Types and characteristics of mixed gases in welding

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The commonly used mixed gases include the following: 1. Ar+He. The advantage of argon is that the arc combustion is very stable with minimal spattering. The advantages of helium gas are a high arc temperature, large heat input into the base metal, and fast welding speed. 2. Using argon as the base gas and adding a certain amount of helium can yield the advantages of both. When welding thick aluminum and aluminum alloys, using an Ar+He mixed gas can improve weld penetration, reduce porosity, and increase productivity. When the plate thickness is 10–20 mm, the volume fraction of He is 50% ； When the plate thickness exceeds 20 mm, He with a volume fraction of 75%–90% is added. When welding copper and copper alloys, an Ar+He mixed gas can improve the wetting of the weld seam and enhance the quality of the weld. His proportion is generally 50% to 75% (by volume). 2. Ar+H2: Adding H2 to argon can increase the arc temperature and raise the heat input to the base metal. When welding stainless steel using a TIG arc or plasma arc, H2 is often added to argon at a volume fraction of 4% to 8% in order to increase the welding speed. The reducibility of the Ar+H2 gas mixture can be utilized to weld nickel and its alloys, in order to suppress and eliminate CO pores in the nickel welds. However, the content of H2 added (by volume fraction) must be below 6%, otherwise it will cause hydrogen pores to form. 3. Ar+N2: When N2 is added to Ar, the temperature of the arc is higher than that in pure argon; this mixture is mainly used for welding copper and copper alloys. Compared with the Ar+He mixture, its advantage lies in the fact that N2 is readily available at a lower cost. The disadvantages are spatter during welding, a rough weld surface, and some smoke generated during the welding process. 4. There are two types of Ar+O2 mixtures: one with a low oxygen content (by volume), ranging from 1% to 5%, which is used for welding stainless steel ； Another type has a higher oxygen content (by volume), which can exceed 20%, and is used for welding low-carbon steel and low-alloy structural steel. Adding 1% by volume of O2 to pure argon for welding stainless steel can overcome the instability of the arc cathode spot (cathode drift) that occurs when using pure argon for welding stainless steel. Cathode drift: The arc itself is characterized by jumpiness and stickiness. In the case of ferrous metals, whose oxide layers on the surface are not as dense as those of alloys such as magnesium and aluminum, the arc always seeks out oxide sites on the microscopic surface of the metal to aggregate around. Due to the jumpiness and stickiness of the arc, it becomes discontinuous, which leads to arc displacement, or cathode drift, and thus results in arc instability. The addition of a certain amount of oxygen causes the metal surface to oxidize during welding, thereby ensuring the stability of the welding arc. 5. Ar+CO2 is widely used in welding carbon steel and low-alloy structural steel; it can improve the impact toughness of the weld metal and reduce spatter. 6. A mixture of Ar, CO2, and O2 can be used to weld low-carbon steel and low-alloy structural steel, delivering good results in terms of weld shape, joint quality, droplet transfer, and arc stability.