Q - I am experiencing problems with porosity in my 5086/5356 aluminum welds.  The porosity is being detected in groove welds by radiographic inspection and is outside the requirements of our acceptance criteria.  What can I do to prevent this porosity problem?

A - Unfortunately, there is seldom a quick answer for resolving problems associated with porosity in aluminum welds.  The reason for this is that a number of conditions relating to material, consumables, welding technique and / or equipment can cause porosity.  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, it is necessary to understand how porosity occurs 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.  During the welding operation, it is easy to introduce Hydrogen unintentionally through contaminants within the welding area.  It is important to understand thoroughly the many sources of these contaminants in order to detect the cause and take the necessary action to resolve porosity problems.

Shielding During the Welding Operation

Exposure and contamination 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 because of inadequate gas shielding during welding. Following are some examples:

• Welding in drafty conditions  - Strong drafts due to open doors or fans directed at the area of welding can remove the shielding gas during the welding operation.
• Excessive spatter buildup inside the gas nozzle when gas metal arc welding -   This condition can restrict gas flow and reduce the efficiency of the shielding gas
• 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.
• 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:  By using argon / helium shielding gas mixtures, reduction in porosity level is sometimes achievable. The advantage of the helium mixtures is the ability of this gas to provide additional heat during the welding process, and consequently, provide the hydrogen a greater opportunity to escape prior to solidification.  The use of helium as an additive, sometimes up to 75%, 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.

Hydrocarbons

Hydrocarbons are other sources of hydrogen and porosity, which may be present on the surface of the plate or on the welding wire. Following are some examples:

• Contamination of plate surfaces – A thorough cleaning of plate surfaces, that have hydrocarbons on them such as lubricants, grease, oil or paint, is necessary to remove contamination prior to welding.
• Exhaust contaminants on the plate surface from compressed air tools used for weld preparation - Make sure these tools  do not exhaust contaminants, oil and moisture onto the plate surface.
• The quality and cleanliness of the aluminum welding wire - Wire must be clean and free of any residual oil used during the wire manufacturing process. If the quality of the welding wire is inferior, it may be virtually impossible to produce acceptable porosity levels.
• Anti-Spatter Compounds – This type of material, applied to the welding nozzle or to the plate surface is not usually recommended for aluminum welding when low levels of porosity are desired.

Moisture

Through a number of sources, Moisture (H₂O), which contains hydrogen, can also be a source of contamination to the welding area. Following are some examples:

• Water leaks - This occurs within the welding equipment, if using a water-cooled welding system. 
• Inadequately pure shielding gas - Shielding gas should meet the minimum purity requirements specified by the appropriate welding code or standard.  Additionally, contamination of shielding gas may also occur from imperfections within the gas delivery line such as leaking pipes or hoses.
• Condensation on plate - High humidity and change in temperature (crossing a dew point) can result in condensation of the plate. When welding in high humidity, it is relatively easy to acquire moisture from rather small fluctuation in temperature.
• Hydrated aluminum oxide – This is another source of moisture and porosity. Aluminum has a protective oxide layer that is relatively thin and naturally forms on any exposed surface.  Properly stored aluminum, with an uncontaminated thin oxide layer, can be easily welded with the inert gas (GMAW and GTAW) processes, which break down and remove the oxide during welding.  When the aluminum oxide is exposed to moisture, potential problems with porosity arise. 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.  This problem can occur on both base material and on welding wire.

Material Preparation Concerns

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.  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.

Cleaning prior to welding

To achieve low porosity levels for x-ray quality welds, it is 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 significantly contribute to porosity problems.

When designing welding procedures intended to produce low levels of porosity, it is important to incorporate degreasing and oxide removal.  Typically, you can achieve this 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.

Conclusion

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.  You must approach a porosity problem 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 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, are all very important variables if low porosity levels are desirable.

Fig 1.  Porosity seen here in a cross-section of a fillet weld in aluminum