Additive Manufacturing of Lightweight Metal Mirrors
Many scientific instruments for earth observation or spectroscopic studies of the earth’s atmosphere are based on metal mirrors. In addition to the requirements for the optical performance and the mechanical characteristics of the mirror, the mass budget is also an important specification. Established approaches for lightweight designs are based on the material removal using cutting technologies. Based on the geometry and the area of material removal, mass savings of 30 % up to 50 % can be achieved. When machining the rear side, a negative impact on the stiffness of the mirror must be taken into account.
A new method of manufacturing metal optics is the powder-bed based technology of Selective Laser Melting (SLM). Individually designed lightweight structures can be realized by this additive manufacturing technology, enabling a mass reduction of up to 70 %. By keeping the outer surface of the mirror almost completely closed, very stiff designs can be achieved.
Complex internal lightweight structures can be designed with a variety of configurations, which can be analyzed and optimized during the CAD process. Besides the traditional periodic structures, topology optimized approaches can be used. These optimized structures are based on the possibility to selectively increase the material fraction in areas of high mechanical stress and save material in other areas. Therefore, it is possible to use tailored designs for specific load cases. The material selection for the SLM process is optimized to enable an athermal design. The aluminum-silicon material with a high silicon content of 40 % can be processed with a very low final porosity of < 0.01 %. The mechanical stability of the additive manufactured mirrors was verified by shock and vibration tests.
The additively made base-body can be handled with the well-established opto-mechanical process chain for metal mirrors. The machining of the optical surface with ultra-precision diamond turning, as well as the coating with electro-less nickel, is possible. After finishing with magnetorheological polishing, achievable shape deviations are below 150 nm peak-to-valley and a roughness of 2 nm rms was achieved.
Parts of the work were funded by the German Aerospace Center (DLR) within the project ultraLEICHT under grant number 50EE1408.
Authors: Nils Heidler, Enrico Hilpert, Johannes Hartung, Stefan Risse