Shielding Gases and Mixtures
Air in the weld zone is displaced by a shielding gas in order to prevent contamination of the molten weld puddle. This contamination is caused mainly by nitrogen, oxygen and water vapor present in the atmosphere. As an example, nitrogen in solidified steel reduces the ductility and impact strength of the weld and can cause cracking. In large amounts, nitrogen can also cause weld porosity. Excess oxygen in steel combines with carbon to form carbon monoxide (CO). This gas can be trapped in the metal, causing porosity. In addition, excess oxygen can combine with other elements in steel and form com- pounds that produce inclusions in the weld metal. When hydrogen, present in water vapor and oil, combines with either iron or aluminium, porosity will result and ”underbead” weld metal cracking may occur. To avoid these problems associated with contamination of the weld puddle, three main gases are used for shielding. These are argon, helium and carbon dioxide. In addition, small amounts of oxygen, nitrogen and hydrogen have proven beneficial for some applications. Of these gases, only argon and helium are inert gases. Compensation for the oxidizing tendencies of other gases is made by special wire electrode formulations. Argon, helium and carbon dioxide can be used alone, in combinations or mixed with others to provide defect free welds in a variety of weld applications and weld processes.
The basic properties of shielding gases that affect the performance of the welding process include:
1) Thermal properties at elevated temperatures.
2) Chemical reaction of the gas with the various elements in the base plate and welding wire.
3) Effect of each gas on the mode of metal transfer.
The thermal conductivity of the gas at arc temperatures influences the arc voltage as well as the thermal energy delivered to the weld. As thermal conductivity increases, greater welding voltage is necessary to sustain the arc. For example, the thermal conductivity of helium and CO2 is much higher than that of argon; because of this, they deliver more heat to the weld. Therefore, helium and CO2 require more welding voltage and power to maintain a stable arc. The compatibility of each gas with the wire and base metal determines the suitability of the various gas combinations. Carbon dioxide and most oxygen bearing shielding gases should not be used for welding aluminium, as aluminium oxide will form. However, CO2 and 02 are useful at times and even essential when GMAW welding steels. They promote arc stability and good fusion between the weld puddle and base material. Oxygen is a great deal more oxidizing tham CO2. Consequently, oxygen additions to argon are generally less than 12 percent by volume whereas 100 percent CO, can be used for GMAW mild steels. Steel wires must contain strong deoxidizing elements to supress porosity when used with oxidizing gases, particularly mixtures with high percentages of CO2 or 02 and especially 100 percent CO2. Shielding gases also determine the mode of metal transfer and the depth to which the workpiece is melted (depth of penetration). Spray transfer is not obtained when the gas is rich in CO2. For example, mixtures containing more than about 20 percent CO2 do not exhibit true spray transfer. Rather, mixtures up to 30 percent CO2 can have a ”spray-like” shape to the arc at high current level but are unable to maintain the arc stability of lower CO2 mixtures. Spatter levels will also tend to increase when mixtures are rich in CO2.
Binary (2) Shielding Gas Mixtures
The addition of srnall amounts of 02 to argon greatly stabilizes the weld arc, increases the filler metal droplet rate, lowers the spray arc transition current, and improves wetting and bead shape. The weld puddle is more fluid and stays molten longer allowing the metal to flow out towards the toe of the weld. This reduces undercutting and helps flatten the weld bead. Occasionally, small oxygen additions are used on non- ferrous applications. For example, it’s been reported by NASA that .1% oxygen has been useful for arc stabilization when welding very clean aluminium plate. Argon-1% O2 – This mixture is primarily used for spray transfer on stainless steels. One percent oxygen is usually sufficient to stabilize the arc, improve the droplet rate, provide coalescence and improve appearance. Argon-2% O2 – This mixture is used for spray arc welding on carbon steels, low alloy steels and stainless steels. It provides additional wetting action over the 1% 02 mixture. Mechanical properties and corrosion resistance of welds made in the 1 and 2% 02 additions are equivalent. Argon-5% O2 – This mixture provides a more fluid but controllable weld pool. It is the most commonly used argon-oxygen mixture for general carbon steel welding. The additional oxygen also permits higher travel speeds.
Ternary (3) Shielding Gas Mixtures
Mixtures containing these three components have been termed ”universal” mixtures due to their ability to operate using short circuiting, globular, spray, pulse and high density type transfer characteristics. Several triple-mixes are available and their application will depend on the desired metal transfer mechanism and optimization of the arc characteristics.
Quaternary (4) Shielding Gas Mixtures
Argon-Helium - Co2- O2 Commonly known as a ”quad mix”, this combination is most popular for high deposition GMAW using the high density metal transfer type arc characteristic. This mixture will give good mechanical properties and operability throughout a wide range of deposition rates. Its major application is welding low alloy high tensile base materials but has been used on mild steel for high productivity welding. Weld economics are an important consideration in using this gas for mild steel welding, in that other less expensive mixtures are available for high deposition welding. Regardless of the type of welding that need to be done, there is a shielding gas that will best suit the requirements.
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