Aerospace Welding and Aero Industry Repair by John Routledge
Engine parts are joined by several methods. Wherever a joint may have to be dismantled there is little alternative to drilling holes and fitting bolts and nuts, the latter being of one of the patented `stop' types that cannot work loose in service. Engine accessories - even some large items such as gearboxes and oil tanks - and bleed connections, fuel injectors, cover plates, and many other items are retained by similar stop nuts held (like all others, with the correct torque) on studs welded to the engine casings. Many engines contain numerous countersunk screws. These always have cross-type heads, such as the Parker-Kalon series. Locking wire, used by the mile in the Second World War, is now seldom seen.
Where engine parts will not need to be separated, the almost universal answer is some form of aerospace welding or bonding; rivets are rare. As in so many processes, human welders have tended to be replaced by computer control. Once the software has been perfected, this should mean absolutely repeatable joints without the shrinkage or distortion that was previously hard to avoid. Among the more traditional forms, used to join sheet metal work are spot and seam resistance welding. In spot welding, twin electrodes, usually of copper, are brought lightly together at each joint and a large DC current passed to cause intense local heating; almost at once the desired temperature is reached, the current is switched off and the electrode pressure increased to make the joint. In seam welding, the parts are moved past copper wheels which by the same process make a continuous joint.
Traditional manual arc or gas welding is almost extinct in engine production, but considerable use is made of tungsten inert-gas, TIG welding. The electrode torch, which forms a DC cathode, is made of consumable thoriated tungsten and fitted with a `gas lens' which shields the weld with inert argon, even though in many cases the entire operation takes place in an atmosphere of high-purity argon. To avoid the workpiece cracking, the arc is started without torch contact and the finishing current is tapered off in a programmed manner.
Electron-beam welding (EBW) faintly resembles an electron microscope, in that a beam of electrons is focussed magnetically on the target in an evacuated (vacuum) chamber. It differs in the much higher power of the beam, the weld being made by the heat generated by the electron impacts. Under computer control, perfect welds can be made between dissimilar materials with virtually no distortion or shrinkage. Electron beam welding is the preferred method for such tasks as joining the turbine nozzle vanes to their rings, or the discs, rings, and spacers of a compressor rotor, or attaching the HP turbine stub shaft (made of stable bearing steel) to the expandable material mating with the turbine disc.
Plasma-arc welding differs from traditional arc welding in that the arc carries with it ionized gas aimed at the weld through a small nozzle. The gas travels at several thousand mph at a temperature of 12,000-18,000°C to produce a weld that is much finer, more accurate, faster and distortion-free, and needs less filler material. Plasma welding does not need a vacuum chamber (though the weld is protected from oxidation by inert gas) and can be done manually, so it is handy for irregular joints, especially where only a few welds must be made and automated operation is not justified.
Diffusion bonding (DB) is a seemingly simple process in which metal parts are heated in a vacuum furnace to the diffusion point, just below the melting temperature.
At this point the metal molecules are mobile, so that when the parts are brought together under high pressure the molecules migrate in both directions across what had been the interface. In effect, the two parts become a single piece of metal, even though the two components might be of dissimilar metals such as a high-nickel alloy joined to a cobalt-based alloy. As there is no local heating, there is no stress or distortion. Diffusion bonding is often combined with super plastic forming SPF, using balloon-like inflation by argon between hot dies to produce startlingly complex engine and nacelle parts which would otherwise have to be made by joining numerous smaller components. An alloy particularly amenable to SPF/DB is 6A1-4V (6 per cent aluminum, 4 per cent vanadium) titanium.
Aero industry repair by laser-beam welding.
Of course, the ubiquitous laser is also capable of making superior welds, and laser-beam welding (LBW) is rapidly gaining in popularity. One reason is that it offers all the advantages of EBW without the time-consuming need to `pump down' a vacuum chamber, so it saves time in series production. Typically, a high power (eg, 2 kW) continuous COZ (carbondioxide) laser is used, under open-shop conditions.
Inertia bonding is an economical way of joining circular parts such as shafts and discs. One of the parts is spun at high speed whilst attached to a heavy flywheel. At an exact predetermined speed it is suddenly forced under high pressure against the mating surface, which is fixed. The instantaneous friction heat and pressure bonds the parts without melting, to give a joint with outstanding physical properties, though finish machining will be needed to remove a ring of metal squeezed outwards at the joint.
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aviation-database.com has lots of resources for the aircraft industry. The web is a vast source of information. Aviation-database collects the industry into one huge database of contacts.
Frequently Asked Questions...
What is an Electric resistance weld (ERW) tube?
This is either a pipe or tube (i.e. muffler pipe for your automtoive is made this way) that has ben bend from flat metal into a tube shape and then welded together to form a solid pipe/tube.
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