Test drive inverters before buying.

Richard Harris, consulting editor

Gardner grinds electrodes longitudinally because grooves direct the ion flow on the surface and aid arc starting. The truncated point controls bead width and limits the heat zone.

Running multiple beads with various tungsten electrodes on 2 3 5-in. strips of type 3003 aluminum tests for reliable arc starts on aluminum.

The electrodes from Diamond Ground Products include (top to bottom) orange — ceriated, blue — lanthanated, red — thoriated, and white — zirconated.

In WDF's testing, the Thermal Arc inverter power source ensured reliable arc starts.

To scroll for his settings on the Pro-Wave 300TSW inverter power source, Gardner pushes buttons through a protective cover. A symbol and lighted LED marks the function being adjusted.

When repairing an aluminum component inside a bowling-alley machine, Gardner relies on the built-in track control on CK Worldwide's Trimline 26 torch, which plugs into the Thermal Arc power source.

When buying inverter power sources, welders should begin by "test driving" specified tungsten electrodes. This policy can minimize or even eliminate the need to re-qualify weld procedures and avoid other aggravations as well. But what things should the testers examine?

Are reliable GTAW arc starts possible on aluminum?
Yes, if welders use an inverter power source that provides a low starting amperage followed by a surge of current to heat the workpiece.

Buyers of inverter power sources should be aware that not all inverters reliably start an arc with some tungsten electrodes or without a specifically ground point. These limitations could require frequent redressing of the electrode point or requalifying the weld procedures.

What creates an arc start?
A reliable arc with gas-tungsten-arc welding (GTAW) on aluminum begins with a basic understanding of the variables affecting the process.

Process variables for GTAW, from Jefferson's Welding Encyclopedia, include the primary variables of arc voltage (arc length), welding current, travel speed, and shielding gas. The amount of energy the arc produces is proportional to the current and voltage. The amount transferred per unit length of weld is inversely proportional to the travel speed. The arc in helium is more penetrating than that in argon. Because these variables interact strongly, it is impossible to treat them independently when establishing welding procedures for fabricating specific joints.

To focus on the arc, CK Worldwide Inc., Auburn, Wash., explains the characteristics of three current types for GTAW: direct-current straight polarity (DCSP), direct-current reverse polarity (DCRP), and alternating current with high frequency (ACHF) stabilization. Each has applications, advantages, and disadvantages. A look at each type and its uses helps welders select the best type for a job. The type of current used has a great impact on the penetration pattern as well as the bead configuration.

DCSP produces deep penetration because it concentrates the heat in the joint area. No cleaning action occurs with this polarity.

DCRP produces good cleaning action as the argon ions flowing toward the work strike with sufficient force to break up oxides on the surface. The electrode overheats.

ACHF combines the good weld penetration on the negative half cycle with the desired cleaning action of the positive half cycle. High frequency reestablishes the arc, which breaks each half cycle.

To start an arc in alternating current welding, to paraphrase Jefferson, the welder does not have to touch the workpiece with the electrode, as in DC welding. The superimposed high-frequency current jumps the gap between the welding electrode and the work, establishing a path for the welding current.

To strike an arc, the welder turns on the power supply and holds the torch about 2 in. above the workpiece. He then quickly swings down the end of the torch toward the workpiece, stopping the end of the electrode about ¹ / 8 in. above the plate. The arc then strikes.

The rapid downward motion provides the maximum gas protection to the electrode and the molten weld metal from atmospheric contamination. Keeping the electrode approximately ¹ /8 in. above the workpiece avoids contaminating the electrode in the molten puddle.

Aluminum welding qualities
"Using AC power to weld aluminum requires a ratio between arc penetration — the negative side of the AC wave — and cleaning action — the positive side of the wave," states Bob Gardner, a shop owner who conducts tests for WELDING DESIGN & FABRICATION.

"Because of wave control on inverter power sources, I can adjust the percent of positive for cleaning or negative for penetration," says Gardner, who is also an AWS-certified welding instructor and certified welding educator.

"The machine settings make the weld easier," he comments. "I can set the AC frequency to focus the arc and the wave balance for penetration and cleaning.

"For aluminum, I normally start with a setting of 65% negative, at 140 amp, 120 cycles/sec Hz, Ar shielding gas preflow at a half second, and after flow at 20 sec," he states.

"The ability to adjust AC frequency for a small focused arc minimizes the heat into the aluminum," says Gardner. "Most aluminum welds are heat sensitive.

Because aluminum melts at 1,220° F and the electrode temperature reaches 6,170° F, its melting point, the welder can quickly burn through the workpiece, then either try to repair the hole or start all over."

How was the test performed?
To test for consistent arc starts on aluminum, Gardner repaired an aluminum casting and ran multiple beads on 2 3 5-in. strips of Type 3003 aluminum.

To repair the casting — a skag or stabilizer, a part of an outdrive assembly for turning a power boat — Gardner prepped the casting joint with a double V. The casting ranged from ¹ /4-in. thick. Gardner wants 100% penetration with one pass on each side of the joint with 4043 filler rod. "I set the machine for 160 amp, 120 Hz, and 65% electrode negative for good cleaning with a ³ /32-in.-diameter zirconated electrode," he explains.

Power-source setup
Equipment and materials for this test include a Pro-Wave 300TSW inverter power source from Thermal Arc (part of Thermadyne Holdings, St. Louis), torch and hand control from CK Worldwide, and tungsten electrodes from Diamond Ground Products Inc., Newbury Park, Calif.

After switching the inverter to AC on the control panel, Gardner pushes buttons, not membrane switches, through a protective cover to scroll for his settings. A symbol and lighted LED marks the function being adjusted. He prefers to start with a frequency between 80 and 120 Hz that gives him a pinpointed arc.

"The adjustable wave balance on most inverters runs from 10% to 65% of electrode negative," says Gardner. "The conventional machines designed for GTAW were unbalanced."

A 90% electrode negative setting gives maximum weld penetration. The 65% setting offers a good balance between cleaning and penetration of heavily oxidized aluminum or magnesium alloys.

In an inverter power supply, solid-state components change incoming 60 Hz current to a variable frequency, typically between 18 to 150 kHz, depending on the manufacturer.

Most manufacturers design constant-current-type power sources for GTAW. These power sources typically have either drooping or nearly true constant-current static output qualities. Static output is a function of the welding current control used in the power source design.

Features on the 300TSW, AC/DC GTAW power supply include a digital amp/volt meter, gas solenoid valve, remote-control capabilities, VRD (voltage-reduction device) circuit, current pulsing and sloping with repeat function, and programmable controls with memory for five welding programs.

The unit is a single/three-phase AC/DC power source with constant current (cc) output. Its capability ranges from 300 down to 5 amp. Precise wave shaping allows the operator to control the cleaning, penetration, and heat input of any aluminum application. Specs include 300 amp, 22 V at 25% duty cycle, or 150 amp, 16 V at 100% duty cycle.

Gardner took advantage of three features on the machine to set and operate it: wave balance and frequency controls as well as pulse mode to pulse the arc.

CK Worldwide provided a complete torch package ready to plug into the Thermal Arc power source. It includes a Trimline 26 200-amp air-cooled torch with a flex head having built-in current control. It uses number 3 Linde torch wear parts, cable, and adapters for the inverter power source.

Tungsten electrode prep
The American Welding Society classifies tungsten electrodes based on chemical compositions in ANSI/AWS A5.12, Specifications for Tungsten and Tungsten Alloy Electrodes for Arc Welding and Cutting. Electrodes are produced with either a clean finish or a ground finish. Chemical cleaning removes surface impurities after the forming operation. Centerless grounding removes imperfections.

Electrode manufacturers offer number of chemical compositions, diameters, and lengths. The Procedure Qualification Specification, if available for the job, states the electrode composition, size, and tip configuration. Welders can also use AWS guides.

For this product test, Diamond Ground Products sent samples of 2% zirconated, 2% thoriated, 2% ceriated, 1.5% lanthanated, 2% lanthanated, Tri-Mix, pure tungsten, and Cryo-T tungsten. The firm offers assorted sizes: 0.020, 0.040, ¹ /16, ³ /32, ¹ /8, ⁵ /32, ³ /16, and ¹ /4-in. diameters by 7 in. lengths, plus special diameters and lengths of uncommon materials.

A recommendation for testing inverters is to try lanthanated or 2% thoriated tungsten with a truncated grind on aluminum. Run beads and repeat to determine multiple start capability.

Gardner likes to use diameter rod because it is easier to grind to match the application. "For most jobs without a PQS, choosing the electrode type — zirconated, 2% thoriated, or pure tungsten — and the tungsten prep is personal, like how you adjust your under-wear," he comments.

The electrodes work with a variety of tip preparations. "Electrodes are ground longitudinally because grooves direct the ion flow on the surface and aid the arc starting," remarks Gardner. "When using GTAW on steel, the pointed tungsten tip is normally ground to 3 3 the diameter of the electrode, while for aluminum the ground tip is tapered with a flattened point. The truncated point minimizes bead width and thus limits the heat-affected zone."

Was the arc start reliable?
The arc repeatedly started, running the beads with each of the electrodes, regardless of tip condition. Even after the testers intentionally dipped the tip in the weld puddle, the contaminated electrode restarted.

"This machine arc starts all tungsten rods on aluminum, point or no point," states Gardner. "I think it could light a matchstick. This inverter is similar to a transformer-type power source for GTAW because it is not electrode sensitive."

For more information call: Thermal Arc (937) 440-0100, CK Worldwide Inc. (800) 426-0877, or Diamond Ground Products Inc. (805) 498-3837.