gas tungsten arc welding equipment and...

OBJECTIVESAfter completing this chapter, the student should be able to:

Demonstrate how to set up a gas tungsten arc (GTA) welding station. Identify different types of tungsten electrodes and explain their uses. List the different GTA welding currents and explain their effects on

welding. List the different GTA welding shielding gases and explain how they are

used.

KEY TERMS

339

Chapter 15Gas Tungsten Arc Welding Equipment and Materials

cleaning action

collet

flowmeter

frequency

inert gas

noble inert gases

postflow

preflow

tungsten

INTRODUCTIONThe gas tungsten arc welding (GTAW) process is sometimes referred to as TIG, or heliarc. The term TIG is short for tungsten inert gas welding. Under the correct welding conditions, the tungsten electrode does not melt and is considered to be nonconsumable. The surface of the metal being welded does melt at the spot where the arc impacts its surface. This produces a molten weld pool.

To make a weld, either the edges of the metal must melt and flow together by themselves or filler metal must be added directly into the molten pool. Filler metal is added by dipping the end of a filler rod into the leading edge of the molten weld pool. Most metals oxidize rapidly in their molten state.

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340 CHAPTER 15

To prevent oxidation from occurring, an inert gas flows out of the welding torch, surrounding the hot tungsten and molten weld metal shielding it from atmospheric oxygen.

GTA welding is efficient for welding metals rang-ing from sheet metal up to 1/4 in. The eyehand coordination required to make GTA welds is very similar to the coordination required for oxyfuel gas welding.

Two of the advantages of GTA welding for weld-ing fabrication are that it can be used to produce very high-quality welds and it can be used to weld on almost any metal. Two of the limitations of GTA welding are the slow welding rate and tedious nature, both of which limit its use to small projects or high-integrity critical welds. Although most other welding processes are faster and less expensive, the clean, neat, slag-free welds GTAW produces are used because of their appearance and ease of finishing.

GTA WELDING EQUIPMENT Four major components make up a GTA welding sta-tion. They are the welding power supply, often called the welder; the welding torch, often called a TIG torch; the work clamp, sometimes called the ground clamp; and the shielding gas cylinder, Figure 15-1. There are a variety of hoses and cables that connect all three of these components together.

GTA WELDING TORCHES GTA welding torches are available water-cooled or air-cooled, Figure 15-2. The heat transfer efficiency for GTA welding may be as low as 20%. This means that 80% of the heat generated does not enter the weld. Much of this heat stays in the torch. To avoid damage to the torch, the heat must be removed by some type of cooling method. Following are some of the advantages of air-cooled GTAW torches:

Lighter weight for the same amperage range Easier to manipulate without the water hoses More portable Easier to maintain No water supply required No water leakage danger

The above advantages of the air-cooled GTAW torches are the disadvantages of water-cooled GTAW torches. This is a case in which the advan-tages of one are the disadvantages of the other.

Some of the advantages of the water-cooled GTAW torches include the following:

Continuous operation without overheating Lower torch temperatures means less tungsten erosionLess torch handle temperature in the welders hands

The above advantages of the water-cooled GTAW torches are the disadvantages of air-cooled GTAW torches.

WELDING MACHINE

ACOFF

ON

GASWATER

ININ

DC

WELDING TORCH

SHIELDINGGAS CYLINDER

WORK CLAMP

OUT OUT

FIGURE 15-1 Gas tungsten gas welding station setup. Cengage Learning 2012

FIGURE 15-2 Power cable safety fuse. ESAB Welding and Cutting Products


Gas Tungsten Arc Welding Equipment and Materials 341

The advantages of air-cooled torches make them the preferred GTA welding torch type for most small shops. The lower operating temperature and con-tinuous operating ability of the water-cooled GTA welding torches make them the preferred torch type for most production welding.

GTA welding torch heads are available in a vari-ety of amperage ranges and designs, Figure 15-3. The amperage listed on a torch is the maximum rating and cannot be exceeded without possible damage to the torch. The various head angles allow better access in tight places. Some of the heads can be swiveled eas-ily to new angles. The back cap that both protects and tightens the tungsten can be short or long, Figure 15-4 and Figure 15-5.

Shielding Gas HoseAir-cooled torches have just the shielding gas hose that needs to be connected to the welding machine or flowmeter, Figure 15-6. The shielding gas hose must be plastic to prevent the gas from being contami-nated. Rubber hoses contain oils that can be picked up by the gas, resulting in weld contamination.

Water Hoses Water-cooled torches have three hoses connecting to the welding machine. In addition to the shielding gas hose, they have two cooling water hoses, Figure 15-7. One hose is for transporting the cooling water to the torch. This allows the head to receive the maximum cooling from the water. The power cable is usually inside the second hose, which is for the return cool-ing water. By running the power cable through the return water line, it is kept cool. This permits a much smaller-size cable to be used because the water keeps it cool. The smaller diameter cable is more flexible.

The water-in hose may be made of any sturdy material. Water hose fittings have left-hand threads, and gas hose fittings have right-hand threads. This pre-vents the water and gas hoses from accidentally being FIGURE 15-3 GTA welding torches. Larry Jeffus

FIGURE 15-4 Short back caps are available for torches when space is a problem. ESAB Welding and Cutting Products

FIGURE 15-5 Long back caps allow tungstens that are a full 7 in. (177 mm) long to be used. ESAB Welding and Cutting Products


342 CHAPTER 15

reversed when attaching them to the welder. The return water hose also contains the welding power cable.

A protective covering can be used to prevent the hoses from becoming damaged by hot metal, Figure 15-8. Even with this protection, the hoses should be sup-ported, Figure 15-9, so that they are not underfoot on the floor. Supporting the hoses reduces the chance of their being damaged by hot sparks.

Cooling WaterThere are two GTA welding torch water-cooling sys-tems. One system is an open system and the other is

ARGONGAS

TORCH

POWER CABLE AND GAS HOSE

CABLE ADAPTER

ARGON HOSEFLOWMETER REGULATOR

WELDING POWER CABLE

POWER SUPPLY

WORK LEAD

FIGURE 15-6 Schematic of GTA welding setup with air-cooled torch. Cengage Learning 2012

TORCH

TORCH CABLE

ARGON HOSE

FLOWMETER REGULATOR

SAFETY FUSE

WATER OUTLET HOSE

ARGONGASWELDING POWER CABLE

POWERSUPPLY

CABLE ADAPTER

WATER INLET HOSE

WORK LEAD RECIRCULATING WATER PUMP

FIGURE 15-7 Schematic of GTA welding setup with water-cooled torch. Cengage Learning 2012

FIGURE 15-8 Zip-on protective covering also helps keep the hoses neat. ESAB Welding and Cutting Products



a recirculating system. The open system uses pota-ble water from the building or shops fresh drinking water supply. The water passes through a pressure regulator, then once through the torch, and then down the drain. Water pressures higher than 35 psi may cause the water hoses to burst. These systems are not water conservative, and many local com-munities, cities, or states have ordinances or laws restricting their use.

Recirculating systems use water pumps to circulate water through the torch. An air-to-water coil and fan in the unit cool the water, Figure 15-10. A low con-ductive antifreeze solution may be added to the water to prevent freezing and corrosion. Only manufacturer-approved antifreeze solutions may be used.

Shielding Gas Nozzles The nozzle or cup is used to direct the shielding gas directly on the welding zone. The nozzle size is deter-mined by the diameter of the opening and its length,

Table 15-1. Nozzles may be made from a ceramic such as alumina or silicon nitride (opaque) or from fused quartz (clear). The nozzle may also have a gas lens to improve the gas flow pattern.

The nozzle size, both length and diameter, is often the welders personal preference. Occasion-ally, a specific choice must be made based upon joint design or location. Small nozzle diameters allow the welder to better see the molten weld pool and can be operated with lower gas flow rates. Larger nozzle diameters can give better gas coverage, even in drafty places.

Ceramic nozzles are heat-resistant and offer a relatively long life. The useful life of a ceramic nozzle is affected by the current level and proximity to the work. Silicon nitride nozzles withstand much more heat, resulting in a longer useful life.

The fused quartz (glass) used in a nozzle is a spe-cial type that can withstand the welding heat. These nozzles are no more easily broken than ceramic ones but are more expensive. The added visibility

FIGURE 15-9 A bracket holds the leads off the floor.Larry Jeffus

WATER TO TORCH

WATER RETURN

WATER FLOWINDICATOR

POWER SWITCH

FIGURE 15-10 Typical GTA welding machine connections. Cengage Learning 2012

in. (mm) in. (mm)

1/16 (2) 1/4 to 3/8 (6 to 10)3/32 (2.4) 3/8 to 7/16 (10 to 11)

3/16 (4.8) 1/2 to 3/4 (13 to 19)

1/8 (3) 7/16 to 1/2 (11 to 13)

Nozzle OrificeDiameter

Tungsten ElectrodeDiameter

Table 15-1 Recommended Cup Sizes


344 CHAPTER 15

with glass nozzles in tight, hard-to-reach places is often worth the added expense. The longer a nozzle, the longer the tungsten must be extended from the collet. This can cause higher tungsten temperatures, resulting in greater tungsten erosion. When using long nozzles, it is better to use low amperages or a larger-size tungsten.

TUNGSTEN ELECTRODES The high melting temperature and good electri-cal conductivity make tungsten the best choice for a nonconsumable electrode. As the tungsten electrode becomes hot, the arc between the electrode and the work will stabilize. Because electrons are more freely emitted from hot tungsten, the very highest tempera-ture possible at the tungsten electrode tip is desired. A balance must be maintained between the tempera-ture required to have a stable arc and one too high that would melt the tungsten.

The thermal conductivity of tungsten is what allows the tungsten electrode to withstand the arc tem-perature well above its melting temperature. The heat of the arc is conducted away from the electrodes end so fast that it does not reach its melting temperature.

Because of the intense heat of the arc, some ero-sion of the electrode will occur. This eroded metal is transferred across the arc, Figure 15-11. Slow ero-sion of the electrode results in limited tungsten drop-lets entering the weld, which are acceptable. Standard codes give the size and amount of tungsten inclu-sions that are allowable in various types of welds. The tungsten inclusions are hard spots that cause stresses to concentrate, possibly resulting in weld failure.

Although tungsten erosion cannot be completely eliminated, it can be controlled. Following are a few ways of limiting erosion:

Have a good mechanical and electrical contact between the electrode and the collet. Use as low a welding current as possible. Use a water-cooled torch. Use as large a size of tungsten electrode as possible. Use direct-current electrode negative (DCEN) current. Use as short an electrode extension from the collet as possible.Use the proper electrode end shape. Use an alloyed tungsten electrode.

Types of Tungsten Electrodes Pure tungsten has a number of properties that make it an excellent nonconsumable electrode for the GTA welding process. These properties can be improved by adding cerium, lanthanum, thorium, or zirconium to the tungsten.

The American Welding Society (AWS) classifies GTA welding as the following:

Pure tungsten, EWP 1% thorium tungsten, EWTh-1 2% thorium tungsten, EWTh-2 1/4% to 1/2% zirconium tungsten, EWZr 2% cerium tungsten, EWCe-2 1% lanthanum tungsten, EWLa-1 Alloy not specified, EWG

See Table 15-2.

TUNGSTEN DROPLETSTRANSFERRINGACROSS THE ARC

BASE METAL

MOLTEN WELD POOL

TUNGSTEN INCLUSIONS INTHE WELD METAL

WELD BEAD

ELECTRODE

FIGURE 15-11 Some tungsten will erode from the electrode, be trans-ferred across the arc, and become trapped in the weld deposit. Cengage Learning 2012



Pure Tungsten, EWP Pure tungsten has the poorest heat resistance and electron emission characteristic of all the tungsten electrodes. It has a limited use with alternating cur-rent (AC) welding of metals, such as aluminum and magnesium.

Thoriated Tungsten, EWTh-1 and EWTh-2Thorium 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 oxide does not further improve current-carrying capacities. It does, however, help with electron emission. This can be observed by a reduction in the electron force (voltage) required to maintain an arc of a specific length. Thorium also increases the serviceable life of the tungsten. The improved electron emission of the thoriated tungsten allows it to carry approximately 20% more current. This also results in a correspond-ing reduction in electrode tip temperature, result-ing in less tungsten erosion and subsequent weld contamination.

Thoriated tungstens also provide a much easier arc-starting characteristic than pure or zirconiated tungsten. Thoriated tungstens work well with DCEN. They can maintain a sharpened point well. They are very well-suited for making welds on steel, steel alloys (including stainless), nickel alloys, and most other metals other than aluminum or magnesium.

Thoriated tungsten does not work well with AC. It is difficult to maintain a balled end, which is required for AC welding. A thorium spike, Figure

AWSClassification

TungstenComposition

TipColor

EWP Pure tungsten GreenEWTh-1EWTh-2EWZrEWCe-2 OrangeEWLa-1EWG

1% thorium added YellowRed2% thorium addedBrown1/4 to 1/2% zirconium added

2% cerium added1% lanthanum added BlackAlloy not specified Not

specified

Table 15-2 Tungsten Electrode Types and Identification

15-12, may also develop on the balled end, disrupting a smooth arc.

CAUTIONThorium is a very low-level radioactive oxide, but the level of radioactive contamination from a thorium electrode has not been found to be a health hazard during welding. It is, however, recommended that grinding dust be contained. Because of concern in other countries regarding radioactive contamination to the welder and welding environment, thoriated tungstens have been replaced with other alloys.

Zirconium Tungsten, EWZr Zirconium oxide (ZrO2) also helps tungsten emit electrons freely. The addition of zirconium to the tungsten has the same effect on the electrode char-acteristic as thorium, but to a lesser degree. Because zirconium tungstens are more easily melted than

THORIUM SPIKE

FIGURE 15-12 Thorium spike on a balled end tungsten electrode. Cengage Learning 2012


346 CHAPTER 15

are not normally available from manufacturers; how-ever, they do provide welding characteristics for these electrodes.

Tungsten Electrode Surface FinishThe type of finish on the tungsten must be specified as cleaned or ground. More information on compo-sition and other requirements for tungsten welding electrodes is available in the AWS publication A5.12, Specifications for Tungsten and Tungsten Alloy Elec-trodes for Arc Welding and Cutting.

FLOWMETER The flowmeter may be merely a flow regulator used on a manifold system, or it may be a combination flow and pressure regulator used on an individual cyl-inder, Figure 15-13 and Figure 15-14.

The flow is metered or controlled by opening a small valve at the base of the flowmeter. The rate of flow is then read in units of cfh (cubic feet per hour).

thorium tungsten, ZrO2 electrodes can be used with both AC and direct current (DC currents). Because of the ease in forming the desired balled end on tho-rium versus zirconium tungstens, they are normally the electrode chosen for AC welding of aluminum and magnesium alloys. Zirconiated tungstens are more resistant to weld pool contamination than pure tungsten, thus providing excellent weld qualities with minimal contamination.

Zirconiated tungstens also have the advantage over thoriated tungsten in that they are not radioactive.

Cerium Tungsten, EWCe-2 Cerium oxide (CeO2) is added to tungsten to improve the current-carrying capacity in the same manner as does thorium. These electrodes were developed as replacements for thoriated tungstens because they are not made of a radioactive material. Cerium oxide elec-trodes have a current-carrying capacity similar to that of pure tungsten; however, they have an improved arc-starting and arc-stability characteristic, similar to that of thoriated tungstens. They can also provide a longer life than most other electrodes, including thorium.

Cerium tungsten electrodes have a slightly higher arc voltage for a given length than does thoriated tungsten. This very slight increase in voltage does not cause problems for manual welding. The higher voltage, however, may require that a new weld test be performed to requalify welding procedures. Cerium tungsten may be used for both AC and DC weld-ing. Cerium electrodes contain approximately 2% of cerium oxide.

Lanthanum Tungsten, EWLa-1 Lanthanum oxide (La2O3) in about 1% concentration is added to tungsten. Lanthanum oxide tungstens are not radioactive. They have current-carrying charac-teristics similar to those of the thorium tungstens, except that they have a slightly higher arc voltage than thorium and cerium tungstens. This does not normally pose a problem for manual arc welding; however, it will usually require that new test plates be produced to recertify weld procedures.

Alloy Not Specified, EWG The EWG classification is for tungstens whose alloys have been modified by manufacturers. Such alloys have been developed and tested by manufacturers to meet specific welding criteria. Specific alloy compositions

FIGURE 15-13 Flowmeter. Controls Corporation of America



Also, when using a line flowmeter, it is important to have the correct pressure. Changes in pressure will affect the accuracy of the flowmeter reading. To get accurate readings, be sure that the gas being used is read on the proper flow scale. Less dense gases, such as helium and hydrogen, will not support the ball on as high a column with the same flow rate as a denser gas, such as argon, Figure 15-16.

Shielding Gas Flow Rate The rate of flow should be as low as possible and still give adequate coverage. High gas flow rates waste shielding gases and may lead to contamination. The contamination comes from turbulence in the gas at high flow rates. Air is drawn into the gas envelope by a venturi effect around the edge of the nozzle. Also, the air can be drawn in under the nozzle if the torch is held at too sharp an angle to the metal, Figure 15-17.

The larger the nozzle size, the higher the permis-sible flow rate without causing turbulence. Table 15-3

The reading is taken from a fixed scale that is com-pared to a small ball floating on the stream of gas. Meters from various manufacturers may be read dif-ferently. For example, they may read from the top, center, or bottom of the ball, Figure 15-15. The ball floats on top of the stream of gas inside a tube that gradually increases in diameter in the upward direc-tion. The increased size allows more room for the gas flow to pass by the ball. If the tube is not vertical, the reading is not accurate, but the flow is unchanged.

FIGURE 15-14 Flowmeter regulator. Controls Corporation of America

(A) (B) (C)

FIGURE 15-15 Three methods of reading the ball and line on a flowmeter: (A) top of the ball, (B) center of the ball, (C) bottom of the ball. Cengage Learning 2012

CO2 Ar He

10 cfh

10 cfh

10 cfh

FIGURE 15-16 Each of these gases is flowing at the same cfh (L/min) rate. Because helium (He) is less dense, its indicator ball is the lowest. Be sure that you are reading the correct scale for the gas being used. Cengage Learning 2012

5 TO 10

AIR

FIGURE 15-17 Too steep an angle between the torch and the work may draw in air. Cengage Learning 2012


348 CHAPTER 15

Nozzle Inside Diameter Gas Flow*

in. (mm) (L/min)cfh

1/4 (6) 1014 (4.76.6)

5/16 (8) 1115 (5.27.0)3/8 (10) 1216 (5.67.5)

7/16 (11) 1317 (6.18.0)1/2 (13) 1720 (8.09.4)5/8 (16) 1720 (8.09.4)

Table 15-3 Suggested Argon Gas Flow Rate for Given Cup Sizes

The postflow is the time during which the gas continues flowing after the welding current has stopped. This period serves to protect the molten weld pool, the filler rod, and the tungsten electrode as they cool to a temperature at which they will not oxidize rapidly. The time of the flow is deter-mined by the welding current and the tungsten size, Table 15-4.

shows the average and maximum flow rates for most nozzle sizes. A gas lens can be used in combination with the nozzle to stabilize the gas flow, thus elim-inating some turbulence. A gas lens will add to the turbulence problem if there is any spatter or contami-nation on its surface.

Preflow and Postflow Preflow is the time during which gas flows to clear out any air in the nozzle or surrounding the weld zone. The operator sets the length of time that the gas flows before the welding current is started, Figure 15-18. Because some machines do not have preflow, many welders find it hard to hold a position while waiting for the current to start. One solution to this prob-lem is to use the postflow for preflow. Switch on the current to engage the postflow. Now, with the cur-rent off, the gas is flowing, and the GTA torch can be lowered to the welding position. The welders helmet should be lowered, and the current restarted before the postflow stops. This allows welders to have post-flow and to start the arc when they are ready.

SHIELDING GAS FLOW

WELDING CURRENT

PREFLOWTIME

WELDING TIME

POSTFLOWTIME

TOTAL GAS FLOW TIME

FIGURE 15-18 Welding time compared to shielding gas flow time. Cengage Learning 2012

Electrode DiameterPostwelding Gas

in. (mm) Flow Time*

0.01 (0.25) 5 sec0.02 (0.5) 5 sec0.04 (1.0) 5 sec

8 sec(2)1/16 3/32 (2.4) 10 sec

15 sec(3)1/820 sec(4)5/32

3/16 (4.8) 25 sec30 sec(6)1/4

Table 15-4 Postwelding Gas Flow Times



separation (fractionation) for purification or refine-ment. Helium offers the advantage of deeper penetra-tion. The arc force with helium is sufficient to dis-place the molten weld pool with very short arcs. In some mechanized applications, the tip of the tung-sten electrode is positioned below the workpiece surface to obtain very deep and narrow penetration. This technique is especially effective for welding aged aluminum alloys prone to overaging. It is also very effective at high welding speeds, such for tube mills. However, helium is less forgiving for manual welding. With helium, penetration and bead profile are sensi-tive to the arc length, and the long arcs needed for feeding filler wires are more difficult to control.

Helium has been mixed with argon to gain the combined benefits of cathode cleaning and deeper penetration, particularly for manual welding. The most common of these mixtures is 75% helium and 25% argon. Although the GTA process was devel-oped with helium as the shielding gas, argon is now used whenever possible because it is much cheaper. Helium also has some disadvantages because it is lighter than air, thus preventing good shielding. Its flow rates must be about twice as high as argons for acceptable stiffness in the gas stream, and proper pro-tection is difficult in drafts unless high flow rates are used. It is difficult to ionize, necessitating higher volt-ages to support the arc and making the arc more diffi-cult to ignite. Alternating-current arcs are very unsta-ble. However, helium is not used with alternating cur-rent because the cleaning action does not occur.

Hydrogen Hydrogen is not an inert gas and is not used as a pri-mary shielding gas. However, it can be added to argon when deep penetration and high welding speeds are

SHIELDING GASES The shielding gases used for the GTA welding process are argon (Ar), helium (He), hydrogen (H), nitrogen (N), or a mixture of two or more of these gases. The purpose of the shielding gas is to protect the molten weld pool and the tungsten electrode from the harm-ful effects of air. The shielding gas also affects the amount of heat produced by the arc and the resulting weld bead appearance.

Argon and helium are noble inert gases. This means that they do not combine chemically with any other material. Argon and helium may be found in mixtures but never as compounds. Because they are inert, they do not affect the molten weld pool in any way.

CAUTION Never allow gases such as O2, CO2, or N to come in contact with your inert gas system. Very small amounts can contaminate the inert gas, which may result in the weld failing.

Argon Argon is a by-product in air separation plants. Air is cooled to temperatures that cause it to liquefy; then its constituents are fractionally distilled. The primary products are oxygen and nitrogen. Before these gases were produced on a tonnage scale, argon was a rare gas. Now it is distributed in cylinders as gas or in bulk in a liquid form. Because argon is denser than air, it effectively shields welds in deep grooves in the flat position. However, this higher density can be a hin-drance when welding overhead because higher flow rates are necessary. The argon is relatively easy to ionize and thus suitable for alternating-current appli-cations and easier starts. This property also permits fairly long arcs at lower voltages, making it virtually insensitive to changes in arc length. Argon is also the only commercial gas that produces the cleaning discussed earlier. These characteristics are most use-ful for manual welding, especially with filler metals added, as shown in Figure 15-19.

Helium Helium is a by-product of the natural gas industry. It is removed from natural gas as the gas undergoes

FIGURE 15-19 Highly concentrated ionized argon gas column. Larry Jeffus


350 CHAPTER 15

the heat distribution between the tungsten electrode and the weld and the degree of surface oxide cleaning that occurs. Figure 15-20 shows the heat distribution for each of the three types of currents.

Direct-Current Electrode Negative (DCEN)DCEN, which used to be called direct-current straight polarity (DCSP), concentrates about two-thirds of its welding heat on the work and the remaining one-third on the tungsten. The higher heat input to the weld results in deep penetration. The low heat input into the tungsten means that a smaller-size tungsten can be used without erosion problems. The smaller-size electrode may not require pointing, resulting in a savings of time, money, and tungsten.

Direct-Current Electrode Positive (DCEP)DCEP, which used to be called direct-current reverse polarity (DCRP), concentrates only one-third of the arc heat on the plate and two-thirds of the heat on the electrode. This type of current produces wide welds with shallow penetration, but it has a strong cleaning action upon the base metal. The high heat input to the tungsten indicates that a large-size tung-sten is required, and the end shape with a ball must be used. The low heat input to the metal and the

needed. It also improves the weld surface cleanliness and bead profile on some grades of stainless steel that are very sensitive to oxygen. Hydrogen additions are restricted to stainless steels because hydrogen is the primary cause of porosity in aluminum welds. It can cause porosity in carbon steels and, in highly restrained welds, underbead cracking in carbon and low-alloy steels.

Nitrogen Nitrogen is not an inert gas. Like hydrogen, nitrogen has been used as an additive to argon. But it cannot be used with some materials, such as ferritic steels, because it produces porosity. In other cases, such as with austenitic stainless steels, nitrogen is useful as an austenite stabilizer in the alloy. It is used to increase penetration when welding copper. Unfortunately, because of the general success with inert gas mixtures and because of potential metallurgical problems, nitrogen has not received much attention as an addi-tive for GTA welding.

TYPES OF WELDING CURRENT All three types of welding current can be used for GTA welding. Each current has individual features that make it more desirable for specific conditions or with certain types of metals. The current used affects

_ _

_3

_2

_

_

HEAT23

HEAT13

HEAT2 HEAT1

HEAT12

+

HEAT13

DCEN(DCSP)

DCEP(DCRP)

AC

+

FIGURE 15-20 Heat distribution between the tungsten electrode and the work with each type of welding current. Cengage Learning 2012



help the electrons get the arc restarted every 1/120 of a second, a very high-frequency (50,000 to 3,000,000 cycles), high-voltage (3000 volts), very low-amperage (100 milliamps) current is added to the lower-frequency (60 cycle), lower-voltage (20 volts), high-amperage (100 amps) welding current, Figure 15-22.

strong cleaning action on the metal make this a good current for thin, heavily oxidized metals. The metal being welded will not emit electrons as freely as tungsten, so the arc may wander or be more erratic than DCEN.

Alternating Current High Frequency (ACHF)Alternating current (AC) concentrates about half of its heat on the work and the other half on the tung-sten. Alternating current is continuously switch-ing back and forth between DCEN and DCEP. This switching takes place once every 1/120 of a second. The complete cycle takes 1/60 of a second, so it makes 60 complete cycles per second, Figure 15-21. Alter-nating current would look like waves on water if you could stretch it out with the peaks of the waves com-ing along every 1/60 of a second. The speed at which current changes back and forth is referred to by three different names that all mean the same thingcycles, frequency, or hertz.

The tungsten electrode is much better at releas-ing electrons than is the metal being welded. This results in an imbalance in the amount of current that flows from the electrode to the work and back. Some welding machines have internal controls that correct this problem; they are called balanced wave machines. There are several advantages of using a balanced wave GTA welder. Two of the advantages are the better arc cleaning of surface oxides and less internal heat in the welder.

Electrons flow from the tip of the tungsten elec-trode for 1/120 of a second. They stop flowing for a very short moment in time before reversing direction and flowing from the work to the electrode. It is this stopping and starting every 1/120 of a second that causes problems with using AC for GTA welding. To

20 V

150,000 SEC TO

13,000,000 SEC

CYCLE TIME

WELDING CURRENT(LOW FREQUENCY, LOW VOLTAGE, HIGH AMPERAGE)

HIGH-FREQUENCY CURRENT(HIGH FREQUENCY, HIGH VOLTAGE, LOW AMPERAGE)

3000

V

VOLT

AG

EVO

LTA

GE

0

FIGURE 15-22 High-frequency arc-starting current shown over the low-frequency welding current. Cengage Learning 2012

ONE FULL CYCLE1/60 SEC

ONE-HALF CYCLE1/120 SEC

FIGURE 15-21 Sine wave of alternating current at 60 cycle. Cengage Learning 2012


352 CHAPTER 15

There are many theories as to why DCEP and the DCEP portion of the AC cycle have a cleaning action. The most probable explanation is that the electrons accelerated from the cathode surface lift the oxides that interfere with their movement. The positive ions accelerated to the metals surface provide additional energy. In combination, the electrons and ions cause the surface erosion needed to produce the cleaning. Although this theory is disputed, it is important to note that cleaning does occur, that it requires argon-rich shield gases and DCEP polarity, and that it can be used to the welders advantage, Figure 15-24.

CAUTIONVery high-frequency, high-voltage, very low-amperage electricity is not dangerous, but it can shock you. High-frequency welding current can easily pass through lots of normal insulating materials. Be careful not to touch the electrode when the high-frequency current is turned on.

The high-frequency (HF) first current ionizes the shielding gas around the tungsten. This appears as a light blue glow, Figure 15-23. Once the shielding gas is ionized, it can conduct the lower-frequency welding current. The term alternating current, high-frequency stabilized (ACHF) is used to describe this GTA welding current.

The high frequency may be set so that it auto-matically cuts off after the arc is established when welding with DC. It is kept on continuously with AC. When used in this manner, it is referred to as alter-nating current, high-frequency stabilized, or ACHF.

Arc Cleaning ActionAs the electrons leave the surface of the metal, they provide some surface cleaning or removal of these oxides. This cleaning action is most important when welding on aluminum.

FIGURE 15-23 The high frequency first appears as a blue glow around the tungsten before the welding current starts its arc. Larry Jeffus

+

0

AC

DCEN

DCEP

DCEN(DCSP)

DCEP(DCRP)

OXIDE LAYER

+

+

FIGURE 15-24 Electrons collect under the oxide layer during the DCEP portion of the cycle and lift the oxides from the surface. Cengage Learning 2012



equipment. The following topics are intended to help explain the significance of the various steps required to assemble and set up a typical GTA welder.

Shaping the Tungsten ElectrodeThe desired end shape of a tungsten electrode can be obtained by grinding, breaking, remelting the end, or using chemical compounds. Tungsten is brittle and easily broken. Welders must be sure to make a smooth, square break where they want it to be located.

Grinding a Tungsten Electrode PointA grinder is often used to clean a contaminated tung-sten or to point the end of a tungsten. The grinder used to sharpen tungsten should have a fine, hard stone. It should be used for grinding tungsten only. Because of the hardness of the tungsten and its brittle-ness, the grinding stone chips off small particles of the electrode. A coarse grinding stone will result in more tungsten breakage and a poorer finish. If the grinder

Hot Start The hot start allows a controlled surge of welding cur-rent as the arc is started to establish a molten weld pool quickly. Establishing a molten weld pool rapidly on metals with a high thermal conductivity is often hard without this higher-than-normal current. Adjust-ments can be made in the length of time and the per-centage above the normal current, Figure 15-25.

REMOTE CONTROLS A remote control can be used to start the weld, increase or decrease the current, and stop the weld. The remote control can be either a foot-operated or hand-operated device. The foot control works ade-quately if the welder can be seated. Welds that must be performed away from a welding station may use a hand or thumb control or may not have any remote welding controls.

Most remote controls have an onoff switch that is activated at the first or last part of the control move-ment. A variable resistor increases the current as the control is pressed more. A variable resistor works in a manner similar to the accelerator pedal on a car to increase the power (current), Figure 15-26. The oper-ating amperage range is determined by the value that has been set on the main controls of the machine.

SETTING UP A GTA WELDEREach manufacturers GTA welding machine and welding torch are assembled and set up differently. Read and follow the manufacturers instructions and safety guidelines any time you are setting up any

WELDINGCURRENT

HOT STARTCURRENT

DOWNSLOPE

WELDINGCURRENT

(B)(A)

FIGURE 15-25 (A) Standard method of starting welding current. (B) Hot start method of starting welding current. Cengage Learning 2012

FIGURE 15-26 A foot-operated device can be used to increase the current. Larry Jeffus


354 CHAPTER 15

Breaking and Remelting TungstenTungsten is hard but brittle, resulting in a low-impact strength. If tungsten is struck sharply, it will break without bending. When it is held against a sharp corner and hit, a fairly square break will result. Figure 15-30, Figure 15-31, and Figure 15-32 show ways to break the tungsten correctly on a sharp corner using a hammer, two pliers, and wire cutters, respectively. Observe the break; it should be square and relatively smooth, Figure 15-33. Once the tungsten has been broken squarely, the end must be melted back so that it becomes somewhat rounded. This is accomplished by switching the welding current to DCEP and strik-ing an arc under argon shielding on a piece of copper. If copper is not available, another piece of clean metal can be used. Do not use carbon, as it will contaminate the tungsten.

is used for metals other than tungsten, particles of these 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.

CAUTIONAny time you use a grinder, always wear safety glasses and follow all grinder safety instructions.

Because of the hardness of the tungsten, it will become hot. Its high thermal conductivity means that the heat will be transmitted quickly to your fingers. To prevent overheating, only light pressure should be applied against the grinding wheel. This will also reduce the possibility of accidentally breaking the tungsten.

Grind the tungsten so that the grinding marks run lengthwise, Figure 15-27 and Figure 15-28. Length-wise grinding reduces the amount of small particles of tungsten contaminating the weld. Move the tungsten up and down as it is twisted during grinding. This will prevent the tungsten from becoming hollow-ground.

CAUTIONWhen holding one end of the tungsten against the grinding wheel, the other end of the tungsten must not be directed toward the palm of your hand, Figure 15-29. This will prevent the tungsten from being stuck into your hand if the grinding wheel catches it and suddenly pushes it downward.

FIGURE 15-27 Correct method of grinding a tungsten electrode. Larry Jeffus

FIGURE 15-28 Incorrect method of grinding a tung-sten electrode. Larry Jeffus

FIGURE 15-29 Correct way of holding a tungsten when grinding. Larry Jeffus



in the compound, a strong alkaline, which rapidly dis-solves the hot tungsten. The chemical reaction is so fast that enough additional heat is produced to keep the tung-sten hot, Figure 15-34. When the tungsten is removed

Chemical Cleaning and Pointing TungstenThe tungsten can be cleaned and pointed using one of several compounds. The tungsten is heated by short-ing it against the work. The tungsten is then dipped

FIGURE 15-30 Breaking the contaminated end from a tungsten by striking it with a hammer. Larry Jeffus

FIGURE 15-31 Correctly breaking the tungsten using two pairs of pliers. Larry Jeffus

FIGURE 15-32 Using wire cutters to correctly break the tungsten. Larry Jeffus

(A) (B)

FIGURE 15-33 (A) Correctly broken tungsten electrode. (B) Incorrectly broken tungsten electrode. Cengage Learning 2012

(A) (B) (C)

FIGURE 15-34 Chemically cleaning and pointing tungsten: (A) Shorting the tungsten against the work to heat it to red hot, (B) inserting the tungsten into the compound and moving it around, (C) cleaned and pointed tungsten ready for use. Cengage Learning 2012


356 CHAPTER 15

machine. The water hoses should have left-hand threads to prevent incorrectly connecting them. Tighten the fittings only as tightly as needed to prevent leaks, Figure 15-36. Attach the cooling water in to the machine solenoid and the water out to the power block.

CAUTIONNever work on a welding machine when the power is on because an electrical shock or arc could occur.

2. The flowmeter or flowmeter regulator should be attached next. If a gas cylinder is used, secure it in place with a safety chain. Then remove the valve protection cap, and crack the valve to blow out any dirt, Figure 15-37. Attach the flowmeter so that the tube is vertical.

3. Connect the gas hose from the meter to the gas in connection on the machine.

4. With both the machine and main power switched off, turn on the water and gas so that the connection

from the chemical cleaner, it must be cooled and cleaned. The chemical will have both cleaned the tungsten and produced a fine point. If the electrode was contami-nated, the chemical compound dissolves the tungsten under the contamination, allowing it to fall free.

Pointing and Remelting TungstenThe tapered tungsten with a balled end, a shape sometimes used for DCEP welding, is made by first grinding or chemically pointing the electrode. Using DCEP, as in the procedure for the remelted bro-ken end, strike an arc on some copper under argon shielding and slowly increase the current until a ball starts to form on the tungsten. The ball should be made large enough so that the color of the end stays between dull red and bright red. If the color turns white, the ball is too small and should be made larger. To increase the size of the ball, simply apply more current until the end begins to melt. Surface tension will pull the molten tungsten up onto the tapered end. Lower the current, and continue welding. DCEP is seldom used for welding. If the tip is still too hot, it may be necessary to increase the size of the tungsten.

Assembling the GTA Welding Station 1. Start with the power switch off, Figure 15-35.

Use a wrench to attach the torch hose to the

FIGURE 15-35 Always be sure thet the power is off when making machine connections. Larry Jeffus

FIGURE 15-36 Tighten each fitting as it is connected to avoid missing a connection. Larry Jeffus

DIRT

FIGURE 15-37 During transportation or storage, dirt may collect in the valve. Cracking the valve is the best way to remove any dirt. Cengage Learning 2012



to the machine can be checked for leaks. Tighten any leaking fittings to stop the leak.

5. Turn on both the machine and main power switches, and watch for leaks in the torch hoses and fittings.

6. With the power off, switch the machine to the GTA welding mode.

CAUTIONTurn off all power before attempting to stop any leaks in the water system.

7. Select the desired type of current and amperage range, Figure 15-38 and Figure 15-39. Set the fine current adjustment to the proper range, depending upon the size of tungsten used, Table 15-5.

8. Place the high-frequency switch in the appropri-ate position, auto (HF start) for DC or continu-ous for AC, Figure 15-40.

9. The remote control can be plugged in and the selector switch set, Figure 15-41.

FIGURE 15-38 Setting the current. Larry Jeffus

FIGURE 15-39 Setting the amperage range. Larry Jeffus

ACDCEPDCENElectrode Diameter

in. (mm)

1050Not recommended1560(1)0.045090102070100(2)1/1680130153090200(2.4)3/32

1002002540150350(3)1/81603004055300450(4)5/32

Table 15-5 Amperage Range of Tungsten Electrodes

FIGURE 15-40 The high-frequency switch should be placed in the appropriate position. Larry Jeffus

FIGURE 15-41 Setting the remote-control switch. Larry Jeffus


358 CHAPTER 15

10. The collet and collet body should be installed on the torch first, Figure 15-42.

11. On the Linde or copies of Linde torches, install-ing the back cap first will stop the collet body from being screwed into the torch fully. A poor connection will result in excessive electrical and thermal resistance, causing a heat buildup in the head.

12. The tungsten can be installed and the end cap tightened to hold the tungsten in place.

13. Select and install the desired nozzle size, Figure 15-43. Adjust the tungsten length so that it does not stick out more than the diameter of the noz-zle, Figure 15-44.

14. Check the manufacturers operating manual for the machine to ensure that all connections and settings are correct.

15. Turn on the power, depress the remote control, and again check for leaks.

16. While the postflow is still engaged, set the gas flow by adjusting the valve on the flowmeter.

FIGURE 15-42 Inserting collet and collet body.Larry Jeffus

ELECTRODE STICKOUT

NOZZLE DIAMETER

FIGURE 15-44 Electrode stickout. Cengage Learning 2012

FIGURE 15-43 Install the nozzle (cup) to the torch body. Larry Jeffus

SUMMARY

One of the prime considerations for gas tungsten arc welding is the cleanliness of the equipment, supplies, base metal, filler metal, the welders gloves, and so forth. When everything is clean, you will find that the welding process proceeds more easily and more successfully.

Another major factor affecting your ability to pro-duce quality welds is the tungsten end or tip shape.

As you practice making the various welds, you will find that keeping the tungsten electrode tip shaped appropriately assists you in producing uniform welds.

Often, new welders feel that there is some sort of attraction between the tungsten electrode, filler metal, and base metal during the welding process because it seems to continually become contaminated.



REVIEW QUESTIONS

What are the two other names used for GTA 1. welding?What are two advantages of GTA welding?2. List four major components that make up a GTA 3. welding station.List three advantages that using an air-cooled 4. GTAW torch might have over using a water-cooled GTAW torch.List three advantages that using a water-cooled 5. GTAW torch might have over using an air-cooled GTAW torch.List the hoses and cables that might be attached 6. to a water-cooled torch.What type of antifreeze solution may be used in a 7. water-cooling system?What is the purpose of the shielding gas nozzle?8. Why is tungsten used as an electrode?9. List five ways to limit tungsten erosion.10. What are the AWS classifications for the follow-11. ing types of tungsten electrodes?a. Pure tungstenb. 1% thorium tungstenc. 2% thorium tungstend. 1/4 to 1/2% zirconium tungsten e. 2% cerium tungsten f. 1% lanthanum tungstenWhich type of tungsten electrode has the poorest 12. heat resistance and emission characteristics?Which type of tungsten electrode has a very low-13. level radioactive oxide additive?

According to Table 15-2, what color tip does the 14. EWCe-2 tungsten electrode have?What is La15. 2O3?What are the units of shielding gas flow?16. How high should the shielding gas flow rate be?17. What is preflow time used for?18. How is argon produced?19. What is the advantage of using helium as a shield-20. ing gas?Nitrogen cannot be used as a shielding gas for 21. GTA welding for what types of materials?What do the following abbreviations mean? 22. DCEN, DCEP, and ACHF?What part of the AC cycle provides surface oxide 23. cleaning?What does a hot start provide to the GTA weld?24. What are the functions of a remote control?25. List the ways of shaping the end of a tungsten 26. electrode.When the end of a tungsten electrode is broken 27. off to remove contamination, how should the broken end look?How does a chemical cleaner remove tungsten 28. contamination?Why must the power be off when you attach the 29. GTA welding torch to a welding machine?What is the maximum distance that the tungsten 30. electrode should stick out of the nozzle?

This almost continuous contamination can be very frustrating. At times it may seem overwhelming; however, with continued practice and diligence you will be able to control this problem. Even experi-enced welders in the field can be plagued from time to time with tungsten contamination. At other times,

they can weld for an entire day without contaminat-ing the tungsten. It is often beneficial for students to realize that tungsten contamination is just part of the process, and they must, therefore, try to ignore the possibility of it happening and concentrate on pro-ducing the welds.


Chapter 15: Gas Tungsten Arc Welding Equipment and MaterialsObjectivesKey TermsIntroductionGTA Welding EquipmentGTA Welding TorchesTungsten ElectrodesFlowmeterShielding GasesTypes of Welding CurrentRemote ControlsSetting Up a GTA WelderSummaryReview Questions

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