Characterization of Residual Stress and Precipitate Evolution in Aluminum 2xxx Self-Reacting Friction Stir Welds

2xxx series aluminum alloys possess attractive properties for structural aerospace applications including high strength to weight ratio, corrosion resistance, and stable cryogenic performance. Solid state joining processes are often employed to reduce weld defects and improve weld performance/consistency as many alloys of this range have poor weldability for traditional fusion based joining techniques. One such process, self-reacting friction stir welding (SRFSW) allows for consistent high-quality, welding of large and curved articles is often used in the construction of large structures such as launch vehicle liquid propellant tanks.

Despite the merits of this process, joint softening (a decrease in mechanical properties related to the base material) remains a persistent issue. This phenomenon has two principal sources: residual stresses formed due to the thermomechanical nature and high restraint conditions involved and coarsening/over-ageing of strengthening precipitates.

This research focuses on the evaluation of aluminum 2xxx SRFSWs via neutron scattering methods. Residual stress was evaluated in the longitudinal, normal, and transverse directions via neutron diffraction and quantified throughout the entire subsurface region and informed by metallography and microhardness studies. Precipitate evolution evaluation was carried out via small-angle scattering (SAS) techniques and supported by scanning electron microscopy (SEM), x-ray diffraction (XRD), and wide-angle X-ray scattering (WAXS) – pending transmission electron microscopy (TEM) and atom probe tomography (APT). Variations in both residual stress and precipitation are observed and described. Residual stress displayed an ‘M-shaped’ profile with highest tensile values occurring in the thermomechanically affected zone (TMAZ). XRD and WAXS identified three secondary phases: θ [theta] and θ’ [theta prime] precipitates (Al₂Cu) and an intermetallic ω [omega] (Al₇Cu₂Fe). SAS indicates that prior to solvation in the stir zone (SZ) the θ’ [theta prime] precipitates coarsen and decrease in volume fraction to the heat affected zone (HAZ) while the θ [theta] precipitates coarsen and increase in volume fraction to the TMAZ.