Porosity in aluminium welds is caused by gas that becomes trapped in the weld pool when the metal freezes before all of the gas in the weld pool has a chance to escape.
The main cause of porosity is entrapment of gases such as air and shielding gases. Gases can be entrapped when turbulence occurs in the weld pool. When welding aluminium by the MIG process, turbulence can occur if too low a welding current is used because large droplets are transferred across the arc. However, excessive currents deposit metal over a gas bubble before it escapes, giving irregular shaped porosity. Hence, the welding current should be sufficiently high to stabilise the droplet transfer, whilst avoiding excessive currents. Erratic wire feeding can also cause turbulence. Erratic wire feeding may be caused by drive-roll slip, excessive bending of the guide liner, using the wrong size liner, kinks in the wire or poorly wound spools.
When using the TIG process porosity is most likely to be caused by contamination or loss of gas shielding.
The main cause for porosity in aluminium is hydrogen, which has very high solubility in molten aluminium but very low solubility in solid, where there is a decrease of solubility in the order of 20 times as solidification takes place. Hydrogen gas is therefore evolved as the weld pool solidifies. If the cooling rate is too high, the gas remains in the metal in the form of porosity. Thus, any compound containing hydrogen and contaminating the filler wire or work surface can cause porosity.
Oil, moisture or other contaminants may be present on the filler wire. In addition, the oxide layer of aluminium tends to get hydrated and improper cleaning of the oxide layer immediately preceding welding could be a cause for porosity. Ensuring that the plate is clean before welding and switching to clean, high quality electrodes will reduce the likelihood of forming porosity.
The amount of porosity depends on how fast the weld pool solidifies. Increasing the welding current and/or decreasing the travel speed will increase the heat input, and help retard the cooling rate, allowing gases to escape from the weld pool and thereby reducing the risk of porosity.
Filler wires should ideally be kept in their packaging until needed; wire that is left out in open workshop conditions will absorb moisture into its oxide layer. It is advisable when TIG welding aluminium to wipe each wire prior to use with a clean rag dipped in acetone.