Preventing voids in tacks/short runs on thin metal

  1. hanvyj Member

    Portsmouth, UK
    Finally got my wire feed working! I'm a total beginner.

    I'm using flux core 0.8 wire on thin sheet metal, hoping to use the 'stitching' method, but mostly I'm messing around getting used to the settings a little.

    A big problem I'm having, especially with tacs, or very short runs, is at the end I get a pronounced void/crater. It can be pretty deep. Seems to consistently happen when I stop the weld.

    Any idea why?

    Here's some examples:


    (Please excuse the big embarrassing hole!)

    Welder was set to the minimum settings.
    Last edited by a moderator: Aug 19, 2020
  2. Jay1st

    Jay1st AdeptusMechanicus wanna be.

    South West FRANCE
    Hi, you should start the learning process on some thicker plate, going with thin sheet is a bad idea to begin with.

    4mm is the minimum, you'll still pierce it if it's too hot, but you'll get the hang of the welder , torch handling, movements and travel speed.
    Clean the surface with a flap wheel, make sur the Negative is on the torch for gasless flux core.
    stuvy and Munkul like this.
  3. Munkul

    Munkul Jack of some trades, Master of none

    Those craters are a mixture of contamination/soot from entrapped slag, and running too cold altogether. It's basically freezing the weld puddle before it has time to form.

    Flux core doesn't like running too cold. It has a sweet spot, and only IF the wire quality is any good.

    You want a nice smooth hiss/hum with fluxcore, at which point you'll only be able to weld 3mm+ thickness without burning through.

    Don't mistake it with "proper MIG" with gas, where you want a bacon frying sound - it's a different animal, and the main reason why people write it off as a spattery mess.

    Flux core in sheet metal will always end in tears, unfortunately. It's simply not a process that works with thin sheet metal, since it needs plenty amps and a steady seam weld to work properly.
    Dcal, 123hotchef and Jay1st like this.
  4. hanvyj Member

    Portsmouth, UK
    No problem, I'll get some gas (or maybe use what little I have left in a bottle), swap the leads and wire, and give it another go. I'll also try with some thicker metal to start off, don't have much thick metal and plan on doing mostly work on cars so I'll probably not be doing much welding in long puddles like you do with 3mm+ steel?

    tom2207 likes this.
  5. tom2207 Member

    uk northern ireland
    spend half an hour and watch some videos , either here or jody at welding tips and tricks on you tube , then you will have an idea of what your looking for
    Patience and Practice.
  6. bricol Member

    N.Yorks, UK
    You can get it to work nicely with thin sheet metal - I used an SIP Migmate for more than a decade to repair and build cars with gasless wire until I read both the welder and wire were crap on here ;) After 20 odd years of use, I treated myself to a GYS welder - and that's seriously impressive with gasless wire, although I now tend not to use gasless wire, as Ihae a BOC account and like not having to do the clean up of flux etc.

    But practice with thicker sheet as suggested.
  7. rich r

    rich r Member

    North Yorkshire
    I've been learning flux core and after a few weeks I can now do reasonable fillet welds on 1.8mm steel without burning through. Like you I tried starting on stuff that was far too thin (1.0mm) - but that extra 0.8mm has made a massive difference. Also having a decent area of metal so the heat can spread out made a massive difference, although you look to be OK from that point of view. I found a bit of 4mm steel and spent a while doing straight lines to get the feel of what power and wire speeds I needed and what angles and travel speeds work best. Just run a few lines then let it cool and grind it back with a flap wheel and repeat. To be fair I have been arc welding with a buzzbox for a while, so had some idea of how the arc and weld pool would behave and how to prepare surfaces. But it's nothing you can't learn from here or YouTube videos.

    I don't have the option of gas (I'm using a Lidl welder :ashamed:) but if you do, you might as well use it as you can see what's happening more easily. Alternatively keep practising with flux core - as noted by a few people, it doesn't have to be a spattery mess. With a bit of practice you can see the weld pool well enough to be able to control it and get pretty good welds with minimal spatter. It's just a lot more difficult than using a shielding gas - but a lot cheaper and more convenient in many situations (eg outside or occasional hobby type use).

    I've very much a beginner, but that £80 flux core Lidl welder combined with some decent wire is proving to meet my needs perfectly well. I don't think I'll ever do bodywork with it, but I can certainly repair exhaust pipes and patch things like tanks and boxes as long as I can get them horizontal. One day I'll practice other positions, but there's no rush for me yet.

    You can get steel offcuts from various metal suppliers on places such as eBay - £10 will usually get you 5kg or so of assorted bits of plate from 1mm up to 5mm. Ideal for practice :laughing: