Weld porosity is the presence of tiny cavities in a piece of welded metal. Porosity can cause all kinds of problems in your welds, so you want to fix the causes of your porosity before they affect your project. Porosity comes in two varieties. Surface porosity, as you'd guess, occurs on the surface of the metal. You can detect surface porosity with the naked eye. The other kind of porosity is subsurface porosity. It occurs within the metal, and you can find it with internal detecting machines only.
Here are a few of the most common causes of weld porosity and solutions you can use to limit porosity in your welds.
Cause: Impurities on the surface of your metal.
Solution: Clean your metal thoroughly before you weld it. You need to make sure it's free of impurities like water, oil, and flux. If you're welding aluminum, you have to make sure you clean off the outside layer of oxide before welding, or it will cause serious porosity.
Cause: Excess shielding gas.
Solution: Use the correct amount of shielding gas. You can usually find that information in the instruction manual for your welding machine or in the literature that comes with your electrodes or electrode wire.
Cause: Damp electrodes.
Solution: Store your electrodes in a clean, dry place. Don't allow them to be exposed to the air (or to liquid water, of course) before you use them.
Cause: Backing bar material that doesn't match the metal you're welding.
Solution: If you're using a backing bar, make completely sure that it's made of the same metal you're welding.
