Aerospace Engineering & Surface Finishing
Aerospace engineering is the field of study that deals with the manufacturing and maintenance of aircraft and spacecrafts. There are two branches of aerospace engineering that overlap each other, namely aeronautical engineering and astronautical engineering. The engineering field deals with the design, construction, development, testing (mechanical, functional, acceptance), and operational parameters of vehicles and machinery that work in the earth's atmosphere or outer space. As the design of flight vehicles depends on several engineering disciplines and a single person can not have expertise in all the disciplines, it is a need to study different aspects of an aerospace vehicle. Science of aerodynamics, structural variations, propulsion system, design, avionics, material selection and availability, control system, and stability under atmospheric changes are all studied for a single aerospace vehicle. Therefore, it needs extensive knowledge of aerospace engineering to input the efforts in making efficient aerospace vehicles. This discipline plays a role in designing to testing aircraft structures and working parts.
Different countries have developed aerospace engineering as an important discipline because of its extensive use in military applications. Transport and fighter aircraft, spacecraft, missiles, and aviation industry equipment need an expert in aerospace engineering to minimise the risk and increase the efficiency of military control.
Difference between Aerospace and Mechanical Engineering
Mechanical engineering is focused on the development of the mechanical design of cars, engines, vehicles, and related automotive applications while aerospace engineering mainly deals with aircraft and spacecraft. It is a more specified mechanical engineering form that narrows to only aircraft-related activities and phenomena. Both engineers can work together to develop an aircraft or satellite that can withstand the changes in the outer environment concerning material, performance, efficiency, and working. In addition to the climate conditions of earth, aerospace engineers have a knowledge of outer space and criteria that are to be fulfilled for aerospace machinery. It adds to the number of parameters that need to be controlled before sending an aircraft to the boundary of the earth's atmosphere or outside it. Mechanical engineers can also work in the aerospace industry because they have a solid foundation and firm knowledge of how vehicles are designed and what principles they work. Both fields require extensive training and practice to be excellent and confident in working for aircraft operations and management. It allows these fields to pursue the right direction and add to their knowledge.
Aerospace Surface Finishing
Aerospace metal finishing processes are used to increase the durability and corrosion resistance of aerospace materials. It increases the strength, product yield, and uniformity of the surface, and reduces the manufacturing cycle. There are different methods of aerospace finishing including mechanical finishing and chemical finishing. Some of the most commonly used processes are centrifugal barrel finishing, deburring, deflashing, passivation, sandblasting, polishing, ultrasonic cleaning, buffing, lapping, and many more. Other methods that involve chemical reagents are electroplating, anodizing, powder coating, electroless plating, spraying, painting, etc. Details of some important surface finishing methods are given below:
During some machining processes, burrs and sharp edges are created in aerospace components. To remove these burrs, deburring processes are required and is a prerequisite for polishing and grinding processes. Aerospace components that are made by using milling machines undergo deburring to remove any burrs and edges along with polishing brushes to remove the cutting marks. It enhances the quality of aerospace components being made. Kemet offers CNC deburring brushes, wheels, cross holes and back burr cutting.
Sucessful Aerospace application processed with Xebec deburring brush
Workpiece: Turbine Disk
Pre-processing: Deburring after grinding with abrasive
Revolutions: 1500 min-1
Depth of cut: 0.5mm
Feed: 2400 mm/min
In this process, loose powders are used as grinding agents with abrasive impact at low speed. It is the finishing process that is used where high precision and close tolerances are required. It can help in finishing the aerospace component close to the required thickness, flatness, parallelism, and very tight tolerance. There are multiple machine sizes offered by Kemet which includes open face or pneumatic lifts from 200mm up to 3m depending on the type of component and its size. Kemet offer bespoke annular grooved lapping plates to make it easier to Lap aerospace fuel and hydraulic systems to a flatness of less than 0.0005mm. (0.5 micron).
Polishing is another technique that uses diamond compound or pastes as an abrasive material with diamond spray and suspension. The purpose of polishing is to obtain a highly reflective surface without any scratches and deformed surfaces. Usually, aerospace components are polished before undergoing optical microscopy analysis. Chemo-textile, silk, and nap cloths are also used for polishing on CMP machines or Electrolytic polishing can be used which requires lesser time for sample preparation but at a higher cost.
Component cleanliness has a significant effect in product quality, efficiency and bottom line results in the Aerospace industry. Kemet’s aim is to provide intelligent and safe parts cleaning solutions with short repayment periods and low production costs. Ultrasonic cleaning has proved to be the most efficient cleaning method in Aerospace parts cleaning.
Passivation is the process in which corrosion resistance of stainless steel parts is increased by removing the iron particles from the surface of the component. Nitric acid or citric acid is used to remove the free iron present on the surface. Passivation builds a layer of shield material by micro-coating and reacting with the base material, either by oxidation or in the air. It creates a shield against corrosion that can last for longer durations. Aluminium, titanium, ferrous materials, nickel, silicon, and stainless steel can be passivated by using anodizing, phosphatizing, nickel fluoridizing, silicon dioxide, and chrome oxide layer, respectively. These standards are ASTM A967 and AMS 2700 where the first method makes use of nitric acid and the second enlists chemical dissolution by treatment with an acid solution.
In the aerospace industry, several parts need to be passivated including landing gear components, stainless steel parts, hydraulic actuators, control rods, exhaust systems of the aerospace engine, and cockpit fasteners. Kemet provides passivation machines and systems that are automated and encapsulated at multi-stage lines. It usually has six to nine stages that are feasible for the normal, routine-sized aerospace industry.
Aerospace Materials and Coatings
Aluminium alloys, high-strength steels, titanium alloys, composites, and fibre-reinforced materials are the aerospace structural materials that are commonly used. 90% of aircraft weight, accounts for the use of these aerospace materials. Aluminium has priority over other materials due to its lightweight and easier processing when compared with steel and other alloys. Light weightiness is the property that accounts for low fuel consumption and the ability for aircrafts to carry more weight. In addition to metal alloys, polymer-based materials are also used for this application.
The required material properties include fatigue resistance, heat resistance, high tensile strength of up to 889 MPa, yield strength of 800 MPa, damage tolerance, high thermal stability, corrosion resistance, and stiffness. Graphene, metal alloys, composites, polymer composites, and glass-fibre reinforced materials are used for these purposes.
To ensure the flawless build-up of aerospace vehicles, there is a need for excellent performing materials with the best methods of production and manufacturing. Foundry and forging along with some metal-forming methods are used for the manufacturing of aircraft parts. The foundry uses heat to melt the metal beyond its melting point while forging heats it less than the melting point. Recent results have shown that forged parts have higher mechanical strength such as 26% higher tensile strength, 37% higher fatigue strength, and yield strength up to 44%. Moreover, there are lower risks of porosity as the part is made to the exact shape after getting the pre-formed sheet or rod. Forged parts show an interlocked grain structure that helps in maintaining better mechanical properties. Simulations are first carried out for both foundries and forging to analyse and check the part's performance. Later on, mechanical testing and non-destructive testing are done to ensure the correct working of parts.
The most commonly used coatings in the aerospace industry are thermal spray coatings being applied as thermal barrier coatings as well as abradable coatings. For thermal barrier coatings, the materials used are of low thermal conductivity and are sprayed on the surface of the airframe. The thickness of these coatings ranges from 100 to 500 µm. Nanocomposite and metal matrix coatings are being used in the aerospace industry. Ni-Ti-based shape memory alloys are one of the most commonly used coatings for this application.
Non-destructive testing has been widely utilised to analyse the components of aerospace vehicles. Multiple techniques are used in the industry to evaluate aircraft parts. NDT can be used to test the in-service aircraft or spacecraft without having an impact on the structural or chemical integrity of the part under test, which is the most appealing characteristic of these techniques. It detects the smallest flaw in the structure of aerospace materials and components.
Aerospace NDT is used to detect geometric flaws such as welding defects, material or coating thickness, delamination, wrinkles, cracks propagation due to corrosion, foreign particles, porosity, and dry areas. It can be performed without opening the whole aerospace component which not only saves the industry time, but also saves money from troubleshooting, opening, and reassembling again. Some of the most commonly used techniques include 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). Different certifications are available in the market that is acquired by the aerospace industry to prove their adherence to international standards. NDT is used in daily routine to test the in-service aerospace components to ensure a safe operation. The quality assurance department makes sure to use the right technique at right the time, to detect the flaw before it creates any damage to the aerospace component. Reliable techniques with experienced personnel are the need of the aerospace industry to compete in the modern world. Depending on the component to be tested, the technique is selected. VT is commonly used due to ease of performance and quick analysis. MPI and LPI are also used during the manufacturing process of aircrafts or spacecrafts.
Aerospace Materials Characterisation
After manufacturing an aerospace material, there is a need for characterisation and testing to evaluate the performance of the material and its durability. For this purpose, multiple techniques are used for characterisations such as microstructural analysis using optical microscopy, scanning electron microscopy, or tunnelling electron microscopy. There are variations in these techniques according to the need of the aerospace industry. It allows us to have an insight into the aerospace material's microstructure. There are two ways in which materials are characterised i.e., spectroscopy and microscopy.
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, etc. Energy dispersive spectroscopy is also used for elemental analysis. It tells us about the impurities or any additional compounds present in the structure of aerospace material. Atomic force microscopy (AFM) are used to trace the surface characteristics of a membrane, film, or material. It gives 1000 times more resolution compared to optical microscopy.
Optical Microscopy in the Aerospace Industry
Kemet offers a variety of optical microscopes to cater to the needs of multiple aerospace components. It includes metallurgical microscopes, stereo microscopes, digital microscopes, and polarising microscopes. Wi-Fi connections can also be used with these microscopes to provide better data transferring modes.
Aerospace Mechanical Testing
Aerospace mechanical testing implies a wide range of components from landing gear to aircraft frame. It is very important to determine the mechanical properties of aerospace materials before their application in actual aircraft. It allows for cost-effective designs and advanced technological orientations of designed aerospace materials. It includes hardness testing, fatigue testing, tensile and compression testing, creeps testing, impact testing, and indentation astrometry.
It involves Brinell, Rockwell, Vickers, Knoop, and Shore hardness testing. Different types of indentors are used in these techniques including diamond, steel ball, or conical shaped. Different ASTM standards are available for these testing methods according to the type of hardness testing.
Fatigue testing is done using cyclic loading within the yield strength limit of the material. ISO 1143:2010 standard is used for fatigue testing.
It describes the tensile strength, yield strength, and related parameters of the material. It is conducted using a universal tensile machine (UTM) where a load is applied to aerospace material and elongation is measured using the software. ASTM E8, D638, and E8-M are the commonly used standards for this testing.
It is conducted in two forms namely Charpy Impact Test and Izod Impact Test. These tests are conducted under the ASTM standards ASTM D883, 12 ASTM D256, and ASTM d1248.
Sample Preparation Methods
For the successful completion of material characterisation and testing, there is a need for sample preparation. Kemet is offering a range of sample preparation machines including surface grinding, automatic milling machines, and pendulum grinding. It is crucial to prepare the sample before optical microscopy, atomic force microscopy, and also for mechanical testing like hardness testing and metallography. One of the major requirements of aerospace materials is fatigue strength as it incurs the maintenance and rebuilds cost on airframe structure. Reduction of direct operating costs is the aim of upcoming future aerospace engineers. Corrosion and bird strike are also the major cause of aircraft failure. Therefore, there is a need for the development and application of new and modified aerospace materials that can withstand these conditions.
Major Aerospace Applications
Aerospace vehicles are useful for human and robotic exploration of space and the universe. It finds applications in long route transportation, communication, and live transmission, climate change analysis, environmental change monitoring, disaster prevention, geolocalisation, and advanced telecommunications. Uncrewed aerial vehicles are also designed to study outer space with more details and precise analysis. Aerospace applications include military, commercial, missile, spaceships, general aviation market, and airlines.