On the Enhanced Creep Performance in Ti6246 Achieved Through Laser Powder Bed Fusion (LPBF) Processing

The high susceptibility of the Ti–6Al–2Sn–4Zr–6Mo wt pct (Ti6246) alloy to microstructural changes stands as a challenge when processed by the laser powder bed fusion (LPBF) technology. However, leveraging the capabilities of the LPBF process to successfully control the microstructure (and/or crystallographic texture) of the Ti6246 could improve mechanical properties, particularly at elevated temperatures. In this study, the creep performance (at 500 °C) of Ti6246 fabricated from three different LPBF processing conditions and heat-treated (HT) at 885 °C were investigated. In the as-built state, all the LPBFed-Ti6246 exhibited columnar microstructures with crystallographic lamellar-like microstructure (CLM), a near-single crystal-like microstructure (SCM), and polycrystalline microstructure (PCM) textures, respectively. At low applied stresses (100–300 MPa), diffusional creep was the dominant deformation mechanism and its resistance depended on grain size. The reference β-forged-HT Ti6246, characterized by large equiaxed grains, exhibited the lowest strain rate compared to the columnar microstructure of SX1 (CLM)-HT, SX2 (SCM)-HT, and SX3 (PCM)-HT. Conversely, dislocation slip governed deformation at high applied stresses (400–580 MPa) and its efficacy depended on the α/β interfaces in the microstructures. Disjointed columnar grains in SX1 (CLM)-HT and the deformation of the polycrystalline grains in SX3 (PCM)-HT indicated that the melt pool boundaries were unstable in the LPBFed-Ti6246. SX2 (SCM)-HT exhibited the longest creep life due to the relatively stable melt pool boundaries and the near < 001 > SCM crystallographic texture parallel to the applied stresses. Shallow ductile dimples and tears and the observation of laser scan tracks characterized the fracture surfaces of the LPBFed-Ti6246. These indicated that failure occurred by intergranular ductile fracture resulting from the formation of microvoids at the melt pool boundaries.