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! (anybody remember the old BOC Autolynx)?
Here i go through a few reasons why this may be happening and how to correct them.
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 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 to the frequency of the electrical circuit. (50-60Hz).
These continual explosions in the same place create a weld pool (molten liquid metal), and we use this pool to fuse the materials.
This means that once the weld pool has developed the wire is then dipping into a liquid. If this 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 popping effect 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.
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 one side more than the other this will cause excessive heat to build upon the edge of the steel getting it very runny. Again once the weld pool gets too liquid, the popping effect can then take place or the weld pool can collapse due to gravity.
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 more massive the explosion.
Suppose your welding wire is the same thickness as the material to be joined. In that case, both essentially have the same melting temperature ( Steel often melts at around 1370 degrees C 2500°F) bringing in the scenario of "what melts first", the material or the wire?
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 fused as now low heat is produced 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, so it takes less welding current to make it blow. A shorter wire sticking out of your contact tip by being closer to the workpiece 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 workpiece enables you to get your weld pool quicker as more heat is produced. You then control this heat, by pressing your trigger for a shorter time. To reduce heat when say you're coming to the end of a joint, by merely drawing the nozzle back a few millimetres reduces the current and your heat will drop. It is possible by moving the torch inwards and outwards (arc length)to fluctuate the current by up to 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, mostly 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.