by Kenneth Alrick
Advanced controls improve AC GTAW aluminum results
If you own a smart phone, chances are you take advantage of its applications and functions to make your life easier–has anyone looked at a paper road map lately? The same holds true for inverter-based GTAW power sources with advanced controls for adjusting the AC output to optimize results for specific applications. Here is a quick overview to get you started.
Balance control: This function adjusts the amount of time the waveform spends in the electrode negative phase (EN, which does the welding work) and the electrode positive phase (EP, which provides the cleaning action necessary to remove surface oxides). With inverters, you can set the balance control from 10 to 90 per cent EN. In many applications, you can set EN at 70 per cent or above, which will allow you to remove heat from the electrode and help maintain a pointed tungsten (more on pointed electrodes below).
Frequency adjustment: With an inverter, you can adjust the welding output frequency, such as from 20 to 200 Hz. Increasing frequency concentrates the arc column and prevents the arc from wandering. As a result, you can increase travel speed, narrow the oxide etch zone and put in a narrower weld bead. To start, experiment with frequencies from 100 to 150 Hz, especially if you want to prevent overheating thin sections and over-welding on fillet welds.
Pulsed AC TIG: A lot of welders confuse AC frequency adjust (the rate at which the current changes from positive to negative) with pulsed welding, which pulses the welding output from a high peak output to a low background output. Pulsing further lowers heat input, which prevents burn-through on thinner materials. It also affects the weld bead profile, specifically the distance between the “stacks of dimes.”
Some power sources enable pulsing the AC or DC output from 0.2 to 2000 Hz. Background current can be adjusted from 1 to 200 per cent of welding current, and the duration of the background pulse can be adjusted from 1 to 99 per cent of total cycle time. As a starting point for aluminum, try setting background current and pulse duration at 50 per cent and experiment from there. Most operators end up with values between 30 and 50 per cent for both parameters. As for pulse frequency for manual welding, start at 1.0 Hz and adjust as desired.
Save your parameters: After dialing in balance control, frequency and pulsing parameters, use the machine’s ability to store parameters for quick recall the next time you encounter the same application.
Tungsten selection: For welding with an inverter, use a ceriated (orange band) or lanthanated (blue band) electrode. The addition of 1.5 to 2.0 per cent of these rare earth elements enables the electrode to maintain a point instead of ball up like a pure (green band) tungsten will. As an added benefit, using these electrodes lets you switch between the AC and DC welding processes without changing electrodes. Because electricity likes to come off a point, using a pointed tungsten promotes a narrow arc cone and more precise welding. Historically, operators chose a 2 per cent thoriated tungsten (red band) when they wanted to weld with a point. However, thorium is a radioactive element; due to safety concerns, this type of tungsten is no longer the best choice.
Tungsten preparation: Prepare the electrode with a tungsten grinder or grinding wheel dedicated to the task to prevent contamination. Grind with the grain (longitudinally) to create a taper that is about two or two-and-a-half times the diameter of the electrode. For welding at 40 amps or below, use a sharp point. For welding at higher amperages, put a small flat on the end of the tungsten to prevent the tip from breaking off and becoming included in the weld bead.
With so many variables to AC GTAW variables to adjust, you’ll want to practice on scrap aluminum before welding on actual parts. However, with just a little added effort, you’ll greatly increase your ability to control the arc, tailor the welding output to match the application at hand and improve productivity and quality. SMT
Kenneth Alrick is senior welding product manager, ESAB Welding and Cutting Products.