Thread: distortion control

Hi,

Clamping the guts out of it would work. But probably can't do that.

Really if it is an awkward shaped thing like that. Smaller stitch welds evenly spaced around the job. Try not to overheat one area too much.

You could try welding bracing to it some how. I dunno maybe a few lengths of box to help hold square etc.

This will be round on the bottom won't it? Can you see it if it is in the water? If so don't be too picky.

Just some ideas.

Is it ally or mild steel by the way?

Hey Rinso21,

I guess the material is mild steel, if so then you need to tack weld the whole thing working from the centre of the areas to be welded always towards an outside edge. The tacks need to be small and spaced around 20mm apart. Start from a centre point and make a couple of tacks on one side of the centre point and then a couple on the other side. Keep working like this as it allows the tacks to cool a little. Distortion is directly proportional to heat input so therefore the idea is to keep the heat input to a minimum. Artificial cooling always helps if this is possible, a piece of heavy aluminium under where you are tacking and eventually welding will conduct the heat away from the weld area. After the whole job is tack welded you then need to sequence weld, again starting from the centre and working outwards, (this drives any accumulated stresses to the outside). The sequence weld can be carried out by welding, say, 50mm then leave a 50mm break then another 50mm. Now go to the other side of the centre point and do the same, continue this technique until the whole thing is welded. Go back after and fill in the unwelded 50 mm areas. Use the aluminium all the time repositioning as you go. If you are using a stick welder, I would suggest a 2.5mm E4112 at around 70 amps, if you can use less current then that will help also.
Good Luck and Regards,
Ted.

When welding sheetmetal I stitchweld 10 - 20mm sections on opposite sides of the sheetmetal and cool down the weld with compressed air

Thank you all for the replies. Much appreciated.

Sorry for not being clearer in my description.
The material is mild steel.
The pontoons are houseboat hulls. Originally round in section, the rusty lower half is being replaced with new "vee" bottom sections, (folded not rolled) with vertical sides to be joined to the original at the widest point (read diameter) and bulkheads to make individual sealed sections. The idea is to add a lot of extra buoyancy for only a little weight gain.

I have a small mig. A Liquidarc 160 which I have had for about 15 years. I know it isn't the best welder available but it has been totally reliable and has done jobs far bigger than it was designed for. However I haven't done much work with light gauge sheet material that is likely to buckle and distort .

Mostly, the joins will be able to be reached inside during installation for application of aluminium heat sink as suggested by Ted. I'm not quite sure how I'll manage to hold the alli in position though. Any ideas?

Thanks again for your inputs.

Cheers
Rinso21

Originally Posted by Rinso21

The pontoons are houseboat hulls. Originally round in section, the rusty lower half is being replaced with new "vee" bottom sections, (folded not rolled) with vertical sides to be joined to the original at the widest point (read diameter) and bulkheads to make individual sealed sections. The idea is to add a lot of extra buoyancy for only a little weight gain.
Rinso21

No real welding experience so I will stay out of that side of things, but is the material folded to form the V at the base?. If so beware that the material will be appreciably thinner at the apex of the V due to drawing in the folding proccess.

Try some magnets and make an attachment out of thin steel -bendable to suit to attach to alloy . Cheers bob

Hi ,
One of your concerns seems to be be the thickness of the metal.For your mig thats heaps thick.

Try to arrange your fit up so that there is no more gap than say half your metal thickness, say 1.6 mm.

If needed include your backing piece as an overlap tacked inside the edge of your new piece .

As this is mig everything must be shiny clean otherwise you will,not may,suffer porosity problems.
I have another suggestion ,but need to go as I am beginning to have a flooding problem, back later

Grahame

sorry for the break
Had a a bit of a problem from Hamish.He filed by bloody yard with water which flooded the laundry and shed. Lotsa tools which I carefully stored on the floor,needed to be to picked up.
To overcome any problems with distortion from either from cutting the original piece off(residual stress) ,or created by welding the new bit on,here is my suggestion .

On the inside tak a bit of 25 x 25 x3mm A section so it forms a backing over the gap.

Space your beads to minimise heat distortion and you can return to fill the missed spaces later. Its called skip welding- run short beads and bugger off to another cool area and weld that and so on.
The stiffener will tuck in behind the existing material and you should be able to pull it in with self tapper screws.


Fill the holes with a squirt of mig later and dress off.
I would also spray the inside with a fish oil or the like to Minimise interior corrosion.

See my mud map below

Cheers
Grahame