Reflowing mig welds

[sabre]

I have heard of a technique that you can reflow mig welds to make them more pleasing to the eye. This is done using tig, or so i am led to believe. I'm just wondering what happens to the structural integrity of the welds? How would it effect the parent metal/ joint?
Any one have any thoughts. As once welded what harm would reflowing the top of the weld do?

 

[kezajones]

why wouldnt you just tig it in the first place

 

[irondarren]

Easy enough to go back of a mig weld, with the TIG, but it would be better to TIG it in the first place. TIG is stronger than mig anyway

 

[Wozzaaah]

A properly done mig weld isn't that bad looking, not quite as pretty as a tig weld but if the mig is set up correctly you can get quite pleasing results. I wouldn't have thought it would have any effect on the structural integrity of the weld as I'd imagine it would only affect the surface appearance of the bead. Never heard of it being done though. I'd imagine you'd do it without filler wire.

 

[sabre]

I saw it once on a bike frame where they didn't want to grind the weld down but wanted it to look smooth, the frame was going to be powder coated so they couldn't use filler

 

[Joval]

Can't you do it MIG over MIG? I have been but I'm only a n00b

 

[Wozzaaah]

The whole point of reflowing is to improve the weld appearance, a mig weld over a mig weld would leave a tall, ugly bead. As Sabre said, it was done to avoid having to grind a weld down. I'd imagine it would look very nice and probably wouldn't take that long to do.

 

[Rusty]

why wouldnt you just tig it in the first place

I think sabre is trying to achieve a perfectly smooth Finnish to the weld, by adding the rod when tig,mig,or gas welding is what gives the semi circle pattern, if you just melt the existing weld surface you should produce a smooth Finnish to the weld.

 

[shenion]

I reflowed some alum MIG welds I had done on some practice sheets. Made them look real nice and was able to fix those cold starts.

 

[Rusty]

Some years ago (many!) as an apprentice i was given a job to weld some straight sections of ducting about 6" diameter and 6' in length
made from 22swg Zintec sheet rolled to produce the tubular sections, the Finnish on the welded joint had to be " flat " and so we were not allowed to use any welding rods ( gas welding ), so we would melt the two edges together at one end welding out-wards about 1/2" then continue to weld in the other direction using a 12" steel rule placed in between the two edges being welded continually moving it in opposite directions side to side to stop the edge of the metal from tucking under the other due to the heat distortion.Finally we would lay the section along a solid length of bar and planished the weld to give a perfect Finnish, ahhhhh those were the days.

 

[Hitch]

Done it a few times on things that would have been better Tiged up. But like Wooz said, set it up properly, and you can get a very smart looking Mig weld

 

[sabre]

I have a mig but not a tig. But i am planning to make a scratch start tig from a dc welder. and if i can. The welds up around the headstock on my trike i wanted to be smooth without grinding down, hence my question about the weld integrity. General impression seems to be that it will not effect the penetration and if any thing it may anneal the chill were the two materials meet, also as i need good penetration there may be a little under cut at the edges of the weld that can be taken care of I'm fine on down hand and verticle but the overhead stuff is a bit tricky and tends to end up lumpy and a bit drippy as well

 

[lovelectricity]

is it just me being thick? or is there no reason you cant make a tig from a mig? get tig gun slap on euro fitting for gas/ power add a foot pedal to switch it on, run pure argon. any info on this half baked idea

 

[shenion]

is it just me being thick? or is there no reason you cant make a tig from a mig? get tig gun slap on euro fitting for gas/ power add a foot pedal to switch it on, run pure argon. any info on this half baked idea

TIG - Constant Current (CC)
MIG - Constant Voltage (CV) (can use CC for spray transfer with voltage-sensing feeder.)

Would have no HF start and CV would probably vaporize the tip of the tungsten for scratch start.

 

[matt1978]

Hello,

Re running a mig weld with just a tig arc, is fine, just as long as your aware that you will be putting a fair bit of heat back into the joint, you shouldn't be doing anything detrimental to the actual weld or the parent material, because you wouldn't be using as many amps if you only want to melt the surface. The only problem you might get is distortion.

Just make sure that the surfaces are really clean and oil free before you start because tig arc is more less forgiving of rust and oil etc and you could end up causing porosity. Would be a good idea to have some 1.6mm dia tig wire to hand just incase.

Ref, the making a mig into tig, you wouldn't be able to do it for a couple of reasons, As Shenion said, you need CC for Tig, where as a stepped voltage transformer machine like I imagine you have is CV only and the OCV (Open circuit Voltage) would be about 30 Volts, where in tig, you only need 10v or so, so it would wreck the Tungsten on ignition. 2nd point is that your mig euro fitting is Plus polarity and you need your tig torch to be on minus or again, you would obliterate the tungsten. Add onto this and you wouldn't have anyway of controling the current (amps).

Get yourself a small dc tig inverter!

 

[lovelectricity]

sorry i messed things right up please look in tig welding (tig fom mig) as i tried to delete the wrong one. whoops. sorry for any confusion but i f**ked up.

 

[lexi]

Am I missing something or am I the only one who does a few roots at low power with the mig
Then turn amps and speed up to blat it over for a decent flat finish....Mmm I`m now waiting on Tachi telling me that`s a right bodge that is!! lol

Alex

 

[matt1978]

no, your dead right there lexi.

Most multi-pass mig welds (Esp butt welds) use lower amps for the root and then higher current for the cap or intermediate runs.

 

[sabre]

This sounds interesting.
I have put roots in on stick but not on MIg as it just builds up to much, when i do it. Care to elaborate on the principle? How low for the amps on the root, how much feed and current for the cap ect.The material is CDS2 tube 1 1/4" 10swg wall
I like the sound of this technique and could be a good alternative to my idea of reflowing and save me some money not having to bodge up a Tig from an Arc welder

 

[lexi]

I think the principle is that once you have put a root or two in you then have thicker metal to aim at with your next and higher power pass.The root you have put down is thicker than your thin tube so the next high power pass spreads it out into joint and you move on with torch before it blows through.

If you had used the higher setting on first pass you would have blown holes in the minimal mass of metal in the joint.........but you prolly know that. I don`t do settings as I just get a feel for what setting the welder will need. A good machine is more forgiving in this respect and my Portamig is good at this.

Alex