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

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 pre-cipitation 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 con-sequences of the rapid L-PBF solidification, were preserved in the direct-aged sample. The resulting non-uniform distribution of eta' 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 pro-moted a high density of well-dispersed eta' 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.

Automated summary

The microstructural evolution and precipitation behavior in age-hardenable TiB2/Al-Zn-Mg-Cu composites manufactured using L-PBF were studied. The composites exhibited a fine equiaxed microstructure with TiB2 particles, resulting in improved hardness compared to unreinforced alloys. The precipitation behavior of the composite was accelerated, mainly due to heterogeneous precipitation on the TiB2 particles and enhanced diffusion along grain boundaries.