Gas Tungsten Arc Welding

Argon when used with DCEP

acts as a cleaning agent. The electrons from the cathode surface lift the oxides that interfere with their movement.

GTA welding is the ____ of all the manual welding processes.

cleanest.

Because there is no ____ used to clean the weld in GTA welding, all surface contamination such as oxides, oil, dirt, and so on, must be cleaned from the part being welded and the filler metal so they do not contaminate the weld.

flux

EWP - Green, Pure Tungsten Electrode

has the poorest heat resistance and electron emission characteristics of all the tungsten electrodes. It can be used with AC for welding metals

GTA can be used to make extremely ___ quality welds.

high

The electrode in GTAW is:

non consumable.

How can the end of a tungsten electrode be shaped?

• Grinding • Breaking • Remelting the end • Chemical Compounds Tungsten is easily broken so the welder must be careful when breaking it.

Why should the grinding marks run lengthwise on the tungsten electrode?

• Lengthwise grinding reduces the amount of small particles of tungsten contaminating the weld.

What is the ideal heat distribution with DCEP Welding Current?

Electrode absorbs 2/3 of the heat. Work absorbs 1/3 of the heat.

1. What materials were weldable only by the GTAW process before GMAW was developed?

Reactive Materials (prone to contamination at welding temps). • Aluminum • Magnesium • Titanium • Some grades of Stainless Steel

Ways to prevent tungsten electrode erosion:

1. Having good mechanical and electrical contact. 2. Using as low a current as possible. 3. Using a water-cooled torch. 4. Using as large a size of tungsten electrode as possible. 5. using DCEN current. 6. Using as short an electrode extension from the collet as possible. 7. Using the proper electrode end shape. 8. Using an alloyed tungsten electrode.

GTAW Process

An arc is established between a nonconsumable tungsten electrode and the base metal which is called the work. The electrode doesn't melt, but the work does at the spot where the arc impacts its surface and produces a molten weld pool where it melts. Since hot tungsten is sensitive to oxygen contamination, a goodinert shielding gas is required to keep the air away from the hot tungsten and molten weld metal.

28. What makes helium difficult to use for manual welding?

Because penetration and bead profile are sensitive to the arc length and the long arcs needed for feeding filler wires are more difficult to control.

GTAW Electrodes

EWP = Pure Tungsten EWTh-1 = 1% Thorium Tungsten EWTh-1 = 2% Thorium Tungsten EWZr = 1/4% to 1/2% zirconium tungsten. EWCe-2 =2% cerium tungsten EWLa-1 = 1% lanthanum tungsten EWG = Alloy not specific

GTAW class summary

GTAW was originally developed for metals that were difficult to weld by the and SMAW processes. Today it is widely used for clean, high-quality, precision welding applications in almost all metals used in construction and maintenance of ships and power plants.

EWZr - Brown, Zirconium Tungsten Electrode

Helps emit electrons freely. Same effect as thorium, but to a lesser degree. Are more easily melted than thorium tungsten so can be used with both AC and DC currents. They can be used with the balled end and are good for AC welding of aluminum and magnesium. Good resistance to weld pool contamination.

EWCeO2- Orange, Cerium Tungsten Electrode

Improves current carrying capacity. Not made of radioactive material. Improved arc starting and arc stability. Slightly higher voltage.

Function of Inert Gas in GTAW welding:

Inert gas provides the needed arc characteristics and protects the molten weld pool. Because flexes are not used, the welds produced are sound, free of slags, and as corrosion resistant as the parent metal.

EWG - No color specified, Alloy not specified Electrode

Manufacturer specific alloys.

EWLa-1- Black, Lonathium Tungsten Electrode

Not radioactive. Improved current carrying capacity. Slightly higher arc voltage than cerium and thorium.

Electrode Tip Shape: DCEN DCEP AC

Pointed Rounded

"How might air be drawn into the shielding gas between the nozzle and the weld?"

The air can be drawn in under the nozzle if the torch is held at too sharp an angle to the metal.

EWTH-1 and EWTH-2 Yellow and Red, Thoriated Tungsten

Thorium oxide (ThO2), when added in percentages of up to 0.6% to tungsten, improves its current carrying capacity. The addition of 1% to 2% of thorium oxides improves electron emission. Thorium increases the serviceable life of the electrode. Easier arc starting. Work well with DCEN. Good for steel, steel alloys, nickel alloys, and most other metals other than aluminum or magnesium. Not suitable for AC.

What does adding thorium oxide do for the tungsten electrode?

What does adding thorium oxide do for the tungsten electrode? • When added in percentages of up to .6% to tungsten, current carrying capacity is improved. • Further addition of thorium oxide to 1-2% helps with electron emission allowing it to carry approximately 20% more current. This results in a reduction of the electrode tip temperature, resulting in less tungsten erosion and subsequent weld contamination. • Also provide a much easier arc starting characteristic than pure or zirconiated tungsten. • They work well with DCEN and maintain a sharpened point well. Do not use with AC. • Very suitable for welding on steel, steel alloys, nickel alloys, and most metals other than aluminum and magnesium.

Gas Tungsten Arc Welding (GTAW)

uses a noncomsumable electrode made of tungsten. In GTA welding the arc between the electrode and the base metal melts the base metal and the end of the filler metal as it is manually dipped into the molten weld pool. A shielding gas flowing from the gun nozel protects the molten weld metal from atmospheric contamination. A foot for thumb remote control switch may be added to the basic GTA welding setup to allow the welder better controls. This remote control switch is often used to start and stop the welding current as well as make adjustments in the power level.

33. Using Table 15-4, determine the minimum gas flow rate for a 1/2'" (13mm) nozzle.

• 17-20 cfh, or 8-9.4 L/min

12. Why should a grinding stone that is used for sharpening tungsten not be used for other metals?

• Because of the hardness of tungsten and its brittleness, the grinding stone chips off small particles of the electrode. A course grinding stone will result in more tungsten breakage and a poorer finish. If the grinder is used for metals other than tungsten, particles (of other metals) may become trapped on the tungsten as it is ground. The metal particles will quickly break free when the arc is started resulting in contamination.

What materials can be used to make nozzles?

• Ceramic such as Alumina or Silicon Nitride (opaque) • Fused Quartz (clear)

What is the heat distribution with DCEN welding current?

• Electrode absorbs 1/3 of the heat. • Work absorbs 2/3 of the heat.

24. What is the heat distribution with AC welding current?

• Electrode absorbs ½ of the heat. • Work absorbs ½ of the heat.

1. What early advancements made the GTA welding process more effective and reduced its cost?

• Improvement in gas composition (switching from helium to tungsten) and a better understanding of the importance of polarity (all three polarities are used, but differ for different applications) improved the effectiveness of the process and reduced its cost

27. Why is argon's ease of ionization a benefit?

• It makes it suitable for alternating current applications and easier starts. It also permits fairly long arcs at lower voltages, making it virtually insensitive to changes in arc length.. Argon is also good for welding in deep grooves because it is denser than air.

Why must AC welding power use high frequencies in order to work?

• The high frequency ensures that a voltage peak will occur reasonably close to the current reversal in the welding arc, creating a low resistance ionized path for the welding current to follow.

2. Which two of tungsten's properties make it the best choice for GTA welding?

• The high melting temperature and good electrical conductivity. Hardness Tensile Strength

What holds the molten ball of tungsten in place at the tip of the electrode during DCEP welding?

• The liquid tungsten surface tension with the larger mass above the molten ball holds it in place like a drop of water on your fingertip.

7. What problem can an excessively large tungsten cause?

• They may result in too low a temperature for a stable arc.

Gas Tungsten Arc Welding

GTAW, TIG