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The increasing adoption of metal Additive Manufacturing in high-performance industries, particularly aerospace, necessitates a comprehensive understanding of fracture-critical properties to ensure structural reliability. However, property variability remains a significant barrier to the qualification and certification of AM components. Additionally, standard post-processing treatments have proven inadequate in consistently enhancing the fracture-critical properties of AM titanium alloys, necessitating modifications to advance the state-of-the-art in post-processing. This study investigates the intra-build variability – the part-to-part variations – of electron beam powder bed fusion Ti6Al4V, focusing on fracture toughness and high cycle fatigue performance, while also evaluating the effectiveness of an optimized low-temperature/high-pressure hot isostatic pressing treatment currently developed in AMS7028 in improving damage tolerance. A systematic design of experiments approach was employed to evaluate the influence of common design parameters – including sample orientation, geometry, and spatial location– on microstructural evolution, defect distribution, and mechanical behavior. Seven builds comprised of over 270+ compact tensile and 155+ high cycle fatigue samples were printed representative of the PBF-EB build space, followed by microstructural characterization, X-ray microcomputed tomography, surface roughness analysis, and fracture toughness and high cycle fatigue testing per ASTM E399 and ASTM E466, respectively.
The average fracture toughness in the as-built condition was 65±6 MPa√m, with preferential crack growth direction the dominant factor influence damage tolerance, resulting in orientation-dependent crack propagation mechanisms contributing significantly to property scatter. Additionally, the spatial location and geometry effects demonstrated strong correlations with microstructural variations, influencing the fracture resistance. Thicker samples exhibited higher fracture toughness, further emphasizing the critical role of part design in mechanical performance. The application of modified low-temperature/high-pressure hot isostatic pressing process effectively enhanced fracture toughness to 74±4 MPa√m by reducing internal defects and increasing ductility while preserving the microstructure and yield strength. However, crack propagation dependent on build direction and location-dependent properties remained, highlight the persistent influence of the as-fabricated microstructure.
High cycle fatigue behavior was primarily governed by surface and subsurface defects, with as-built surface roughness contributing to the largest variation in fatigue life. While HIP treatment alone was insufficient to enhance fatigue performance, combining LTHIP, machining, and shot peening significantly improved fatigue strength up to 400 MPa, surpassing cast Ti6Al4V properties. Despite these improvements, intra-build design parameters continued to impact fatigue scatter, with 6mm samples exhibiting 50 – 60 % reduction in fatigue life compared to the 9mm samples due to increased sensitivity to defects.
This research provides crucial insight into the interplay between part design, build design, microstructure, and fracture-critical properties in PBF-EB Ti6Al4V. The findings highlight the need for intentional build strategies, where sample orientation, geometry, and spatial location influenced should be considered when adopting PBF-EB components in load-bearing applications. Furthermore, by implementing traceability of design parameters and PBF-EB-specific post-processing, consistent and repeatable mechanical performance can be achieved. By demonstrating that PBF-EB Ti6Al4V can achieve fracture toughness and fatigue strength comparable to cast and wrought alloys under optimized condition, this study aids in advancing the metal AM for safety-critical applications in aerospace and beyond.