Custom 18 gallon aluminum fuel tank

[crash]

Sometimes it's not about weight savings. I am building to the same spec of DOM with 4130. It's simply about strength. ANYTHING 4130 or otherwise will benefit from a normalizing process after it has been completely welded out. Problem is that it is expensive and difficult to find someone that can bake an entire frame. That said, I have used both MIG and TIG with 4130 and had excellent results. If you aren't careful with TIG, you get too much heat into the area. Because you are having to move along with the MIG, you don't see this issue as much. Also with TIG the gaps need to be REALLY tight, whereas with MIG you can squirt the filler material in there pretty easily and come out with a good strong weld. I learned from a pro a long time ago that the filler metal should be substantially more flexible than the parent metal to avoid cracking and preserve strength. I generally over build EVERYTHING as I feel my life isn't worth a few pounds of lightness.

So to answer your question, in my experience, MIG is fine. I personally use a soft rod/filler and straight CO2 for extra penetration.

My frame is a mixture of 4130 DOM and 1020 DOM.