Why is Aluminum Welding so Vulnerable to Porosity But Steel Welds Are Not ? 6

tc7;
Lincoln electric has a decent article on the How's of Welding Aluminum and pitfalls;


Also, background information related to filler metals and types of Al alloys for welding;


In addition, the porosity you are observing may actually be associated with cracks or dross. The 6061 alloys are considered weldable but precautions are required especially with technique.

Regarding welding of steels, you don't have the issues with aluminum oxide formation and subsequent dross to contend with as you do with aluminum.

Hello Met-
The Lincoln first article mentions "hydrated oxides" . . . where is that coming from? we take very special care to remove the oxide layer, which of course we can't see, but assume it is there. But 'hydrated oxides', this sounds like new bad news. So if we are cleaning the area to be welded immediately before welding, are hydrated oxides even possible?

tc7,

Metals like titanium and aluminum DESPARATELY want to form oxides during welding. Unless the welding is being conducted in a vacuum or inert gas environment, oxides will form. Water vapor in the air contributes to the hydrated oxides. Iron on the hand, only sort of wants to form oxides.

By saying, ". . .formation of oxides during welding. . . ", I imagine that material ahead of the weld pool, just beyond the shielding gas envelope is capable of forming oxides due to the high heat input and oxides thus formed will soon get injested into the weld? This must be the source of dross formation that MetEngr was mentioning. But I would think the area is also generally too hot for hydration to occur. I don't believe that oxides, by themselves, will contribute to porosity.

I have heard and read elsewhere that high welding heat input will draw Hy into the soluable puddle and subsurface porosity will result. . . if true, what is this source of the Hy?

Thanks

Aluminum oxidizes very rapidly and the oxide "skin" on the surface is relatively strong and continuous, so it does not break-up into microscopic pieces but rather large flakes. The flakes will behave like cracks. This oxide skin is what makes aluminum fairly corrosion resistant even though the metal is highly reactive.

Iron also forms hydroxides.
The orange stuff that first appears on bright steel when left out in the rain is iron oxide hydroxide (I think that's what its called). It is a transition compound; the Fe will be further reduced to form one of several oxide types. Iron would form oxides during welding, but silicon and manganese additions protect it. Its all about thermodynamics and a guy named Ellingham.

The green stuff that forms on copper roofs after many years is copper hydroxide, a.k.a. malachite (also a mineral). There is another copper hydroxide called azurite, which is turquoise in colour. Both are well known to rock collectors. Stuff I learned to spot in the course of many failure investigations of brass cooling water tubes.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

We have done lot on AA6061-T6 with GTAW but we have not faced porosity problem because thorough cleaning of base metal, filler metal, no waiting time for starting of welding, humidification place (at 45), position of angle are very important to avoid porosity because solubility of hydrogen is more in high temperature and less at room temp. weld with high current sothat slow cooling will reduce porosity
Pandithan

Improper storage of filler metal, and exposer to high humidity, improper shaving of filler metal after drawing(In Manufacturing) may lead to formation of Hydrated Aluminium oxide layer, which is porous in turn will absorbs the moisture. Check Shielding moisture content also
Pandithan

Hello Pandithan-
Thankyou for your input.
What is your method of "thorough clenaing" ? are you dipping material into an acide bath or vapor degreaseer or steam cleaning prior to welding operations?

Our precleaning consists of wiping down the joint with acetone, filing the joint face and SS wire brushing followed by a final swipe with isopropyl alcohol. When grinding is necessary, only a carbide burr bit is used. We wipe done the tig rod with acetone followed by alcohol swipe. Our welding rod is very high quality and almost polished in appearance. Our gas is certified UHP and very dry - I tested it one bottle last week with a moisture analyser and it was indeed dry, measuring 2ppm. Don't know what else could be done (with any practicality) to eliminate Hy from the equation.

That's what I would call overkill on the cleaning, never seen that done so rigourously before.
I suppose all cleaning agents are OK for use in this application (?)

Oxide layer forms within a few hours, so I also suppose cleaning is done immediately before welding (including SS wire brushing and all prep, immediately before welding)?

If so, is your base material sound (moulded Al. often still contains moisture in some form)?
Filler is also OK for use with your base material(s)?

Your weldor's gloves are clean/dry as well?

One thing to watch out for is the relative humidity in the room when using acetone or alchohol. The fast evaporation can cause condensation on the surface of the part, which is the last thing you want. Better shielding and use of backup gas will give you improved results.
If that fails go to oxy gas torch welding with flux.
B.E.

King-
"overkill" is what the management is telling me also, but we have to meet a very tough aerospace x-ray standard (AWS D17.1 class A). If I knew where the Hy was creeping in from, maybe I could adjust. All materials are either wrought or extrusions.

Berkshire-
"fast evaporation causing condensation" - hadn't heard that one before. If RH is ~70% in a shop with ambiemnt temps ~75-80deg F, why/how does fast evap cause condensation?

and by "backing gas" are you talking about a trailing gas arrangement?

Thanks folks, I appreciate the interest.

The isopropanol may be mixed with water (70:30 is common). Acetone is better anyway, so I would replace the alcohol wipe with acetone.

Both acetone and isopropyl alcohol are water-hungry and quite low boiling. Surface evaporation of these materials might reduce surface temperature below the dewpoint, which may lead to water vapour from the air being drawn into the evaporating solvent, leaving a tiny amount of water on the surface- but unless you're really sloshing on the solvent, the amount would be truly tiny if the solvent itself was dry, since there isn't much time for water to condense.

If a container of alcohol wipes is left out, exposed to the atmosphere, I would imagine the wipes would draw in quite a bit of water vapour as they sit there if your RH is 70% or above.

Normal rubbing alcohol is, as mentioned, quite wet- over 30% water. If the wipes are intended for cleaning respirators or disinfecting small wounds etc. such that they contain wet isopropanol, they will be no good for use as a drying agent.

Aluminum is very reactive with oxygen but protected by its adherent, durable, rapidly forming oxide layer as others have mentioned, and this oxide's properties are the big problem for welding. The oxides do hydrate over time, but it's not instantaeous.

One thing that you didn't mention was your handling technique for hot filler metal. The natural tendency to pull filler metal out of the purge stream has to be resisted until the metal is cool, or else you need to cut off and discard the part of the filler metal that was hot- at least an inch of it- before using it again.

Titanium, zirconium and tantalum have the added challenge that they are reactive with both oxygen and nitrogen when hot, which aluminum doesn't suffer from. Both the oxides and nitrides lead to embrittlement.

Molten Metal just hit on the head as to why water beads on the surface. From a practical point of view, the first wipe tends to flash off clean, the problem comes when the operator thinks its not quite clean enough, and wipes again on an already chilled surface. especially at 70% RH
B.E.

And by backing gas I was talking about trailing gas, and a gas fed back up bar behind the part, or gas fed into a closed or partially closed compartment.
B.E.

Check the purity of your shielding gas.

Ron-
We pay an exorbitant price for UHP gasses with <1ppm allowable moisture content and in fact did check a couple of new bottles two weeks ago with a digital moisture analyzer. They indicated moisture in the 2 - 2.7ppm range which is higher than what the certification papers said, but still super dry. For the moment, my gas quality is not under suspicion.

Comment back to moltenmetal –
your point about dipping the rod in and out of the shielding is well taken and we instruct everyone to keep the rod IN the shielding stream and we have a 4-second postflow standard. Admittedly it is difficult for a production guy to maintain this awareness especially after fatigue sets in, which is quick, especially when you’re constantly bombarded by the high freq noise of these inverters.


Is anyone who works to aerospace standards performing any type of acid washing, vapor degreasing or steam cleaning prior to delivery to the weld operations ??

In the past I have used acetone for the initial cleaning to remove heavy grime and hydrocarbons. After the initial precleaning, we flushed the surfaces with 90% isopropyl alcohol. That was followed by wire brushing the weld groove and adjacent surfaces with an austenitic stainless steel brush. When abrading the surfaces with the brush, push in one direction and then pick the brush up on the return stroke to prevent pushing oxide into the soft aluminum. Draw file saw cut and sheared edges to remove the crevices that tend to retain oxides and cutting fluids. Use a vixen file to prevent "loading" the file teeth.

We used to soak all tools used to clean or prepare the aluminum in isopropyl alcohol to ensure they were free of all hydrocarbons.

After wire brushing or draw filing the surfaces, we flushed the areas with isopropyl alcohol once again to wash off any debris.

The welder's gloves must be clean. We found nylon gloves work best and non-cellulous wipes are best. Cellulous is a source of hydrogen if introduced into the welding arc. It is best to eliminate any potential sources of cellulous or hydrocarbons.
Make sure the welders are wearing clean gloves when cleaning or handling the cleaned aluminum or tools.

We discovered one unexpected source of contamination. The welders had a habit of rubbing their ungloved finger along the edge of the draw filed surface to make sure there were no burrs, etc. In doing so, they were recontaminating the surface.

Hydrogen is the most likely culprit when the porosity is silver in color.



Best regards - Al