welding an aluminum spar

[Captain Bill]

I have recently discovered a crack in my mast where a tang for the gooseneck was welded on. The crack is along the edge of the existing weld. I have been researching having this re-welded and understand that welding weakens the aluminum near the weld by removing the temper from the adjacent aluminum. Then again the crack isn't supplying any strength at all, so if I get it welded am I really any worse off. I suppose it could even be cracking because of the original weld weakening the metal in that area. Can anyone here with experience welding aluminum spars offer me any advice? Just to be clear the tang is original to the mast which is about 15 years old. I assume the mast was properly heat treated after the original weld, but I don't know that for sure.

[a64pilot]

I'd stop drill the crack and have it welded, I'd be very astonished to learn it had any heat treat after welding. Be very difficult to heat treat an entire spar.

[Suijin]

Ideally you would put a sleeve in. The usual process is to match the sleeve to the profile/contour of the mast, drill a few 1/2" holes in the exterior of the mast and tack weld the whole thing together. You could conceivably do the same thing on the exterior as well. Easier but uglier.

The issue is that nothing you outside of adding more metal is going to do anything but make matters worse. More welding is just going to weaken the existing aluminum.

I've seen a lot of this...tangs welded onto booms or masts that then end up weakening them. The biggest offender it seems is tangs for rigid boom vangs. It's just asking for trouble if it's not robustly reinforced.

All this said I"m not a welder. Maybe a good one can repair that but I'm not sure how without adding more metal to the mast.

[Lepke]

Certified in aluminum many years ago... Welds shrink when cooling. If you take two strips of mast thickness material and butted them together, and start welding from one end, by the time you get to the middle, the far end of the butt would be overlapped.
Just rewelding may not last as long as originally, new stresses along with the old ones. Usually production welding is done with wire, and after the original setup, done with less skilled welders, and rarely does post welding heat treatment get applied. Heat treating usually is heating the entire part uniformly and slowly cooling. Done in an oven. Finding one for a mast would be tough.
In repairing, I would drill the crack at both ends to relieve stress, grind the crack to clean metal, and Tig weld. Tig uses much less heat, so less shrinkage as the metal cools. Also skip weld. Short welds in opposite directions counter some of the shrinkage. Also slow cooling of the weld. In my day we wrapped the part in an asbestos blanket or dropped it into a tank of loose asbestos. All without any knowledge of the hazards. It's hard with a torch on aluminum because it transfers heat so well.
If it was my mast, I'd look into fitting some sort of reinforcement, gussets or a plate fitted to the mast shape to spread the stress over a larger area. Short welds. Full welding would just lead to more stress.

[Cheechako]

Yes, it's the heat effected zone. Entire boats made of aluminum are welded together. They are not heat treated after of course. Just get it welded by someone who knows what they are doing and move on.

[a64pilot]

If you were that worried, you could also cut the tabs off, weld the cracks and put a "fish plate" on the outside, it's nothing more than a doubler. The welds can be polished and it look quite nice if you take your time, look factory.
Drilling the holes and welding a doubler to the inside is a frequent aircraft tubing repair, the welds I believe are called Rosette welds.
Unless starting a trans Pacific or something I think I'd weld it and " fly it and watch it"

[Cheechako]

That "tang" is very thick compared with what a spar thickness is. That can be problematic also as getting the thicker piece to weld well requires too much heat for the thinner piece..... thus your problem.

[UNCIVILIZED]

Probably the best way to go about it is to sleeve the mast internally, along with using a large bolted & bonded flange to attach the new gooseneck. Sleeving is adding a section of matching profile tube to the inside of the spar to strengthen it. And on racing boats it's often done from the mast's butt, pretty much all of the way to the 1st set of spreaders on a 2-4 spreader rig. So as to strengthen the mast in the areas of; the vang, the gooseneck, & the spinnaker pole track.

With the new gooseneck being bolted, & bonded, it gives you 2 types of attachments. Either of which should be sufficiently strong on it's own. And if/when one or the other attachment method begins to work loose, you'll be able to notice it, & properly reattach things again. Plus the use of an oversized flange in this case spreads out the loadings over a much bigger area, as well as creating a much bigger area of attachment. Ergo lower loadings on the attachment itself.

There's a bit of info on this kind of thing in the Dashew's book Offshore Cruising Encyclopedia, vol. II which is gratis at SetSail FPB » Free Books Though most riggers should know of doing such things.


PS: It's also not at all uncommon to attach sleeving to a mast in this way, or in fact to connect 2 halves of a mast together with a sleeve thusly. Especially on boats where the mast is too big to ship other than as freight on the deck of a cargo ship, & or one which commonly gets taken apart & reassembled. Such as on some racing boats which get shipped to different racing venues (areas) a lot.

[Delancey]

Might be worth checking with someone who has experience brazing aluminum?

Temperatures for brazing are much lower (600f?) and works through capillary action as opposed to welding which requires grinding the joint to achieve penetration. I believe in some respects brazing can be stronger than welding.

[Cheechako]

My mast on my 47 footer with in mast furling was cracking at the cutout for the furler drum. I had the mast rebuilt, painted rerigged etc. They put a doubler in the area on the outside, using structural epoxy and many rivets. Outside is a lot easier to put a doubler on that fits the mast profile well. But at the boats age, I suspect just rewelding will stop the cracking and be good for another decade or two.

[Cherod]

Originally Posted by Cheechako

My mast on my 47 footer with in mast furling was cracking at the cutout for the furler drum. I had the mast rebuilt, painted rerigged etc. They put a doubler in the area on the outside, using structural epoxy and many rivets. Outside is a lot easier to put a doubler on that fits the mast profile well. But at the boats age, I suspect just rewelding will stop the cracking and be good for another decade or two.

Grind original weld off and take back to clean metal , then get a good Tig welder to weld it back up , easy job and will (/ should ) look better than the original production Mig weld .

[Cherod]

Looking closely at that crack it has been caused by the weak spot stop crater which is common with cheap production mig welding , pity you did not notice it earlier as it is an easy repair at that stage .

[Snowpetrel]

That looks nasty! I like the internal and external sleeves idea for piece of mind, I guess if the mast is already sleeved in its lower portion cutting the gooseneck tangs off and bonding a big doubler with new gooseneck tangs outside would be Ok and wouldnt need any welding or mast removal.

Prehaps rewelding alone would be ok, pregaps not? It would take a very good welder to do it properly, and even then a doubler inside would be very good insurance.

[UNCIVILIZED]

One other possible option would be to have lugs welded around & to the fittings. Much as is done on bicycle frames where the tube sections meet. They serve to spread the loads out over a much greater area of metal, in addition to pretty much doubling the thickness of the metal in & around the joint. This without creating any hardspots. And if it works on bike frame tubing, which has incredibly thin, fragile walls...

The theory behind them is much the same as gussets. Just spread over a larger area, due to the fragility of the tubing. Though commonly they're brazed on, so as not to melt the tubing, nor destroy it's heat treatment. https://www.bing.com/images/search?q...ug&FORM=HDRSC2
http://sfbikerescue.com/wp-content/g...bike-lug-2.jpg

[model 10]

So totally beside the point but why didn't they put fittings in the track groove, instead of going cheap and welding a tab on the mast.
As a half ass welder, I would V out the cracks with a die grinder and have somebody with a TIG ( a big one), weld it up.