PERCUSSIVE WELDING 



889 



4 6 8 10 12 



SEPARATION IN INCHES 



16 

 XlO-3 



Fig. 2 — Three sigma limits of Ijreakdovvu voltages between a plane surface 

 and (a) a flat end and (b) pointed end of a round cross-section wire. 



excess of that which one woukl compute as average heating. As a result 

 \'er3' high temperatures occur in the arc region and material is evapo- 

 rated. This somewhat increases the arc length and the arc moves on to a 

 new spot. As a result of this mechanism some material is burned off in 

 the arcing period and the arc duration is therefore longer than the 

 initiation separation divided by the approach velocity. It is quite diffi- 

 cult to predict the amount of material evaporated. In lieu of more ex- 

 tensive experiments it can be stipulated that the evaporated material 

 should be proportional to the energy input minus the energy directly 

 radiated to the surroundings. 



It is of interest, however, to note that the phenomenon of evaporation 

 tends to increase the spread of arc duration as computed from the initia- 

 tion separation alone. This is simply due to the fact that a large initiation 



