Hi all, i m well aware of successful aluminum welding practices now and still eager to increase my welding knowledge more. I know why ac is used for aluminum welding, why cleaning of all hydrocarbons and moisture is necessary, what ss wire brushing does etc etc. But what confuses me is that yes there are two mandatory operations prior to welding which are cleaning of all hydrocarbons and moisture by acetone or alcohals ( which do not remove oxide layer only hydrocarbons and moisture unless chemical etching is done) followed by SS wire brushing which removes aluminum oxide but my question is why this wire brushing is done? Is nt it redundant? The oxide layer i read somewhere is less 0.01 mm, cant the DCEP half cycle in the balanced waveform remove it all?
One of my colleague argue that it is done just to favor the increase of DCEN time. And if thats true then was this argument valid before the development of unbalanced waveforms?
Any response will be highly appreciated.
Thanx all
This is an interesting conversation,
The main question would be: "Are your current cleaning processes fit for purpose?"
I don't have much confidence (in 30 years of welding aluminum) in the DC+ side of the half cycle doing a complete job of removing aluminum oxide. With sinewave, balanced or unbalance square wave, or asymmetric current profiles.
Is the thickness of oxide exactly the same on each piece of base metal you weld? No!
Different batches, lots, and mills will produce different corrosion/oxide thicknesses. This does not even take castings into account.
That being said:
Much depends on the acceptance/quality criteria you demand of your product.
If you are working to procedures qualified by testing... It is inadvisable to stray from the material preparations that were carried out during your initial procedure qualification testing.
Some procedures have been developed to produce sound welds directly over anodized surfaces, while others require tremendous process control of surface prep. Fit for purpose and proven to do so.
If you are to eliminate a step in surface prep, it should be done thoughtfully via testing of all possible surface conditions to be seen in production. Without this, how can you defend any defects that might be found post production?
I would concur with Lawrence. My experience with Aluminum is that cutting corners on surface prep will bite you in the ass eventually.
What would be best for wiping the area before welding? Ive heard the argument go both ways for Acetone and Isopropyl Alcohol... some say acetone leaves a film some say alcohol does.. which is correct?
Would the content of hydrogen in either of them have anything to do with how they could contaminate or react with the welding? Or is hydrogen so free its gone right as it evaporates?
J
I have just spent the past 6 years involved in welding Aluminium pipe using DCEN which as we all know is not recommended. Obviously, all oxide removal was manually done and critical. We were operating on 100% xray to B31.3 specs and had a 98+% pass rate. Just throwing it out there that you can weld Ally on DCEN......
Lawrence thank you, i understood the main point. Fit for purpose and proven to do so.
46
yes the cleaning is critical prior using DCEN but the oxide reforms immediately and DCEN does not give any arc cleaning so how to avoid LOF and oxide inclusions in radiography?
Its off topic i knw but i have understood what lawrence said so i would like to contunue.
Thank you
A danger or caution exists with power SS wheels and that would be the burnishing the oxide versus removing it.
Yes, the oxide does reform pretty quick, surprisingly few problems with either LOF or LORF but DCEN does offer greater penetration, even more with Helium gas. Nearly all welds were orbital. Prep'd pipes were isolated as much as possible from atmos after worked and before welding commenced, bagging the weld area etc. I might add, no purging gas was used. Weld Program criteria was obviously a major variable and much time was spent refining these before qualifying, approval and issuing.
90% of the maximum thickness of aluminum oxide forms within 24 hours of a thorough cleaning operation, i.e., chemical cleaning in sodium hydroxide, followed by a nitric acid flush, followed by a clear water rinse, drying, and wire brushing.
If the rate of reaction is a linear functions (it isn't, but for the sake of simplicity, let's assume it is true), 45% of the maximum oxide thickness forms in 12 hours, 22% of the maximum thickness forms in the first 6 hours, 11% of the maximum oxide thickness forms in 3 hours.
Let's assume we use a three hour window to complete the welding after cleaning and drying. How clean is clean? What quality level is required? What is the application? What does our test results indicate?
Wire brushing is required to eliminate as much oxide as practical. The rate of reaction is dependent on temperature and is exponential, so each completed weld bead must be wire brushed to remove the oxides that form immediately upon exposure to air if the "best" weld quality is expected.
Generally, it is agreed that the aluminum oxide melts at temperature roughly 3 times that of the melting temperature of the "clean" aluminum, i.e. 1220 degrees F. compared to roughly 3600 degrees F. Generally it is agreed that aluminum oxide is the same density as clean aluminum, thus it does not "float" to the surface of the weld pool as iron oxide would. The fact that the aluminum oxide has a higher melting point, it doesn't decompose as iron oxide would, and there are few elements that effective deoxidize the molten aluminum weld pool means cleaning have high priority is high quality welds are required.
Best regards - Al
Thats why we remove the oxide layer as much as possible before welding leaving a less work for arc cleaning?
803056
The density of aluminum oxide is more than that of aluminum so can it be seen on radiograph? I m asking because i have done some R & D. After chemical etching and wire brushing exactly as u described above i asked welder for root pass weld. Then i got it radiographed, it showed nothing. After one day the welder did the second pass with same balance control but without prior oxide cleaning with wire brush, just degreasing was done. I got it radiographed and this time radiographed showed porosity. I repeated this practise once again and got same results in radiograph of root pass and complete ( root plus first pass). I suspect that this might be oxide inclusions ,not porosity. The confusing thing is can we see aluminum oxide inclusions on x ray? How would they be diffrent?
Huge bundle of thanx and regards to you in advance
I havent seen radiographs, the inspector emailed me the report.
A microscopic examination should reveal whether you are detecting porosity or oxide inclusions. A marcoetch might do the trick if they are rather large and can be seen at low magnification.
I recently had SEM performed on a crack sample. It is amazing what it revealed.
Al
803056
you mean to say that we cannot see aluminum oxide inclusions in x ray? Or we does see but cant differentiate?
Radiography is fine for detecting materials that are thinner or thicker, denser or less dense. Tungsten is easily detected when viewing a radiograph because the density of tungsten is on the order of 9 to 10 times denser than aluminum. 2 mm thick aluminum is easily distinguishable from 10 mm thick because the thickness is 5 times greater. However, if aluminum oxide and aluminum are nearly the same, well, not so easily differentiated.
A common sensitivity used in radiography is 2%. That simply means that the smallest abnormally that is detectable on the film is 2% of the material thickness. So a defect such as porosity would have to represent a 2% reduction in base metal thickness to show up on the film. The change in density would also have to be represent the same order of magnitude in order to be detectable at that sensitivity.
Al
Thank u 803056
BTW what does the SEM on crack sample revealed?
In this case, the crack occurred before the component was hot dip galvanized and the chemistry of the material and debris on the crack interface.
Further SEM scans on parent metal provided a chemical analysis of the base metal to confirm the correct base metal was used for fabrication.
Best regards - Al
We had a three hour weld time limit from initial pipe pep.....
Excellent information. I would also note that mechanical cleaning is recommended on aluminum as the oxide layer is quite porous and may soak up interstitial elements and hydrocarbons. A piece of aluminum that is cleaned and artificially oxidized would weld better than something that has been weathered even at the same oxide layer thickness.
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