The faster your travel speed the better chance of full penetration and fusion only if you stay on the leading edge of the puddle. A note of caution, it’s very easy with GMAW to slow down and still see push-through (mock penetration) if you have a root opening but that can fool you because there may be no fusion. The molten puddle can literally ooze through and look as if you’ve achieved full penetration when in reality, you’ve only masked cold-lap (lack of fusion). It’s been my experience that as little as a 1/16” difference in the wire’s position from the leading edge of the puddle will take you outside the margin of error.

To help you gain confidence, run a few simple mock-up fillet (inside corner joint) test welds and break them. Use two pieces of your plate and make a “T”. Give yourself a little root opening (gap) between the two just as you would with the actual work and single-pass weld only one side of the “T”. Take a sledgehammer and break the “T” over onto itself and take a look at your penetration and sidewall fusion. Did you penetrate into that little gap? Did you pull any base metal in the break (good) or did the weld metal come away smooth from the sidewall (bad)? It’s a pretty simple but very effective proof test with the GMAW-SC process. I’ve actually tossed a completed 3/8” fillet weld test (multiple 2G) on the concrete floor and had it break into two pieces because of lack of fusion (smooth sidewall) and the welder actually did ooze the root in. Looked great but was absolutely worthless and didn’t have to use the test press!