An introduction to TIG welding

Tungsten inert gas welding is a welding technique that builds on the principles of conventional arc welding; it uses argon gas to protect the metal filler material. A high current is applied to melt the filler material into a liquid state on the base material being welded, thereby creating a pool that allows for metallurgical bonding between the metal to be welded and the filler material. Since argon gas is continuously supplied during the high-temperature melting process, the filler material does not come into contact with oxygen in the air, which prevents oxidation of the filler material. As a result, this technique can be used to weld non-ferrous metals such as copper, aluminum, and alloy steel.

Gas shielded arc welding, abbreviated as gas shielded welding or GMAW, is a fusion welding process that uses an arc as the heat source and gas as a shielding medium. During welding, the shielding gas creates a gas shield around the arc, separating the arc and the molten pool from air, preventing the effects of harmful gases and ensuring stable arc combustion. Depending on the specific circumstances, different gases can be used for gas shielded welding. Common shielding gases include carbon dioxide, argon, helium, hydrogen, and mixed gases. The advantages of gas shielded welding are: a good arc trajectory, easy centering, and the ability to perform welding in all positions as well as automated welding ； The arc heat is concentrated, resulting in a small molten pool; welding speed is high, the heat-affected zone is narrow, deformation of the welded parts is minimal, crack resistance is strong, and the quality of the weld is good. The downside is that welding is not suitable in windy areas, as the arc light radiation is strong. TIG welding is divided into two types based on the electrode used: gas tungsten arc welding with a consumable electrode and gas tungsten arc welding without a consumable electrode. 1. Working principle and characteristics of gas tungsten arc welding Without melting the electrode, gas tungsten arc welding involves an arc burning between a non-melting electrode (usually a tungsten electrode) and the workpiece. An inert gas that does not react chemically with metals (often argon) flows around the welding arc, creating a protective gas shield that keeps the tip of the tungsten electrode, the arc, the molten pool, and the metal at high temperatures away from contact with air, thereby preventing oxidation and the absorption of harmful gases. Thus, a dense weld joint is formed, which has excellent mechanical properties. The characteristics of tungsten inert gas welding are as follows. 　　(1) It can weld metals and alloys with highly reactive chemical properties. The inert gases argon or helium do not react chemically with chemically active metals such as aluminum, titanium, magnesium, copper, nickel, and their alloys, even at high temperatures; nor do they dissolve in liquid metals. Welding methods protected by slag (such as shielded metal arc welding or submerged arc welding) find it difficult to weld these materials, or it is even impossible to weld them. 　　(2) Welded joints with good mechanical properties can be obtained. The weld metal obtained using this welding method has high purity, few gas and gas-metal inclusions, and few welding defects. This welding method is commonly used to weld low-carbon steel, low-alloy steel, and stainless steel, where high requirements are placed on the quality of the weld metal. 　　(3) Suitable for welding thin parts and small components. (4) It can be welded on one side with double-sided formation and welded in all positions. 　　(5) Low welding productivity. 　　The welding current used in tungsten inert gas welding is limited by the current-carrying capacity of the tungsten electrode; as a result, the arc power is low, the arc penetration is weak, the weld depth is shallow, and the welding speed is slow. Additionally, the tungsten electrode needs to be replaced frequently during the welding process. 2. Working principle and characteristics of gas metal arc welding: The welding wire is fed in through a wire feeder, while the electrode tip serves to conduct electricity; an arc is generated between the base material and the welding wire, causing both to melt. An inert gas, argon, is used to protect the arc and the molten metal during the welding process. The difference between it and TIG welding is that in the former, the welding wire serves as the electrode, which is continuously melted and fed into the weld pool; after cooling, it forms the weld seam ； The other is the shielding gas; with the application of gas tungsten arc welding technology, shielding gases have evolved from pure argon to a wide range of mixed gases, such as an argon-rich shielding gas composed of 80% Ar and 20% CO2. Generally, the former is called MIG, and the latter is called MAG. In terms of their operation methods, the most widely used ones at present are semi-automatic GMAW and welding with an argon-enriched gas shield, followed by automatic GMAW. 　　Compared with tungsten inert gas welding, gas metal arc welding has the following characteristics. 　　(1) High efficiency: due to its high current density, heat is concentrated, resulting in a high deposition rate and fast welding speed. Additionally, it is prone to arcing. 　　(2) Enhanced protection is required. Due to the intense arc light and large amount of smoke, enhanced protection measures are necessary. 3. Protective gas 　　(1) The most commonly used inert gas is argon. It is a colorless, odorless gas; its concentration in air is 0.935% by volume. Argon has a boiling point of -186°C, which lies between the boiling points of oxygen and helium. Argon is a by-product of oxygen production in oxygen plants, which is obtained through the distillation of liquid air. Bottled argon is used for welding in our country, with a filling pressure of 15 MPa at room temperature. The gas cylinder is painted gray and marked with the word “Argon”. The chemical composition requirement for pure argon is: Ar≥99.99% ； He≤0.01％ ； O2≤0.0015% ； H2≤0.0005% ； Total carbon content ≤ 0.001% ； Moisture content ≤ 30 mg/m3. 　　Argon is an ideal shielding gas; it is 25% denser than air, which helps to protect the welding arc during flat welding and reduces the consumption of shielding gas. Argon is a gas with extremely low chemical reactivity; it does not react chemically with metals even at high temperatures, thus eliminating the oxidation and burnout of alloying elements and the various problems that arise therefrom. Argon is also insoluble in liquid metal, so it does not cause porosity. Argon is a monatomic gas that exists in atomic form; at high temperatures, there is no molecular decomposition or atomic heat absorption. Argon has a low specific heat capacity and thermal conductivity, which means it absorbs little heat itself and also transfers little heat outward. As a result, the heat in the arc is not easily dissipated, allowing the welding arc to burn steadily with heat concentrated, which facilitates the welding process. 　　The disadvantage of argon is its high ionization potential. When the arc space is filled with argon, it is difficult to ignite the arc, but once ignited, the arc remains very stable. 　　(2) Helium (He). Helium is present in very small amounts in air, accounting for only 0.0005% by volume, and its density is about 1/10 that of argon. Therefore, to achieve good protection, the flow rate must be increased. 　　When protected with helium, the arc voltage is much higher than that with argon, and the heat generation of the helium arc is significantly greater than that of the argon arc. Therefore, helium shielded welding can be used to weld thick workpieces and materials with good thermal conductivity, such as copper and copper alloys, and it is also employed for the high-speed, mechanized welding of stainless steel pipes. 　　However, the cost of helium extraction is high, so its application is limited. 　　(3) Mixed gas. Adding a small amount of another gas or two gases to a gas can be beneficial in refining the droplets, reducing spatter, improving arc stability, altering the penetration depth, and raising the arc temperature. Therefore, gas shielded welding with an argon-based mixed gas is widely used, such as Ar 80% + CO2 (5–20%)، Ar 95% + O2 (1–5%)， Ar 80% + N2 20%， Ar + H2， Ar + He， Ar 80% + CO2 15% + O2 5%, etc. 4. Non-melting electrodes (1) Requirements for electrode materials in gas shielded welding with non-melting electrodes ① Ability to withstand high temperatures, without melting during the welding process. 　　②The electrode must have a high electron emission capability, and must facilitate arc initiation as well as the stable combustion of the arc. 　　Based on these requirements, tungsten is a relatively ideal electrode material. 　　(2) Characteristics of common tungsten electrode materials. The non-fusing electrode materials used in tungsten inert gas welding include pure tungsten electrodes, thorium-tungsten electrodes, cerium-tungsten electrodes, lanthanum-tungsten electrodes, zirconium-tungsten electrodes, yttrium-tungsten electrodes, etc. The first three of them are the most common. 　　①The pure tungsten electrode is the non-fusing electrode with the longest history of use. However, it has some disadvantages: first, its electron emission capability is poor, requiring a high no-load voltage from the power supply ； Second, it has poor resistance to burnout, a short service life, and requires frequent replacement and regrinding of the tungsten tips. It is currently mainly used for AC welding of aluminum, magnesium, and their alloys, taking advantage of its excellent ability to break down oxide films. 　　②Thorium-tungsten electrodes are formed by adding a certain amount of thorium oxide (ThO2) to tungsten. It has a high electron emission capability, requires a low arc voltage, makes arc initiation easy and stable, **and extends the service life of the tungsten electrode. However, thorium oxide (THO2) has trace radioactivity. 　　③Cerium-tungsten electrodes are made by adding less than 2% cerium oxide (CeO) to tungsten. Its main features are: no radioactivity, increased allowable current, strong thermoelectron emission capability, stable arc, concentrated heat generation, long service life, and easy maintenance of the tip shape. 　　5. Type and polarity of current Argon arc welding can be performed using either direct current or alternating current. When using direct current, the positive polarity of DC is the most widely applied. The characteristics of the arc vary significantly depending on the type and polarity of the current. 　　(1) DC reverse polarity gives rise to two extremely important physical phenomena, namely “cathode breakdown” and “tungsten electrode overheating”. 　　①Cathode crushing effect. When the current is in reverse polarity in DC mode, since the workpiece acts as the cathode, positive ions in the arc space move toward the welding pool and its surrounding areas. The heavier positive ions collide with the surface of the welding pool with great force, releasing a large amount of energy. The energy released when positive ions collide with the cathode is greater than the energy released when electrons collide with the anode surface. Under the impact of positive ions, the oxide film on the metal surface is damaged; it may even decompose and evaporate, leaving the base material surface near the liquid metal clean and shiny. After cooling, there is no oxide film on the weld surface, giving it an attractive appearance. This is cathodic disintegration, which is widely used in the welding of metals with highly reactive chemical properties, such as aluminum, magnesium, and their alloys. 　　②Tungsten electrode overheating: Since the tungsten electrode acts as the anode, electrons strike it at high speeds, generating a large amount of heat that raises the temperature of the electrode. This reduces its service life; therefore, it is rarely used except for welding aluminum-magnesium alloys. 　　(2) Direct current positive polarity 　① The workpiece acts as the positive pole; all the energy released upon electron bombardment is converted into heat. This results in a deep but narrow welding pool, which facilitates metal bonding. Welding internal stresses and deformation are minimal, and the welding efficiency is high. 　　②Tungsten electrodes are not prone to overheating, have a long service life, and can handle high current values. 　　③Tungsten electrodes have a strong ability to emit electrons, resulting in a stable arc. 　　④There is no cathode breakdown effect, so it cannot be used to weld aluminum, magnesium, and their alloys; however, it is widely used for welding carbon steel, low-alloy steel, stainless steel, nickel-based alloys, titanium alloys, copper alloys, and others.

Gas shielded arc welding in which pure tungsten or activated tungsten (thorium-tungsten, cerium-tungsten, zirconium-tungsten, lanthanum-tungsten) is used as the non-melting electrode; abbreviated as TIG welding. Gas shielded arc welding is a welding method that uses an arc as a heat source and gas to protect the weld pool. The role of gas is primarily to protect the molten metal from harmful elements in the air such as oxygen, nitrogen, and hydrogen, as well as from moisture. However, it also has an impact on the stability of the arc, the form of droplet transfer, and the activity of the molten pool. Therefore, the use of different gases results in different metallurgical reactions and process effects. The main features of gas shielded arc welding are a visible arc, a small molten pool, ease of mechanization and automation, and high productivity. Welding robots that developed rapidly in the 1970s were mainly used for resistance spot welding and gas-shielded arc welding. Gas shielded arc welding is suitable for welding metals such as steel, aluminum, and titanium. It is widely used in the manufacturing of products like automobiles, ships, boilers, pipelines, and pressure vessels, especially in applications where high quality or welding in all positions is required. Gas shielded arc welding can be classified into tungsten inert gas shielded welding and metal inert gas shielded welding according to the type of electrode. Tungsten inert gas welding is abbreviated as TIG welding. A tungsten rod is used as the electrode, with argon or helium as the shielding gas. The base material is melted by an arc to form a joint; filler wire can also be added if necessary (Figure 1: Tungsten Inert Gas Shielded Welding). Tungsten inert gas welding is characterized by a stable arc and easy control of the input energy. Therefore, it is often used for welding workpieces that require high dimensional accuracy, whose materials are prone to overheating and embrittlement, and that are susceptible to oxidation in air. GMAW uses a continuously fed wire as the electrode, with argon, carbon dioxide, or a mixed gas serving as the shielding gas (Figure 2: GMAW). Compared with tungsten inert gas welding, this type of welding offers higher productivity and is more widely used; it ranks only after shielded metal arc welding and submerged arc welding, and shows a trend of further development. The welding wire can be either solid wire or flux-cored wire. Melted electrode gas shielded welding can be further divided into inert gas shielded welding, carbon dioxide gas shielded welding, and mixed gas shielded welding, depending on the type of shielding gas used. ①GMAW: Argon or helium is used as the shielding gas. Inert shielding gases do not participate in the metallurgical reactions within the molten pool; they are suitable for welding various metal materials with high quality requirements or those that are prone to oxidation, such as stainless steel, aluminum, titanium, zirconium, etc., but they are costly. ②Carbon dioxide gas shielded welding: uses carbon dioxide as the shielding gas. At high temperatures, carbon dioxide decomposes to release oxygen, which enters the molten pool; therefore, appropriate amounts of deoxidizing agents such as manganese and silicon must be added to the welding wire. The main advantage of this shielded welding method is its low cost, but it can only be used for welding carbon steel and low-alloy steel. ③Mixed gas shielded welding: The shielding gas is primarily argon, with an appropriate amount of carbon dioxide (15–30%) or oxygen (0.5–5%) added. Compared with carbon dioxide gas shielded welding, this type of shielded welding has a wider range of welding parameters, better weld shape, and higher quality ； Compared with GMAW, the molten pool is more active, resulting in better metallurgical reactions.