If you have not yet mastered MIG welding of mild steel, you may need to do so before you start welding aluminium using the GMAW (MIG) process. When MIG welding aluminium, you use the same torch angle and gas flow rate (20-30 CFH).
Aluminium is a difficult metal to weld with a MIG welder as it requires more heat than mild steel (usually in the 21 to the 24-volt range). The minimum thickness of aluminium you should try is around 14 ga. to 18 ga. Anything thinner than this and you will need a TIG welder.
With a TIG welder, you can actually weld soda cans together. Welding aluminium with a wire welder is notorious for accidental burn through, and if you don't move fast enough, the weld will go right through the piece.
That's why you shouldn't try welding thin pieces of aluminium. When you are MIG welding mild steel, you can and should move the MIG torch slowly enough to achieve a deep depth of fusion. With aluminium, however, the melt pool is similar to a "wet foil". You will not see a molten, hot weld with aluminium. It takes time to feel when the aluminium weld becomes too hot. This is why you can easily damage the workpiece if you are not careful.
A good rule of thumb is to use roughly the same voltage setting as for mild steel, but double the travel speed. As I said before, you will start off moving fast with an aluminium MIG gun until you get used to it. MIG welding steel uses a short-circuit transfer process, which means that the wire actually creates a short circuit at the joint, resulting in a molten weld. When welding aluminium you are using spray transfer in most cases. Spray transfer is a method of actually spraying fine particles of aluminium wire into the molten pool. Most MIG welders handle this process. All you have to do is increase the voltage and use the correct gas mixture.
If you are going to MIG weld aluminium with a wire feed welder, you will need a spool gun. Why not put a spool of aluminium wire in the MIG machine? Because MIG aluminium wire is much softer than steel and tends to form a bird's nest in the cable lining. Steel wire is not as hard as mild steel wire, so you have to use a spool gun to compensate for this. Spool guns are great for DIY welders and usually only hold a small 1lb roll inside. The advantage of using a spool gun is that the aluminium wire does not have to go through the MIG hose, where it could get chewed up.
When welding mild steel you usually use the so-called C25 gas (25% CO2 and 75% argon). For MIG welding of aluminium, you will use a pure argon shielding gas. This will allow a spray transfer process to take place. For thicker aluminium (1/2" or more), 25% to 75% helium can be added. This allows for deeper penetration into the workpiece.
MIG welding aluminium requires that you set the machine to DCEP (DC positive). With this polarity setting (called reverse polarity), electrons are passed from the machine through the earth cable and back through the gun. It is important to set this setting correctly or your weld will not come off.
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