Fabricating Ti–22Al–25Nb intermetallic with ductility higher than 25% by advanced printing technique: Point-forging and laser-deposition

Abstract

PF-LD is an advanced printing technique which introduces point-forging (PF) into the process of conventional laser-deposition (LD). Ti–22Al–25Nb intermetallic fabricated by PF-LD technique presented the excellent combination of room-temperature ultimate tensile strength (UTS: 1050.1–1169.3 MPa) and elongation after failure (EL: 20.8%–25.7%). An elaborately designed controlled trial by simultaneous use of LD and PF-LD techniques was employed to investigate the effect of PF on LD microstructural evolution. The results pointed out that the occurrence of static recrystallization (SRX) was the essential reason for the ultra-high mechanical properties. Both mechanisms of continuous-SRX (CSRX) and discontinuous-SRX (DSRX) played an important role in grain refinement. By in-situ forging and subsequent re-heating, the coarse columnar grains were first broken into fine equiaxed grains through progressive subgrain rotation. After two-pass PF-LD, the solidification texture <100>//DD disappeared while no obvious recrystallization texture was found. In subsequent PF-LD process, micron-scale CSRX grains were developed within original equiaxed grains through repeated polygonization and progressive subgrain rotation while DSRX grains appeared along grain boundaries driven by the differential dislocation density. We elucidated the excellent mechanical properties by detailed microstructural analysis. The results revealed that the finer equiaxed grains and the reasonable phase composition (O + BCC) greatly retarded the process of crack nucleation and propagation resulting in the typical micro-void accumulation ductile fracture while both high-density dislocations and finer recrystallized grains were beneficial to strength enhancement.