Push or Drag Angle with GMAW to Reduce Porosity – The Answer Is: It Depends

Rather than chalking up this difficulty to the electrode or perhaps some equipment issue and accepting this as our fate, I dug in my heels and attempted to discover the cause of the weld porosity. Based on all that I could gather, the consensus in the universe of welding literature is that inadequate shielding gas coverage is considered to be the number one culprit in causing weld porosity in GMAW. The message that comes across clearly is that, most of the time, the problem boils down to shielding gas turbulence.

The simplest way in which turbulence can be created is quite simply that the gas flow rate can be either too high or too low. If the gas flow rate happens to be too low, the laminar flow of the shielding gas does not pose enough of a barrier between the weld and the atmosphere, and the gas is easily pushed out of the way by ordinary air movement. If the gas flow rate is too high, turbulence can cause air –comprised mostly of nitrogen and oxygen – to be aspirated into the weld pool.

Whether the gas flow is too high or too low, the end result is the same – oxygen and nitrogen can chemically react with constituents in the weld pool and can form the voids termed as porosity.

In addition to improper gas flow rate, a second possible way for gas flow to be disrupted is that the contact-tip-to-work distance (CTWD) might be too long. A long CTWD can create an environment where swirling eddy currents are created, thereby sweeping air into the weld pool. Similarly, a wind velocity of just a few miles per hour can have the same effect. A slight breeze is often just enough to displace shielding gas and introduce air into the weld.

Additionally, a weave width that is too wide can leave molten weld metal exposed without shielding before it has a chance to solidify.

A third mechanism for shielding gas turbulence is improper arc voltage – whether it’s too low or too high. Based on my experience, a voltage that is too low seems to have the greater effect on porosity. If the voltage is too low, the electrode tends to short-circuit to the puddle. When this happens, there can be an excessive level of spatter ejected from the resulting agitated puddle. This spatter disrupts shielding gas, which can also cause air to be sucked into the weld.

The opposite situation – where the voltage is too high – can lead to wander in a direction perpendicular to the direction of travel, and the arc effectively “escapes” the umbrella of shielding gas.

With all of our focus on how turbulence affects shielding gas flow, it can be easy to overlook another potential cause of weld metal porosity: the base material. High levels of sulfur, phosphorus, or other impurities from the base material can enter the weld pool. And, if the conditions are right, these impurities can lead to gas pocket formation.

Additionally, base materials can have contaminants such as mill scale, oils, and cleaning solvents adhered to the surface, leaving the weld susceptible to porosity. Fortunately, the use of welding filler materials containing deoxidizers such as silicon and manganese can help alleviate these conditions by combining with base metal contaminants to form slag, which then rises to the surface of the weld. However, this in-and-of-itself may not be enough to solve porosity problems.

Now that we’ve demonstrated that we conducted our due diligence, you can take my word on it that all of these issues had been addressed numerous times. We confirmed that there were no issues with gas shielding, whether it be the inappropriate gas flow rate, a CTWD to long, the wrong arc voltage, or weave width that was too wide.

Furthermore, the shielding gas line, welding gun, and cable lead were checked for leaks and none could be found. And finally, the base material was ground to remove nearly all of the mill scale as well as other surface contaminants just prior to welding. So by now, as one can see, we were being sufficiently challenged. What in the world was causing the porosity?

Then inspiration struck me on a recent Saturday afternoon while I was attending Lincoln Electric’s Family Day with my wife and parents at our headquarters here in Cleveland. During this event, which is held biennially, my company encourages all employees to bring their family members so that they can see where we work for 50 to 60 hours a week.

I was fortunate to have the opportunity to show my family the manufacturing floor, our testing laboratory, and of course, my office in the basement. But for me the highlight of the day began when we stopped by my old stomping grounds in our Application Engineering lab.