Hello Welding Experts!
Appreciate your inputs here.
We are using Argon-Helium mixture shielding gas to weld aluminium.
I recently bought AWS A5.32M:2011, Gases & Gas Mixtures for Fusion Welding. Table 4 Minimum requirement on Purity & Moisture states Inert gas 99.99% purity with a foot note stating “For certain applications a higher purity and lower dew point may be recommended to avoid possible oxidation and contamination.”
However, the old D1.2:1997 Structural Welding Code-Aluminium, states minimum purity for Argon is 99.998% and Helium 99.995%.
So the two purity level choices are 99.99% or 99.999% which have significant cost difference.
Does anybody have idea what happens to the aluminium weld quality if the Argon and Helium shielding gas CO2 level is at 5ppm, i.e. 99.99% purity?
Smut along the sides of the aluminum weld is usually the result of the magnesium in the filler metal vaporizing in the arc, cooling and falling to the surface of the base metal to either side of the weld. It appears black because the vary small nodules of magnesium absorb the light, thus appearing black. The selection or purity of the argon or argon helium mix will have little influence on the appearance of "smut" along side of the welds.
The purity of the shielding gas, i.e., the difference between 99.99 or 99.999 percent purity, will have very little influence in the appearance of the weld. What will be affected is the strength of the joint. Aluminum is a very effective deoxidizer. Any moisture in the shielding gas will disassociate in the arc into the basic chemical components, i.e., oxygen, carbon, or hydrogen. The hydrogen will manifest itself as porosity that appears silver on the inside surfaces if the specimen is broken open. The oxygen will oxidize the aluminum and result in brittle aluminum oxides in the weld. Aluminum oxide has about the same density as the base metal so it stays in solution, i.e., it doesn't float out of the weld. The oxide also has a higher melting temperature than the molten aluminum, so it doesn't decompose. If the gas is sufficiently contaminated with moisture (too high a dew point), it will not achieve the required mechanical properties when subjected to tensile testing or it will fail the guided bend tests. It is doubtful there will be visual evidence of a problem when examining the welds unless the shielding gas is not in compliance with the purity requirements of AWS A5.32.
If there is a concern for the visual attributes, i.e., discoloration or discolored porosity, I would look for other issues such as leaks in the system or insufficient cleaning prior to welding, not the purity (low moisture) of the shielding gas.
Al