Spatial heterostructure engineering and enhanced mechanical isotropy in hybrid additive manufacturing of high-strength aluminum alloys via different cyclic interlayer friction stir processing strategies

To address the dilemma between high-defect susceptibility of fusion-based additive manufacturing (AM) and the process-interruption sensitivity of solid-state AM for the high-strength AA7075 alloys, this study reports a hybrid manufacturing technology. It applies cyclic interlayer friction stir processing (C-IFSP) during laser directed energy deposition (LDED). The fundamental advancement lies in establishing a thermomechanical strategy that couples defect elimination and microstructural heterogeneity control, enabling in-situ architectural design of grain size and precipitate distributions across multiple length scales. Two contrasting C-IFSP strategies, unidirectional (UC-IFSP) and reciprocating (RC-IFSP), were systematically investigated. Both eliminated LDED-entrapped pores (porosity reduced by ∼4 and ∼7 orders of magnitude, respectively), but divergent thermomechanical transients yielded markedly different heterostructures: UC-IFSP produced laminated ultrafine/fine-grained layers along the build direction, whereas RC-IFSP established a bimodal grain structure with additional transverse heterogeneity. Consequently, RC-IFSP achieved synergetic enhancement of strength and ductility in the transverse direction while reducing strength anisotropy by 21.7%, reconstituting the strengthening mechanism from nanoprecipitate-dominated (wrought plate) to a synergistic strengthening of precipitate, dislocation, fine-grain, and heterogeneous deformation. This work establishes a pathway for defect-free, hierarchically controlled AM of high-strength Al alloys through spatially programmed thermomechanical processing.