and the wire brush becomes contaminated and unsuitable for its intended purpose. Removal of Aluminum Oxide Aluminum alloys rapidly develop a self-limiting oxide film when exposed to air. The aluminum oxide on the material’s surface has a melting point in excess of 3600°F, which is around 2400°F above the melting point of pure aluminum base material. Because of this large difference in melting temperatures, the oxide film can impede fusion between the filler alloy and base alloy. Flakes of oxide can become entrapped during the welding process and form inclusions within the completed weld. Aluminum with an uncontaminated thin oxide layer can be successfully welded with the inert-gas welding processes such as GMAW or gas tungsten arc welding, which break down and removes the thin oxide during welding. Potential problems arise when the aluminum oxide has been exposed to moisture. For high-quality welds, it is usually necessary to remove the aluminum oxide mechanically just prior to welding. Removing the oxide layer by brushing with a stainless steel wire brush is probably the most common method of oxide removal. Other methods used include scraping, filing, machining, or grinding. Be careful to use only tools that are clean and free of contaminants such as oil and grease. An alternative to mechanical removal of aluminum oxide is chemical removal by immersion in alkaline (caustic) solution, followed by a water rinse, and then a nitric acid and water rinse. The use of chemical cleaning, however, is much less common as the handling and disposal of these chemicals is often seen as a restricting inconvenience. Storage of Aluminum Welding Wire All aluminum welding wire should be stored correctly to achieve highquality welds. The most common problem that can cause contamination to welding wire is exposure to moisture, which will produce porosity. This can occur quite easily if the wire is subjected to abrupt changes in temperature at high humidity. Obtaining wire from a cool location and immediately unpacking it in a warm, humid area will subject the wire to condensation from crossing the dew point. This moisture produces hydrated aluminum oxide on the surface of the wire and, consequently, causes low-quality, porous welds. Also, at very high humidity, small differences between the aluminum wire temperature and air temperature can cause moisture condensation. For example, at a relative humidity of 90% and ambient air temperature of 70°F, the weld wire or aluminum plate need only be 3°F lower in temperature than the ambient air for it to cross the dew point and produce moisture — Fig. 4. Consequently, when attempting to produce welds that are required to be low in porosity, it is favorable to store aluminum welding wire in a heated area with a uniform temperature and low humidity. Conclusion Determining the actual cause of porosity within a specific welding operation is not always easy. Without an understanding of the basic principles relating to this problem, it can be an extremely time consuming and often frustrating process. If you have a problem with porosity, you must approach it from an organized problem-solving standpoint and work through the possibilities, based on knowledge of the various sources of hydrogen, until finding and eliminating the cause. The very important items that should be considered if low porosity welds are to be made and maintained are as follows: correctly cleaning the aluminum parts prior to welding; using proven welding procedures and well-maintained equipment; working in a suitable welding environment; and employing high-quality shielding gas along with aluminum welding wire free from contamination. 28 WELDING JOURNAL / APRIL 2015 TONY ANDERSON is director of aluminum technology, ITW Welding North America. He is a Fellow of the British Welding Institute (TWI), a Registered Chartered Engineer with the British Engineering Council, and holds numerous positions on AWS technical committees. He is chairman of the Aluminum Association Technical Advisory Committee for Welding and author of the book Welding Aluminum Questions and Answers currently available from the AWS. Questions may be sent to Mr. Anderson c/o Welding Journal, 8669 NW 36th St., #130, Miami, FL 331666672; tony.anderson@millerwelds.com. For info, go to www.aws.org/adindex WJ
Welding Journal | April 2015
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