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A - There are two shielding gases commonly used for arc welding aluminum, and these are argon and helium. These gases are used as pure argon, pure helium and various mixtures of both argon and helium.
Excellent welds are often produced using pure argon as a shielding gas. Pure argon is the most popular shielding gas and is often used for both gas metal arc and gas tungsten arc welding of aluminum. Mixtures of argon and helium are probably the next common, and pure helium is generally only used for some specialized GTAW applications.
When considering a shielding gas for welding aluminum, we need to consider the differences between argon and argon helium mixtures. In order to understand the effect of these gases on the welding operation, we can examine the properties of each gas in fig 1.
We can see immediately that the ionization potential and the thermal conductivity of the helium shielding gas is much higher than that of argon. These characteristics have the effect of producing greater heat when welding with additions of helium in the shielding gas.Shielding Gas for Gas Metal Arc Welding
For GMAW the additions of helium range from around 25% helium up to 75% helium in argon. By adjusting the composition of the shielding gas, we can influence the distribution of heat to the weld. This, in turn, can influence the shape of the weld metal cross section and the speed of welding. The increase in welding speed can be substantial, and as labor costs make up a considerable amount of our overall welding costs, this can relate to a potential for significant savings. The weld metal cross section can also be of some consequence in certain applications. Typical cross sections for argon and helium are shown in fig 2.
Tests have shown that the relatively narrow cross section of the pure argon shielded weld has a higher potential for gas entrapment and, consequently, can contain more porosity. The higher heat and broader penetration pattern of the helium/argon mixtures will generally help to minimize gas entrapment and lower porosity levels in the completed weld.
For a given arc length, the addition of helium to pure argon will increase the arc voltage by 2 or 3 volts. With the GMAW process, the maximum effect of the broader penetration shape is reached at around 75% helium and 25% argon. The broader penetration shape and lower porosity levels from these gas mixtures are particularly useful when welding double-sided groove welds in heavy plate. The ability of the weld bead profile to provide a wider target during back chipping can help to reduce the possibility of incomplete joint penetration that can be associated with this type of welded joint.
Pure argon shielding gas will typically produce a completed weld with a brighter, shinier surface appearance. A weld made with a helium/argon mixture would usually require post weld wire brushing to obtain a similar surface appearance. Because of aluminum’s high thermal conductivity, incomplete fusion can be a likely discontinuity. Helium shielding gas mixtures can help to prevent incomplete fusion and incomplete penetration because of the extra heat potential of these gases.Shielding Gas for Gas Tungsten Arc Welding
When considering the shielding gas for gas tungsten arc welding with alternating current (AC), pure argon is the most popular gas used. Pure argon will provide good arc stability, improved cleaning action, and better arc starting characteristics when AC - GTAW aluminum.
Helium / argon mixtures are sometimes used for their higher heat characteristics. Gas mixtures, usually 25% helium and 75% argon are sometimes used and can help to increase travel speeds when AC - gas tungsten arc welding. Mixtures of more than 25% helium for AC – gas tungsten arc welding are used, but not often, as they can tend to produce instability, under certain circumstances, in the AC arc.
Pure helium or high percentages of helium (He-90%, Ar-10%) shielding gas are used primarily for gas tungsten arc machine welding with direct current electrode negative (DCEN). Often designed as seam welders, the combination of GTAW - DCEN and the high heat input from the gas used can provide fast welding speeds and outstanding penetration. This configuration is sometimes used to produce full penetration butt welds, welded from one side only, onto temporary baking with no vee-groove preparation, just a square edged plate.
Conclusion:In answer to your questions, there are a number of choices available for gases and gas mixtures that can be used to weld aluminum. The choice is usually based on the specific application. Generally speaking, the high helium content gases are used for GMAW welding on thicker materials and GTAW welding with DCEN. Pure argon can be used for both GMAW and GTAW welding and is the most popular of the shielding gases used for aluminum. The helium content gases are usually more expensive. Helium has a lower density than argon and higher flow rates are used when welding with helium. It is possible to increase welding speeds in some circumstances by using helium and/or helium/argon mixtures. Therefore, the extra cost of the helium mixtures may be offset by your improved productivity. You should try the different gas types and choose the one that best suites your specific application.