workpiece can produce significant variation in shielding gas efficiency. Too large a standoff distance can reduce gas shielding efficiency. • Establishing and maintaining the correct shielding gas flow rate — Designed to provide the most efficient gas coverage, it should be high enough to ensure adequate shielding but not too high that it can cause turbulence in the weld pool during welding. Note on Shielding Gas and Porosity A reduction in porosity levels is sometimes achievable through the use of argon/helium shielding gas mixtures. The advantage of helium mixtures over pure argon is the ability to provide additional heat during the welding process, and consequently, provide the hydrogen a greater opportunity to escape prior to solidification of the aluminum weld metal. The use of helium as an additive to argon, sometimes up to 75% helium, can help to provide reduced porosity levels; however, the best line of defense against unacceptable porosity levels is to remove the source of hydrogen contamination. Material Preparation Concerns Other potential contamination problems are associated with material preparation. Cutting or grinding methods, which may deposit contaminants onto the plate surface or subsurface, cutting fluids, grinding disc debris, and saw blade lubricants are all areas of concern. Material preparation methods need to be evaluated as controlled elements of the welding procedure and not changed without revalidation. Certain types of grinding discs, for example, can deposit particles within the aluminum that will react during welding and cause major porosity problems. Additionally, aluminum filler material should be stored in an area that will prevent it from becoming contaminated by hydrocarbons or moisture. Preventing Porosity through Cleaning Prior to Welding To achieve low porosity levels for Xray quality welds, it is important to understand the methods available for the effective removal of hydrocarbons and moisture from the weld area, and incorporate the appropriate methods into the welding procedure. If these contaminants are present in the weld area during welding, they will produce hydrogen and significantly contribute to porosity problems. When designing welding procedures intended to produce low levels of porosity, it is important to incorporate cleaning prior to welding that addresses the removal of hydrocarbons (degreasing) and aluminum oxide removal. Removal of Hydrocarbons Base material is frequently formed, sheared, sawed, and machined prior to welding. If a lubricant is used during any of these preweld operations, complete removal of the lubricant prior to welding is essential if high-quality welds are required. Since it is important to remove lubricants before welding, it is advantageous to use the minimum amount of lubricants in preweld operations. Sawing and machining aluminum can often be performed dry. Hydrocarbons, if present, can be removed by a number of the following methods: wiping with solvents such as acetone or alcohol, detergent-spray degreasing, steam degreasing, or wiping with a mild alkaline solution. Solvent cleaners are possibly the most popular method used to remove hydrocarbons. Most hydrocarbon solvents are highly volatile and evaporate quickly. A hydrocarbon solvent suitable for preweld cleaning must dissolve oil and grease readily, evaporate quickly, and not leave a residue. One product called ZeroTri ®, a heavy-duty degreaser by LPS Laboratories, has proven to work well. Care must be taken not only in the selection of the correct solvent but also in its use. Adequate ventilation is essential, and the manufacturer’s recommendations should be carefully followed. Flammable chemicals are, obviously, dangerous in the presence of welding arcs. If material has been subjected to contamination from hydrocarbons, remove the hydrocarbons before wire brushing the part to remove aluminum oxide. Wire brushing on an oily or greasy surface tends to smear the contaminants into the surface, For info, go to www.aws.org/adindex APRIL 2015 / WELDING JOURNAL 27
Welding Journal | April 2015
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