Through-thickness microstructure and residual stress distributions in additive friction stir deposited Aluminum 7075

Abstract

Aluminum 7075-T651 was printed by additive friction stir deposition (AFSD) onto 7075-T651 substrate with one-directional passes to form a $\sim$190 mm wall $\sim$27 mm in height. Neutron residual stress mapping of the as-printed sample was performed in the longitudinal (LD), transverse (TD), and normal (ND) directions of the build at the longitudinal midplane of the wall with 6 mm LD x 3 mm TD x 3 mm ND voxels. These data were contextualized with microhardness and plastometry mapping, powder X-ray diffraction, electron backscatter diffraction, and scanning transmission electron microscopy. Peak tensile stresses were observed in the base plate 4–5 mm below the base plate-AFSD interface with maximum values of 168 ± 20 MPa LD, 152 ± 16 MPa TD, and 129 ± 16 MPa ND. Peak compressive stresses were located in the deposit, with maximum values −63 ± 19 MPa LD, −39 ± 16 MPa TD, and −67 ± 16 MPa. Gaussian process regression was used to estimate the strength and residual stress values at congruent points throughout the analysis plane. The region of peak residual tensile stress was found to reach 32% of the von Mises yield criterion. The spatial distribution of hardness and strength in the part was found to be independent of grain size effects ($d_{50}$ $\sim$ $1\mu m$) but to correlate with the spatial distribution of work hardening, solution strengthening, and precipitation strengthening, reaching an apparent steady state 15 mm below the final AFSD surface.