Why is mig welding with a spool gun so much harder than mig welding steel?
Well its not always harder but things definitely tend to go wrong more than on steel
much more difficult than mig welding steel.
Everything that makes a difference mig welding steel, makes a lot bigger difference with a spool gun on 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.
1.wire feed speed : welding aluminum with a spool gun is way more sensitive to wire feed speed than migging steel. Just a tad 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. When the wire burns back and fuses to the contact tip, partys over. Thats a big time waster and its easy to go thru lots of tips.
2. Stickout: with mig welding steel using short circuit transfer, , a short stickout works best . A longer stickout is needed for Aluminum Welding with mig. about 3/4″ seems to work, but sometimes slightly more works better. but just a bit too much , and you can lose coverage.
3. gun angle: push, dont pull. pushing the puddle using a 10-15 degree gun angle is what works. Pulling makes 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. One comment on the youtube video mentioned a situation where the internal bore of a large ship propeller where lots of pulling the puddle was done. Soot a plenty but the welds machined clean and x rayed without porosity. Soot is not preferred but not the end of the world.
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 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 on aluminum, too much gas coming out of the torch messes with the arc.
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.
Not so with aluminum welding.
7. using a clean contact tip:
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.
preheat on thick aluminum makes a big difference. But even on 1/4″ thick aluminum, cold metal welds way different than warm 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.