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Why do I blow Holes when MIG welding?

WHY DO I BLOW HOLES WHEN MIG WELDING?

Blowing Holes,  is the eternal dilemma when welding car body panels. Technicians have been fighting this problem ever since the MIG MAG welder was introduced to  body shops nearly 40 years ago! 

Here we go through a few reasons why this may be happening and how to correct them.

 

Dip Transfer weld explanation

REASON ONE.

The action of DIP transfer welding is very aggressive as the process requires the wire to "short circuit"  onto the material to generate the explosion and and create heat.

This explosion melts off a set length of wire and deposits it onto the material. That would be it were it not for the fact that you have a wire feed unit that replaces the blown wire and creates a short again. This continual action happens 50 or 60 times per second due the frequency of the electrical circuit. (50-60Hz). 

These explosions mean that once the weld pool is developed the wire is then dipping into a liquid pool. If this  pool then becomes too runny the dipping of the wire creates a popping effect much like a needle popping a balloon and this can pop it causing a hole. This is more easily obtained in thin steals as it’s easier to get the weld pool excessively liquid. 

Always observe the weld pool size and how liquid it gets, we need heat to weld correctly and fuse to our joint, but too much heat can mean we either distort the vehicle panel or we do in fact "blow a hole". Check out what is dip transfer welding

 Why do I blow holes ? by welding on one side!

REASON TWO 

When welding a butt weld on thin steels the joint preparation requires that there is a 1 mm gap between the two pieces of steel to be joined.  Correct welding would mean the welding wire is placed right in the middle of these two pieces of steel forming an equal liquid pool each side of the gap. If for any reason you heat up one side more than the other this will cause excessive heat to be developed on the edge of the joint meaning the edge will get  too liquid as the heat builds up, causing it to collapse due to gravity. 

REASON THREE

Welding wire diameter could be another factor. The size of the welding wire determines the amount of welding current that can be generated. Dip transfer welding or "short circuit" is created because we "short" the welding wire onto the material. Due to the voltage selected and the diameter of the welding wire this will create an explosion.  The thicker the diameter of the welding wire the larger the explosion.

If your welding wire is the same thickness as the material to be welded then they both essentially have the same melting temperature ( Steel often melts at around 1370 degrees C 2500°F). This would bring the scenario of "what melts first",  the material or the wire?

Dip Transfer Arc Length Explanation

Generations of automobile welders and technicians have struggled with this when welding thin vehicle body panels. (some of the new steel panels are 0.67mm thick) meaning they go for thinner welding wires or increase their arc length to reduce the heat developed.

This could mean that the panel joint is incorrectly welded as now insufficient heat is generated in the weld pool.  

Arc length affects the amount of welding current you get. A long arc length means the wire isn't under as much tension and so it takes less welding current to make it blow. A shorter wire sticking out of your contact tip by being closer to the work piece means that it now takes a little more current to make it blow. 

We need heat to create the weld,  but what we don't want is too much time of heat. It's the length of time you weld that causes the problem not the amount of current or the diameter of the welding wire.  Welding closer to the work piece enables you to get your weld pool quicker as more heat is produced.  You then control this greater heat input by pressing your trigger for a shorter time. To reduce heat when say your coming to the end of a joint simply draw the nozzle back a few millimetres and your current will drop. It is possible by moving the torch inwards and outwards to fluctuate the current by 20 Amps.

Heat therefore is reduced by trigger on or weld on time not by reducing the welding current.  The stop start trigger action is a great way to weld thin steels,  especially when butt or groove welding as the cooling cycle can be controlled by how long you have the trigger off. This is how we reduce distortion whilst still ensuring the root of the weld is sufficiently penetrated.

  Check out my post " how to weld on a car" for more tips and tricks.

 

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