Achieving high strength in Ti65 alloys via TiB₂ addition during LDED and simulation of β grain evolution

In this work, the Ti65 matrix composite by adding TiB₂ reinforcement (TMC1) is fabricated via Laser directed energy deposition (LDED). The yield strength and ultimate tensile strength of TMC1 are 1150.5 MPa and 1215.5 MPa, representing increases of 20.2% and 12.0% compared with the Ti65 alloy. The improved yield strength is mainly derived from grain refinement strengthening with a contribution of 160 MPa. The grain evolution model for TMC1 in LDED is established by incorporating the thermo-fluid flow (TFF) model for powder melting and solidification process. The model reveals the different stage effect of TiB₂ reinforcements, in which solute boron modulates the nucleation and growth of β grains within the solid–liquid coexistence region, while TiBw suppress β grain coarsening via the grain boundary pinning effect in the solid-phase region. During multi-layer deposition, boron in TMC1 promotes an early CET. The small equiaxed grains retained after remelting act as nucleation sites for the subsequent deposited layer, effectively suppressing the cross-layer epitaxial growth of columnar grains. Additionally, the misorientation angle significantly influences the grain boundary migration of high-angle grain boundaries (HAGBs) and low-angle grain boundaries (LAGBs) of TMC1 in the solid phase region.