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<p> Environmental Barrier Coatings (EBCs) protect ceramic gas turbine engine components from corrosion by high temperature water vapor, but the coatings often form complex metastable microstructures upon plasma spray deposition. In ytterbium disilicate (YbDS) and its yttrium-doped counterpart (Y/YbDS), two coatings compatible with SiC/SiC parts, plasma spray forms a largely cracked, mechanically weak amorphous phase comprising up to ~80% of the coating’s volume. Therefore, the coatings must undergo a pre-service heat treatment to crystallize into stable phases and heal cracks. During the treatment, however, interplay between thermal expansion and crystallization contraction can cause vertical cracks which expose the component to the corrosive atmosphere. Remedial treatments with long, high temperature holds (~1300 ºC) can both crystallize the coating and heal existing cracks. However, these temperatures cause unnecessary grain growth that reduces the structural integrity of the coating over its lifetime.</p> <p>Here we propose an alternate heat treatment informed by experiments and modelling that removes metastable phases, heals cracks, and reduces time at temperature to prevent significant grain growth. First, we determine crystallization and phase change kinetics by applying the Ozawa-Flynn-Wall and Vyazovkin kinetic methods to differential scanning calorimetry (DSC) data. Next, we track locations and microstructural effects of phase evolution using correlative Raman spectroscopic mapping, scanning electron microscopy (SEM), and X-Ray diffraction (XRD). We interpret the formation of three distinct phases – a major phase of stable β-YbDS, and minor phases of stable Χ2-YbMS and metastable α-YbDS – within the existing framework of kinetic theory and quantify differences in their transformations between YbDS and Y/YbDS. We find that cracks in the coating heal through the crystallization of the amorphous phase and the transformation of the metastable phase although the mechanisms remain unclear. Each phase transformation causes a bulk volumetric change which we measure using dilatometry and use to calculate delamination stresses during a simulated heat treatment. Lastly, we determine the viability of our heat treatment compared to the industry standard.</p>

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<p> The Selective Laser Melting (SLM) process is a manufacturing technique that facilitates the  production of metallic parts with complex geometries and reduces both materials waste and lead  time. The high tunability of the process parameters in SLM allows the design of the as-built part’s  characteristics, such as controlled microstructure formation, residual stresses, presence of pores,  and lack of fusion. The main parameter in the SLM process that influences these parts’  characteristics is the transient temperature field resulting from the laser-matter interaction.  Nanosecond pulsed lasers in SLM have the advantage of enabling rapid and localized heating and  cooling that make the formation of ultrafine grains possible. This work shows how different pulse  durations can change the near-surface microstructure and overall mechanical properties of metallic  parts. The nanosecond pulses can melt and resolidify aluminum parts’ near-surface region to form nanograined gradient structures with yield strengths as high as 250.8 MPa and indentation  strengths as high as 725 MPa, which are comparable to some steel's mechanical properties. Knowing that the nanosecond pulsed lasers cause microstructure refinement for high-purity metals,  the microstructure variations effects were also investigated for the cast iron alloy. Cast iron was  used alone and mixed with born or boron nitride powders to induce the precipitation of  strengthening phases only enabled under high cooling rates. Although producing parts with  superior mechanical properties and controlling the precipitation of strengthening phases, the SLM  process with nanosecond pulsed lasers is still accompanied by defects formation, mainly explained  by the large thermal gradients, keyhole effect, reduced melt pool depth, and rapid cooling rates.  Ideally, a smooth heating rate able to sinter powder grains, facilitating the heat flow through the  heat-affected zone, followed by a sharper heating rate that generates a fully molten region, but  minimizes ablation at the same time are targeted to reduce the porosity and lack of fusion. Then, a  sharp cooling rate that can increase the nucleation rate, consequently refining the final  microstructure is targeted in the production of strong materials in SLM with pulsed lasers. This  work is the pioneer in controlling the transient temperature field during the heating and cooling  stages in pulsed laser processing. The temperature field control capability by shaping a nanosecond  laser pulse in the time domain affecting defects formation, residual strains, and microstructure was  achieved, opening a wide research niche in the additive manufacturing field.  </p>

<p dir="ltr">The ability to change the shape of aerodynamic surfaces is necessary for modern aircraft, both to provide control while performing maneuvers and to meet the conflicting requirements of various flight conditions such as takeoff/landing and level cruise. These shape changes have traditionally been accomplished through the use of various mechanical devices actuating discrete aerodynamic surfaces, for example ailerons and flaps. Such control surfaces and high-lift devices are generally limited to their specific functionality and create surface discontinuities which increase drag and aircraft noise. Broadly speaking, the design and study of morphing wings typically seeks to improve the performance of aircraft by completing one or more of the following objectives: reducing the drag from discontinuities in the aerodynamic surface of the wing by closing hinge gaps and creating smooth transitions, reducing weight and/or mechanical complexity by integrating mechanism functionality into compliant structures that can bear aerodynamic load and maintain shape adaptability, and providing unique or optimal functionality to the aircraft by allowing it to adjust its aerodynamic shape to meet the needs of various flight conditions with conflicting objectives and constraints.</p><p dir="ltr">The concepts proposed in this work represent potential methods for addressing these objectives. In each case, a compliant structure with multiple stable states is embedded into the wing. Exploiting elastic structural instabilities in this way provides the advantage that a structure can be made relatively stiff while still allowing for large deformations. In the first case, the development of a 3D-printable rib with an embedded bistable element creates a truss-like 2D structure that allows for modification of the airfoil. Switching states of the elements changes their local stiffness, and therefore the global stiffness of the system. By optimizing the topology of the airfoil, a passive deflection of the trailing edge can be leveraged to change the camber to leverage different lift characteristics for varying operating conditions. Primary work on this concept has included the construction of multiple experimental demonstrators for validating the concept through static structural and wind tunnel testing. In the second case, a cellular material has been investigated incorporating a bistable unit cell with a sinusoidal arch. This provides a metamaterial that can exhibit large, reversible deformations with as many stable configurations as there are rows in the honeycomb. This metamaterial is incorporated into a beam-like structure which can serve as a spar for a spanwise morphing wing, providing sufficient bending and torsional stiffness, particularly when utilized at the wing tip. Extending and retracting the wing by switching the states of the honeycomb rows provides a significant change to the wing’s induced drag and wing loading, making it ideal for optimal flight in both loitering and cruising conditions. Contributions to this concept have been the development and characterization of the bistable unit cell and honeycomb, as well as the design and analysis of the metabeam and morphing wing concept.</p>

<p dir="ltr">The process of metal additive manufacturing is characterized by the layer-by-layer construction of components, where each individual layer may be subjected to distinct thermal variations, resulting in differences in cooling rates and thermal gradients. These variations can impact the microstructure and, subsequently, mechanical properties of the final product, especially as the height of the build increases. In the present investigation, an evaluation was undertaken to ascertain the impact of build height on the structural integrity of Ti-6Al-4V samples produced using the laser powder bed fusion (LPBF) technique. The study encompassed a comprehensive examination of microstructural features, the microhardness measurement, as well as an evaluation of defect characteristics including size, location, and distribution, with respect to the build height. Tensile and fatigue tests were conducted to elucidate the potential dependence of fatigue and tensile failures on the build height. Two groups of specimens were fabricated: the first, underwent continuous fabrication, while the second involved a pause at the half height, with the process resuming after a 24-hour interval. The results of this investigation unveiled a discernible influence of the height of the build on the structural integrity of components under cyclic loading. Most fatigue specimens were observed to exhibit failure in the upper portion of the gage section with respect to the build direction. Analyses of microstructure revealed a consistent grain morphology in alignment with the build direction, and a uniform distribution of hardness throughout the build height was noted. However, for the specimens in the first group, more process-induced defects were detected within the top half of the gage section in comparison to the bottom half, while there was no noticeable difference in the distribution of defects in the second group. The results suggest that in LPBF process, as the build height is increased, there is a higher likelihood of process-induced defect formation, ultimately resulting in a reduction in structural integrity at greater build heights.</p>

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<p>Demand for lightweight, cost-effective, structural components is driving the development of continuous fiber thermoplastic tow preforms, also known as 3D-tow or tow reinforcements, to add material performance to hybrid-molded structures as an alternative to metal components. Tow reinforcements offer the performance advantages of continuous fiber composites within molded structures. The tow reinforcements also feature more tailorability of performance compared to fabric or organo-sheet reinforced hybrid-molded structures, improving their potential for design optimization. However, the added complexity of 3D-tow reinforcement structure requires the development of unique design principles and computer aided engineering (CAE) methodologies to effectively design components which meet manufacturing and performance requirements. </p> <p><br></p> <p>A systematic evaluation of design considerations was performed comparing parts manufactured with various design features, configurations, and materials. Choosing the structural profile and balance of material properties was shown to be an important component of achieving the desired performance especially where the tow reinforcement must work in conjunction with the overmolding material to provide structural performance. </p> <p><br></p> <p>By experimentally testing representative structures with features found on automobile components and molded sports equipment, performance was evaluated for a range of material combinations and reinforcement content. Tow reinforcements were made from continuous glass or carbon fiber reinforced PA6 prepreg tape and injection overmolded with unfilled or glass fiber filled PA6 adding a shear web and rib structures. Tow reinforcement significantly reduced warpage, and in tensile loading, demonstrated potential for 340\% strength increase over parts without tow. However, three-point bend performance was dominated by the overmolding material. High strain to break overmolding materials are recommended to avoid premature overmolding material cracking. </p> <p><br></p> <p>Tensile performance of tow reinforced structures is not accurately captured by conventional modeling processes. When wrapped around load introduction points, the fibers of a thick tow traverse a shorter distance at the inner radius than the outer radius leading to waviness on the inner region of each wrap. The Hsaio and Daniel model was used to predict local elastic properties of the wavy fiber composite and spatially varying material properties were applied to 3D finite element models of a suspension link. Neglecting fiber waviness overpredicted experimental tensile stiffness and strength by 36\% and 33\% respectively while modeling waviness overpredicted stiffness and strength by only 9\% and 14\% respectively. Tow wrap configuration, waviness propagation, and material parameters have significant effect on tensile performance while the tow has little effect on compressive performance.</p> <p><br></p> <p>In addition to fiber waviness, tow bundles also spread to reconcile path length differences. A method for accounting for tow spread orientations was developed and combined with fiber waviness modeling techniques. The effects of simulating the resulting fiber orientations and effective elastic properties was used to model representative beams in tension and bending load cases and compared to previous experimental results. Accounting for fiber waviness in tension demonstrated greatly improved part stiffness predictions. Spread tow bundles improved predicted strength and stiffness over simulations where tow was constrained to a uniform cross section. Increased tow reinforcement increased bending stiffness, but failure behavior was significantly influenced by the overmolding material.  </p> <p><br></p> <p>The studies in this work identified key performance attributes of 3D-tow reinforced hybrid composite structures. Design principles and modeling techniques were developed in this work, providing improved performance predictions which brings the technology closer to widespread adoption. </p>

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