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The Theory of a MIG weld on A Vehicle


The theory of how to MIG/ MAG weld on a car!

I teach a lot as you may or may not know, and over the years, I’ve narrowed down the way I inform students or even older body technicians how to weld vehicles correctly. 

In the back of a technicians mind when welding the quarter panel joint on that really expensive vehicle, is “don’t blow a hole”.  Secondly is the distortion of the panel after,  because that will determine how much filler goes into the joint to cover it.

MIG welding the new vehicles steels require a particular technique and understanding, and here I go through the theory of how to perform a  weld correctly.


Mig Plug Weld


When you arc up with a MIG torch on a piece of metal,  due to the heat involved,  a weld pool is formed.  This molten liquid forms the shape of a circle ( if the torch hasn't moved) and will grow in size the longer you stay in the same position. We call this a weld "pool".

If you then release your trigger,  move the torch, then perform another weld you again form another circle. How far you jump will alter the appearance of your weld and the amount of heat you put into the material, for best results its best to aim for half of the width of the pool.

weld movement.

Example - if your weld pool diameter is 8mm, then your pool width is 4mm. Consequently, the required movement between weld pools is 2mm.  It is also possible on thinner steels to move up to 4mm and still obtain root fusion with no gaps. Jumping larger than this will reduce heat and the weld will look flatter but you won't achieve full root penetration and have possible gaps between your welds to the rear of the panel. 

example of the root of a weld

The "root" is the essential part of a weld, and it's necessary that you fully penetrate as this is where welds can break.  Remember you are grinding down the front of the weld for appearance. Therefore if you haven't welded the root correctly, your weld will be weak.



Welding believe it or not is about HEAT!  We need heat to weld the joint. The weld, therefore, needs to be hot!  What we don’t do though is weld hot for an extended period!


To weld the 0.6 to 1.0mm vehicle body panels on modern vehicles we need to weld in a “pulsed” action whereby the technician will press the MIG torch trigger for a  second or so and then release it.

This pulsed action is perfectly acceptable and is good practice, but what you need to think about is hot and cold cycles.  When you are welding,  its a “hot cycle” and when you are not that’s cold.

 hot cold hot cold weld technique

I have called this "Tik Tok" welding and when teaching always inform technicians or apprentices to count when they are welding.  Count how long you press the trigger (hot) and how long you release it (cold). This technique will give you the technician/welder a consistency to your weld bead appearance and also enables you to learn memory function. 


Mig Plug Weld


When a welder arcs up, the material is cold,  it isn’t going to blow a hole,  so don't move,  observe your weld pool and its size and stay in one place 

Let the weld pool form its circle and get to the size you require.   If we say we want an 8mm bead, simply wait till the weld pool gets too 8mm all the while counting how long this takes. 

If it takes 2 seconds, then that is your “Hot” number.  Now move your torch half the width of your pool size and hot weld again. Repeat over and over until your weld bead is produced.

  An example of a pulsed mig weld

The other part of this is the “cold cycle”, the time between your hot cycles. To reduce blowing holes and also reduce distortion, your cold cycle time needs to be two to three times longer than you’re hot. 

So if the hot is a two, then the cold needs to be a four or a six. If you stick to this techniqueyour weld bead will look consistent and uniform. 

After a while,  you will learn memory function, whereby you automatically do this counting without thinking and move your torch the required distance just by feeling.  

How fast the weld pool forms depends on the material thickness and what voltage and wire setting you have set on the welder. 

heat affected zone of a MIG weld

Always observe the heat-affected zone (HAZ) and its size and always aim to keep this to a minimum. The HAZ is an excellent indicator of how much heat you are putting onto the material.

The other important factor to be aware of when welding thin steels are not to perform your next "welding circle" while your last weld pool is still molten.

This will mean you have created more heat as the time between your hot welds is reduced, but it also means your chances of blowing a hole are increased.

The type of welding we use for cars is what we call "Dip Transfer".  The welding Wire is essentially a fuse, and when it shorts on the surface of the steel, it creates an explosion which blows or melts the welding wire.  

Take a look at another post I have written on this by clicking here

What you have to be aware of though is this dipping motion of the wire into the pool creates a popping effect much like a needle popping a balloon if you let the weld pool get too liquid. This is another way you can blow holes in your panel.



 0.6mm  MIG welding wire

The drift towards 0.6mm welding wire was caused by the MIG welding machines of the day having too high an open circuit voltage for welding vehicles.  This meant when using 0.8mm, it was a little too hot and hard to control from blowing holes.  So 0.6mm was better for car welding? 

Yes, the 0.6mm welding wire enabled a lower current to be utilised and here lay the problem, not enough heat is generated in the weld pool to weld the root of the weld. You see the surface of the weld isn’t as important when welding a butt weld, well not as important as the root of the weld and this was the mistake that people made.

The lower welding current meant that the root of the panel wasn’t full melted and more of the weld bead was sat on the top of the panel. Okay, it required a little more grinding off than normal but at least I didn’t blow a hole, right?

The strongest part of a butt weld is the root, that’s the part of the joint to the rear of the panel. Full root fusion is a MUST, but the trouble is on a vehicle you cant look at it after you have welded the joint. Therefore a technician has to rely on experience and his technique to judge has he welded it correctly.

REMEMBER you need heat to weld the root of your joint but what you don't need is the heat to be there for a long TIME. 

You can control the heat, not by the voltage switch on your welder but by how long you weld for. The longer you weld the more heat builds up in the pool and on the panel, this and the popping effect of the wire dipping into this pool is the reason you blow holes!

Take your time and increase the length of your cold time between welds to reduce the chances of you causing distortion and ultimately blowing holes. 

Move consistent lengths between weld pools and weld for the same length of time each time. This will give a more uniform weld bead appearance that is not only pleasing to the eye but also ensures weld integrity.

This technique can be applied to thicker steels by simply increasing the hot (TIK) part of the weld in relation to the cold (TOK) part of the weld.


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