High Resolution Microstructural Analysis of Friction Stir Weld Joints between Ni-Based Alloy 625 and Mild-Carbon Steel A516

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Date

2016-05

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The Ohio State University

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Abstract

Welds between Ni-based alloys and steels are often sought after for the combination of inexpensive material with moderate to high strength in combination and superior corrosion resistance, namely within industries requiring high-performance materials functioning in harsh, corrosive environment exposure. Fusion welds between the two alloys are currently used in industry, and Solid-State welds between these alloys may be a potential candidate to join these materials. Friction Stir Welds (FSW) between Ni-based alloy 625 and Low-Carbon Steel ASTM A516 have been studied previously, but not extensively under micron-scale resolutions. Advanced electron microscopy characterization is used to analyze these joints at resolutions up to approximately 10 nm. This study follows the development and application of Transmission Kikuchi Diffraction (TKD), a microscopy method akin to Electron BackScatter Diffraction (EBSD) in the Scanning Electron Microscope (SEM) on samples at the interface of FSW between Ni-based Alloy 625 and Steel A516. Analyses are run in combination with Energy Dispersive X-ray Spectroscopy (X-EDS), and microstructural, crystallographic, and chemical data are reported within this study. Crystallographic interface of iron Ferrite and nickel Austenite is observed with TKD at the weld interface. Fine, possibly strained, 300 nm grains adjacent to this interface are indicative of martensite or residual strains from the FSW process, and precipitates in the Austenitic matrix can be seen within grains and grain boundaries. A gradient elemental interdiffusion region of iron, nickel, chromium is detected by X-EDS within the FCC Austenite region, and precipitates appear to contain niobium and molybdenum.

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Denman Research Forum, Engineering 2nd Place

Keywords

Dissimilar Metal Welds, Transmission Kikuchi Diffraction, Electron Backscatter Diffraction, Nickel, Steel, Friction Stir Weld

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