Aluminum welding using a spool gun is much more difficult than mig welding steel.
Is it because of Murphys law?
... you know the one ...
"everything that can go wrong... will go wrong"
Well it may not be that bad, but I can tell you this for a fact...
Everything that makes a difference when you are mig welding steel, makes a lot bigger difference when you are mig welding aluminum.
Here are some of the heavy hitters that can make a big difference:
• wire feed speed • stickout • gun angle • cleanliness of metal • gas flow • purity of gas • using a clean contact tip • ground • internal inductance setting of power source • temperature of metal being welded
A closer look
1.wire feed speed : aluminum welding with a spool gun is very sensitive to wire feed speed. Slightly too much wire feed speed and the arc is not smooth. A smooth humming or hissing spray transfer is the goal with little or no sputtering sound. But just slightly not enough wire speed and the wire tries to burn back to the tip. Once that happens, you need to change tips.
2. Stickout: with mig welding steel using spray transer, , a short stickout is the way to go. A longer stickout is needed for Aluminum Welding with mig. about 3/4" seems to work, but sometimes slightly more works better.
3. gun angle: push, dont pull. pushing the puddle with a 10-15 degree gun angle is what works. Pulling makes for a sooty weld. Sometimes the weld is ok inside, and the soot can be wire brushed easily. But sometimes pulling causes oxides to be trapped in the puddle and causes porous welds.
4. cleanliness of metal: I will get some arguments on this one. If the metal is new and clean and free of oils. A good weld can be made without any cleaning. I will repeat that just in case someone thinks its a typo. If the metal is new and clean and free of oils. A good weld can be made without any cleaning.
If the metal is not new and has been outside growing a thick layer of oxidation, wire brushing with a stainless wire brush, or even some type of abrasive wheel that does not leave residue might help a lot.If the aluminum is heavily oxidized like you might find on a part subjected to salt air or water, cleaning with an abrasive wheel is a must, followed by an acetone or alcohol wipe...I prefer acetone.
5. gas flow: the rate of gas coming out of the mig gun is set at the flowmeter. With steel, enough is enough but more than enough usually does not do any harm until you get really high. Its just wasteful.But for mig welding aluminum, gas flow rate makes a lot of difference,. Too little flow, and the weld will be sooty with specks of black oxidation in the weld. Too much, and the puddle gets very erratic. The arc has trouble heating and melting the metal with all that cool argon blowing on it. The arc will often be very loud with lots of fine freeze lines and ripples in the bead.
6. purity of gas: Slightly contaminated gas, whether contaminated in the cylinder or as it comes out the mig nozzle sometimes goes unnoticed on steel. Its very forgiving.A slight leak in a connection, a pinhole in a gas hose, or moisture in the argon can all cause plenty of problems for aluminum welding with mig.
7. using a clean contact tip:aluminum mig wire is soft. any burrs on the contact tip can restrict wire feed and cause problems.
8. ground: A good ground is important with any mig welding job but especially important when migging aluminum. Also , make sure the aluminum part you are welding has a direct ground because arcing can dig deep and cause lots of damage...especially to machined surfaces.
9. internal inductance setting of power source: not all mig welding power sources are specifically designed for aluminum welding. Every mig welder has either an inductance setting knob...or an internal preset inductance. Inductance affects whether an arc is soft and sweet or harsh and digging, or somewhere in between. That is why finding the sweet spot settings for aluminum mig welding can be hard.
For higher end machines that were designed with aluminum welding in mind, are more forgiving and have much bigger sweet spot settings.
Anyone who has done much mig welding with a spool gun knows that even one small movement of the wire speed knob on the spool can make all the difference.
10. Temperature of metal.
when mig welding steel of any thickness greater than say 3/16" , it takes 3 or 4 beads done back to back to get the metal hot enough that the puddle acts very differently. The steel could be cold or hot and it welds similar.
Just a slight preheat of aluminum makes a huge difference in the way it welds. The first bead could be too cold, and the second bead, too hot..conductivity is the reason. Aluminum is much more conductive than steel.
If you are thinking this is an article trying to discourage you from mig welding aluminum, you are wrong. I am just trying to be real and lay it out there the way I see it.No need to sugar coat it. Anyone can buy a mig welder with a spool gun regardless of skill level ...and thats a recipe for discouragement without the proper heads up .