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Up Topic Welding Industry / Inspection & Qualification / Porosity in aluminum PQR RT
- - By BryantB (*) Date 10-08-2020 16:51 Edited 12-04-2020 16:18
Parent - By 803056 (*****) Date 10-10-2020 00:08
Porosity is usually associated with hydrogen. You need to be very fastidious about cleaning to remove all hydrocarbon or other potential sources of hydrogen. That would include something often overlooked, the wipes used to degrease the aluminum. Any lint from paper, cotton, etc. are sources of hydrogen the resulting porosity it causes.
The gloves the welder wear should be chrome tanned leather rather than oil tanned work gloves. Don’t handle the filler rod with the bare hands.
Don’t use any abrasives that utilize aluminum oxide (this includes “ScotchBrite”.
Just a few of my favorite things to keep in mind.

Parent - - By Jim Hughes (***) Date 10-14-2020 15:27
what welding process are you using? It really depends on the welding process you are using. If its GMAW then using 100% Argon tends to have more porosity, and depending on what code you are grading the film to can pose a problem. Using Helium/Argon mix not as much. If your using AC TIG then, cleaning, high feq. intensity and pre-heat could be an issue, using a higher pre-heat temp can help a lot, if you are using TIG DCEN then we are back to shielding gas. (TIG DCEN with Helium is used for thicker material greater than 1/4")
Parent - - By Lawrence (*****) Date 10-15-2020 15:12
Porosity in aluminum is always hydrogen..... No matter if GMAW, GTAW, or any other process.... Always hydrogen

How the hydrogen gets into (or more accurately doesn't get out of" the weld is the big mystery we each must solve.

Oxides can also sometimes be misconstrued as porosity on RT... especially when the highest standards for defect size are in place,,,, But it really doesn't matter as best practices will eliminate both oxides and hydrogen from the finished weld.

I very strongly but respectfully disagree with Mr. Hughs...   Preheat should be avoided in all but the most special cases in aluminum welding...  

Molten Aluminum is "thirsty" for hydrogen and will drink it up any way it can (laymens terms here)  Meaning the longer the molten pool is exposed to any kind of atmosphere, the more hydrogen the weld drinks up....  When the weld cools the hydrogen is forced out, unless there is so much of it that the pool freezes before the hydrogen escapes.    

So with GTAW a slow travel speed often produces a weld that has a "grainy" bumpy surface on the crown and especially at terminations... That is the little hydrogen bubbles that were racing to the top but didn't make it....  Those little bits of roughness can be through the entire thickness of the weld if the operator is going too slowly.    In the 21st century, welding power supplies are available that can bring heat to the weld zone fast, so the operator does not have to "wait" for the base metal to heat up....  If you cannot do this, you have the wrong equipment to do work that has this kind of high inspection criteria.

As Al mentioned, there can be other contaminations that result in indications that appear like poroisity,,,,   Aluminum oxides from abrasives, hydrocarbons in solvents, paints etc.    Castings that have not had adequate prep will offer indications like this as well, especially if the surfaces are not dealt with.

The Cliff notes:   Clean       HOT       FAST

No matter which process you choose... Those three things are the key to excellent RT results in an aluminum weld.

We have not even scratched the surface of how preheat can expand the degraded performance zone in heat treatable aluminum alloys and the very real possibility that an acceptable RT on an aluminum weld that was produced with too much heat input and preheat can have only 50% of the strength that was designed for the joint and proven in the PQR.    If there was no preheat in the PQR (meaning a real engineer did not demand preheat) don't do it on a weldment that has a high standard for inspection criteria or performance.
Parent - - By BryantB (*) Date 10-16-2020 14:19 Edited 12-04-2020 16:18
Parent - By 803056 (*****) Date 10-17-2020 02:18
Generally, hydrogen will effuse into the environment, but holding the weldment at a slightly elevated temperature will allow more hydrogen to escape. That doesn't eliminate any porosity that has already formed during welding. It should improve the tensile test results if past testing has revealed evidence of fish eyes on the fractured surface. This is more common with steels.

- - By 262CWI Date 10-18-2020 18:03
Bryant pay special attention to the advice given by two of the most knowledgeable giants in the industry (AL & Lawrence) when they speak "you listen".

Please share any informational data regarding "newer mill processing of aluminum hydrogen is not removed as much as it was before".

Not doubting, just would like to see some data.

A Vote for Trump is a Vote for a DicKtator
Parent - - By BryantB (*) Date 10-19-2020 17:25 Edited 12-04-2020 16:21
Parent - By BryantB (*) Date 10-28-2020 16:16 Edited 12-04-2020 16:21
Up Topic Welding Industry / Inspection & Qualification / Porosity in aluminum PQR RT

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