Aluminum weld porosity

[DReimer]

We have been having some problems with porosity in some of our aluminum welds. One of our engineers has done extensive research into causes and has implemented some procedural changes which have helped. We still seem to get some porosity, however, no matter what we do. In some cases we'll get perfect welds and porous welds using the same settings, prep and material.

I'm not asking for suggestions on how to cure the porosity (although I'll take them!) as we've already investigated just about every variable. But rather, does anyone here have a specification or inspection standard by which an acceptable level of porosity is identified? (The application is industrial machinery, and the welds are not safety critical.)

I want to develop a quality standard that will allow porosity to some extent, but I need a metric to determine a pass or fail cutoff. Pores per linear inch? Per square inch? Any ideas?

Thanks,
Dean

 

[tbuelna]

DReimer,

I would suggest that you review the available MIL-STD's for non-destructive inspection. Typically, the acceptance criteria for a particular process (dye penetrant, radiographic, mag particle, etc.) are dependent upon the criticality of the end use.

Regards,
Terry

 

[Timelord]

DReimer,

This is a long shot, but I'ver got to ask. What kind of cover gas are you using during the welding process? Some weld shops use gas mixes that can lead to porosity in the welds because they are cheaper than argon. Aluminum TIG really needs argon or maybe a special mix of argon & helium. Talk to your welding supply dealer and see what he recommends for the alloys you are welding.

Timelord

 

[boo1]

Typical Al porosity is caused by contamination. Look at your process and identify the source. The surfaces must be clean. Wire brush (Stainless steel) the surface oxides and solvent clean. The rod must be stored correctly to prevent corrosion. As Timelord indicated the shield gas must be adaquate.


MIL-STD-2035A (SH) 15 MAY 1995 NONDESTRUCTIVE TESTING ACCEPTANCE CRITERIA

http://assist2.daps.dla.mil/quicksearch/

 

[DReimer]

Thanks for the suggestions.

Timelord: we use a helium/argon gas mix, and have tried different mix ratios and flow rates to no effect. Our aluminum welding is typically MIG.

boo1: oxides came up in the investigation. We changed our weld preps and added additional grinding prior to welding. This has improved the problem but we still get some. This leads us to conclude that it isn't the sole problem.

We have two different MIG welders that we've used, and we see porosity on both. Early in our investigation we found that our wire-feed gun had some issues. We sent it for repair and in the interim we had a loaner gun that showed a huge improvement in porosity (unfortunately the design of the gun wasn't well suited to the welding we are doint). Our original gun didn't take well to repair so we had it replaced, and for two weeks had no porosity problems. Now the porosity has come back.

Like I said, we've exhausted just about every avenue. It is likely something to do with the gun, but until we can isolate a problem we will live with the porosity. I just need a cutoff level above which we can reject welds, and a simple, repeatable way to measure such a cutoff level.

Thanks,
Dean

 

[strokersix]

I recall something about MIG guns requiring a teflon liner when welding aluminum. Also, I have a felt doohickey (lubricator/wire cleaner?) near the driving rolls on my machine. If you have this on yours perhaps it's contaminating the wire and gun inside?

A couple more long shots.

 

[unclesyd]

Al porosity is mainly from H2 contamination. The H2 is soluble in liquid Al and essentially 0 solubility in solid Al. The rub comes when you trying to find the source of the H2. One of the more common culprits is humidity in the air or wet gas (rare) or a very small leak in a water cooled torch. In high humidity environments one has to watch the temperatures of base metal and electrodes.

Al oxidizes no matter what you do so it needs to be brushed off with a SS brush, but don't get too far ahead.

An old trick with wire is to wipe off a piece of electrode with a fine denier white cloth it will be black. Expose it to a UV light if it glows it's grease, no glow Al oxide/hydroxide. More than likely a predominance of the hydroxide.

Relative clean Al that is giving porosity problems and obvious moisture has been eliminated might be helped by slowing down the welding process to allow the H2 to escape the melt pool.

The AWS, ASME, Aluminum Association, and others all have standards for porosity in Al welds. A lot more is allowed in most cases than you would think.

Anecdotal:
We have a large number of large Al storage tanks and silos that were site-built in the 60's. The Al storage tanks like to ate our lunch as 2 of three were built in the summer with our high humidity. We actually put up a windsock for the welders to watch. This was in the days when bad welds were chipped out with V chisel and air hammer. The consensus of opinion at the time was that a slight breeze was contaminating the He shielding gas. Several years later when we learned of the H2 problem potential we actually ran break test where we contaminated the He with few ppmv of H2. The results were dramatic above 20 ppmv.

 

[CudjoeCodger]

As you know, be sure no one is contaminating the wire brush between uses on the Al.

 

[metengr]

I concur with unclesyd, here is some additional information regarding porosity in Al welds;

http://www.esabinternational.com/kcentre/templates/T131.asp?item_id=98489

 

[ornerynorsk]

clean, clean, clean!!! Just as the other posts indicated, you cannot get aluminum clean enough prior to welding. Grinding prior to welding requires a lot of cleaning afterwards, because you've got microscopic particulate, which will contaminate badly. You're better off using a machining process for bevel/prep if you can. It cleans up much easier.

 

[rorschach]

There was one sentence that the original poster made that sticks in my mind. the torch head was replaced and for a short period of time there was no porosity. This makes me suspect that there is something about the gun or the wire feeder that is contaminating the welding wire. lubricant on the wire? dirty drive rollers? anti-spatter spray that got inside the torch head? Clearly something seemingly inconsequencial got overlooked. Have you watched any of these welds prepped/made? Is there something the welder or fitter is doing you haven't paid attention to before that might be relavent?

 

[mshimko]

Can I ask the application? Is it repairs to Navy surface ships with aluminum superstructures?

 

[DReimer]

mshimko: application is aluminum frames for packaging machinery. Nothing so exotic as ships or Ferraris.

Everyone: thanks for all the suggestions. I think we might focus on gun. I had a look before going home on Friday and there was definitely some spatter in the gun. I don't know if it is the culprit (doesn't seem enough to impede gas flow) but we'll try giving it a clean.

Dean

 

[aamoroso]

Often overlooked (although mentioned here, its worth metioning again):

Definitely control the wire brushes, they should be stainless and only used on clean aluminum parts. Wipe the parts prior to brushing to ensure the oil on the parts isn't getting onto the brushes. Always wipe the surfaces prior to brushing. Miller has supplied us with a nice video on aluminum welding when we purchased our Al Mig from them, I am sure they would make one available to you if you ask nicely.

Turn off any forced air fans around the welder, a fan blowing on the welder will cause contaminated atmosphere around your weld which will result in porosity problems.

Clean welds, clean bushes, clean gasses...

Fill what's empty. Empty what's full. And scratch where it itches.

 

[unclesyd]

Welding Al shapes for frames can also be tricky due to eddies or vortices generated by the gas flow from the torch. This is especially troublesome in high humidity conditions, either natural or generated. If you have stray air currents just try a little test, a 2' dia. tube to flat plate with the tube in the vertical.

Not taking anything away the cleanliness rules a little bit of water vapor goes a long way in formation of porosity.
The Al welding wire has to kept above the dewpoint, i.e. you can't bring a roll or tube of wire from an air-conditioned area and start welding in non-conditioned area. The Al wire needs to be at ambient conditions of the weld site. It should be brought up to temperature while sealed/covered. Our Al wire was stored at around 110°F in a storage cabinet in an air condition fab shop, so if used in the shop or taken to the field it would be above the dew point.

One big no no is to machine Al with kerosene/lard as the lubricant if it's going to welded on. If the you can see a rainbow sheen on the Al after machining guess what

 

[dkelly]

DReimer,
I know that you stated that your weld engineer looked at everything, but over the years I have seen so many complex problems turn out to be something so simple that you could cry. If you can make the problem disappear by changing a gun then that is where you need to spend 80% of your effort. Is there spatter build up in the nozzle? Do the welders use a corn oil based anti spatter in the nozzles? How many people have access to the gas mixture valves? What is the preventative maintenace like on the weld equipment. Good Luck and don't give up!
dkelly