Blowing Holes in MIG Welding: A Battle That Has Lasted for Decades!
Blowing holes is a persistent challenge faced by welders when MIG welding car body panels and its even more relevant now as car body panels have gotten thinner. This issue has plagued welders ever since the introduction of MIG/ MAG welders to body shops nearly 40 years ago
The phenomenon of blowing holes occurs when undesired holes form in the welded area, compromising the integrity of the joint. Its caused by the weld pool becoming too hot and gravity pulling the the liquid. It is mainly found in thinner materials and not just in steel. It creates a major headache for the welder in how to fix it and subsequently the fix increases the heat put into the panel. A viscous circle which we can help avoid by understanding whats going on as we weld.
Anybody remember the old BOC Autolynx ?
MIG or MAG?
People call it MIG welding when in fact in the UK we MAG weld as we use an “active” gas with an Argon, Co2/O2 mix for steel. This mixture of gases increases the heat in the weld pool and so actively changes the weld pool, hence we call this MAG (metal Active Gas). Therefore if the gas indeed makes the weld pool hotter then the amount of gas flow becomes important.
Too much flow will increase the weld pool heat and therefore increase the problems of potentially blowing holes. The correct gas flow is 14LPM and this is measured at the torch using a flow meter.
You can also use flow meters at the gauge for more accuracy.
REASON ONE: The dip transfer Process is Aggressive and Explosive!
The DIP Transfer welding process is known for its aggressive nature. It involves the wire "short circuiting" onto the material, creating an explosion that generates intense heat. This explosion melts a set length of wire, depositing it onto the material. A wire feed unit continuously replaces the blown wire, repeating the explosive process at a frequency of 50 to 60 times per second (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 our joint, but too much heat can mean we either distort the vehicle panel, or we do in fact "blow a hole".
REASON TWO: The nature of the butt weld and heat distribution
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 overly liquid, the popping effect can then take place or the weld pool can collapse due to gravity.
Welding Wire Diameter Matters: Size Does Make a Difference!
The diameter of the welding wire plays a significant role in the welding process. The size of the wire determines the amount of welding current that can be generated. In the DIP transfer welding process, the wire is "shorted" onto the material, creating an explosion. The thicker the diameter of the welding wire, the hotter 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?
This brings us to the old dilemma, which welding wire is best 0.6mm or 0.8mm?
Which Wire Size is best?
Generations of welders have struggled with this issue when welding thin vehicle 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 a lower heat is produced in the weld pool giving the weld bead a more pronounced appearance requiring excessive grinding, and all the extra heat that involves.
To combat this lower heat produced, technicians tend to weld for longer on each pool /pulse causing excessive heat and potential distortion.
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.
Reason Three: Incorrect Arc Length and more or less heat.
Arc length is incredibly important especially when welding thinner steels. The arc length is the distance from the contact tip to the work piece and it pays a role in amount heat is produced.
Holding the torch closer to the workpiece means the welding wire has less “stick out” and this means the wire is under more tension and therefore more current is required to blow this length of wire. This will make the weld pool hotter, faster.
Pulling the torch away and increasing the “arc length” or “stick out” and so the wire is under less tension meaning less current is required to blow the wire.
Moving the torch in and out like this, increasing and decreasing the arc length is how we control the weld pool when welding for longer lengths.
Heat Control: Trigger Time and Arc Length Take the Lead!
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 you use.
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.
If you are required to reduce heat in the pool ie, when you're coming to the end of a weld you can increase the arc length by merely drawing the nozzle back a few millimetres and this in turn 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.