Improving TIG Aluminum Results

If you weld aluminum or magnesium and haven't experimented with the newest generation of AC TIG welding inverters, there's a good chance you're leaving money on the table.
New TIG inverters give fabricators and manufacturers a broader "set of tools" that they can use to solve their welding challenges and business issues. They are:

• ?Independent amperage control during electrode negative (EN) and electrode positive (EP) portions of the AC cycle to precisely control heat input into the work and the electrode.
• ?AC waveform shaping to adjust the wave shape. Shapes such as advanced squarewave, soft squarewave, sine wave and triangular wave provide different arc and puddle characteristics.
• ?Adjustable output frequency to control the width of the arc cone (hence weld bead width), as well as control the arc force for better direction control over the weld puddle.
• ?Extended balance control from 30 to 99 percent electrode positive, adjusting the amount of "cleaning action" used to remove oxides.

These tools have enabled a Florida fabricator of aluminum docks to increase productivity by 18 percent, a California wheelchair manufacturer to increase productivity 30 to 50 percent, a Nebraska tube mill to increase travel speeds by 10 percent and a Florida marine product fabricator to cut welding time in half. In every case, the newly acquired TIG inverters:

• ?Increased productivity to help meet company goals
• ?Solved problems that were never before solvable
• ?Provided rapid payback (typically in months) and solid ROI

Ideally, the job of any welder is to create a weld bead with just the right profile and do it as quickly and as cost efficiently as possible. In fact, doing anything else reduces competitiveness and wastes consumables and precious labor time. For example, if the leg of a fillet weld should be 3/16 in but the welder over-welds and makes a 5/16 in weld instead, the joint would require a 177 percent increase in weld metal deposition and shielding gas use, as well as take 177 percent more arc-on time to complete!

Given the fact that a conventional TIG welder creates a relatively broad arc cone and there's nothing the operator can do to change it, the above scenario occurs more frequently that most people realize. Further, given the scarcity and cost of trained TIG welders, does any company truly want to apply 1.77 people for a job that 1.0 person could do if they were given the right tools? Of course not! Here's how advanced AC TIG technology provides welders with a new set of tools.

Independent Amperage Control

Independent amperage control of the EN and EP portions of the AC cycle allows the operator to direct more or less energy into the work piece, as well as take heat off the tungsten (see Figure 1).

Figure 1

For example, when welding thick aluminum, the operator can put 350 amps of EN into the work and only 175 amps of EP into the tungsten. This provides faster travel speeds, faster feed of filler rods, deeper penetration, and the potential to eliminate pre-heating. A California manufacturer of vacuum chambers cut production time by up to two-thirds using this technology.

Independently increasing EN while maintaining or reducing EP permits the use of smaller diameter tungstens to more precisely direct heat to the weld. The Nebraska tube mill superintendent reports that the company went from a 5/16-in electrode, which was a special order and cost $46 each, to an off-the-shelf 1/4-in tungsten that cost $26. Further, taking heat off the electrode extends tungsten life and reduces the need redress the tungsten, and it reduced the amount of helium used in the gas mix by 10 cu/ft per hour.

Independently increasing EN amperage while maintaining or reducing EP amperage also narrows the arc cone, nearly eliminating the etched zone at the toes of welds. Independently increasing EP amperage while maintaining or reducing EN amperage produces a wider arc cone, wider bead and shallower penetration.

Choose your Wave

Changing the shape of AC wave (see Figure 2) significantly changes the arc and puddle characteristics, as well as penetration profile.

Figure 2

Waveforms available today include:

• ?Sine wave, which gives the arc a "traditional" and "soft" feel. This arc provides good wetting action and sounds quieter. Note that the wave uses a square, or fast, transition through the zero amperage point, which eliminates the need for continuous high frequency.
• ?Advanced squarewave, which gives fast transitions for a responsive, dynamic arc with better focus and directional control. Chose this wave shape for a fast-freezing weld puddle, deep penetration and fast travel speeds.
• ?Soft squarewave, which provides a softer, "buttery" arc with maximum puddle control and good wetting action. The puddle is more fluid than the advanced squarewave puddle and more controllable than the sine wave puddle.
• ?Triangular wave, which creates a quick peak of amperage while reducing overall heat input. This leads to quick puddle formation, making it good for thin aluminum and reduced weld distortion, as well as faster travel speeds.

AC Frequency Control

If you think of welding current as water coming out of a garden hose, then frequency control would be like adding an adjustable brass nozzle to control the spray pattern. Watering a flowerbed calls for a wide, soft pattern to prevent damage, while rising dirt off the driveway calls for a focused, driving blast of water.

Conventional AC TIG welders have a fixed output of 50 or 60 Hz, but advanced TIG inverters adjust the welding output frequency from 20 to 400 Hz.

Increasing frequency narrows the arc cone and increases arc force. This creates deeper penetration and it narrows the weld bead (see Video clip 1).

Higher frequencies increase travel speed and prevent over-welding, often leading to productivity increases of 15 to 20 percent or more. They help when welding in corners, on root passes and fillet welds.

In a fillet weld with conventional TIG technology, the arc is prone to "wandering" between the two plates as the current seeks the path of least resistance (see Video clip 2).

Conversely, higher frequencies direct the arc precisely at the joint without any wandering (Video clip 3), giving the operator much greater control over the weld puddle and weld bead placement.

A good starting point for general welding would be 75 to 150 Hz. For a fillet weld with full penetration, increase the frequency. For build up work and to spread the heat over a wider area, start at 60 Hz and adjust lower from there.

Extended Balance Control

Using AC to TIG weld aluminum evolved from the need to remove the oxide layer that forms on its surface. The newest TIG inverters, because of their advanced power switching capabilities, provide extended balance control. Operators can fine tune the duration of the EN portion of the cycle from 30 to 99 percent.

Greater amounts of EN create a deeper, narrower weld bead, better joint penetration and a smaller etched zone. This helps when welding on thick material or when appearance (i.e. a minimal etched zone) is important. Setting an inverter's EN duration to the maximum level creates the potential to deliver more heat into the work, permitting faster travel. Lesser amounts of EN (e.g., more EP) remove more oxide and create a shallower, wider bead.

Some companies have even experimented with welding ferrous metals using AC TIG with a high percentage of EN. These metals tend to be contaminated, such as saturated with an oil, and benefit from just a few percentage points cleaning action.

Synergy

Any one of the new tools noted above provide benefits worth exploring. Combined, they can revolutionize an AC TIG welding operation. Benefits include:

• ?More parts out the door without adding more operators
• ?Better weld quality with fewer rejects
• ?Lower consumables cost
• ?Fast payback and strong ROI
• ?More satisfied operators, because these new tools give them more control and make welding easier

In conclusion, note that new TIG inverters draw about one-quarter of the primary power of traditional TIG welders. They eliminate the need for costly upgrades to primary service when adding more welders, they free up power to run other equipment and they can lower utilities¾all benefits that you can take to the bank.

John Luck is a product manager for Miller Electric Mfg. Co., 1635 W. Spencer Street, P.O. Box 1079, Appleton, WI 54912-1079, 920-734-9821, www.millerwelds.com.