Enabling Oxide Dispersion Strengthening via In-situ Reaction During Laser Powder Bed Fusion Additive Manufacturing: Methods and Mechanisms
Type of DegreePhD Dissertation
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Oxide dispersion strengthened (ODS) steels have continuously attracted interest over the past few decades for their potential applications in fission and fusion nuclear systems, high-temperature heat exchangers, and gas turbines. While powder metallurgy (PM) is the typical process for manufacturing this class of materials, it possesses a great limitation in producing complex geometry for reasonable cost and time. Laser powder bed fusion (L-PBF) additive manufacturing (AM) offers great advantages over PM, enabling complex component design, high production rate, and reduced design iteration. Research has been carried out to fabricate ODS steels by laser AM. Challenges still exist in producing the high-density oxide dispersion during laser solidification. In this dissertation, the fabrication of ODS steels by (L-PBF) was systematically studied. Different pathways of oxygen intrusion and processing parameters were investigated. The goal is to develop methods and mechanistic understandings to utilize in-situ oxidation to form nanoscale oxides in the steel alloy during laser solidification. First, in-situ oxidation by reacting with a gas-phase oxygen donor was explored. Factors influencing the oxide dispersion in AM steels such as alloy composition, oxygen getter elements, atmospheric oxygen partial pressure, and laser AM parameters were studied. The reasons that present high-density nano oxide dispersion were identified. Second, a reactive AM method was developed to produce AM ODS stainless steel by reacting Y-containing stainless steel powder with solid-phase oxygen donors, like low melting temperature oxides (MoO3 and Fe2O3). High thermal stability was achieved, and the underlying strengthening mechanism was investigated. Third, a different laser mode, pulse wave (PW) emission laser, was used to reduce the oxide spattering during laser melting and thus increase the oxide density in AM ODS steel. Pulse laser showed a great advantage in refining the oxide size and microstructure. The effects of PW laser power, pulse interval, pulse width, and repetition rate on oxide dispersion and steel microstructure were systematically explored. The mechanistic insights on how pulse laser helps oxide dispersion was obtained. Last but not least, the melt pool simulation was performed to understand the effects of pulse laser on melt pool dynamics and temperature distribution, which are believed to be the key controlling factors that affect spattering behavior and phase transformation during laser solidification.