Additive manufacturing (AM) or 3D printing is a promising industrial technology that enables rapid prototyping of complex configurations. Powder Bed Fusion (PBF) is one of the most popular AM techniques for metallic materials. Until today, only a few metals and alloys are available for AM, e.g., titanium alloys, the most common of which is Ti-6Al-4V. After optimization of PBF parameters, with or without post processing such as heat treatment or hot isostatic pressing, the printed titanium alloy can easily reach tensile strengths of over 1100 MPa due to the quick cooling of the AM process. However, attributed to the unique features of metallurgical defects and microstructure introduced by this AM process, their fatigue strength has been low, often less than 30% of the tensile strength, especially in very-high-cycle regimes, i.e., failure life beyond 10⁷ cycles. Here, based on our group’s research on the very-high-cycle fatigue (VHCF) of additively manufactured (AMed) Ti-6Al-4V alloys, we have refined the basic quantities of porosity, metallurgical defects, and the AMed microstructure, summarized the main factors limiting their VHCF strengths, and suggested possible ways to improve VHCF performance.

With regards to Thermal Protection System, the thermal-structural responses of the modern ablative materials are of primary importance in many aspects, including material selection and sizing. Due to the intricate nature of its porous structure and complicated thermal conditions, ablative materials may exhibit unexpected behavior, which can potentially lead to material failure. In this study, two solvers -- a direct simulation Monte Carlo solver and a finite-volume based Material Response solver -- are coupled together to predict the micro-scale thermal-structural performance of a Thermal Protection System material. In this approach, individual fibers are modeled at the micro-scale, which provides valuable knowledge of the porous media behavior. Non-uniform boundary conditions, including the heat flux and external force, are captured by the Direct-Simulation Monte-Carlo solver, and the detailed thermal and structural performance of the fiber is captured by the Material Response solver. The results show that individual fibers do not fail based on temperature gradient and applied aerodynamic forces. However, it is shown that attachment points of the fibers are the most vulnerable. This vulnerability can lead to breakdown of the binders, which would separate fibers and cause material collapse.

Defects and stress ratio play critical roles in the fatigue behavior of additively manufactured alloys. This study presents the first comprehensive investigation of the high-cycle fatigue (HCF) behavior of laser powder bed fused (L-PBF) 6061 Al alloy, with explicit consideration of both defects and stress-ratio effects. Fatigue tests conducted at 100 ± 5 Hz under stress ratios of R = -1, 0, and 0.2 yielded S-N curves showing a monotonic decrease in fatigue strength with increasing stress ratio. At a given fatigue life, the fatigue strength followed σmax (R = 0.2) > σmax (R = 0) > σmax (R = -1), or σa (R = -1) > σa (R = 0) > σa (R = 0.2). The lower ΔK observed at R = 0.2 than at R = -1 and 0 indicates that defect-controlled crack initiation is governed not only by defect size but also by stress ratio. Moreover, low stress ratios (R = -1 and 0) promoted pronounced dislocation multiplication and grain coarsening in the crack propagation region, whereas the higher stress ratio (R = 0.2) suppressed such microstructural evolution. A modified Chaboche model explicitly incorporating defects and stress ratio was developed. This model accurately predicts the HCF life of L-PBF 6061 Al alloy across all stress ratios and satisfactorily reproduces the observed scatter in fatigue life. These findings provide new insight into the HCF behavior of L-PBF 6061 Al alloy and highlight the critical roles of defects and stress ratio in governing fatigue performance.

Defects and stress ratio play critical roles in the fatigue behavior of additively manufactured alloys. This study presents the first comprehensive investigation of the high-cycle fatigue (HCF) behavior of laser powder bed fused (L-PBF) 6061 Al alloy, with explicit consideration of both defects and stress-ratio effects. Fatigue tests conducted at 100 ± 5 Hz under stress ratios of R = -1, 0, and 0.2 yielded S-N curves showing a monotonic decrease in fatigue strength with increasing stress ratio. At a given fatigue life, the fatigue strength followed σmax (R = 0.2) > σmax (R = 0) > σmax (R = -1), or σa (R = -1) > σa (R = 0) > σa (R = 0.2). The lower ΔK observed at R = 0.2 than at R = -1 and 0 indicates that defect-controlled crack initiation is governed not only by defect size but also by stress ratio. Moreover, low stress ratios (R = -1 and 0) promoted pronounced dislocation multiplication and grain coarsening in the crack propagation region, whereas the higher stress ratio (R = 0.2) suppressed such microstructural evolution. A modified Chaboche model explicitly incorporating defects and stress ratio was developed. This model accurately predicts the HCF life of L-PBF 6061 Al alloy across all stress ratios and satisfactorily reproduces the observed scatter in fatigue life. These findings provide new insight into the HCF behavior of L-PBF 6061 Al alloy and highlight the critical roles of defects and stress ratio in governing fatigue performance.

Additive manufacturing (AM) or 3D printing is a promising industrial technology that enables rapid prototyping of complex configurations. Powder Bed Fusion (PBF) is one of the most popular AM techniques for metallic materials. Until today, only a few metals and alloys are available for AM, e.g. titanium alloys, the most common of which is Ti-6Al-4V. After optimization of the PBF parameters, with or without post processing such as heat treatment or hot isostatic pressing, the printed titanium alloy can easily reach tensile strengths of over 1100 MPa due to the quickly cooling of AM process. However, attributed to the unique features of metallurgical defects and microstructure introduced by this AM process, their fatigue strength has been low, often only not less than 30% of the tensile strength, especially in very-high-cycle regime, i.e. failure life beyond 107 cycles. Here, we have reviewed the current state of research on the very-high-cycle fatigue (VHCF) of additively manufactured (AMed) titanium alloys, refined the basic quantities of AMed microstructure and metallurgical defects, summarized the main factors limiting their VHCF strengths, and suggested possible ways to improve VHCF performance.