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?
Then I bought a spool of .035″ diameter 5356 wire to butt weld 3/8″ 6063 aluminum strips with spray mode (24.5 volts, 700 ips, 100% argon, 40 cfh) and was surprised to see that the black residue level was maybe 10 times higher than with the 4340 wire. I ground the welds flat, then repaired a few holes with Tig welding and found a huge amount of the black soot inside the weld bead. What was causing it? What could it be? The tig welds were ending up with black powder on top. When I went over the whole weld, the black powder seemed to be everywhere inside the weld.
I even planned an experiment welding aluminum inside of a transparent tent filled with argon to see if atmospheric carbon dioxide was the source of the black powder.
You can see examples of aluminum mig weld beads above on the right, all done on 1/4″ thick 5052 alloy. All beads are done with a recessed tip, 700 ips wire feed speed. The top row shows beads of .035″ diameter 5356 wire, voltages ranging from spray mode 25 volts (left) to 19 volts (right) short circuit mode. The bottom row shows beads run with .035 4340 wire with voltages ranging from 19 volts (left) to 25 volts (right). You can easily see the black powdery residue surrounding the weld bead and how much worse it is with the 5356 wire.
Searching Google leads me to this Miller Forum that discusses the black residue but doesn’t say what it is or why it is there. They say that it’s normal, just wipe it off and keep going.
Miller Welding Aluminum Tips has some good welding tips for mig welding aluminum but doesn’t address the black powder residue.
I called up Ed Craig, the expert at WeldReality.com and asked him about the black powdery residue. His site has a wealth of information on aluminums and welding aluminums and states that the black residue is comprised of oxides of aluminum and magnesium: however, I have checked, there are no black aluminum and magnesium compounds! Some aluminum and magnesium compounds are white or pale yellow, most are colorless. Lapis lazuli (deep blue) is the only exception. The oxide coatings on aluminum and magnesium are all colorless.
Ed Craig kindly answered the phone and took the time to respond to my questions. He said not to use spray transfer mode: my welding technique was not breaking the oxide layer. I should reduce the arc length by reducing the voltage to get crackle without spatter. Also, increase argon flow rate to 50 cfh.
I wish I could tell you that he solved my problem but it was just more of the same, plus invested some time, metal & argon. You can see the results in the image to the right above. Note that the non-spray mode welds (all below 24 volts) are all cleaner. But I doubt it has anything to do with breaking the aluminum oxide layer.
The Fabricator also says the black powder is magnesium oxide! Apparently this myth has been passed from welder to welder for a long time.
Mig Welding Aluminum Problem – Black Powder Residue Identified
I have a chemistry degree from 1979. Let’s look through the chemical composition of these two metals and try and solve this mig welding aluminum problem.
|5356||balance||4.5-5.5%||.5-1.1%||.25% max||.40% max||.1% max||.1% max||.05-.20%|
A quick search through Wikipedia finds that Manganese (Mn) is the culprit. Manganese dioxide (MnO2) is brown-black and Manganese (III) oxide (Mn2O3) is brown or black. The manganese level in the 5356 wire is 10-20 times higher than in the 4043 wire! Manganese is added to the wire for stiffness, but it is also an excellent oxygen scavenger. (Manganese is often added to steel for that reason alone, to bind up any residual oxygen and sequester it from the iron.)
Manganese oxide – do you want this in your welds? Probably not. Therefore, I would recommend to never use the spray mode with 5356 wire. It seems to be okay with the 4043 wire. Do not weld over the black residue – clean it off as best you can before proceeding.
What causes those white fumes when Mig Welding Aluminum?
That’s the zinc. You don’t want to breathe that. It will give you flu-like symptoms. You will also get white fumes when you braze with a zinc chloride flux, or try to tig weld galvanized steel. Invest in a quality air cleaner or welding fume extractor. Wear a breathing mask rated for welding. Be safe out there.