Flux-cored wire gas shielded welding (FCAW-G for short) is a very widely used welding process. It is widely used in the welding of low carbon steel, low alloy steel and other various alloy materials in heavy manufacturing, construction, shipbuilding, offshore facilities and other industries. FCAW-G welding process often uses 100% pure CO2 or 75%~80% Ar and 20%~25% CO2 mixed gas as shielding gas. So when implementing flux-cored wire gas shielded welding, which shielding gas should be chosen, CO2 or Ar/CO2 mixed gas? Each type of shielding gas has its own advantages and disadvantages. When choosing welding shielding gas, we must focus on cost, quality, productivity and other factors. Sometimes the choice of protective gas contradicts these factors. This article mainly describes the advantages and disadvantages of FCAW-G in the selection of two basic shielding gases in welded steel.

Before discussing the advantages and disadvantages of shielding gas selection, it is best to review some basic knowledge. It should be noted that this article only discusses a few types of protective gas. For a more comprehensive introduction, please refer to ANSI/AWSA5.32/A5.32M. The technical requirements for shielding gas are specified in the welding shielding gas specification, including testing, packaging, identification, and acceptance. In addition, it also includes some useful information such as ventilation and ventilation in the welding process, comprehensive consideration of safety requirements.

Working principle of shielding gas

One of the main functions of all shielding gases is to isolate oxygen, nitrogen and water vapor in the air and protect the welding pool and electrodes. The shielding gas enters through the welding torch, sprays out from the welding nozzle, surrounds the electrode, replaces the air around the electrode, and forms a temporary shielding gas shield around the molten pool and arc. Both CO2 gas and Ar/CO2 mixed gas can achieve this purpose.

These shielding gases promote the formation of the arc plasma zone, which is the current channel for the welding arc. The type of shielding gas also affects the arc heat conduction and the magnitude of the arc force exerted on the molten pool. On these issues, the performance of CO2 and Ar/CO2 mixtures are not the same.

Characteristics of protective gas
CO2 and Ar react differently in arc heat. Analyzing these differences can help understand how the characteristics of each gas affect the welding process and welding deposits.

Ionization potential. The ionization potential is the amount of energy required to ionize the gas (for example, to convert the gas into a charged ion state), so that the gas can conduct electricity. The lower the ionization potential, the easier the arc will ignite and remain stable. The ionization potential of CO2 is 14.4eV, and the ionization potential of Ar is 15.7eV. Therefore, CO2 shielding gas ignites the arc more easily than Ar shielding gas.

Heat Conduction. The heat conduction of gas refers to the ability of gas to conduct heat energy. Its quality will affect the method of droplet transfer (such as jet transition and large droplet transition), arc shape, weld penetration and arc temperature distribution. CO2 gas has higher thermal conductivity than Ar gas and Ar/CO2 mixed gas.

Reactivity. The reactivity of the gas refers to whether the gas chemically reacts with the molten welding pool. Gas can be roughly divided into two categories: inert gas and active gas. Inert gas does not react with other elements in the welding pool. Ar is an inert gas. The reactive gas will combine with or react with other elements in the weld pool to form new compounds. At room temperature, CO2 is an inert gas, but in the arc plasma area, CO2 will be decomposed to form carbon monoxide (CO), oxygen (O2) and some independent oxygen atoms (O). Therefore, CO2 becomes an active gas under the arc and can oxidize with other metals. Ar/CO2 mixed gas is also an active gas, but its activity is lower than that of CO2.

When other welding specification parameters are the same, the welding fumes produced by different shielding gases are also different. Specifically, Ar/CO2 shielding gas produces less welding fumes than CO2 shielding gas, because CO2 is oxidizing. In addition, due to different welding occasions and welding sequence, the amount of welding fumes is different.

Introduction of inert gas
Although inert gas can provide protection for the weld pool, they are not suitable for iron-based metals (such as low carbon steel, low alloy steel, stainless steel, etc.) flux-cored wire gas shielded welding. For example, if only Ar is used as shielding gas to weld stainless steel, the weld performance will become very poor. This is because the use of inert gas protection will cause the arc length to increase and the outer steel skin of the electrode to melt prematurely. The arc range is increased and difficult to control, resulting in weld deposits. Therefore, when flux-cored wire gas shielded welding is used for welding iron-based base metals, a mixed gas shield of inert gas and active gas is usually used.
Introduction of CO2/Ar gas mixture

In North America, the welding of stainless steel flux-cored wire gas shielded welding often uses Ar/CO2 mixed gas as shielding gas, of which Ar accounts for 75% and CO2 accounts for 25%. Sometimes 80% Ar and 20% CO2 are also used, but this mixing ratio is not commonly used. Some gas-shielded flux-cored wires need to be protected with a mixture of 90% Ar and 10% CO2. However, if the Ar content in the mixed shielding gas is less than 75%, it will damage the arc performance, so it is necessary to ensure the percentage of Ar in the shielding gas. In addition, mixed gas tanks with non-standard Ar/CO2 percentage configurations are usually more difficult to obtain than standard percentage mixed gas tanks (such as 75%Ar/25%CO2 or 80%Ar/20%CO2).

Due to the active nature of CO2, when using Ar/CO2 mixed gas shielding for flux-cored wire shielded welding, the welding electrode alloy is deposited in the weld metal to a higher degree than using pure CO2 gas shielding. This is because CO2 reacts with the alloy to form oxides, which together with the oxides in the flux form slag. The core of the electrode must include some active elements, such as manganese (Mn) and silicon (Si), among others, it can also be used as a deoxidizer. Some of these alloys react with the free oxygen obtained by CO2 ionization to form oxides that stay in the slag instead of staying in the weld metal. Therefore, the content of Mn and Si in welding deposited metal using Ar/CO2 mixed gas is higher than that using CO2 gas protection.

The higher the content of Mn and Si in the weld deposited metal, the higher the weld strength and the lower the weld elongation, and the Sharp V-notch impact toughness will also change accordingly. Simply change the shielding gas from CO2 to Ar/CO2 mixed gas, the tensile and yield strength will increase by 7~10ksi, and the elongation will drop by 2%. It is very important to understand this. With the increase of Ar content in the shielding gas, the weld strength will increase and the toughness will decrease.

Since shielding gas will affect the final performance of the weld, AWS D1.1/D1.1M:2008, Steel Structure Welding Regulations stipulate a series of specific requirements to ensure the performance of the weld. For all welding, the choice of shielding gas must be consistent with AWS A5.32/A5.32M standards. The AWS classification of FCAW-G welding consumables (A5.20/A5.20M and A5.29/A5.29M) specifies the upper limit of the strength of the weld deposited metal. The selected welding shielding gas must ensure that the welding result does not exceed these specified upper strength limits, which also depends on the design of the welding rod and welding process. For the unmodified welding procedure specification, D1.1:2008 requires specific filler and shielding gas composition to support the test data.

D1.1:2008 entry 3.7.3 specifies two forms of support: one is the use of shielding gas for electrode classification purposes; one is that the filler metal manufacturer's data is consistent with AWS A5 requirements and is consistent with WPS requirements The protective gas is consistent. If these two conditions are not met, D1.1:2008 requires that mixed shielding gas be evaluated.

Classification of filler metals according to gas type

Since 2005, the flux cored metal classification of the American Welding Association has incorporated the shielding gas type into the classification symbol of the electrode. The AWS number of the low carbon steel FCAW-G electrode is EXXT-XX, and the last symbol refers to the type of shielding gas. If the last digit is C, it means that the protective gas is CO2; if it is M, it means Ar/CO2 mixed gas (for example, E71T-1C or E71T-1M). For low-alloy steel electrodes, the shielding gas symbol is the same as the final symbol of the deposited metal composition (such as E81T1-Ni1C). In contrast, self-shielded flux-cored electrodes do not require any shielding gas, and there is no shielding gas code in its classification number (such as E71T-8).

Some electrodes can only be protected with CO2. Some electrodes can only be protected with Ar/CO2 mixed gas. There are also some electrodes that can be protected by CO2 gas or Ar/CO2 mixed gas at the same time. In this case, the electrode must meet two classification requirements.

Selection of FCAW-G protective gas
When welding flux cored wire, whether to choose CO2 gas protection or Ar/CO2 mixed gas protection needs to consider the following three aspects.

1) Cost of protective gas

In general, 80% of the total welding cost is labor and management expenses, and 20% is the material cost. The cost of shielding gas accounts for about 1/4 of the material cost, or 5% of the total welding cost. Assuming that the cost of shielding gas is the only decisive factor, then the replacement of Ar/CO2 shielding gas with CO2 shielding gas can greatly reduce the welding cost. However, usually other costs also affect the total cost of welding, which will be discussed later.

CO2 is cheaper than Ar/CO2 because it can be obtained at low cost. The resources of CO2 in the world are extensive and abundant. CO2 can usually be obtained as a by-product of other processes. For the welding industry, on the one hand, CO2 can be obtained by processing or separation of natural gas, on the other hand, CO2 can also be obtained by air. Because the content of Ar in the atmosphere is less than 1%, a large amount of air needs to be processed and processed to extract a certain amount of Ar, and a special air separation device is required to process the air. Air separation devices consume a lot of electricity and need to be placed in special areas.

2) The influence of welder's preference and productivity

When welding with the same type and size of welding wire, the arc obtained by using Ar/CO2 shielding gas is smoother, weaker and less spattered than when welding with CO2 shielding gas, so it is deeply loved by welders. When welding with CO2 shielding gas, the welding arc easily generates large droplet transition (the droplet is usually larger than the diameter of the welding wire), resulting in unstable arc, discontinuous, and large splash. Ar/CO2 mixed gas protects the transition from splash droplets (the droplets are usually smaller than the wire diameter), resulting in a more stable and continuous arc with less spatter.

Another feature of Ar/CO2 mixed gas protection also increases the welders' preference for it. Compared with the use of CO2 shielded gas welding, its thermal conductivity is lower, so it can maintain the heat and liquidity of the molten pool. This can make the reaction of the molten pool more thorough, and the welding toe part of the weld seam can be melted more easily. When performing extraordinary position welding (such as uphill welding or overhead welding), the use of Ar/CO2 is more attractive, because the welder with poor technology can also control the arc and improve the welding productivity.

When Ar/CO2 mixed gas shielded welding is used, due to the higher Ar content, it emits more heat to the welder than CO2 shielded gas welding. This means that the welder feels hotter while welding. In addition, the welding torch will also be hotter (the duty cycle of the welding gun under Ar/CO2 shielding gas is lower than that of the CO2 shielding gas), which requires the use of a larger welding gun or the same type of welding gun and its vulnerable components. Replace more frequently.

3) Welding quality

As discussed earlier, the use of Ar/CO2 mixed shielding gas can maintain the heat and liquidity of the molten pool compared to welding with CO2 shielding gas, making the reaction of the molten pool more thorough, and the weld toe part of the weld is more easily melted full. Therefore, it greatly improves the weld forming ability and weld quality.

In addition, Ar/CO2 mixed gas protection has little spatter during welding, the quality of the weld seam is greatly improved, and the time and cost of cleaning after welding are reduced. The lower amount of spatter also improves the cost of ultrasonic weld inspection. If there is too much spatter, in order to ensure the accuracy of ultrasonic testing, the spatter must be cleaned in advance.

Another quality issue that affects the appearance of the weld is the sensitivity of the shielding gas to gas marks. Gas marks, defects similar to earthworm crawls or chicken scratches, are small grooves that are sometimes distributed on the surface of the weld. They are caused by dissolved gases in the weld metal. These gases are removed before the molten pool solidifies, but are trapped under the solidified slag.

Ar/CO2 mixed gas protection has higher gas trace sensitivity than simple CO2 gas protection. The splash transition characteristics of Ar/CO2 shielding gas resulted in a large amount of fine droplets, which increased the surface area of the droplets and caused the weld metal to dissolve a large amount of gas. In addition to the protective gas type will affect the sensitivity of gas marks, there are other factors, but they are not within the scope of this article. Commonly used protective gases for some major applications

For many years, the protective gas of FCAW-G on some major occasions has gradually formed a standard. For example, in high-welding welding applications of flat welding and horizontal welding, CO2 gas protection is usually used, because in these welding positions, Ar/CO2 mixed gas protection does not have much advantage.

The shipbuilding industry also generally likes to use CO2 gas protection, because the arc characteristics of CO2 gas protection better burn off the base material primer. In the offshore construction industry in North America, downward welding of T-shaped, Y-shaped and K-shaped bevel welds requires a smooth weld profile and small welding spatter, so Ar/CO2 mixed gas protection is more suitable. If more than one gas shielded welding process is used in the construction workshop, such as GMAW and FCAW-G, the shielding gas of the two processes is usually standardized. Sometimes, in order to obtain better spatter rate and pulsed arc transition, many manufacturers also choose Ar/CO2 mixed gas protection for GMAW welding.

Conclusion
When choosing a shielding gas for FCAW-G applications, one should not only consider the cost of the gas, but the three aspects discussed in this article. How does each gas type affect the total welding cost? Which kind of gas can reduce the cost per meter of weld? Some manufacturers have found that Ar/CO2 mixed gas protection can improve the quality and productivity of welds. Other manufacturers believe that the advantages of Ar/CO2 mixed gas shielded welding are not ideal or not as low as CO2 gas shielded welding. However, for other manufacturers, CO2 has a low cost, which is very suitable for some welding occasions. For users of the FCAW-G process, the choice of shielding gas should be based on how this gas affects the cost, quality, and productivity of the welding operation. Once the shielding gas is selected, the FCAW-G electrode should be suitable for welding with this gas.

Selection of shielding gas for flux-shielded wire gas shielded welding