Welding on aluminium hull. Which alloy, which wire?

Hi,
Say you have an aluminium hull... and want to weld something to it... a bracket, a bulkhead, whatever.
I believe the exact alloy is defined through the grade, 5000 or whatever.
How careful should I be in "mixing different grades" regarding corrosion and mechanical soundness in relation to the dissimilarity between them, the existing material, the material to be added and the wire of the welder.
In other words, would just a sheet of aluminium for marine application and any aluminium wire be reasonable? Against analysing the existing aluminium and getting the exact same grade and using the most suitable wire? which should also be similar if not the exact same also...

 

Alloy 4043 is designed for welding 6xxx series aluminum alloys. It may also be used to weld 3xxx series alloys or 2xxx alloys. 4043 has a lower melting point and more fluidity than the 5xxx series filler alloys, and is preferred by most welders because it “wets and flows better” and it’s less sensitive to weld cracking with the 6xxx series base alloys. 4043 can also be used for welding castings. 4043 also makes brighter looking MIG welds with less smut because it doesn’t contain magnesium. 4043 gives more weld penetration than 5356, but produces welds with less ductility than those made using 5356. However, 4043 is not well suited for welding Al-Mg alloys and should not be used with high Mg content alloys such as 5083, 5086 or 5456 because excessive magnesium-silicide (Mg2Si) can develop in the weld structure to decrease ductility and increase crack sensitivity. (One exception to this rule is 5052, which has a low magnesium content.)

Alloy 5356 has become the most commonly used of all aluminum filler alloys because of its good strength and its good feed-ability when used as a MIG electrode wire. It is designed to weld 5xxx series structural alloys and 6xxx series extrusions, basically anything other than castings, because castings are high in silicon. Its one limitation is that 5356 is not suitable for service temperatures exceeding 150 degrees Fahrenheit (65 degrees Celsius). The formation of Al2Mg at elevated temperatures at the grain boundaries makes the alloys prone to stress corrosion. For components that will be anodized after welding, 5356 is recommended over 4043, which turns jet black when anodized.

Pillaged from the Lincoln Welding site

 

Ok! exactly the push I needed!
Thanks for the reply!
I had a subject in my degree exactly about these things and now its all gone... I remember it was mostly about the in-between the grains in the metal....

 

5356, have liked current production Hobart, as well as alcotec. Have had some sporadic qc on even good names in the last 3 years, seems like the chaos is subsiding but some garbage snuck out over the last few years. Upside the price has dropped clost to a third in the last couple months.

 

Very!!

There should never be a - whatever!

You must establish the exact alloy you have to weld to and what ally you are welding to it.

If you you think this is childs play, you should read this:
“Report on the investigation of the failure of a mooring of a bollard from the Class V Star Clipper resulting in a fatal accident at St. Katherine’s Pier, River Thames, London, on 2 May 2004,” MIAB Report No. 3/2005, Southampton, United Kingdom, February 2005

An excerpt is noted in "Altered Properties", Proboat Magazine Feb/March 2014 No. 147.
Noting how a person was killed, because a fabricator did not establish the correct grade of alloy he was using or welding to, and hence the correct filler wire when welding on a bollard to a passenger vessel.

If you are welding a coat hook...it matters little. But anything more...get the correct advice...i.e what is the grade and temper of each.

 

Absolutely, due diligence must be met.

From what I've seen, here and there, aluminium boats will, probably, be made of 5000 series for sheets and 6000 for extrusions, and maybe, 5083 sheets and 6082 extrusions.

So do trace elements alter the electric potential? Is the aluminium oxide conductive? I'm not even addressing the structural aspect!

I mean, in the end, the correct perspective, is just to acknowledge the situation and abide to what is considered to be the best solution. I mean, actually understanding it, is a bit to much to take in if all one wants is to weld something in a hull. And just knowing what is considered to be the best solution is sometimes a puzzle in itself.

 

You can make a lot of assumptions...but if the structure elements that require to be welded are "major structure" then you really best establish what is what, and no longer assume.
But if you're just using for coat hangers...aaahh..that's a different issue and an 'assumption'...would not do much harm if incorrect.

 

Unless it's home built by the uninformed the hull plating is a 5000 series as will some of the external and below waterline extrusions. It's not terribly uncommon to see 6061 extrusions (flat bar, tube etc) above the water line for various finish type situations. Hull is probably 5086 as that's the most popular but could be 5052 or 5083, again this all doesn't matter as your faced with only two options when you walk into a store or get online. 4043 or 5356 that's your options, for 5,xxx series plate you want 5356.

My assumption is your using a small spool gun as rarely do folks have a pulse mig box rocking a water cooled push pull gun. The miller 100 that's also rebranded across Hobart and esab doesn't push 5356 which is an unfortunate discovery when opening the manual for many. Need at least the 150 gun to burn 5356, it's a bit more unfortunately (price has almost doubled on it since my last 150 was bought). But that said I really liked the 150 gun I have one aboard my boat and I've run quite a bit of wire through it. It's not my aluma pro.... but combined with a shorty tank and a 215 is small enough to go with me everywhere.

Again I can't understate this enough unless you're ordering 200 lb spools for push-pull guns for highly dedicated saltwater boat builders ( even then many will just rock 5356) you will only have two options and you want the 5000 series for 5000 series and you want 4043 if you're welding to a 6061 extrusion. It won't be more complicated than that because that's what you'll be presented with.

 

This is exactly the information I was looking for, and couldn't find.

After some more digging I found in the Bureau Veritas site some info on this subject - NR561 HULL IN ALUMINIUM ALLOYS, but their not very specific...

As a rule, the aluminium alloys used for the construction of aluminium ships are as follows:
for rolled or extruded products:
- series 5000: aluminium-magnesium alloy
- series 6000: aluminium-magnesium-silicon alloy.
for cast products:
- aluminium-magnesium alloy
- aluminium-silicon alloy
- aluminium-magnesium-silicon alloy.

...As a general rule, the heat affected zone (HAZ) is to be taken extending over 25 mm on each side of the weld axis.
...Aluminium alloys of series 5000 in O condition (annealed) are not subject to a drop in mechanical strength in the welded areas.
...Aluminium alloys of series 5000 other than condition O are subject to a drop in mechanical strength in the welded areas. The mechanical characteristics to be considered are normally those of condition O.
...Aluminium alloys of series 6000 are subject to a drop in mechanical strength in the vicinity of the welded areas.​

Regarding the spool gun, if necessary, one can always rent I guess... For every thing else, rebrands sound fine!

 

A buddy of mine put A synthetic core in his little Bernard gun ran some 3/64th wire through a short wire feeder and said it worked good enough. Never really asked what good enough was, but he usually does decent repair work.

Said the liner was only about 30 bucks on Amazon and he just kept the Box lined up with where he was working so there was no heavy bends. Might be an option for a small project.

 

For a pillager, you sound pretty good. I'd have to go back to the books to even try to argue, but seriously doubt I would find anything to argue about.

I was aware of the problem with cracking of the high Mg alloys around welds exposed to temperatures above 150°F. There was a huge problem around 2000 with cracking of 5083, 5086 and 5456 on around 300 vessels traced to mill processing. There was an investigation that resulted in the issuing of ASTM B 928 which requires corrosion testing and a different heat treatment designation from the ASTM B209 high strength high Mg 5000 series. The B 928 link is a detailed report on the issue with far more detail than should be repeated here. I was not aware that the SCC problem could be caused by the filler wire as well, although it makes sense that it could. I have had to review weld requirements for a number of other Al alloys, but never the high Mg 5000 alloys. After reading your post I found warnings about welding with 5356 as well, even if the ASTM B928 material is used. The recommend weld alloy from Lincoln for high Mg alloys is 5554.