Low-cycle Fatigue Behavior and Microstructural Characterization of Laser Powder Bed Fusion Processed Ti-55511 Alloy: Influence of Surface Machining Dyuti Sarker and Sugrib Kumar Shaha The Metallurgy Master Mind LLC Naperville, IL, 60565 USA e-mail: info@t3ms.com; phone: +1 877 503 8367
This study examines the microstructural characteristics, tensile response, and low-cycle fatigue (LCF) behavior of near-beta titanium alloy Ti-55511 fabricated by Laser Powder Bed Fusion (L-PBF). Two surface conditions, as-built (AB) and as-built machined (ABM), were compared to determine the influence of surface finishing on mechanical performance. Microstructural analyses using scanning electron microscopy and X-ray computed tomography revealed that machining significantly reduced surface-connected porosity and improved homogeneity. Monotonic tensile testing showed enhanced strength and ductility for the ABM condition, with yield and ultimate tensile strengths increasing from 895 MPa and 960 MPa (AB) to 980 MPa and 1110 MPa (ABM), respectively, accompanied by an increase in fracture strain from 19% to 22%. Fatigue testing indicated that ABM specimens exhibited longer fatigue lives at all strain amplitudes, confirming the beneficial role of surface machining in mitigating early crack initiation. The Coffin–Mansonand Jahed–Varvani energy models were used to correlate experimental fatigue data, both demonstrating strong predictive capability for fatigue life estimation. Fractographic analysis revealed that fatigue cracks in AB samples typically initiated at surface pores and un-melted regions, while ABM specimens showed delayed crack initiation, refined striation, and more stable fatigue crack growth zones. The surface machining significantly enhances the cyclic performance of L-PBF Ti-55511 alloy by reducing defect sensitivity and improving microstructural integrity. These findings provide valuable insights into optimizing post-processing strategies for additive-manufactured titanium components used in aerospace and high-performance structural applications. Keywords: Laser Powder Bed Fusion (L-PBF); Ti-55511; Near-beta titanium alloy; Surface machining; Low-cycle fatigue (LCF); Manson–Coffin model; Jahed–Varvani energy model; Fatigue fracture; Additive manufacturing; Porosity characterization.
Learning Objectives:
Understand how surface machining influences the microstructure, tensile response, and low-cycle fatigue behavior of Laser Powder Bed Fusion processed Ti-55511 alloy, highlighting defect reduction and improved fatigue life prediction accuracy.
Evaluate the effectiveness of Coffin–Manson and Jahed–Varvani energy models in predicting low-cycle fatigue life of L-PBF Ti-55511 alloy under varying surface conditions and microstructural integrity levels.
