Laser powder bed fusion (LPBF) is a widely used and well-developed approach in additive manufacturing. To meet the high material performance requirements of fourth-generation nuclear power reactors, the combination of LPBF processing with oxide dispersion strengthening (ODS) is currently of interest for the design and development of new materials. In this approach, nanoscale \(\hbox {Y}_{2}\hbox {O}_{3}\) particles are dispersed into the feeding powders to produce LPBF-ODS materials. Oxygen exposure and the introduction of oxygen into the solvation cell during LPBF are usually considered as detrimental processes that are impossible to eliminate completely. However, our understanding of these unavoidable processes is still limited. In this study, we developed a new LPBF-ODS design approach based on in situ oxygen content regulation during the LPBF process. The oxygen content of the environmental chamber was artificially adjusted using an online monitoring system to activate reactions between oxygen and the metallic elements for the in situ formation of dispersed oxide particles. Four batches of LPBF 304 L stainless steel samples were successfully processed under different oxygen levels to investigate the reinforcement effect of in situ chemical alloying. The results show that dispersed oxide particles were formed with an average nanoscale size of approximately \(46 \mathrm {~nm}\) through the LPBF in situ alloying approach. The increase in the number density of oxide particles to 11.4 particles \(/ \upmu \textrm{m}^{2}\) as the oxygen content increased played a role in refining and stabilizing the cellular structure. The yield strength of the in situ alloyed ODS material was enhanced (to up to \(\sim 675~\textrm{MPa}\) ) while its ductility was not significantly degraded (elongation of up to \(\sim 39 \%\) ). These tensile properties are competitive within the ranges reported for ODS alloys prepared by mechanical alloying. The main mechanisms for yield strength enhancement through interactions between nanoscale oxide particles and dislocation entanglement cells were analyzed. This study provides a new approach for the future preparation of high-performance LPBF-ODS alloys.
