Mig Welding Aluminum Problem – Black Powder Residue
I had a struggle recently trying to solve a mig welding aluminum problem. I do a lot of aluminum mig welding, primarily with 4043 wire and always wonder, what is that black powdery residue on the edges of the weld? Was it soot? If so, where would the carbon be coming from? Was the argon sucking in carbon dioxide from the atmosphere (venturi principle) and contaminating the weld?
It’s been a very busy month in the sculpture shop. I am only working on one project, a slightly smaller version of my Pas de Deux sculpture design. I am trying to keep careful account of the time required so that I can more accurately price these sculptures. Looks like it’s going to end up being about 80 hours. It is a pleasure to work on only one project. I am naturally a high-focus person and, when I am trying to do too many projects at once, I can feel like the Linda Blair character in the first Exorcist movie (1973), with my head spinning around. Multi-tasking can be very exhausting but sometimes it’s the only choice, running several tests and procedures, in the sculpture shop, one after another.
The path to success is paved with many failures: here’s the story of one such learning experience using a crack detector kit.
In July, I began working on a 84″ tall double strip mobius. I thought it would be an easy quick project, but I was still working on it in September. The sculpture ended up being 10′ tall.
The concept of the mobius is that it is a strip with a 180 degree twist in its length prior to the ends being joined. This creates a strip with only one side, versus a normal loop that has two sides. Here’s a Wikipedia article explaining the mobius concept more graphically.
With a 30″ high double strip mobius, I can take one continuous 96″ slit strip of 10 gauge metal and shape it with the roller and the body bender (me and my muscles working with a vise).
With a 120″ high double strip mobius, I had to break the strips down into six 96″ lengths of 1/4″ aluminum, shape each piece, then line it up and butt weld to its ends and adjoining strips. It sounds easy and it seemed easy too, until I started trying to get the end loops to meet up. I could do it fine as a non-mobius (Nobius) but the mobius design requires a 180 degree flip. I wasn’t talking directly with the client – my wife was – so I assumed the client was a math-freak like me and wanted the mobius concept. Later I was to learn that he had no idea what the word mobius meant. Live and learn.
Then the strips had to be butt welded and/or side welded. Most people run a bead and leave it at that, but I wanted the ribbons to look seamless, beadless and non-welded, so after running a bead, I ground the bead flat. The bead would look terrific, then I would grind it flat and pinholes, small cracks and voids would appear where the two pieces of metal met.
Porosity is a major bugaboo of welding. Porosity is identified as tiny pits and voids in a weld. Porosity weakens the welded area, kind of like the perforations on stamps.
Common causes of porosity are moisture or oil on the surface of the metal, hydrogen dissolved in the hot puddle or oxygen (from oxide) on the surface of the metal, dirty mig wire and similar. In my case, since the porosity was at the bottom of the weld, it was probably caused by oxygen, since I had cleaned the weld with a flap wheel and acetone just prior to welding. Where did the oxygen come from? Aluminum reacts very quickly with oxygen, especially when it is clean and bright, so it must have come from the newly formed oxide layer. Breaking up the aluminum oxide layer with a dedicated (for-aluminum-only) stainless steel brush seconds before doing the weld helps.
Of course, the flap wheel could have ground in some dirt, and the acetone-soaked cloth could have left some contamination. Sometimes it is difficult to track down the true cause of porosity.
To repair the porosity, I was enlarging the holes with a drill, cleaning with the stainless steel brush, and re-migging the hole, starting about 1/2″ away and running it in. It’s very hard to see what path you should be taking with the helmet on, so I drew red lines on each side of the problem area to keep me lined up. See the photo to the left.
Sculpture Crack Detector – What Could Possibly Go Wrong?
I’m an over-achiever/perfectionist with a fair level of obsessive compulsive disorder. I had bought some crack detector – Magnaflux’s SpotCheck– at my local welding supply house a number of years before, but never tried it. Since this sculpture would be going to a high-wind area, I wanted to remove as much porosity as possible, to minimize the chance of cracks forming later.
I pre-cleaned the areas with acetone, sprayed on the red dye as lightly as possible (hard to do – this is a super low-viscosity oil and comes out of the nozzle at high speed), then sprayed on the SpotCheck Developer after a minute or so. It certainly worked. However, it did not reveal any hidden cracks or voids that I had not already seen with the naked eye. To the right is a photo showing two aluminum ribbons both butt welded and side/butt welded with the red dye penetrant very visible.
So then I had to clean off the crack detector. I went through about twenty clean rags soaked in acetone. The crack detector dye was down in the voids and wouldn’t come out. I drilled out the voids, cleaned again with a clean cloth and acetone, scratched with the stainless steel brush, welded the holes, ground the welds flat – the voids were still there – maybe even worse than before!
It took about a week of this repeated effort to finally get the voids fixed. A week of wasted effort, but I learned a lesson – an expensive lesson. Don’t use Crack Detector!
Crack Detector – What I should have done
I should have used the Tig welder on DC- with a 2% lanthanated sharp tip. This works great – much better than the balled tip AC method for Tig welding aluminum. And much better than repairing the voids with Mig. The heat goes right to the void, melts it flat, easy to fill the puddle.
For butt welds in 1/4″ aluminum I’ve learned to clamp a 1/4″ aluminum plate to the back of the butt weld. Then start the weld on the backing plate and run it on to the butt weld, then off the other side. This avoid the cold cracking at the start of the bead and the crater problem at the end.
I also learned to set the Trim on the Mig welder to the lowest possible setting, with the wire feed speed on a very high setting. I did the Mig welding with a Lincoln Electric Power Mig 300 with the cobra push-pull gun and 4043 wire. The metal was all 5052 aluminum.
Of course, I should never have used the SpotCheck crack detector. You would have thought my welding supply house would have warned me. Guess no one bothered to tell them either.
I should have used my high pressure washer with a high-powered soap to clean it off. Unfortunately, when in a rush, the best ideas sometimes don’t occur to me right when I need them. That’s why I keep a notebook and write the ideas down. Hopefully I will remember immediately what I need to remember when the next opportunity comes up.
More dye showed up again when I lacquered the metal with acrylic. The acrylic sucked the dye right out all the places I hadn’t been able to find before! Embarrassing.
Crack Detector – What is it good for?
That’s it – I hope I’ve persuaded you to not make the same mistake with crack detector as I did!
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