Skip to Main Content
 

Global Search Box

 
 
 
 

Files

File List


40357.pdf (15.18 MB)

ETD Abstract Container

Abstract Header

Effects of Advanced Surface Treatments on Microstructure, Residual Stress and Corrosion-Fatigue Behavior of Aluminum Alloy 7075-T6

Sharma, Anurag
http://orcid.org/0000-0003-0065-5122
http://rave.ohiolink.edu/etdc/view?acc_num=ucin162765884039947

Abstract Details

2021, PhD, University of Cincinnati, Engineering and Applied Science: Materials Science.
Aluminum 7xxx series alloys exhibit a combination of high mechanical strength as well as decent corrosion resistance and are widely utilized in aircraft structures. However, high strength 7xxx series alloys, like AA7075 in the T6 heat-treated condition, is susceptible to failures from fatigue, corrosion, stress corrosion cracking and corrosion-fatigue from the mechanical loading and saline environments these structures are exposed to during service. To address these shortcomings, the effects of advance surface treatment processes of Laser Shock Peening without coating (LSPwC) and Ultrasonic Nanocrystal Surface Modification (UNSM) on the mechanical behavior, corrosion properties and near-surface microstructure changes of Al 7075-T6 alloy were investigated. These treatments induce high compressive residual stresses which results in enhancement of the fatigue life of the material and has a positive impact on the corrosion resistance. A series of experiments were conducted to study the impact of these surface treatments on residual stress, microstructural evolution and, in turn, their effects on strengthening, fatigue, corrosion, and corrosion-fatigue properties. The near-surface microstructure in Al 7075-T6 alloy after these surface treatments were characterized by advanced electron microscopy techniques. LSPwC led to remarkable near-surface microstructure composed of a ~2 µm wide newly solidified matrix recast surface layer embedded with O-rich Al nanoparticles (NPs) with the same close-packed orientation relationship (OR) as the surrounding Al matrix, together with a nano-scale aluminum oxide layer formed on the outermost surface. The formation mechanism is associated with high-pressure surface ablation leading to melting, vaporization, and shock-assisted rapid solidification during the LSPwC process. The close-packed OR between NPs and matrix is believed to be due to surface energy minimization. These unique near-surface microstructural changes induced by LSPwC led to a substantial improvement in corrosion properties, as manifested by the increase in corrosion potential, ~3x reduction in corrosion rate and nearly 3x higher polarization resistance. Intense shockwave generated by LSPwC treatment induced high residual compression (-387 MPa) and high plastic strain in the near-surface region which enhanced the yield strength by ~41% and fatigue strength by ~36% when tested in air medium. The combined effect of enhanced corrosion resistance and high residual stress induced by LSPwC, further resulted in a massive improvement of up to ~6.5x in corrosion-fatigue life. Severe plastic deformation caused by the UNSM process, using ultrasonic impacts at 20 kHz, induced high compressive stress (~ 484 MPa) on surface and improved surface hardening by ~39%. Significant improvement of ~36% was achieved in fatigue strength in air medium without compromising the corrosion properties of the baseline material. UNSM also improved the corrosion-fatigue behavior of alloy by increasing the fatigue life by up to ~3.6x when tested in naturally aerated 3.5 wt.% NaCl solution. Overall, this study demonstrated that both surface treatments could significantly enhance the mechanical properties of this alloy in both air and corrosive environment. UNSM induced higher plastic strain on the surface which resulted in higher compressive stress and hardness on surface in comparison to LSPwC. Both showed similar improvements in fatigue strength when tested in air medium. But in the corrosive environment, LSPwC surpassed UNSM in improving the corrosion and corrosion-fatigue performance of Al7075-T6 alloy, because of the unique near-surface microstructural changes, high levels of compressive residual stress and nearly 3x higher depth of compression caused by the process. Hence, LSPwC was able to restore the fatigue life of the alloy in the corrosive environment.
Vijay Vasudevan, Ph.D. (Committee Chair)
Yao Fu, Ph.D. (Committee Member)
Ashley Paz y Puente, Ph.D. (Committee Member)
Matthew Steiner, Ph.D. (Committee Member)
165 p.
Materials Science
Aluminum Alloy 7075-T6; Laser shock peening without coating; Ultrasonic Nanocrystal Surface Modification; Electron Microscopy; Residual Stress; Corrosion-Fatigue

Recommended Citations

Citations

  • Sharma, A. (2021). Effects of Advanced Surface Treatments on Microstructure, Residual Stress and Corrosion-Fatigue Behavior of Aluminum Alloy 7075-T6 [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin162765884039947

    APA Style (7th edition)

  • Sharma, Anurag. Effects of Advanced Surface Treatments on Microstructure, Residual Stress and Corrosion-Fatigue Behavior of Aluminum Alloy 7075-T6. 2021. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin162765884039947.

    MLA Style (8th edition)

  • Sharma, Anurag. "Effects of Advanced Surface Treatments on Microstructure, Residual Stress and Corrosion-Fatigue Behavior of Aluminum Alloy 7075-T6." Doctoral dissertation, University of Cincinnati, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=ucin162765884039947

    Chicago Manual of Style (17th edition)

ucin162765884039947
246
© 2021, all rights reserved.
This open access ETD is published by University of Cincinnati and OhioLINK.