Welding, especially Aluminum alloys, removal of surface oxides is a very important element when high-quality welds or deposits are to be achieved by gas tungsten arc welding (GTAW)/TIG in an open atmosphere. The melting temperature of aluminum oxide (2050°C) is much higher than that of aluminum alloys. The high melting temperature of aluminum oxide and its tenacious behavior in the weld pool can cause weld defects. The presence of the oxide layer on the molten pool can cause fusion defects, inclusions, and porosities.

To remove the oxides during melting of aluminum alloys by GTAW, alternative current (AC)/DCEP can be used. During the positive polarity at electrode, the cathodic cleaning of oxides occurs.

Mechanism of Cathodic Cleaning during aluminum welding.

The positive ions that are accelerated toward the aluminum cathode are assumed to “sputter” the surface oxide layer. Sputtering is the ejection of substrate atoms from their lattice under the effect of high-energy bombarding particles.  In the case of the sputtering of cathode  material  in the plasma of arcs and glow discharges,  the  energetic  positive  ions striking the surface are responsible for the removal  of atoms  from the  substrate  — Fig. Some of the energetic incident ions can penetrate into the interatomic space of the substrate material lattice, experience a number of collisions (Fig), and even start a cascade of collisions inside the material. They can also accelerate the surface atoms of substrate into the material, and these atoms can ignite multiple collisions themselves. A portion of these atoms and ions can bounce upward and knock out the surface atoms.

Conclusion:

• AC or DCEP in GTAW process is used for welding metals where oxide cleaning is essential- Al & Al-alloys welding and Ti-alloys.
• Bombardment of ions is responsible for cleaning up of oxide layer from the surface.

Reference: Cathodic cleaning of oxides from Aluminum surface by Variable-Polarity arc, R. Sarrafi and Kovacevic

Keep reading, Happy welding

Thank you

KP Bhatt