Aluminations (2): Shedding Light on Aluminum Welding Issues
May 1, 2007
This issue of Aluminations discusses important topics related to welding thin aluminum, including advanced AC TIG controls, tungsten preparation and machine selection.
TIG welding thin aluminum? Don't ball your tungsten - sharpen it. To advance everyone's knowledge, this aluminations answers questions on welding thin material, describes the operation and benefits of AC balance control and discusses selecting a TIG welder based on amperage requirements.
Using AC to TIG weld aluminum evolved from the need to remove the oxide layer that forms on its surface. The electrode positive (EP) portion of the AC cycle, in which electricity flows from the work to the tungsten, "blasts" off surface oxides. The electrode negative (EN) portion of the cycle does the actual welding, directing heat from the tungsten into the metal.
When Miller engineers invented the Squarewave AC output, they also discovered that an unbalanced AC wave form works best for many applications. That's why lighter-duty machines, like the Syncrowave® 180 SD, feature a fixed balance control set for more penetration (60% EN) than cleaning (40% EP), as shown in Fig. 1.
Miller also invented adjustable balance control. This feature permits tailoring the EN/EP ratio to match an application. For example, Miller's Syncrowave® 250 and Syncrowave® 350 LX let you adjust EN values from 45 to 68% (32 to 45% EP).
Greater amounts of EN create a deeper, narrower weld bead and better joint penetration. This helps when welding thick material, or it may permit faster travel speeds. Conversely, greater EP values remove more oxide and create a shallower, wider bead (see Fig. 2). On materials that have a heavy oxide layer or cast aluminum, increasing the cleaning action minimizes the chance of foreign particles becoming included, promoting a better weld.
No hard rules exist for setting balance control. The typical error involves over-balancing the cycle. Too much EP creates a large ball on the end of the tungsten. Consequently, the arc loses stability and you can't control arc direction or the weld puddle; arc starts also degrade. Too little EP results in a scummy weld puddle. Add more cleaning action if the puddle looks like it has black pepper flakes floating on top.
When choosing a TIG welder, consider how much amperage you need for a given thickness. To weld aluminum less than 3/16 in., a 15 to 180 amp machine like the Syncrowave 180 SD works well. For thicker metal, or if you want adjustable balance control, the Syncrowave 250 with its 5 to 310 amp range makes sense. Consider the Syncrowave 350 LX and its 3 to 400 amp output for work on heavy sections, or when you need a full-featured machine with pulsing control, Lift-Arc™ and a sequencer option.
For more specifications on Miller's Syncrowave family or information on TIG welding, call 1-800-4-A-MILLER (800-426-4553, ext. 604).
Questions & Answers
Q. How do I pick a TIG welder for thin gauge aluminum?
A. First, recall that you need about 1 amp of heat for every .001 in. of metal (e.g., you need a welder that goes down to 15 amps AC for work on .015 in. aluminum). Next, consider a machine with a reputation for a stable arc and good starts at low amperages, such as one of Miller Electric's Syncrowave® units. Lastly, note that many fabricators use high-end TIG welders for fine detail work because of their low amperage capabilities.
Q. I keep making holes in thin material. What can I do?
A. Try a set-up that gives you finer control over amperage adjustments. If your foot pedal and front panel amperage control have a leader/follower relationship, limit output on the machine (e.g., if you need 20 amps, set the machine at 40). Now the entire range of foot pedal motion only controls a fraction of the welder's output. In other words, 1 in. of travel might change the heat by 5 amps, not 50.
Q. Should I ball a pure tungsten electrode for welding thin material?
A. No. Instead use a 3/32 in. tungsten with 2% cerium (2% thorium is second choice), grind it to a point and put a small land on the end. Compared to a balled tungsten, a pointed electrode provides greater arc control and lets you direct the amperage precisely at the joint, minimizing distortion.
View Online Arc Shaping Videos
Frequency, or Hz, is the number of times the AC TIG arc switches between electrode negative and electrode positive in one second. Miller's Dynasty™ 300 DX inverter-based TIG power source permits adjusting output frequency from 20 to 250 Hz. Conventional TIG machines have a frequency fixed to that of the 60 Hz primary power.
Increasing the frequency narrows the shape of the arc cone and increases the arc force. This stabilizes the arc, reduces arc wandering and provides excellent directional control over the arc. On lap and T-joints, using a higher frequency lets you establish the weld puddle exactly at the root (view 200 Hz arc shaping video). This can ensure good penetration, control bead width and minimize the etched zone. With a 60 Hz output on fillet welds, the wider arc dances from plate to plate. The puddle starts at the toes of the weld and flows toward the center (view 60 Hz video). On some joints, you're almost compelled to over-weld to ensure penetration at the root.
Other Aluminations Articles
Issue 1 - See if an inverter-based AC/DC TIG machine that incorporates advanced Squarewave technology can improve your operation.
Issue 2 - How to TIG weld thin aluminum, adjust balance control and determine amperage requirements.
Issue 3 - New push-pull feeder technology improves MIG welding.
Issue 4 - Why you should consider pulsed MIG for your aluminum welding jobs.
Issue 5 - When you control the shape of an AC TIG weld bead, you control profitability.