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A - Unfortunately there is seldom a quick answer for resolving problems associated with porosity in aluminum welds. The reason for this is that porosity can be caused by a number of conditions relating to material, consumables, and / or equipment. It is often necessary to address this problem through a process of elimination, evaluating each of the potential problem areas in order to identify the true cause.
When investigating this type of problem, there is a distinct advantage in understanding how porosity is formed, and how to identify and eliminate these causes.
Porosity is a result of hydrogen gas becoming entrapped within the solidifying aluminum weld puddle and leaving voids in the completed weld. Hydrogen is highly soluble in molten aluminum, and for this reason, the potential for excessive amounts of porosity during arc welding of aluminum is considerably high. Reduction in porosity level can sometimes be achieved through the use of argon / helium shielding gas mixtures. The advantage of the helium mixtures are associated with the ability of this gas to provide additional heat during the welding process, and consequently, allowing hydrogen a greater opportunity to escape prior to solidification. The use of helium as an additive can help to provide reduced porosity levels; however, our best line of defense against unacceptable porosity levels is to remove the source of (hydrogen) contamination.
Hydrogen can be unintentionally introduced during the welding operation through contaminants within the welding area. Exposure of the molten weld metal to the surrounding atmosphere during the welding operation is one consideration when examining a porosity problem. This situation may occur as a result of inadequate gas shielding during welding.
1. Welding in drafty conditions due to open doors or fans directed at the area of welding. Strong drafts can remove the shielding gas during the welding operation.
2. Excessive spatter buildup inside the gas nozzle. This condition can restrict gas flow and reduce the efficiency of the shielding gas.
3. Using the incorrect standoff distance. This is the distance from the end of the nozzle to the surface of the work piece and changes in this distance can produce significant variation in shielding gas efficiency.
4. Establishing and maintaining the correct shielding gas flow rate. This should be designed to provide the most efficient gas coverage.
Other sources of hydrogen and porosity are hydrocarbons such as lubricants, grease, oil, or paint and moisture that can contaminate the plate and or welding wire.
The quality and cleanliness of the aluminum welding wire can be a major factor. If the welding wire is of inferior quality it may be virtually impossible to produce acceptable porosity levels.
To achieve low porosity levels for x-ray quality welds, it is also important to understand the methods available for the effective removal of hydrocarbons and moisture from the weld area, and to incorporate the appropriate methods into the welding procedure. If these contaminants are present in the weld area during welding, they will produce hydrogen and greatly contribute to porosity problems.
Moisture (H2O), which contains hydrogen, may be introduced to the welding area through a number of sources.
1) Water leaks within the welding equipment, if using a water-cooled welding system. 2) Inadequately pure shielding gas. Shielding gas should meet the minimum purity requirements specified by the appropriate welding code or standard. Shielding gas may also become contaminated from imperfections within the gas delivery line such as leaking pipes or hoses.
3) Condensation on plate or wire from high humidity and change in temperature (crossing a dew point). Information provided in Fig 1 shows the temperature differences required at various humidity levels in order to cross a dew point. When welding in high humidity, it is relatively easy to acquire moisture from rather small fluctuation in temperature.
4) Another source of moisture and porosity is hydrated aluminum oxide. Aluminum has a protective oxide layer that is relatively thin and naturally forms on any eposed surface. Correctly stored aluminum, with an uncontaminated thin oxide layer, can be easily welded with the inert-gas (GMAW and GTAW) processes, which breaks down and removes the oxide during welding. Potential problems with porosity 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 that are required to be relatively porosity free.
When designing welding procedures intended to produce low levels of porosity, it is important to incorporate degreasing and oxide removal. Typically, this is achieved through a combination of chemical cleaning and/or the use of solvents to remove hydrocarbons followed by stainless steel wire brushing to remove contaminated aluminum oxide.
Other potential contamination problems are associated with material preparation. Cutting or grinding methods, which may deposit contaminants on to the plate surface or sub-surface, cutting fluids, grinding disc debris, and saw blade lubricants are all areas of concern. These material preparation methods should be closely 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.
Correct cleaning of the aluminum parts prior to welding, use of proven procedures, well maintained equipment, high quality shielding gas, and a high quality aluminum welding wire that is free from contamination, all become very important variables if low porosity levels are desirable. Porosity is typically detected by radiographic testing of completed welds. However, there are other methods that can be used to evaluate porosity which do not use radiographic equipment. The nick-break test for groove welds (fig 2 and 3) and the fracture break test for fillet welds can be extremely useful on test plates when evaluating a new cleaning method, during preliminary procedure development and for day to day weld quality verification.
Determining the actual cause of porosity within a specific welding operation is not always a straightforward exercise. Without an understanding of the basic principals relating to this problem, it can be an extremely time consuming and often a frustrating process. We need to approach a porosity problem from an organized problem-solving standpoint, and work through the possibilities, based on our knowledge of the various sources of hydrogen, until we find and eliminate the cause.