What's your frequency

Before the advent of inverter-based gas tungsten arc welding (GTAW) machines, frequency control was rarely thought of as a way to improve aluminum welding. The current that came from the wall—60 hertz—was the same current that went into the weld. Since then countless manufacturers have sworn off this mentality after seeing firsthand the benefits of adjustable output frequency.

In alternating current (AC) GTAW, frequency refers to the number of times that the direction of the electrical current completes a full cycle every second, expressed in hertz. Frequency is represented by a sine wave, which depicts the current flow rising and falling as it reverses direction.

Direct current (DC) cannot be used with nonferrous metals because of the oxide layer that forms on the surface of the base material. In direct current electrode negative (DCEN) GTAW, the current flows from the tungsten electrode to the work surface, and the positively charged argon gas ions flow from the work surface to the tungsten. DCEN works well for steel and other common ferrous metals, but the oxide layer that forms on nonferrous metals such as aluminum and magnesium melts at a higher temperature than the base metal. Trying to weld with this process causes the base metal underneath the oxide layer to liquefy while the surface remains hard and impenetrable.

Direct current electrode posititive (DCEP) solves the oxide problem because the current flows from the workpiece to the tungsten, lifting the oxide off the material in the arc zone. DCEP alone provides the oxide cleaning action and very little penetration. Because the heat is concentrated on the tungsten instead of the workpiece, DCEP also causes the tungsten to ball up at the end.

AC, then, combines DCEN and DCEP to provide good heat penetration with cleaning action. Historically, though, AC has posed an obstacle to GTAW because the arc frequently extinguishes itself as the current reaches a zero point before reversing directions. Without any current passing between the tungsten and the base metal, the arc simply goes out.

Improvements in transformer-based GTAW machines created the square wave, which increased the amount of time the arc spends at full-current flow in both DCEN and DCEP. Square-wave technology eliminated the tendency for the arc to extinguish when the current came to a halt as it reversed directions by making the transition very quickly. This greatly improved the stability of the arc and made square-wave technology the preferred method for GTAW of aluminum and other materials that form an oxide layer, such as magnesium.

The second major revolution in frequency technology came with the invention of the inverter, which created the ability to increase or decrease output frequency beyond the standard 60 Hz, which is the standard frequency delivered to every outlet in the U.S. (other countries, such as Germany, England, and France, deliver AC power at 50 Hz). The inverter also allowed for the development of the advanced square wave, which decreases the time it takes for the current to reverse directions, increasing arc stability even more and eliminating the need for continuous high frequency.

Inverter Basics

A traditional power source uses a large, heavy transformer to turn high-voltage, low-amperage primary power into the low-voltage, high-amperage power needed for welding. An inverter power source takes input power, filters it to DC, and, increases its frequency up to 100,000 Hz with fast, solid-state switches. Input power then is transformed into usable, and then transforms it into useable welding power with an advanced level of arc control. The higher frequency allows inverter-based machines to have much smaller transformers, which greatly reduces the overall size and weight of the machines.

The range of available Most inverter-based power sources provide AC output frequencies between 20 Hz and 150 Hz. Miller inverter TIG machines provide AC output frequencies between 20-400 Houtput frequencies varies widely by manufacturer. Some companies offer machines with frequencies from 20 to 100 Hz, while others make machines outputting 20- to 400-Hz. Power sources could be designed to provide frequencies outside of the 20 to 400 Hz range, but very few welding situations would benefit from such frequencies. In general, 120 to 200 Hz is a suitable frequency for most aluminum welding.

Controlling the Arc Cone

Increasing frequency above 60 Hz causes the current to change direction more often, which means that it spends less time per cycle in both DCEN and DCEP mode. By spending less time at each polarity, the arc cone has less time to expand.

AC Frequency control controls the width of the arc cone. Increasing the AC frequency provides a more focused arc with increased directional control. Decreasing the AC frequency softens the arc and broadens the weld puddle for a wilder weld band.

An arc cone at 400 Hz is much tighter and more focused at the exact spot the electrode is pointing than an arc cone operating at 60 Hz (see Figure 1). The result is significantly improved arc stability, ideal for fillet welds and other fit-ups requiring precise penetration.

Combined with adjustable balance control to increase the electrode negative polarity—resulting in deeper penetration and tungsten that doesn't ball up—high AC frequency can weld very tight joints with good penetration and without the risk of laying down too much filler metal. Workpieces with wide gaps to fill or that require buildup will benefit from the softer, wider arc cone that results from lower frequencies (Figure 1).

Unlike other types of waveform control, such as balance and amplitude, frequency control provides good penetration at both low and high frequencies. The primary difference between the two is the width of the arc cone and resulting weld bead.