WELDING

Exercise 1

Read this text and learn the words in bold

There are number of methods of joining metal articles together depending on the type of metal and the strength of the joint which is required. Satisfactory joint is brazed, riveted or welded for strong permanent joints.

Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld pool) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.

The simplest method of welding two pieces of metal together is heating. The ends are heated to a white heat (for iron, about 1300° C ) in a flame, when the metal becomes plastic. The ends are then pressed or hammered together. Care must be taken to ensure that the welded surfaces are clean, for dirt will weaken the weld. A high temperature causes oxidation, and a film of oxide is formed on the heated surfaces. For this reason a flux is applied to the heated metal. At welding heat, the flux melts, and the oxide particles are dissolved in it together with any other impurities which may be present. The metal surfaces are pressed together and the flux is squeezed out of the weld. 

The heat for welding is generated in several ways, depending on the sort of metal which is being welded and on its shape. An extremely hot flame can be produced from an oxy-acetylene torch. For some welds an electric arc is used. In this method, an electric current is passed across two electrodes and the metal surfaces are placed between them . The work itself constitutes one electrode and the other is an insulated filler rod. An arc is struck between the two and the heat which is generated melts the metal at the weld. A different method known as spot welding is usually employed for welding sheets or plates of metal together. Two sheets or plates are placed together with a slight overlap, and a current is passed between the electrodes. Today the new welding methods include laser welding and electronic beam welding.

Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding can be done in many different environments, including open air, under water and in outer space. Regardless of location, however, welding remains dangerous, and precautions must be taken to avoid burns, electric shock, eye damage, poisonous fumes, and overexposure to ultraviolet light.

Until the end of the 19th  century, the only welding process was forge welding, which blacksmiths had used for centuries to join metals by heating and bounding them. Arc welding and oxyfuel welding were among the first processes to develop late in the century, and resistance welding followed soon after. Welding technology advanced quickly the early 20th century as World War I and World War II drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like shielded metal arc welding, now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding, submerged arc welding, flux-cored arc welding and electroslag welding. Developments continued with the invention of laser beam welding and electron beam welding in the latter half of the century. Today, the science continues to advance. Robot welding is becoming more common in industrial settings, and researches continue to develop new welding methods and gain greater understanding of weld quality and properties.

Exercise 2

Exercise 3

Exercise 6

Read the text about arc welding and learn the words in bold

Arc welding refers to a group of welding processes that use a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct (DC) or alternating (AC) current, and consumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and/or an evaporating filler material.

Shielding gases are inert or semi-inert gases that are commonly used in several welding processes, such as, gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW). Their purpose is to protect the weld area from atmospheric gases, such as oxygen, nitrogen, carbon dioxide, and water vapor. Depending on the materials being welded, these atmospheric gases can reduce the quality of the weld or make the welding process more difficult to use.Other arc welding processes use other methods of protecting the weld from the atmosphere as well - shielded metal arc welding, for example, uses an electrode covered in a flux that produces carbon dioxide when consumed, a semi-inert gas that is an acceptable shielding gas for welding steel.

A welding power supply is a device that provides an electrical current to perform welding. Welding usually requires high current (over 80 amps) and it can need above 12, 000 amps in spot welding. Low current can also be used; welding two razor blades together at 5 amps with gas tungsten arc welding is a good example. A welding power supply can be as simple as a car battery and as sophisticated as a modern machine based on silicon controlled rectifier technology with additional logic to assist in the welding process.

Gas tungsten arc welding (GTAW), commonly known as tungsten inert gas (TIG) welding, is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld. The weld area is protected from by a shielding gas (usually an inert gas such as argon) in order to keep it`s strength, and filler metal is normally used. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma. GTAW is most commonly used to weld thin sections of stainless steel and light metals such as aluminium, magnesium, and copper alloys. The process gives the welder greater control over the weld than competing procedures such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, it is significantly slower than most other welding techniques.

Filler Metal

A filler metal is a metal added in the making of a joint through welding, brazing, or soldering. Four types of filler metals exist - covered electrodes, bare electrode wire or rod, tubular electrode wire and welding fluxes. Sometimes nonconsumable electrodes are included as well, but since these metals are not consumed by the welding process, they are normally excluded.

Covered electrodes are used extensively in shielded metal arc welding and are major factor in that method`s popularity. Bare electrode wires are used in gas metal arc welding and bare electrode rods are used in gas tungsten arc welding. Tubular electrode wire is used in flux-cored arc welding. Welding fluxes are used in submerged arc welding.

Exercise 7

Exercise 8

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Shielded Metal Arc Welding (SMAW) 

Shielded metal arc welding (SMAW), also known as stick welding, is a manual arc welding process that uses a consumable electrode coated in flux to lay the weld. An electric current, in the form of either alternating current or direct current from a welding power supply, is used to form an electric arc between the electrode and the metals to be joined. As the weld is laid, the flux coating of the electrode disintergrates, giving off vapors that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination.

Because of the versatility of the process and the simplicity of its equipment and operation, shielded metal arc welding is one of the world`s most popular welding processes. It dominates other welding processes in the maintenance and repair industry, and though flux-cored arc welding is growing in popularity, SMAW continues to be used extensively in the construction of steel structures and in industrial fabrication. The process is used primarily to weld iron and steels (including stainless steel) but aluminium, nickel and copper alloys can also be welded with this method.

SMAW is performed by striking an arc between a coated-metal electrode and the base metal. Once arc has been established, the molten metal from the tip of the electrode flows together with the molten metal from the edges of the base metal to form a sound joint. This process is known as fusion. The coating from the electrode forms a covering over the weld deposit, shielding it from contamination. Therefore the process is called shielded metal arc welding. The main advantages of shielded metal arc welding are that high-quality welds are made rapidly at a low cost.

Shielded metal arc welding equipment typically consists of a constant current welding power supply and an electrode, with an electrode holder, a work clamp, and welding cables (also known as welding leads) connecting the two.

Metal Inert Gas/ Gas Metal Arc Welding (GMAW) 

Gas metal arc welding (GMAW), is also known as, metal inert gas (MIG) welding, is a semi-automatic or automatic arc welding process in which a continuous and consumable wire electrode and a shielding gas are fed through a welding gun. A constant voltage, direct current power source is most commonly used with GMAW, but constant current systems, as well as alternating current, can be used. There are four primary methods of metal transfer in GMAW, each of which has distinct properties and corresponding advantages and limitations.

GMAW was applied to steels because it allowed for lower welding time compared to other welding processes. Today, GMAW is commonly used in industries such as the automobile industry, where it is preferred because it can be used in many different ways and it is quick.

Exercise 9

Exercise 10

Read and learn about the welding symbols