Direct measurements of laser absorptivity during metal melt pool formation associated with powder bed fusion additive manufacturing processes

Direct calorimetric measurements are used to study the effective optical absorptivity at 1070 nm laser wavelength for bulk and powder-coated discs of industrially relevant metals. Effective absorptivity is plotted as a function of nominal laser power from 30 up to 600 W for scanning velocities of 100, 500, and 1500 mm/s. The absorptivity versus power curves of the bulk materials typically shows a reduction in effective absorptivity until the beginning of the formation of a keyhole-type surface depression that is associated with an increased absorption of the laser light in the growing keyhole until a saturation value is reached. For powders, an additional plateau of higher absorptivity can be observed for low laser power, until the curves qualitatively collapse when full melting of the powder tracks is achieved. It is shown that, under conditions associated with laser powder-bed fusion additive manufacturing, absorptivity values can vary greatly, and differ from both room temperature powder layer measurements and liquid metal estimates from the literature.