Doctor of Philosophy, University of Akron, 2020, Mechanical Engineering

In this research, an innovative surface engineering process, ultrasonic nanocrystal surface modification (UNSM), was used to process a 7075-T651 aluminum alloy. It was observed that UNSM led to better surface finish, higher surface hardness, and the generation of an oxide layer on the surface of this alloy, in addition to beneficial compressive residual stresses in the near-surface region. Rotating–bending fatigue tests showed that UNSM processing significantly improved the fatigue performance of 7075-T651 aluminum alloy in both the low-cycle fatigue regimes and high-cycle fatigue regimes. Besides, the oxides that formed in the surface layer after UNSM treatment were found to prevent pitting corrosion of 7075-T651 aluminum alloy in a 3.5 wt% NaCl solution. Therefore, the fatigue performance of the UNSM-treated samples did not deteriorate after corrosion in a 3.5 wt% NaCl solution. These results demonstrate that UNSM is a robust surface modification method that can improve the rotating–bending fatigue resistance and pre-corrosion fatigue resistance of the 7075-T651 aluminum alloy. Furthermore, the fatigue life extension of the pre-damaged 7075-T651 aluminum alloy through UNSM treatment was investigated for the first time. It was found that the fatigue life of 7075-T651 aluminum alloy deteriorates after being immersed in a corrosion solution. However, UNSM treatment significantly recovered the fatigue life of the pre-corroded samples. Removal of corroded surface layer, the introduction of work-hardened surface region and compressive residual stress by UNSM treatment played a compelling role in the restoration of fatigue life of the pre-damaged sample. This result indicated that UNSM is a feasible method for rejuvenation of corrosion-damaged 7075-T651 aluminum alloy. UNSM also showed its powerful effectiveness to rejuvenate pre-fatigued 7075-T651aluminum alloy.

Committee: Yalin Dong (Advisor); Chang Ye (Committee Member); Guo-Xiang Wang (Committee Member); Qixin Zhou (Committee Member); En Cheng (Committee Member) Subjects: Mechanical Engineering

MS, University of Cincinnati, 2018, Engineering and Applied Science: Materials Science

This study investigates the effects of laser shock peening (LSP) and ultrasonic nanocrystal surface modification (UNSM) on residual stress, near surface modification, and hardness of Inconel 718 (IN718) specimens manufactured by selective laser melting (SLM) techniques. IN718 is a nickel-based Ni-Cr-Fe superalloy. It has a unique set of properties that include good workability, corrosion resistance, high-temperature strength, favorable weldability, and excellent manufacturability. Additive manufacturing (AM) techniques, in particular, the laser assisted AM techniques have been developed and adopted in the industry in the past three decades. The LSP and UNSM are the recently developed mechanical surface treatment techniques that cause the severe plastic deformation on the surface. This in turn induces deep compressive stresses and forms a fine-crystalline surface layer in the specimen that improves the hardness, strength, and fatigue life. In this study, the SLM technique is used to manufacture IN718 super-alloy specimens. SLM parts are well known for their high tensile stresses in the as-built state, in the surface or subsurface region. These stresses have a detrimental effect on the mechanical properties, especially on the fatigue life. LSP and UNSM as a surface treatment method are applied on heat-treated specimens and as-built specimens. Heat-treated specimens are those samples which are fully annealed to relieve all the inbuilt stresses. They are heat treated at 955°C for one hour followed by furnace cooling. After LSP and UNSM treatment, optical microscope and electron back scattered diffraction (EBSD) is used to characterize the microstructures of both heat-treated and as-built specimens. A nanoindentation test is performed to determine the local properties like the hardness of as-built and heat-treated specimens. Afterward, the hardness along the distance from the LSP and UNSM treated surface is also defined. After UNSM treatment, compressive residual str (open full item for complete abstract)

Committee: Jing Shi Ph.D. (Committee Chair); Yao Fu (Committee Member); Vijay Vasudevan Ph.D. (Committee Member) Subjects: Materials Science