Choose a welding tool with a fine degree of voltage and wire feed speed control. Better yet, choose one into which you can input your base metal thickness and the diameter of the welding wire you are using, allowing the machine to automatically set the appropriate voltage and wire feed speed. Too low voltage results in poor penetration, while too high voltage can result in burn-through. Too-high wire feed speed results in an overly wide weld bead, while too-low wire feed speed results in insufficient amperage.
Use the shielding gas recommended by your welding tool manufacturer, in the appropriate proportions, to protect your molten weld pool from contamination that would cause porosity in your weld. Generally, 100 percent argon is used to weld aluminum, while a mixture of 20 percent CO2 and 80 percent argon is used for steel.
Clean the surface of the aluminum or steel with a stainless steel brush. This prevents surface oxides on the material from contaminating the weld.
Ramp up the current at the beginning of a weld, then reduce it, to avoid excessive heat build-up. This will help prevent problems with lack of fusion that exist until enough energy is put into the weld. Alternatively, consider pre-heating aluminum to 300 to 400 degrees Fahrenheit before welding, to facilitate fusion.
Travel rapidly through the metal you are welding, particularly if it is aluminum. Slow travel speed results in heat build-up.