Master of Science (MS), Ohio University, 2022, Mechanical Engineering (Engineering and Technology)

Laser Powder Bed Fusion (L-PBF) is a branch of metal additive manufacturing technologies which has become increasingly more popular due to the geometric freedoms and strategic design methods which it allows. L-PBF produces metallic components to near net shape within a single process step while simultaneously allowing for the creation of complex geometries and internal structures which are not readily produced by other manufacturing techniques. Not without issues, L-PBF produces materials with preferential directions of growth in the underlying material microstructure as well as undesirable phase content in many cases. While techniques exist to change microstructure of L-PBF materials, many rely on post-processing or in situ control over the flow of heat. This thesis documents the development and analysis of a novel technique separate from previous methods which allows for in situ modification of grain structure produced in LPBF without the need of complex modification of the machine. Ultrasonic vibrations are introduced to the build process as an added parameter, hypothesizing that in situ ultrasonic cavitation will reduce grain size and modify the formation of secondary phases in a way that is beneficial to the as-manufactured material properties.

Committee: Brian Wisner (Advisor) Subjects: Mechanical Engineering

Master of Science, The Ohio State University, 2022, Materials Science and Engineering

The processing parameters used during electron beam melting powder bed fusion (EBM-PBF) fabrication can create spatio-temporal variations in thermal gradients and solidification rates which impact the evolution of microstructure and mechanical properties in a material. Therefore, a systematic in-depth understanding of process structure property correlations for material in use is a prerequisite for wide industrial adoption of EBM-PBF fabrication. Haynes 282, a recently developed Ni based superalloy known for its high temperature applications in industrial gas turbine engines, is a promising candidate for fabrication via EBM-PBF on account of its good weldability. A rapid qualification of EBM-PBF manufactured Haynes 282 for industries requires a well tiered framework to quantify impact of variations in individual processing parameters on microstructural and mechanical identifiers. In this study, a high throughput multiscale characterization was performed to quantify impact of processing parameters, such as build height, scan velocity and column thickness, on size and morphology evolution of gamma grains (γ), gamma prime (γ') precipitates and carbides. Vickers microhardness testing was used to correlate variations in microstructural features with mechanical properties. Further, the effect of a two-step ageing (1050 ⁰C/2 hours/air cooling + 788 ⁰C/8 hours/air cooling) and a one-step ageing (800 ⁰C/4 hours/air cooling) post process heat treatment on microstructure and Vickers hardness was evaluated. A bimodal distribution of γ' precipitates was observed in both as-deposited and heat-treated states. Backscattered electron imaging performed using a scanning electron microscope (SEM) revealed a decrease in size of primary γ' ( 40%) along the build direction (BD) for the as-deposited state. Local variations in γ' size along BD were observed after heat treatments. Energy dispersive X-ray spectroscopy results revealed presence of discrete, blocky Ti, Mo rich MC type carbide (open full item for complete abstract)

Committee: Carolin Fink (Advisor); Gopal Viswanathan (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2018, Welding Engineering

Subsea high pressure equipment used in production of oil and gas is routinely clad with nickel base alloys for corrosion protection. In the equipment with partial clad for sealing purpose, dissimilar metal interfaces are possibly exposed to the production fluids containing H2S. After cladding, a high hardness heat affected zone (HAZ) is produced in the base metal adjacent to the fusion boundary and is possibly susceptible to hydrogen assisted cracking (HAC) and sulfide stress cracking (SSC). National Association of Corrosion Engineers (NACE) standard MR0175/International Standard Organization (ISO) 15156 requires that HAZ hardness should be less than 22 HRC or 250 VHN. Postweld heat treatment (PWHT) is applied to reduce the HAZ hardness to meet this requirement. However, PWHT causes the carbon to diffuse from the base metal to the weld metal and pile up in a narrow region adjacent to the fusion boundary, possibly causing interface embrittlement. Also, prolonged PWHT can overtemper the base metal and impair its strength. Therefore, the optimal PWHT conditions need to be determined, which reduce the HAZ hardness to meet the industry standard, do not harm base metal strength, and do not increase the HAC and SSC susceptibility near or at the fusion boundary. In this work, nickel base Alloy 625 overlays on F22 (2.25Cr-1Mo) steel and AISI 8630 steel, or F22/625 and 8630/625 dissimilar metal welds (DMWs), were studied. A wide range of PWHT conditions indicated by Hollomon-Jaffe Parameter (HJP) was investigated to determine an optimal balance between HAZ softening and interface embrittlement. Vickers hardness testing revealed that the CGHAZ hardness decreases with the HJP increase due to martensite decomposition. There is a secondary hardening effect in F22 CGHAZ. The hardness of the planar growth zone (PGZ) of the interface and the weld metal increases with HJP, and the PGZ hardness increases at a higher rate than the weld metal. Nanoindentation and optical microsc (open full item for complete abstract)

Committee: John Lippold (Advisor); Boian Alexandrov (Committee Member); David Phillips (Committee Member) Subjects: Engineering; Materials Science; Metallurgy

Doctor of Philosophy, The Ohio State University, 2009, Oral Biology

Although NiTi rotary instruments are very popular for endodontic treatment, instrument separation is still a challenge in clinic. A new NiTi rotary instrument has recently been marketed that is machined from M-Wire that has been subjected to a proprietary novel thermomechanical processing procedure. The manufacturer has claimed that this new M-Wire instrument has considerably improved flexibility and resistance to cyclic fatigue, compared to conventional rotary instruments that are machined from superelastic (SE) austenitic NiTi wire. Clinical use has confirmed that these new M-Wire rotary instruments have outstanding clinical fatigue resistance. However, the mechanism for the improved clinical performance of these instruments is unknown.The objective of this study was to employ a variety of metallurgical laboratory techniques to determine the origin of these improved mechanical properties for the new rotary instruments. Specimens from as-received M-Wire instruments, clinically used M-Wire instruments, and conventional instruments were prepared for evaluation. The temperature range for phase transformation was examined by differential scanning calorimetry (DSC). Vickers hardness measurements were made since hardness variations for the same type of alloy has been found to correlate with variations in mechanical properties. The microstructures of the NiTi alloys were revealed by acid etching and examined with an optical microscope and a scanning electron microscope. Wear resistance of clinically used M-Wire instruments was investigated by examining their surfaces with an SEM. In a complementary study, bright-field images of M-Wire blanks were obtained by scanning transmission electron microscopy (STEM).DSC study showed that M-Wire instruments have much higher Af (austenite-finish) temperatures (over 40°C) than conventional superelastic rotary instruments (below room temperature), and are a mixture of martensite, R-phase and austenite at room temperature. The Vickers h (open full item for complete abstract)

Committee: William Brantley (Advisor); William Johnston (Committee Member); Sarandeep Huja (Committee Member); John Nusstein (Committee Member) Subjects: Dental Care; Materials Science

Doctor of Philosophy, The Ohio State University, 2006, Dentistry

The microstructures, Vickers hardness, tensile and fatigue properties of six commercial palladium–silver dental casting alloys were studied: Rx 91; Super Star; W-1, Aries, IS 64, and d.SIGN 59. Specimens simulating a coping for a maxillary central incisor restoration, and tensile test bar specimens meeting ADA Specification Nos. 5 and 38 were cast. Three kinds of heat treatment were conducted: the entire simulated porcelain-firing (SPF) process; the cumulative effect of each individual step of the SPF process, and isothermal annealing for 0.5 hour at step intervals of 50°C from 400° to 950°C. Optical microscopy, and scanning and transmission electron microscopy were employed to study the porosity, microstructures and fracture surfaces. After the SPF heat treatment, two different precipitate morphologies were observed in alloys with relatively lower and higher indium contents, respectively: spherical precipitates and platelet-like precipitates. During the SPF process after initial alloy oxidation, the Vickers hardness reached highest values in Aries, IS 64 and d.SIGN 59; after each of the subsequent steps, the Vickers hardness had no substantial changes. After isothermal annealing, Super Star showed two hardness peaks approximately at 500° and 650°C, while the other five alloys exhibited one hardness peak in the temperature range from 550° to 700°C. The TEM microstructures of as-cast and SPF heat-treated W-1 and Rx 91 were similar: solid solution matrix, ordered face-centered cubic granular precipitates, and ordered fine-scale face-centered tetragonal precipitates. After isothermal annealing, Rx 91 exhibited the same TEM microstructure as in the as-cast condition. Super Star, however, had a complicated TEM microstructure, and two kinds of new precipitates were observed corresponding to the two hardening peaks. In the presence of porosity up to 4%, Aries, IS 64, and d.SIGN 59 exhibited acceptable mechanical properties for clinical use. The fatigue limits of Super Star (open full item for complete abstract)

Committee: William Brantley (Advisor) Subjects: