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GTAW Welding Aluminum with Alternating Current-High Frequency

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GTAW Welding Aluminum with Alternating Current - High Frequency

Gas Tungsten Arc Welding (GTAW) process utilizes heat generated by an electric arc maintained between the workpiece and a nonconsumable ungsten electrode. The arc is enveloped by a stream of inert gas. GTAW weld quality is primarily controlled by workpiece, filler wire, electrode quality, type of power source, and welder technique. Discussed below are several important items that must be addressed in order to produce high quality welds.

High Frequency - The high frequency mode will initiate and maintain the arc during the zero crossing of the A.C. sine wave. Three positions exist on most GTAW machines: 1) Start - This mode helps arc initiation without making actual contact to the work with the electrode. The “Start” mode is most often used in D.C. welding. 2) Continuous - This also helps initiate the arc and continues throughout the process to maintain the arc during periods when current (amperage) is at the zero crossing point of the sine wave. This mode is most often used in A.C. welding. 3) Off - The high frequency system does not engage during any part of the process in this mode. Contact between the electrode and work surface must occur before the arc can be initiated. A “Touch Start Practice” to initiate the arc can cause contamination of the electrode in the GTAW process. The “Off” mode is often used for stick welding (SMAW) where scratch starting will initiate the arc.

Using a Balanced Wave Transformer - The balanced wave designation or Balanced Wave Control indicates that the power supply can alter the current sine wave in the A.C. mode. A normal sine wave will show an even division between each polarity’s dwell time. With equipment supported by the Balanced Wave Control, the dwell time can be extended during either the positive or negative cycle. (Figure #1).

Square Wave designates the shape of the current wave. It appears in a square, cyclical wave pattern, rather than the standard smooth sine wave cycle. The ability to produce the square wave allows the power supply to utilize the Balanced Wave Control feature to create an arc that can add either more penetration or more cleaning action. Since penetration occurs during the negative side of the wave cycle (electrode negative - EN), and cleaning occurs during the positive side of the wave cycle (electrode positive - EP) a change in portions of cycle will increase the desired characteristics. This process is accomplished by using SCR’s (Silicon Controlled Rectifiers), which act as control gates for current of either EN or EP. Precise timing must be controlled as these gates open and close. This results in a crisp arc as polarity transitions through the zero part of the cycle.

Torch Angles and Bead Placement: The proper manipulation of the welding torch is very important in making a good weld. (Figure #2) The torch is held like a pencil to improve control. When welding in the flat position, the hand should be placed lightly on a surface, so that the hand can move across the joint evenly. Movement of the torch by the fingers alone usually results in incorrect torch angles and a poor weld. When adding filler wire, the wire should be gripped in the fingers. The hand should be as close as possible to the arc to hold the wire steady. The wire should move in conjunction with the torch movement. When adding wire, move the wire with the thumb through the fingers. The end of the wire should extend 6 to 8 inches from the hand. Too much extension of the filler wire results in a wobbly wire end making the puddle uneven and allowing the filler wire to become contaminated. Adding wire to the puddle requires steadiness and concentration to place the right amount of material at the right place, at exactly the right time. Torch angles vary only slightly depending on weld position. The torch is generally held 10 degrees forward from perpendicular to the weld, thus pushing into the point and in the direction of travel. The filler rod is added ahead of the weld pool 10 to 25 degrees from the plane of the weld bead.

Weld Tip (Electrode): - Pure, Zirconiated, and Ceriated are the recommended tungsten electrodes for use in A.C. welding. Thoriated electrodes are generally reserved for D.C. welding of products such as low alloy steels, stainless, and other high heat-input welding. Thoriated tungsten will handle a higher current than pure tungsten, although it does not retain the balled shape required for A.C. welding of aluminum.

The Pure Tungsten electrode is often recommended and used for A.C. welding on aluminum and magnesium. These contain a minimum of 99.5% tungsten, with no alloying elements intentionally added. By using high purity tungsten, current carrying capability is diminished, although it maintains a clean, balled end which provides good arc stability.

Zirconiated tungsten electrodes have arc stability characteristics that are similar to pure tungsten besides the higher current carrying capability found in the thoriated tungsten. This electrode provides a good balance of properties. It is more resistant to contamination than pure tungsten and better for radiographic-quality welding applications than thoriated tungsten.

Ceriated tungsten electrodes have an addition of 2% cerium oxide (CeO2) which helps to reduce electrode burn-off. In performance, the ceriated electrodes will react much like pure tungsten by providing a stable arc and reducing the amount of tungsten “spitting.” These characteristics allow this electrode to perform well on aluminum in balanced wave machines (A.C.) and on steel in the D.C. mode.

Cold Wire Feeders: - Cold wire feeders are used in manual, semi, and fully automatic welding operations where filler wire is required. They are adaptable to both hard and soft wires. Various types of drive mechanisms and guides are used to adapt the units to different diameters of filler wire. The units may be equipped with rheostats, or the more accurate, digital tachometer drive to give consistent smooth feed and readout. All systems use a drive roll mechanism that requires smooth, U-grooved drive rolls for aluminum wires. Conduits with varying diameter liners are used to feed the wire from the feeder to the manipulator. Conduits require replacement when changing wire diameters, when worn, or if contaminated by dirt or oil.