Description

1. TASK 70-31-00-310-501 Welding And Brazing

   1. General

      1. Welding

         1. This TASK gives general data on welding as a procedure to repair parts at overhaul. The types of welding that follow are the types used to make repairs on aircraft jet engines:

            1. TIG welding.

            2. Plasma arc welding.

            3. Electrical resistance welding.

            4. Electron beam welding.

         2. Tungsten inert gas (TIG) welding uses an electrical arc, which moves between a permanent tungsten electrode and the work-piece, to surface weld components; which are to be attached together or increased in dimension. To give protection and prevent oxidation of the tungsten electrode, melted metal and hot areas, a protective inert gas (For example argon, helium, or a mixture of the two) goes through a circular nozzle on the torch. The procedure can be manual or automatic. Usually a filler wire, applied by hand or through a wire supply device, is used.

         3. Plasma arc welding is different to TIG welding. An initial arc is made between a tungsten electrode and a circular copper nozzle; which is kept cool. To increase the energy, an inert gas (Usually argon) is blown directly into the circle made between the electrode and the nozzle. It goes out as a high-energy arc, that is then transferred to the work-piece to be welded. As with TIG welding, inert gas is added through a circular nozzle; to supply the full torch head. Plasma arc welding is best used for automatic welding procedures; when it can be used with or without a filler material. Because of the higher energy used, the heat affected area is smaller and component distortion is less.

         4. Electrical resistance welding uses the Joule heat that occurs when a high current, supplied by electrodes attached with clamps, goes through an electrically resistant work-piece. When the electrode force, welding current and time, are all correct for each other the necessary heat (Used to weld the component surfaces together) starts at the mating faces of the sheets to be welded; where the contact resistance is largest. Two or more sheets can be welded together (When the correct welding equipment is used) and welds can be of the spot, stitch (overlapping spots), continuous seam, or roll-spot type.

         5. Electron beam welding uses a fusion welding technique, which is done in a vacuum. With this procedure an electron beam, of high kinetic energy, moves through a magnetic lens; where it is put into focus on the surface of the component to be welded. The kinetic energy of the electrons changes to heat energy, when they touch the component; which causes the material, in or near the beam, to melt. The melted metal then becomes cool and solid, to make the joint. This procedure is usually only used on easy joints (For example, straight single-plane joints or circular joints with rotary equipment); but new computer-controlled equipment also permits the welding of three dimension joints, which are not easy. The very high energy used, in this procedure, permits very thin joints to be made with the minimum of distortion; when compared with the TIG and plasma arc welding procedures.

      2. Brazing

         1. Soldering and brazing are heat procedures to bond materials together. When this occurs a liquid phase results usually from a brazing material which melts at the interfaces of the parts to be joined. Brazing material is a filler material (usually an alloy), which becomes a solid and a liquid at temperatures below the temperature which the base metals to be joined become solid. During the brazing procedure the filler material fills a given brazing joint by surface or capillarity force. For the joint to be filled the surfaces must be clean. This is done by different procedures referred to in the brazing procedure. For soldering and brazing procedures fluxes will be applied to make and keep the interfaces to be joined clean of grease and oxides. High temperature brazing is brazing without fluxes in a vacuum, hydrogenic air or in an air of inert gas. This prevents oxidation of the interfaces to be joined. Much of the time the brazing materials will be applied into the joint or near to the joint. If you do not do that you will have to apply the brazing material into the joint while heat is applied to the parts. Then the brazing material will melt at the joint. Different procedures, has the brazing material in different forms such as foil, powder, paste, bar or preform. Brazing paste is a mixture of brazing powder and binder.

            For high temperature procedures you apply usually powder, paste and foil. Nickel and cobalt base alloys and other special materials are common brazing materials.

            For good wetting of the surfaces of the parts to be joined a special distance is necessary, this is up to 0.05 mm for nickel and cobalt base alloys and from 0.05 to 0.15 mm. for other special materials. If different materials have to be joined you also have to look at the different heat expansion coefficients.

      3. Quality control

         1. Correct inspection procedures for the brazed parts are such as visual, penetrant and x-ray inspection (nondestructure) and prepare and calculate the micro sections (destructure). Important points for the quality of the brazed parts are porosity, large areas without filler and the type of brazing seam structure. The maximum defect dimension, must always be followed.