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9/27/01 AC 43.13-1B CHG 1 roll over, cold lab, or unfued weld metal. Check underside of welded joint for defects. TABLE 4-12. Current and polarity selection for inert gas welding. ALTERNATING DIRECT CURRENT STRAIGHT Polarity N.R. N.R. N.R. N.R. N.R. N.R. 1 1 1 MATERIAL Magnesium up to 1/8 in. thick.................................................. Magnesium above 3/16 in. thick ............................................... Magnesium Castings................................................................ Aluminum up to 3/32 in. thick .................................................. Aluminum over 3/32 in. thick ................................................... Aluminum Castings ................................................................. Stainless Steel .......................................................................... Low Carbon Steel, 0.015 to 0.030 in. ..................................... Low Carbon Steel, 0.030 to 0.125 in. ..................................... 1 Recommended N.R. Not Recommended 4-79. MICROFISSURES Cracks in parts and materials can vary from tiny microfissures, that are visible only with magnification, to those easily identified by unaided eyes. Micro- fissures are the worst type of defect for two reasons; they are often hard to detect, and they produce the worst form of notch effect/stress concentration. Once they form, they propagate with repeated applications of stress and lead to early failures. Every possible means should be used to detect the presence of cracks, and en sure their complete removal before welding operations proceed. (See figure 4-26.) 4-80. NONDESTRUCTIVE TESTING or evaluation is advisable in critical applications. Nondestructive testing methods such as; mag netic particle, liquid penetrant, radiography, ultrasonic, eddy current, and acoustic emission can be used; however, they require trained and qualified people to apply them. 4-81. PRACTICES TO GUARD AGAINST Do not file or grind welds in an effort to create a smooth appearance, as such treatment causes a loss of strength. Do not fill welds with solder, brazing metal, or any other filler. When it is necessary to weld a CURRENT With High- Frequency Stabilization 1 1 1 1 1 1 N.R. FIGURE 4-26. Common defects to avoid when fitting and welding aircraft certification cluster. joint which was previously welded, remove all of the old weld material before rewelding. Avoid welding over a weld, because reheating may cause the material to lose its strength and become brittle. Never weld a joint which has been previously brazed. 4-82. TORCH SIZE (Oxyacetylene weld ing). When using oxyacetylene welding, the torch tip size depends upon the thickness of the material to be welded. Commonly used Par 4-78 Page 4-55PDF Image | AFS-640
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