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AC frequency is one of the most useful TIG controls for aluminum, but it is not a magic setting. It changes how tightly the arc is focused, how wide the weld spreads, and how easy it is to control the puddle. The best frequency depends on the joint, material thickness, fit-up, and whether you need penetration or a narrow, tidy bead.
For most aluminum work, begin around 80 to 100 Hz. Lower frequencies produce a softer, wider arc that covers more area. Higher frequencies produce a tighter arc that concentrates heat in a smaller spot. Adjust frequency only after you have suitable amperage, tungsten size, gas flow, and AC balance.
What AC Frequency Changes
During AC TIG welding, the current alternates between electrode-negative and electrode-positive portions of the cycle. Frequency controls how many complete cycles occur per second. A 60 Hz setting changes polarity 60 times each second; 120 Hz changes it twice as often.
Low frequency, commonly 50 to 80 Hz, gives a broader arc and a larger molten puddle. This can help when you need to wash the edges of a wide joint, but it also spreads heat and makes thin aluminum easier to overheat.
High frequency, usually 100 to 150 Hz, narrows and stiffens the arc. It improves control on thin sheet, tight corners, and small fillet welds. The trade-off is less coverage. If your torch angle or travel speed is poor, a narrow arc can leave one edge unfused.
Frequency does not replace correct AC balance. Balance controls the proportion of electrode-negative versus electrode-positive current. Too much cleaning action can overheat the tungsten and reduce penetration; too little can leave oxide contamination. On many modern machines, a starting point near 70 to 75% electrode negative is reasonable, but the displayed percentage varies by manufacturer.
Starting Frequency by Aluminum Joint
| Joint or job | Starting frequency | Why it works | Watch for |
|---|---|---|---|
| Thin butt joint, 1/16 inch or less | 100–150 Hz | Focused arc limits heat spread and helps prevent burn-through | Insufficient edge fusion if travel is too fast |
| Medium-thickness butt joint | 80–120 Hz | Balances penetration with manageable bead width | Warping from excessive amperage or slow travel |
| Lap joint | 80–110 Hz | Provides enough width to heat both overlapping surfaces | Trapped oxide, lack of fusion at the upper edge |
| Outside corner | 100–140 Hz | Keeps the arc controlled at an edge that melts quickly | Burning away the corner or making a concave bead |
| Inside corner or fillet | 80–120 Hz | Moderate focus helps reach both legs of the joint | Arc shadowing and cold sidewalls |
| Thick plate or wide joint | 60–90 Hz | Broad arc improves edge washing and puddle coverage | Excessive heat input and a wandering puddle |
These are starting ranges, not fixed rules. A 1/8-inch outside corner with a large gap may need a tighter, faster arc than a close-fitting 1/8-inch butt joint. Make a short test weld on a piece of the same alloy and thickness before welding the finished part.
Thin Sheet and Butt Joints
For aluminum sheet from roughly 0.040 to 0.063 inch, try 100 to 140 Hz with a short arc, modest amperage, and a sharp or lightly truncated tungsten. Higher frequency helps keep the arc from wandering across the sheet. Use a foot pedal or torch amperage control if possible, because the heat builds quickly once the joint is hot.
Fit-up matters more than a small frequency change. Keep the gap close and consistent. A large gap forces you to add more filler and heat, which increases distortion and burn-through risk. If the puddle repeatedly drops through the joint, increase travel speed, reduce peak amperage, improve fit-up, or use a backing bar before simply raising frequency.
On thicker butt joints, 70 to 100 Hz often gives better edge coverage. A frequency that is too high can create a narrow bead with good penetration in the center but incomplete fusion along the bevel or joint edges.
Lap, Fillet, and Corner Joints
Lap joints need enough arc width to heat both layers. Start around 80 to 100 Hz and aim the arc slightly toward the thicker or lower piece. A common failure is a bead that appears to sit on the upper sheet while barely bonding to the lower sheet. Clean both surfaces, remove the oxide mechanically with a dedicated stainless brush, and make sure the overlap is tight.
Inside fillets benefit from a controlled arc around 90 to 120 Hz. Keep the tungsten centered over the root and use enough torch angle to direct heat into both legs. If one side stays unmelted, lower the travel speed or adjust torch position before dropping frequency dramatically.
Outside corners melt rapidly because the joint exposes more edge to the arc. A frequency near 100 to 140 Hz helps keep the puddle small. Reduce amperage as the corner heats, and add filler consistently. A wide, low-frequency arc can wash away the corner before enough filler is deposited.
How to Tune Frequency at the Weld
Set your machine to a sensible starting point, then change only one control at a time. If the arc is too broad, the puddle is difficult to place, or the sheet is overheating away from the joint, increase frequency by 10 to 20 Hz. If the bead is narrow, the edges are not tying in, or the arc feels overly sharp for a broad joint, reduce frequency by the same amount.
Do not use high frequency to hide poor cleaning. Black pepper-like contamination, gray oxide islands, and a dirty tungsten usually point to inadequate surface preparation, contaminated filler, excessive cleaning action, or poor shielding. Check those causes first.
Equipment and Practical Trade-Offs
A TIG welder with adjustable AC frequency and balance is worthwhile if you regularly weld aluminum, especially thin sheet or varied joint designs. If you weld occasional 1/8-inch brackets, a basic AC/DC inverter with a fixed or limited frequency range can be perfectly adequate. Spend money on stable arc control and a usable foot pedal before chasing an unusually wide frequency range.
For aluminum, a water-cooled torch is useful when welding long seams or material above roughly 1/4 inch. For short repairs, an air-cooled torch is cheaper and simpler. A decent AC/DC TIG welder for aluminum should provide adjustable frequency, balance, and post-flow.
Use pure argon, normally about 15 to 20 cubic feet per hour indoors without drafts. Excessive gas flow can create turbulence and draw air into the shielding zone. A clean TIG tungsten electrode set, dedicated stainless brush, and correctly sized filler rod will usually improve results more than another 20 Hz of adjustment.
Always verify the machine’s manual because frequency, balance, and amperage displays are not standardized. Start with a coupon, watch the puddle, and let joint fit-up and fusion—not bead appearance alone—decide whether the setting is correct.