[Anton]

some of our aluminum tig welds tend to crack when we weld the thicker aluminum eg. 30mm thick welded to 5mm aluminum. Would it be better to weld it with pulse?

[weldin mike 27]

Hey,

Welcome to the show. Please give us a little info about yourself , and also the exact parameters of your job. we wont be able to comment helpfully on such little info.

Mick.

[Mike]

Welcome to the forum Anton.

[Otto Nobedder]

Anton,

That 30mm material is quite a big heat sink. Have you tried pre-heating the heavy material? Maybe even some post-heating to slow the cool-off? When the weld cools rapidly (because of that big mass of aluminum absorbing the heat), cracking is more likely.

Also, some alloys are more prone to this "chill cracking". One example from my own experience, on 6061, I found 5356 filler less likely to crack than 4043. It seems counter-intuitive that a harder alloy would crack less, so my guess is that the shrink rate was a better match to the base metal.

Just some thoughts.

Steve S

[rake]

I too have found success with pre and post heating of aluminum when one member is greatly heavier than the other.
Post heat especially. Controlled cool downs in an oven can be beneficial also.

[Anton]

I am an artisan welder at Southern Spars South Africa. We manufacture yacht masts and supply the world.
Yes I definitely pre-heat the thick material, must admit I haven't post-heated. I am using 5356 filler rod. Cant use an oven either because masts and x-beams are somewhere between 5 and a 100 meters long. Some of the parts I have to weld is even thicker 50 to 70mm. We had a rep come to see us the other day and he recons on thick aluminum like that you have to use a tig pulse machine. As the rich people buy bigger yachts our material gets thicker and thicker. Even if I pre-heat it and weld it at about 320 amps my machine struggles to cope, if I put the amps higher the machine overheats an cuts out. I am scared that if we do buy a new machine that can pulse the problem might not be solved.

[Otto Nobedder]

Anton,

The pulse will not likely help with the origin of the problem, but it will earn the salesman a fine commision. Pulse is more about fine control of the heat input.

There's more than one way to skin this cat, though. The preheat/post-heat can be done with electric induction heating, or heating with resistive strips like heat-trace cables. Study how P-91 chrome pipe is pre-heated, interpass-heated, and post-heated for controlled cooling for some ideas.

Steve S

[weldin mike 27]

Hey,

a video on induction heating,

http://www.youtube.com/watch?v=zc-wq95Clso

mick

[Anton]

How do i paste a photo here? so i can show you some welding

[noddybrian]

There's a post showing how to upload in the instructions part of the forum - but when replying to a post - directly under the dialogue box is a small box with " upload attachment " in - click this & use " browse " box to locate image file - then click " add file" - then just need to place in appropriate part of post - hope this helps - & of coarse - welcome to the forum.

[Otto Nobedder]

Noddybrian's spot on. Two notes: after clicking "add the file", when you decide where to place the image, click "place inline". Also, there's a 250K limit per picture.

Steve

[Anton]

This is 32mm aluminum that I weld without pre-heating - it does not crack.

32MM2.jpg (36.13 KiB) Viewed 1108 times

This is 32mm aluminum that I weld without pre-heating - it does not crack.

[Anton]

This is 40mm thick - this I do pre-heat it also doesn't crack

[Anton]

This is the problem one - the one that cracks. I do pre-heat. As soon as I stop welding to get another rod it starts cracking - I can see the crack when I stop - so therefore post-heating wont stop the problem either. The thick material is 30mm welded to about 6mm. It cracks all down the side of the weld on the 6mm material.

[Anton]

Ok so you can't see the crack on the photo but it has cracked on the bottom of the weld. Maybe the 2 materials aren't heated up to the exact same temperature? Can that cause the cracks - I don't think so.

[RichardH]

This is the problem one - the one that cracks. I do pre-heat. As soon as I stop welding to get another rod it starts cracking - I can see the crack when I stop - so therefore post-heating wont stop the problem either. The thick material is 30mm welded to about 6mm. It cracks all down the side of the weld on the 6mm material.

Anton,
First, thanks for sharing the problem here - I'm sure to learn something from this discussion.

My observation is completely unqualified, but just considering the physics... Since the 6mm material has so much more surface area, and aluminum conducts heat so well, I'd think the 6mm tube would cool at a much faster rate than the 30mm slug. I've seen techniques used for castings where the part is buried in sand and the whole lot pre-heated to make the cool-down very slow; or completely blanketed except for the joint being welded. Is that even practical here?

[weldin mike 27]

Hey,

If the cooling rate of the 6mm material is an issue, is it possible to rotate the part 90degrees so that the thick part is flat and the heat will travel up into the 6mm material. Or maybe try a different joint design, such as a beveled butt weld,(even with a back up bar) with the fillet welded on top in an second pass. This Maybe rubbish?

Mick

[Anton]

I do keep the thick part flat at the bottom when I weld it.

[MinnesotaDave]

My first thoughts are that it is a highly restrained joint that cannot move freely when it expands and contracts (bottom of tube is also closed and welded).
So when the weld metal shrinks it cracks on the thinner side at the toe.
Could be similar to why a socket weld cannot have the tube jammed in tight.

Disclaimer, I'm not an engineer nor have I welded a mast before. Just seems it has to be a joint design problem since other areas of the same material do not crack and pre-heat has been used without good result.

[Otto Nobedder]

This is the problem one - the one that cracks. I do pre-heat. As soon as I stop welding to get another rod it starts cracking - I can see the crack when I stop - so therefore post-heating wont stop the problem either. The thick material is 30mm welded to about 6mm. It cracks all down the side of the weld on the 6mm material.

Do you treat the "end of a rod" the same as the "end of a weld"? Meaning, do you fully button-off (fill the crater at the end of) the weld, just as you would if the point where you need more rod were actually the end of the weld? I ask because leaving a crater at the end while you pause to grab a fresh rod creates the same stress risers that cause undersized tacks to crack.

It's important here for a reason mentioned above... the 6mm carries/radiates it's latent heat FAR faster then the 30mm section. I do see the crack in the picture, and it's on the leg of the weld on the 6mm side.

Suggested above was a way to insulate the 6mm side of the joint to slow the cooling. Perhaps a reverse is in order... putting the 30mm section vertical, and burying the 6mm tube in sand or Vermiculite nearly to the weld depth, perhaps using something as simple as a cardboard box cut and taped to fit.

Just thinking out loud...

Steve S

[kiwi2wheels]

Are you using straight argon or an argon/helium mix ?

[Anton]

I'm using straight argon.
yes i do fully button off when stopping to get a new rod.
joint design problem - yes that i suppose is possible but how would one design it?

[weldin mike 27]

Hey,

I posed this issue to a pen pal of mine who is a welding inspector/ supervisor and in-training metallurgist,

His response was very long winded, but offers some good points,


A lot of these guys make a point, if u r going to weld aluminum over 12mm thick, u must pre heat to increase the cooling rate as ally is the second best conductor of thermal heat (copper is the first best hence why most of all electrical cable are copper)
The 4 thousand series ally Is a high based silicon grade and should only be welded using the 6 thousand grade because the silcon is only compatible with its self. The silicon provides go deoxidance, and fluidity. Altho it is prone to cracking due to the fact it has a lower melting point then magnesium, which is the main element in the 5 thousand series.

What these guys haven't addressed is are these plates fixed ? Is there any distortion taking place, cause if the joint is fixed and the weld metal starts to solidifies it contracts and shrinks by 9% of its original liquid volume. It will start from the side of the weld (part of the weld in contact with base metal) and solidifies to the center causing it to split or tear strait in the center.

Post weld heat treatment, I would most highly NEVA recommend that, Eva. The trick the ally is to go in hot and go in fast. U must always try and keep the heat imput as low as possible because once you pour heat into the material, weather it be by welding or by torch u lower the mechanical properties, and ally dosent really have "great" properties to start off with.

Pulse is not as answer as it is designed for low spatter, thin material, out of positional welding and dissimilar materials.

I would say check the filler materials and external restraint on the joint and apply preheat no greater then 200 degrease and at lease 4 inches from the joint and they will be sweet. Also make sure the travel speed is good and you cover the molten weld pool as much as possible with shielding gas while it's solidifying.

Hope it helps

Mick

[TamJeff]

Every time I have come across this situation, the answer has always been heat, full penetration and weld size with regard to scale of the parts. Big heat, big tungsten, big amps and big welds. In other words, the parts preheated enough and a machine able to keep that joint wet throughout the motion of a significant weave.

I've used electric hot plates used for cooking to rest heavy plate on to get it good and hot. By hot, I mean like that of the 2nd pass on a typical weldment. Probably sounds kind of hokey but, whatever it takes. I've tried everything else. It's always come down to heat, penetration and weld size.

[kiwi2wheels]

If you can get a 60% helium/argon mix, it may be worth trying. If you have a draught free weld area , maybe even try pure helium.

Helium, and preheating, would considerably increase your heat input.