Formability, densification behavior and hierarchical grain structure of laser-directed energy deposition of TiB reinforced titanium matrix composites

Abstract

Objective to obtain the homogeneous microstructure and the high strength-ductility has always been the significant considerations for the additively manufactured titanium alloys, but it is still limited to its huge columnar β grains (>1 mm). Designing equiaxed grain is still a critical method to overcome the challenge. This work successfully in-situ developed 5 vol% TiB reinforced titanium matrix composites (TMCs) with tailorable columnar to equiaxed grain transition (CET) and network structures through controlling laser energy densities (Ev) of the laser-directed energy deposition (L-DED) process. The deposited TMCs (230.8 J/mm3) showed the highest relative density of 97.683%. The sufficient Ev (∼250.0 J/mm3) induced the significant CET and the large hierarchical networks (41 μm). The 5 vol% TiB dramatically reduced the β grain size by over 150 μm, obtaining a refined grain size of 29.8–42.5 μm. TiB/α-Ti interfaces exhibited the favorable bonding and presented the uniform strain distribution without noticeable strain gradient. This work obtained the highest ultimate tensile strength of 1138 MPa (176.4 J/mm3) and remained an acceptable ductility. The good strength-ductility synergy was achieved at the Ev of 230.8 J/mm3 due to the high densification and the CET. The grain refinement was mainly responsible for the excellent strength of TMCs. The CET coupled with hierarchical coarsening networks were advantageous for improving ductility at high level of Ev (≥230.8 J/mm3). This study confirmed that changing solidification condition could further induce the significant CET for the additively manufactured TiB/Ti6Al4V composites.