One of the tricks to tig welding 18ga lap joints is getting a tight fitup.
... and that means making sure there are no gaps in the lap joint.
With carbon steel and stainless steels, this is usually fairly easy because tack welds can be made without filler metal while you either clamp the joint tight with welding clamps...or hold the joint tight with a tool in the other hand while you prop the tig torch and make a tack.
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I like to make quick burst tacks aka flash tacks for tig welding 18ga sheet metal when I can because its a huge time saver when building something.
In order to make flash tacks, I set the amperage to 1.5 to 2 times the normal welding amperage and then either bump the foot pedal very quickly or do a quick on/off using a torch switch to make a super fast, small, and clean tack weld.
Some tig welders are equipped with a spot timer for making quick flash tacks like this but you can do it manually. ( it works really well with a torch switch)
18 ga cold rolled steel is a little under ,050” (1.2mm) so normal amperage following the one amp per one thousandths of thickness would be around 50 amps
I set the main amperage to around 80 to try to keep the tack size smaller than the final weld.
I chose a standard #5 cup for the tack welding with only 10cfh has at 80 amps.Not only does 10cfh save argon, but a soft low flow of argon really helps with a stable arc start which is important for accurate flash tacks.
After tack welding every inch or so, I switched to a #8 clear cup and lowered amps to 63 for the actual welding.
I went full pedal for most of the welding.
I used the backstep method welding about 1/2 of each side before backing up and welding into the previous start.
The Backstep welding method helps to prevent distortion on thin metal.
The thinner the metal, the shorter each bead before back stepping.
I have seen a few rare situations where .012" thick stainless was
welded to .020" thick stainless where we had to limit each backstep bead
to 1/2" or so)
For this 18ga cold rold steel application, I was tig welding a 3 inch square to a larger piece of 18ga so I welded 1-1/2" at a time before backstepping.
80 amps for tack welds
63 amps with .045" ER70S-2