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Residual Stress Enhancement of Additively Manufactured Inconel 718 by Laser Shock Peening and Ultrasonic Nano-crystal Surface Modification

Sidhu, Kuldeep S
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1535464760914267

Abstract Details

2018, MS, University of Cincinnati, 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 stress as high as -1080 MPa is induced in the surface of the heat-treated specimens; whereas the surface residual stress in the as-built specimens reaches -975 MPa. Surface hardness also shows the significant increase, which has an increment from 625 HV to 941 HV for the heat-treated specimens, and 468 HV to 951 HV for the as-built specimens. Similarly, after LSP treatment, the compressive residual stress is induced in the heat-treated specimens with the high energy density of 8.60 GW/cm2 is -850 MPa. The compressive residual stress induced with the low energy density of 6.37 GW/cm2 is -480MPa. The increase in hardness is from 625 HV to 839 HV for treatment with high energy density, and 750 HV for treatment with low energy density. With increase in laser energy density, compressive residual stress and hardness after LSP treatment. The as-built specimens after LSP treatment with the energy density of 8.60 GW/cm2 show the compressive residual stress of -875 MPa, and the surface hardness increases from 468 HV to 853 HV. The compressive residual stress induced by UNSM treatment is higher compared to LSP. However, the effected depth is higher for LSP treatment. With UNSM treatment, the effected depth is 530 µm for the as-built specimens, and 550 µm for the heat-treated specimens. Compared to UNSM treatment, the effected depth is in-between 700-750 µm for the as-built specimens, and 870 µm for the heat-treated specimens with LSP treatment using high energy density. This is because LSP generated shockwaves can affect material to a deeper depth, while no shockwaves are produced in the UNSM process and the depth of the modified layer is thus smaller than LSP. Compressive residual stress induction can lead to the improvement in fatigue life; therefore, LSP and UNSM prove promising techniques for improving mechanical properties and fatigue life of the super-alloys produced using SLM technique.
Jing Shi, Ph.D. (Committee Chair)
Yao Fu (Committee Member)
Vijay Vasudevan, Ph.D. (Committee Member)
96 p.
Materials Science
Laser Shock Peening; Ultrasonic Nano-crystal Surface Modification; Additive Manufacturing; Compressive Residual Stress; Inconel 718; Selective Laser Melting

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Citations

  • Sidhu, K. S. (2018). Residual Stress Enhancement of Additively Manufactured Inconel 718 by Laser Shock Peening and Ultrasonic Nano-crystal Surface Modification [Master's thesis, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1535464760914267

    APA Style (7th edition)

  • Sidhu, Kuldeep. Residual Stress Enhancement of Additively Manufactured Inconel 718 by Laser Shock Peening and Ultrasonic Nano-crystal Surface Modification. 2018. University of Cincinnati, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1535464760914267.

    MLA Style (8th edition)

  • Sidhu, Kuldeep. "Residual Stress Enhancement of Additively Manufactured Inconel 718 by Laser Shock Peening and Ultrasonic Nano-crystal Surface Modification." Master's thesis, University of Cincinnati, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1535464760914267

    Chicago Manual of Style (17th edition)

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