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 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.
Find out more here: Avoiding Porosity When Welding Aluminium
