Graduate Thesis Or Dissertation

 

Selective Laser Melting of Duplex Stainless Steel 2205: Microstructure, Mechanical Properties, and Corrosion Performance Öffentlichkeit Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5h73q332v

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  • The aim of this work was to assess the viability of duplex stainless steel 2205 components built via selective laser melting for seawater applications. First, a comprehensive additive manufacturing assessment was done. The duplex stainless steel 2205 components were selectively laser melted from gas atomized powder (D90 <45m) using a 250 W laser in nitrogen environment. A relative density of 99.1±0.3% was achieved, but a heat treatment was necessary to restore the balanced duplex microstructure. A comparison of electron backscatter diffraction, optical microscopy, and magnetometer phase analysis techniques was done yielding ferrite contents of 51.7±0.7 %, 53.0±4.8 %, and 55.7±1.0 %, respectively. The grain orientation before and after heat treatment was heavily influenced by the layerwise building and scan strategy, which resulted in a [001] ferritic texture. However, the austenite had a mixed orientation. The processing conditions did not affect the bulk chemical composition, however elemental segregation was observed. Characterization of bulk properties was done on samples built from optimized parameters. While the measured microhardness of 258±8 HV was comparable to wrought, the ultimate tensile strength (622±19 MPa) and % elongation (21.3±1.4 %) were inferior. A high surface roughness was observed across at all processing conditions (> 5μm Ra). Finally, a study on corrosion performance was done. Two build orientations (parallel and perpendicular to build direction) and as-built (no heat treatment) and annealed conditions were studied. Parts manufactured by SLM were characterized for corrosion properties in a 3.5% NaCl electrolyte and compared to results obtained for wrought DSS 2205. The measured corrosion rates from linear polarization resistance were < 1 μm/year for all conditions. The as-built conditions showed anisotropic corrosion rates due to the preferred crystallographic orientation of the different build orientations. The anisotropy was not replicated on the annealed conditions, possibly due to elemental segregation of Si and Mn, or porosities. A novel finding was that the as-built parallel condition showed superior performance to wrought (0.33±0.10 μm/year vs 0.59±0.10 μm/year corrosion rate). No steady state pitting behavior occurred during the cyclic polarization (CP) test for any conditions tested, although certain SLM conditions showed metastable pitting (up to 10-5 A/cm2 measured during CP), likely due to open surface pores. The exception was as-built, parallel to build direction, which showed no metastable pitting due to the strongly resistant [110] ferrite texture.
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