Hello Shawn, the 75/25 will work for welding with ER70S series solid wires on mild steel. It will also work with many of the flux-cored gas-shielded wires. Many of the flux-cored stainless steel wires will also work with the 75/25 shielding gas. For GTAW, on all materials, straight argon is suggested, a combination of argon and helium, or straight helium. As you go from argon to a mix of argon/helium and then on to straight helium the energy transfer across the arc increases. Using this same progression the flowrates must increase and the cost of gases increases also. In some instances and for special purposes, a small percentage of hydrogen might be used in the shielding gas when welding on certain stainless steels. For GMAW with aluminum the shielding gases are pretty much the same as GTAW, starting with argon, on to argon/helium mixes and finally using straight helium.
     There have been numerous threads devoted to discussions on shielding gases on the forum here. If you do some searching on this topic you will probably find a host of great information to further answer your questions. Best regards, Allan

aevald explained it well.

75/25 was and still is a very common shielding gas for GMAW on mild steels. Generally the practice was discouraged on stainless steels due to carbon uptake from the CO2 this could cause carbide precipitation or intergranular corosion.

also when using 75/25 it prevents the ability to operate in spray transfer at practical settings, so it generally increases spatter and reduces weld appearance to a degree.  However it has long been acceptable on mild steels and is still a common hobbyist gas and a significant improvement from 100% CO2

Not much to say other than confirm what has been said.  75-25 is a splendid choice for mild steel.  I'll take it any day over pure CO2.  Less spatter, smoother welds, better all around performance.  But it is more expensive.  Main reason many go with the CO2.  I'll take performance over cost in many cases, this is one of them.

Have a Great 4th,  Brent

Not gonna work to well . You will get a great deal of sparkling and difficult to control puddle. 75/25 Ar/He will work OK.

You can give it a try but I think you will find it unpleasant. I tested 3 brothers from texas on a new boiler construction job many years ago. After listening to about 5 minutes of whining about how we bought JUNK" tig rigs that all leaked, I asked them if they checked the gas . They said it was fine and had tried a few different bottles (All Ar/CO2) .They had set aside about 5 or 6 weldcraft WP9 rigs and thought they ALL leaked.

I could see the big silver band around the top and just grinned. After telling them they needed to the check the gas they threatened me and explained what great welders they were. They had no idea what the big silver band around the top meant. Didn't ask em if they could read as I figured they were a little tense at the time. Don't know if they ever figured out what was wrong. They never got to test.