Design of a transmissive optical system of a laser metal deposition three-dimensional printer with metal powder

A transmissive optical system of a three-dimensional (3D) printer using the laser metal deposition (LMD) technique with a metal powder is proposed here, together with its design method. An f-number of the focused laser beam emitted from the optical system should be large enough for the laser beam not to be absorbed or reflected by the metal powder injection nozzle. On the other hand, owing to the Lagrange-Helmholtz relation, the f-number should be small enough to form a minute laser spot diameter for high-resolution building. A temperature rise induced by the high-power laser may cause damage or a thermal lens effect on the optical system, which can be reduced by using lenses having plano-surfaces contacting thermally with holders. A formula of the relationship of the thermal lens effect with an f-number of each lens is derived to determine the lens specifications. A formula of longitudinal spherical aberration (LSA) of these lenses is also derived, and the total LSA is shown to be able to be zero with an acceptable f-number of the focused laser beam. The prototype LMD 3D printer with 6 kW laser is fabricated and shows favorable properties, such as a minimum building width of 0.3 mm and a building speed of over 500 cc/h.