Weld repairs to aircraft and component parts use such equipment as gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), plasma arc welding, and oxyacetylene gas welding. When repairs of any flight-critical parts are required, it is extremely important to make the weld repairs equal to the original weld. Identifying the kind of metal to be welded, identifying the kind of welding process used in building the part originally, and determining the best way to make welded repairs are of utmost importance. Aerospace welding certification for personnel in the UK is CAA BCAR A8-10.
After the weld is completed, the weld must be inspected for defects. All these things are necessary in order to make an airworthy weld repair.
Aircraft welding is one of the three commonly used methods of joining metals without the use of fasteners. Welding is done by melting the edges of two pieces of metal to be joined and allowing the molten material to flow together so the two pieces will become one.
Aerospace Brazing is similar to welding in that heat is used to join the material; but rather than melting, the metal is heated only enough to melt a brazing rod having a much lower melting point. When this brazing rod melts, it wets the surfaces to be joined, and when it cools and solidifies, it bonds the pieces together.
Soldering is similar to brazing except that brazing materials normally melt at temperatures above 425 °C (800 °F), while solders melt at temperatures considerably lower.
Aircraft weld repairs
The next step in making aircraft weld repairs is to decide the best process to use, considering the available state-of-the-art welding equipment, and then deciding the correct weld-filler material to use. Before any weld repairs can be made, the metal parts to be welded must be cleaned properly, fitted and jigged properly, and all defective welds must be removed to prepare for an aircraft quality weld repair.
Most large business or agencies conduct their own certification tests, or they have an outside testing lab validate the certification tests.
Pitfalls are many, for example, when using GTAW equipment, a weld can be contaminated with tungsten if the proper size electrode is not used when welding with direct current reverse polarity. Another example, the depletion of the inert gas supply below the critical level causes a reduction in the gas flow and will increase the danger of atmospheric contamination.
Electric welding equipment versatility requires careful selection of the type current and polarity to be used. Since the composition and thickness of metals are deciding factors, the selection may vary with each specific application. Metals having refractory surface oxide films (i.e., magnesium alloys and aluminum and its alloys), are generally welded with alternating current (AC), while direct current (DC) is used for carbon, low alloy, non-corrodible, and heat-resisting steels.
Oxyacetylene gas equipment is suitable for welding most metals. It is not, however, the best method to use on such materials as stainless steel, magnesium, and aluminum alloys; because of base metal oxidization, distortion, and loss of ductility.
If oxyacetylene is used for welding stainless steel or aluminum, all flux must be removed, as it may cause corrosion.
Clean parts to be welded with a wire brush or other suitable method prior to welding. Do not use a brush of dissimilar metal, such as brass or bronze on steel. The small deposit left by a brass or bronze brush will materially weaken the weld, and may cause cracking or subsequent failure of the weld. If the members are metallized, the surface metal may be removed by careful sandblasting followed by a light buffing with emery cloth.
Visually inspect the completed weld for the following:
The weld has a smooth seam and uniform thickness. Visual inspection should be made of the completed weld, to check for undercut and/or smooth blending of the weld contour into the base metal.
The following steels are readily weldable; plain carbon (of the 1000 series), nickel steel (of the Society of Automotive Engineers (SAE) 2300 series), chrome-nickel alloys (of the SAE 3100 series), chromemolybdenum steels (of the SAE 4100 series), and low nickel-chrome-molybdenum steel (of the SAE 8600 series).
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