"Spray Transfer is Hot ... "
The Mig welding techniques used for short circuit mig are different from those used in spray mig.
For short circuit mig, either straight co2 welding gas or a mix like 75/25 argon/co2 welding gas is used.
Usually, depending on the thickness of the pieces being welded, 17-23 volts is used. ( for thin sheet metal, voltage might be as low as 14 using smaller diameter wires.)
I like to use as high a voltage and wire speed setting as I can tolerate because above all else...above making a pretty weld, I want to make a strong weld that penetrates.
Short circuit mig is so versatile because it is able to be adjusted really cold for sheet metal, or really hot for thick metal.
But there are no mig welding police that will show up at your door if you weld too cold for the sake of making a pretty weld.
Spray transfer mig is hot.
Using .035" wire , it takes more than 25 volts to transition into spray mode. using 90/10 argon/co2 welding gas.
you cannot use 75/25 welding gas for spray. It will not work very well at all except under special circumstances. You need 90/10 or some other argon mixed gas with less than 20% co2.
Other gases that can be used for spray arc are 98/2 argon/o2 and 95/5 argon/co2 .
these are not the only mixes that allow spray arc but they are pretty common.
For my purposes, the only reason to use spray transfer mig is for jobs where deep penetration is needed along with fast travel speed.
I like short circuit mig overall because it is more versatile and more forgiving....with less heat input.
I have been able to get around the risk of lack of fusion using sound mig welding techniques and by setting the machine to the higher range of recommended settings for a given thickness.