Mig welding thin metal and distortion

[optima21]

I as having a play at welding some 0.9mm steel (zintec) sheet in different ways to see how it would distort. the sections welded were about 120mm long x 100mm thick, and were clamped together using intergrip fasteners like these

http://www.ebay.co.uk/itm/140689265011

using my clarke 151te on the minimum power settings using argoshield light gas

on all the test peices I used one tack weld at each end and 3 tacks in the centre (roughly equally spaced).they were wire bushed at the edge to remove most of the zinc coating. the plates were only held in one corner while welding.

I'm only do welding for a hobby, so my my welding wont be to the standards of some of you guys out there .

so the first attempt was to run a bead straight across and the results weren't pretty, with the heat blowing holes in the metal and it I then tried to fill the holes with weld the distortion increased (big blob on the left).
and the view from above

and below

and the distortion

so then I tried again using the same settings but this time pulsing the trigger, by welding one tack, waiting for a short while (less than a second) and doing an adjoining tack and repeating this across the join

the view from above

and below

definately looking better, but still with distortion

 

[optima21]

I then tried to weld the two plates together using tacks that were done roughly an inch apart, then allowing the plate to cool down for 2 or 3 minutes to allow the panel to cool, to see if less heat input would reduce the distortion. the bead wasn't as pretty as the second test and the penetration didnt look as good either

from above

and below

and the distortion (no better than no.2)

 

[mike 109444]

You are not allowing enough cooling time !
Try a damp cloth every few welds

 

[optima21]

I'll have to try again tomorrow thanks,although Im not wanting to be doing that with a wet rag in the middle of winter, so might try with compressed air too.

I also tried with a dc tig, using 20amps, 1.0mm rod and 1.6mm electrode, and the plates being butt welded together. I definately need more practice with this.

from above

and below

and showing the distortion, which appears to be about the same as the mig

Im going to have to weld a repair section into an almost flat panel on my car, so distortion may be an issue if I cant control it.

 

[Kent]

There are so many ways to manage this. You do what you need to when you can. Depends on the job in question
You can often make it work for you on some tasks

 

[hotrodder]

Im going to have to weld a repair section into an almost flat panel on my car, so distortion may be an issue if I cant control it.

What are we talking about, a door skin or something else? Can you get at the back of the panel or are their bits of car/inner structure in the way? Obviously no use if the entire panel has little shape but sometimes it can make more sense to cut out a larger area so that welds can be placed in areas with more shape to naturally resist distortion or because it'd make life easier from a finishing POV- access for dollies, easier to finish etc

Small test coupons tend to behave differently to real parts due to restraint but at the end of the day all welds shrink and most of the shrinkage happens as the weld freezes. While limiting heat build up with compressed air or a wet rag can help minimise/localise distortion it ain't gonna eliminate it. Traditional coachbuilding methods accept this and fix any distortion afterwoods by planishing the weld i.e. stretching it which corrects the cause of distortion. Obviously it takes a fair bit of practice to not make distortion worse by over stretching things etc and is much easier said than done
With MIG there's usually too much weld reinforcement for hammering to have much in the way of positive effects. While TIG isn't usually the weapon of choice for repairing rusty cars the welds can be planished out albeit not as easily as when oxyfuel is used... yeah, more potential for distortion but the weld is softer again as everything got hotter and, if correcting distortion as you go, remains hotter if you weld an inch and immediately planish it

 

[optima21]

its for the boot lid on my herald, I'd already modified it once,

so I've now got a big section missing, access behind the panel is good as there is a tubular steel frame running around edge of the boot lid.

although its taken a while but Ive got the rest of the panel to the correct profile, and its taken several hours to get to this stage,but its been an interesting learning curve with the panel beating

and the start of the new section, which is about 600mm long, but the curve of the boot lid against the flat of new section leaves a gap of 10mm at the ends

trying to do this a second time is bad enough, Im hoping that I wont have to try it for a third time

 

[Wendelspanswick]

I had the same issues to overcome when making a radiator cowl for my Austin 7 special. In the end I used the TIG/planish method trying to keep the heat input to a minimum.

http://www.mig-welding.co.uk/forum/threads/new-radiator-surround-for-my-austin-big-7-special.42940/
Heat sinks on the back of the weld would help as well but are difficult to use with intergrips.

 

[normspanners]

I used to know an old panel beater next door to my unit, many years ago, he will be long gone now, he was an old style lead filler craftsman, and he used to gas weld patches in and leave virtually no distortion it was fantastic to see, I asked him several times to teach me, but was always unfortunately, too busy, he said he didnt really know how, he just did it, but one thing he told me was, he kept welding from start to finish and would not stop for anything.
He could brush paint a panel and it would have a better finish than any spray job. His work was fantastic, but with just basic tools and he hardly ever used a mig.

 

[trewjohn2001]

I find MIG stitch welding painfully slow work if I go slow enough to avoid distortion. I never let the surface of the panel get hot to the touch - obviously it will around the immediate weld area but you know what I mean. I try to weld in other places on the car or have another job on the go in the garage while I stitch in a patch. I no good at panel beating so I have to take my time to avoid distortion. I´m sure for a good body guy its faster for them to have at it with a tig and take out the distortion later on. No way could I make money charging out the time it takes me to put a patch in. Worst part is chasing around pin holes with a torch on the back of the panel after grinding down - I don´t like using the "cold" stitch mig method - I want to try MIG brazing for body panels.

 

[mike 109444]

They (Edd China) did show the use of some heat absorbing goop, looked like a cross between wall paper paste and porridge. Was built up about an inch away from the weld and would take up (by evaporating the moisture in it ?) the heat. just a thought.

 

[mike 109444]

Bit steep but.....
http://www.ebay.co.uk/sch/i.html?_f...t+paste.TRS0&_nkw=welding+heat+paste&_sacat=0
ps hope things have changed !!

 

[marlinspike]

Frost Restoration sell the Cold Front welding paste too. I have seen it recommended on the Mini forum and it is reusable I believe so perhaps better value than it first appears.

I bought a thick piece of copper on eBay which helps to dissipate the heat. Only works where you can get the copper behind the patch though.

 

[Kayos]

When i did bodywork many years ago, i left a small gap then did a series of small tacks, one end of the join then the other, with cooling in between

Stopped all the distortion problems and meant after a run over with a flap disc they usually just needed a bit of filler and primer

 

[Chris Stevens]

The ideal way to weld thin steel with a MIG set is to have one with wire feed pulse feature. This can be set up perfectly for job and material, and while its possible to get pretty much the same result with triggering that tends to take longer, and can be pretty fiddly. Most Murex Tradesmigs come with pulse, and often appear used on Ebay for less than £300. I can weld very thin off-road bike exhaust systems using pulse feature, quite quickly and without any major problems with burning holes.

 

[Wendelspanswick]

The ideal way to weld thin steel with a MIG set is to have one with wire feed pulse feature. This can be set up perfectly for job and material, and while its possible to get pretty much the same result with triggering that tends to take longer, and can be pretty fiddly. Most Murex Tradesmigs come with pulse, and often appear used on Ebay for less than £300. I can weld very thin off-road bike exhaust systems using pulse feature, quite quickly and without any major problems with burning holes.

The problem with using a stitch timer feature is that they don't account for heat soak, so the first stich lacks pentration and the last stitch can burn through as the material has become heat soaked.
Stitching by manual use of the trigger means you can alter the stich time to allow for temperature build up.

 

[Chris Stevens]

Cant say I have ever had any problems with the pulse setting, and find it easier and faster than triggering. Done an awful lot of exhausts using pulse, but not used it for much else, so might be better to trigger on slightly thicker car tinwork?

 

[Gareth J]

I played around with the stitch function on my migatronic the other day. I didn't find it all that useful mainly as the gas on it kept flowing between pulses. No doubt less archaic welders stop the gas flow too but seemed like a bit of a waste!

 

[Chris Stevens]

Gas flowing cools the weld,and seems to make pulse function work even better on very thin material. To get the very best out of pulse, you need to play around with the settings for a while, but you will soon find the right ones for various different jobs, which will become a good deal easier using pulse.

 

[Wendelspanswick]

Gas flowing cools the weld,and seems to make pulse function work even better on very thin material. To get the very best out of pulse, you need to play around with the settings for a while, but you will soon find the right ones for various different jobs, which will become a good deal easier using pulse.

I wouldn't call it pulse as that tends to confuse things with the frequency settings found on higher end Migs, where the base voltage alternates with a higher voltage, stitch or spotweld timer is a better term unless you were talking about a proper pulse setting?