OK Here is a really dumb one!

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Why do you have to use different shielding gases?For instance........MIG welding I am using Argon/CO2 mix for mild steel.  Why do I need tri-mix for ss?  Or pure argon for AL what is the big deal with a little 25% CO2 in the mix?I can understand that some gases would provide a 'hotter' arc, like helium, kinda like different gases in a laser, or propane vs. acetylene.Inquiring minds want to know???????????????Thanks
Reply:You don't "need" to use different gasses, but using the right mix for the right job improves both ease of welding and quality of welding.Kalroy"It's all about the OODA Loop."
Reply:you can weld ss with 75/25 just fineChuckASME Pressure Vessel welder
Reply:Well I confess I tried a little SS with the argon/co2 mix and it really splattered and looked like a turkey had **** on the joint.But I still don't see what the functional difference is.
Reply:So what you guys are saying is "It really doesnt matter what inert gas you use". That is not really true. Some gases and mixtures are better than others for certain applications. What he wants to know is "Why?" I cant answer that because I am not really sure myself. Revenos
Reply:I've never MIG welded, but I TIG, and pure argon works fine for everything. Helium mixes are used for thicker aluminum though.
Reply:I personally use 95/5.  95% argon and 5% oxygen. No spatter at all and burns hotter than **** !     VERY good penetration but is alot brighter
Reply:Originally Posted by RevenosSo what you guys are saying is "It really doesnt matter what inert gas you use". That is not really true. Some gases and mixtures are better than others for certain applications. What he wants to know is "Why?" I cant answer that because I am not really sure myself. Revenos
Reply:One thing a lot of people don't realize is that CO2 is NOT a truly inert gas, therefore any welding done with a shielding gas containing any amount of CO2 is not, technically, MIG welding (the "I" in MIG stands for "Inert"). This is why the AWS renamed MIG welding to GMAW (Gas-Metal Arc Welding), because it would allow for the use of non-inert shielding gases like 75/25.In the intense heat and electric field of the arc, some of the CO2 molecules disassociate into Carbon and O2. While large amounts causes serious problems, small quantities of oxygen in the gas can be very beneficial to welding mainly mild steel by among other things, increasing the heat of the arc (like emerikwelding said), and helping to smooth out the arc. However, true spray-type transfer can only be achieved with no more than 15% CO2.I can't really comment on welding stainless since I haven't had the opportunity to weld any... yet.As far as aluminum, it absolutely HAS to be GMAW/MIG welded with a truly inert shielding gas such as argon, helium, or a mix of the two. When I first started welding I attempted to weld some Aluminum using 75/25. This was partly because the welding supply place was closed that day (and Tractor Supply Co. didn't carry pure argon) but mostly because I thought that the small amount of CO2 (25% is not a small amount when it comes to welding gas mixes) was "inert enough." It wasn't, all I ended up with was some spongy black and gray pseudo-beads with no meaningful penetration that were extremely weak. What I believe was happening was that in the heat of the arc the aluminum (which is highly reactive at those temperatures)  was oxidizing very badly by stripping the oxygen atoms from the CO2 molecules while the remaining carbon was responsible for the black deposits in the "weld."Visit Tensaiteki.com
Reply:Dudes!Wouldn't it be great if you could use your 75 Ar/25 CO2 for all your MIG and TIG of steel, stainless, aluminum, and titanium!  Well you can, but you would be an idiot and you welds would be have the appearance and quality of pigeon caulk.Please see this Welding Journal article published by the American Welding Society.  These are sound recommendations for Gas Metal Arc Welding (GMAW) gas mixtures.http://files.aws.org/wj/2006/02/wj20...j200602-46.pdfYeah, you can deviate from these recommendations, but your results may suffer.Notice you do not use any reactive gas mixes (oxygen or carbon dioxide) on aluminum, and the percentages used for stainless steel and nickel alloys is limited to 2% O2 and 5% CO2 max.  Guess what, these gasses will oxidize molten metal, duh, you're adding Oxygen!  A small addition of reactive gas causes a small amount of oxidizing in the arc, adding heat and arc stability.And you know what, CO2 has Carbon in it!  Carbon disassociates from the CO2 molecule in the arc and it combines with chromium in the melt to form chromium carbides.  This is not a good thing, it reduces the corrosion resistance of the stainless steel.  This is why there are low carbon grades of stainless such as 304L and 316L.  And this is why there are stabilized grades such as 321 and 347, that contain titanium and niobium additions to preferentially combine into carbides, thus leaving the chromium free for corrosion resistance.  So, this is one reason 75/25 (Ar/CO2) is not recommended for stainless steel welding.  The other reason is excessive oxidation.
Reply:This is a perfect example of how the science of welding can explain or solve the problems in the shop. Gas selection from metal to metal is also a way to engineer the welding results for position of welding as well as "metal transfer" modes used for a given weldment. i.e. You can't "spray" with more than 15% CO2 (as stated above) and you can't "spray" out of position - Flat only, cuz the puddle stays liquid too long.It's all so confusing!Weldtek
Reply:Good points Weldtek, you are absolutely correct.Modern pulsed current power supplies so allow out of position "pulsed spray" transfer welding.Many people look at welding as an art, and I agree with this.  But since anybody can pick up a stinger and make an arc on some scrap steel, there is alot of bogus welding wisdom, tips and tricks to be spouted.  Luckily there are some people smarted than me that are continuing to uncover the science behind the welding, and this is helping to ensure it is not a "black art".
Reply:Originally Posted by TensaitekiOne thing a lot of people don't realize is that CO2 is NOT a truly inert gas, therefore any welding done with a shielding gas containing any amount of CO2 is not, technically, MIG welding (the "I" in MIG stands for "Inert"). This is why the AWS renamed MIG welding to GMAW (Gas-Metal Arc Welding), because it would allow for the use of non-inert shielding gases like 75/25.. "
Reply:Hey thanks guys the last few answers where some great info.So Tri Mix could be used for  mild steel and SS.  Because of the low 2.5% CO2.   Would it be a better general purpose gas than Argon/CO2?
Reply:i use 80/20 argon/co2 it's a good common mix run's a little hoter than 75/25 and with less spatterChuckASME Pressure Vessel welder
Reply:I use 95/5 at work and it seems smoother than the 75/25 I use at home.  I believe I'm gonna swap over on the next bottle.DewayneDixieland WeldingMM350PLincoln 100Some torchesOther misc. tools
Reply:Weld metal conductivity -the ability of the metal to absorb and conduct the concentrated heat away from the arc site is one reason for the use of various ga mixtures.Different types of metal - alumininium,  s/ steel  and carbon steel metals -all have different rates of heat conductivity, therefore the need to accomodate this, with the use of various gas mixes.A change in a gas mix can alters the cross sectional weld bead profiles giving change to bead heights ,widths and penetrations into the respective weld metals.These gas mixes are among the possible changes to weld procedures that are termed "variables".Does this help explain the reasons for the range of gas mixes?I hope it helps.Grahame