TIG steel with AC?

[evfreek]

Hi. I just connected a DC welder to my HF box and TIG torch to see the difference. There is not much difference in the quality of the weld. It is more important, it appears, to select correct amperage. The arc is much quieter and smoother. It is also more focussed and predictable. I used DCEN (the standard polarity for TIG). Now I understand why the people in the welding class complained about the sound of AC arcs for welding aluminum.

[Jose Gomez]

High frequency is commonly used with DC tig to initiate the arc without having to touch the electrode to the base metal. This is common with tig welding because the high frequency is capable of jumping across the gap between the tungsten and the base metal, thereby eliminating the need for "Scratch starting" or initiating the arc by striking the electrode across the base metal like a match (which typically contaminates the electrode). with DC positive or negative the arc only has to be initiated once because the current is only flowing in one direction, so no significant diference in weld performance aside from a slightly more stable and focused arc will be noticed.
Where HF is essential is when welding with alternating current. With AC the direction of current flow reverses 60 times per second (60 hertz typicaly) which means that the arc ignites in the positive cycle, then extinguishes itself, and then re-ignites in the negative cycle. all of this happens 60 times a second, which is where HF comes into play. HF continuously bridges the gap between the electrode and the base metal thereby keeping a "pathway" open for the arc to remain lit while the welder is transitioning between the positive and negative cycles. this constant switching of flow direction is where the AC arc gets its signature BUZZZZZZ.

The reason that almost all tig welding in steel and stainless is done with Direct current electrode negative is that 70 percent of the heat generated by the arc is focused on the positive side of the circuit. this is why the electrode remains sharp when used with DC electrode - and it "balls up" or melts when used DC electrode+ or AC. When the electrode balls up the arc is much less focused and tends to wander more.

As for you argon consumption, im not sure what you have your flow rate set at, but you can run as low as 15 to 20 CFH, anything more is a waste.

Kind a long winded explination, but I hope it helps.

[evfreek]

Hi Jose. Thanks for the reply. I do notice the tungsten getting blunter early on AC. for steel. I just read that AC is often used for brazing with silicon bronze. It is probably a heat into the metal control issue. Perhpas the tungsten is getting blunt because of lack of fine motor control. Time will tell as that skill develops. Just need to practice more. Looking at the charts on the Miller web site, the current capacity of thoriated tungsten is only a little less for AC than it is for DCEN. Much less, though for DCEP. I notice that the heating rate is much more directional with DC. The puddle gets much more slow to establish for angles off 90 degrees.