It’s the switching of electron flow from en to ep that breaks up the oxidized layer. When electrons flow one way the heat is directed into the metal, but is resisted by the oxidation. When it switches direction it directs the heat more into the tungsten but also the change in direction of flow breaks up the oxidation layer. The faster you switch between the two becomes a balancing act, between cleaning and penetration.
I have more questions than answers
Josh
Josh
It's actually quite interesting that during the DCEN portion the electrons do indeed traverse the entire aluminum oxide layer -- this is evidenced by the pure aluminum substrate indeed melting underneath, while the oxide layer stays in place, still in it's intact "skin" layer on the top. It's only when the base metal becomes the emitter on the DCEP cycle that the oxide layer is actually broken up, and floats out of the way. I found out the reason is because it is believed when the base metal becomes the emitter, there is an electric field that is created that is not created the same way during the DCEN cycle. On the DCEP cycle, the electric field causes the oxide layer (which is normally a dielectric) to suffer dielectric-breakdown and then is "forced" to become a conductor, which causes physical breakdown of it's own grain-boundaries, and then floats out of the way as the aluminum underneath becomes molten.Poland308 wrote:It’s the switching of electron flow from en to ep that breaks up the oxidized layer. When electrons flow one way the heat is directed into the metal, but is resisted by the oxidation. When it switches direction it directs the heat more into the tungsten but also the change in direction of flow breaks up the oxidation layer. The faster you switch between the two becomes a balancing act, between cleaning and penetration.
Source: Cathodic Cleaning of Oxides from Aluminum Surface by Variable-Polarity Arc, SUPPLEMENT TO THE WELDING JOURNAL, JANUARY 2010, R. SARRAFI AND R. KOVACEVIC
