Aluminum Workshop: Explaining the causes of black smoke

Q: I get a lot of black smoke on the surface of my aluminum welds. What is it, and how can I get rid of it?

A: I can tell you what the black smoke isn’t. It is not soot and it doesn’t contain carbon, as most people think. It is, in fact, a mixture of very fine particles of aluminum oxide and magnesium oxide (I am assuming you are using a 5XXX filler such as 5356).

The boiling points of both aluminum and magnesium are much lower than the temperatures encountered in a welding arc, so quite a lot of aluminum and magnesium evaporate into the arc and then cool and condense at the arc’s outer edges. If they do so within the inert shielding gas blanket, they are then incorporated into the weld. If they travel outside the inert shielding gas blanket, they immediately oxidize into aluminum or magnesium oxide and condense on the cooler weld surface.

The presence of large quantities of black smut indicates there is a problem with the shielding gas blanket. You will always get a small quantity of smut at the edges of the weld, if you’re using gas metal arc welding (GMAW) because you are actually transferring metal across the arc or if you are using a 5XXX filler alloy, which contains magnesium that oxidizes more readily. Always remember that the weld itself should appear bright and shiny. If it is black, you may need to check the following:

• Shielding gas flow rate. Use at least 20 SCFH for gas tungsten arc welding (GTAW) and 35 SCFH for GMAW.
• Torch angle. Always use a push angle; never drag the torch.
• The gas nozzle-to-work distance. Try to keep it between 0.5 and 0.75 in.
• Welding environment. If you can feel a breeze, it’s probably blowing the shielding gas away. Put up shields, turn off fans, and close doors.