I look for TIG welding machines that can start down at 5 amps and weld down to at least 5 amps.
AC is good for aluminum welding, but welded aluminum usually needs heat treating and aluminum frames are normally bought to save money, not so much for custom frames.
I don't think aluminum frames sell well.
Aluminum is fine for machine made factory frames at a price point, but isn't that popular for people buying handmade frames.
Titanium has a good following with money and welded with DC.
Some cheap TIG welding machines can start at 10 amps, but I like to see a machine that can start at 5 amps.
The Miller Multimatic 220, like many multifunction welding machines, can only go down to 20 amps in TIG welding mode.
MIG welding generally requires about twice as many amps / as much power compared to TIG welding and that produces about twice as much heat and the current is flowing in the other direction.
Some people claim they can get away with MIG welding thin wall chrome moly and maybe they can.
I haven't tried it.
Not many years ago the welding industry called for weld filler to have equal or greater alloys of the base metal, such as chromium, molybdenum and sometimes nickel and those wires generally require stress relieving when MIG welded and can be more crack-prone.
More recently, with the acceptance of plain carbon steel weld filler on chrome moly, MIG welding has been more successful, but may not look as good as TIG welds.
For welding steel frames, I recommend a good DC TIG welding machine.
TIG welding thin steel results quit a bit of dilution of the weld from the base metal, adding strength to the weld.
MIG welds pick up fewer alloys, less carbon and dilution from the base metal and rely more on the strength of the welding wire.
With the welds twice as thick as the tubes, strength shouldn't be an issue though.
It is more difficult to judge penetration of MIG welds compared to TIG welds.
This is one reason MIG welds are discouraged in aviation welding, but they have become more popular recently.
Most European tubing companies still recommend chrome moly TIG filler for welding their steel tubes.
What they recommend is called CrMo-1.
This is similar to US grade ER80S-B2, but has slightly less chromium and about twice as much manganese.
CrMo1 has specs for both as welded and stress relieved, where ER80S-B2 only has specs for being stress relieved.
The only source of Euro spec CrMo-1 in the US I know of is Weld Mold, as Weld Mold 9211T and they only sell it for TIG welding.
https://www.weldmold.com/9211T.html
US ER80S-B2 from Weld Mold is Weld Mold 215-T CM and is NOT RECOMMENDED for welding bike tubes.
http://www.weldmold.com/215TCM.html
They are both made for welding chrome moly pipe, but the Euro spec CrMo-1 is more crack resistant.
Plain carbon steel weld fillers are fine though and chrome moly and weld fillers are normally reserved for high temperature applications in the US these days.
For welding steel, stainless and titanium frames I recommend a good quality DC TIG welding machine.
I started brazing as a teenager in the 1970's and experimented with TIG welding chrome moly bike frames in 1985 at university in Belgium as a materials engineering student.
Welding chrome moly frames in 1985 wasn't common at all.
Frame tubing wasn't made specifically for welding.
I had a frame building business in Italy from 1987 to 2015, when I returned to the US.
I have no experience MIG welding frames, but I see some people online MIG welding recumbent bikes and such with generic tubing of unknown thickness.
Generally TIG welding is recommended for bike frames.
I think those multifunction welding machines are best for motor vehicles.
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