Investigating multi-scale heterogeneity in multi-layer additive friction stir deposition of high-strength aluminum alloys

Abstract

This study investigates the application of multi-layer Additive Friction Stir Deposition (AFSD) for the manufacturing of an AA7075 wall. A particular focus is placed on the material’s structural integrity, including, to the best of our knowledge, the first detailed characterization of the interface between the substrate and the deposited material. The diversity of analytical techniques used provides a detailed understanding of the evolution of microstructure during deposition and as a function of material height.

Scanning electron microscopy in conjunction with X-ray diffraction allows for the observation of the evolution of the microstructure, revealing a smooth transition linked to a mechanical gradient. A crystallographic analysis reveals inter- and intra-layer texture variations, indicating that dynamic recrystallization and restoration mechanisms are concomitantly at work in the deposited material zone, as a function of the vertical distance from the tool. Hardness and tensile measurements indicate a non-negligible evolution from the substrate to the last deposited layer, resulting from the overaging of the ${\eta }^{\prime }$ phase. Finally, a detailed analysis of the interface between the substrate and the deposited material is proposed, which reveals a disturbed microstructure characterized by local heterogeneities in hardness due to significant variations in texture, grain size, and precipitation. All the results are intended to provide highly instructive data regarding microstructural evolution due to thermal cycling both in the deposited material and in the substrate, particularly in the context of the application of the repair of damaged parts.