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INTRODUCTION OF WELDING Bhupinder Singh

Welding Principle

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welding and its various types of welding proccess

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Page 1: Welding Principle

INTRODUCTION OF WELDING

Bhupinder Singh

Page 2: Welding Principle

WELDING A joining process that produces a coalescence of metals (or non-metals) by heating them to the welding temperature,

• with or without the application of pressure, or by pressure alone, and • with or without the use of filler metals

Welding is a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone, and with or without the use of filler material. Welding is used for making permanent joints. It is used in the manufacture of automobile bodies, aircraft frames, railway wagons, machine frames, structural works, tanks, furniture, boilers, general repair work and ship building.

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BRIEF HISTORY OF WELDING • Late 19th Century

Scientists/engineers apply advances in electricity to heat and/or join metals (Le Chatelier, Joule, etc.)

• Early 20th Century Prior to WWI welding was not trusted as a method to join two metals due to crack issues

• 1930’s and 40’s Industrial welding gains acceptance and is used extensively in the war effort to build tanks, aircraft, ships, etc.

• Modern Welding the nuclear/space age helps bring welding from an art to a science

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Base metal: The metal to be welded or cut. May be referred to as the “work piece”. Weld metal: The portion of the base metal that has been melted during welding Heat-affected zone (HAZ): That portion of the base metal that has not been melted during welding, but whose mechanical properties and/or microstructure have been altered by the heat of welding or cutting. Joint: The junction of members or the edge of members that are to be joined. Usually bevelled or otherwise designed for welding. Welding electrode: A component of the welding circuit that terminates at the arc. May also be the source of filler metal. Polarity: Manner in which the electrode holder and work piece connection are connected to the electrical supply. • DCEN : Direct current electrode negative. (straight polarity) • DCEP : Direct current electrode positive. (reverse polarity) Spatter : Droplets of electrode material that land outside the weld fusion area and may or may not fuse to the base material Porosity : Small volumes of entrapped gas in solidifying weld metal

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WELDING OPERATION

Setting of the job: Parts to be welded are cleaned and the joint prepared. Joint preparation depends upon the thickness of work pieces. Thin sheets can be joined by an edge or flange-joint. Sometimes, a lap or fillet joint can be used. A sheet of higher thickness but not exceeding 4.5 mm may be welded with a butt joint without any joint preparation. Different kind of joints commonly used in welding are illustrated below:

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WELDING POSITIONS These are four welding positions from the point of view of the welder. These affect execution of sound welding. These positions are: • Downhand welding position: This is the most comfortable position for welder to work in

and he is able to produce welds of a good quality.

• Horizontal welding position (on a vertical surface).

• Vertical welding position (on a vertical surface).

• Overhead welding position (say on the ceiling of a room): This is the most difficult welding position. Not only the operator has to crane his neck upwards and raise his arm to maintain arc, it is also difficult as molten metal tends to fall down due to gravity.

For important jobs, manipulators are used, which are capable of turning over the jobs and as much welding is done in down hand welding position as possible.

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WELDING POSITIONS

FLAT

HORIZONTAL

VERTICAL

OVERHEAD

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CLASSIFICATION Welding means the process of joining two metal parts together to give a sound and strong joint. The welding process is subdivided into two main classes.

• Fusion Welding • Pressure Welding

1. Fusion welding, which involves heating the ends of metal pieces to be joined to a temperature high enough to cause them to melt or fuse and then allowing the joint to cool. This process is somewhat similar to casting process. The joint, after the fused metal has solidified will result in a strong joint.

2. Pressure welding, which involves heating the ends of metal pieces to be joined to a high temperature, but lower than their melting point and then keeping the metal pieces joined together under pressure for sometime. This results in the pieces welding together to produce a strong joint.

There are many sub classifications of welding under each head. Sub classification is done according to the source of heat required for fusion or pressure welding. (a) Gas welding (b) Electric arc welding, and (c) Electric resistance welding.

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CLASSIFICATION OF WELDING PROCESSES:

Arc welding • Carbon arc • Metal arc • Metal inert gas • Tungsten inert gas • Plasma arc • Submerged arc • Electro-slag Gas welding • Oxy-acetylene • Air-acetylene • Oxy-hydrogen Resistance welding • Butt • Spot • Seam • Projection • Percussion

Thermit welding Solid state welding • Friction • Ultrasonic • Diffusion • Explosive Newer welding • Electron-beam • Laser Related process • Oxy-acetylene cutting • Arc cutting • Hard facing • Brazing • Soldering

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Weld Symbol (Corner Joints)

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Weld Symbol (Fillet Joints)

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Weld Symbols (Butt Joints)

Backing

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Arc welding

• Arc Welding Principle

• Arc Welding Equipments and Machines

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Arc welding Principle • Arc welding is the welding process, in which heat is

generated by an electric arc struck between an electrode and the work piece.

• Electric arc is luminous electrical discharge between two electrodes through ionized gas.

Any arc welding method is based on an electric circuit consisting of the following parts:

• Power supply (AC or DC);

• Welding electrode;

• Work piece;

• Welding leads (electric cables) connecting the electrode and work piece to the power supply.

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Arc welding

• Equipments:

• A welding generator (D.C.) or Transformer (A.C.)

• Two cables- one for work and one for electrode

• Electrode holder

• Electrode

• Protective shield

• Gloves

• Wire brush

• Chipping hammer

• Goggles

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Arc Welding Equipments

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Arc welding

Advantages – Most efficient way to join

metals

– Lowest-cost joining method

– Affords lighter weight through better utilization of materials

– Joins all commercial metals

– Provides design flexibility

Limitations • Manually applied, therefore high

labor cost.

• Need high energy causing danger

• Not convenient for disassembly.

• Defects are hard to detect at

joints.

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WELDABILITY OF A METAL

• Metallurgical Capacity

– Parent metal will join with the weld metal without formation of deleterious constituents or alloys

• Mechanical Soundness

– Joint will be free from discontinuities, gas porosity, shrinkage, slag, or cracks

• Serviceability

– Weld is able to perform under varying conditions or service (e.g., extreme temperatures, corrosive environments, fatigue, high pressures, etc.

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Oxyacetylene Welding (OAW)

The oxyacetylene welding process uses a combination of oxygen and acetylene

gas to provide a high temperature flame.

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Oxyacetylene Welding (OAW)

• OAW is a manual process in which the welder must personally control the torch movement and filler rod application

• The term oxyfuel gas welding outfit refers to all the equipment needed to weld.

• Cylinders contain oxygen and acetylene gas at extremely high pressure.

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Typical Oxyacetylene Welding (OAW) Station

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Various Gas Combinations

•Oxygen and acetylene

•oxygen and hydrogen (low melting points-like aluminum, magnesium), the reason is that temperature produced is low

•Oxygen with coal gas

•Oxygen and natural gas or propane

•Purity of O2 should be 99.5 %

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•The filler metal may be of same composition or nearly same of parent metal

•Oxygen and acetylene is used coz both produces high temperature and form an inert gas envelope consists of CO2 and H2O vapours, prevents oxidation.

• High pressure: both cylinders are used

• Low pressure: O2 cylinder and acetylene is generated with action of water and carbide in a low pressure acetylene generator

• Pressure on oxygen cylinder: 125 to 140 kgs

• Pressure in acetylene cylinder: 16 to 21 kgs

• Acetylene cylinders are called as dissolved acetylene coz it has a capacity to dissolve 25 times its own volume of acetylene for every atm pressure applied

• Transferring with help of trolley only

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• Two types of blow pipes

• High pressure (mixing chamber & nozzle)

• Low pressure (O2 at high pressure carry acetylene with itself)

• High pressure welding: 0.35 kg/cm2

• Cutting: 3.5 kg/cm2

• Acetylene: 0.14 kg/cm2

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Oxygen Cylinders

• Oxygen is stored within cylinders of various sizes and pressures ranging from 2000- 2640 PSI. (Pounds Per square inch)

• Oxygen cylinders are forged from solid armor plate steel. No part of the cylinder may be less than 1/4” thick.

• Cylinders are then tested to over 3,300 PSI using a (NDE) hydrostatic pressure test.

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Cylinder Transportation

• Never transport cylinders without the safety caps in place

• Never transport with the regulators in place

• Never allow bottles to stand freely. Always chain them to a secure cart or some other object that cannot be toppled easily.

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Oxygen Cylinders

• Oxygen cylinders incorporate a thin metal “pressure safety disk” made from stainless steel and are designed to rupture prior to the cylinder becoming damaged by pressure.

• The cylinder valve should always be handled carefully

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Pressure Regulators for Cylinders

• Reduce high storage cylinder pressure to lower working pressure.

• Most regulators have a gauge for cylinder pressure and working pressure.

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Pressure Regulators for Cylinders

• Regulators are shut off when the adjusting screw is turn out completely.

• Regulators maintain a constant torch pressure although cylinder pressure may vary

• Regulator diaphragms are made of stainless steel

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Regulator Hoses

• Hoses are are fabricated from rubber

• Oxygen hoses are green in color and have right hand thread.

• Acetylene hoses are red in color with left hand thread.

• Left hand threads can be identified by a grove in the body of the nut and it may have “ACET” stamped on it

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Check Valves & Flashback Arrestors

• Check valves allow gas flow in one direction only

• Flashback arrestors are designed to eliminate the possibility of an explosion at the cylinder.

• Combination Check/ Flashback Valves can be placed at the torch or

regulator.

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Acetylene Gas

• Virtually all the acetylene distributed for welding and cutting use is created by allowing calcium carbide (a man made product) to react with water.

• The nice thing about the calcium carbide method of producing acetylene is that it can be done on almost any scale desired. Placed in tightly-sealed cans, calcium carbide keeps indefinitely. For years, miners’ lamps produced acetylene by adding water, a drop at a time, to lumps of carbide.

• Before acetylene in cylinders became available in almost every community of appreciable size produced their own gas from calcium carbide.

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Acetylene Cylinders

• Acetylene is stored in cylinders specially designed for this purpose only.

• Acetylene is extremely unstable in its pure form at pressure above 15 PSI (Pounds per Square Inch)

• Acetone is also present within the cylinder to stabilize the acetylene.

• Acetylene cylinders should always be stored in the upright position to prevent the acetone form escaping thus causing the acetylene to become unstable.

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Acetylene Cylinders

• Cylinders are filled with a very porous substance “monolithic filler” to help prevent large pockets of pure acetylene form forming

• Cylinders have safety (Fuse) plugs in the top and bottom designed to melt at 212° F (100 °C)

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Acetylene Valves

• Acetylene cylinder shut off valves should only be opened 1/4 to 1/2 turn

• This will allow the cylinder to be closed quickly in case of fire.

• Cylinder valve wrenches should be left in place on cylinders that do not have a hand wheel.

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Typical torch styles

• A small welding torch, with throttle valves located at the front end of the handle. Ideally suited to sheet metal welding, Can be fitted with cutting attachment in place of the welding head shown. Welding torches of this general design are by far the most widely used. They will handle any oxyacetylene welding job, can be fitted with multiflame (Rosebud) heads for heating applications, and accommodate cutting attachments that will cut steel 6 in. thick. A full-size oxygen cutting torch which has all valves located in its rear body. Another style of cutting torch, with oxygen valves located at the front end of its handle.

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Typical startup procedures

• Verify that equipment visually appears safe IE: Hose condition, visibility of gauges

• Clean torch orifices with a “tip cleaners” (a small wire gauge file set used to clean slag and dirt form the torch tip)

• Crack (or open) cylinder valves slightly allowing pressure to enter the regulators slowly

• Opening the cylinder valve quickly will “Slam” the regulator and will cause failure.

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Typical startup procedures

• Never stand directly in the path of a regulator when opening the cylinder

• Check for leaks using by listening for “Hissing” or by using a soapy “Bubble” solution

• Adjust the regulators to the correct operating pressure

• Slightly open and close the Oxygen and Acetylene valves at the torch head to purge any atmosphere from the system.

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Typical startup procedures

• Always use a flint and steel spark lighter to light the oxygen acetylene flame.

• Never use a butane lighter to light the flame

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Oxy acetylene cutting

In oxy-fuel cutting, a cutting torch is used to heat metal to kindling temperature. A stream of oxygen is then trained on the metal, and metal burns in that oxygen and then flows out of the cut as an oxide slag. Torches that do not mix fuel with oxygen (combining, instead, atmospheric air) are not considered oxy-fuel torches and can typically be identified by a single tank (Oxy-fuel welding/cutting generally requires two tanks, fuel and oxygen). Most metals cannot be melted with a single-tank torch. As such, single-tank torches are typically used only for soldering and brazing, rather than welding.

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Cutting • A cutting torch has a 60- or 90-degree angled head with orifices

placed around a central jet. • The outer jets are for preheat flames of oxygen and acetylene. • The central jet carries only oxygen for cutting. • The use of a number of preheating flames, rather than a single flame

makes it possible to change the direction of the cut as desired without changing the position of the nozzle or the angle which the torch makes with the direction of the cut, as well as giving a better preheat balance.

• The flame is not intended to melt the metal, but to bring it to its ignition temperature.

• The torch's trigger blows extra oxygen at higher pressures down the torch's third tube out of the central jet into the work piece, causing the metal to burn and blowing the resulting molten oxide through to the other side.

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Flame Settings

• There are three distinct types of oxy-acetylene flames, usually termed:

– Neutral

– Carburizing (or “excess acetylene”)

– Oxidizing (or “excess oxygen” )

• The type of flame produced depends upon the ratio of oxygen to acetylene in the gas mixture which leaves the torch tip.

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Pure Acetylene and Carburizing Flame profiles

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Neutral and Oxidizing Flame Profiles

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Flame definition • The neutral flame is produced when the ratio of oxygen to acetylene,

in the mixture leaving the torch, is almost exactly one-to-one. 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.

• The excess acetylene flame , as its name implies, is created when the proportion of acetylene in the mixture is higher than that required to produce the neutral flame. Used on steel, it will cause an increase in the carbon content of the weld metal.

• The oxidizing flame results from burning a mixture which contains more oxygen than required for a neutral flame. It will oxidize or ”burn” some of the metal being welded.

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Resistance Welding

• Raising temperatures of two pieces to fusion point and applying a mechanical pressure to join them.

• Strong electric current of high amperage and low voltage is passed. (100–100,000 A)

• Resistance offered to the flow of electric current resulting into raising of temperature.

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Types

• Spot Welding

• Butt Welding

• Flash Welding

• Seam welding

• Projection welding

• Percussion welding

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Terms used

• The pressure required to hold the two pieces in between the electrodes is termed as weld pressure.

• The pressure required to forge or squeeze the metal pieces together to form the weld, during plastic state of metal is termed as forge pressure.

• Hold time-current flow through the metal pieces to raise temperature

• Squeeze or forging time-Mechanical pressure is applied

• Hold time or cooling time-Metal pieces hold under pressure, allow metal to solidify

• Off time- Mechanical pressure is released and removed.

• All above 4 are termed as Time of Application

• Contact area of electrode

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Factors influencing

• Some factors influencing heat or welding temperatures are the proportions of the work pieces, the coating or the lack of coating, the electrode materials, electrode geometry, electrode pressing force, weld current and weld time.

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Application

• Resistance welding is used for:- • Joining sheets, bars, rods and tubes.

• Making tubes and metal furniture.

• Welding aircraft and automobile parts.

• Making cutting tools.

• Making fuel tanks of cars, tractors etc.

• Making wire fabric, grids, grills, mash weld, containers etc.

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Limitation

• Initial equipment costs

• Lower tensile and fatigue strengths

• Lap joints add weight and material

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Heat Source • Heat generated in an electrically conductive work piece depends upon three factors:

• amount of electric current,

• resistance of the work piece

• the period of time of the current passing through the work piece.

• The amount of heat can be calculated using the three factors on the basis of the following equation:

Q = I2 R t • Q = heat generated, joules

• I = current, amperes

• R = total resistance of the work piece, ohms

• t = total duration of heat input (weld time), seconds

• The squared current, and the duration of heat input and resistance directly proportionally.

• A part of the heat generated is used for melting the metal i.e. the creation of the weld, and a part is conducted to the surrounding work piece and electrodes.

• Heat input in resistance welding is controlled by adjusting welding current and weld time. The work piece has its own specific resistance and thermal conductivity coefficients which depend on the material and cannot thus be adjusted. However, the electrode force used and the surface properties of the work pieces – such as the thickness of the oxide layer, cleanness and possible coatings – have an impact on the total resistance of a work piece.

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Spot Welding

• Definition Spot welding is a resistance welding process in which overlapping sheets are joined by local fusion at one or more spots by the heat generated, by resistance to the flow of electric current through work pieces that are held together under force by two electrodes, one above and the other below the two overlapping sheets.

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• Applications of Spot Welding - (i) Spot welding of two 12.5 mm thick steel plates has been done satisfactorily as a replacement for riveting. (ii) Many assemblies of two or more sheet metal stampings that do not require gas tight or liquid tight joints can be more economically joined by spot welding than by mechanical methods. (iii) Containers such as receptacles and tote boxes frequently are spot welded. (iv) The attachment of braces, brackets, pads or clips to formed sheet metal parts such as cases, covers, bases or trays is another application of spot welding. (v) Spot welding finds application in automobile and aircraft industries.

• Advantages of Spot Welding - (i) Low cost, (ii) High speed of welding, (iii) Dependability, (iv) Less skilled worker will do, (v) More general elimination of warping or distortion of parts, (vi) High uniformity of products, (vii) Operation may be made automatic or semiautomatic, and (viii) No edge preparation is needed

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Seam Welding

• Definition Seam welding is a resistance welding process wherein coalescence at the faying surfaces is produced by the heat obtained from resistance to electric current (flow) through the work parts held together under pressure by electrodes. The resulting weld is a series of overlapping resistance-spot welds made progressively along a joint by rotating the circular electrodes

• Principle of Operation The seam welding is similar to spot welding, except that circular rolling electrodes are used to produce a continuous air-tight seam of overlapping welds. Overlapping (spot) welds are produced by the rotating electrodes and a regularly interrupted current.

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• Applications of Seam Welding - (i) Girth welds can be made in round, square or rectangular parts. (ii) Except for copper and high copper alloys, most other metals of common industrial use can be seam welded. (iii) Besides lap welds, seam-welding can be used for making butt seam welds too.

• Advantages of Seam Welding (i) It can produce gas tight or liquid-tight joints. (ii) Overlap can be less than for spot or projection welds. (iii) A single seam weld or several parallel seams may be produced simultaneously.

• Disadvantages of Seam Welding (i) Welding can be done only along a straight or uniformly curved line. (ii) It is difficult to weld thicknesses greater than 3 mm. (iii) A change in the design of electrode wheels is required to avoid obstructions along the path of the wheels during welding.

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• Upset-welding (UW) is a form of resistance welding process using both heat and pressure to perform quickly a weld, even with relatively large joint areas.

• Heat is produced from the resistance to the passage of electric current at the contact interface of those surfaces.

• Upset-welding can be used only if the parts to be welded have the same cross-sectional area.

• Many different shapes like wire, bar, strip and tubing of various materials can be joined end to end by Upset-welding. Coalescence is produced simultaneously over the entire area of the abutting surfaces or progressively along a joint.

• An example of a progressive process is that used in the manufacture of pipes or tubes in a continuous production line

• Advantages of Upset-welding:

• Fast process

• Ease of parameters control (only current, time and force)

• High quality, absence of typical fusion defects

• Metallurgical properties comparable to those of hot worked material.

• Simple, sturdy and reliable equipment operated by unskilled workers

• Tolerance for minor alloy deviations

• Large selection of materials, including difficult to weld ones.

• Limitations:

• Equipment generally suitable to one type of applications only

• Peak current drawn from power line.

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Projection Welding

• Projections are previously provided at desired location on the surface of one of work piece.

• As current is passed, projections got melted and the work piece pressed together to complete weld.

• This method is able for producing simultaneous spot welds.

• Special designed welds are also possible with this method.

• Process is suitable or economical for large scale productions.

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Percussion Welding

• For very thin objects (0.08 to 0.35 mm)

• Parts are holded at a small distance with their faces opposite to each other.

• Arc is produced by bringing them into contacts at fast speed.

• Metal gets fused under the impact.

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THANK YOU

Bhupinder Singh