Optimizing TiB₂ inoculation strategies to achieve isotropic properties in AA6061 fabricated by interlayer-paused additive manufacturing

Additive manufacturing (AM) has become an important technology for producing metallic parts, but the ultrafast solidification often triggers coarse columnar grains and severe hot cracking. Our previous work demonstrated that an appropriate interlayer pause (IP) strategy during laser melting deposition (LMD) can effectively alleviate hot cracking. However, the grains remain textured and filiform, leading to anisotropic mechanical properties. Building on the established optimal IP, this study introduces and optimizes TiB₂ inoculation strategies for the LMD-fabricated AA6061. We found 2 wt% nano-TiB₂ inoculation combining IP successfully achieves isotropic high strength and ductility (longitudinal: 301 ± 3 MPa, 8 ± 2 %; transverse direction: 310 ± 5 MPa, 7 ± 1 %). In contrast, 6 wt% nano-TiB₂ inoculation and 2 wt% micro-TiB₂ inoculation under the same IP yield inferior properties characterized by evident anisotropy. Microstructural investigations reveal the 2 wt% nano-TiB₂ inoculation combining IP promotes a dense and uniform distribution of nano-TiB₂ inoculants, which helps to eliminate cracks and results in an ultra-fine equiaxed microstructure. Conversely, excessive inoculation of 6 wt% nano-TiB₂ leads to severe particle agglomeration, forming large TiB₂ clusters. Similarly, with 2 wt% micro-TiB₂ inoculation, numerous oversized TiB₂ inoculants are observed. Consequently, both inoculation strategies can impair metallurgical bonding and re-induce various metallurgical defects, such as cracks. Furthermore, they can limit the efficiency of columnar to equiaxed transformation (CET), resulting in a relatively coarse microstructure consisting of partially columnar grains. We anticipate that the design strategy developed in this work can be extended beyond Al alloys to achieve isotropic mechanical performance.