Defects and microstructure are key concerns in Additive Manufacturing (AM) of metal components. Further understanding of contributions from build design to metal quality is needed. For this investigation, multiple builds were designed and printed to determine the relative influence of part thickness, spacing, and location on the tensile properties of Grade 5 Ti6Al4V produced by electron beam melting (EBM). Including both vertically and horizontally oriented specimens, a suite of mechanical properties were evaluated. Both the elongation at failure and tensile toughness were found to be highly correlated with part thickness. For the vertical orientation, these properties were further influenced by part location, with non-linear dependence on height and a linear dependence on radial distance from the center of the build plate. The metal with horizontal orientation did not exhibit the same relationships, with only mild correlations with height and radial position. For the vertical orientation, the elongation at failure and toughness were primarily dependent on characteristics of the lack of fusion defects, whereas microstructure was the dominant contributor to properties for the horizontal orientation. Visual maps representing property distributions over the build space were constructed. These tools can contribute to design decisions and lead to more consistent part performance for AM metals.

Powder reuse has become a central issue in the pursuit to industrialize metal Additive Manufacturing (AM). Furthermore, build porosity is a critical concern to component reliability and damage tolerance of the metal. However, the contributions of powder reuse to metal porosity has received very limited attention. In this study, the porosity resulting from Powder Bed Fusion-Electron Beam Melting (PBF-EB) AM of Ti6Al4V was characterized over a series of 30 build cycles (consisting of similar to 480 h cumulative build time) using X-ray Micro Computed Tomography (mu CT). Investigated were the volume fraction of pores (i.e. the porosity), as well as the pore size, shape, and spatial distribution. The most prevalent pores identified were: i) those originating from the gas atomized powder, and ii) those caused by incomplete fusion of the melt pool. Although there was a minor reduction in the pore size distribution with powder reuse, the overall average volumetric porosity was 0.10 +/- 0.02% and there was no significant change with increasing reuse. An inverse relationship between pore diameter and sphericity was found, with large pores posing greater effective stress concentration. Whereas the greatest pore density was located at the transition between the contour and melt beams, approximately 0.5-1.0 mm adjacent to the surface of parts, the largest pores were located within the interior hatching region. Hence, despite progressive deformation of the particles and an increase in oxidation, there was no discernable change in metal porosity with powder reuse in PBF-EB AM of Ti6Al4V.

Powder reuse is recognized as a key to industrialization of metal additive manufacturing (AM), which necessitates that changes in the structural behavior of metal with reuse are clearly understood. In this investigation, the mechanical properties of Ti6Al4V resulting from Electron Beam Melting (EBM) were evaluated as a function of powder reuse over 30 build cycles. The metal was characterized in the horizontal and vertical build orientations under uniaxial tension to failure in the as-built and machined conditions. Results showed that there was an increase in strength and decrease in ductility of the metal with powder reuse, resulting from rising oxygen content of the powder. The elongation at failure and tensile toughness of the metal exhibited anisotropy, which increased substantially with powder reuse. For the horizontal orientation, the elongation at failure decreased by over 60% in the 30 builds, which was one and a half times greater reduction than that for the vertical orientation. The anisotropy in ductility with reuse is suspected to result from changes in microstructure with oxygen content, specifically changes in the grain boundary alpha phase. Additionally, the powder recovery system (PRS) was identified as a contributor to powder oxidation.

Porosity is one of the primary concerns in metal powder bed fusion Additive Manufacturing (AM). While investigations concerning defects in metal AM are common, there is limited understanding concerning the spatial distribution of pores in the metal, and how they evolve with powder reuse. In this study, the porosity in Ti6Al4V produced by Electron Beam Melting (EBM) AM was characterized by X-ray Micro Computed Microtomography (μCT) over a series of 30 build cycles (~480 hr cumulative build time). Investigated were the volume fraction of pores (i.e. the porosity), their size, shape, and location distribution. The most prevalent pores identified were those originating from the gas atomized powder and pores caused by incomplete fusion of the melt pool. The overall average volumetric porosity was 0.10±0.02%, with no significant dependence on powder reuse. A general tightening of the pore size distribution was seen with reuse including slight increases in the 10th and 50th percentile and a reduction in the 90th percentile. However, some consistency in the size distribution was noted, with some builds developing extreme size pores. An inverse relationship between pore diameter and sphericity was found, with large pores posing greater effective stress concentration. Regarding spatial distribution, the largest pore density developed around 0.5 mm adjacent to the surface of parts, at the transition between the contour and melt beams. Based on results of this study, µCT is recommended for quality control of metal parts, and models that account for these defects in defining the damage tolerance of components are needed.

Additive Manufacturing (AM) processes for metals are advancing at a rapid pace. Among many attractive qualities, AM relaxes design constraints and can significantly reduce material waste in comparison to subtractive manufacturing processes. However, there are some fundamental issues that must be addressed for metal AM to become prevalent in aerospace. In powder bed fusion AM, powder reuse from previous build cycles is desired to improve process economics. However, there is limited understanding of the contributions from powder reuse to particle and part quality. The present study investigates this topic in electron beam melting (EBM) powder fusion AM of a titanium alloy (Ti6Al4V) over 30 cycles of build and powder reuse. Results show that nearly all aspects of the process are influenced by powder reuse. Specifically, the particle size distribution tightens, largely due to fewer with small diameter. Particle damage increases with reuse, which includes surface deformation (reduction in sphericity), partial melting and/or particle fusion and fracture. In regard to the built metal, the microstructure exhibits increasingly finer basket weave and greater surface area to volume ratio of a with reuse. Yet, there are no apparent trends in a-lath thickness or volume. In the analysis of composition, while substantial changes in the Al and V content are apparent, or in Fe, H and N, the O concentration of the powder increased significantly with reuse. In fact, it exceeded the concentration limit (0.2%) in just 11 build cycles. Overall, powder reuse should be considered carefully in the development of titanium parts for performance critical applications by EBM AM.

Fish scales serve as a dermal armor that provides protection from physical injury. Due to a number of outstanding properties, fish scales are inspiring new concepts for layered engineered materials and next-generation flexible armors. While past efforts have primarily focused on the structure and mechanical behavior of ontogenetic scales, the structure-property relationships of regenerated scales have received limited attention. In the present study, common carp (Cyprinus carpio) acquired from the wild were held live in an aquatic laboratory at 10° and 20°C. Ontogenetic scales were extracted from the fish for analysis, as well as regenerated scales after approximately 1 year of development and growth. Their microstructure was characterized using microscopy and Raman spectroscopy, and the mechanical properties were evaluated in uniaxial tension to failure under hydrated conditions. The strength, strain to fracture and toughness of the regenerated scales were significantly lower than those of ontogenetic scales from the same fish, regardless of the water temperature. Scales that regenerated at 20°C exhibited significantly higher strength, strain to fracture and toughness than those regenerated at 10°C. The regenerated scales exhibited a highly mineralized outer layer, but no distinct limiting layer or external elasmodine; they also possessed a significantly lower number of plies in the basal layer than in the ontogenetic scales. The results suggest that a mineralized layer develops preferentially during scale regeneration with the topology needed for protection, prior to the development of other qualities.

As metal Additive Manufacturing (AM) becomes more widely adopted in the aerospace and orthopedic industries, there is increasing demand to improve part quality and reduce overall cost. The high cost of powder feedstock has raised interest in recovering unmelted powder in the build chamber and its reuse in subsequent builds. While degradation in powder properties with recovery and reuse can cause degradation in part properties, this topic has received rather limited attention. In this study the properties of Ti6Al4V metal powder are evaluated over 30 build cycles in Electron Beam Melting (EBM) AM. The morphological, microstructural, mechanical, and chemical changes are evaluated in cross-sectioned powder particles and compared to isolated control samples to understand the mechanisms of degradation. Results show that in response to the elevated build chamber temperature, the powder undergoes a sub-beta-transus aging heat treatment with powder reuse. Based on nanoindentation hardness measurements, the particles undergo an increase in near-surface hardness (up to 2 GPa) with respect to the core. Moreover, tint etching revealed an oxidized surface layers consistent with alpha case formation. The particle hardening appears to result from oxygen diffusion during powder recovery and not work hardening related to the mechanical aspects of that process. These results demonstrate the importance of managing/mitigating oxidation of metal powder feedstock to improve its reusability and increasing its overall lifetime.

Among many dermal armors, fish scales have become a source of inspiration in the pursuit of "next-generation" structural materials. Although fish scales function in a hydrated environment, the role of water and intermolecular hydrogen bonding to their unique structural behavior has not been elucidated. Water molecules reside within and adjacent to the interpeptide locations of the collagen fibrils of the elasmodine and provide lubrication to the protein molecules during deformation. We evaluated the contributions of this lubrication and the intermolecular bonding to the mechanical behavior of elasmodine scales from the Black Carp (Mylopharyngodon piceus). Scales were exposed to polar solvents, followed by axial loading to failure and the deformation mechanisms were characterized via optical mechanics. Displacement of intermolecular water molecules by liquid polar solvents caused significant (p ≤ 0.05) increases in stiffness, strength and toughness of the scales. Removal of this lubrication decreased the capacity for non-linear deformation and toughness, which results from the increased resistance to fibril rotations and sliding caused by molecular friction. The intermolecular lubrication is a key component of the "protecto-flexibility" of scales and these natural armors as a system; it can serve as an important component of biomimetic-driven designs for flexible armor systems. STATEMENT OF SIGNIFICANCE: The natural armor of fish has become a topic of substantial scientific interest. Hydration is important to these materials as water molecules reside within the interpeptide locations of the collagen fibrils of the elasmodine and provide lubrication to the protein molecules during deformation. We explored the opportunity for tuning the mechanical behavior of scales as a model for next-generation engineering materials by adjusting the extent of hydrogen bonding with polar solvents and the corresponding interpeptide molecular lubrication. Removal of this lubrication decreased the capacity for non-linear deformation and toughness due to an increase in resistance to fibril rotations and sliding as imparted by molecular friction. We show that intermolecular lubrication is a key component of the "protecto-flexibility" of natural armors and it is an essential element of biomimetic approaches to develop flexible armor systems.

Metal additive manufacturing (AM) is being rapidly adopted in the aerospace and biomedical industries. Powder bed fusion AM processes are leading this trend. To maximize process economy, excess “unmelted” powder retrieved from the build chamber is used in subsequent build cycles. The metal properties and component reliability could undergo degradation with powder reuse. This study investigates the effects of powder reuse on fracture surface characteristics of Ti6Al4V specimens fabricated by electron beam melting AM over 30 sequential build cycles. Optical microscopy and scanning electron microscopy were used to evaluate the changes in fracture surface features of tensile failures with powder reuse. Macroscopically, slant fractures were most common in early builds, which transitioned to orthogonal fracture surfaces with poorly defined shear lips with increasing reuse. Regardless of the build number, the fracture origins were consistently from the as-built surfaces. Microscopically, ductile features such as micro-void coalescence were evident throughout the 30 build cycles. However, increasing flute content with reuse suggests that rising oxygen levels causes solution strengthening and limits the participation of active slip systems. These results highlight the importance of surface roughness and powder oxidation to metal performance in AM, and the evolution of fractographic features with powder reuse.

Natural dermal armors are serving as a source of inspiration in the pursuit of "next-generation" structural materials. Although the dynamic strain response of these materials is arguably the most relevant to their performance as armors, limited work has been performed in this area. Here, uniaxial tension and transverse puncture tests were performed on specimens obtained from the scales of Asian carp over strain rates spanning seven decades, from 10(-4) to 10(3) s(-1). The importance of anatomical variations was explored by comparing the performance of scales from the head, middle and tail regions. In both loading orientations, the scales exhibited a significant increase in the resistance to failure with loading rate. The rate sensitivity was substantially higher for transverse loading than for in-plane tension, with average strain rate sensitivity exponents for measures of the toughness of 0.35 and 0.08, respectively. Spatial variations in the properties were largest in the puncture responses, and scales from the head region exhibited the greatest resistance to puncture overall. The results suggest that the layered microstructure of fish scales is most effective at resisting puncture, rather than in-plane tension, and its effectiveness increases with rate of loading. X-ray microCT showed that delamination of plies in the internal elasmodine and stretching of the fibrils were key mechanisms of energy dissipation in response to puncture loading. Understanding contributions from the microstructure to this behavior could guide the development of flexible engineered laminates for penetration resistance and other related applications.

Fish scales are laminated composites that consist of plies of unidirectional collagen fibrils with twisted-plywood stacking arrangement. Owing to their composition, the toughness of scales is dependent on the intermolecular bonding within and between the collagen fibrils. Adjusting the extent of this bonding with an appropriate stimulus has implications for the design of next-generation bioinspired flexible armours. In this investigation, scales were exposed to environments of water or a polar solvent (i.e. ethanol) to influence the extent of intermolecular bonding, and their mechanical behaviour was evaluated in uniaxial tension and transverse puncture. Results showed that the resistance to failure of the scales increased with loading rate in both tension and puncture and that the polar solvent treatment increased both the strength and toughness through interpeptide bonding; the largest increase occurred in the puncture resistance of scales from the tail region (a factor of nearly 7×). The increase in strength and damage tolerance with stronger intermolecular bonding is uncommon for structural materials and is a unique characteristic of the low mineral content. Scales from regions of the body with higher mineral content underwent less strengthening, which is most likely the result of interference posed by the mineral crystals to intermolecular bonding. Overall, the results showed that flexible bioinspired composite materials for puncture resistance should enrol constituents and complementary processing that capitalize on interfibril bonds.