Additive Manufacturing Technologies at Fraunhofer IOF

Additive Manufacturing of Metal Optics

Additively made scanning mirror using topology optimized design for Arges GmbH.
© Fraunhofer IOF

Additively made scanning mirror using topology optimized design for Arges GmbH.

Applications for metal optics include scientific instrumentations for earth observation, scan mirrors for laser material processing, or high power tools e.g. for EUV-lithography. The commonly used subtractive approaches are subject to severe limitations regarding the design of those mirrors. Additive Manufacturing (AM) of metal optics using selective laser melting is used to realize optimized mirror bodies. The layer wise built up of parts by AM is able to avoid these limitations.

After extensive process studies, optimized parameters for the processing of the powder based raw material have been evaluated. The processing of aluminium-silicon material with a high silicon content of up to 40 % (AlSi40) is possible, realizing a very low porosity of 0.05 %. Studies of material samples show a Young’s modulus of 100 GPa and a tensile strength Rm of 260 MPa. The mechanical values are better than the conventionally manufactured semi-finished products. The coefficient of thermal expansion of AlSi40 is matched to the successively applied polishing layer, minimizing thermal induced bending effects. Computer tomographic measurements (CT) enable the geometric testing of internal volumes and complex structures of the manufactured parts.

omputer tomographic measurement of AM high power mirror with internal cooling channels for optiX fab GmbH.
© Fraunhofer IOF

omputer tomographic measurement of AM high power mirror with internal cooling channels for optiX fab GmbH.

The direct integration of cooling channels into high power optics enables a uniform temperature distribution even for high thermal loads. The disadvantages of conventional methods such as additional bonds or uneven distances to the optical surface can be eliminated by AM.

Topologically optimized designs allow scan mirrors to achieve improved torsional frequencies (12 kHz), enabling higher scan speeds and thus more efficient laser material processing.

The reduction of mass while retaining the mechanical stiffness of mirrors is a major advantage for space applications. Mass savings of up to 70 % can be achieved.

The additively made mirror bodies are processed using the established production chain for optical components including diamond turning, coating, and polishing. After the final processing using the established technology chain (diamond processing – magnetorheological polishing – chemical-mechanical polishing), shape deviations of less than 150 nm peak-to-valley and surface roughnesses of < 1 nm RMS have been achieved.

 

Acknowledgement

The research project “AM-OPTICS” is funded by the German Federal Ministry of Education and Research (BMBF) within the Program “Innovations for Tomorrow’s Production, Services, and Work” (02P15B204) and managed by the Project Management Agency Karlsruhe (PTKA).

 

Authors: Nils Heidler, Enrico Hilpert, Stefan Risse

Additive Manufacturing of Lightweight Metal Mirrors

Additive manufacturing lightweight demonstrator.
© Fraunhofer IOF

Additive manufacturing lightweight demonstrator.

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.

 

Acknowledgement

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