Let’s first define the basic cause of weld porosity in aluminum and then go on to summarize the major causes. All porosity in aluminum welds is caused by hydrogen in the weld. Though solid aluminum has a very low solubility for hydrogen, molten aluminum has a very high solubility for hydrogen. While the weld metal is in the molten state, it absorbs a lot of hydrogen. It will then try to expel the hydrogen as the weld metal solidifies. If the weld is solidifying even moderately quickly, it can’t rid itself of the hydrogen, which stays behind and forms porosity. So, where exactly does this hydrogen come from? After all, there is only a very small amount of hydrogen in the air. The hydrogen comes from one of two possible sources – either water vapor or hydrocarbon contamination (i.e., oils, greases, lubricants, and solvents). If any of these substances gets into the arc, the heat from the arc breaks them down into hydrogen and other substances. There’s the cause of your porosity. Minimizing porosity is mostly about cleanliness and housekeeping. There are actually many possible causes of porosity. Following are the top four culprits and how to prevent them from happening. Poor Preweld Cleaning. Be sure to degrease the part before you weld. Wipe down the surface of the material with a clean, lint-free rag saturated with a good degreasing solvent. Acetone and toluene are among the best. Alcohols are not good degreasers and should not be used for preweld cleaning. Following degreasing, the aluminum surface is then deoxidized, usually by using a stainless steel wire brush. Mistakes here are the most common cause of porosity. Insufficient Shielding Gas Flow Rate. For gas metal arc welding (GMAW), the flow rate should be at least 35 SCFH, although flow rates up to 60 SCFH can be used, especially with big gas nozzles. Argon/helium gas mixtures require even higher flow rates. For gas tungsten arc welding (GTAW), the flow rate should be at least 25 SCFH. Incorrect Nozzle-to-Work Distance. Simply put, this distance should be as small as possible, preferably in the range of 1/2 to 5/8 in. Longer nozzle-to-work distances can suck in surrounding air and other contaminants. Improper Gun Angle. Aluminum welding always requires a leading gun angle. If you try to use a drag angle or even a 90-degree angle, you will get porosity in your welds.