CT-based material distribution analysis and correlation with residual stress distribution in heterogeneous composite structure fabricated via additive friction stir deposition

A heterogeneous aluminium composite structure was fabricated via additive friction stir deposition using alternating Al6061-T6 and Al7075-T6 feedstock rods. The deposit was designed to achieve periodic layering of dissimilar alloys through a bidirectional deposition strategy. Characterisation of the as-deposited structure was performed using X-ray computed tomography, optical and electron microscopy, Vickers hardness mapping, and neutron diffraction. X-ray computed tomography revealed compositionally partitioned layers with minimal mixing and asymmetrical Al7075 distribution across the deposit, attributed to feedstock sequencing and preference for Al7075 to be deposited on the advancing-side of the rotating tool. Elemental mapping confirmed zinc-rich regions corresponding to Al7075. Electron backscatter diffraction identified refined equiaxed grains and localised shear at Al6061 and Al7075 interfaces. Hardness mapping showed regions of high and low hardness aligned with the individual alloys. Neutron diffraction residual stress characterisation revealed increased tensile residual stress in the alternating-alloy deposit compared to a single-alloy reference, driven by the higher yield strength of Al7075. These findings demonstrate that additive friction stir deposition can be used to produce compositionally and mechanically heterogeneous aluminium structures and highlight the need to explore process control to reduce alloy distribution asymmetry for the manufacturing of layered and functionally graded components.