Achieving uniform microstructure and excellent mechanical properties in TiB2/7055 composite via additive friction stir deposition and heat treatment

Additive friction stir deposition (AFSD) provides an innovative solution for the additive manufacturing of Al-Zn-Mg-Cu alloys and composites with poor printability due to their high hot crack sensitivity. The present work successfully fabricates TiB2/7055 composite deposit as high as 50 mm by AFSD, and systematically investigates the evolution of microstructure and mechanical properties during AFSD and heat treatment. The as-deposited composite exhibits fine equiaxed grains and dispersed TiB2 particles, being different from the feedstock with elongated grains and non-uniformly dispersed particles. Both the size of the primary eta-Mg(Zn,Cu,Al)(2) precipitates and hardness in the as-deposited composite show a significant height gradient along building direction, while the bottom region shows the largest precipitates with the lowest hardness and the top region shows the smallest precipitates with the highest hardness. After heat treatment, these non-uniform features can be eliminated, and the eta-Mg(Zn,Cu,Al)(2) precipitates dissolve into matrix. The fine grains exhibit excellent stability during heat treatment due to the pinning effect of dispersed TiB2 particles. Numerous homogenous nano-sized precipitates (similar to 4.8 nm, GPII zone and eta' phase) form at grain interior, and the interface precipitates (eta-Mg(Zn1.5Cu0.5)) form at the interface between TiB2 particle and Al matrix. In addition, the heat-treated composite achieves uniform mechanical properties across different height positions, with a hardness of 219 HV, tensile strength of 644-650 MPa, and ductility of 13-14.6 %. The corresponding strengthening and toughening mechanisms are thoroughly discussed. These findings provide valuable insights for the fabrication of Al matrix composites with an excellent strength-ductility combination.