Brackets are the important components of space vehicles. Brackets serve the purpose of carrying loads and supporting the structures. Mounting brackets are found in large numbers in aerospace vehicles and satellites. Due to their lightweight and thermal conductivity, these are generally used in satellites. Various brackets are used to carry loads of electronic goods, cameras, batteries, antennas, and various satellite components. One of the applications of a bracket used on the outer surface of the satellite is shown in Figure 1. Depending on the application, the shapes of the mounting brackets can be hanging type or supporting type [1]. On the other hand, a suitable, sturdy bracket or stand is necessary for the proper functioning of the satellite dish [2]. A sturdy wall bracket prevents the structure from being damaged. The usage of a suitable bracket has advantages like ease of assembly, optimal reception, and easy alignment in the required direction. Although a sturdy and strong bracket is required for the applications, the weight of the bracket can be a major concern, especially in the case of aerospace and satellite applications. A high strength-to-weight ratio is essential as every kilogram carried into space will add to the total cost of the flight depending on the carrier system and the orbit to be reached. Weight minimization in every aspect adds great value to the design of bracket structures used in space vehicles. Weight reduction of the part can be addressed in three ways: material selection, material design, and material processing. The favorable choice of materials for making satellite and aircraft brackets are the advanced functional materials like Al-based, Ti-based, stainless steel, NiTi-based and composite materials. The material design is considered depending on the loading conditions and also on the structure, and thus, structure optimization of the part should is of great importance. Finally, the choice of material processing technology has a vital role to play as it decides the total cost of the project. For a finalized choice of material, the focus would be on the integration of structure optimization and processing of the material chosen. The resultant complex geometries are extremely difficult to process by conventional manufacturing techniques. Brackets constructed by conventional metal cutting do not fulfill the desire of achieving a high strength-to-weight ratio in parts produced because conventional metal cutting may not optimize the component’s weight and stress factor [3]. Hence, there arises a need for efficient processing techniques that can accommodate complex geometries in addition to structural optimization. One of the best ways to achieve this is through Additive Manufacturing (AM). Owing to the design freedom and also the capability of processing complex geometries, the world is seeing greater interest in the processing of mounting brackets through additive manufacturing technologies [4].

Additive Manufacturing (AM) is a process in which a 3D solid object is built by adding the material layer-over-layer ^[6][7][8][6,7,8]. The schematic of the AM steps is shown in Figure 2. A sliced 3D CAD model is directly converted into a solid model on the building platform of the machine. Hence, the process is also called 3D printing. During the making of the product, the material is added and consolidated into solid-phase layer-by-layer in the height direction. Hence the name layered manufacturing process. The most attractive aspect of AM is its ability to process complex geometries easily which is known as a solid free-form manufacturing process [9].

AM is a recent technology and the alternatives to AM are the traditional manufacturing process that includes Subtractive Manufacturing (SM) and Formative Manufacturing. The SM uses a material removal approach, In contrast to AM, the part in SM is prepared out of the solid material block by removing the part of material using material removal processes like cutting, grinding, planing, shaping, drilling, milling, and slotting, etc. The AM technology was first attempted in 1981 by a Japanese Hideo Kodama for processing a photo-hardening polymer. Later in 1983, Charles W. Hull printed a teacup using the Stereolithography Apparatus SLA-1. In the initial days, the technology was imagined for making the prototypes of the products during the product development [10]. It was a solution for quickly making the products which otherwise consumed a huge amount of time and eventually added huge product development costs. With this technology, prototypes were quickly prepared and it was known as Rapid prototyping. The technology was evolving and attempts were made to process metals, ceramics, and other materials [11]. After about 12 years, the first powder bed has been developed by MIT using inkjet print heads and the term 3D printing began to be used [12]. In 1997, AeroMat produced a Metal 3D printer that used a laser to fuse Titanium powders, the process was called Laser Additive Manufacturing (LAM). As of today, the technology is matured to a greater extent and the world has witnessed the processing of almost all kinds of materials viz., polymers, metals and alloys, ceramics, composites, concrete, chocolates, sand, bio-materials, etc., using this technology. AM offers several advantages over traditional manufacturing technologies. The various advantages are explained below.

Although AM is evolving gradually, technology has gained tremendous interest in the last decade. As a part of industrial revolution 4.0, digital manufacturing involving AM appears to be the leading technology for manufacturing products on customized and mass levels. The current world is witnessing a revamp in manufacturing industries due to the implementation of AM [20]. The choice of suitable AM technique for making products mainly depends on (a) the material to be processed, (b) the size and shape of the product, and (c) the end characteristics of the product [21]. The success of making the product using AM technology requires greater experience in Design for Additive Manufacturing (DFAM) which makes use of the design of freedom of AM. DFAM mainly aims to optimize the product design by decreasing the total amount of support structures and the support material; decreasing the total mass of the product by incorporating lattice structures, print consolidation, and part orientation, eventually decreasing the total print time and total cost. Hence, the implementation of DFAM plays a vital role in making products using AM. Owing to the various advantages of AM and DFAM, it is easy to create high strength-to-weight ratio products. This is an important contribution to aerospace industries in meeting the unabated demand for lightweight and strong structural applications. Again, the applications of AM are spread on a large spectrum of industrial domains that include, automotive, healthcare, electronics, durable goods, cosmetics, architecture, etc.

2. Materials Used in Brackets

Material selection is a critical aspect of the aerospace component and system design cycle. It affects many of the performance factors of aircraft such as payload, flight performance, safety, reliability, structural efficiency, energy consumption, disability, lifecycle cost, and recyclability. The material for aerospace structural applications must consist of mechanical, physical, and chemical properties, such as rigidity, high strength, damage resistance, fatigue durability, high thermal stability, low density, excessive corrosion, and oxidation resistance. The material selection depends on various operating conditions like loading conditions, temperature, moisture, corrosion, noise, etc., of that particular component or system. For the brackets used in the wings of an aircraft that sustains bending during service along with torsion, vibration, tension, and fatigue, a material possessing high tensile and compressive strength, stiffness, vibration modes, and buckling strength will be suitable. Similarly, the combustion chamber always interacts with the fluid which is subjected to high temperature and pressure. So, the material for this should have oxidation resistance and principally thermal properties [22]. Despite matching the primary service requirements, the improvement of structural capability in aerospace structural design becomes remarkably important because the function of lightweight arrangement provides an exponential boost in aircraft performance wherein AM plays a significant role in meeting such unique demand. Another effective and more advanced way to achieve lightweight material is structural optimization. In this method, the material is distributed to reduce material, and structural performance is enhanced ^[23][24][25][23,24,25]. In addition to structural capability, improved acceleration performance, energy efficiency, flight endurance, payload, and decreased life cycle cost and greenhouse gas emissions can also be achieved by the application of DFAM [26]. The various materials used for making brackets and their processing using AM techniques are discussed below.

2.1. Al-Based Alloys

Al6061 is a lightweight material yet high in strength as compared to other aluminum alloys. The important factors to be considered are deflection and stress distribution. In general, good ductility and corrosion resistance, relatively high basic strength and rigidity, low price, and simple manufacturability and durability make advanced aluminum alloys an excellent choice of lightweight materials in many aerospace structural applications such as upper and lower wing skins, wing stringers, and fuselage skin, etc. [27]. On their first human-crewed flight in 1903, Al was the first choice for the engine components and cylinder block for the Wright brothers. For the first time, an Al-alloy was heat-reinforced, a discovery that set Aluminium’s dominance in aerospace engineering ^[28][29][28,29]. The aluminum added substance layer manufactured (ALM) section saves weight and decreases the making time. More prominent solidness gives better guiding exactness toward mounting receiving wires. Airbus Defence and Space in the UK attempted its first space-qualified aluminum 3D printed segments. The optimized antenna bracket for TMTC is shown in Figure 3. The 3D printed segments being created by the UK group are high-performance structures that cannot be manufactured by conventional techniques. The advanced structural bracket made of Al-alloy was installed in Eurostar E3000 broadcast communications satellites. The bracket is a single-piece laser liquefied part weighing 35% less than the conventionally manufactured part that consists of four sections and 44 rivets. In addition, the ALM section is 40% stiffer. Further, as compared to the part processed using conventional machining techniques, the least wastage is observed. The section is used for mounting the telemetry and telecommand (TMTC) reception apparatuses onto the satellite. From the flight qualification testing of the structure, it was found fit to be flown on an impending satellite [30]. The innovative design in the 8 U Cube Satellite included DFAM-based lattice structures to improve the protection. Some of the latest developments include alloying additions such as Gd, La, Sm, and Yb to improve the high-temperature performance and corrosion resistance of cast Mg alloys ^[31][32][31,32].

2.2. Ti-Based Alloys

Titanium alloy is a preferred choice of engineering material after aluminum to manufacture aircraft frames and engine components. Although It has paved its way into the aerospace industry manufacturing Ti products have challenges. Firstly, the high cost of alloy, which is nearly eight times that of Al-alloys used in industries. Secondly, machining Ti-alloy is quite a difficult and expensive process due to its high strength and hardness. Hence, an adaptation of Ti alloys is considered only in critical components of aircraft. They are preferred where high strength, corrosion resistance, and space constraint are the deciding parameters for designing components, and also cost of the product must not be that crucial. In general, Ti-alloys are used mainly in manufacturing engine components and the mechanical structure of aircraft. Titanium combinations have a boundless edge over different metals. It has high rigidity, great crack strength, and fatigue resistance, with some erosion resistance, heat resistance, cryogenic embrittlement restriction, and low thermal coefficient. This makes the Ti-alloy an appropriate alternate choice for steel and Al-based compounds used in aircraft applications. Figure 4 shows a simple design of the additively manufactured Ti-alloy bracket. The processing of Ti-alloy-based brackets using AM is also challenging. Benedetti et al. [34] found high porosity and surface roughness along with poor fatigue properties in the Ti-6Al-4V-alloy-made components processed by Selective Laser Melting (SLM). The relationship between porosity, microstructure, and mechanical properties is also analyzed in terms of process parameters and then improving the processing parameters to achieve 99.5% relative density in CP-Ti-made components [35]. Atar et al. [36] reported improvement in compressive strength, toughness, and tensile strength for SLMed structure. Sample production of Ti-6Al-4V alloy using Selective Laser Sintering (SLS) followed by hot pressing was attempted by Das et al. [37]. The Ti alloy structure processed by SLM provides the best strength-to-weight ratio and is about half as heavy-duty steels or Ni-based superalloys. The vast majority of aircraft frames today are made of Ti-alloys. These alloys have a very specific reinforcing body and can suppress the development of weakness on these grounds; they are considered an ideal replacement for steel and aluminum in frames. To prevent catastrophic fatigue, they are used as thin, narrow bracket rings around the aircraft fuselage of aluminum, similar to the belly bar. One of Ti alloys’ main applications in the aerospace industry has been the use of (α + β) Ti3Al2.5V for hydraulic pipes, aircraft floors, piping systems, etc. [38]. Ti-6Al-2Zr-2Sn-2Mo-2Cr-0.25Si is being produced for US F-22 aircraft and joint strike missile projects. Ti alloys such as Ti-3Al-8V6Cr-4Mo-4Zr (β-C) and Ti-15V-3Cr-3Sn-3Al are preferred for generating sources for multi-aircraft activation systems [39]. Currently, Boeing has created 200 unique parts for ten aeroplane stages utilizing AM, and it has delivered around 20,000 sections for military and business aeroplanes, including 32 distinct segments for its 787 Dream liner planes. General Electric, the world’s biggest fly motor provider, produces fuel spouts for a great many fly motors that are processed to have 25% less weight and multiple times more solidness than best-in-class parts, which were recently made by welding 20 unique parts. SLM is considered the most solid AM strategy that can deliver lightweight parts for aviation applications with diminished CO₂ discharge. Tomlin and Myer have demonstrated the useful suitability of the Electron Beam Melting (EBM) technique for making Airbus A320 nacelle pivot section segments. The utilization of Ti instead of steel brings about a 63% weight decrease.