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There have been a number of posts about welding annodized aluminum over the years here. And also about Asymetric power supplies (Dyn
Here is a link to an article from welding magazine.. It discusses both topics and actually gets down to parameter and wave form adjustment. It's an obvious advertizement for Miller but the article is pretty good.http://weldingmag.com/mag/birdsall_marine_rides/
Just to complement the excellent Lawrence's advice.
The anodized layer of aluminum consists mainly in aluminum oxyde. The melting point of pure aluminum is roughly 640ºC (your translate into Farenheit). The melting point of aluminum oxyde is 3500 ºC or so.
So, when the base metal and the TIG welding rod are molten, the anodized layer isn't, it's still solid. So, if special precautions aren't taken, it will show up as inclusions on the finished weld.
Giovanni S. Crisi
Sao Paulo - Brazil
Actually, Birdsall Marine were "late comers" to the table when it comes to GTAW welding anodized aluminum. The process of "bump welding" anodized aluminum was developed by Pipewelders of Ft. Lauderdale, FL many years ago. I was actually taught the process by the son of the owner of Pipewelders (Edison Irving), long before Birdsall opened their doors.
For those interested, there was an interesting/informative article written by Mike Sammons (Product Mgr for Miller Electric) that appeared in Metalformingmagazine back in 1999. If you'd like to do a little homework here you can go to (not good with links) http://archive.metalformingmagazine.com/1999/11/miller.pdf
Good stuff there.
I worked with Mike Sammons from Miller when Aerowave was being Beta tested in our facillity.. I think he is top shelf, a responsive and knowledgable guy who actually answers his phone... There is a good series of short articles by Mike on the Miller website called "Aluminations" He discusses Hybred GTAW and GMAW both.
Thanks. I'll check the articles out.
Don't know Mike personally, but seemed to be very knowledgeable and reports what he hears/sees. Miller has a lot of "good guys" on their staff.
I would love to qualify a welding procedure using anodized aluminum. There is a big difference in simply making a pretty weld bead and depositing a weld that passes all the nondestructive and destructive tests.
Giovanni provided some important information in his comment that anodizing is simply creating a thick oxide layer on the aluminum substrate for additional corrosion protection or for developing a particular color of finish. He also mentioned the difference in melting temperature of the aluminum base metal and the oxide of aluminum.
What hasn't been mentioned is the aluminum oxide has the same approximate density as the base metal, thus it is difficult if not impossible to see in a radiograph and it does not flow to the surface of the weld. Instead, it remains entrapped within the weld causing it to be brittle.
I'm not saying the new machines on the market are not amazing, but I have yet to see any of my clients attempt to qualify their aluminum procedures with the anodized layer intact. I have seen numerous incidences where the welder didn't wire brush the weld joint before welding, make the weld (they looked marvelous) and the bend samples snapped like glass. We never got to the tensile testing.
Maybe someone here has been through this with anodized aluminum. If so, I would like to hear your results. Were you able to pass NDT and all the destructive tests?
Until I see actual test results I usually regard the statements made by manufacturers and salesman to parallel those made by the snakeoil salesmen of days gone by.
Best regards - Al
Perhaps the tuna towers are just way over built. They seem to stay together OK in spite of the inertial loading of a sport fishing boat pounding through waves.
Bend & tensile test results would be interesting to see compaired to similar welds made with the anodizing removed.
Welds done by most tower builders (Pipewelders Included) are seldom if ever x-rayed. They are however torture tested in one of the harshest laboratories in the world. That lab is called the open ocean. You wouldn't believe the punishment some owners put these towers through. If you haven't done a fair amount of offshore fishing, it's hard to imagine the stresses these towers are subjected to. Some boats are more difficult to build towers on, due to the amount of flex in the hull itself.
If the towers didn't hold up. You wouldn't see the number that you do on million dollar +++ yachts.
Actually, the greatest cause for failure in a marine tower comes from the inexperienced tower builder who employs a double pass weld to correct a mistake in his first pass. When you see a failure of this kind, it's invariably in the HAZ, not in the bead itself. Welding anodized aluminum (bump welding) is a single pass weld. Only get one chance to do it properly. In the better shops, when a mistake happens, that portion of the tower is cut out and replaced.
Additionally, we're not talking about inexpensive structures here either. A full tuna tower on a 65" Sportfisherman will run in the neighborhood of 100K +.
I don't doubt what you say, but has anyone actually qualified the welding procedure using anodized aluminum alloy?
I've seen some nasty looking welds hold up to harsh punishment without failing. That only tells me a crappy weld is still pretty good, but it says nothing of the mechanical properties.
The weld area is pretty large relative to the wall thickness of the tubes. If I had to guess I would think the actual loading on the welds are in the single digits KSI, not approaching the strength of the parent material. As Sundown points out fatigue can be an issue.
Aluminum typically fails by fatigue at some point unless the loading is strictly static. I wouldn't call a tower on a boat a static load.
Best regards - Al
Definately NOT a static load.
Just out of curiosity... What grade Aluminum are the builders using these days? If I had to guess, I would say 5XXX but then again, I don't fabricate "Tuna Towers" ;)
I do agree with Al's observation with respect to the mechanical properties being validated through testing especially with anodized Aluminum welds that are subjected to various dynamic (cyclical) loads... Hmmm, very interesting indeed! :)
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