Hi MBsims, Pjseaman, rugg,!

High-frequency on continuous mode is going to be higher than 60Hz or 60 full cycles per second on all of the equipment I worked on years ago but, you also have to remember that the High-frequency is being used to maintain or stabilize the arc only... Allow me to use this excerpt from "Modern Welding Technology" fourth edition (1998), written by the late Howard B. Cary, a Prentice Hall publication, ISBN# 0-13-241803-7.
On page 322 it states:"One of the major problems in the early days of GTAW was arc rectification in the welding pool, especially when welding on aluminum. Rectification occured because the tungsten was a better emitter than the aluminum weld pool and caused the arc to miss half waves of the AC cycle. This resulted in a very unstable arc. The DC or rectified component of the welding current created problems in the welding machine. this problem was overcome by SUPERIMPOSING a high- frequency stabilizing current - on the welding current. This ionized the arc gap and partially corrected the rectification problem. It also aided in arc starting. The high-frequency current ionized the arc gap so that when the welding current goes through zero," (AC sine wave zero point between the positive and negative half cycle) "the arc would be reestablished instantaneously. This high-frequency current is provided by a high-frequency oscillator, which consists of a high-frequency transformer, a spark gap, a high-voltage capacitor and resistance, and a coupling coil." I wish I could paste into my response figure 10-46 to show you a simplified diagram of the high-frequency spark gap arc stabilizer... oh well. if you live near a good library, you'll be able to find it in the book. Mr. Cary continues to explain: " The high-frequency current is a broad-based signal with a fundamental frequency of about 2 MHz.
The spectrum is very rich in harmonics, with frequencies to 20MHz components. The frquency is determined by the charging time of the capacitor and by the 50 or 60 Hz line frequency."

Now to answer your next question rugg, "does the high freq contin operate in AC only as well?"... The best way to answer that is to try welding in AC without the high freq continuous engaged, then try welding with it engaged and you'll see the difference between the 2, then try welding with the power source set for DCEN or stright polarity, with the high freq continuous engaged, then disengaged... You will then see that No, the high frequency does operate in both AC and DC but, is only required for use when AC welding of Aluminum... When set to high frequency start when GTAW using DC, (electrode negative primarily) the result is a superior arc starting capability as compared to "lift arc start". You will also notice that once the arc is established, the superimposed high frequency current will no longer be present and the DC current is what maintains the arc transfer across the gap.

As far as your last question is concerned rugg, it all depends on alot of factors involved because, aluminum dissapates heat input better than stainless steel does...It'll take less heat input to melt aluminum than stainless however, it may take more or even less heat input to weld Al depending on the thickness of the surface oxide layer and alloying elements involved... Joint design consideration must also be taken into account... Using a pulsed welding current can help to minimize distortion in both AL and SS as can copper or brass heat sinks but without knowing all of the factors or details involved, I can only tell you that the two metals dissapate heat input at different rates, and the worst of the two is SS.

Respectfully,
SSBN727
Run Silent... Run Deep!!!

60 cycle is the frequency power is delivered at out of the wall. In the past an AC power source would have consisted of a transformer, which has no ability to change frequency hence welding current is also delivered at 60 cycles. These machines then superimpose a high frequency, high voltage delivered through a high resistance to avoid danger to the operator. I know neither the frequency nor the voltage but would guess something in thousands of cycles. The purpose is to generate an ionized area around the electrode so that the arc can start without the electrode touching the work, and also to avoid loss of the arc during zero crossings in AC. Only the first is necessary in DC since there are no zero crossings then and so high frequency is used only to start.

Modern inverter machines generate the AC (as square waves) from internal DC. This allows much more control over the output, the frequency can be easily varied, generally in the 20 to 200 cycle range I believe. It is also easy to vary the positive / negative timing. Since the power is delivered as square waves the zero crossings have very short duration, probably too short for the ionized gas generated by the arc to dissipate. In addition to that square waves analyzed mathematically contain a substantial high frequency component. The sharper the corner the higher the frequency. I do not know if that is enough to start the arc or not but it would contribute to the restarting of the arc at zero crossings. I doubt that these machines ever use continuous high frequency and indeed high frequency may not be necessary on these machines at all.
Bill