What shielding gas should be used when brazing aluminum alloys?

What shielding gas should I use when brazing aluminum alloys in a vacuum brazing furnace? Some suggest argon, while others say helium is best. Can the same gas be used for both processes?

Two shielding gases are commonly used for brazing aluminum: argon and helium. These gases include pure argon, pure helium, and argon-helium mixtures.

Using pure argon as the shielding gas generally produces high-quality welds. Pure argon is the most commonly used shielding gas and is commonly used for gas metal arc brazing and gas tungsten arc brazing of aluminum. Argon and helium mixtures are probably the second most common, while pure helium is usually only used for specialized gas tungsten arc brazing applications.

Shielding Gas for Gas Metal Arc Brazing
For gas metal arc brazing, the amount of helium added ranges from 25% to 75% of the argon. By adjusting the shielding gas composition, we can influence the heat distribution of the weld. This, in turn, affects the shape of the weld metal cross-section and the welding speed. Increased welding speeds can be significant, and since labor costs contribute significantly to our overall welding costs, this can represent significant savings potential. In some applications, the cross-sectional area of ​​the weld metal also plays a role.

The relatively narrow cross-section of pure argon shielded welds is more susceptible to gas entrapment and, therefore, higher porosity. The higher heat and wider penetration pattern of a helium/argon mixture generally help minimize gas entrapment and reduce porosity in the weld.

The Effect of Adding Helium

Pure argon shielding gas generally produces brighter, more shinny welds. Welds made with helium/argon mixtures often require post-weld brushing to achieve a similar surface appearance. Due to the high thermal conductivity of aluminum, lack of fusion can result in discontinuities. The additional thermal potential of a helium shielding gas mixture helps prevent both lack of fusion and incomplete penetration.

Shielding Gases for Gas Tungsten Brazing

When considering shielding gases for alternating current (AC) gas tungsten brazing, pure argon is the most commonly used shielding gas. In alternating current (AC) gas tungsten arc brazing (GTAW) of aluminum, pure argon provides good arc stability, improved cleaning, and better starting characteristics.

Helium/argon mixtures are sometimes used due to their higher thermal conductivity. A mixed gas, typically 25% helium and 75% argon, is sometimes used to increase brazing speeds with AC GTAW.

Pure helium or a high helium concentration (90% helium, 10% argon) shielding gas is primarily used for direct current negative electrode (DCEN) GTAW. The combination of GTAW and DCEN, along with the high heat input of the gases used, is often designed for seam welding, providing fast welding speeds and excellent penetration. This configuration is sometimes used for full-penetration butt welds on temporary bake plates, welding from only one side without V-groove preparation and using a square-edged plate.

Conclusion
To answer your question, there are many options for gases and gas mixtures used for welding aluminum. The choice often depends on the specific application.