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A - I must start by stating that the storage and preparation of all materials that are welded, including carbon steel, is important and can have significant effect on the finished quality of the welding. The basic rules for cleanliness, removal of contaminants from the welding area, and suitable storage and handling of filler material, should be applied rigidly when welding all materials if we are attempting to produce high quality welding. I will follow the above statement by identifying the fact, that yes, there are differences between these two materials, carbon steel and aluminum, and that some of these differences are reflected in issues relating to storage and preparation.
As we consider the storage and preparation of aluminum base alloys and filler alloys, we can recognize two areas of potential problem. First, aluminum oxide, how it forms, reacts under certain conditions, and how it can be removed, and secondly, contamination from hydrocarbons, their source and removal.
Aluminum Oxide - Probably the most important issue to understand about aluminum regarding storage and preparation is the nature and characteristics of its surface oxide film. Aluminum alloys rapidly develop a self-limiting oxide surface film upon exposure to air. This aluminum oxide on the material’s surface has a melting point in excess of 3600 F or 2400 F above the melting point of pure aluminum base material. Because of this large difference in melting temperature, the aluminum oxide film can prevent fusion between filler alloy and base alloy and/or flakes of oxide can become entrapped during the welding process, as inclusions within the completed weld.
Aluminum, with an uncontaminated thin oxide layer, can often be easily welded with the inert-gas (GMAW and GTAW) processes, which breaks down and removes the thin oxide during welding. Potential problems arise when the aluminum oxide has been exposed to moisture. The aluminum oxide layer is porous and can absorb moisture, grow in thickness, and become a major problem when attempting to produce welds of high quality that are required to be relatively porosity free.
For high quality welds, it is usually necessary to remove the aluminum oxide mechanically just prior to welding. This is often achieved by brushing with a stainless steel wire brush, but can also be achieved by scraping, filing, machining or grinding. Care must be taken to employ only tools that are clean and free of contaminants such as oil and grease. An alternative to removal of aluminum oxide mechanically is chemical oxide removal. Immersion in alkaline (caustic) solution, followed by a water rinse, then nitric acid and water rinse. The use of chemical cleaning, however, is becoming less common as the handling and disposal of these chemicals is often seen as a restricting inconvenience.
Hydrocarbons - Another issue relating to storage and preparation, is the presence of hydrocarbons on the surface of base material and or filler alloy. Base material is frequently formed, sheared, sawed and machined prior to the welding operation. 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 in preweld operations. Sawing and machining of aluminum can often be performed dry. Hydrocarbons, if present, can be removed by a number of 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 oil and grease. Most hydrocarbon solvents are highly volatile and evaporate quickly, but the water-based cleaners must be thoroughly wiped away or heat dried. A hydrocarbon solvent suitable for preweld cleaning must dissolve oil and grease readily, evaporate quickly and not leave a residue. Care must be taken, not only in the selection of the correct solvent, but, also in its use. Adequate ventilation is essential, and the manufacturers recommendations should be followed carefully. Flammable chemicals are obviously dangerous in the presence of welding arcs.
It should be recognized that if material has been subjected to contamination from hydrocarbons, they need to be removed before wire brushing the part to remove aluminum oxide. Wire brushing on an oily or greasy surface tends to smear the contaminants in to the surface and, in addition, the wire brush becomes contaminated and unsuitable for its intended purpose.
The amount of preweld cleaning required is largely dependent on how much care is taken to keep the material clean and dry in storage and in subsequent handling operations during fabrication and before welding. Some manufacturers have been able to control their handling operations adequately enough so that only wire brushing the joint area is required prior to welding.
Storage of Filler Wire – Both spooled GMAW and straight length GTAW welding wire should be stored correctly. The most common problem is the exposure of wire to moisture. This can occur quite easily if the wire is subjected to abrupt changes in temperature at high humidity. Acquiring wire from a cool atmosphere and immediately unpacking it in a warm, humid atmosphere will subject the wire to condensation from crossing the dew point. This moisture will produce hydrated aluminum oxide on the surface of the wire and, consequently, will result in poor weld quality containing porosity. It is favorable to maintain filler wire in a heated area with a uniform temperature. For example a light bulb inside a storage cabinet can produce adequate heat to prevent condensation on the aluminum filler alloy.
Metalworking Methods – Considerations when cutting and beveling are different for aluminum than for steel. Probably the most common method of thermally cutting steel, oxy fuel gas cutting, is not suitable for cutting aluminum. Plasma arc cutting is perhaps the most common method used for cutting aluminum. It is important to recognize that plasma arc cutting can affect the quality of the cut edge on some of the aluminum alloys. The partial melting of the grain boundaries can result in micro cracking in the cut edge (see fig 1). The 2xxx, 6xxx, and 7xxx series (heat-treatable) alloys are particularly prone to this type of cracking, whereas the 1xxx, 3xxx and 5xxx series (nonheat-treatable) alloys are not. The cracking tendency increases with metal thickness because thick metal imposes greater restraint on the solidifying metal. Some welding standards require that both the roughness and the cracking zone be removed by machining the plasma cut edge to a depth of 1/8 inch (3.2 mm) before incorporating the edge into a welded joint.
Other methods of cutting aluminum are becoming popular. Laser cutting can produce very high quality cut edges, and abrasive water jet cutting is also capable of producing excellent results. Other cutting methods for aluminum include sawing. Circular saws may be either portable or floor mounted, and band saws are used extensively for preparing weld samples and cutting smaller parts. Tooth shapes have been developed by saw blade manufacturers that perform very well on aluminum. Blades recommended for aluminum have more rake and clearance than those for steel. Most gouging is performed on aluminum with mechanical tools. Straight line back gouging of groove welds is probably best performed by using a rotary cutter machine designed especially for this purpose. Some fabricators have chosen to adapt a small portable power saw for back gouging, replacing the saw blade with a cutting blade ground to the required shape. Tungsten carbide cutters are standard for all gouging machines.
Fig 1. Micro cracking at the edge of an aluminum plate after plasma cutting