rel="nofollow" href="#ulink_b9bfcf02-6000-51ab-b3bf-0417624a445a">TIG Welding
Tungsten inert gas (TIG) welding was first developed in the 1940s to weld aluminum and magnesium. Today it is used to weld many materials, including a variety of steels. TIG creates an arc between a non-consumable tungsten electrode and the workpiece and uses an inert gas, usually argon, to shield the molten puddle. A DC or AC power source supplies the electric power. The tungsten is held in place with a collet inside a TIG torch. The argon shielding gas protects the molten puddle as well as the tungsten, which may be 6,000 degrees F at the tip. The arc itself is much hotter than the oxyacetylene flame. It is 12,000 to 15,000 degrees F on the outer part of the arc to twice that near the tip of the tungsten. A major advantage of the process is the ability to have a stable arc exist from very low currents (3 amps) up to maximum torch capacity, which can be over 500 amps on automatic machine torches.
Fig. 1.3. TIG welding uses an inert gas, usually argon, to shield the molten puddle, which is created by an arc between a non-consumable tungsten electrode and the workpiece. Electric power is supplied by a DC or AC source. A major advantage of the process is the ability to have a stable arc at settings as low as low 3 amps.
For gas tungsten arc welding, the official AWS designation for TIG welding is GTAW. The common term TIG for tungsten inert gas is used throughout the book. Note: If taking welding courses and the instructor insists on the use of GTAW, use it!
Oscar Kjellberg, the founder of ESAB, invented and was issued a patent for stick electrode welding in 1904. Its use grew rapidly through the 1920s, 1930s, and 1940s and became the leading welding method until displaced by MIG welding.
Stick electrode welding is simple to use, requiring only a power source and an inexpensive electrode holder. It is still preferred when welding outdoors because it can make acceptable welds in significantly more wind than gas shielding processes. The AWS Bridge Welding Code, for example, specifies maximum wind speeds of 5 mph for MIG and TIG welding, while stick welding is allowed up to 20 mph. However, when TIG welding, a gas lens should be used for up to a 4-mph wind. In addition, TIG is more sensitive to the need for quality shielding than MIG.
Stick welding utilizes a simple power source, such as an AC welding transformer. DC power is also widely used and for welding outdoors; portable engine-driven DC generators are often employed. All these power sources are called constant current, referring to setting the desired welding current level, which stays relatively fixed regardless of the arc voltage. Thus, welding starts at a high voltage needed to initiate an arc, and reduces to 20 to 30 welding volts. In addition, when the stick electrode is shorted to the work at the start, the maximum current is only slightly more than the preset valve. While welding, as the arc length varies with the manipulation of the melting electrode, the current remains relatively constant. Therefore, if the arc length is varied, the voltage changes, but the welding current, which controls weld penetration, remains close to the preset level. In addition to the power source, the stick electrode holder is the only other equipment needed to make a stick weld.
Fig. 1.4. Stick electrode welding is simple—it only requires a power source and an inexpensive electrode holder. The heart of the process is the stick electrode that has a metal rod in the center, which is coated with a mixture of flux ingredients, small amounts of metal alloy, and a liquid binder. The coated rod is baked to harden the binder.
The heart of the process is the stick electrode itself. For steel welding, the center core is typically a non-alloyed steel rod. The rod is cut into short lengths, such as 14 inches. Flux ingredients are mixed with small amounts of metal alloy and a binder, often liquid sodium silicate. The dough-like mixture is extruded around the core wire. The coated rod is baked to harden the binder. A short section at the end has the coating removed, so the electrical power can be transferred to the core rod. When an arc is struck between the core wire and the workpiece, the flux melts and some gaseous products, such as carbon dioxide, are formed, and this helps protect the weld puddle from oxidation and nitrogen contamination. The flux ingredients melt and form a slag that floats to the top of the weld puddle and protects it from atmospheric contamination as it cools. A variety of electrode types are available. Some can weld high-strength steels and match their strength and toughness.
Fig. 1.5. Oscar Kjellberg, founder of ESAB, invented (and received a patent for) stick electrode welding in 1904. Stick welding became more prevalent through the 1920s, 1930s, and 1940s and became the leading welding method until displaced by MIG welding. ESAB grew to be a worldwide leader in the welding field.
Note the official AWS designation for stick welding is SMAW for shielded metal arc welding. The common term “stick welding” is used throughout this book.
In 1950, Gibson, Muller, and Nelson, working at the Airco development laboratories, patented the MIG (metal inert gas) welding process. Over the ensuing decades it evolved, and today it is used to deposit more than 60 percent of the filler metal in the United States.
Fig. 1.6. The MIG welding process utilizes a constant-voltage power supply. The output is similar to a car battery in which the voltage stays relatively constant as current rises. A small-diameter wire, typically .035 or .045 inch, is fed from a spool through a flexible cable and MIG gun. The wire exits the gun and current flows to an arc, which forms between the wire and workpiece.
The MIG process utilizes a constant-voltage power supply. Similar to a car battery in output characteristics, the voltage stays relatively constant as current rises. A small-diameter solid wire, typically .030 to .045 inch, feeds from a spool through a flexible cable and MIG gun. The MIG gun has a copper nozzle that directs shielding gas to protect the weld from oxidation and nitrogen contamination. The wire exits the front of the gun through a copper contact tip where it picks up electrical power. Current flows from the copper contact tip to the small diameter wire. As current passes through the wire on the way to the workpiece, resistant heating increases the temperature to perhaps to 500 degrees F. Then an arc forms between the end of the wire and the workpiece, and as a result, the arc melts both the wire and the workpiece. Unlike TIG welding that requires careful hand manipulation to maintain the arc length, MIG arc length is maintained automatically. However, the weldor must still control the distance from the MIG nozzle to the work to achieve proper welding performance. This is covered in detail, with examples, in Chapter 6.
MIG welding can be used for steel, stainless steel, aluminum, and some other materials. The official AWS designation for MIG welding is GMAW (gas metal arc welding), but the common term MIG is used throughout this book. For those outside of the United States, the term MIG is only used when 100 percent argon or argon-helium shielding gas mixtures as used when welding aluminum. For any shielding gas that includes oxygen or an oxygen compound, such as carbon dioxide, MAG (metal active gas) is used. My purest friends cringe when I use MIG at AWS Section talks. I tell them it is far better than calling the process wire welding!
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