The only thing I know about yachts is they are a big hole in the water you throw money into I do know a bit about motorcycle frames and the best in the business design frames to "bend like a tree in the wind" (make that a mast then ) one trick used is when a big thick piece of material has to be welded to a thin piece of material a 'pad' is employed. By that I mean a piece of the thin material is first welded to the thick section then the 2 thin sections are welded together. Very useful on tubing. I would say that big long mast is sucking heat out of the weld to fast , the whole mast would have to be pre/post heated.
When I'm finished with the first run it has a crack all along the toe of the weld against the thin material. So I decided to give it a second weld all along the toe to cover up the crack - it still cracks but much less - still not exceptable.
I cant use 4043 filler cause it must get anodized.
When pre-heating I concentrate on heating the thick material only - because the 2 are close together the thin one automatically gets pre-heated in the process.
I pre-heat for about 5 to 10 minutes (don't know what temp but it is very hot) with a blow flame.
Does this part have to go thru any kind of post weld heat treat? No it doesn't.
Does one need to wire brush ally when tig welding? As far as I know the machine does that for you (the reversed polarity or high frequency or something like that).
You could try the wire brush but I don't think that will help your situation. What about trying this filler wire. Maxal 4943. http://www.maxal.com/4943_datasheet.pdf
I re-read your post here and on weldingweb and the only conclusion I can come to is this,
Pre-heat to 200-350 degrees. I agree with the suggestion to add more filler per dip, although I am not saying this will cure your problem. This 4943 is supposed to be really good stuff. I just got some today as samples to try so I cant speak right now as to how well this will perform. I plan on playing with 4943 tonight. 4043 is the filler most commonly used for 6061 and 6062. The problem I cant figure out is if it cracks right after you stop welding wouldn't that mean it is shrinking fast (already mentioned I know). I wish I had your exact material and thicknesses and I would try it out myself. I know you are anodizing after weld but maybe 4043 will be your only choice.
One other question....is this a new design or have they done this particular joint before?
This is what I did to try and get the heat to distribute a bit more evenly. It still cracks.
Anton,
When you added these blocks to the thin material, did you preheat them? If so, I'd expect them to help slow the rate of cooling for the thin material so it shrinks near the same rate as the thick lug. If they're not preheated, then they will act as a heatsink and pull heat out of the thin material even faster.
What about using heat fence like you would use for body panels to keep them from warping? I haven't been around that kind of work in a very long time, but it did work well on sheet metal. If I remember right it was not all that expensive. It was like a rope that you formed to the panel around where you were welding and it kept the heat local. Might work.
I've been reading the posts and still think it's because the joint is highly restrained.
I think this because pre-heat did not work so it may be another issue.
The large block will not bend over with the shrinking of that one weld.
The tube cannot move because one end is uncut, the other end is held round by the flat plate welded on.
As a result the weld cracks the thinner member as it shrinks.
My theory to fix the problem is to bevel the tube, make a full penetration root pass, then a fillet weld. This is additional to the pre-heat.
Any pros able to confirm this theory in practice or would the same cracking occur?
Has a welding engineer been consulted?
Dave J.
Beware of false knowledge; it is more dangerous than ignorance. ~George Bernard Shaw~
Syncro 350
Invertec v250-s
Thermal Arc 161 and 300
MM210
Dialarc
Tried being normal once, didn't take....I think it was a Tuesday.
To answer some of your questions:
When I lift my welding shield the crack is there.
4043 is not an option.
No this 32mm to 6mm we have not done this before.
4943 can it be anodized and have no color change?
Yes I did preheat the blocks - I made everything the same temp
Ok what I have done:
After I welded it right around - I then turned my amps down by about 70 and welded one run right around again over the crack. This is not ideally what I want to do but it worked.
On the next one I welded, it obviously cracked again so like the previous time I welded over the crack again BUT this time it cracked again so I welded it a 3rd time over the crack before the crack was gone - shit - this is not how I want to do this?
What I will try next time: (some of the things you suggested).
Normally I would weld the bottom part first (the underneath side) which is much smaller than the top side and it doesn't petrude very much (more like a but weld). Next time I will weld the top side first (this side requires a lot more heat than the bottom side). This should allow for more movement in the thick material when welded (less restrained). Obviously I will preheat before I do any welding. The 2 flat plates that cover the end of the tube, I will leave them off until all the welding is done and then only weld them on. I will bevel the tube (I did not do this in the previous welds), make a full penetration root pass, then weld over it all again. It sounds like the last run should be a thick run? (someone mentioned heat, penetration and the size (big) of the weld).
Lets hope for the best! Any other suggestions let me know. I will let you know what happened after I tried this. Hell I hope this works!!!!
I do know a bit about motorcycle frames and the best in the business design frames to "bend like a tree in the wind" (make that a mast then 
) one trick used is when a big thick piece of material has to be welded to a thin piece of material a 'pad' is employed. By that I mean a piece of the thin material is first welded to the thick section then the 2 thin sections are welded together. Very useful on tubing. I would say that big long mast is sucking heat out of the weld to fast , the whole mast would have to be pre/post heated.
