Fracture Toughness Enhancement of a Laser Powder Bed Fusion Manufactured Ti-55511 Alloy with a Heat-Treatment-Tailored Hierarchical Microstructure

Abstract

Near-β titanium alloy Ti–5Al–5Mo–5V–1Cr–1Fe (Ti-55511), fabricated using laser powder bed fusion, typically exhibits heterogeneous microstructures characterized by strongly textured columnar prior-β grains, resulting in low fracture resistance during crack growth. To address this, in this work, a three-step heat treatment was developed, consisting of solution treatment near the β transus temperature, followed by a double-stage aging process. This treatment produced a hierarchical α phase structure, with uniformly distributed tertiary α phases in the equiaxed β grains. Subsequently, the LPBF-fabricated Ti-55511 samples demonstrated an excellent combination of mechanical properties, with a yield strength exceeding 1100 MPa, ductility greater than 10%, and fracture toughness exceeding 76.8 MPa·m1/2. This study elucidated the mechanisms by which multi-stage heat treatments govern the formation of nano-scale secondary α phase, sub-micron α phase, and primary α phase microstructures. Consequently, the hierarchical microstructure imparted the material with an excellent combination of strength and toughness.

Keywords: laser powder bed fusion (LPBF), Ti-55511 alloy, fracture toughness, isotropic mechanical properties, hierarchical microstructure