Full-Tensor Neutron Diffraction Residual Stress Analysis of Additive Friction Stir Deposited Aluminum 7075

Additive Friction Stir Deposition (AFSD) is a solid-state additive manufacturing process capable of producing fully dense parts without fusion-related defects. However, this process generates complex residual stress states through asymmetric frictional heating, severe plastic deformation, and multi-layer thermal cycling. Despite this complexity, prior residual stress characterizations have assumed that stress fields align with the principal deposition directions, leaving shear stress contributions unexamined.

This study presents spatially resolved, full strain and stress tensor measurements of an Al 7075 AFSD deposit obtained via neutron diffraction. 36 crystallographic orientations were measured at 57 positions across a central cross-sectional plane to over-determine the six independent strain tensor components. Normal stress components reached magnitudes up to 64 MPa while shear components (up to 14 MPa, ~22% of peak normal) exhibited patterns consistent with the rotational process kinematics. Three distinct stress regions were identified along the build direction: a high-deviatoric-stress baseplate-deposit interface, a thermally relaxed central zone, and low-magnitude upper layers. These results provide the first experimental verification that the principal-axis-alignment assumption is approximately valid for AFSD, with shear components constituting a minor but physically interpretable fraction of the total stress state.