Purging AL-6XN Tack Welds

THE IMPORTANCE OF PURGING WHEN WELDING AL6XN

AL-6XN alloy is selected for its high corrosion resistance. This corrosion resistance is obtained by the formation of a very thin, stable, oxide film that is rich in chromium. This film forms at normal temperature and pressure. To maintain the corrosion resistance, it is very important to protect the chromium oxide film from contamination during the fabrication process. This is why we must purge the back side of the weld in AL-6XN material. Purging prevents the formation of oxides and, by doing so, protects the material from losing its corrosion resistance.

HOW TO PURGE:
  1. Apply an argon shielding gas to the inside diameter (ID) of the assembly. To do this, both ends of the tube should be plugged. The gas purging time will vary depending on the volume of atmosphere that needs to be evacuated. Allow enough gas-flow time to exchange four times the original held volume. There are other methods that can be used to ensure that the ID of the assembly has been properly purged for high purity applications. In some cases, the escaping gas should be monitored with an oxygen meter.
  2. To avoid any other oxidation, purging time should continue until the weld seam cools down.
  3. Skin tacks should be very light to avoid penetrating the wall of the tubing. Full penetration tacks require a high purity of purging gas and should be avoided.