Aerospace Welding and Aero Industry Repair

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 pipe welder  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.