Umm nope, not half way on the peddle.... and most metals requier 1amp per .001" of thickness for proper penatration so being that 1 1/2 sch 40 pipe is .109", that would mean that you would need about 109amps.... When I weld sch. 40 pipe at work, I have the machine set to 150amps and the peddle is full compressed to start and and then backed off to about 3/4 depressed for most of the weld... Im sorry, what I typed before was wrong about the 120 to 140... Its really 100 to 120 range... My readout on the machine is very accurate and the machine is accurate cause it is a requerment by Seamens/Westinghouse and NASA that our machines be tested, calibrated and documented every 90 day.
As for 1/4" aluminum and only 120 amps, your not getting any penatration on that... what frequancy and ballance are you set at when welding 1/4" at 120amps? What setup are you using for AL, the oldschool pure tungsten balled or have you changed over to pointed Lanthanated?
Thats even more strange that its only turning gray when you add filler.. When you add filler, you actually cool the weld so it should be even more less likely to show signs of being over heated.... The only things that come to mind is that either your using Sh*t filler (which I doubt), or your heat is too cold which is causing a you to have to stay in one spot to long thus cause the over heat... Low heat and prolonged time will actually equate to over heating of the weld.. Try using more heat and a faster depo rate.. also, what is your gas flow set to?
Just out of curiousity though, what material are the pipes and Els, 304L or 316L? and what Filler are you using?