Spark stage of welding arc discharge binding on an aluminum surface

The article presents a study of the spark stage of binding of the welding arc discharge on the surface of aluminum covered by oxide film. The experiments have shown the formation of cathode- and anode-directed leaders in a nonuniform field between the pin cathode (tungsten) and the plate (aluminum). It has been found that, if the cathode spot occurs against the background of uniform discharge glow, then the spark channel forms, as a rule, in two stages. At the first stage, a diffuse channel bound to the cathode spot that forms in the gap. At the second stage, a highly conductive contracted spark channel spreads from the cathode side along the diffuse channel; the brightness of that spark channel is comparable to that of the cathode plasma glow. From the color spectrum, it has been found that intensive aluminum emission takes place in the domain of binding spots already at the stage of avalanche-streamer spark formation. The estimated calculation of the heat flux rate in the binding spot of the spark discharge has shown values of $10^6$–$10^8$ W/cm$^2$, i.e., comparable to laser heating parameters. The spark discharge exerts a significant thermal impact consisting of melting of the surface in the spark binding zone and the development of the recrystallization process of an amorphous film matrix in the zone of thermal effect. Electron diffraction through the thin film layer in the thermal effect zone has shown clear concentric rings corresponding to the polycrystal $\gamma$-phase of $\rm Al_2\rm O_3$. According to transmission electron microscope data, the average size of $\gamma$-phase grains in the surface film layer after impact by the spark is $d = 8$–$15$ nm, whereas the volume of the produced $\gamma$-phase is at least $70\%$. The stable thermodynamic $\alpha$-phase in the melting zone has been fixed. By the moment of arc discharge excitation, the entire aluminum surface in the spark-binding zone has been free of oxide film.