For the majority of cases, you'll optimize the Tungsten's arc stability and cathodic penetration effects by properly preparing and profiling the included angle to subtend ~ 30-45-60deg. An an option, you may slightly blunt the Tungsten's tip. In most cases, blunting the tip prior to welding out Aluminum aids in combating disintegration at the tip that primarily results from evaporation at LTE (local thermal equalibirum) within the plasma column during electron/ionic transfer. Note that blunting, if used for aluminum, should be small/small unlike shown in the image. IMPO. Using a larger diameter Tungsten (ie. 1/8") with REMs (rare-earth metal oxides), the propensity for erosion of the Tungsten {thermionic cathode} is significantly diminished sans a blunt tip. Lastly, there are certain cases where we profile the tip with two separate included angles.entity-unknown wrote:....Recap: Are we saying a tungsten grind that matches a sharpened fine point pencil is bad for Al and instead I need a flattened end?...
The image, captured from the body of literature, represent what we employ 90+ percent of the time for steel/stainless steel and aluminum. Again, with a smaller blunting (if used). We profile based on our application (i.e. joint geometry, position, etc), the type/thickness of the materials to be welded, and the filler selected.
You have to "test and learn" and determine what works for your welding style and your application.
"For a cathode to support an arc, some process must exist which provides current continuity at the cathode-plasma boundary. To escape from the metal the electrons should either overcome an energy barrier that exists at the cathode-plasma boundary (work function) or tunnel through this barrier. The first process is thermionic electron emission. It requires a hot cathode temperature for a substantial number of electrons to be emitted. The second process, field electron emission, requires high electric fields at the cathode surface to make the barrier low and thin and thus intensify electron tunneling. In the considered type of arc, the intermediate case takes place: the cathode is hot and its work function is lowered by a strong electric field at the surface (the Schottky Effect). Both factors (hot cathode and high electric field) play important roles in the electron emission process in thermionic arcs. However,the electric field while being strong enough to reduce thework function is not strong enough to provide any substantial electron tunneling." (Valerian Nemchinsky, 2014).