Hi Zeek!

I copied a pasted this from Alcotec, a division of ESAB and this is based on recommendations from Tony Anderson who is one of the leading authorities on Aluminum welding fabrication best practices... Although no particular type of oxide powder is recommended such as Aluminum, Zirconium oxide or silicon carbide there is some very helpful information and it seems to point out that using a stainless steel wire brush, or wire brush wheel as being the preferred method... Anywho, here's what Alcotec recommends:

GRINDING:

· Wheel grinding is not recommended since it smears the surface of aluminum and can deposit organic binders from the wheel during grinding.

· Disc grinding can be used with grit size, 30 to 50 preferred, and speeds of 4,000 to 6,000 rpm's. Only flexible discs should be used and grinding pressures should be moderate to prevent surface heating or smearing of the aluminum. Lubricants or coolants must not be used.

"BASE METAL CLEANING

Moisture:

· Minute traces of moisture on aluminum can produce severe weld porosity. Both the welding filler metal and the base metal should be brought into the welding area 24 hours in advance to allow all material temperatures to equalize. A dew point test should be done prior to welding (See discussion on "Atmospheric Conditions Affect Weld Quality"). If preheating must be used, heat no higher than 1500F and remember that oxy-fuel flames produce water as a byproduct of combustion.

Lubricants:

· Before oxides can be removed from aluminum, the metal must be degreased. This is best done with a solvent. Toluene is the best general solvent for this purpose. Acetone is a poor solvent for oils and greases and is less effective than toluene. Chlorinated solvents are also good degreasers but are not recommended for this application because they present environmental problems and their vapors can decompose into toxic or poisonous gases in the presence of heat. Weld joints should be washed with solvent prior to assembly and wiped dry using clean cloth such as cheesecloth. Shop rags should not be used since they contain soaps and other organic compounds from the washing and conditioning processes used to treat them. Do not use compressed air to blow off or to dry solvent cleaned areas since it often contains moisture and oil.

Oxides:

· Wire Brushing:

Oxide removal must be done after degreasing and is best done with a stainless steel wire brush. Wire brushes must be frequently cleaned with the same solvent as the base metal. Wire brushing can be done by hand or with a power brush. If power is used, keep rpm’s and pressures low to avoid heating and smearing the surface metal. Compressed-air power brushes should exhaust their air to the rear, not forward towards the brush where the compressed air can contaminate the base metal

· Chemical Cleaning:

Chemical cleaning deoxidizes and etches the aluminum. These cleaners contain acids and can present problems in handling and disposal. If they are used, the base metal must be thoroughly rinsed and dried and should be milled or wire brushed prior to welding.

· Etch Cleaning:

This process uses a hot sodium hydroxide etch and nitric acid rinse. It effectively removes heavy oxides, rough machined, sawed, or smeared surfaces, and hydrocarbons. However, the process leaves a porous surface containing hydrated oxides that absorb moisture during storage faster than an as-fabricated mill surface. This surface should be milled or wire brushed prior to welding."

Here's the link to this page online:

http://www.alcotec.com/us/en/solutions/Aluminum-Storage-and-Preparations-for-Welding.cfm

This article covers atmospheric conditions which affect weld quality:

http://www.alcotec.com/us/en/education/knowledge/qa/Atmosperic-Conditions-Affect-Weld-Quality.cfm

Here is another article which IMHO, is a MUST READ for you and if I remember correctly, I have already posted this article from Alcotec for you in an earlier thread sometime last year,.. But then again, I could be mistaken and instead posted this information for someone else instead which really isn't important as for you to read this article which really compliments the first one I posted here in this reply...

So please read this because I strongly believe that this will help you along with using a similar procedure as recommended in the attached article which Lawrence posted earlier in this thread regarding the type of filler wire to use, and to even go to the extent of "peeling' the oxide surface layer via a die which is surrounded by an inert atmosphere so that once the oxide layer is "peeled" away there is on chance of another surface oxide layer replacing it afterwards and shielding the filler wire throughout it's journey through every last inch of travel from the spool all the way until prior to it entering the weld pool, because then the inert shielding gas form the GTAW torch will sufficiently protect the surface of the wire from being contaminated... Therefore ensuring that the filler wire is free of having a surface layer of aluminum oxide!!!

If you also take into consideration of the moisture and dew point concerns as well as making sure that the base metal surface is free from any residual hydrocarbons, and also ensuring that the same type of residues and moisture concerns are not present in any point within the filler wire feeding delivery system, then the chances of producing almost no micro-pores at all within the weld will be increased substantially!!! :) :) :) Especially in Ultra High Vacuum Applications (UHV).

Once again before I distract you from this article any further, please consider what is being emphasized here:

'Problems with porosity in groove welds

COMMON ALUMINUM PROBLEMS - QUESTIONS AND ANSWERS

QUESTION: I am having problems with porosity in my aluminum groove welds. I am currently unable to pass radiographic inspection. I am failing this inspection because of excessive porosity in the weld. We are using the GMAW process and welding ½ inch thick 5083 base material with an ER5183 filler alloy and pure argon shielding gas. How can I correct this porosity problem and improve the x-ray quality of my welds.

ANSWER: 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 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 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 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 (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." Here's the link to this article:

http://www.alcotec.com/us/en/education/knowledge/qa/Problems-with-porosity-in-groove-welds.cfm

Here's one last article for you to read which may be helpful also... Here's the title & link:
Smoke and Twist Testing of Aluminum Welding Wire... Smoke Testing Aluminum Welding Wire for Surface Contamination:

http://www.alcotec.com/us/en/education/knowledge/qa/Smoke-and-Twist-Testing-of-Aluminum-Welding-Wire.cfm

Finally, here's an interesting article from the fabricator and if you would, please read this part where the author writes this:

"When choosing a wheel for an application, also consider the hardness of the material you'll be grinding.
To understand this, let's look at how abrasive grains and bonding agents work on a wheel. During grinding, dulled grains in the abrasive wheel create friction and heat that melt the resin bond that holds the abrasive. This releases the dulled grain and exposes a new, sharp one.

A soft-grade abrasive is designed to release new grains more quickly than a hard-grade abrasive, which means that a soft grade will remove the maximum amount of stock while creating the least amount of heat in the base material.

Conversely, a hard-grade abrasive sheds its dulled grains more slowly, so it removes less stock. The rubbing of the dulled grains against the base material increases the amount of heat in the base material.

The softness of the steel makes the stock easier to remove with less heat buildup. Soft steel is less resistant to the abrasive and, as a result, doesn't cause as much friction.

A hard abrasive wheel lasts longer than a soft one, so using the hardest-grade abrasive contributes to longer wheel life. Therefore, when grinding soft steels, harder abrasives can be used effectively.

A zirconia alumina abrasive wheel works somewhat differently. Instead of being released by friction, the grain in a zirconia alumina wheel actually fractures to expose sharp, new facets that continue to cut. The stock removal rate of these wheels is similar to a soft-grade abrasive while offering the extended life of a hard grade.

In addition to grain type, match the grain size, or grit, to the work. Use a large grain for soft material such as mild steel and a small grain for harder material. Use large grain for more aggressive stock removal and a small grain for hard metals to avoid loading the grit, or clogging the grinding wheel's grit with the metal being ground."

Anywho here's the link to the article from the Fabricator:

http://www.thefabricator.com/article/cuttingweldprep/selecting-the-right-abrasives-for-your-operation

Well, that's about all I have for now so I hope this will be helpful for you and anyone else interested also. ;)

Respectfully,
Henry