Controlling microstructure in an object created by metal powder additive manufacturing is disclosed. During additive manufacturing of one or more objects using an irradiation beam source system, for each respective layer in a selected range of layers including a cross-sectional area of the one or more objects including the selected object, a duration controller controls actuation of each irradiation device to maintain constant a sum of: an irradiation device melting time, an irradiation device idle time, and a recoating time expended applying a new powder material layer, while otherwise maintaining all other operation parameters of each irradiation device constant.