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AC 43.13-1B CHG 1 9/27/01 factors, the selection may vary with each spe cific application. Metals having refractory sur face oxide films (i.e., magnesium alloys and aluminum and its alloys), are generally welded with alternating current (AC), while direct cur rent (DC) is used for carbon, low alloy, non- corrodible, and heat-resisting steels. General recommendations covering current and polar ity are shown in table 4-12. (b) 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 al loys; because of base metal oxidization, dis tortion, and loss of ductility. NOTE: If oxyacetylene is used for welding stainless steel or aluminum, all flux must be removed, as it may cause corrosion. 4-76. ACCURATELY IDENTIFY THE TYPE OF MATERIAL TO BE RE PAIRED. If positive identification of the material is not possible, contact the aircraft manufacturer or subject the item to a metallur gical laboratory analysis. Before any welding is attempted, carefully consider the weldability of the alloy, since all alloys are not readily weldable. The following steels are readily weldable; plain carbon (of the 1000series), nickel steel (of the Society of Automotive En gineers (SAE) 2300 series), chrome-nickel al loys (of the SAE 3100 series), chrome- molybdenum steels (of the SAE 4100 series), and low nickel-chrome-molybdenum steel (of the SAE 8600 series). 4-77. PREPARATION FOR WELDING. a. Hold elements to be welded in a welding jig or fixture which is sufficiently rigid to pre vent misalignment due to expansion and con traction of the heated material and which positively and accurately positions the pieces to be welded. b. Clean parts to be welded with a wire brush or other suitable method prior to weld ing. 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 fol lowed by a light buffing with emery cloth. 4-78. INSPECTION OF A COMPLETED WELD. Visually inspect the completed weld for the following: (a) The weld has a smooth seam and uniform thickness. Visual inspection shall be made of the completed weld to check for undercut and/or smooth blending of the weld contour into the base metal. (b) The weld is tapered smoothly into the base metal. (c) No oxide has formed on the base metal more than 1/2 inch from the weld. (d) There are no signs of blowholes, poros ity, or projecting globules. Many military specifications, as well as American Society of Testing Materials (ASTM) codes, specify ac ceptable limits of porosity and other types of defects that are acceptable. (e) The base metal shows no signs of pitting, burning, cracking, or distortion. (f) The depth of penetration insures fusion of base metal and filler rod. (g) The welding scale is removed. The welding scale can be removed using a wire brush or by sandblasting. Remove any Page 4-54 Par 4-75PDF Image | AFS-640
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