Multiscale Analysis of Al-Li Alloys: On the Impact of the Friction Stir Welding Process

Aerospace components undergo a series of manufacturing process steps which can have a detrimental or beneficial impact on the mechanical performance and conformance of a given part. Extensive studies have been done on the impact of friction stir welding process on Aluminium-based alloys, yet the impact of the process on the material is not fully understood. The main reason being that new tools emerge which allow a new understanding around materials and processes.

State-of-the-art experimental techniques are exploited in this work to reveal an in-depth understanding of the impact of the process on Aluminium 2099 alloy at different length scales. Initially, Scanning Electron Microscopy (SEM), Electron Back-Scattered Diffraction (EBSD) and Energy Dispersive Spectroscopy (EDS) is performed to reveal the microstructural morphology at micro and macro-levels.

Further study utilizing FIB-SEM and coupled Transmission Electron Microscopy (TEM)-EDS was needed to reveal the changes in dislocations and precipitate structures within the grains of the base metal, heat affected zone and weld metal, where the weld region suffered from lower hardness and strength.

TEM-EDS analysis satisfactorily reveals the nano-structural changes related to the welding process, but these techniques are unable to confirm the presence of Lithium in this alloy. Thus, in the next stage the authors use Secondary ion mass spectrometry (SIMS) and Atom Probe Tomography (APT) to reveal the chemistries and structures which contain Lithium at the nanoscale, so the particles and base metal are analyzed adequately and efficiently.

Though all analytical techniques were helpful to characterize the impact of the welding on the material, the macro- and micro-scale techniques are not able to clearly resolve the reduction in the mechanical properties in the weld. Alternatively, the combination of TEM/EDS, SIMS and APT provides more clear evidence and answers.