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Metal additive manufacturing in aerospace: A review

THE OPTO-MECHANICAL DESIGN PROCESS Introduction Conceptualization Performance Specifications and Design Constraints Preliminary Design Design Analysis and Computer Modeling Error Budgets and Tolerances Experimental Modeling Finalizing the Design Design Reviews Manufacturing the Instrument Evaluating the End Product Documenting the Design References ENVIRONMENTAL INFLUENCES Introduction Parameters of Concern Environmental Testing of Optics References OPTO-MECHANICAL CHARACTERISTICS OF MATERIALS Introduction Materials for Refracting Optics Materials for Reflecting Optics Materials for Mechanical Components Adhesives Sealants Special Coatings for Opto-Mechanical Materials Techniques for Manufacturing Opto-Mechanical Parts References MOUNTING INDIVIDUAL LENSES Introduction Considerations of Centered Optics Cost Impacts of Fabrication Tolerances Lens Weight and Center of Gravity Location Mounting Individual Low-Precision Lenses Mountings for Lenses with Curved Rims Mountings Interfacing with Spherical Surfaces Elastomeric Mountings for Lenses Mounting Lenses on Flexures Alignment of the Individual Lens Mounting Plastic Lenses References MOUNTING MULTIPLE LENSES Introduction Multielement Spacing Considerations Examples of Lens Assemblies with No Moving Parts Examples of Lens Assemblies Containing Moving Parts Lathe Assembly Techniques Microscope Objectives Assemblies Using Plastic Parts Liquid Coupling of Lenses Catadioptric Assemblies Alignment of Multi-Lens Assemblies Alignment of Reflecting Telescope Systems References MOUNTING WINDOWS AND FILTERS Introduction Conventional Window Mounts Special Window Mounts Mounts for Shells and Domes Conformal Windows Filter Mounts Windows Subject to a Pressure Differential References DESIGNING AND MOUNTING PRISMS Introduction Geometric Relationships Designs for Typical Prisms Kinematic and Semikinematic Prism Mounting Principles Mounting Prisms by Clamping Mounting Prisms by Bonding Flexure Mounts for Prisms References DESIGN AND MOUNTING SMALL, NONMETALLIC MIRRORS, GRATINGS, AND PELLICLES Introduction General Considerations Semikinematic Mountings for Small Mirrors Mounting Mirrors by Bonding Flexure Mounts for Mirrors Multiple-Mirror Mounts Mountings for Gratings Pellicle Design and Mounting References LIGHTWEIGHT NONMETALLIC MIRROR DESIGN Introduction Material Considerations Core Cell Configurations Cast Ribbed Substrates Slotted-Strut and Fused Monolithic Substrates Frit-Bonded Substrates Low-Temperature Bonded Substrates Machined-Core Substrates Contoured-Back Solid Mirror Configurations Thin Face Sheet Mirror Configurations Scaling Relationships for Lightweight Mirrors References MOUNTING LARGE, HORIZONTAL-AXIS MIRRORS Introduction General Considerations of Gravity Effects V-Type Mounts Multipoint Edge Supports The Ideal Radial Mount Mercury Tube Mounts Strap and Roller-Chain Mounts Push-Pull Mounts Comparison of Dynamic Relaxation and Finite-Element Analysis Techniques References MOUNTING LARGE VERTICAL-AXIS MIRRORS Introduction Ring Mounts Air Bag (Bladder) Mounts Multiple-Point Supports Metrology Mounts References MOUNTING LARGE, VARIABLE-ORIENTATION MIRRORS Introduction Mechanical Flotation Mounts Hydraulic/Pneumatic Mounts Center-Mounted Mirrors Mounts for Double-Arch Mirrors Bipod Mirror Mounts Thin Face Sheet Mirror Mounts Mounts for Large Space-Borne Mirrors References DESIGN AND MOUNTING OF METALLIC MIRRORS Introduction General Considerations of Metal Mirrors Aluminum Mirrors Beryllium Mirrors Mirrors Made from Other Metals Mirrors with Foam and Metal Matrix Cores Plating of Metal Mirrors Single-Point Diamond Turning of Metal Mirrors Conventional Mountings for Metal Mirrors Integral Mountings for Metal Mirrors Flexure Mountings for Larger Metal Mirrors Interfacing Multiple SPDT Components to Facilitate Assembly and Alignment References OPTICAL INSTRUMENT STRUCTURAL DESIGN Introduction Rigid Housing Configurations Modular Design Principles and Examples A Structural Design for High Shock Loading Athermalized Structural Designs Geometries for Telescope Tube Structures References ANALYSIS OF THE OPTO-MECHANICAL DESIGN Introduction Failure Predictions for Optics Stress Generation at Opto-Mechanical Interfaces Parametric Comparisons of Annular Interface Types Bending Effects Due to Offset Annular Contacts Effects of Temperature Changes Effects of Temperature Gradients Stresses in Cemented and Bonded Optics Due to Temperature Changes Some Effects of Temperature Changes on Elastomerically Mounted Lenses References APPENDIX A: UNITS AND THEIR CONVERSION APPENDIX B: SUMMARY OF METHODS FOR TESTING OPTICAL COMPONENTS AND OPTICAL INSTRUMENTS UNDER ADVERSE ENVIRONMENTAL CONDITIONS APPENDIX C: HARDNESS OF MATERIALS APPENDIX D: GLOSSARY INDEX

Opto-mechanical systems design

In the last 10 years, freeform optics has enabled compact and high-performance imaging systems. This article begins with a brief history of freeform optics, focusing on imaging systems, including marketplace emergence. The development of this technology is motivated by the clear opportunity to enable science across a wide range of applications, spanning from extreme ultraviolet lithography to space optics. Next, we define freeform optics and discuss concurrent engineering that brings together design, fabrication, testing, and assembly into one process. We then lay out the foundations of the aberration theory for freeform optics and emerging design methodologies. We describe fabrication methods, emphasizing deterministic computer numerical control grinding, polishing, and diamond machining. Next, we consider mid-spatial frequency errors that inherently result from freeform fabrication techniques. We realize that metrologies of freeform optics are simultaneously sparse in their existence but diverse in their potential. Thus, we focus on metrology techniques demonstrated for the measurement of freeform optics. We conclude this review with an outlook on the future of freeform optics.

Freeform optics for imaging

The Fine Guidance Sensor (FGS) is one of the four science instruments on board the James Webb Space Telescope (JWST). FGS features two modules: an infrared camera dedicated to fine guiding of the observatory and a science camera module, the Near-Infrared Imager and Slitless Spectrograph (NIRISS) covering the wavelength range between 0.7 and 5.0 μm with a field of view of 2.2' X 2.2'. NIRISS has four observing modes: 1) broadband imaging featuring seven of the eight NIRCam broadband filters, 2) wide-field slitless spectroscopy at a resolving power of rv150 between 1 and 2.5 μm, 3) single-object cross-dispersed slitless spectroscopy enabling simultaneous wavelength coverage between 0. 7 and 2.5 μm at Rrv660, a mode optimized for transit spectroscopy of relatively bright (J > 7) stars and, 4) sparse aperture interferometric imaging between 3.8 and 4.8 μm enabling high contrast ("' 10-4) imaging of M < 8 point sources at angular separations between 70 and 500 milliarcsec. This paper presents an overview of the FGS/NIRISS design with a focus on the scientific capabilities and performance offered by NIRISS.

The JWST Fine Guidance Sensor (FGS) and Near-Infrared Imager and Slitless Spectrograph (NIRISS)

The fabrication chain for the development of an afocal all aluminum telescope using four anamorphic aspherical mirrors is described. The optical and mechanical design are intended to achieve an enhanced system integration with reduced alignment effort by arranging two optical surfaces monolithically on common mirror bodies. Freeform machining is carried out by a hybrid fabrication approach combining diamond turning and diamond milling in the same machine setup. A direct figure correction of diamond turned aluminum mirrors by magnetorheological finishing is presented, resulting in high-precision athermal mirror modules with excellent figure properties. The interferometric system test highlights the diffraction limited telescope performance and the feasibility of the chosen approaches for freeform machining and mechanical integration.

Development, fabrication, and testing of an anamorphic imaging snap-together freeform telescope

This article presents a new and individual way to generate opto-mechanical components by additive manufacturing, embedded in an established process chain for the fabrication of metal optics. The freedom of design offered by additive techniques gives the opportunity to produce more lightweight parts with improved mechanical stability. The latter is demonstrated by simulations of several models of metal mirrors with a constant outer shape but varying mass reduction factors. The optimized lightweight mirror exhibits 63.5% of mass reduction and a higher stiffness compared to conventional designs, but it is not manufacturable by cutting techniques. Utilizing selective laser melting instead, a demonstrator of the mentioned topological non-trivial design is manufactured out of AlSi12 alloy powder. It is further shown that – like in case of a traditional manufactured mirror substrate – optical quality can be achieved by diamond turning, electroless nickel plating, and polishing techniques, which finally results in

Precision manufacturing of a lightweight mirror body made by selective laser melting

Metal mirrors are used for spaceborne optical systems, such as telescopes and spectrometers. In addition to the optical performance, the mechanical needs and the mass restrictions are important aspects during the design and manufacturing process. Using the additive manufacturing process, optimized internal lightweight structures are realized to reduce the weight of the system while keeping the mechanical stability. A mass reduction of ≈60.5  %   is achieved. Using the aluminum silicon alloy AlSi40, the thermal mismatch of the mirror base body to a necessary electroless nickel-polishing layer is minimized. Based on an exemplary mirror design, the optimization of the interior lightweight structure is described, followed by the manufacturing process from additive manufacturing to diamond turning, plating, and polishing. Finally, the results of surface metrology and light scattering measurements are presented. A final form deviation below 80 nm p  .    −  v  .   and a roughness of ∼1  nm rms could be demonstrated.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-58/issue-9/092613/Design-additive-manufacturing-processing-and-characterization-of-metal-mirror-made/10.1117/1.OE.58.9.092613.pdf

Design, additive manufacturing, processing, and characterization of metal mirror made of aluminum silicon alloy for space applications

Freeform surfaces are becoming an increasingly exciting opportunity in optical design, in particular when correcting systems with off-axis geometries. Nevertheless, especially when coming to commercial use, the challenges for manufacturing are difficult to handle. The optical quality of a system is perturbed by typical deformations, such as localized figure errors and regular mid-spatial-frequency ripples, that come from the diamond-turning process. In this proposal, we investigated a workflow for analyzing the impact of real optical surfaces on the optical performance for even complex systems. Based on a simple and robust description, the surface is implemented back into the design. While the more localized deviations are analytically described by radial basis functions, the residual ripple structures are covered by a new approach based on the power spectral density. The reimport of optical surfaces back into the design software allows simple estimations for the requirements on manufacturing and the analysis of the realistic impact on system performance.

Description and reimplementation of real freeform surfaces.

The assembly effort of an optical system naturally relies on the degrees of freedom and the maximum allowable tolerances each optical surface introduces into the overall budget. Snap-together approaches traditionally can be regarded as attractive solutions for IR systems having moderate tolerances, where the required precision is achieved by simultaneously machining optical surfaces and mounting interfaces in a single machine setup. Recent improvements in manufacturing and metrology enable a transfer of the assembly approach to shorter wavelength applications, where sub-aperture figuring techniques are used in combination with suitable amorphous polishing layers to achieve the increased requirements on figure and finish. A further decrease of the assembly effort is gained by machining several optical surfaces on common mechanical substrates and fixing the relative position with uncertainties as low as the machine precision. The article presents the fabrication of large electroless nickel coated aluminum mirror modules having two functional freeform surfaces and references for metrology and system integration. The modules are part of an all metal anamorphic imaging telescope operating in the visual spectral range. Presented methods open up a rapid and reliable assembly of metal mirror based VIS telescopes to be used in ground and space based astronomy or remote sensing applications.

Johannes Hartung

Papers

Development, fabrication, and testing of an anamorphic imaging snap-together freeform telescope

Precision manufacturing of a lightweight mirror body made by selective laser melting

Design, additive manufacturing, processing, and characterization of metal mirror made of aluminum silicon alloy for space applications

Description and reimplementation of real freeform surfaces.

Fabrication of metal mirror modules for snap-together VIS telescopes