Before the advent of inverter-based TIG welders, no one even thought about frequency control as a way to improve aluminum welding. The current that came from the wall was the same current that went into the weld—60 Hertz. Since then, countless manufacturers have sworn off 60 Hertz after seeing first hand the benefits of adjustable output frequency.
In AC TIG welding, frequency refers to the number of times every second that the direction of the electrical current completes a full cycle, expressed in Hertz. Frequency is represented by a sine wave, which represents the current flow rising and falling as it reverses direction (see graphic below).
Direct current cannot be used with non-ferrous metals because of the oxide layer that forms on the surface of the base material. In Direct Current Electrode Negative (DCEN) TIG welding, 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 types of ferrous metals, but the oxide layer that forms on non-ferrous 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 work piece 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 being concentrated on the tungsten instead of the work piece, DCEP also causes the tungsten to ball up at the end.
Alternating Current, then, combines DCEN and DCEP to provide good heat penetration with cleaning action. Historically, though, Alternating Current has posed an obstacle to TIG welding because the arc would frequently extinguish itself as the current reached a zero point before reversing direction. Without any current passing between the tungsten and the base metal, the arc would simply go out.
Improvements in transformer-based TIG machines created the “squarewave,” which increased the amount of time the arc spent at full current flow in both DCEN and DCEP. Squarewave 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 squarewave technology the preferred method for TIG welding 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 creates the ability to increase or decrease output frequency beyond the standard 60 Hz, which is the standard frequency delivered to every outlet in the United States (other countries, such as Germany, England and France, deliver AC power at 50 Hz). The inverter also allowed for the development of the advanced squarewave, which further decreases the time it takes for the current to reverse directions, further increasing arc stability and eliminating the need for continuous high frequency.
A traditional power source uses a large, heavy transformer to turn high voltage, low amperage primary power into the low voltage, high amperage power used for welding. An inverter power source takes input power, filters it to DC, and, using fast solid-state switches, increases its frequency up to 100,000 Hz, and then transforms it into useable welding power with an advanced level of control over the arc. The increased frequency allows inverter-based machines to use much smaller transformers, which greatly reduces the overall size and weight of the machines.
The range of available output frequencies varies widely by manufacturer. Some companies offer machines that range from 20 to 100Hz, while others make machines ranging from 20 to 400 Hz. Most inverter-based power sources provide AC output frequencies between 20 Hz and 150 Hz. Power sources could be designed to provide frequencies outside of the 20 to 400 Hz range, but there are very few welding situations it would yield any practical benefits. In general, 120 to 200 Hz provides an ideal frequency for most aluminum welding.
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.
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 diagram). The result is significantly improved arc stability, ideal for fillet welds and other fit ups requiring precise penetration.
Combined with adjusting the balance control to increase the electrode negative polarity—resulting in deeper penetration and tungsten that doesn’t ball up—high AC frequency provides the ability to weld very tight joints with good penetration and without the risk of laying down too much filler metal.
Work pieces with wider gaps to fill or that require build up will benefit from the softer, wider the arc cone that results from lower frequencies (see diagram).
Unlike other types of waveform controls, such as balance and amplitude control, frequency control provides good penetration in both low and high frequency. The primary difference between the two is the width of the arc cone and consequent weld bead.
Although all machines with adjustable frequency are inverter-based, not all inverter machines offer frequency control. The bottom line results of the balance and frequency controls of an inverter-based TIG welder compared to a traditional TIG welder are increased productivity, reduced weld costs, more consistent welds and improved bead appearance.
A1A Dock Products, a Hollywood, Fla. manufacturer of aluminum ladders, is just one among many examples of companies that have taken advantage of advanced TIG controls. Although their existing power sources were in perfect working order, A1A purchased two Dynasty® 300 packages and managed to increase their productivity by 18 percent and calculated a 8.8 month Return On Investment. The difference was the increased frequency, set at 150 Hz, and balance control, set at 75 percent electrode negative, that allowed them to increase their production by eight ladders per day.
"With the Dynasty, you can weld much more quickly," says Derek Grundler, A1A’s production manager. ""The Dynasty creates a much narrower weld bead than a conventional TIG. It allows us to direct the arc where conventional machines spread out the arc. The technology also lets us sharpen the tungsten like a pencil point and maintain a point, so the arc comes off of the tip" instead of dancing around a balled tungsten and creating a wide bead.
The significance of maintaining a narrow weld bead cannot be overstated. A welder who lays down a 5/16-in. bead where a 3/16-in. bead is called for takes three times longer per foot—1 minute and 39 seconds compared to 36 seconds—and lays down 177 percent more weld metal than necessary. Added up over the course of a year, advanced TIG controls provide thousands of dollars in material and labor savings. Figuring out the ideal settings for a specific application might take a little more time than simply adjusting voltage and amperage, but once you get the parameters right, the results are well worth the effort.