The manufacturers of MIG welding tools recommend specific wire diameters corresponding to the thickness of the metal that you are welding. The general rule is: for thin metals, use a smaller diameter wire, and for thick metals, use a larger diameter wire. Material 16 gauges in thickness, for instance, requires wire .03 or .035 inch in diameter. Material that is 24 gauges, conversely, requires wire .024 inch in diameter. A MIG wire spool will have both the alloy and the diameter of the wire marked on the label.
Aluminum wire is a high tensile-strength alloy, offering good feedability and strong welds. For best results, use aluminum wire with a spool gun and with argon shielding gas. Stainless steel wire, otherwise known as 308L alloy, can be used to MIG weld 301, 302, 304, 305, and 308 stainless base metals. Stainless steel wire must be used either with a mix of gases, such as the combination of helium, argon, and CO2, or with spray shielding gas.
You can weld steel to two kinds of carbon steel MIG spools: flux core and solid core. Flux-cored carbon steel wire requires no shielding gas, and is an excellent choice when the weld will be subjected to outdoor weather and windy conditions, or when welding dirty, rusty or painted materials. Flux-cored wire is used to weld material that is at least 18-gauge or thicker. Solid-core wire, on the other hand, is used to weld thinner metals that are 22 gauge or thinner, and is used indoors or for auto manufacturing. Solid-core wire may be used with the shielding gas combination of 75 percent Argon and 25 percent CO2.
Using flux-core wire tends to produce more spatter, and necessitate more heat-control, than using solid-core wire. Heat builds up rapidly, especially with the lack of shielding gas, increasing the risk of both welding burns and fire. The shielding gas used in solid core welding, on the other hand, has its disadvantages. Shielding gas can be easily disrupted, and results in the whole procedure being less portable, due to difficulty in transporting shielding gas containers. Further, if shielding gas does not adequately shield the weld, porosity can result, weakening the weld.