Shielding Gas Purity for Aluminium welding

Date 07-15-2016 00:35

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?

By Lawrence (*****)

Date 07-15-2016 14:02

Welcome to the forum Psalm !

That very small difference is notable "in my opinion" only when the highest order of inspection criteria is involved.

I think in most typical aluminum cases... "you shall not want"

By waqasmalik (**)

Date 07-16-2016 16:08

The difference between 99.99 and 99.999 is the resolution of the equipment which measures purity. The former has value in two digits after point and latter has in three. You cant say one is pure than other, depends on instrument.

By Psalm23 (*)

Date 07-18-2016 08:24

Actually the EINECS or EC classify the gas purity by 99.99%, 99.999%, 99.9999%,etc.. so depending on the maker of which level of gas purity, they will use accordingly the appropriate gas analyzer sensitivity (resolution). Not the other way around.

By Psalm23 (*)

Date 07-18-2016 07:04

Thanks Lawrence, thats right, it does seem "i shall not want" the 99.99%, but im looking for objective reference to back it up in lieu of an actual verification test since this cannot be ordered in few cylinder quantity and will take at least 2 months.

By Lawrence (*****)

Date 07-18-2016 08:38

I can't think of anything more objective than tens of thousands of welding procedures, all around the world, every day, being completed to a variety of standards, year after year.

By Psalm23 (*)

Date 07-18-2016 23:57

do you mean all those are using the 99.999% purity?

By fschweighardt (***)

Date 07-19-2016 12:14

If you are working to D1.2, the requirements in that standard supercede the A5.32 purity requirements. Also, your initial comments about 5PPM CO2 would result in an global purity of 99.9995%, if CO2 is the only impurity

By Psalm23 (*)

Date 07-19-2016 23:55

It does narrow down to 99.999% since CO2 is not the only impurity being considered. In the mix also are even higher tolerance for H2O and O2. Yes, will have to follow the D1.2:1997, since D1.2:2014 and A5.32L:2011 doesnt say it supersedes D1.2:1997 shielding gas requirement. Thanks guys. :smile:

By fschweighardt (***)

Date 07-20-2016 01:01

Do you have a customer defined edition of tbe d1.2 you have to work to, or are you setting up procedures on your own

By Psalm23 (*)

Date 07-20-2016 08:22 Edited 07-20-2016 08:52

I will be recommending our internal shielding gas quality specs. A guideline for our buying or supply contract since we've experienced weld oxidation issues lately like excessive smut and discoloration, which i suspect was due to CO2 (since the O2 is within spec).

By MBSims

Date 07-30-2016 00:18

I thought 99.999% (ultra high purity) was only needed for straight polarity welding of aluminum.

By 803056

Date 07-30-2016 14:19 Edited 08-03-2016 13:06

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