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This overview highlights some of the key aspects regarding materials qualification needs across the additive manufacturing (AM) spectrum. AM technology has experienced considerable publicity and growth in the past few years with many successful insertions for non-mission-critical applications. However, to meet the full potential that AM has to offer, especially for flight-critical components (e.g., rotating parts, fracture-critical parts, etc.), qualification and certification efforts are necessary. While development of qualification standards will address some of these needs, this overview outlines some of the other key areas that will need to be considered in the qualification path, including various process-, microstructure-, and fracture-modeling activities in addition to integrating these with lifing activities targeting specific components. Ongoing work in the Advanced Manufacturing and Mechanical Reliability Center at Case Western Reserve University is focusing on fracture and fatigue testing to rapidly assess critical mechanical properties of some titanium alloys before and after post-processing, in addition to conducting nondestructive testing/evaluation using micro-computerized tomography at General Electric. Process mapping studies are being conducted at Carnegie Mellon University while large area microstructure characterization and informatics (EBSD and BSE) analyses are being conducted at Materials Resources LLC to enable future integration of these efforts via an Integrated Computational Materials Engineering approach to AM. Possible future pathways for materials qualification are provided.

As the metal additive manufacturing (AM) industry moves towards industrial production, the need for qualification standards covering all aspects of the technology becomes ever more prevalent. While some standards and specifications for documenting the various aspects of AM processes and materials exist and continue to evolve, many such standards still need to be matured or are under consideration/development within standards development organizations. An important subset of this evolving the standardization domain has to do with critical property measurements for AM materials. While such measurement procedures are well documented, with various legacy standards for conventional metallic material forms such as cast or wrought structural alloys, many fewer standards are currently available to enable systematic evaluation of those properties in AM-processed metallic materials. This is due in part to the current lack of AM-specific standards and specifications for AM materials and processes, which are a logical precursor to the material characterization standards for any material system. This paper summarizes some of the important standardization activities, as well as limitations associated with using currently available standards for metal AM with a focus on measuring mission-critical properties. Technical considerations in support of future standards development, as well as a pathway for qualification/certification of AM parts enabled by the appropriate standardization landscape, are discussed.

The fracture toughness and fatigue crack growth behavior of two as-vacuum arc cast high-entropy alloys (HEAs) (Al 0.2 CrFeNiTi 0.2 and AlCrFeNi 2 Cu) were determined. A microstructure examination of both HEA alloys revealed a two-phase structure consisting of body-centered cubic (bcc) and face-centered cubic (fcc) phases. The notched and fatigue precracked toughness values were in the range of those reported in the literature for two-phase alloys but significantly less than recent reports on a single phase fcc-HEA that was deformation processed. Fatigue crack growth experiments revealed high fatigue thresholds that decreased significantly with an increase in load ratio, while Paris law slopes exhibited metallic-like behavior at low R with significant increases at high R . Fracture surface examinations revealed combinations of brittle and ductile/dimpled regions at overload, with some evidence of fatigue striations in the Paris law regime.

This preliminary work documents the effects of test orientation with respect to build and beam raster directions on the fracture toughness and fatigue crack growth behavior of as-deposited EBM Ti-6Al-4V. Although ASTM/ISO standards exist for determining the orientation dependence of various mechanical properties in both cast and wrought materials, these standards are evolving for materials produced via additive manufacturing (AM) techniques. The current work was conducted as part of a larger America Makes funded project to begin to examine the effects of process variables on the microstructure and fracture and fatigue behavior of AM Ti-6Al-4V. In the fatigue crack growth tests, the fatigue threshold, Paris law slope, and overload toughness were determined at different load ratios, R , whereas fatigue precracked samples were tested to determine the fracture toughness. The as-deposited material exhibited a fine-scale basket-weave microstructure throughout the build, and although fracture surface examination revealed the presence of unmelted powders, disbonded regions, and isolated porosity, the resulting mechanical properties were in the range of those reported for cast and wrought Ti-6Al-4V. Remote access and control of testing was also developed at Case Western Reserve University to improve efficiency of fatigue crack growth testing.

Functional and mechanical properties of novel biomaterials must be carefully evaluated to guarantee long-term biocompatibility and structural integrity of implantable medical devices. Owing to the combination of metallic bonding and amorphous structure, metallic glasses (MGs) exhibit extraordinary properties superior to conventional crystalline metallic alloys, placing them at the frontier of biomaterials research. MGs have potential to improve corrosion resistance, biocompatibility, strength, and longevity of biomedical implants, and hence are promising materials for cardiovascular stent applications. Nevertheless, while functional properties and biocompatibility of MGs have been widely investigated and validated, a solid understanding of their mechanical performance during different stages in stent applications is still scarce. In this review, we provide a brief, yet comprehensive account on the general aspects of MGs regarding their formation, processing, structure, mechanical, and chemical properties. More specifically, we focus on the additive manufacturing (AM) of MGs, their outstanding high strength and resilience, and their fatigue properties. The interconnection between processing, structure and mechanical behaviour of MGs is highlighted. We further review the main categories of cardiovascular stents, the required mechanical properties of each category, and the conventional materials have been using to address these requirements. Then, we bridge between the mechanical requirements of stents, structural properties of MGs, and the corresponding stent design caveats. In particular, we discuss our recent findings on the feasibility of using MGs in self-expandable stents where our results show that a metallic glass based aortic stent can be crimped without mechanical failure. We further justify the safe deployment of this stent in human descending aorta. It is our intent with this review to inspire biodevice developers toward the realization of MG-based stents.

This paper investigated the Mg-rich phase precipitation behavior and the corrosion performance throughout the thickness direction within the stirred zone (SZ) of friction stir welded (FSW) AA5083 alloy after 175 °C/100 h sensitization. For the as-welded SZ, the recrystallized grain size gradually decreased from the top surface (5.5 μm) to the bottom (3.7 μm). The top and bottom of the SZ maintained relatively high levels of deformed grains and accumulated strain induced by either shoulder pressing or pin stirring. After 175 °C/100 h sensitization, 100 nm thick β′-Al3Mg2 precipitates were present along the grain boundaries (GBs) in the SZ. The bottom of the SZ exhibited more continuous precipitates along GBs due to the fine grain size and the large fraction of high-angle grain boundaries (0.724%). Although the as-welded SZ exhibited excellent corrosion resistance, it became extremely vulnerable to intergranular cracking (IGC) and stress corrosion cracking (SCC) after sensitization. The large SCC susceptibility indices of the SZ samples ranged from 66.9% to 73.1%. These findings suggest that sensitization can strongly deteriorate the corrosion resistance of the Al-Mg FSW joint, which is of critical importance for the safety and reliability of marine applications.

This paper explores the influence of sample thickness and build orientation on the microstructure and mechanical properties of electron beam melting (EBM) additive manufactured Ti-6Al-4V titanium alloy and compared to previously published work on SLM-processed material. In particular, the various mechanical properties (tensile yield strength, ultimate tensile strength) were investigated with attempts to correlate with various microstructural features, including lamellae thickness, porosity and the size of prior-beta grains. However, it is shown that the surface exerts a dominant effect on mechanical properties with as-deposited surfaces. These observations provide the possibility for the further improvement of processing/property relations with as-deposited surfaces.

Results are presented on the shock response of a zirconium-based bulk metallic glass (BMG), Zr41.25Ti13.75Ni10Cu12.5Be22.5, subjected to planar impact loading. An 82.5-mm bore single-stage gas-gun facility at Case Western Reserve University, Cleveland, OH, was used to conduct the shock experiments. The particle velocity profiles, measured at the back (free) surface of the target plate by using the velocity interferometer system for any reflector (VISAR), were analyzed to (i) better understand the structure of shock waves in BMG subjected to planar shock compression, (ii) estimate residual spall strength of the BMG after different levels of shock compression, and (iii) obtain the Hugoniot elastic limit (HEL) of the material. The spall strength was found to decrease moderately with increasing levels of the applied normal impact stress. The spall strength at a shock-induced stress of 4.4 GPa was 3.5 GPa while the spall strengths at shock-induced stresses of 5.1, 6.0, and 7.0 GPa were 2.72, 2.35, and 2.33 GPa, respectively. The HEL was estimated to be 6.15 GPa.

We report a low-temperature inkjet printing and plasma treatment method using silver nitrate ink that allows the fabrication of conductive silver traces on poly(vinyl alcohol) (PVA) film with good fidelity and without degrading the polymer substrate. In doing so, we also identify a critical salt loading in the film that is necessary to prevent the polymer from reacting with the silver nitrate-based ink, which improves the resolution of the silver trace while simultaneously lowering its sheet resistance. Silver lines printed on PVA film using this method have sheet resistances of around 0.2 Ω/□ under wet/dry and stretched/unstretched conditions, while PVA films without prior treatment double in sheet resistance upon wetting or stretching the substrate. This low resistance of printed lines on salt-treated films can be preserved under multiple bending cycles of 0–90° and stretching cycles of 0–6% strain if the polymer is prestretched prior to inkjet printing.

Although the benefits of titanium aluminides for intermediate service temperature applications were well conceived and significant research and development activities were conducted in the past four decades, they remained as developmental materials due to barriers associated with melting, processing, scale-up, and cost. Demanding requirements of efficient aero-engines and extensive risk reduction demonstrations paved the path for commercial introduction of gamma titanium aluminides. The single most attractive current application is for low pressure turbine blades (LPTBs) in advanced aero-engines replacing conventionally cast nickel superalloys. This paper provides an overview of recent progress, producibility challenges, and opportunities. The successful journey of gamma (γ) TiAl LPTB development from laboratory demonstrations to production insertions in mass-produced commercial jet engines will be described. Collaboration and integrated product development were identified as the most critical needs for rapid maturation and implementation of γ-TiAl into aerospace applications. An integrated computational materials engineering modeling framework and toolsets developed under a collaborative US Air Force Metals Affordability Initiative project between industry, government, and academia will be illustrated. Model-based optimization of material and processing for achieving desired performance goals will be highlighted.