The basic differences between mixed gas welding and pure argon gas are arc stability and cost. Pure argon (purity ≥99.996%) provides a stable arc (±1.2V voltage fluctuation) in TIG welding, suitable for non-ferrous metals such as aluminum and magnesium, and the oxidation rate of the weld is only 0.3% (2.5% when welding stainless steel under mixed gas). If Ar+2%O₂ mixed gas is utilized to weld carbon steel in MIG, then arc energy density is increased by 15%, penetration depth is increased by 1.2mm (as against penetration depth of 3.5mm using pure argon and 4.7mm using mixed gas), and welding speed is increased to 45cm/min (which is only 35cm/min using pure argon).
Chemical activity difference was gigantic. pure argon gas, as it is an inert gas, will not react to the molten pool, but is unable to eliminate steel welding spatter issue (spatter rate of around 8%). 15-25%CO₂ Ar/CO₂ blend addition (i.e., C25) leads to the evolution of CO gas through the reaction of oxidation between carbon and lower the spattering rate below 3%, and improve the fluidity of molten pool (reduces the surface tension by 28%), and also the tensile strength of weld from 450MPa up to 520MPa (ASTM A370 standard test). Japan Kobe Steel experiments show that when welding 5mm thick low carbon steel plate, Ar+20%CO₂ mixture gas improves the welding wire melting efficiency from 82% to 94%, and the cost per unit length of welding material is reduced by 12%.
The cost contrasts sharply with gas consumption. The unit price of pure argon gas is about 0.8/m
(industrial quality), and Ar+50.6/m³, if in case the annual consumption by a plant is 5,000m³, the gas mixture will be able to save $10,000. Pure argon, however, is not replaceable with welding aluminum – its density (1.784kg/m³) is larger than air, the protection spreads up to a distance of 15cm (only 10cm in the case of gas mixture due to CO₂ density difference), and porosity in the case of 6061 aluminum alloy decreases from 1.2% to 0.3% (as per the ISO 10042 standard).
The heat input is differentiated from the respective conditions. In welding thin plates (≤3mm) with pure argon, the heat input control is accurate (2.1mm in the heat affected zone), and the arc Ar+He temperature (30%-50% helium) is increased from 10,000K to 16,000K due to the high ionization energy of helium (24.6eV as opposed to 15.8eV of argon) suitable for welding thick copper plates (≥8mm). Penetration depth was 40% higher. In the Boeing FAA certification report, argon gas should be used in its pure form for welding aerospace titanium alloy parts because it avoids hydrogen embrittleness (hydrogen mixing probability <0.001ppm), and trace hydrogen in the gas mixture (0.5-2%) may cause a loss of 18% of titanium alloy ductility.
There were significant differences in environmental adaptability. In plateau region (altitude above 3,000 meters), pure argon gas dilutes protective effect due to low atmosphere pressure, and flow rate needs to be adjusted from 15L/min to 20L/min, and active substance of oxygen in Ar+O₂ mixed gas can counteract ionization stability. Maintain arc length error ±0.5mm (pure argon error ±1.2mm). The welding example of the Antarctic research station shows that Ar+2%N₂ mixed gas can prevent the freezing of liquid argon (freezing point -189 ° C) at -40 ° C, and the low-temperature impact toughness of 304 stainless steel welding is still 45J (pure argon welding only 32J).
Regulations and safety standards differ. The EU EN ISO 14175 standard mandates that the pressure vessel welding in nuclear power plants shall utilize pure argon gas (impurity content ≤10ppm) and Ar+8%CO₂ mixed gas in the automobile industry has 75% market share due to cost advantages. United States OSHA specifies that the concentration of argon in welding of confined spaces shall be <19.5% (anti-asphyxiation risk), and the concentration of CO₂ in the mixed gas can be strictly controlled by the flow meter (error ±0.2%) so that the concentration of CO₂ in the working area will not be above the safety threshold of 5,000ppm.
Cases from industry substantiate the economy. Tesla Shanghai factory applies pure argon gas (annual amount 120,000m³) for Model Y aluminum alloy body welding, the cost of welding one car is 18, if using the mixed gas is reduced to 14, but the scrap rate of porosity will be increased from 0.5% to 3%, and the total cost will be raised by $9.6/unit. Conversely, Sany Heavy Industry uses Ar+18%CO₂ to weld structural steel of the excavator, and it can increase the efficiency of the production line by 22% and reduce the gas cost by ¥860,000 yearly. It is selected according to material properties, process requirements and overall life cycle cost balance.