Fall 2012 (contents):

1. Thar she blows

2. TIG Welding

3. Dress for Success


Thar She Blows

In the good old days when slaughtering whales was a busy profession, lookouts on the ships would yell "thar she blows" when they saw water vapor spout from a surfacing whale. A similar cry (using my writer's liberty) goes out when a welder notices the welding arc wander from its intended path. In welding jargon, such unusual movements are referred to as arc blow. Arc blow can be the cause of multiple problems including spatter, porosity, lack of fusion, and undercut.

Arc blow tends to occur more commonly in materials that can be magnetized, specifically when interacting magnetic fields are present that are not symmetrical around the DC arc. Keep in mind that even if you are welding non-magnetic materials, the fixtures made of steel can affect the welds. When the arc encounters and unbalanced magnetic field, it tries to deflect to a position - a corrective action of sorts - which will compensate for the mismatched fields. The most commonly observed arc blow occurs during welding of steels that have some level of residual magnetism from prior manufacturing operations and handling. Even though individual parts may not appear to be heavily magnetized, when two of them are setup for welding with a narrow gap in between, the lines of force may orient themselves and produce a much stronger effect. The situation worsens when making a root pass in thick plates with a groove, where the part geometry produces even more focusing of the magnetic lines of force across the root.

Arc blow can also occur in seam welds where magnetic lines crowd around in front of the weld (unwelded portion of the seam) while the lines of force are much more openly spread behind the weld (welded portion). In this situation, the arc tries to move away from the crowded lines, essentially against the weld travel direction; this behavior is referred to as back blow. Direction of arc blow is also affected by location of the current return connection on the workpiece; the arc will blow away from it if only one connection location is present.

Excessive shielding gas flow rate can also push the arc around. Unstable gas flow not only affects the arc, but it also has the potential to draw air into the weld. The arc can also wander if one of the parts has a thicker oxide layer in the weld zone as compared to the other. Options available to reduce effect of arc blow include use of AC current, use multiple earth connections, de-magnetization of the material prior to or during welding, use short arc length, control gas flow rate, and proper cleaning of parts and electrodes before welding. With so many choices, there is no reason why a good engineer should have the need to shout out "thar she blows".


TIG Welding

TIG stands for Tungsten Inert Gas - a welding process that uses a tungsten electrode and an inert shielding gas. This process was called Heliarc welding in the early days when helium was the inert gas of choice to weld aircraft parts during World War II. The process is now referred to by its new acronym - GTAW - or Gas Tungsten Arc Welding. By dropping the word "inert", the powers-to-be have paved the way for use of gas mixtures that are not completely inert. For example, one can add small amounts of hydrogen to argon for welding stainless steels; hydrogen produces a reducing action on surface oxides and produces a cleaner looking weld. A mixture of argon and up to 5% hydrogen can be used safely without any special needs for venting. Nitrogen can also be added to argon to help stabilize austenite in stainless steels.

During TIG welding, an arc is formed across the tungsten tip and the parts to be welded. The arc is struck by a high-frequency starting pulse and then maintained by controlling the arc length - either manually or by a feedback controller and integrated motion system that maintains a fixed voltage across the arc. Amount of total energy input into the weld is controlled by voltage and current flow across the arc. However, the energy dissipation in not symmetrical across the arc and depends on direction of current flow. You can read more about this topic in "The arc or welding" from the Spring 2012 Weld Nugget. TIG welding can be used in both manual and automated modes. One can make autogenous welds (no filler added) or use a filler wire - again introduced manually or as a continuous wire feed in automated welds.

Since bare filler is used, the welds are free of slag and often require no cleaning after welding. As long as the user keeps the tungsten tip out of the weld puddle, chance of contamination from inclusions is very low. The inert shielding gases used also help to give the welds a bright shiny finish. TIG welding can be used for practically all structural metals but is frequently limited to thicknesses less than 0.25"; at higher gauges, other welding process are more economical.   Even with thicker sections, TIG welding is commonly used to make the root pass for groove welds where the need for full penetration without the risk of burn-through is important. With such unique abilities, TIG continues to play an important role in the world of welding.


Dress for Success

Most arc welds made with filler have a weldment that produces a positive reinforcement. This is not always obvious in butt welds but is quite pronounced in fillet welds (see schematic).  

  • Even though a positive reinforcement is required to ensure a strong weld and avoid crack formation, a convex reinforcement can produce a sharp junction at the toe of the weld. Such a transition can lead to high localized stresses which will likely cause failure initiation in tensile and in fatigue. The toe can be smoothed or "dressed" with a TIG or Plasma torch pass, usually without filler, to introduce a slight concave profile which will reduce stress concentration. Dressing will also help to blend in any localized undercuts or slag inclusions. If fatigue failures are a key concern, you may just have to dress your welds for success.