Straight answers to the questions people ask most when choosing and using MIG welding wire.
How do I choose the right MIG wire?
Start with the metal you are welding, because the wire must suit it: steel, stainless steel, aluminium and other alloys each need their own type. Then consider whether you will use shielding gas, the thickness of the material, the welding position and the strength and finish you need.
Check that the wire diameter and spool size suit your machine. As a rule, choose a wire whose composition matches or is compatible with the parent metal, and follow the wire maker's guidance on the gas to use.
What is the difference between solid wire, flux-cored wire and gasless wire?
Solid wire is a plain wire that needs shielding gas, and gives clean welds with little slag, so it is the usual choice for general fabrication and sheet metal. Flux-cored wire is a hollow tube filled with flux and alloying powders. Gas-shielded flux-cored wire usually runs with gas, deposits metal quickly and handles thicker plate well.
Gasless (self-shielded) flux-cored wire makes its own shielding as the flux burns, so it can be used outdoors without a gas bottle. It leaves slag to remove and tends to give more fume and spatter.
What wire diameter should I use?
The right diameter depends on the thickness of the metal, the current you will use and the machine. Thinner wire suits thin sheet and lower currents and gives good control in all positions, while thicker wire carries higher currents and suits heavier material, particularly in the flat position.
Your machine and torch limit the choice, because the drive rolls, liner and contact tip must all match the wire size. Check the machine's recommended range, and do not use a diameter much outside it.
Which shielding gas goes with which wire?
For solid steel wire, a mix of argon and carbon dioxide is the usual choice, and pure carbon dioxide can also be used with some wires and for some work. For aluminium, the gas is normally argon, sometimes with helium added.
For stainless steel wire, gases that are mostly argon are common, but the exact mix depends on the wire and the maker's recommendation, so check the wire data sheet.
Gas-shielded flux-cored wires are designed for a particular gas, so use what the maker recommends. Self-shielded wires need no gas. The wrong gas can cause porosity, a poor arc or weld problems.
What do the letters and numbers on a wire's specification mean?
Wire is classified under standards that describe its type and composition, and for steel wire, the strength it gives. The classification tells you the kind of metal, whether it is solid or cored, and which gas it is designed for. Different standards use different codes, so the same wire may carry more than one.
Wires with more deoxidising elements, such as silicon and manganese, tend to cope better with light surface rust or mill scale. The data sheet for the wire gives the details and the right gas.
When should I use gasless flux-cored wire?
Gasless wire suits outdoor and site work in wind, where shielding gas would be blown away, and it is handy when a gas bottle is awkward to move around. It can weld through light rust and surface dirt better than solid wire.
The trade-offs are slag to chip off, more spatter, and more fume, so ventilation and extraction matter. The polarity is often reversed compared with solid wire, so check the wire maker's instructions and your machine's setup.