Machinery's Handbook, 31st Edition
1600 Gas Tungsten Arc Welding Filler Metals.— Specifications covering composition and mechanical properties for GTAW filler metal are published by the American Welding Society under the following classifications:A5.7, copper and copper alloys; A5.9, chromium and chromium nickel; A5.10, aluminum; A5.14, nickel; A5.16, titanium; A5.18, carbon steels; A5.19, magne sium; and A5.28, low-alloy steels. Filler metals must be kept dry and clean if they are to be used satisfactorily. Shielding Gases.— Inert gases such as argon, and argon + helium mixtures are most com monly used for GTAW. Helium provides greater thermal conductivity and additional arc voltage potential than argon, and is normally added to argon when more weld energy is required for improved penetration and increased mechanized welding travel speeds. Argon gas mixtures containing 30 to 75 percent helium provide benefits for manual weld ing of aluminum over 3 ∕ 8 in. (9.6 mm) thick; mechanized welding of aluminum where high speeds are required; mechanized welding of carbon and stainless steels where good penetration is needed; mechanized welding of stainless steel where good penetration and faster speeds are required; and for copper of 1 ∕ 4 in. (6.4 mm) thickness and thicker. Shielding gas purity for GTAW welding is important. Welding-grade argon is supplied at a purity of at least 99.996 percent and helium is produced to a minimum purity of 99.995 percent. However, shielding gases may be contaminated due to poor cylinder filling practices. If impure gas is suspected, the following test is suggested. With the HF and power on, create an arc without welding and hold the arc for about 30 seconds. Examine the electrode tip for signs of unusual coloration, oxidation, or contamination, which result from impurities in the shielding gas. PLASMA ARC WELDING (PAW) When an electric current passes between two electrodes through certain gases, the energy of the gas molecules is increased so that they accelerate and collide with each other more often. With increases in energy, the binding forces between the nuclei and the electrons are exceeded, and electrons are released from the nuclei. The gas now consists of neutral molecules, positively charged atoms, and negatively charged electrons. The plasma gas is said to be ionized, so that it is capable of conducting electric current. Plasma forms in all welding arcs but in plasma arc welding it is generated by a series of events that begins with inert gas passing through the welding torch nozzle. High-frequency current is then generated between the tungsten electrode (cathode) and the torch nozzle (anode), forming a low-current pilot arc. The ionized path of this nontransferred arc is then transferred from the tungsten electrode to the work, and a preset plasma current is generated. The above sequence of events provides the ionized path for the plasma current between the electrode and the work so that arcing between the electrode and the nozzle ceases. (Nontransferred arcs may be used for metal spraying or nonmetallic welds.) Forcing the ionized gas through the small orifice in the nozzle increases both the level of ionization and the arc velocity, and arc temperatures between 30,000 and 50,000 ° F (16,650 and 27,760 ° C) are generated. Gases for Welding.— Argon is the preferred gas for plasma arc welding (PAW) as it is eas- ily ionized and the plasma column formed by argon can be sustained by a low voltage. The low thermal conductivity of argon produces a plasma column with a narrow, concentrated hot core surrounded by a cooler outer zone. Argon plasmas are suited to welding steel up to 1 ∕ 8 in. (3.2 mm). For thicker materials, requiring a hotter arc and using higher current melt- in technique, a mixture of argon 25 + helium 75 percent may be used. Additions of helium and hydrogen to the gas mixture improve heat transfer, reduce porosity, and increase weld travel speed. For welding materials thinner than 1 ∕ 8 in. thick by the plasma gas keyhole method (full penetration welds), gases may contain up to 15 percent hydrogen with the
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