The properties and components of welding shielding gases have a great impact on welding quality. Common welding shielding gases have different properties and different impacts on welding speed, weld penetration and arc stability. Therefore, in the actual production process, a variety of influencing factors should be considered comprehensively to select appropriate shielding gases to provide reliable guarantee for welding quality.

I. Introduction

Team Introduction
In the arc welding process, if the protector is not added, oxygen (O2) or other gases in the atmosphere will invade the arc and molten pool, and react with the metal melted at high temperature, resulting in welding defects, affecting the adaptation performance of the product. The main function of welding gas is to protect the metal to be welded from other gases and impurities, to ensure the quality of welding products; On the other hand, the properties of welding shielding gas have corresponding effects on welding speed, weld penetration, forming, welding smoke and arc stability. Welding gas can be divided into inert gases (such as He) and non-inert gases (such as CO2) according to the degree of gas activity. According to the component composition, it can be divided into single component gas (such as: GTAW- using pure Ar as a protective gas) and multi-component mixed gas (such as: GMAW- using pure 75%Ar+25%CO2 as a protective gas); According to the strength of oxidation gas can be divided into strong oxidizing gas, weak oxidizing gas, reducing gas, neutral gas; In addition, in addition to the gas activity degree, component composition, oxidation tendency, the ionization energy and thermal conductivity of the protective gas are also important basis for selecting the protective gas.

Second, the selection and application of welding protective gas

Team Introduction
(A) GTAW welding gas selection and application
Argon (Ar), because it is an inert gas, does not react chemically with other elements at room temperature and high temperature, and has good arc stability, low ionization voltage, beautiful weld formation, and low cost, it is suitable as a protective gas for various metal welding in GTAW. At present, the metals that can use argon as welding protection gas in GTAW include all carbon steel stainless steel and almost all non-ferrous metals: aluminum, copper, nickel, titanium, zirconium and their alloys, and the welding effect is excellent. Although the use of pure argon as a GTAW protective gas can obtain excellent welded joints, but due to its low arc energy, the welding penetration force and welding speed can not meet the actual needs, at this time, argon can be mixed with other gases to improve the welding penetration force and welding speed, such as Ar+He or Ar+H2.
(2) Selection and application of GMAW welding gas

1, ordinary carbon steel (structural steel) GMAW protective gas selection
For ordinary carbon steel, or structural steel used for the production of steel structures, if it is not strict requirements for welding quality and welding appearance, CO2 is usually used as a protective gas, which is also known as CO2 gas shielded welding, which has the advantages of high production efficiency, good welding quality, low cost and strong practicality. It should be noted that the soluble mass fraction of water in liquid CO2 is 0.05%, and the excess water is sunk in a free state at the bottom of the bottle. This water is volatilized with CO2 during the welding process and mixed with CO2, directly into the welding zone. Therefore, water is the most important harmful impurity in CO2 gas. The purity and quality of CO2 gas should meet the requirements.
For the occasion of high welding quality, when non-destructive testing or pressure test is required, Ar+CO2 can be used as a protective gas, and the change of the component of the protective gas can affect the spatial form of welding arc, arc energy density, droplet transition mode, welding wire melting characteristics and splashing during welding process. It can also improve the stability of arc and transition during welding and the wetting of liquid metal and molten pool, improve weld forming, reduce spatter, eliminate and prevent defects, and improve the performance of weld joints.

2, stainless steel GMAW protective gas selection
The selection of welding gas in stainless steel GMAW is not only based on factors such as the type of stainless steel and welding position, but also must consider other factors such as back molding, welding combination, and droplet transition form in order to obtain the best welding effect.
Pure argon can only be used for TIG welding stainless steel, but not for MIG welding stainless steel. Because the surface tension of the molten drop and molten pool of stainless steel is large, the liquid metal flow of molten pool is poor, the surface of the weld can not spread and wet, and the weld bead forming is poor.
When 1-2% oxygen is added to argon, the droplet of stainless steel and the surface tension of the molten pool are reduced, the fluidity of liquid metal in the molten pool is enhanced, the spreading wettability of the weld surface is improved, the weld depth and width are moderate, and the weld bead formation is beautiful. 0-1% suitable for austenitic stainless steel, 0-2% suitable for ferritic stainless steel; The 0-2% molten pool has better fluidity than the 0-1% molten pool, which is suitable for the spray transition and pulse transition of stainless steel welding wire, and is suitable for the flat welding and flat fillet welding of stainless steel welds.
When 2-5%CO2 is added to argon, it is concerned that there is a tendency to increase carbon. The test proves that CO2 is less than 5%, and the carbon content of the weld is less than 0.03%, which is still below the level of ultra-low carbon. The stability of the arc is good, the oxidation is weakened, the alloying elements are burned less, and there is no carburization tendency. Suitable for short circuit transition and pulse transition of stainless steel welding wire. When 2-5%CO2 is added to argon, it is suitable for the combination process of TIG bottom welding +MAG filling cover welding of stainless steel pipes, all-position welding, short circuit transition, and smooth and beautiful weld.
The advantages of the three-component gas mixture are more prominent, such as the three-component gas mixture of Ar+5%CO2+2%O2, strong arc concentration, good single-side welding of the weld, suitable for stainless steel welding with high technical requirements; It can be divided into Ar+He+CO2 mixed gas, in which helium can increase the weld penetration, improve the welding speed, and reduce the deformation of welds. The gas mixture of Ar+CO2+N2 is a new process developed in Europe and the United States, in which nitrogen can increase the penetration depth and width of the weld.
3, aluminum alloy GMAW protective gas selection
For aluminum alloy suitable for welding, Ar is usually used as a protective gas in GMAW, it is worth noting that aluminum alloy has high requirements for the purity of welding gas, if the protective gas is not up to, there will be black oxide on both sides of the weld, affecting the appearance of the welding quality. If you want to obtain a greater welding depth and welding speed, you can add a certain proportion of He to Ar. Because of the large heat transfer coefficient of He, the arc voltage is higher than that of Ar at the same arc length. High arc temperature, large base metal heat input, high melting speed. Suitable for welding thick aluminum plate, can increase penetration, reduce porosity, improve production efficiency. However, if the proportion of He is too large, more spatter will occur.
4, other metals and alloys GMAW protective gas selection
For copper and copper alloy GMAW, in addition to using pure Ar as welding protection gas, a certain proportion of nitrogen can be added to Ar gas, which can reduce production costs, and can also play a protective role, but there is a certain splash and smoke, and the forming is poor. For nickel and nickel alloy GMAW, in addition to using pure Ar and Ar+He as welding protection gas, a small amount of hydrogen can also be added to Ar gas, which can also improve welding efficiency. For titanium and titanium alloy GMAW, because Ti and N, H, O have strong tubercularity, so only pure Ar and Ar+He can be used as welding protection gas.

Iii. Conclusion

Team Introduction
In summary, welding shielding gas plays a particularly important role in gas shielding welding, and the selection of welding shielding gas directly affects the quality, efficiency and cost of welding production. Due to the diversity of welding materials, the selection of welding gas is also more complicated. In actual production, it is necessary to consider the welding material, welding method, droplet transition form, welding position, and the required welding effect in order to select the most suitable welding gas and achieve the best welding results.