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

[tc7]

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.

 

[StoneCold]

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

 

[Tmoose]

Modern TIG welders offer AC balance to change the cleaning VS penetration trade-off.

http://www.millerwelds.com/resources/articles/index.php?page=articles82.html

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

 

[vanci]

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.

 

[ornerynorsk]

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.

 

[tc7]

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.

 

[gtaw]

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

 

[vanci]

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.

 

[tc7]

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.

 

[ornerynorsk]

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.

 

[gtaw]

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

 

[OGMetEngr]

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/

 

[gtaw]

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

 

[pandithan]

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