Welding Technology

08AAE18 WELDING TECHNOLOGY

UNIT - I INTRODUCTION Comparison between casting and welding processes Definition of welding as per AWS Weldability Heat affected zone weld decay, Basic welding positions and joint types Welding symbols Types of welding processes

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Comparison between Casting and Welding processes 1. Welding is more economical and is a much faster process as compared to casting. 2. Fabricated mild steel structures are lighter as compared to cast iron. 3. Fabricated mild steel structures have more tensile strength and rigidity as compared to cast iron. 4. Welding can join dissimilar metals and thus in a complicated structures, different part of the structure can be fabricated with different materials.

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Comparison between Casting and Welding processes (Continued) • • • • • •

The design of a welded structure is simpler as compared to that of cast part Man power requirement is less in welding process as compared to casting process. Structural shapes not easily obtainable with casting can be produced by welding without much difficulty. Fabrication by welding saves machining costs involved in cast parts Cost of standard rolled sections is much less as compared to that of a casting . Making changes in an already cast structure is extremely difficult, but in welding is possible. 3

Definition of welding as per AWS

The welding is a process of joining two similar or dissimilar metals by fusion, with or without the application of pressure and with or without the use of filler metal.

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Weldability The ability of a material to be welded under imposed conditions into a specific, suitable structure and to perform satisfactorily for its intended use. (OR) The capacity of a material to be welded under the fabrication conditions imposed into a specific, suitably designed structure and to perform satisfactorily in the intended service.

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HEAT AFFECTED ZONE • Heat-affected zone (HAZ) is the area of base material, either a metal or a thermoplastic which has had its microstructure and properties altered by welding or heat intensive cutting operations. • The heat from the welding process and subsequent recooling causes this change from the weld interface to the termination of the sensitizing temperature in the base metal.

• The spread and magnitude of property change depends primarily on the base material, the weld filler metal, and the amount and concentration of heat input by the welding process. 6

HEAT AFFECTED ZONE (Continued) • If the diffusivity is high, the material cooling rate is high and the HAZ is relatively small and vice versa . A low diffusivity leads to slower cooling and a larger HAZ. The amount of heat inputted by the welding process plays an important role.

• Processes like oxyfuel welding use high heat input and increase the size of the HAZ. Processes like laser beam welding and electron beam welding give a highly concentrated, limited amount of heat, resulting in a small HAZ. • Arc welding falls between these two extremes, with the individual processes varying somewhat in heat input. 7

Heat affected zone

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Weld Decay Weld decay is a form of intergranular corrosion, usually of stainless steels or certain nickel-base alloys, that occurs as the result of sensitization in the heat-affected zone during the welding operation. The corrosive attack is restricted to the heat affected zone (HAZ). Positive identification of this type of corrosion usually requires microstructure examination under a microscopy although sometimes it is possible to visually recognize weld decay if parallel lines are already formed in the heat affected zone along the weld.

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• During welding of stainless steels, local sensitized zones (i.e., regions susceptible to corrosion) often develop. Sensitization is due to the formation of chromium carbide along grain boundaries, resulting in depletion of chromium in the region adjacent to the grain boundary. • If this depletion drops the chromium content below the necessary 12 wt% that is required to maintain a protective passive film, the region will become sensitized to corrosion, resulting in intergranular attack. • This type of corrosion most often occurs in the HAZ. Intergranular corrosion causes a loss of metal in a region that parallels the weld deposit. This corrosion behavior is called weld decay.

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Factors influence weld Decay • • • • • • • • • • •

Weldment design Fabrication technique Welding practice Welding sequence Moisture contamination Organic or inorganic chemical species Oxide film and scale Weld slag and spatter Incomplete weld penetration or fusion Porosity Cracks (crevices)

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Welding positions

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Weld Joints

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Welding Symbol

Reference Line (Required element)

Always Horizontal

Arrow Line

Reference Line (Required element)

Arrow

Reference Line must always be horizontal, Arrow points to the line or lines on drawing which clearly identify the proposed joint or weld area.

Reference Line (Required element)

Arrow

Tail The tail of the welding symbol is used to indicate the welding or cutting processes, as well as the welding specification, procedures, or the supplementary information to be used in making the weld.

All the way Around A circle at the tangent of the arrow and the reference line means welding to be all around.

Field Weld Symbol

A flag at the tangent of the reference line and arrow means Field Weld.

OTHER SIDE

ARROW SIDE

Fillet Weld (Arrow Side Only)

Fillet Weld (Other Side)

Size of Fillet Weld Noted

1/4 1/4

Example of Double Bevel Groove weld

Depth of preparation or groove

1/4 (5/16)

1/4

(5/16)

Depth of penetration

Plug or Slot Weld Symbol Arrow Side

What does this symbol Represent? 5/16

5/16

Chain Intermittent Fillet Weld Weld both sides each end and 10 inches center to center in between

1/4

2-10

1/4

2-10

10 in

Staggered Intermittent Fillet Weld Weld ends than 10 inch centers staggered each side

1/4

2-10

1/4

2-10 10 in

10 in

Welding symbol nomenclature

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UNIT – II GAS AND ARC WELDING • Gas welding, principle and equipment, applications and selection, • Arc welding, principle, electrodes, energy source characteristics.

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• Oxy-fuel welding, commonly referred to as oxy welding or gas welding is a process of joining metals by application of heat created by gas flame. • The fuel gas commonly acetylene, when mixed with proper proportion of oxygen in a mixing chamber of welding torch, produces a very hot flame of about 5700-5800°F. • With this flame it is possible to bring any of the so-called commercial metals, namely: cast iron, steel, copper, and • aluminum, to a molten state and cause a fusion of two pieces of like metals in such a manner that the point of fusion will very closely approach the strength of the metal fused. • If more metal of like nature is added, the union is made even stronger than the original. This method is called oxy-acetylene welding.

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Chemistry of Oxy Acetylene Process • The most common fuel used in welding is acetylene. It has a two stage reaction; the first stage primary reaction involves the acetylene disassociating in the presence of oxygen to produce heat, carbon monoxide, And hydrogen 2C2H2 + 2O2 = 4CO + 2H2 + Heat • A secondary reaction follows where the carbon monoxide and hydrogen combine with more oxygen to produce carbon dioxide and water vapor. 4CO + 2H2 + 3O2 = 4CO2 + 2H2O + Heat • When combine above equations it is noticed that about 5 parts of oxygen is necessary to consume 2 parts of acetylene 2C2H2 + 5O2 = 4CO2 + 2H2O + Heat

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• In operation, one part of oxygen is supplied through the torch and the remaining 1.5 parts is obtained from the surrounding air atmosphere (secondary reaction).

• When the secondary reaction does not burn all of the reactants from the primary reaction, the welding processes produces large amounts of carbon monoxide, and it often does.

Oxy Fuel welding Gases To be suitable for welding operations, a fuel gas, when burned with oxygen, must have the following: • High flame temperature • High rate of flame propagation • Adequate heat content • Minimum chemical reaction of the flame with base and filler metals • propane, liquefied petroleum gas (LPG), natural gas, propylene, hydrogen and MAPP gas, Acetylene 38

To be suitable for welding operations, a fuel gas, when burned with oxygen, must have the following: – High flame temperature – High rate of flame propagation – Adequate heat content – Minimum chemical reaction of the flame with base and filler metals – propane, liquefied petroleum gas (LPG), natural gas, propylene, hydrogen and MAPP gas, Acetylene

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• Acetylene differs from those hydrocarbons in a way that its molecule is made up of two carbon atoms and two hydrogen atoms, the carbon atoms are joined by what chemists call a ”triple bond”. • When acetylene reaches its kindling temperature; the bond breaks and releases energy. • In other hydrocarbons, the breaking of the bonds between the carbon atoms absorbs energy. The triple bond is the reason that when acetylene and oxygen are mixed and ignited, the flame can reach the temperature of 5700°F to 6300 °F

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Advantages of Oxyacetylene Process • Does not require electricity; • The equipment is portable, easy to transport; • Welder has considerable control over the rate of heat input, the temperature of the weld zone, and the oxidizing or reducing potential of the welding atmosphere; • Oxyacetylene process is ideally suited to the welding of thin sheet, tubes, and small diameter pipe. It is also used for repair work, maintenance and in body shops; • Dissimilar metals can easily be joined; • Can also be used for preheating, cutting metal, case hardening, soldering and annealing.

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Limitations of Acetylene • Acetylene becomes extremely dangerous if used above 15 pounds pressure. Pure acetylene is self-explosive if stored in the free state under a pressure of 9.4 pounds per square inch (psi) • The process is typically slower than the electrical arc-welding processes

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Basic equipments of Oxy Acetylene Welding 1. Oxygen gas cylinder (green) 2. Acetylene gas cylinder (maroon/red) 3. Oxygen pressure regulator 4. Acetylene pressure regulator 5. Oxygen gas hose(Blue) 6. Acetylene gas hose(Red) 7. Welding torch or blow pipe with a set of nozzles and gas lighter 8. Trolleys for the transportation of oxygen and acetylene cylinders 9. Set of keys and spanners 10. Filler rods and fluxes 11. Protective clothing for the welder (e.g., asbestos apron, gloves, goggles, etc.)

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• Oxy-fuel apparatus consists of two cylinders (one oxygen and one acetylene) equipped with two regulators, pressure gauges, two lengths of hose, and a blow torch. • The regulators are attached to cylinders and are used to reduce and maintain a uniform pressure of gases at the torch. • The gases at reduced pressure are conveyed to the torch by the hoses. The regulators include high pressure and low pressure gauges to indicate the contents of the cylinder and the workingpressure on each hose. • When the gases reach the torch they are there mixed and combustion takes place at the welding tip fitted to the torch.

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Oxygen Cylinder

Acetylene Cylinder 46

Oxygen cylinder is drawn from a piece of high strength steel plate and is available in common sizes of: o 244 cu ft (for industrial plants); o 122 cu ft; o 80 cu ft The oxygen volume in a cylinder is directly proportional to its pressure. In other words, if the original pressure of a full oxygen cylinder drops by 10% during welding, it means 1/10th of the cylinder contents have been consumed. Oxygen cylinders are usually painted green and are screwed right handed.

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• The oxygen cylinder valve is made largely of brass with right hand threads. • Its outlet is threaded and machined to comply with standards set by the Compressed Gas Association (CGA) and the American National Standards Institute (ANSI). • Every oxygen cylinder valve is also equipped with a bursting disk which will rupture and release the contents of the cylinder if cylinder pressure should approach cylinder test pressure (as it might in case of a fire). • In order to protect cylinder valve from getting damaged, a removable steel cap is screwed on the cylinder at all times when the cylinder is not in use. • The cylinder valve is kept closed when the cylinder is not in use and even when cylinder is empty.

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ACETYLENE GAS CYLINDER • An acetylene cylinder is also a solid drawn steel cylinder and the common sizes are 300, 120 and 75 cubic feet. • Cylinder pressure is 250 PSI when filled. An acetylene cylinder is painted maroon and the valves are screwed left handed (with grooved hex on nut or shank).

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• Acetylene is extremely unstable in its pure form at pressure above 15 PSI. • This instability places special requirements on the storage of acetylene. Acetylene cylinders are packed with porous material (balsa wood, charcoal, corn pith, or portland cement) that is saturated with acetone to allow the safe storage of acetylene. • These porous filler materials aid in the prevention of high-pressure gas pockets forming in the cylinder. • Acetone, a colorless, flammable liquid, is than added to the cylinder until about 40 percent of the porous material is saturated. • Acetone is a liquid chemical that dissolves large portions of acetylene under pressure without changing the nature of the gas and is a liquid capable of absorbing 25 times its own volume of acetylene gas at normal pressure. • Being a liquid, acetone can be drawn from an acetylene cylinder when it is not upright. 50

•

•

•

•

Two very important things to remember about dissolved acetylene cylinders: First, acetylene cylinders should always be stored in the upright position to prevent the acetone form escaping thus causing the acetylene to become unstable. Second, withdrawal rate “not to exceed 1/10 (one-tenth) of the capacity of the cylinder per hour during intermittent use and no more than 1/15 (one-fifteenth) of the capacity of the cylinder per hour. If acetylene is withdrawn too rapidly, quite a lot of acetone may come with it, in vapor or droplet form, and the cylinder may cool down so much that it cannot sustain the high rate. This will affect your torch flame, and will mean that your supplier must replenish the acetone in the cylinder more frequently. Many acetylene cylinder valves are not equipped with hand wheels, and must be operated by a wrench. The wrench should always be left in place while the cylinder valve is open. Acetylene cylinders should be opened only 1/3 to ¼ of a turn when in use. 51

• An acetylene cylinder is protected by number of fusible plugs, which melt at 220°F (104°C). These plugs melt and release the pressure in case the cylinder is exposed to excessive heat. • Small cylinders (the 10 cu-ft. and 40 cu-ft. sizes) have one fusible metal channel located in the cylinder valve. The large cylinders normally used in welding and cutting, with capacities ranging up to nearly 300 cubic feet of acetylene, have two to four plugs, located in both top and bottom of the cylinders. • If a cylinder is exposed to a fire, one or more safety devices will melt and allow the acetylene and acetone to escape and burn gradually. If it did not have such a safety device, a full acetylene cylinder exposed to a fire would rupture and release its contents all at once, perhaps explosively.

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OXYGEN & ACETYLENE PRESSURE REGULATORS • The pressure of the gases obtained from cylinders is considerably higher than the gas pressure used to operate the welding torch. The purpose of using a gas pressure regulator is: • To reduce the high pressure of the gas in the cylinder to a suitable working pressure, and • To produce a steady flow of gas under varying cylinder pressures. • A pressure regulator is connected between the cylinder and the hose leading to welding torch. Desired pressure at the welding torch may be somewhere up to 35 psi for oxygen and 15 psi for acetylene.

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GAS HOSES & CLAMPS • The hoses used to make the connections between the torch and the regulators must be strong, nonporous, light, and flexible enough to make torch movements easy. The most common type of cutting and welding hose is the twin or double hose that consists of the fuel hose and the oxygen hose joined together side by side. • Size is determined by the inside diameter, and the proper size to use depends on the type of work for which it is intended. Hose used for light work has a 3/1 6 or 1/4 inch inside diameter and one or two plies of fabric. • For heavy-duty welding and cutting operations, use a hose with an inside diameter of 5/1 6 inch and three to five plies of fabric. Single hose is available in the standard sizes as well as 1/2, 3/4, and 1 inch sizes. These larger sizes are for heavy-duty heating and for use on large cutting machines.

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Oxygen hoses are green in color and have right hand thread. Acetylene hoses are red in color with left hand thread. The nut on the acetylene connection has a notch that runs around the center, distinguishing it from the nut on the oxygen connection. This is a safety precaution to prevent hoses from being hooked up the wrong way. 56

Check Valve • A check valve lets gas flow in one direction only and is positioned at the torch inlet, and at the regulator outlet. • The purpose of check valve is to prevent flame or oxygen-fuel mixture being pushed back into cylinder and causing backfire, flashback and explosion. • Backfire: A backfire is caused by the flame going out suddenly on the torch. A backfire may occur when: • The tip is touched against the work piece; • If the flame setting is too low; • If the tip is dirty, damage or loose, or; • If the tip is overheated.

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WELDING TORCH & BLOW PIPE • A welding torch mixes oxygen and acetylene in the desired proportions, burns the mixture at the end of the tip, and provides a means for moving and directing the flame.

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Welding Nozzles or Tips

• The welding nozzle or tip is that portion of the torch which is located at the end of the torch and contains the opening through which the oxygen and acetylene gas mixture passes prior to ignition and combustion. Depending upon the design of the welding torch, the interchangeable nozzles may consist of: • a) Either, a set of tips which screw onto the head of the blowpipe, or • b) As a set of gooseneck extensions fitting directly onto the mixer portion of the blowpipe.

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Filler Metals • Filler metals are used to supply additional material to the pool to assist in filling the gap (or groove) and it forms an integral part of the weld. • Filler rods have the same or nearly the same chemical composition as the base metal and are available in a variety of compositions (for welding different materials) and sizes. • These consumable filler rods may be bare, or they may be coated with flux. The purpose of the flux is to retard oxidation of the surfaces of the parts being welded, by generating gaseous shield around the weld zone. • The flux also helps to dissolve and remove oxides and other substances

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Types of Welding Flames • In oxyacetylene welding, flame is the most important tool. The flame must be of the proper size, shape and condition in order to operate with maximum efficiency. Three distinct types of flames are possible on adjusting the proportions of acetylene and oxygen: • Neutral Flame (Acetylene & oxygen in equal proportions) • Oxidizing Flame (Excess of oxygen) • Reducing Flame (Excess of acetylene)

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Neutral Flame

• A neutral flame is produced when the ratio of oxygen to acetylene, in the mixture leaving the torch, is almost exactly one-to-one. The temperature of the neutral flame is of the order of about 5900ºF.

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• Characteristics of Neutral flame: • The neutral flame is obtained when approximately one volume of oxygen and one volume of acetylene are mixed. It’s termed “neutral” because it will usually have no chemical effect on the metal being welded. It will not oxidize the weld metal; it will not cause an increase in the carbon content of the weld metal. • Neutral flame is obtained by gradually opening the oxygen valve to shorten the acetylene flame until a clearly defined inner cone is visible. • Neutral flame is used for most welding operations and for preheating during cutting operations. When welding steel with neutral flame, the molten metal puddle is quiet and clear; the metal flows easily without boiling, foaming, or sparking.

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• There are two clearly defined zones in the neutral flame. The inner zone consists of a luminous cone that is bluish-white. The inner cone is where the acetylene and the oxygen combine. • Surrounding this is a light blue flame envelope or sheath. This neutral flame is obtained by starting with an excess acetylene flame in which there is a "feather" extension of the inner cone. • When the flow of acetylene is decreased or the flow of oxygen increased the feather will tend to disappear. The neutral flame begins when the feather disappears. The neutral flame is commonly used for the welding of: • Mild steel • Stainless steel • Cast Iron • Copper • Aluminum 64

Carburizing or Reducing Flame • If the volume of oxygen supplied to the neutral flame is reduced, the resulting flame will be a carburizing or reducing flame, i.e. rich in acetylene. • A reducing flame can be recognized by acetylene feather which exists between the inner cone and the outer envelope. • The outer flame envelope is longer than that of the neutral flame and is usually much brighter in color.

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Characteristics of Reducing or carburizing flame

• An excess of acetylene creates a carburizing flame. The reducing or carburizing flame is obtained when slightly less than one volume of oxygen is mixed with one volume of acetylene. • This flame is obtained by first adjusting to neutral and then slowly opening the acetylene valve until an acetylene streamer or "feather" is at the end of the inner cone. • The length of this excess streamer indicates the degree of flame carburization. For most welding operations, this streamer should be no more than half the length of the inner cone.

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• The feather is caused by incomplete combustion of the acetylene to cause an excess of carbon in the flame. • The carburizing flame may add carbon to the weld metal and will tend to remove the oxygen from iron oxides which may be present, a fact which has caused the flame to be known as a “reducing flame”. • With iron and steel it produces very hard, brittle substance known as iron carbide. This chemical change makes the metal unfit for many applications in which the weld may need to be bent or stretched. • Metals that tend to absorb carbon should NOT be welded with reducing flame. • The reducing flame is typically used for welding high carbon steel and hard facing operations or backhand pipe welding techniques. When used in silver solder and soft solder operations, only the intermediate and outer flame cones

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• Since this flame provides a strong reducing atmosphere in the welding zone, it is useful for those materials which are readily oxidized like oxygen free copper alloys. • It is also used for high carbon steels, cast iron and hard surfacing with high speed steel and cement carbides. • A reducing flame has an approximate temperature of 5500°F (which is lowest among all the three flames). • A reducing flame may be distinguished from a carburizing flame by the fact that a carburizing flame contains more acetylene than a reducing flame. • A carburizing flame is used in the welding of lead and for carburizing (surface hardening) purposes. A reducing flame, on the other hand, does not carburize the metal; rather it ensures the absence of the oxidizing condition. • It is used for welding with low alloy steel rods and for welding those metals, (e.g. non ferrous) that do not tend to absorb carbon. This flame is very well used for welding high carbon steel. 68

Oxidizing Flame • The oxidizing flame is the third possible flame adjustment. It occurs when the ratio of oxygen to acetylene required for a neutral flame is changed to give an excess of oxygen. This flame type is observed when welders add more oxygen to the neutral flame.

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• The presence of excess oxygen in this flame creates undesirable oxides to the structural and mechanical detriment of most metals. It is useful for welding copper base alloys, zinc base alloys, cast iron, manganese steel etc.

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Characteristics of an Oxidizing flame • The oxidizing flame is produced when slightly more than one volume of oxygen is mixed with one volume of acetylene. To obtain this type of flame, the torch should first be adjusted to a neutral flame. The flow of oxygen is then increased until the inner cone is shortened to about onetenth of its original length. • When the flame is properly adjusted, the inner cone is pointed and slightly purple. • An oxidizing flame can also be recognized by its distinct hissing sound. The temperature of this flame is approximately 6300ºF (3482ºC) at the inner cone tip. • An oxidizing flame can be recognized by the small white cone which is shorter, much bluer in color and more pointed than that of the neutral flame. The outer flame envelope is much shorter and tends to fan out at the end on the other hand the neutral and carburizing envelopes tend to come to a sharp point.

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• An oxidizing flame burns with a decided loud roar. An oxidizing flame tends to be hotter than the other two flames. This is because of excess oxygen which causes the temperature to rise as high as 6300°F and not heat up as much thermally inert carbon. • When applied to steel, an oxidizing flame especially at high temperatures tends to combine with many metals to form hard, brittle, low strength oxides. This indicates that the excess oxygen is combining with the steel and burning it. Moreover, an excess of oxygen causes the weld bead and the surrounding area to have a scummy or dirty appearance. This flame will ruin most metals and should be avoided, except as noted below.

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• An oxidizing flame is of limited use in welding. It is not used in the welding of steel. A slightly oxidizing flame is helpful when welding most • Copper base metals • Zinc base metals, and • A few types of ferrous metals, such as manganese steel and cast iron • A stronger oxidizing flame is used in the welding of brass or bronze. The oxidizing atmosphere, in these cases, creates a base metal oxide that protects the base metal. For example, in welding brass, the zinc has a tendency to separate and fume away. The formation of a covering copper oxide prevents the zinc from dissipating.

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Advantages of Gas Welding • Welder has considerable control over the rate of heat input, the temperature of the weld zone, and the oxidizing or reducing potential of the welding atmosphere; • As the source of heat and filler metal are separated, the metal deposition can be easily controlled and heat properly adjusted giving rise to a satisfactory weld; • Welding equipment is portable and can be operated at remote places. Besides gas welding, the equipment can be used for preheating, post heating, braze welding, torch brazing and it is readily converted to oxygen cutting; • Weld bead size and shape and weld puddle viscosity are also controlled in the welding process because the filler metal is added independently of the welding heat source; • Gas welding is ideally suited to the welding of thin sheet, tubes, and small diameter pipe. It is also used for repair welding. Thick section welds, except for repair work, are not economical. 75

Applications of Gas Welding • For joining of thin materials. The process is used extensively for soldering copper tubing; • For joining materials in whose case excessively high temperatures or rapid heating and cooling of the job would produce unwanted or harmful changes in the metal; • For joining materials in whose case extremely high temperatures would cause certain elements in the metal to escape into the atmosphere; • For joining most ferrous and nonferrous metals, e.g., carbon steels, alloy steels, cast iron, aluminum, copper, nickel, magnesium and its alloys, etc; • In automotive and aircraft industries. In sheet metal fabricating plants, etc.

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Limitations of gas welding • • • • • • • • • •

Gas flame takes a long time to heat up the metal than an arc; Flame temperature is less than the temperature of the arc; Slower speed of welding compared electric arc welding; Heavy sections cannot be joined efficiently; For heavy sections proper penetration may not be achieved; Refractory metals (e.g., tungsten, molybdenum, tantalum, etc.) and reactive metals (e.g., titanium and zirconium) cannot be gas welded; Flux used in the filler metal provides fumes which are irritating to the eyes, nose, throat and lungs; More safety is recommended in gas welding; Acetylene and oxygen are expensive gases; Prolonged heating of the joint may results in large HAZ. This often leads to increased grain growth, more distortion and, in some cases, loss of corrosion 77

Arc welding process A group of welding processes wherein the metal or metals being joined are coalesced by heating with an arc, with or without the application of pressure and with or without the use of filler metal.

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Processes

Heat

Shielding

Filler

Shielded Metal Arc Welding

Electric arc

Inert gas-flux

consumable electrode

Gas metal arc welding

Electric arc

Inert gascylinder

Consumable wire

Flux core arc welding

Electric arc

Inert gascylinder

Consumable wire

Gas tungsten arc welding

Electric arc

Inert gascylinder

Manual rod

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The type of current and the polarity of the welding current are one of the differences between arc welding processes. – SMAW

Constant current (CC), AC, DC+ or DC-

– GMAW

Constant voltage (CV) DC+

– FCAW

Constant voltage (CV) DC-

– GTAW

Constant Current (CC) ), AC, DC+ or DC-

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Amperage Output • The maximum output of the power supply determines the thickness of metal that can be welded before joint beveling is required. • 185 to 225 amps is the optimum output amperage is a common size. • For an individual weld, determined by the – Thickness of the metal, – Type of joint, – Welding position – Type of electrode.

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Duty cycle •

The amount of continuous welding time a power supply can be used is determined by the duty cycle of the power supply. • Duty cycle is based on a 10 minute interval. • Many power supplies have a sloping duty cycle.

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Effect of Amperage too high • • • • • • • •

Excessive penetration, Burn through, Porosity, Spatter, Deep craters, Undercut, Electrode overheats, High deposition (positional welding difficult).

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Effect of Amperage Too Low • • • • • • •

Poor penetration or fusion, unstable arc, irregular bead shape, Slag inclusion, porosity, electrode freezes to the weld, possible stray arc strikes.

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Effect of Voltage too high • • • • • •

Porosity Spatter Irregular bead Slag inclusion Very fluid weld pool positional welding difficult.

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Effect of Voltage too low • • • • • • •

Poor penetration, Electrode freezes to work Possible stray arcs Fusion defects Slag inclusions Unstable arc Irregular bead shape.

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• Travel speed too slow

•

Excessive deposition

• •

Slag inclusions Irregular bead shape.

• Travel speed to fast • • • • •

Narrow thin bead Slag inclusion Fast cooling Undercut Poor fusion and Penetration

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Applications • • •

Construction, pipelines, shipbuilding, fabrication job shops. Used for: Steels, stainless steels, cast irons. Not used for aluminum and its alloys, or copper and its alloys (energy density is too high).

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ELECTRODE IDENTIFICATION • Arc welding electrodes are identified using the A.W.S, (American Welding Society) numbering system and are made in sizes from 1/16 to 5/16 . • An example would be a welding rod identified as an 1/8" E6011 electrode. • The electrode is 1/8" in diameter • The "E" stands for arc welding electrode.

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• Examples: • E60xx would have a tensile strength of 60,000 psi E110XX would be 110,000 psi • The next to last digit indicates the position the electrode can be used in. • EXX1X is for use in all positions • EXX2X is for use in flat and horizontal positions • EXX3X is for flat welding

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• The last two digits together, indicate the type of coating on the electrode and the welding current the electrode can be used with. Such as DC straight, (DC -) DC reverse (DC+) or A.C.

• • • • • • • • • • • •

EXX10 DC+ (DC reverse or DCRP) electrode positive. EXX11 AC or DC- (DC straight or DCSP) electrode negative. EXX12 AC or DCEXX13 AC, DC- or DC+ EXX14 AC, DC- or DC+ EXX15 DC+ EXX16 AC or DC+ EXX18 AC, DC- or DC+ EXX20 AC ,DC- or DC+ EXX24 AC, DC- or DC+ EXX27 AC, DC- or DC+ EXX28 AC or DC+ 91

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Power Sources for Arc Welding

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Power Sources for Arc Welding

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Power Sources for Arc Welding Before studying the types of power sources, one must understand the following basic features of the arc welding power sources: • Static characteristic curves (Volt-Amp curves) • Open-circuit voltage (OCV) • Static and dynamic characteristics • Current ratings and duty cycle specifications • Classes of insulation • Power factor

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Static Characteristic Curves • These curves show the relation between load voltage and load current under various load conditions. Hence they are also called Volt-Amp (V-A) curves. • To obtain such a curve, a pure resistive load (usually water load) is connected across the output terminals of the power source. • The load is gradually varied from the minimum or no-load condition to the maximum or short-circuit condition, and the voltage across the load and the current passing through the load are accurately measured.

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Characteristic curves of welding power sources 97

• A machine giving a drooping curve is said to have drooping characteristics and is described as a constant current (CC) type machine, because small variations in the voltage caused by variations in arc length during welding, do not significantly change the output current. • A machine with a relatively flat curve is described as a constant voltage (CV) or constant potential (CP) type machine, because in this case small variations in load voltage caused by arc fluctuations result in substantial changes in the current output. Normally CV machines are designed for DC welding. A CV power source giving AC output is not suitable for arc welding. • The terms constant current and constant voltage are technically incorrect; yet they are used and accepted by the welding industry.

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• Drooping or constant current type power sources are conventionally used for manual metal arc welding (MMAW) or gas tungsten arc welding (TIG). • The obvious reason being that with this type of characteristics, the welding current remains substantially constant, irrespective of small variations in arc length and consequent slight change in arc voltage, which are unavoidable even in the case of a skilled welder. As the welding current is fairly steady, the weld quality is consistent.

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• Flat or constant-voltage type power sources are conventionally used for semi-automatic/fully automatic processes involving a continuous electrode fed at a constant rate (MIG/CO2 welding, flux cored arc welding and submerged-arc welding) • The flat type power source together with a continuous electrode fed at a constant wire-feed speed form essentially a self regulating arc. The arc length and weld current are interrelated in such a way as to correct sudden changes. • For example, arc length variation is determined by the difference between melting rate and feeding rate of the electrode wire. The voltage drop across the arc is directly proportional to arc length. • A small change in arc voltage results in a very large change in the welding current. This in turn increases the melting rate of the wire and quickly restores the arc length to normal.

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Open-circuit Voltage • Open-circuit voltage (OCV) is the voltage across the output terminals of the power source when it is under no-load condition. Hence it is also termed as no-load voltage. • In CV type power sources, OCV is not significant, but in the case of CC type machines, both AC and DC, OCV plays a very important role in ensuring easy arc starting and good arc stability. • Higher the OCV, better is the arc stability. However, high OCV poses the danger of electric shock to the welder and hence its value is restricted to 100 V maximum by IS:4559,

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• Commercially available welding rectifiers have OCV values generally ranging between 65 and 80 V, while welding generators have variable OCV values in the 40-90 V range. • Welding transformers meant for heavy duty applications are generally designed for OCV of 60 to 70 V, while low-cost limitedservice class transformers have OCV as low as 50 V. • With such low OCV transformers, it is difficult to strike and maintain an arc. • Therefore electrodes having a high proportion of arc stabilisers in their coating are used. Some transformers are designed to give OCV of 100 V in addition to the normal OCV value, so that they can operate satisfactorily with certain basic low-hydrogen electrodes.

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Static and Dynamic Characteristics • The static characteristics describe the relation between voltage and current under various fixed load conditions. • In contrast, the dynamic characteristics describe the relation between voltage and current under changing load conditions. The instantaneous variation in arc voltage with change in welding current over an extremely short interval of time (say, 1 microsecond). • Good dynamic characteristics enable the power source to provide extremely rapid changes in its output voltage and current under changing arc conditions. • This is of special significance for the power source, because the welding arc is never in real steady state, but is subjected to severe and rapid fluctuations due to constant small variations in arc length, arc voltage and welding current.

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Current Ratings and Duty Cycles • Power sources are of various current ratings and duty cycles, and are rated by the manufacturers on the basis of their current output at specific duty cycles. • Duty cycle is defined as the percentage of a five-minute interval that it operates at a given current setting. • A 60% duty cycle means that the arc is in action for three minutes out of a five-minute time period. • In other words, a cycle of five minutes comprises a period of three minutes of welding load followed by a period of two minutes of noload operation. • Continuous operation at rated currents for 36 minutes out of one hour is not a 60% duty cycle, because the rating is based on successive five-minute intervals.

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Classes of Insulation

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ARC WELDING TRANSFORMERS

• The transformer is the most popular type due to its low cost, simple construction and ease of maintenance. • This welding machine which always delivers AC output for welding, is mainly a step-down type of transformer, which converts the highvoltage low-current industrial supply into low-voltage and highcurrent required for welding. • It generally operates on single-phase supply, i.e. 220 V singlephase, or two lines of 440 V three-phase supply. • The basic principle of a transformer: If two electrically conductive coils, which are electrically insulated from each other are wound on a common core made up of a magnetic material and an alternating voltage is given to one of the coils, an alternating flux is built up, which induces voltage in the second coil. The relation between input voltage and induced voltage is given by the formula:

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Input voltage (V1)

No. of turns on supply winding =

Induced voltage (V2) No. of turns on induced winding • In an arc welding transformer, the number of primary turns is considerably larger than the number of secondary turns. This results in low output voltage and high current required for welding. • Heavy duty industrial machines are rated for 200 to 500 Amps. at 60% duty cycle and operate on 440 V mains supply, whereas those meant for lower duty cycles are rated for 50 to 200 Amps. and operate on 220 V supply. • Heavy duty transformers used for automatic submerged arc welding are rated for 1,000 to 1,500 Amps. at 100% duty cycle

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The welding transformer essentially consists of:

• Main transformer assembly • Frame • Ventilating system (natural air cooling, forced air cooling or oil cooling) • Current control mechanism • The main transformer consists of primary and secondary coils and a magnetic core made up from thin laminations of special silicon-alloy steel. • The laminated core results in low core losses and improved overall efficiency. • The primary and secondary coils are made up of copper or aluminium. Aluminium coils provide advantages of low weight and economy. • As the conductivity of aluminium is less than that of copper, conductors of heavier cross-section are used for aluminium windings. 109

• Natural and forced air-cooled transformers are commonly used in industries. • However, if the environment is corrosive or if metallic dust particles are likely to enter the coils in service, oil-cooled transformers are preferred. • Various mechanisms are used to control and adjust the welding current to the desired value. In the early days, a bank of very high wattage resistors was added in the output circuit. • By using various parallel series combinations the effective resistance could be changed resulting in variation in the output current. • This method of current control is totally outdated mainly because considerable electrical power is wasted in the resistor bank and hence the efficiency is very low.

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Control of welding current In the modern industrial welding transformers, welding current is controlled by any of the following five methods: • A) Tapped choke: Tapping on a reactor in the secondary circuit. • B) Moving coils: Changing the magnetic coupling between primary and secondary by physically changing the position of coils. • C) Magnetic shunt: Changing the magnetic coupling between primary and secondary by putting a movable magnetic shunt. • D) Moving core: Moving the iron core in the reactor. • E) Saturable reactor: Putting saturable reactor unit in the secondary circuit.

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• Method (A) uses a tapped reactor, which does not give a continuous current output demands for certain important applications. The limited number of taps restricts the values of output current available. However, this system is relatively efficient and suitable for general fabrication and repair work.

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• Method (B) changes the reactance of the transformer by changing the relative positions of the coils. • Moving one coil away from the other increases the amount of leakage flux flow between them, thereby increasing the leakage reactance of the coils. This reduces the current output. Bringing the coils nearer results in increased current output. The change in positions of coils is brought about by a lead screw which facilitates continuous adjustment of current.

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• Method (C) employs a low magnetic resistance path which diverts part of total lines of force linking with the secondary coils. • The movement of this magnetic shunt causes the leakage flux to vary and thereby adjusts the output current. On large machines, the movement of the magnetic shunt can be conveniently motorized.

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• Method (D) uses a moving iron core in the reactor instead of in the main core. The moving core changes the air gap which changes the reactance. The larger the air gap, smaller the impedance and higher the output.

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• Method (E) eliminates all the moving parts with their service problems, but is more expensive. In this system, secondary reactor impedance is controlled by regulating the saturation level of the core electrically. • The system uses a rectifier bridge and a rheostat to control the DC current in the control coil. When there is no DC current flowing through the control winding, the impedance is maximum and the output is minimum.

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Arc welding generators • The welding generator was the first power source successfully developed for industrial use. • It is a rotary type machine driven by an electric motor or an internal combustion engine. The latter is either a diesel engine or a petrol engine and is air-cooled or water-cooled. • The welding generator, whether driven by a motor or an engine, forms a single integral unit. • The welding generator is a rugged and reliable DC machine which gives high quality welds. • The engine-driven type is specially developed for use in the open air and at site where regular power supply is not available. The welding generators commonly used in industry have outputs of 200 to 600 Amps. • The welding generators which were developed were basically DC generators with carbon brushes and commutators. 120

• When a conductor moves in a magnetic field, it cut the magnetic lines of force (flux) and, as a result alternating voltage is generated in the conductor. • Alternating voltage is then converted to direct volt by means of a device called commutator and collected together by a set of carbon brushes to get the required output. • The welding generator consists of a stationary frame called a stator, and a rotating armature, called a rotor. The frame, also called yoke, contains an extremely efficient magnetic circuit consisting ,of various poles. • Each pole is surrounded by a fine gauge wire coil and is so connected that the poles form a north/south combination. The power required to energize the poles is either taken from the exciter or the machine can be self-excited type, which makes use of the residual magnetism of the magnetic field.

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• The output voltage is controlled by the rheostat in series with the main poles. This increases or decreases the main magnetic lines of force and thereby brings about a change in the output voltage. • The carbon brushes which ride on the surface of this commutator pick up the -generated current and convert it into DC. Cables fastened to the brush holders carry the DC power from the brushes to the two output terminals to which electrode cable and earth cable have to be connected. • Since welding generators give DC output, provision has to be made for the reversal of polarity to suit welding conditions. In most cases, this is done through a reversing switch or by physically changing the output connections (i.e. electrode cable and earth cable). The output terminals are marked +ve and -ve or electrode and work. • The reversing switch changes the direction of the main field and thereby changes the output polarity.

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ARC WELDING RECTIFIERS • The welding rectifier, like the welding generator, provides DC for welding but unlike the welding generator, it has no moving parts. It consists of a step down voltage transformer with means to rectify AC to DC. • The transformer can be single-phase (or three phase) type, which converts the mains supply to a low voltage supply on the secondary side. • The method of controlling the current is usually in the AC section between the transformer and the rectifier set. The welding rectifier can be designed for constant current (CC) as well as for constant voltage (CV). • CC type rectifiers meant for manual welding are usually rated for any value between 200 and 600 Amps, while the CV types meant for automatic welding are rated between 300 and 1,500 Amps

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CC Type Rectifier • In this type, the current is controlled through variable inductance or impedance. The various methods available for varying the impedance are: (a) moving coil, (b) moving shunt, (c) saturable reactor or magnetic amplifiers, (d) tapped reactor, (e) moving reactor core and (f) solid state. • Rectifiers are used for converting AC to DC. A rectifier is a device which conducts easily in one direction, while offering a high resistance in the other, direction. • Today silicon diode rectifiers are preferred to selenium or germanium rectifiers, because they have greater rectification efficiency. Silicon diodes are smaller than selenium diodes and are hermetically sealed and mounted on a suitable fin for cooling.

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CV Type Rectifier • This type is usually designed to give a slightly sloping, instead of a perfectly flat, volt-ampere curve. While some machines are designed with one fixed slope, others are provided with an adjustment to tailor the slope of the V-A curve as desired. • Slope adjustment must take care that the dynamic characteristics of the machine are not affected. For this purpose, adjustable inductors may be placed in the DC portion of the circuit. • In CV machines, the slope is usually achieved by changing taps on reactors in series with the AC part of the circuit. • Slope control may be provided by carbon brushes, attached to a lead screw, contacting the reactor turns. • This variable reactor provides continuous adjustment of slope. Another method of control uses magnetic amplifiers or: solid$tate devices to electrically regulate output voltages.

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GAS TUNGSTEN ARC (TIG) WELDING (GTAW) • Gas tungsten arc welding (TIG welding or GTAW) is a process in which the joining of metals is produced by heating therewith an arc between a tungsten (non consumable) electrode and the work. • A shielding gas is used, normally argon. TIG welding is normally done with a pure tungsten or tungsten alloy rod, but multiple electrodes are sometimes used. • The heated weld zone, molten metal, and tungsten electrode are shielded from the atmosphere by a covering of inert gas fed through the electrode holder. Filler metal may or may not be added. • A weld is made by applying the arc so that the touching work piece and filler metal are melted and joined as the weld metal solidifies. 126

• This process is similar to other arc welding processes in that the heat is generated by an arc between a non consumable electrode and the work piece, but the equipment and electrode type distinguish TIG from other arc welding processes. Striking the arc may be done by any of the following methods: • Touching the electrode to the work momentarily and quickly . • Using an apparatus that will cause a spark to jump from the electrode to the work. • Using an apparatus that initiates and maintains a small pilot arc, providing an ionized path for the main arc.

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• High frequency arc initiation occurs when a high frequency, high voltage signal is superimposed on the welding circuit. • High voltage (low current) ionizes the shielding gas between the electrode and the workpiece, which makes the gas conductive and initiates the arc. Inert gases are not conductive until ionized. • When welding manually, once the arc is started, the torch is held at a travel angle of about 15 degrees. For mechanized welding, the electrode holder is positioned vertically to the surface. • To start manual welding, the arc is moved in a small circle until a pool of molten metal forms. The establishment and maintenance of a suitable weld pool is important and welding must not proceed ahead of the puddle. • Once adequate fusion is obtained, a weld is made by gradually moving the electrode along the parts to be welded to melt the adjoining surfaces.

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• Solidification of the molten metal follows progression of the arc along the joint, and completes the welding cycle. • Welding is stopped by shutting off the current with foot-or-handcontrolled switches that permit the welder to start, adjust, and stop the welding current. • They also allow the welder to control the welding current to obtain good fusion and penetration. • Welding may also be stopped by withdrawing the electrode from the current quickly, but this can disturb the gas shielding and expose the tungsten and weld pool to oxidation.

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The major operating variables summarized briefly are: • Welding current, voltage, and power source characteristics. • Electrode composition, current carrying capacity, and shape. • Shielding gas--welding grade argon, helium, or mixtures of both. • Filler metals that are generally similar to the metal being joined and suitable for the intended service.

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GTAW Equipment • Power Source • Torch • Gas Cylinder • Tungsten Electrode

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TIG Welding Power Source • •

• •

Gas tungsten arc welding uses a constant current power source, meaning that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. The polarity of the GTAW system depends largely on the type of metal being welded. Direct current with a negatively charged electrode (DCEN) is often employed when welding steels, nickel, titanium, and other metals. Direct current with a positively charged electrode (DCEP) is less common, and is used primarily for shallow welds since less heat is generated in the base material. Instead of flowing from the electrode to the base material, as in DCEN, electrons go the other direction, causing the electrode to reach very high temperatures. As a result, a larger electrode is often used. However, the ionized shielding gas flows toward the base material, removing oxides and improving the quality and appearance of the weld. 135

Electrode • The electrode used in GTAW is made of tungsten or a tungsten alloy, because tungsten has the highest melting temperature among metals, at 3422 °C. As a result, the electrode is not consumed during welding, though some erosion (called burn-off) can occur. • The diameter of the electrode can vary between 0.5 mm and 6.4 mm (0.02-0.25 in), and their length can range from 75 and 610 mm (3-24 in).

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AWS Class

Color

Alloy

EWP

Green

None

EWCe-2

Orange

~2% Cerium oxide

EWLa-1

Black

1% Lanthanum oxide

EWLa-1.5

Gold

1.5% Lanthanum oxide

EWLa-2

Blue

2% Lanthanum oxide

EWTh-1

Yellow

~1% Thorium oxide

EWTh-2

Red

~2% Thorium oxide

EWZr-1

Brown

0.15-0.4% Zirconium oxide 137

Shielding gas • GTAW to protect the welding area from atmospheric gases such as nitrogen and oxygen, which can cause fusion defects, porosity, and weld metal embrittlement if they come in contact with the electrode, the arc, or the welding metal. • The gas also transfers heat from the tungsten electrode to the metal, and it helps start and maintain a stable arc. • Argon-helium mixtures are also frequently utilized in GTAW, since they can increase control of the heat input while maintaining the benefits of using argon. • Noramlly, the mixtures are made with primarily helium (often about 75% or higher), with the remainder being argon.

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Applications • The process is widely used for welding of pressure vessels, heat exchangers and pipes where tightness is important since the process produces welds with very low pore fractions. • TIG-welding is also beneficial for welds with frequently starts and stops and for short welds due to excellent quality with low porosity. • DC TIG (negative electrode) could in addition also be beneficial for welding thin walled structures and profiles.

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Advantages and Disadvantages Advantages • • • • • • •

High quality and precision welding Pin point control Aesthetic weld beads No spark or spatter No flux or slag No smoke or fumes Weld more metals and alloys

Disadvantages • • • • •

Slower travel speed than others Lower filler metal deposition Hand – eye coordination required is high Brighter UV rays than other processes Costly equipment 140

Metal Inert Gas Welding • It is an arc welding process wherein coalescence is produced by heating the job with electric arc established between a continuously fed metal electrode and the job. • No flux is used but the arc and molten metal are shielded by an inert gas, which may be argon, helium, carbon dioxide or a gas mixture. • This process uses a bare wire consumable electrode wire, typically 0.8 -1.6 mm diameter • Is continuously fed from a coil through a specially designed welding gun. • The possibility of atmospheric contamination is eliminated by introducing a shielding gas. • When an inert gas is used for shielding the welding, the process is known as metal inert-gas (MIG) welding • Argon is an efficient shielding gas, being inert Argon does not chemically react with the weld metal 141

• The MIG-process uses a direct current power source, with the electrode positive (DC, EP). By using a positive electrode, the oxide layer is efficiently removed from the aluminium surface, which is essential for avoiding lack of fusion and oxide inclusions. • The shielding gas, which is usually carbon dioxide or mixtures of carbon dioxide and argon, protects the molten metal from reacting with the atmosphere.

MODES OF METAL TRANSFER • Short Circuit Transfer • Globular Transfer • Spray transfer

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Short circuit transfer

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Description • Droplets transferred during short-circuits • Weld pool of high viscosity • About 70 droplets transferred per second • Low current and Low voltage Applications: • Thin sheet welding • All position welding 144

Globular Transfer • Medium Current • Medium Voltage Application • Down hand position welding

• Sheet thickness > 2 mm.

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• • • •

Description Electrode material transferred in large globules; not free from short circuits Weld pool of low viscosity About 100 droplets transferred per second

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Spray transfer

•High Current; High voltage No short-circuit on droplet transfer Weld pool of low viscosity About 100 to 300 droplets transferred per second

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• DCEP (Direct Current Electrode Positive) is used for most stainlesssteel welding. • Ar 1 or 2% argon-oxygen mixture is recommended for most stainless steel spray arc welding. • Electrode diameters as great as 1/16-in., but usually 0.045", 0.035", and 0.030", are used with relatively high currents to create the spray-arc transfer. • A current of approximately 300-350 amperes is required for a 1/16in. electrode, depending on the shielding gas and type of stainless wire being used. • The degree of spatter is dependent upon the composition and flow rate of the shielding gas, wire-feed speed, and the characteristics of the welding power supply.

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Shielding gases for MIG-welding • For thicknesses: < 12.5 mm: Argon • 12.5 to 25 mm Argon or Argon/Helium mixtures • 25 mm Argon/Helium mixtures or Helium

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Advantages of MIG • The best choice when cosmetic appearance is an issue since it provides lower spatter levels than flux-cored. The arc is soft and less likely to burn through thin material. • The lower spatter associated with MIG also means no slag to chip off and faster cleaning time. • MIG is the easiest type of welding to learn and is more forgiving if the operator is somewhat erratic in holding arc length or providing a steady travel speed. • If you are skilled and get specific proper guns, shielding gas, liners, drive rolls, and electrode, MIG can weld a wider range of material including thinner materials and different materials such as stainless, nickel alloys or aluminum.

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Disadvantages of MIG • Since a bottle of external shielding gas is required, MIG may not be convenient for portability. • MIG also requires additional equipment such as a hose, regulator, solenoid (electric valve) in the wire feeder and flow meter. • The welder’s first job is to prepare the surface by removing paint, rust and any surface contamination. • MIG has a soft arc which will not properly weld thicker materials (10 gauges would be the maximum thickness that MIG could soundly weld with the 115 volt compact wire feed • As the thickness of the material (steel) increases, the risk of cold lapping also increases because the heat input needed for good fusion is just not possible with these small machines.

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Applications of MIG-welding • MIG-welding is a general purpose welding process for welding of aluminium and applicable in most cases in all welding positions from about 1 mm sheet thickness to thick walled sections. • MIG-welding also offers high quality welds with a high productivity.

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Electro Slag Welding • • •

• • •

Electroslag welding is a welding process wherein coalescence is produced by molten slag which melts the filler metal and the surfaces of the work to be welded. Electroslag welding is initiated by starting an arc between the filler metal/electrode and the work. The molten metal pool remains shielded by the molten slag which moves along the full cross section of the joint as the welding progresses. Electroslag welding is a mechanized method of making vertical and near-vertical welds, with a maximum slope 150C of the vertical. It is intended for welding very thick materials (40mm and up), although it can also be used for thinner materials. Electroslag welding is used mainly to join low carbon steel plates and/or sections that are very thick. It can also be used on structural steel if certain precautions are observed.

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•

The process is started by striking an arc between the electrode and the workpiece.

•

Flux is fed into the joint, and melts to form a bath of slag that increases in depth as more flux is added.

•

When the temperature of the slag, and thus also its electrical conductivity, has increased sufficiently, the arc is short-circuited and the current is carried by the molten slag, maintaining its temperature by resistive heating.

•

The molten metal is prevented from escaping the joint by water-cooled copper shoes, which may be fixed or arranged to travel with the welding head.

•

The weld is formed between them and the surfaces of the joint. The welding head moves up the joint as welding progresses.

•

One or more filler wires may be used, depending on the thickness of the plate. If the material is very thick, the welding head may weave.

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• The filler wire in this version of electroslag welding is guided into the weld pool through a tube that melts and contributes to the filler material as welding proceeds. • The tube may be coated, to provide slag to keep the depth of the slag pool constant. • The advantage of this method is the welding head can be fixed, with the length (height) of the weld being determined by the length of the tube, which can be up to a metre long.

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Benefits • • • • • • • • • • •

Low cost for joint preparation Single pass, regardless of the plate thickness No angular deformation when making butt joints Little transverse shrinkage Little risk of hydrogen embrittlement. Joint preparation is much simpler than other welding processes. Thicker steels can be welded more economically. ESW gives extremely high deposition rates. Residual stresses and distortion produced are low. Flux consumption is very low. No spattering or intense arc flashing occurs.

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Plasma Arc Welding • Plasma arc welding is an arc welding process wherein coalescence is produced by the heat obtained from a constricted arc set between a tungsten/alloy tungsten electrode and the water-cooled (constricting) nozzle (non-transferred arc) or between a tungsten/alloy tungsten electrode and the job(transferred arc). • The process employs two inert gases, one forms the arc plasma and the second shields the arc plasma. Filler metal may or may not be added.

Plasma is commonly known as fourth state of matter after solid, liquid and gas. This is extremely hot substance which consists of free electrons, positive ions, atoms and molecules, it conducts electricity. 161

Plasma welding

Equipment • Welding torch • Power source • Control equipment Welding torch The same basic requirements apply here as for TIG welding torches are generally water-cooled.

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• All plasma torches are water cooled, even the lowest-current range torch. • This is because the arc is contained inside a chamber in the torch where it generates considerable heat. • If water flow is interrupted briefly, the nozzle may melt. • During the nontransferred period, the arc will be struck between the nozzle or tip with the orifice and the tungsten electrode. • Manual plasma arc torches are made in various sizes starting with 100 amps through 300 amperes. • The normal combination of gases is argon for the plasma gas, with argon plus 2 to 5% hydrogen for the shielding gas only for austenitic stainless steels.

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• The torch utilizes the 2 percent thoriated tungsten electrode. Since the tungsten electrode is located inside the torch, it is almost impossible to contaminate it with base metal. • Control console. A control console is required for plasma arc welding. The plasma arc torches are designed to connect to the control console rather than the power source. • The console includes a power source for the pilot arc, delay timing systems for transferring from the pilot arc to the transferred arc, and water and gas valves and separate flow meters for the plasma gas and the shielding gas. • The console is usually connected to the power source and may operate the contactor. It will also contain a high-frequency arc starting unit, a nontransferred pilot arc power supply, torch protection circuit, and an ammeter. • The high-frequency generator is used to initiate the pilot arc. Torch protective devices include water and plasma gas pressure switches which interlock with the contactor. 166

• PAW has a higher energy concentration. Its higher temperature, constricted cross-sectional area, and the velocity of the plasma jet create a higher heat content. • The other advantage is based on the stiff columnar type of arc or form of the plasma, which doesn´t flare like the gas tungsten arc. These two factors provide the following advantages: • (a) The torch-to-work distance from the plasma arc is than for gas tungsten arc welding. This is important operation. • (b) High temperature and high heat concentration of allow for the keyhole effect, which provides complete single pass welding of many joints.

less critical for manual the plasma penetration

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• In this operation, the heat affected zone and the form of the weld are more desirable. The heat-affected zone is smaller than with the gas tungsten arc, and the weld tends to have more parallel sides, which reduces angular distortion. • The higher heat concentration and the plasma jet allow for higher travel speeds. • The plasma arc is more stable and is not as easily deflected to the closest point of base metal. Greater variation in joint alignment is possible with plasma arc welding. • Plasma welding has deeper penetration capabilities and produces a narrower weld. This means that the depth-to-width ratio is more advantageous.

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Power Source • Plasma welding employs DC, with a drooping characteristic, as for TIG welding. • Open circuit voltage should be at least 80 V. • HF generator In principle, the purpose of the HF generator is the same as in TIG welding. • When used in plasma welding, the HF generator does not normally strike the main arc: instead, it strikes a pilot arc as a non-transferred arc, with the current flowing between the electrode and the inner gas nozzle. • The pilot arc, in other words, can be maintained in air: as the torch approaches the workpiece, the main arc strikes and the pilot arc is extinguished.

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Shielding Gas • Argon/helium mixtures result in a higher energy in the plasma jet at constant current. • The mixture must contain at least 50 % helium if any significant difference is to be noted. On the other hand, mixtures containing more than 75 % helium have the same characteristics as pure helium. • Pure argon, or argon/helium mixtures, are well suited to the welding of mild steel and reactive metals (titanium, aluminium, zirconium etc.), for which hydrogen or nitrogen cannot be used.

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Advantages • Butt welds possible in thick materials (8 mm) without the use of fillers. • Fusion welding possible even in very thin materials (0.03 mm). • Low heat-affected zone and little distortion. • High arc stability at low arc currents. • Little sensitivity to arc length variations as a result of the concentrated arc. • Assessment of the weld quality possible while welding is in progress. • High metallurgical quality in comparison with that of conventional TIG welded materials.

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Disadvantages • Infra-red and ultraviolet radiations necessitate special protection devices. • Welder need of ear plugs because of unpleasant, disturbing and damaging noise. • More chance of electrical hazards are associated with this pro-butt welds. • The process is limited to metal thickness of 25mm and lower for butt welds. • Inert gas consumption is high

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• The major limitations of the process have to do more with the equipment and apparatus. • The torch is more delicate and complex than a gas tungsten arc torch. Even the lowest rated torches must be water cooled. • The tip of the tungsten and the alignment of the orifice in the nozzle is extremely important and must be maintained within very close limits. • The current level of the torch cannot be exceeded without damaging the tip. • The water-cooling passages in the torch are relatively small and for this reason water filters and deionized water are recommended for the lower current or smaller torches. • The control console adds another piece of equipment to the system. This extra equipment makes the system more expensive and may require a higher level of maintenance.

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Applications • • • •

Welding steel rocket motor cases Nuclear submarine pipe system Welding nickel and high nickel alloys Higher production rates based on faster travel speeds result from plasma over gas tungsten arc welding. Tubing made of stainless steel, titanium, and other metals is being produced with the plasma process at higher production rates than previously with gas tungsten arc welding. • Most applications of plasma arc welding are in the low-current range, from 100 amperes or less. The plasma can be operated at extremely low currents to allow the welding of foil thickness material. • (c) Plasma arc welding is also used for making small welds on weldments for instrument manufacturing and other small components made of thin metal. It is used for making butt joints of wall tubing. 176

• Tool Die & Mold Repair: Modern micro-arc is used for tool, die and mold repair. • Strip Metal Welding: The plasma process provides the ability to consistently transfer the arc to the workpiece and weld up to the edges of the weld joint. This is especially advantageous in high volume applications where the material outgases or has surface contaminants. • Tube Mill Welding: Tube mills produce tube and pipe by taking a continuous strip of material and rollforming the edges upwards until the edges of the strip meet together at a weld station. At this point the welding process melts and fuses the edges of the tube together and the material exits the weld station as welded tube.

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• Regarding thickness ranges welded by the plasma process, the keyhole mode of operation can be used only where the plasma jet can penetrate the joint. In this mode, it can be used for welding material from 1/16 in. (1.6 mm) through 1/4 in. (12.0 mm). • Thickness ranges vary with different metals. The melt-in mode is used to weld material as thin as 0.002 in. (0.050 mm) up through 1/8 in. (3.2 mm). • Using multipass techniques, unlimited thicknesses of metal can be welded. Note that filler rod is used for making welds in thicker material.

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Laser Beam welding • Laser beam welding is defined as a welding process wherein coalescence is produced by the heat obtained from the application of a concentrated coherent light beam impinging upon the surface to be joined. • Laser is a device for concentrating light waves into narrowly defined highly intense beam that can impart tremendous energy on a small area to produce fusion for welding purpose. • The laser beam is focused by a lens or mirrors into a point only a few tenths of a diameter in order to provide a high energy density. • The focus point is arranged to fall on, or slightly below, the surface of the workpiece. • The material immediately melts, with some even being vaporised. • The vaporised metal in the hole forms a plasma which, being a good absorber of the incident light, improves energy absorption and so efficiency of the process.

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• The term Laser is an acronym for Light Amplification Stimulation Emission of Radiation. A medium, either gaseous or solid, is excited to emit a monochromatic (single wavelength) coherent source of light. • This light can be focused to a point source, called spot size, resulting in very high power densities, capable of vaporizing various materials. • By controlling the power density, through the laser power and spot size, and with the assistance of gases, laser cutting and welding can be achieved certain dissimilar metal combinations. Advantages of LBW include high travel speeds, minimal heat affected zones, high mechanical properties, low distortion, no slag or spatter and the process is automated. Thick (>1 inch) single pass welds can be achieved with high powered CO2 systems. Nd:YAG lasers can be delivered via fiber optics and thus can be manipulated by robotics and can weld complex structures. 181

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• Laser beam welding has high power density (on the order of 1 MW/cm2) and high heating and cooling rates. • The spot size of the laser can vary between 0.2 mm and 13 mm, though only smaller sizes are used for welding. • The depth of penetration is proportional to the amount of power supplied, but is also dependent on the location of the focal point: penetration is maximized when the focal point is slightly below the surface of the workpiece. • A continuous or pulsed laser beam may be used depending upon the application. Millisecond-long pulses are used to weld thin materials such as razor blades while continuous laser systems are employed for deep welds.

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• LBW is a versatile process, capable of welding carbon steels, HSLA steels, stainless steel, aluminum, and titanium. • Due to high cooling rates, cracking is a concern when welding highcarbon steels. • The weld quality is high • The speed of welding is proportional to the amount of power supplied but also depends on the type and thickness of the workpieces. • The high power capability of gas lasers make them especially suitable for high volume applications. LBW is particularly dominant in the automotive industry.[1][2] • the laser beam can be transmitted through air rather than requiring a vacuum, • the process is easily automated with robotic machinery, • x-rays are not generated, and

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• This is achieved by keyhole welding methods, which ensure complete penetration.

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Welding speed depends on the laser power, and can be high when thin materials are being welded: up to 10-50 m/min and even 100 m/min when welding foils.

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Laser welding is, in other words, fast: about twice as fast as plasma welding and eight times faster than TIG welding. Pore-free highstrength welds, excellent dimensional tolerances and high productivity make the method superior to most others in many applications.

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CO2 Laser Nd:YAG Laser

CO2 Laser • laser generates its light in a tube through which a mixture of gases (including flows, producing a wavelength of 10.6 µm The energy input is by means of an electric discharge through the gas. • It can produce a high power output, and so is popular for welding and cutting applications. The light is usually conveyed to the welding head and focused by mirrors. • A shielding gas (often helium or an argon/helium mixture) is used to protect the lens and the weld: it helps to limit the amount of energyabsorbing plasma formed above the surface of the joint. In this respect, helium is to be preferred, due to its high ionisation energy.

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The major drawback to laser beam welds is the slow welding speeds (25 to 250mm/min) resulting from the pulse rates and puddle sizes at the fusion point. Laser welding is limited to depths of approximately 1.5mm and additional energy only tends to create gas voids and undercuts in the work.

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Carbon arc welding • Carbon Arc Welding (CAW) is a welding process, in which heat is generated by an electric arc struck between an carbon electrode and the work piece. The arc heats and melts the work pieces edges, forming a joint. Carbon arc welding is the oldest welding process. If required, filler rod may be used in Carbon Arc Welding. End of the rod is held in the arc zone. The molten rod material is supplied to the weld pool. Shields (neutral gas, flux) may be used for weld pool protection depending on type of welded metal.

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• CARBON ARC WELDING (CAW) utilizes what is considered to be a nonconsumable electrode, made of carbon or graphite, to establish an arc between itself and either the workpiece or another carbon electrode. • However, this electrode erodes fairly quickly and generates carbon monoxide (CO) gas that partially replaces the air around the arc, thereby providing the molten weld with some protection. • The CAW process, which uses either single or twin electrodes, most closely resembles gas-tungsten arc welding (GTAW), where the arc is used only as a source of heat. • The single-electrode arrangement usually operates with direct current (dc), electrode negative (straight polarity), using most dc power supplies.

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• The twin-electrode arrangement usually operates with alternating current (ac), generally with small ac power supplies. Figure shows typical configuration.

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Advantages • Heat input to the workpiece can be easily controlled by changing the arc length. • Workpiece distortion is negligible. • Process can be easily mechanized. • Process is simple and good welding skill can be acquired in short time. • Total welding cost is less as compared to other welding processes. Equipment required for carbon arc welding is simple and easily available. • Process is very suitable for butt welding of thinner workpieces (1-2 mm thickness).

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Disadvantages • There are chances of carbon being transferred from electrode to weld metal, thus causing a harder weld deposit in case of ferrous materials. • In the absence of proper electrode geometry and in confined spaces arc blow results which gives poor welds with blowholes and porosity. • A separate filler metal is needed; which (when used) slows down the welding speed.

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Applications • Carbon arc welding process can be used for welding steel, aluminium, nickel, copper and a good number of other alloys. • Carbon arc can also be employed for brazing, preheating and postheating of the welded joints. • Carbon arc welding can be used for repairing castings. On many applications, however, carbon arc welding has been replaced by TIG welding.

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Atomic Hydrogen Welding • Atomic hydrogen welding (AHW) is an arc welding process that uses an arc between two metal tungsten electrodes in a shielding atmosphere of hydrogen. • The electric arc efficiently breaks up the hydrogen molecules, which later recombine with tremendous release of heat, reaching temperatures from 3400 to 4000 °C. • This device may be called an atomic hydrogen torch, nascent hydrogen torch or Langmuir torch. The process was also known as arc-atom welding. • The heat produced by this torch is sufficient to weld tungsten (3422 °C), the most refractory metal. • The presence of hydrogen also acts as a gas shield and protects metals from contamination by carbon, nitrogen, or oxygen, which can severely damage the properties of many metals. It eliminates the need of flux for this purpose. 195

• Atomic hydrogen welding is a thermo - chemical welding process in which the work pieces are joined by the heat obtained on passing a stream of hydrogen through an electric arc struck between tow tungsten electrodes. • The arc supplies the energy for a chemical reaction to take place, thereafter heat is obtained for welding. Filler rod may or may not be used during the process. • The equipment consists of a welding torch with two tungsten electrodes inclined and adjusted to maintain a stable arc. • Annular nozzles around the tungsten electrodes carry the hydrogen gas supplied from the gas cylinders. • AC power source is suitable compared to DC, because equal amount of heat will be available at both the electrodes. A transformer with an open circuit voltage of 300 volts is required to strike and maintain the arc.

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• The work pieces are cleaned to remove dirt, oxides and other impurities to obtain a sound weld. Hydrogen gas supply and welding current are switched ON. • An arc is struck by bringing the tow tungsten electrodes in contact with each other and instantaneously separated by a small distance, say 1.5 mm, such that the arc still remains between the two electrodes. • As the jet of hydrogen gas passes through the electric arc, it disassociates into atomic hydrogen by absorbing large amounts of heat supplied by the electric arc. (Endothermic Reaction) • The heat thus absorbed can be released by recombination of the hydrogen atoms into hydrogen molecule (H2) • Recombination takes place as the atomic hydrogen touches the cold work piece liberating a large amount of heat. (Exothermic reaction)

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• The arc is maintained independently of the workpiece or parts being welded. • The hydrogen gas is normally diatomic (H2), but where the temperatures are over 600 °C (1100 °F) near the arc, the hydrogen breaks down into its atomic form, simultaneously absorbing a large amount of heat from the arc. • When the hydrogen strikes a relatively cold surface (i.e., the weld zone), it recombines into its diatomic form releasing the energy associated with the formation of that bond. • The energy in AHW can be varied easily by changing the distance between the arc stream and the workpiece surface. • This process is being replaced by shielded metal-arc welding, mainly because of the availability of inexpensive inert gases.

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Advantages of Atomic Hydrogen Welding: • Intense flame is obtained which can be concentrated at the joint. Hence, less distortion. • Welding is faster. • Work piece do not form a part of the electric circuit. Hence, problems like striking the arc and maintaining the arc column are eliminated. • Separate flux / shielding gas is not required. The hydrogen envelop itself prevents oxidation of the metal and the tungsten electrode. It also reduces the risk of nitrogen pick - up.

Disadvantages of Atomic Hydrogen Welding: • Cost of welding by this process is slightly higher than with the other process. • Welding is limited to flat positions only.

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Applications of Atomic Hydrogen Welding: • Atomic hydrogen welding is used in those applications where rapid welding is necessary, as for stainless steels and other special alloys.

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Solid state Welding • • • •

Joining takes place without fusion at the work piece interface No liquid or molten phase is present at the joint Two surfaces brought together under pressure For strong bond, both surfaces must be clean: – No oxide films – No residues – No metalworking fluids No metalworking fluids – No adsorbed layers of gas – No other contaminants

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Types of Friction Welding • Rotary • Rotary friction welding is the most common form of friction welding and has become the • Industry standard for a number of processes including welding API drill pipes and drill rods, joining of axle cases and spindles and welding of piston rods. • Rotary friction welding involves holding one component still while spinning the other component and brining the two together. •

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• The rotary friction welding process is inherently flexible, robust and tolerant to different • qualities of materials. The parameters involved are the rotational speed, time and force • applied. There are optimum parameters for each particular weld that Thompson Friction • Welding have calculated through years of experience. However, as the process is • inherently robust and flexible, deviations on these parameters can still give a good weld. •

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• Linear • In linear friction welding the same principles apply as rotary. One component is held still while the other is moved at speed and the two are brought together. The difference is that the moving component does not rotate, it is made to oscillate laterally. • The weld times for parts are similar no matter how big the part. • Due to the geometry, rotary friction welding does not have any friction in the centre of the rotating part. • This portion of the weld must heat up conventionally rather than due to friction. This is not the same for a linear friction weld. Friction occurs throughout the weld surface. • This means that weld times are very quick and do not vary hugely from part size to part size.

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• Speed is an important parameter in determining the maximum interface temperature and hence the final joint metallurgy. • High speeds produce overheated structures whereas low speeds can produce insufficient heating. • The friction pressure in conjunction with the peripheral speed determines the thermal conditions established in the weld region, and the rate at which metal is extruded radically to form an upset • For most materials, there is wide range of combinations of speed and pressure that may be used to give excellent mechanical and metallurgical integrity in the weld • The duration of the heating is selected so as to ensure that the faying surfaces are cleaned by friction and the weld zone temperature is raised to achieve the required plasticity for solid state pressure welding. • The forge pressure is selected with respect to the hot strength of the materials being welded because sufficient pressure must be used to hot work the region and to consolidate the interface. 210

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Advantages • • • • •

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Weld monitoring can insure 100% weld quality and produces a 100% cross sectional weld area Far superior weld integrity compared to MIG welding Limited operator training require – full automation also possible The weld cycle is fully controlled by the machine Friction welding is a solid state process and does not suffer from inclusions and gas porosity. Friction welding required no consumables therefore becomes more cost effective Friction welding typically will complete a full cross sectional weld in 15% of the time it take MIG welding to produce an 85% cross sectional weld. Friction welding requires no special weld interface preparation No post machining is needed for friction welded components in many cases Dissimilar materials can be joined with no alloying of the material 212

Disadvantages • The use of this process is restricted to flat and angular butt welds, where one part is normal to the other part. • So far the process has been applied only to the joining of small pieces in the form of bar stock. • Sometimes, quite a heavy flash is forced out in all inertia and friction welds. • If tubing is welded, flash may have to be removed from inside the joint.

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Applications • Trailer axles – welding spindle to the case. Advantages of this include – fast production time – extremely accurate weld – required machinery footprint reduction

• Piston rods – welding the eye or yoke to the shaft. • API drill pipes and drill rods – welding of connectors to pipes and rods.

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Ultrasonic Welding • Ultrasonic welding is a solid state welding process where in coalescence is produced by the local application of high frequency vibratory energy to the work piece as they are held together under pressure. • Ultrasonic welding bonds the work piece parts together by vibrating them against each other at high frequency under pressure. • A frequency converter converts 50 cycle line power into high frequency electrical power and a transducer change the high frequency electrical power into vibratory energy which is transmitted to the joint through the welding tip attached to the transducer. The tip oscillates in the plane of the joint interface 215

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Power Generation

Ultrasonic Welding Power Generation • Frequency is transformed to vibration energy through the transducer. • Energy requirement established through the following empirical relationship. – E = K (HT)3/2 – E = electrical energy – H = Vickers hardness number – T = thickness of the sheet

Electrical energy

Frequency Converter

Vibratory transducer

Sonotrode Tip and Anvil Material High Speed Tool Steels Used to Weld • Soft Materials • Aluminum • Copper • Iron • Low Carbon Steel

Hardenable Nickel-Base Alloys Used to Weld • Hard, High Strength Metals and Alloys

• Ultrasonic vibrations combined with the static clamping force, induce dynamic shear force stresses in the workpieces, then local plastic deformation of joint materials occurs at the interface. • Oxide coating and other surface films are shattered and dispersed so that intimate contact and bonding of the workpiece surface takes place.

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Welding equipment • Transducer (magnetostrictive and electrostrictive) • Anvil • Force application device

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• Transducer converts the electrical power to elastic vibratory power and a coupling system including the sonotrode tip which conducts vibration to the weld zone. • Magnetostrictive transducer materials change length under the influence of varying magnetic flux density. Transducers are rugged and serviceable for continuousduty operation but they have a low practical overall efficiency . • Electrostrictive ceramic material and is capable of changing dimensions when subjected to an electrical field parallel to the plane of polarization.

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• An anvil serves as a backing for the workpieces and provides the necessary reaction to clamping force. An anvil must not permit compliance of the workpiece with the applied vibration . To prevent energy loss the anvil assembly should be structurally isolated from the welder frame. • A force application device applies static load normal to the plane of weld surface. • Spring –actuated for very small machines • Pneumatically –actuated in medium size machines • Hydraulically actuated with larger units. 226

Advantages • Surface preparation is not critical • No defects are produced from arc, gases and filler metals. • Minimum surface deformation result • Very thin materials can be welded • Thin and thick sections can be joined together • To weld glass is impossible by any other means. • The equipment is simple and reliable and only moderate skill is required of the operator. • Dissimilar metals having vastly different melting points can be joined. 227

Disadvantages • Ultrasonic welding is not economically competitive when other processes can be used to do the same job. • Materials being welded tend to weld to the tip and anvil. • Due to fatigue loading, the life of equipment is short. • Hard materials will fatigue under the stresses necessary for welding. • Very ductile materials will yield under ultrasonic strain without sliding.

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Applications • • • •

Joining of electrical and electronic components. Hermetic sealing of volatile substances. Welding aluminium wire sheet . Fabricating unclear fuel elements.

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