March 11, 2008
Q: I'm having problems with excessive porosity when TIG welding aluminum. My friend says it comes from water vapor in the air (it's very humid here) being broken down by the arc. Is this true? It sounds pretty far-fetched.
A: I hope you didn't bet on this one. If you did, get ready to pay up. All porosity in aluminum welds is caused by hydrogen. Liquid aluminum can dissolve a lot of hydrogen, but solid aluminum can dissolve almost no hydrogen. So a molten aluminum weld sucks up any hydrogen around. As it solidifies, the weld tries to get rid of the hydrogen. If it can, everything is OK. If it can't, you get porosity.
So where does the hydrogen come from? Basically, there are two sources. The first is any sort of hydrocarbon, like oils, greases, and solvents. The second is water vapor. The temperature in a welding arc is more than 10,000 degrees F, hot enough to break down these substances into hydrogen plus something else.
So how do you minimize porosity in aluminum welds? First, clean the aluminum with a good degreaser, and make sure the solvent has completely evaporated before you weld. Check that there are no water or air leaks in the torch or cables. Close all nearby windows and doors so there are no breezes (even a slight breeze can deteriorate the shielding gas coverage and suck moist air into the arc). If you brought your base metal in from outside, make sure there is no water condensation on it. You need to look at lots of other things too, but they all fall into the housekeeping category. If you take care of these during weld preparation, your job will be a lot easier.
As for welding in very humid conditions, I must admit that high humidity does make it more difficult to make porosity-free welds. However, this is usually because your housekeeping has gotten sloppy. If you return to basics and complete the necessary weld preparation, you should be able to make porosity-free welds even at very high levels of humidity.
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