Doctor of Philosophy, The Ohio State University, 2015, Welding Engineering

Abstract Increasing the steam temperature and pressure in coal-fired power plants is a perpetual goal driven by the pursuit of increasing thermal cycle efficiency and reducing fuel consumption and emissions. The next target steam operating conditions, which are 760°C (1400°F) and 35 MPa (5000 psi) are known as Advanced Ultra Supercritical (AUSC), and can reduce CO2 emissions up to 13% but this cannot be achieved with traditional power plant construction materials. The use of precipitation-strengthened Nickel-based alloys (superalloys) is required for components which will experience the highest operating temperatures. The leading candidate superalloys for power plant construction are alloys 740H, 282, and 617. Superalloys have excellent elevated temperature properties due to careful microstructural design which is achieved through very specific heat treatments, often requiring solution annealing or homogenization at temperatures of 1100 °C or higher. A series of postweld heat treatments was investigated and it was found that homogenization steps before aging had no noticeable effect on weld metal microhardness, however; there were clear improvements in weld metal homogeneity. The full abstract can be viewed in the document itself.

Committee: John Lippold (Advisor); Boian Alexandrov (Committee Member); Antonio Ramirez (Committee Member) Subjects: Materials Science; Metallurgy

Doctor of Philosophy, The Ohio State University, 2019, Materials Science and Engineering

High-temperature metallic materials such as nickel-based and titanium alloys are attractive as skin structures for aerospace vehicles. They can allow significant performance improvement and mass reduction in aircraft. However, there are substantial challenges in welding and forming them affordably for service. This project examines the use of impulse-based methods, as enabled by the vaporizing foil actuator method, for the impact welding and precise shaping of alloys Ni - 718, Ni - 625, Ni - 230, and Ti 6242. The mechanical properties and weld microstructure of four similar and Six dissimilar VFA spot welding combinations are presented and analyzed. Microhardness measurements showed the absence of a heat affected zone (HAZ). The dissimilar Ni - Ni joints and Ni - Ti joints exhibited high loads to failure in lap-shear tests and show great potential for applications involving transition joints, repair welding, medical devices, and more. The VFA method is cheap, safe, fast, durable, and marks the advancement in the solid-state joining of dissimilar nickel and titanium systems. Nickel alloys typically exhibit low springback during quasi-static forming processes. However, the large amounts of strain hardening that occurs during these operations often requires a second annealing stress relief operation. Titanium alloys are commonly known to exhibit high springback levels due to the high strength to stiffness ratios of titanium alloys. Sheet metals components are usually shaped by hot or superplastic forming. This process is expensive and has long lead-times. This work examines an athermal process to relax or remove the residual stresses and elastic strains in sheet metals. All the materials explored, especially titanium showed significant improvements in shape conformance when processed through the VFA method. Recent shock-based calibration studies have provided some insight into the previously unconfirmed mechanism of springback relief. The driving hypothesis f (open full item for complete abstract)

Committee: Glenn Daehn (Advisor); Stephen Niezgoda (Committee Member); Xun Liu (Committee Member) Subjects: Engineering; Materials Science