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Title: Selective laser melting of a ferritic oxide dispersion strengthened steel
Author: Boegelein, Thomas
ISNI:       0000 0004 5356 7748
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2014
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Oxide dispersion strengthened (ODS) steels contain a fine dispersion of nano-sized, typically Y based, oxide particles which result in the material displaying significantly better creep, irradiation and oxidation resistance when compared to conventional alloys. Thus, such materials are considered as candidate structural materials for a number of applications in the fossil and nuclear energy sectors and in other high-temperature applications. ODS steels are currently produced by powder metallurgy which includes mechanical alloying (MA) of master alloys or elemental powder, hot extrusion or hot isostatic pressing (HIP) followed by a final heat treatment. Recent studies revealed that Y added during MA in the form of yttria (Y2O3) breaks down and the elements go into supersaturated solution in the Fe matrix; and Y based dispersoids form during fabrication of the alloy. In this work, an additive manufacturing method, selective laser melting (SLM), was applied to as-MA ODS-PM2000 (Fe-19.0wt.%Cr-5.5Al-0.5Ti-0.5Y2O3) powder. SLM produces almost fully dense solid freeform components by successively melting thin layers of metal powder. In order to investigate the feasibility of SLM in an ODS alloy environment, a number of builds were fabricated. These included a complex thin walled structure, coatings on Inconel IN939 (Ni-22.5wt.%Cr-19.0Co-3.7Ti-2W-1.9Al-1.0Nb-1.4Ta-0.15C), a nickel based superalloy, and optimised wall and solid builds. A wide range of microstructural and mechanical characterisation techniques were carried out on these builds with the focus to study the fundamentals of SLM in ODS environment. The most important finding of this work was that a fine homogeneous dispersion of globular shaped nanoscopic particulates could be retained in the SLM build configurations investigated. Indications were found that there is a very low number of dispersoids in the deposited layer after it was put down. Repeated heating cycles during SLM deposition of further slices resulted in coarsening and growth of existing precipitates, but probably also in nucleation and growth of new dispersoids in the α-Fe matrix. Such heating cycles and post-build annealing trials resulted in modification of initially multiphased dispersoid compounds including originally a number of elements, such as O, Al, Si, Ti, Cr, Fe and Y, into structures having significantly increased concentrations of Al and Y. After post-build annealing, the particles were most frequently of the compound type yttrium aluminium monoclinic, Y4Al2O9. SLM processing parameters were developed leading to a relative density of >99.5 % for wall builds having different thicknesses and of >98.5 % for solid builds. Electron backscatter diffraction (EBSD) was conducted and revealed a strong [001] fibre texture along the growth direction of a wall build. For annealed walls, values of the 0.2% offset yield strength YS0.2 up to those of recrystallized conventional produced PM2000 could be achieved. Fracture behaviour and the individual key parameters determined, YS0.2 and Young's modulus were anisotropic due to this texture. In coatings, Y-rich dispersoids could be retained. When oxidised isothermally in laboratory air at 1100°, the SLM deposit PM2000 formed a mainly α-Al2O3 (alumina) scale, which was similar to conventionally fabricated PM2000. Oxidation at 870°C, however, resulted in different scale morphologies between both variants. Those were Al-rich equiaxed structures and nodules and Ti-rich needles for conventional PM2000. On the other hand, the SLM material exhibited Al-rich platelet structures and Al-rich equiaxed crystals in pores. The observations of this work confirmed the feasibility of SLM in ODS alloy environment, which may motivate further studies in this field.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TA Engineering (General). Civil engineering (General)