Neutron diffraction analysis of residual stress distribution in the lubricant-free TR-AFSD AA7075 repair coupled with SPH simulations

Abstract

This work examines the residual stress in high-strength aluminum alloy repaired by lubricant-free additive friction stir deposition (AFSD) using the same aluminum alloy feedstock. Specifically, a milled groove in an AA7075-T651 substrate was repaired using the twin rod additive friction stir deposition (TR-AFSD) without using any graphite lubricant on the feedstock materials, which is required for conventional square feedstock AFSD. Residual stress distribution in the repaired substrate at different depths was quantified via neutron diffraction, where the distribution of longitudinal residual stress in the TR-AFSD repair was found comparable to materials subjected to other friction-based processes, with an M-shaped or bell-shaped distribution. The tensile longitudinal residual stress, with a peak of 171.3 MPa, spanned the center region around 36 mm, while compressive longitudinal residual stresses, ranging between -112.9 MPa and -12.3 MPa, were balanced outside the center at both the advancing side and retreating sides. The transverse and normal residual stresses were consistent across the repair, with smaller magnitudes between -52 MPa and 68.3 MPa. The non-destructive and high penetration depth nature of the neutron diffraction method enabled the calculation of von Mises stress by interpreting the three measured orthogonal residual stresses as the principal stresses. By normalizing the calculated von Mises stress to the microhardness, this quantified ratio indicates the influence of the embedded residual stresses relative to the material's strength. The higher normalized ratio observed at a deeper depth closer to the bottom of the repair, suggests that the magnitude of residual stresses is closer to the material's strength, indicating a higher potential for residual stress-induced failure at this location. We also calibrated the state-of-the-art smooth particle hydrodynamic (SPH) TR-AFSD process model to predict the von Mises stress distribution in the TR-AFSD AA7075 repair. The experimentally measured residual stress, coupled with the SPH simulation, could further help the research community to minimize the tensile region and alleviate substrate distortion in materials subjected to friction-based processes.