Aircraft and Aerospace Engineering: Enhancing Surface Finishing and Component Maintenance
Aerospace engineering pertains to the academic discipline that encompasses the manufacture and maintenance of aircraft and spacecraft. This discipline is further divided into two branches, namely aeronautical and astronautical engineering, which are interrelated. The aerospace engineering field involves the creation, development, construction, testing (mechanical, functional, and acceptance), and operational parameters of machinery and vehicles that function in the earth's atmosphere or outer space. As the design of flight vehicles is dependant on multiple engineering disciplines, it is necessary to study various aspects of an aerospace vehicle. This includes a comprehension of aerodynamics, structural variations, propulsion systems, avionics, design, material selection and availability, control systems, and stability under atmospheric changes for a single aerospace vehicle. Therefore, extensive knowledge of aerospace engineering is essential to successfully design and create efficient aerospace vehicles. This discipline plays a crucial role in the design and testing of aircraft structures and components.
Numerous countries have recognised aerospace engineering as a vital discipline due to its extensive use in military applications. Aerospace engineering experts are required to design, develop, and maintain transport and fighter aircraft, spacecraft, missiles, and aviation industry equipment to minimise risk and increase military control efficiency.
Distinctions Between Aerospace and Mechanical Engineering
Mechanical engineering is focused on developing the mechanical design of cars, engines, vehicles, and related automotive applications, whereas aerospace engineering primarily deals with aircraft and spacecraft. Aerospace engineering is a more specific form of mechanical engineering that concentrates solely on activities and phenomena related to aircraft. Both mechanical and aerospace engineers can collaborate to create aircraft or satellites that can withstand the changes in the external environment, including material, performance, efficiency, and functionality. In addition to the earth's climate conditions, aerospace engineers have knowledge of outer space and the criteria that aerospace machinery must meet. This adds to the number of parameters that need to be controlled before deploying an aircraft to the boundary of the earth's atmosphere or beyond. Mechanical engineers can also work in the aerospace industry because they possess a firm foundation and extensive knowledge of vehicle design and principles of operation. Both fields require substantial training and practise to excel and feel confident working in aircraft operations and management, allowing them to enhance their knowledge and move in the right direction.
Aerospace Surface Finishing
Aerospace surface finishing processes play a crucial role in enhancing the durability and corrosion resistance of aerospace materials. These processes also improve the strength, yield, and uniformity of the surface, while reducing the manufacturing cycle. Various techniques are employed for aerospace finishing, including both mechanical and chemical methods. Some of the most commonly used mechanical processes include centrifugal barrel finishing, deburring, deflashing, passivation, sandblasting, polishing, ultrasonic cleaning, buffing, lapping, and others. Meanwhile, chemical finishing techniques comprise electroplating, anodising, powder coating, electroless plating, spraying, painting, and more. The following section provides detailed information on some important surface finishing methods used in the aerospace industry.
Deburring for Aerospace Engineering
Deburring refers to the process of removing burrs and sharp edges that are produced during machining processes on aerospace components. It is a necessary step that precedes polishing and grinding processes. Milling machines used for producing aerospace components require deburring to eliminate burrs and edges, while polishing brushes are used to remove cutting marks. This enhances the overall quality of the aerospace components. Kemet provides a range of CNC deburring brushes, wheels, cross holes and back burr cutting tools to meet industry requirements. A successful aerospace application was processed with Xebec deburring brush, which involved the use of an A11-CB40M brush on an Inconel turbine disk. The deburring process was performed after grinding with abrasive, at a depth of cut of 0.5mm, feed rate of 2400 mm/min, and revolutions of 1500 min-1.
Lapping
In the aircraft and aerospace industry, Kemet Diamond Flat Lapping Systems are used in many areas. Kemet Diamond Flat Lapping systems can be utilised to lap metal to metal faced seals. For precise shoulder lapping of fuel pump gear faces, Kemet annular grooved plates are effective. Similarly, hydraulic parts can be accurately lapped using Kemet Diamond Flat Lapping systems and Kemet annular grooved plates. Engine seals and bearings can be precisely lapped using Kemet Diamond Flat Lapping systems. Landing gear can be effectively lapped using Kemet Diamond Flat Lapping systems and intricate internal faces/diameters can be polished using Kemet Diamond paste and Kemet Helilaps. Manufacturers for mechanical seals and engine seals use Kemet Diamond Flat Lapping Systems extensively. Tungsten carbide, stellite, ceramic, silicon carbide, carbon, and brass are the frequently processed materials. Under production conditions, seal rings with a diameter of up to 12" can be effectively lapped to achieve mirror surfaces with a flatness of 2 or 3LB. Manufacturing and reconditioning companies for pumps utilise Kemet Diamond Flat Lapping Systems to produce and maintain/repair flat seal faces on mechanical seals and flow control equipment.
Kemet Portable hand Lap kits are used for lapping and polishing small aircraft parts, while Kemet Diamond Lapping Machines and Systems are used for larger components. In Belgium, F100 Carbon & Metal Engine Seals are lapped on Kemet Pneumatic Lift Diamond Lapping Machines with Kemet Iron/Kemet XP Composite plates. Hydraulic parts for landing gears are lapped on Kemet Diamond Lapping Machines with Kemet XP Composite plates. In Holland, fuel pumps are shoulder lapped using Kemet Diamond Lapping Machines with annular grooved Kemet Iron Composite plates. Hydraulic parts, engine seals, and air cabin pressure flaps are shoulder lapped using Kemet Pneumatic Lift Diamond Lapping Machines with annular grooved Kemet Iron Composite plates and Kemet Diamond Lapping Machines with Kemet Composite Plates. In Spain, general engine components are lapped on Kemet Lapping Machines. In Singapore, metal-faced seals are lapped on the Kemet Diamond System. Carbon seals are lapped with Al/Ox on Cast Iron Composite plates on a Machine. Hydraulic components, engine seals, and landing gear are lapped on Kemet Large Sized Diamond Lapping Machines with Cast Iron plates. In Malaysia, engine seals are lapped, while in Thailand, general aviation components are lapped. In Hong Kong, general components are lapped on Kemet Medium Diamond Lapping Machines. In Australia, mechanical seals and hydraulic parts, as well as seals and carbons for the aircraft industry, are lapped using Kemet Lapping Machines and Kemet Hand Lapping Plates with Diamond Slurry/Compound. Hand lapping is mainly involved in the tooling side for the helicopter industry, while engine and hydraulic
Kemet International’s ISO assured Diamond Slurries and Compounds enable precision lapping of aircraft components to be performed in production and service environments. Kemet's diamond products are formulated using a unique combination of diamond powder, chemical carrier, and specific grading and concentration. This blend is designed to achieve maximum stock removal and surface finish, while also providing advantages such as easy cleaning, resistance to high temperatures, and lubrication to prevent drying out.
Polishing
Polishing is a technique that involves using diamond compounds or pastes with diamond sprays and suspensions as abrasive materials. Its objective is to achieve a highly reflective surface without any scratches or deformed areas. Typically, aerospace components undergo polishing before optical microscopy analysis. CMP machines use chemo-textile, silk, and nap cloths for polishing, or electrolytic polishing can be employed to reduce sample preparation time, albeit at a higher cost.
Ultrasonic Cleaning
Ultrasonic cleaning is an essential component of aerospace manufacturing since component cleanliness plays a significant role in product quality, efficiency, and profitability. Kemet provides intelligent and safe parts cleaning solutions with low production costs and rapid repayment periods. Ultrasonic cleaning has proven to be the most effective method for aerospace parts cleaning.
Passivation
Passivation is a process that enhances the corrosion resistance of stainless steel parts by eliminating iron particles from the component's surface. Nitric acid or citric acid is used to remove free iron on the surface. Passivation creates a shield against corrosion that can last for longer durations by developing a layer of shield material through micro-coating and reacting with the base material through oxidation or in the air. Different materials like aluminium, titanium, ferrous materials, nickel, silicon, and stainless steel can be passivated by using anodising, phosphatising, nickel fluoridising, silicon dioxide, and chrome oxide layer, respectively. In the aerospace industry, several parts require passivation, such as landing gear components, stainless steel parts, hydraulic actuators, control rods, exhaust systems of aerospace engines, and cockpit fasteners. Kemet provides passivation machines and systems that are automated and encapsulated at multi-stage lines, usually having six to nine stages, which are suitable for the normal, routine-sized aerospace industry.
Aerospace Materials and Coatings
Aerospace structural materials commonly used include aluminium alloys, high-strength steels, titanium alloys, composites, and fibre-reinforced materials, which make up about 90% of the aircraft's weight. Aluminium is preferred over other materials due to its lightweight and ease of processing. To meet the necessary material properties, graphene, metal alloys, composites, polymer composites, and glass-fibre reinforced materials are used. The production of flawless aerospace vehicles requires high-performing materials and manufacturing methods. Foundry and forging are commonly used for aircraft parts manufacturing, with foundry melting the metal beyond its melting point and forging heating it less than the melting point. Recent studies have shown that forged parts have higher mechanical strength and lower risks of porosity, with interlocked grain structures maintaining better mechanical properties. Simulations are carried out for both foundry and forging, followed by mechanical and non-destructive testing.
Thermal spray coatings are the most commonly used coatings in the aerospace industry for thermal barrier and abradable coatings. Materials used in thermal barrier coatings have low thermal conductivity and are sprayed on the airframe's surface, with a thickness range of 100 to 500 µm. Nanocomposite and metal matrix coatings are also being used, with Ni-Ti-based shape memory alloys being one of the most commonly used coatings.
Aerospace NDT
Aerospace NDT refers to non-destructive testing methods that are extensively used for assessing the components of aerospace vehicles. Multiple techniques are employed in the aerospace industry to evaluate aircraft parts without having any negative impact on their structural or chemical integrity. This is the most attractive aspect of these techniques. The smallest flaw in the structure of aerospace materials and components can be detected with NDT.
Aerospace NDT techniques are used to identify geometric flaws, including welding defects, material or coating thickness, delamination, wrinkles, cracks caused by corrosion, foreign particles, porosity, and dry areas. These tests can be conducted without opening the whole aerospace component, which saves both time and money. Ultrasonic testing (UT), magnetic particle inspection (MPI), liquid penetrant inspection (LPI), visual testing (VT), eddy current testing (ET), radiographic testing (RT), shearography, thermography, and acoustic emission testing (AE) are some of the most commonly used techniques. The aerospace industry acquires various certifications to comply with international standards. NDT is used daily to test in-service aerospace components to ensure safe operation. The quality assurance department ensures the use of the right technique at the right time to detect the flaw before it creates any damage to the aerospace component. The aerospace industry needs reliable techniques with experienced personnel to remain competitive in the modern world. The technique selected depends on the component to be tested; VT is commonly used due to its ease of performance and quick analysis, while MPI and LPI are used during the manufacturing process of aircraft or spacecraft.
Aerospace Materials Characterisation
Characterisation and testing are necessary to evaluate the performance and durability of aerospace materials after manufacturing. Various techniques are used, including microstructural analysis through optical microscopy, scanning electron microscopy, and tunnelling electron microscopy, with variations according to the industry's needs. Spectroscopy and microscopy are the two ways in which materials are characterised.
Spectroscopy in the Aerospace Industry
Spectroscopy involves the use of multiple spectrums, such as X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis spectroscopy, Raman spectroscopy, and energy dispersive spectroscopy for elemental analysis. It detects impurities and additional compounds present in the structure of aerospace materials. Atomic force microscopy (AFM) is also used to trace surface characteristics with 1000 times more resolution compared to optical microscopy.
Optical Microscopy in the Aerospace Industry
Kemet provides a variety of optical microscopes, including metallurgical, stereo, digital, and polarising microscopes, to cater to the needs of various aerospace components. These microscopes can also utilise Wi-Fi connections to improve modes of data transfer.
Aerospace Mechanical Testing
Aerospace mechanical testing covers a wide range of components, including landing gear and aircraft frames, to determine the mechanical properties of aerospace materials before their use in actual aircraft. This allows for cost-effective designs and advanced technological orientations. The testing includes hardness, fatigue, tensile and compression, creeps, impact, and indentation astrometry.
Different types of hardness testing are used, such as Brinell, Rockwell, Vickers, Knoop, and Shore hardness testing, with different ASTM standards available for each type. Fatigue testing is done using cyclic loading within the yield strength limit of the material, with ISO 1143:2010 standard being used. Tensile testing is conducted using a universal tensile machine (UTM), with commonly used standards including ASTM E8, D638, and E8-M. Impact testing is conducted in the form of Charpy Impact Test and Izod Impact Test under ASTM standards ASTM D883, 12 ASTM D256, and ASTM d1248. Sample preparation is also necessary for successful material characterisation and testing, with Kemet offering a range of sample preparation machines.
The major requirements of aerospace materials include fatigue strength, corrosion and bird strike resistance, and reduction of direct operating costs. Aerospace vehicles have various applications, including long route transportation, communication, climate change analysis, environmental change monitoring, disaster prevention, geolocalisation, and advanced telecommunications. They also include military, commercial, missile, spaceships, general aviation market, and airlines.
