A closeup micrograph of a TIG weld etch zone on 6061 aluminium (pulsed at 200 amps peak current, 5 amps sustain current, 50% pulse duty cycle, AC, 65% electrode negative, pure argon shield gas). These intentionally suboptimal conditions produce an widened etch zone where the normal layer of aluminium oxide has been removed from the surface, leaving exposed metallic aluminium capable of being welded.