|Cary, Howard B. & Scott C. Helzer. Modern Welding Technology. Upper Saddle River, New Jersey: Pearson Education, 1979: 74-80.||"The gas tungsten arc welding process utilizes the heat of an arc between a nonconsumable tungsten electrode and the base metal. The arc develops intense heat, approximately 11,000°F."||6100 °C|
|TIG Handbook. Miller Electric Mfg Co., 4 June 2007.||"The GTAW process can produce temperatures of up to 35,000°F, The torch only contributes heat to the work piece."||19,426 °C|
|Francoeur, Michael. Gas Tungsten Arc Welding. Robotic Industries Association, 2005.||"The arc comprises both the current and plasma, along with some generated metallic vapors, etc. Plasma temperature at the electrode can be tens of thousands of degrees Fahrenheit depending on amperage. Within the arc region it can be ten to twenty thousand degrees Fahrenheit. Considerable heat is lost by radiation and in this regard arc efficiency drops."||5,500–11,100 °C|
|D. Farson, R. Richardson, X. Li, Infrared Measurement of Base Metal Temperature in GTAW, American welding Society, November 2002: 2.||"Within this region, the temperature of the electrode should increase quickly from room temperature up to a possible temperature of above 3000 °C (5432 °F) due to the electrical resistance and arc heating."||3000 °C|
|Jeffus, Larry F. Welding Principles and Applications. Clifton Park, NY: Thomson Delmar Learning, 2002: 48.||"The arc temperature, around 11,000 °F (6000 °C), is much higher than the melting temperature of tungsten but not much higher than its boiling temperature of 10,600°F (5900°C)"||6000 °C|
Introduction to Welding
Welding is the process of fusing two metal pieces into an alloy by melting them. The fundamentals of arc welding include the work piece or the base metal in contact, the welding torch which holds the electrode at the tip, shielding gas, a power source, and a coolant system. The most important aspect of arc welding is the welding arc that is responsible for melting the base metal. The two methods of starting an arc includes the touch start method and the high frequency start. The touch start involves the contact between the electrode (connected to a power supply) and the work piece. This allows electrons to flow from the anode (usually the work piece) to the cathode (usually the electrode). When the electrode is then pulled slightly away, an arc is formed because of dielectric breakdown or ionization of the shielding gas. The disadvantage of this method is that it causes impurities and poorer quality welds. The high frequency start, on the other hand, doesn't involve contact. The high frequency produced in the electric field would cause excitation and collisions between the atoms of the shielding gas. Ionization of the gas used is now easier because the high frequency oscillations made it possible for the gas atoms to reach its ionization potential (necessary voltage to take an electron away from a gas atom).
In order to produce high quality welds, TIG Welding (Tungsten Inert Gas Welding), also called GTAW (Gas Tungsten Arc Welding) is used. This method involves the use of a nonconsumable tungsten electrode. Since Tungsten has a melting point of approximately 3400 °C, it is harder to melt than most metals; it is nonconsumable or would not melt in the arc. Once the arc is started, the tungsten electrode is usually negative (cathode) and the work piece is positive (the anode). When electrons are emitted (also called thermionic emission) from the electrode, they are attracted to the positive work piece and travel in that direction. Meanwhile, they collide with nearby shielding gas atoms and cause thermal ionization of some of the shielding gas atoms. As a result, these less in population but heaver positive ions (from the gas) are attracted by the negative electrode and travel in that direction. In TIG welding, the electrode is not always the cathode or negative pole; it can also be the anode. Welders change the pole to do something called the cleaning effect. When the tungsten electrode is positive and the work piece is negative, the heavier positive ions travel towards the work piece instead. The resulting positive ion bombardment causes an etched surface.
DC or AC?
The type of power supply used, either DC, a direct current or AC, an alternating current depends on the materials being fused. A DC current is usually used to weld steel, nickel, and titanium. This is the more usual procedure and also the one described earlier in TIG Welding. The AC, on the other hand, is used for welding magnesium and aluminum. The AC method causes the electrode to alternate between positive and negative throughout the welding process. Since the electrons are now traveling in alternating directions, the tungsten electrode will not overheat. In addition, the positive ion bombardment would clean the work piece of impurities half the time (when electrode is positive and base metal negative).
TIG, as its name suggests, uses inert gases for its shielding gas. Shielding gases are used to prevent nitrogen and oxygen from causing fusion defects, porosity, and metal embrittlement if they come in contact with the electrode, the arc, or the welding metal. The inert gases that are most commonly used are argon and helium because they are the cheapest of them all. Helium has a higher ionization potential (24.5 V) than argon (15.7 V). Ionization potential is the voltage needed to remove an electron from a gas atom.
Advantages of Helium
- The arc column (a large portion of the arc length) is larger and can cause deeper penetration because the heat is directed at a small area.
- Since the column is larger, it has more power and heat.
- Since the penetration is high, the weld speed can be higher than when using argon. Heavier base metals can be wielded due to the power of using helium.
Disadvantage of Helium
- It is harder to start an arc because of the high ionization potential of helium.
Advantage of Argon
- It is much easier to start an arc because of argon’s lower ionization potential.
Disadvantages of Argon
- The arc column is smaller, creates less penetration and a large heat directed area, allows slower weld speed, and can only weld light metals.
Determinants of Temperature
- Region: The arc length or arc gap (between the work piece and the torch) is separated into 3 sections: the cathode drop, arc column, and anode drop. The arc column contains the most heat. Within this column/arc there is an inner core called the plasma and an outer flame. The plasma has the most heat while the surrounding outer flame is cooler. The temperatures within the tungsten electrode decrease as we measure the temperature further away from the tip of the electrode.
- Diameter of the central plasma: The greater the diameter, the greater the temperature.
- Amount of current passing through the arc: As the current increases, the temperature increases and depth of penetration increases.
- Shielding atmosphere: Argon is the commonly used shielding gas but if helium is added, the temperature of the arc will increase and promote greater penetration and greater welding speeds.
- Electrode Size and type
- A Theory: There's a theory that if the cross-sectional area of the arc column is reduced/constricted, temperatures increase. The logic behind it is that the temperature and conductivity must increase in order for the current to be maintained.
Anthony Ho -- 2007