Gas welding of 4130 steel and aluminum with a torch is quite possible. I have a Henrob torch setup that is specifically marketed for welding, and I've used it with good success on both steel and aluminum. For aluminum, I feel its' best use in in gas welding sheetmetal where the weld area is going to be ground down and the panel shaped, as it has a larger Heat Affected Zone (HAZ) than a TIG weld, and you can use the torch to anneal the entire panel for workability.

The key to gas welding aluminum is flux. The oxide layer on aluminum melts at a higher temperature than the base aluminum, which is why the prior poster got his pile of molten goo. Cleaning the pieces VERY well to remove the oxide layer just before welding, and using a specific aluminum flux that breaks down this oxide layer, are key. Take a look at https://www.tinmantech.com/index.php for some excellent info on gas welding and a source for aluminum welding flux. He also sells a green lens that's supposed to filter the colors that the flux emits when it burns to allow better vision when welding aluminum.

Now, after saying all of that, I will say that I also own a TIG, and I think for the kind of thin wall tubing and "visual quality" people are looking for in bike tubing welds, the TIG is superior. I have never managed a weld using the torch that exhibited the "roll of quarters" look I can get with the TIG. Certainly aircraft 4130 and aluminum panels were welded for decades with a torch and proved structurally sound, but I'll be using the TIG on the frame I'm preparing to weld, for aesthetics and finer control if nothing else.

Facinating, I guess gas-welding aluminum can be done! That would also explain why I had 0 success: I needed flux! We were using bare aluminum welding rods, but we did try coating it in borax (works for purifying steel, I guess).

Here is a site about welding bicycles. https://www.bikewebsite.com/weld.htm
They even have a section about welding aluminum. The site also answered my question why steel isn't typically gas welded in the bicycle applicaiton. (hint, creates a larger heat-affected-zone than other methods) I don't think that will stop me from trying to gas weld a bicycle, but it is good to know.

Thanks for enlightening me about gas-welding aluminum, KendallF!

I don't see the problem with a larger heat affected zone, it is something people talk about largely when they don't understand what is going on, there are lots of factors. Gas remains popular with aircraft builder, what more does one need to know. I know a guy who TIGs helicopter booms for a living. It's all good.

Good site by the way!