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Revealing the precipitation behavior of crack-free TiB2/Al-Zn-Mg-Cu composites manufactured by Laser Powder Bed Fusion

Journal Contribution - e-publication

The microstructural evolution and precipitation behavior in additively manufactured age-hardenable TiB2/Al-Zn-Mg-Cu composites were investigated. As-built, direct-aged, solution-treated, and T6 heat-treated samples were characterized at macro, micro, and nano-scales to provide fundamental insights on the effect of the non-equilibrium L-PBF solidification microstructure and the presence of TiB2 reinforcement particles on the precipitation behavior. Crack-free and near fully dense (99.7%) TiB2/Al-Zn-Mg-Cu composites were successfully manufactured using L-PBF. The composites exhibited a fine equiaxed microstructure containing nanometer- and submicrometer-sized TiB2 particles, possessing an improved hardness compared to unreinforced Al-Zn-Mg-Cu alloys. The heterogeneous chemistry and the heterogeneously distributed dislocation density, both direct consequences of the rapid L-PBF solidification, were preserved in the direct-aged sample. The resulting non-uniform distribution of η’ precipitates led to a hardness of 200 ± 5 HV, showing a 40 HV increase compared to the as-built sample. A solution heat treatment enabled the dissolution of interdendritic segregated elements in the as-built sample, resulting in a chemically homogeneous FCC-Al matrix. Simultaneously, it decreased the dislocation density and induced the formation of needle-shaped Al7Cu2Fe phases. The subsequent ageing treatment promoted a high density of well-dispersed η’ nanoprecipitates, leading to an overall Vickers hardness of 215 ± 2 HV for the T6 sample. The TiB2/Al-Zn-Mg-Cu composite exhibited an accelerated precipitation behavior compared to the unmodified Al-Zn-Mg-Cu alloy, mainly due to heterogeneous precipitation on the TiB2 particles and the thermal expansion mismatch dislocations in the vicinity of the TiB2 particles, as well as enhanced diffusion along high-angle grain boundaries.
Journal: Additive Manufacturing
ISSN: 2214-8604
Volume: 66
Publication year:2023