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

It’s often said that you never know when inspiration will strike. But when it does, you had better be ready. This happened to me recently while we were in the midst of grappling with a true welding challenge, and I was fortunate enough to be able to harness that inspiration.

The situation was this: We were charged with the task of testing several “lots” of .045 in ER2209 duplex stainless steel solid gas metal arc welding (GMAW) wire. The testing, as specified by the customer, including mechanical testing (to determine tensile strength, yield strength, and percent elongation), Charpy V-notch impact testing, and deposit chemical analysis.

In order to accomplish this, we welded two, single-V-groove test plates in the 1G position using a pulsed MIG output waveform. The reason that two welded test plates were required was due to the possibility that the customer could choose between either of two different shielding gases in their fabrication shop.

The shielding gas used for the first test plate was a “tri-mix” blend of 55 percent helium/ 43 percent argon/ 2 percent carbon dioxide_, while the second shielding gas was the more traditional 98 percent argon / 2 percent carbon dioxide. And the welding technique used was one that utilized a “forehand” or “push” angle, where the arc is ahead of the body of the torch in the direction of travel speed.

Now let’s get to that challenge that I mentioned. We were confronted with the dilemma of excessive weld porosity, as determined through a radiographic X-ray examination of the weld test plates. The type of porosity I’m referring to here are the small, but visible, holes sometimes found inside a weld. These are formed as a consequence of trapped gas pockets.

In the context of this article, we’re only discussing porosity encountered in the welding of conformance test plates, where porosity is merely a nuisance and a challenge to be overcome. However, when porosity is present in “field” welds, such as in structural steel applications, these weld imperfections can be a recipe for a much more serious defective weld condition. For example, these porosity holes can be starting points for fatigue cracks that reduce the service life of welded components.

The results that we were seeing were independent of which of the two shielding gas blends was being used, as the number of round discontinuities counted on the X-ray film was the roughly same. Further, attempts to change the welding parameters – e.g., wire feed speed, voltage, and travel speed – had limited impact on the problem. As such, the welded test plates were always on the razor edge of being acceptable. We were at a loss to explain what could possibly be the source of the problem.