Forget the Oxygen!!!
Al is spot on when it comes to cleanliness... If there's any sort of hydrocarbon - even the grease from your fingers or from the workspace area... For example, any sort of spraying or vapors from something else going on close by that could become airborne and mix it up with the
humidity... And then eventually land on to the Aluminum as a thin layer of
moisture mixed with whatever else is in the air can be a source... Remember that the thin
layer of Aluminum oxide is porous as opposed to steel which is not!
Moisture can become entrapped within that
porous layer of Aluminum oxide which can grow to become thicker resulting in
porosity and inclusions of the oxides in the weld and moisture mixed with hydrocarbons = a witches brew when welding Aluminum...
So clean, clean, clean - then wire brush with a stainless steel brush that's dedicated to be used only for Aluminum in order to clean off the oxide layer that will still be there even after cleaning... This must be done or else all of the cleaning you do alone will not substantially improve your weld quality and eliminate the greater than usual porosity problem you're currently experiencing... This is from Alcotec: "Atmospheric Conditions Affect Weld Quality":
http://www.alcotec.com/us/en/education/knowledge/qa/Atmosperic-Conditions-Affect-Weld-Quality.cfm"What shielding gas should I use when welding aluminum?"
http://www.alcotec.com/us/en/education/knowledge/qa/What-shielding-gas-should-I-use-when-welding-aluminum.cfm"Storage and Preparation of Aluminum Base Alloys and Filler Alloys"
http://www.alcotec.com/us/en/education/knowledge/qa/Storage-and-Preparation-of-Aluminum-Base-Alloys-and-Filler-Alloys.cfmWelding 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...
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...Water leaks 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... Shielding gas may also become contaminated from imperfections within the gas delivery line such as leaking pipes or hoses...
Condensation on plate or wire from
high humidity and change in temperature
(crossing a dew point)... When welding in
high humidity, it is relatively easy to acquire
moisture from rather small fluctuation in temperature...
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 exposed 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 oxides 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 re-validation... 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, 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... I hope this helps Martin.
Respectfully,
Henry
We have been experiencing what we call summer porosity with our welding of aluminum extrusion. Our welding supply vendor is recommending adding oxygen to our Argon mixture instead of Helium (because Helium is in shortage). Has anyone heard of this, I have always been taught that the shielding gas is to protect from oxygen and nitrogen.
