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

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

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

(OP)
To beat posrosity in our aluminum welds (primarily GTAW), we take extraordinary measures to clean the plates, rod, strctly adhere to procedures, avoid welding in high humidity conditions (when possible), pay exhobitantly for UHP argon and helium, use mild preheat and clean, clean, clean, etc., etc.

As a general rule, for reasons not understood by anybody, we have bigger porosity issues on 6061-T6 and less so on 5086 or any 5xxx.

In contrast, when welding steel (by any process) we spend about 5% of the cleaning and preparation effort that we spend on aluminum and have NEVER seen a porosity issue. Ever.

So my question is a technical/scinetific inquiry and not a process question - "Why is Aluminum Welding so Vulnerable to Porosity But Steel Welds Are Not ? "

Thanks for any insight.

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

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

http://www.lincolnelectric.com/en-us/support/weldi...

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

http://www.lincolnelectric.com/en-us/support/weldi...

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.

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

(OP)
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?

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

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.

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

(OP)
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

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

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.

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

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?"

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

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

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

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

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

(OP)
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.

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

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?

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

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.

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

(OP)
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.

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

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

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

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.

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

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.

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

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.

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

Check the purity of your shielding gas.

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

(OP)
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 ??

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

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

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

I will mention this, because no one else has. When you weld aluminum if you run the high frequency unit all the time it helps clean the weld as you go along.

Regards
StoneCold

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

Modern TIG welders offer AC balance to change the cleaning VS penetration trade-off.
http://www.millerwelds.com/resources/articles/inde...

I am not sure if oxidation and the cracking tendencies of hydrogen on aluminum are related.

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

I agree your cleaning is sufficent however, I would consider trying denatured alcohol or use only acetone. Also, I see no info regarding joint design. Are we talking open root grooves, fillets, seal welds, in plate or pipe? What is the material thickness 1/16" or 1" or ...? This all plays a role as your welding technique could be contibuting to the porosity.

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

Any pre-heat? Thoriated tungsten? Are your stainless steel brushes embedding any material that the final alcohol wipe would not remove?

Just a couple of thoughts.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

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

(OP)
Vanci & Ornerynorsk-
typical test paltes are 3/8" thick with a 1/4" backing plate, welds are CJP. Bevels are 30-35 degrees (60-70 incuded angle) 0-1/16" root face (land). Tungstens used have been ceriated, lanth (1.5% and 2%) and zirconium. tried 250 deg preheat. We are now using single gut files and new ss brushes, aclcohol wipe and 10X visual on all bevels before welding. Haven't seen any significant changes yet.
Thanks.

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

Alcohol wipe? Exactly hat is an alcohol wipe?

If it is what I think it is, you can be leaving lint behind. The lint can be a serious source of hydrogen which can be a major cause of porosity.

I recommend flushing the joint and adjacent surfaces with a spray of alcohol. It has work nicely in the past.

Best regards - Al

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

I think we got off topic. To answer your initial question which I think has already been done is: Aluminum and some of its alloying elements are more reactive with the envirnoment/ atmosphere on a molecular or atomic level than steel especially when heated to melting temp.
As far as your specific application, have you tried using puling and gas lens with large cup? A high pulse rate will agitate the puddle which in turn may help bring some of the trapped gases to the surface. The gas lens will improve gas coverage. Also, possibly welding slower with less filler. In my mind you need keep the puddle molten longer to allow the gases to come to the surface before the puddle solidifies. Another option is using a 70% argon/ 30% helium blend, no idea if it will help, but its something else to try if your desperate.

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

(OP)
Thanks Al-
First of all I need to apologize to all for my sloppy posting on 20 May 13 - was in a hurry and typed that out in 30-seconds without proofreading. If I could have edited it later that night when I saw what was written, I certainly would have.
Secondly no, the ‘wipes’ are not what your’re thinking, again it was my hasty jargon that may have confused. We’re only using scotch-brites and lint free cloth’s when we wipe down the areas with alcohol and acetone.

Thanks Vanci-
Have tried all the variations you suggested ( pulse, 70/30, moving slow, moving fast, big rod dia., small rod dia., etc.)
I don’t we think we really addressed the original question, I was hoping for someone to speak on the why and how of solubility of hydrogen in aluminum as compared to steels.

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

There is an article on this very subject in Metallurgical and Materials Transactions A
September 1989, Volume 20, Issue 9, pp 1785-1791.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

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

Porosity typically forms at the liquid-solid interface at the trailing edge of the molten weld pool. Gas is typically highly soluble in liquid metal. As the liquid solidifies, the solubility decreases by a couple of orders of magnitudes. The "bubble" forms at the liquid/solid interface and rises to the surface where it escapes into the environment. The gas that fails to escape freezes within the metal where it is identified as porosity, i.e., a gas filled void within the solidified metal.

Hydrogen goes into solution easily with aluminum because pure aluminum is face centered cubic. To say it another way, the FCC is an empty box that easily accommodates the monatomic hydrogen atom. Iron is body centered cubic at room temperature, so it does not accommodate the hydrogen as easily.

Aluminum and hydrogen can form various hydroxides since both are chemically active.

There is a lot more to the subject. Rather than providing you with a welder’s suppositions, it might be better to steer you in the right direction regarding the solubility of hydrogen in metals. I did a quick Internet search and found several research papers on the subject. You would be better served to read papers on the subject written by the experts that are researching subjects related to your inquiry.

Best regards - Al

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

This site has some good information on hydrogen in materials. Has information on steels and aluminum including solubility, permeability, etc.


Technical Reference for Hydrogen Compatibility of Materials:

http://www.sandia.gov/matlsTechRef/

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

Scotch-Bright: What is the abrasive composed of, aluminum oxide perhaps?

It may have nothing to do with porosity, but it may introduce aluminum oxide into the area to be welded.

Rather than "lint free", you might want to try flushing the area with a spray. Any cellulous material or synthetic material is going to provide a potential source of hydrogen once the material is broken down by the welding arc.

Best regards - Al

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

Try with Half bead technique to eliminate porosity.Use water cooled torch to avoid temp raise in torch. AC with square wave power source.
Pandithan

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