The manufacture of aircraft engines, whether piston or jet, involves the conversion of raw materials into extremely reliable precision machines. The highly stressed operating environments associated with air transport require the use of a broad range of high-strength materials. Both conventional and unique manufacturing methods are utilized.
Aircraft engines are primarily constructed of metallic components, although recent years have seen the introduction of plastic composites for certain parts. Various aluminium and titanium alloys are used where strength and light weight are of primary importance (structural components, compressor sections, engine frames). Chromium, nickel and cobalt alloys are used where resistance to high temperature and corrosion are required (combustor and turbine sections). Numerous steel alloys are used in intermediate locations.
Since weight minimization on aircraft is a critical factor in reducing life-cycle costs (maximizing payload, minimizing fuel consumption), advanced composite materials have recently been introduced as light-weight replacements for aluminium, titanium and some steel alloys in structural parts and ductwork where high temperatures are not experienced. These composites consist primarily of polyimide, epoxy and other resin systems, reinforced with woven fibreglass or graphite fibres.
Virtually every common metalworking and machining operation is used in aircraft engine manufacture. This includes hot forging (airfoils, compressor disks), casting (structural components, engine frames), grinding, broaching, turning, drilling, milling, shearing, sawing, threading, welding, brazing and others. Associated processes involve metal finishing (anodizing, chromating and so on), electroplating, heat treating and thermal (plasma, flame) spraying. The high strength and hardness of the alloys used, combined with their complex shapes and precision tolerances, necessitate more challenging and rigorous machining requirements than other industries.
Some of the more unique metalworking processes include chemical and electrochemical milling, electro-discharge machining, laser drilling and electron-beam welding. Chemical and electrochemical milling involve the removal of metal from large surfaces in a manner which retains or creates a contour. The parts, depending upon their specific alloy, are placed in a highly concentrated controlled acid, caustic or electrolyte bath. Metal is removed by the chemical or electrochemical action. Chemical milling is often used after forging of airfoils to bring wall thicknesses into specification while maintaining the contour.
Electro-discharge machining and laser drilling are typically used for making small-diameter holes and intricate contours in hard metals. Many such holes are required in combustor and turbine components for cooling purposes. Metal removal is accomplished by high-frequency thermo-mechanical action of electro-spark discharges. The process is carried out in a dielectric mineral oil bath. The electrode serves as the reverse image of the desired cut.
Electron-beam welding is used to join parts where deep weld penetration is required in hard-to-reach geometries. The weld is generated by a focused, accelerated beam of electrons within a vacuum chamber. The kinetic energy of the electrons striking the work-piece is transformed into heat for welding.
Composite plastic fabrication involves either “wet” lay-up techniques or the use of pre-impregnated cloths. With wet lay-up, the viscous uncured resin mixture is spread over a tooling form or mould by either spraying or brushing. The fibre reinforcement material is manually laid into the resin. Additional resin is applied to obtain uniformity and contour with the tooling form. The completed lay-up is then cured in an autoclave under heat and pressure. Pre-impregnated materials consist of semi-rigid, ready-to-use, partially-cured sheets of resin-fibre composites. The material is cut to size, manually moulded to the contours of the tooling form and cured in an autoclave. Cured parts are conventionally machined and assembled into the engine.
Inspection and Testing
In order to assure the reliability of aircraft engines, a number of inspection, testing and quality-control procedures are performed during the fabrication and on the final product. Common non-destructive inspection methods include radiographic, ultrasonic, magnetic particle and fluorescent penetrant. They are used to detect any cracks or internal flaws within the parts. Assembled engines are usually tested in instrumented test cells prior to customer delivery.
Health and Safety Hazards and Their Control Methods
Health hazards associated with aircraft engine manufacture are primarily related to the toxicity of the materials used and their potential for exposure. Aluminium, titanium and iron are not considered significantly toxic, while chromium, nickel and cobalt are more problematic. Certain compounds and valence states of the latter three metals have indicated carcinogenic properties in humans and animals. Their metallic forms are generally not considered as toxic as their ionic forms, typically found in metal finishing baths and paint pigments.
In conventional machining, most operations are performed using coolants or cutting fluids which minimize the generation of airborne dust and fumes. With the exception of dry grinding, the metals usually do not present inhalation hazards, although there is concern about the inhalation of coolant mists. A fair amount of grinding is performed, particularly on jet engine parts, to blend contours and bring airfoils into their final dimensions. Small, hand-held grinders are typically used. Where such grinding is performed on chromium-, nickel- or cobalt-based alloys, local ventilation is required. This includes down-draft tables and self-ventilating grinders. Dermatitis and noise are additional health hazards associated with conventional machining. Employees will have varying degrees of skin contact with coolants and cutting fluids in the course of fixing, inspecting and removing parts. Repeated skin contact may manifest itself in various forms of dermatitis in some employees. Generally, protective gloves, barrier creams and proper hygiene will minimize such cases. High noise levels are often present when machining thin-walled, high-strength alloys, due to tool chatter and part vibration. This can be controlled to an extent through more rigid tooling, dampening materials, modifying machining parameters and maintaining sharp tools. Otherwise, PPE (e.g., ear muffs, plugs) is required.
Safety hazards associated with conventional machining operations mainly involve potential for physical injuries due to the point-of-operation, fixing and power transmission drive movements. Control is accomplished through such methods as fixed guards, interlocked access doors, light curtains, pressure-sensitive mats and employee training and awareness. Eye protection should always be used around machining operations for protection from flying chips, particles and splashes of coolants and cleaning solvents.
Metal-finishing operations, chemical milling, electrochemical milling and electroplating involve open surface tank exposures to concentrated acids, bases and electrolytes. Most of the baths contain high concentrations of dissolved metals. Depending upon bath operating conditions and composition (concentration, temperature, agitation, size), most will require some form of local ventilation to control airborne levels of gases, vapours and mists. Various lateral, slot-type hood designs are commonly used for control. Ventilation designs and operating guidelines for different types of baths are available through technical organizations such as the American Conference of Governmental Industrial Hygienists (ACGIH) and the American National Standards Institute (ANSI). The corrosive nature of these baths dictates the use of eye and skin protection (splash goggles, face shields, gloves, aprons and so on) when working around these tanks. Emergency eyewashes and showers must also be available for immediate use.
Electron-beam welding and laser drilling present radiation hazards to workers. Electron-beam welding generates secondary x-ray radiation (bremsstrahlung effect). In a sense, the welding chamber constitutes an inefficient x-ray tube. It is critical that the chamber be constructed of material or contain shielding which will attenuate the radiation to the lowest practical levels. Lead shielding is often used. Radiation surveys should be periodically performed. Lasers present ocular and skin (thermal) hazards. Also, there is potential for exposure to the metal fumes produced by the evaporation of the base metal. Beam hazards associated with laser operations should be isolated and contained, where possible, within interlocked chambers. A comprehensive programme should be rigorously followed. Local ventilation should be provided where metal fumes are generated.
The major hazards related to the fabrication of composite plastic parts involve chemical exposure to unreacted resin components and solvents during wet lay-up operations. Of particular concern are aromatic amines used as reactants in polyimide resins and hardeners in epoxy resin systems. A number of these compounds are confirmed or suspected human carcinogens. They also exhibit other toxic effects. The highly reactive nature of these resin systems, particularly epoxies, gives rise to skin and respiratory sensitization. Control of hazards during wet lay-up operations should include local ventilation and extensive use of personal protective equipment to prevent skin contact. Lay-up operations using pre-impregnated sheets usually do not present airborne exposures, but skin protection should be used. Upon curing, these parts are relatively inert. They no longer present the hazards of their constituent reactants. Conventional machining of the parts, though, can produce nuisance dusts of an irritant nature, associated with the composite reinforcement materials (fibreglass, graphite). Local ventilation of the machining operation is often required.
Health hazards associated with test operations usually involve radiation (x or gamma rays) from radiographic inspection and noise from final product tests. Radiographic operations should include a comprehensive radiation safety programme, complete with training, badge monitoring and periodic surveys. Radiographic inspection chambers should be designed with interlocked doors, operating lights, emergency shut-offs and proper shielding. Test areas or cells where assembled products are tested should be acoustically treated, particularly for jet engines. Noise levels at the control consoles should be controlled to below 85 dBA. Provisions should also be made to prevent any build-up of exhaust gases, fuel vapours or solvents in the test area.
In addition to the aforementioned hazards related to specific operations, there are several others worthy of note. They include exposure to cleaning solvents, paints, lead and welding operations. Cleaning solvents are used throughout manufacturing operations. There has been a recent trend away from the use of chlorinated and fluorinated solvents to aqueous, terpine, alcohol and mineral spirit types due to toxicity and ozone depletion effects. Although the latter group may tend to be more environmentally acceptable, they often present fire hazards. Quantities of any flammable or combustible solvents should be limited in the workplace, used only from approved containers and with adequate fire protection in place. Lead is sometimes used in airfoil forging operations as a die lubricant. If so, a comprehensive lead control and monitoring programme should be in effect due to lead’s toxicity. Many types of conventional welding are used in manufacturing operations. Metal fumes, ultraviolet radiation and ozone exposures need to be evaluated for such operations. The need for controls will depend upon the specific operating parameters and metals involved.