrystallization (DRX) during creep as well as during the FSW process. Various stages of the development of dislocation networks into a cellular dislocation structure and finally into dislocation free recrystallized grains were recorded. The friction stir welded material, which has already recrystallized during welding, undergoes DRX during creep so that ultrafine grains are being created concurrently. Precipitation processes at the friction stir welded material occur as well during creep. The instability of the microstructure during creep and exposure to high temperature plays an important role in the analysis of the creep results. The influence of the above microstructure changes occurring during creep on the creep behavior will be referred to and discussed.