MIG (GMAW)MIG welders are mostly used for carbon steel although is possible to weld stainless and aluminium with the correct wire. The shielding gas used depends on which metals are to be welded, for carbon steel the best results come from a 20/80 mix of CO2 and Argon (Fogon or Mison are the AGA/Linde brands). When welding alloys such as stainless and aluminium it is best to use either pure argon or if deep penetration is requried mixtures of helium /argon up to 50/50 can be used. Never use gas containing hydrogen while welding stainless steels as this can cause hydrogen embrittlement. Pure CO2 is commonly used but with the current prices of 20/80 Fogon it can hardly be justified because of spatter and weld quality. A typial flow figure for the shielding gas is around 10lpm (21 CFH), consult manufacturer specifications for more detailed information on flow rates. It is possible to use mig welders without shielding gas, in these cases a flux cored wire is used. Using Flux cored wire is only practical where it is to windy for gas, the flux causes more splatter and slag. MIG Wire typically has a diameter between 0.6mm (.024") and upwards to 1.2mm (.046), the most common is 0.8mm (.030"). Aluminium wire is typically thicker, 1mm(.036) is good for medium thickness (2-6mm). Welding aluminium requires almost double the current of steel for the same thickness, consider using a high amp output welder.
Selecting a MIG welder depends on the job it is intended for, while a 150Amp welder is fine for fixing your car body it is way to small for boatbuilding. The main areas of attention should be, Amperage, Gas ability*, Duty Cycle**, Mains hookup***, wire size ability, transformer/inverter.
MIG welding is less sensitive to unclean/rusted surfaces than other welding forms although clean surfaces withh always produce better results. MIG welding can be done in any position including overhead and unlike stick welding it does not require special electrodes/wire. With todays welders MIG welding is the easist method to master, any person can learn to produce adequate welds in a couple or hours of practise. Getting the wire speed and current correctly adjusted can prove a bit tricky for beginners, the reason for the lack of a "general guide" is the this varies based on your machine and the type of weld. Keep in mind that welding "into a corner" will require more power than welding "the edge of a corner". Welding a flat joint is somewhere in between the two. Good welders can tell if the power/wire speed are correctly adjusted by the sound, what you want is a "crackling sound" not an "electrical buzz".
As with any welding it is good practise to tack-weld**** the objects before laying down the final welds.
For more information on MIG welding Lincoln Electric has a grea article at www.lincolnelectric.com/knowledge/articles/content/migvsfluxcored.asp Mig Welding Videos* There are "MIG" welders which can only use flux-cored wire, even though the price might be attractive they are not worth it. If price is the exit criteria you are better off buying a Stick welder.
** Duty cycle is specified as a percentage. It is a ratio of the number of minutes you can continuously weld out of a 10 minute period. For the rest you have to let the machine cool down.
*** Big Welders require a substrancial amount of electricity, there are several types of electrical outlets available, be sure your welder mathes your mains.
The main types are, 110V, US 220V (180°), 220V, 380V, 400V, 415V, 440V, 480V,550V,575V. On top of this it can be 50hz or 60hz and if they are three phase they can be wye or star configuration.
**** Tack Welding, before laying down any heavy welds start by fastening the items with tack-welds, these are "dot-welds" placed at all ends/corners. Typically the entire piece/joint is tack welded before putting down any other welds, this reduces stress and distortion.
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