Tungsten inert gas (TIG) or gas tungsten arc welding (GTAW) of aluminum, aluminum alloys and similar metals is well know in the art and is discussed, for example, in the Welding Handbook, Volume Two, Seventh Edition, 1978, of the American Welding Society. In A.C. TIG welding, a problem well known to the art is that the electrode and the metal workpiece have different abilities to emit electrons. The electrode emits electrons more readily than the metal workpiece, especially for aluminum, aluminum alloys and similar metals. Thus, during the A.C. current half cycle wherein the welding arc is maintained by electron flow from the electrode (electrode negative) to the workpiece (referred to in the art as the straight polarity or weld half cycle) there is little resistance caused by the characteristics of the electrode or the workpiece to the formation or maintenance of the arc. In contrast, during the A.C. current half cycle wherein the welding arc is maintained by electron flow from the workpiece to the tungsten electrode (referred to in the art as the reverse polarity or clean half cycle) the tendency of the workpiece, particularly aluminum or aluminum alloys, to poorly emit electrons frequently results in non conduction or poor conduction of the current during this half cycle. This phenomena is referred to as arc rectification and results in poor quality welds. Accordingly, the art has sought welding current supplies which will reliabily and inexpensively solve this problem of arc rectification and provide a stablizied welding arc for TIG A.C. arc welding of aluminum, aluminum alloys and similiar metals.
In the art of TIG A.C. arc welding of aluminum, aluminum alloys and similar metals, it is desirable for the welding current waveform to approach a square waveform because the rapid zero crossing time reduces arc outages during crossover therefore enhancing ignition of the opposite polarity. Also, the ratio of peak current to RMS current is reduced therefore allowing a given electrode to be used at higher RMS currents without tungsten splitting.
As used herein, positive current means that the electrode is positive with respect to the work (reverse polarity). Negative current means that the electrode is negative with respect to the work (straight polarity).
Furthermore, it is desirable that the welding power supply have the capability to vary the width or time period of adjacent pairs of positive (reverse polarity) and negative (straight polarity) current half cycles. If the width or time period for the straight polarity current half cycle is greater than the time period or width for the adjacent reverse polarity current half cycle, greater penetration of the workpiece for a given current may be achieved if this is desired. If the width or time period for the reverse polarity half cycle is greater than the width or time period for the adjacent straight polarity current half cycle, greater cleaning may be achieved if this is desired. That is, it is desirable that the welding power supply permits the essentially square wave current adjacent half cycles to be made unsymmetrical in width wherein the sum of the widths of adjacent pairs of positive and negative half cycles equal 360.degree. of the line voltage frequency.
An attempted prior art solution to the problem of arc rectification in TIG A.C. welding is described in U.S. Pat. No. 4,038,515--Risberg. In U.S. Pat. No. 4,038,515, an SCR controllable full wave rectifier bridge is connected at its A.C. or external terminals in series between the output terminal of a transformer and the welding electrode. An inductor is connected across the D.C. or internal terminals of the bridge. This inductor provides an energy storage source to induce an arc current and acts as a filter to provide a quasi square wave arc current.
A problem arises in this attempted prior art solution when attempting to switch the current flow from straight to reverse polarity. When the bridge is operated to cause this current reversal, free-wheeling of the inductor connected in the bridge occurs. The power source voltage is suppose to provide for electron flow from the aluminum workpiece to the tungsten electode to provide a reverse current to back bias two of the SCR's in the bridge and end the free-wheeling. In practice, the aluminum workpiece does not want to emit electrons. Hence, this prior art solution will frequently fail to reestablish the arc upon switching from straight to reverse polarity and thus results in an unstable arc.
In order to overcome this problem, the art will use high frequency starting mode voltage in order to force conduction of electrons from the aluminum workpiece to the tungsten electrode upon the switching of the welding current from straight to reverse polarity. While this improves the performance of this prior art power supply, arc rectification can still occur upon the switching which degrades the quality of the resulting weld.