Additive manufacturing build units with process gas inertization systems

A build unit for additively manufacturing three-dimensional objects may include an energy beam system having one or more irradiation devices respectively configured to direct one or more energy beams onto a region of a powder bed, and an inertization system including an irradiation chamber defining an irradiation plenum, one or more supply manifolds, and a return manifold. The one or more supply manifolds may include a downflow manifold configured to provide a downward flow of a process gas through at least a portion of the irradiation plenum defined by the irradiation chamber, and/or a crossflow manifold configured to provide a lateral flow of the process gas through at least a portion of the irradiation plenum defined by the irradiation chamber. The return manifold may evacuate or otherwise remove process gas from the irradiation plenum defined by the irradiation chamber.