Previous research into refractory high-entropy alloys (RHEAs) often focused on optimizing alloys with solid solution phases by adjusting elemental compositions and refining microstructure. To be suitable for critical structural applications, formation of secondary phases, such as those seen in the microstructures of many superalloys, is an area which is still in the early stages of exploration for RHEAs. In this work, a new Mo5Nb35Ti30V30 RHEA is manufactured via laser powder bed fusion and subsequently heat treated, inducing the formation of a TiCN phase, initially on cell and grain boundaries (GBs) after 1 hour. After prolonged 24-hour heat treatment the TiCN on the GBs coarsens and the cellular substructure is removed. Samples are then compression tested, all showing ductile failure. Due to the strengthening caused by interstitial elements in the body-centered cubic (BCC) matrix phase, recovery of the cellular substructures and micron-scale TiCN on GBs, the 24-hour heat-treated samples showed increased compressive strength and ductility compared to the as-built samples. TiCN largely grows at a 45 deg misorientation angle about the [100] axis in the BCC matrix phase, hence Kernel average misorientation (KAM) maps show dislocation pile up at the phase boundaries and at the high angle grain boundaries in the recovered microstructure. Susceptibility of RHEAs to atmospheric interstitial infiltration is a concern in the RHEA field; however, this work shows that, if controlled, exposure to these elements can result in beneficial dual-phase microstructures, interstitial strengthening and improved material properties as a result.

This study aims to utilize a combined machine learning (ML) and CALculation of PHAse Diagrams (CALPHAD) methodology to design hardmetal matrix phases for metal-forming applications that can serve as the basis for carbide reinforcement. The vast compositional space that high entropy alloys (HEAs) occupy offers a promising avenue to satisfy the application design criteria of wear resistance and ductility. To efficiently explore this space, random forest ML models are constructed and trained from publicly available experimental HEA databases to make phase constitution and hardness predictions. Interrogation of the ML models constructed reveals accuracies >78.7% and a mean absolute error of 66.1 HV for phase and hardness predictions respectively. Six promising alloy compositions, extracted from the ML predictions and CALPHAD calculations, are experimentally fabricated and tested. The hardness predictions are found to be systematically under- and overpredicted depending on the alloy microstructure. In parallel, the phase classification models are found to lack sensitivity toward additional intermetallic phase formation. Despite the discrepancies identified between ML and experimental results, the fabricated compositions show promise for further experimental evaluation. These discrepancies are believed to be directly associated with the available databases but, importantly, have highlighted several avenues for both ML and database development.

Additive manufacturing (AM; 3D printing) in aluminium using laser powder bed fusion provides a new design space for lightweight mirror production. Printing layer-by-layer enables the use of intricate lattices for mass reduction, as well as organic shapes generated by topology optimisation, resulting in mirrors optimised for function as opposed to subtractive machining. However, porosity, a common AM defect, is present in printed aluminium and it is a result of the printing environment being either too hot or too cold, or gas entrapped bubbles within the aluminium powder. When present in an AM mirror substrates, porosity manifests as pits on the reflective surface, which increases micro-roughness and therefore scattered light. There are different strategies to reduce the impact of porosity: elimination during printing, coating the aluminium print in nickel phosphorous, or to apply a heat and pressure treatment to close the pores, commonly known as a hot isostatic press (HIP). This paper explores the application of HIP on printed aluminium substrates intended for mirror production using single point diamond turning (SPDT). The objective of the HIP is to reduce porosity whilst targeting a small grain growth within the aluminium, which is important in allowing the SPDT to generate surfaces with low micro-roughness. For this study, three disks, 50 mm diameter by 5 mm, were printed in AlSi10Mg at 0 degrees, 45 degrees, and 90 degrees with respect to the build plate. X-ray computed tomography (XCT) was conducted before and after the HIP cycle to confirm the effectiveness of HIP to close porosity. The disks were SPDT and the micro-roughness evaluated. Mechanical testing and electron backscatter diffraction (EBSD) was used to quantify the mechanical strength and the grain size after HIP.

Space telescopes are required to be lightweight and small without compromising high optical performance. A Metallic mirror is one of the technologies that can meet launch conditions, the harsh space environment and achieve the optical requirements of an imaging payload and have been widely used from JWST to new space payloads. Flexible mounting pads are one of the geometrical designs within a metallic mirror that is a very critical part which mounts the mirror to the supporting structure. Flexible pads improve optical stability by reducing screw pressure from mounting and increase vibration endurance by creating more flexibility in the design. This study will use Finite Element Analysis to optimize the shape of flexible pads, examining the effects on mechanical and optical performance by varying geometric dimensions in a parametric study under multiple scenarios from manufacturing to operating in orbit. The results highlight the parameters that have the biggest impact on mechanical and optical performance in each scenario and describe the relation between the parameters that affect mechanical and optical performance that improve the understanding of the opto-mechanical design of metallic mirrors. Finally, the design will be optimized with multiple objectives to get the most optimal design based on all scenario's conditions. The parametric study could be analyzed with the sensitivity study, response surface, and optimization. The results show the parameters that have the most impact on performance and show its effect on performance in various conditions such as manufacturing load, grounded based stability with screw pressure, natural frequency, thermal load, and gravity release. The optimization process can lead to the improvement of the optical design. This study improves understanding of opto-mechanical design of the flexible pads in metallic mirrors, which can be applied to other metallic mirror designs.

Lightweight optical manufacture is no longer confined to the conventional subtractive (mill and drill), formative (casting and forging) and fabricative (bonding and fixing) manufacturing methods. Additive manufacturing (AM; 3D printing), creating a part layer-by-layer, provides new opportunities to reduce mass and combine multiple parts into one structure. Frequently, modern astronomical telescopes and instruments, ground- and space-based, are limited in mass and volume, and are complex to assemble, which are limitations that can benefit from AM. However, there are challenges to overcome before AM is considered a conventional method of manufacture, for example, upskilling engineers, increasing the technology readiness level via AM case studies, and understanding the AM build process to deliver the required material properties. This paper describes current progress within a four-year research programme that has the goal to explore these challenges towards creating a strategy for AM adoption within astronomical hardware. Working with early-career engineers, case studies have been undertaken which focus on lightweight AM aluminium mirror manufacture and optical mountings. In parallel, the aluminium AM build parameters have been investigated to understand which combination of parameters results in AM parts with consistent material properties and low defects. Metrology results from two AM case studies will be summarised: the optical characteristics of a lightweighted aluminium mirror intended for in-orbit deployment from a nanosat; and the AM build quality of wire arc additive manufacture for use in an optomechanical housing. Finally, an analysis of how surface roughness from AM mirror samples and build parameters are linked will be discussed.

Additive Manufacturing (AM) has several potential advantages for astronomical instrumentation: particularly the ability to create custom parts with optimised geometries that cannot be produced with traditional manufacturing. The goal of the EU H2020 funded OPTICON (Optical Infrared Coordination Network for Astronomy; grant agreement 730890) A2IM (Additive Astronomy Integrated-component Manufacturing; PI H. Schnetler) project completed in June 2021, was to develop prototypes demonstrating these benefits. This paper presents the design and additive manufacture of a piezoelectric stack actuator driven, monolithic flexure for the active array of the Freeform Active Mirror Experiment (FAME). Flexure geometry had previously proved difficult to repeatedly produce and AM was considered as a potential solution. Two AM processes were used: powder bed fusion where metal powder is bonded using a laser, and binder jetting where powder is bonded using a polymer adhesive. A topology optimised, flexure hinged frame was designed based on the minimum feature size of each AM machine. This geometry was produced in Aluminium (AlSi10Mg), Titanium (Ti64Al4V) and Stainless Steel 316L. Porosity is a known issue with AM and Hot Isostatic Pressing (HIP): a post process whereby parts are subject to increased temperature and pressure was identified as a way of reducing this, thereby increasing the predictability of flexure behaviour and suitability for vacuum applications. Conformity of AM parts to their original geometry was assessed using external dimensional metrology. X-ray Computed Tomography (XCT) was used to identify internal porosity.

Details of a programme to investigate the outgassing rate of additively manufactured (AM) aluminium alloys are presented. AM has significant potential benefits to applications in ground- and space-based instrumentation, particularly in mass optimisation, part consolidation and increased design freedom. However, its use in high-risk projects is often curtailed by lack of heritage and an imperfect understanding of the materials. The programme goal was to address one of the most significant topics preventing wider adoption of AM technology in cryogenic and space-based applications; uncertainty about material outgassing. The sensitivity of outgassing rates to various key parameters was characterised, including print method, post-processing and geometrical complexity. Correlation of outgassing rates against other measurable properties, such as sample porosity and surface roughness, was also investigated via the use of X-ray computed tomography and profilometry. Finally, the test apparatus, experimental design and implications of the findings on design and process control are discussed.

Additive manufacture (AM), also known as 3D printing, builds an object, layer-by-layer, from a digital design file. The primary advantage of the layer-by-layer approach is the increase in design-space, which enables engineers and scientists to create structures and geometries that would not be practical, or possible, via conventional subtractive machining (mill, drill and lathe). AM provides more than prototyping solutions: there are a broad range of materials available (polymers, metals and ceramics); software capable of creating lightweight structures optimised for the physical environment; and numerous bureaux offering AM as a service on a par with subtractive machining. In addition, AM is an ideal method for bespoke, low-count parts, which are often the foundation of astronomical instrumentation. However, AM offers many challenges as well as benefits and, therefore, the goal of the OPTICON A2IM Cookbook is to provide the reader with a resource that outlines the scope of AM and how to adopt it within astronomical hardware, with an emphasis on the fabrication of lightweight mirrors. The Cookbook was an open access deliverable of the EU H2020 funded OPTICON (Optical Infrared Coordination Network for Astronomy; grant agreement #730890) A2IM (Additive Astronomy Integrated-component Manufacturing; PI H. Schnetler) work package and it was completed in June 2021. This paper will introduce the Cookbook, its scope and methodology, and highlight the paradigm shift required to design and AM lightweight mirrors for astronomy and space-science.

Fabricating mirrors using additive manufacturing (AM; 3D printing) is a promising yet under-researched production route. There are several issues that need to be better understood before AM can be fully adopted to fabricate mirror substrates. A significant obstacle to AM adoption is the presence of porosity and the influence that has on the resultant optical proprieties. Several batches of high-silicon aluminium (AlSi10Mg) samples were created to investigate the relationships laser parameters, laser paths and build orientations have with the porosity. The results showed that eliminating defects relies on a complex interaction of the process parameters and material properties, with the residual heating from the laser proving to be a significant factor. In addition, the use of a hot isostatic press is investigated and some full prototypes of the Cassegrain CubeSat were produced.

Lightweight, aluminum, freeform prototype mirrors have been designed and fabricated by a Thai led team, with UK support, for intended applications within the Thai Space Consortium (TSC) satellite series. The project motivation was to explore the different design strategies and fabrication steps enabled by both conventional (mill, drill, and lathe) and additive (3D printing) manufacture of the prototype substrates. Single Point Diamond Turning was used to convert the substrates into mirrors and optical metrology was used to evaluate the different mirror surfaces. The prototype criteria originated from the TSC-1 satellite tertiary mirror, which is designed to minimize the effect of Seidel aberrations before the beam enters the hyperspectral imager. To converge upon the prototype designs, Finite Element Analysis (FEA) was used to evaluate the different physical conditions experienced by the prototypes during manufacture and how these influence the optical performance. The selected designs satisfied the mass and surface displacement criteria of the prototype and were adapted to either the conventional or additive manufacturing process. This paper will present the prototype design process, substrate manufacture, optical fabrication, and an interferometric evaluation of the optical surfaces comparing the conventional and additive manufacturing processes.

In the search for applications for alloys developed under the philosophy of the High Entropy Alloy (HEA)-type materials, the focus may be placed on applications where current alloys also use multiple components, albeit at lower levels than those found in HEAs. One such area, where alloys with complex compositions are already found, is in filler metals used for joining. In soldering (<450 °C) and brazing (>450 °C), filler metal alloys are taken above their liquidus temperature and used to form a metallic bond between two components, which remain both unmelted and largely unchanged throughout the process. These joining methods are widely used in applications from electronics to aerospace and energy, and filler metals are highly diverse, to allow compatibility with a broad range of base materials (including the capability to join ceramics to metals) and a large range of processing temperatures. Here, we review recent developments in filler metals relevant to High Entropy materials, and argue that such alloys merit further exploration to help overcome a number of current challenges that need to be solved for filler metal-based joining methods.

The additive manufacturing of metals requires optimisation to find the melting conditions that give the desired material properties. A key aspect of the optimisation is minimising the porosity that forms during the melting process. A corresponding analysis of pores of different types (e.g. lack of fusion or keyholes) is therefore desirable. Knowing that pores form under different thermal conditions allows greater insight into the optimisation process. In this work, two pore classification methods were trialled: unsupervised machine learning and defined limits. These methods were applied to 3D pore data from X-ray computed tomography and 2D pore data from micrographs. Data were collected from multiple alloys (Ti-6Al-4V, Inconel 718, Ti-5553 and Haynes 282). Machine learning was found to be the most useful for 3D pore data and defined limits for the 2D pore data; the latter worked by optimising the limits using energy densities.

Additive manufacturing (AM) methods and post processing techniques are promising methodologies considering that it is now possible to print in a wide variety of materials using processes much refined from those originally available twenty years ago. To date the uptake of AM in Astronomy is relatively low compared to other application areas, aviation being one such example. Due to the rapid progress made in additive manufacturing and the lack of its adoption in Astronomy, there are many opportunity to deploy new fabrication processes. In this paper, we outline the project and report the results of our investigation to make use of additive manufacturing and novel materials in the fabrication of multi-functional integrated components fit for use in astronomy instrumentation, which can operate in cryogenic environments and space application.

Additive Manufacturing (AM; 3D printing) for mirror fabrication allows for intricate designs that can combine lightweight structures and integrated mounting. Conventional lightweight structures utilise cubic or prismatic unit cells, which do not provide uniform support at the edge of curved mirrors. We present a new circular lattice based upon cylindrical coordinates and how this lattice has been incorporated within an 80 mm diameter mirror intended for use in a 3U CubeSat telescope. Several design iterations are explored, which include prototype mirrors produced in a titanium alloy and a finite element analysis of the one of the design iterations.

A possible answer to the challenge brought by the construction of the next generation 40 m-class telescopes was the case study of FAME (Freeform Active Mirror Experiment). As the new instruments typically increased in both volume and complexity, the use of highly aspherical freeform surfaces could be a great solution as these systems are built up from fewer parts and can achieve higher performance. The idea of FAME was to create a thin face sheet which is then deformed to the nominal shape utilizing actuators mounted on the back of the mirror and acting parallel to the optical surface. The test phase of the FAME prototype revealed the complexity of the design and its sensitivity to manufacturing and assembly processes. As part of the characterization it was very difficult to predict correctly how the system behaves which is due to the several interfaces between the actuators and the face sheet. These experiences led to the development of a new structure that eliminates the strict tolerance chain obtained from a variety of components mounted on one another. It also means that the design for conventional manufacturing technologies should be left behind, and additive manufacturing must be introduced. This paper gives a brief overview how the lessons learned from the previous development is matched with the new design approach of the same component using topology optimization, additive manufacturing of metals and post processing of 3D printed parts. This work is funded under the OPTICON H2020 INFRAIA-2016-2017/H2020-INFRAIA-2016-1 Grant Agreement 730890.

Additive manufacturing (AM) offers many advantages, including material savings, lightening, design freedom, function integration, etc. In the case of cellular materials, regular structures (lattice and honeycomb) are particularly important due to their ability to reduce weight. However, the design process and FEM analysis of this type of structure is very high time-consuming. In order to mitigate this problem, we propose a modelling method, called "Equivalent Continuum Material", based on the treatment of a cellular material as a continuous mass. This document describes the method and presents examples of applications, to facilitate and understand its use.

Additive manufacturing (AM) methods and post processing techniques are promising methodologies considering that it is now possible to print in a wide variety of materials using processes much refined from those originally available twenty years ago. To date the uptake of AM in Astronomy is relatively low compared to other application areas, aviation being one such example. Due to the rapid progress made in additive manufacturing and the lack of its adoption in Astronomy, there are many opportunity to deploy new fabrication processes. In this paper, we outline the project and report the results of our investigation to make use of additive manufacturing and novel materials in the fabrication of multi-functional integrated components fit for use in astronomy instrumentation, which can operate in cryogenic environments and space application.

Design for additive manufacture (AM; 3D printing) is significantly different than design for subtractive machining. Although there are some limitations on the designs that can be printed, the increase in the AM design-space removes some of the existing challenges faced by the traditional lightweight mirror designs; for example, sandwich mirrors are just as easy to fabricate as open-back mirrors via AM, and they provide an improvement in structural rigidity. However, the ability to print a sandwich mirror as a single component does come with extra considerations; such as orientation upon the build plate and access to remove any temporary support material. This paper describes the iterations in optimisation applied to the lightweighting of a small, 84mm diameter by 20mm height, spherical concave mirror intended for CubeSat applications. The initial design, which was fabricated, is discussed in terms of the internal lightweighting design and the design constraints that were imposed by printing and post-processing. Iterations on the initial design are presented; these include the use of topology optimisation to minimise the total internal strain energy during mirror polishing and the use of lattices combined with thickness variation i.e. having a thicker lattice in strategic support locations. To assess the suitability of each design, finite element analysis is presented to quantify the print-through of the lightweighting upon the optical surface for a given mass reduction.

Additive manufacturing (AM; 3D printing) is a fabrication process that builds an object layer-upon-layer and promotes the use of structures that would not be possible via subtractive machining. Prototype AM metal mirrors are increasingly being studied in order to exploit the advantage of the broad AM design-space to develop intricate lightweight structures that are more optimised for function than traditional open-back mirror lightweighting. This paper describes a UK Space Agency funded project to design and manufacture a series of lightweighted AM mirrors to fit within a 3U CubeSat chassis. Six AM mirrors of identical design will be presented: two in aluminium (AlSi10Mg), two in nickel phosphorous (NiP) coated AlSi10Mg, and two in titanium (Ti64). For each material mirror pair, one is hand-polished and the other is diamond turned. Metrology data, surface form error and surface roughness, will be presented to compare and contrast the different materials and post-processing methods. To assess the presence of porosity, a frequent concern for AM materials, X-ray computed tomography measurements will be presented to highlight the location and density of pores within the mirror substrates; methods to mitigate the distribution of pores near the optical surface will be described. As a metric for success the AlSi10Mg + NiP and AlSi10Mg mirrors should be suitable for visible and infrared applications respectively.