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Spot welding heat illustration

Why Do I Blow Holes When Spot Welding On Vehicles?

The reason you blow holes in your steel when spot welding on vehicles is because you have high resistance in your weld joint, (stack)  and not enough pressure to control the heat that the resistance generates in that stack.

 

 

 

Resistance Welding

Your spot welder isn't officially called a spot welder,  its a resistance welder.   It uses the resistance in a weld joint to generate heat.  This heat builds up at the highest resistance (between the plates to be welded) until the material to be welded is molten. This molten metal is then squeezed together between the electrodes and allowed to go cold, forming a spot.

 

 

Spotwelding weld stack information

The Stack

In the welding stack, you have three resistances 

  1. The first layer of material to be welded (steel)
  2. An air gap - even when the steels are pressed together while welding there is still, in theory, an air gap to electricity.
  3. Last layer of material to be welded.

 

The stack above consists of three resistances, the two sheets of steel and the air gap. Even with the pressure of the electrodes forces those two sheets of steel together under a microscope and for understanding our example there is still an air gap.

The resistances in the steel in this joint will generate heat as the welding current is passed through them but the crucial point here is the centre, the air gap.

Air is VERY HIGH RESISTANCE, and as such this centre point is the place of most resistance, therefore it generates the most heat. This heat will build up as the current is passing through it till the metal becomes liquid, the pressure from the electrodes then squeezes this liquid together forming a ball or spot.

In a weld stack if the air gap is high then the heat produced at this point will be very high. If there isn't enough pressure to control this heat then expulsion will occur (sparks and splash outs). If the gap is too large then a hole may blow.

 

Air Gaps Can Be Produced By Many Things.

1,  If your electrode doesn't fit into the indentation of an old spot weld then this will create a large air gap under your weld electrode,  panel explosion is inevitable as too much heat will be generated where the electrode meets the steel.

spot weld bad placement of elctrodes

 

2, Old paint not removed completely from a panel (especially to the rear because you can't see it) can cause a steel explosion due to the high resistance under your weld electrode. 

spot welding over paint

3, Spotweld removal drills or “ZIP” drill cut into the rear panel when removing the front panel.  This creates an air gap problem when you come to replace your new panel. Sometimes you would be advised to grind off your old panel rather than drill off to reduce the possibility of this fault.

spot welding after spotweld drill has been used

All of these faults will cause explosions and because the panel you are welding on is generally the thinnest, this is the one that blows, so creating a hole.

 

Explosions due to bad electrode shape or condition.

dirty spot electrode bad welding picture

The electrode is the main electrical contact for the current to flow, good electrical contact is critical so that the only heat produced is in the centre of the welding stack.

A badly conditioned or unclean welding electrode creates surface resistance between the electrode and the surface of the steel. This, in turn, will create extra unwanted heat at the electrode contact point.  Normally the operator will see "sparks" or "splash" as an indication of this extra heat, but if the heat generated is excessive it could ultimately blow a hole.

 

Glues and paints in between your steel panels.

These glues and paints are insulators in that they won't allow electrical current to flow through them. This wouldn't normally be a problem in a normal joint as there is no alternative route for the current to flow, but in a car, there are lots of alternative current paths.

Electrical current can travel great distances if it's able too, so it looks for the best (least resistive)  route to flow and return to the welder via the other electrode at the back of the panel. The greater the distance this current has to travel through the steel to return (through the rear electrode) the more heat is generated at the electrodes contact point.

If you do not have enough electrode tip pressure to control this extra heat then a hole in your panel may be observed.

This effect is called Shunting!

Shunting when spot welding on a vehicle during repair is very important for you to understand as it can be the biggest source for blowing holes in your panel, shunting can help you and it can also hinder you in making a good weld.

So let's look at shunting and how it works when spot welding.

spot welding shunting effect

As you can see the welding current would rather travel through the steel via an older spot weld (shunt) than jump the higher resistance gap you are trying to weld.

This is bad as although on the outside the indentation and weld appearance on the steel may look like you have produced a good weld, in the middle (the critical part) your weld nugget size is reduced due to the loss of some of your weld current. 

Manufacturers understand this and alter the weld gap between spot welds to compensate for this shunting.

The distance between the spot welds will vary according to the resistance qualities of the steel being welded. The lower the steels electrical resistance the larger the gap and vice versa. This brings us to the importance of weld nugget size but we will cover this in another blog post at a later date.

Shunting though for the reduction of blowing holes in your panel, needs to be understood to reduce the chances of you blowing holes in your new panel.

So in conclusion, blowing holes in your panel when spot welding during vehicle repair is not the fault of the spot welding machine, its the fault of :

  • Your joint preparation,
  • Your electrode placement
  • shunting 

 

 

 

 

Header Picture with thanks from ARO Technology

Last Illustration picture with thanks to Renault Training.

 

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