vibrations during welding. What's happening?

Factual Questions
1

I do TIG/GTAW welding at home from time to time. Whereas stick or wire-feed welding typically requires you to set the current level before you begin welding, TIG welding allows the user to adjust current flow in real time; my particular welder (a Miller Econotig) offers DC current outputs ranging from 30 to 160 amps, controlled via a foot pedal.

Sometimes when I’m welding on a part that has a small footprint or warped base, the part starts rocking. It’s not from me touching it; I can cause the rocking just by applying the arc to it and ramping the current up. I don’t believe it’s related to AC current; I’m welding with DC current, and although there probably is some AC ripple, it’s likely to be relatively small, and the rocking frequency of the item doesn’t correlate to the 120-Hz I[sup]2[/sup] sine wave (it’s much lower, usually more like 5-10 Hz).

The part is grounded by resting on the aluminum surface of my welding table (the welder’s ground clamp is attached to the edge of this table).

Why does this rocking happen?

2

maybe thermal expansion causing dimensional changes it the metal. the curved surface causes the spot being affected to move.

3

My guess will be that you are generating a magnetic field that maybe lifting the part and then it falls back since the current decreases. Does it happen when you weld small non-magnetic parts ?

You can try a titanium shield around your torch to reduce such effects.

4

Never welded aluminum at home, so don’t know.

Are you talking about the shield-gas nozzle? It’s currently alumina. What would the titanium shield do for me?