welding ppt bmp unit 3

05/03/2023BASICS OF MANUFACTURING PROCESS

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Unit 3- Welding process


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L12 Welding: Introduction, classification of welding

processes. Gas-welding, types of flames and their applications.

L13 Electric -Arc welding and its applications,

Resistance welding and its typesL14 Soldering and brazing processes and their uses

Lecture plan – unit 3welding section


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It is process of joining two similar or dissimilar metals with the application of heat and with or without the application of pressure.

Welding definition


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Welding is a process of joining two or more similar or dissimilar metals by melting them locally at the place of intended joint. Filler material may be added and a pool of molten metal is made.

This pool of metal becomes a strong joint after solidification. Sometimes help of pressure along with heating is taken to produce a weld.

Flux is added to remove impurities and protect the weld from oxidation.


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Pressure welding- resistance welding

Non pressure welding- arc welding, gas welding

Types of welding processes


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Gas Welding 1. Oxy-acetylene welding 2. Air acetylene welding Oxy-hydrogen welding 4. Oxy-fuel weldingElectric Arc Welding Metal Arc welding Carbon arc welding Tungsten inert gas welding Metal inert gas welding Submerged arc welding Plasma arc welding Fluxed cored arc welding Electro slag welding

Classification of welding


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Resistance Welding Spot welding Seam welding Projection welding Flash weldingSolid Friction welding Ultra sonic welding Diffusion welding Explosive welding


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Thermo-chemical Welding Thermit welding Atomic hydrogen weldingLow temperature State Welding Cold welding welding process Soldering Brazing Bronze Welding


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Welding Source of heat

Arc welding electric arc Gas welding gas Resistance welding electric current Laser welding laser Friction welding friction Thermit welding chemical reaction ( exothermic reaction)

Welding types on the basis of source of heat


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Arc welding


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In this method, metal pieces to be welded are heated locally to the melting temperature by creating an electric arc and then allowed to solidify to form welded joint. Additional metal for filling the weld is provided by core wire of electrode (As in metal arc welding) or by filler rod (As in case of carbon arc welding).

For creating an arc low voltage high amperage AC or DC current is used.


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Filler material is used to supply extra molten metal to fill the gap between joints. Filler material has similar composition and lower melting point then the base material.

Flux is some time used to remove the oxides formed during welding process by combining with the impurities and forming slag, which floats on the molten metal. Molten slag covers the weld and protects it from oxidation.


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The potential difference should be such that the heat generated at positive terminal is 2/3 and at negative terminal is 1/3. The temperature of the arc is 3700oC to 4000oC. In DC welding two types of polarities are used.

In DC welding two types of polarities are used.Straight Polarity: welding electrode is kept

negative and work piece is kept positive.Reverse Polarity: In this work piece is

negative and electrode is positive.

POLARITY IN DC ARC WELDING


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Arc welding


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Power supply- A.C. OR D.C SUPPLY ELECTRODE HOLDER ELECTRODE- BARE ELECTRODE AND

COATED ELECTRODE PROTECTIVE SHIELD GLOVES WELDING CLOTHES

Arc welding accessories


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Various equipment used in Electric arc welding: Following equipment are needed in Electric arc welding

High current Low Voltage AC or DC Source : In DC arc welding current is generated by a DC

generator. The generator can be driven by means of an electric motor of by means of a petrol or Diesel engine. They can be portable or standing type. The voltage required is 60-80V for striking the arc and 15-25volts for maintaining the arc.

In AC arc welding a step down transformer is used which transforms AC supply from 440 Volts AC to 80-100 Volts AC.


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Apart from transformer following equipment are also needed( as shown in figure of arc welding set up)

Well insulated electrode holder Wire cables and cable connectors Welding helmet and hand screen or shield Safety Goggles Weld chipping hammer Earthling Clamp Hand Gloves Apron and Sleeves Wire brush, chipping hammer

Arc welding equipment


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ARC WELDING EQUIPMENT and accessories


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ARC WELDING EQUIPMENT and accessories used in arc welding


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Electrode holder- to hold the electrode in arc welding


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Arc welding helmet and goggles to protect eyes and skin from ultraviolet and infrared rays emitted by welding arc


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Welding helmet, goggles, Welding gloves, wire brush, chipping hammer


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ARC WELDING


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Suitable for variety of application and positions High metal deposition rate Less training required for welders and less

supervision needed. Equipment are easy to use and are less costly Equipment are portable and can be used in

confined places Auxiliary gas shield is not required Suitable for most commonly used metals and

alloys

Advantages of Electric Arc welding


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Incorrect welding may result into low weld toughness

Lack of fusion of metal if weld parameters are incorrect

Slag is to be removed after every pass in multipass welding

Disadvantages of electric arc welding


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Arc welding application


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Arc welding application


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In this process heat for melting is supplied by combustion of a fuel gas with oxygen. The fuel gas can be acetylene, hydrogen, propane or butane.

The molten metal from the edges of metal flows and solidifies together in the weld and a continuous joint is obtained. A filler metal may be added when the metal is flowing to fill the cavity.

GAS WELDING


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GAS WELDING EQUIPMENT and its accessories


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Gas cylinders- oxygen( black colour) and acetylene ( red colour)gas cylinders

Hose pipes Pressure regulators Welding torch or blow pipe with needles

valves

Gar welding accessories( as shown in figure)


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Oxy Acetylene Welding : This is used for welding almost all metals because it produces comparatively higher temperatures and also an inert gas envelope consisting of CO2 and H2O is formed around the weld, which prevents liquid metal from oxidation. The temperatures obtained are of the order of 3200oC -3500oC.

Oxy acetylene gas welding


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FIGURE – GAS WELDING


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Pressure Regulators: Oxygen and acetylene cylinders are fitted with pressure regulators to reduce pressure of gases as per the requirement.

Blow pipes or Torches: Torch for high pressure and low pressure system differ in design. The low pressure torch has an inbuilt ejector which draws acetylene from the low pressure generator and mixes with the Oxygen. In high pressure torch there is only a mixing provision for the two gases. The torch is fitted with regulating valves for oxygen and acetylene.

Equipment used in Oxyacetylene welding & cutting


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Other equipment: Other equipment needed for gas welding are

Keys for cylinders and valves Hoses for Oxygen and Acetylene Spanners for connecting hoses Safety equipment like goggles, Screens, Lather

hand glows and lather apron Chipping hammer goggles Wire brush, Spark lighter etc. Trolley to carry oxygen cylinders

Gas welding equipment


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GAS WELDING


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Oxidising Flame : Oxidizing flame is obtained when the quantity of oxygen is more than that required for perfect combustion. The inner cone is pointed and very short in this case. Such flame is not required except for welding brass. Maximum temperature of about 3400oC - 3500oC at the tip of inner cone is obtained.

Neutral Flame : When quantity of oxygen is so adjusted that it is just sufficient for complete combustion a neural flame is obtained. The length of the inner cone becomes more. Most of the metals are welded using this flame. Maximum temperature of about 3200oC - 3300oC at the tip of inner cone is obtained.

Carburising Flame When the quantity of oxygen is less than required for complete

combustion we get carburizing flame. This 2800oC - 3150oC at the tip of inner cone is obtained. type of flame is used for welding lead.

Gas welding flames


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GAS WELDING FLAMES


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Mainly two types of weld joints are made by gas welding :-◦ Butt Welds i.e. welding two pieces of metal

without any overlap.◦ Fillet Welds i.e. joining two pieces perpendicular

to each other or with an over lap.

Types of Weld Joints


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Butt joint and fillet joint


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Some coommon welding joints


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Common welding positions


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Gas welding application


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Resistance welding processes are pressure welding processes in which heavy current is passed for short time through the area of interface of metals to be joined. The basic governing equation

H = I 2 R T, H- total heat generated These processes differ from other

welding processes in the respect that no fluxes are used, and filler metal rarely used.

Resistance welding


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All resistance welding operations are automatic and, therefore, all process variables are preset and maintained constant.

Heat is generated in localized area which is enough to heat the metal to sufficient temperature, so that the parts can be joined with the application of pressure. Pressure is applied through the electrodes.


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The heat generated during resistance welding is given by following expression:

H = I 2 R T Where, H is heat generated I is current in amperes R is resistance of area being welded T is time for the flow of current.

Working principle of resistance welding


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Figure – resistance welding


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Spot weldingSeam welding Butt weldingProjection welding

Types of resistance welding


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In resistance spot welding, two or more sheets of metal are held between electrodes through which welding current is supplied for a definite time and also force is exerted on work pieces

Spot welding


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Figure – spot welding


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Figure – spot welding machine


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Application of spot welding


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Spot welding use


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Spot welding in automobile using robot welding


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In seam welding overlapping sheets are gripped between two wheels or roller disc electrodes and current is passed to obtain either the continuous seam i.e. overlapping weld nuggets or intermittent seam i.e. weld nuggets are equally spaced

Seam welding


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Figure – seam welding


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Figure – Seam welding machine


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Figure – Seam welding application


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In this process projections are provided in one of the work piece at the place of intended welding. Thus surface of the work pieces come in contact with each other only at the tip of these projections. As the current is switched on the projection are melted and the work pieces are pressed together to complete the weld. The melted projection form the spot welds simultaneously. The electrodes and the holding fixtures are suitably designed as per the shape of the weld.

Projection welding


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Figure – projection welding


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Projection welding set up


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Butt welding


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BUTT WELDING MACHINE


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Butt welding set up


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Flash butt welding


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Very little skill is required to operate the resistance welding machine

Process is suitable for high production rate HAZ (Heat affected Zone) is very small. No filler metal is required Possible to weld two dissimilar metals or

sheets of different thickness Semi-automatic equipment are available No consumable other than electricity is

required.

Advantages of Resistance welding


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Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint. Filler metal has relatively low melting point. Bond is through wetting action. Resulting joints are not as strong as the base metal but they have adequate strength.

It is often confused with welding but the difference between them is, in soldering the work piece is not melted, they are joined using a filler metal, but in welding work piece is joined by melting.

SOLDERING AND BRAZING


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This method is used for joining metals particularly when they are in the form of sheets or wires, by using another metal or alloy which has fairly low melting point compared to the base metal to be joined. The metal or alloy used for this purpose is known as solder. There are two types of solders soft solder and hard solder.


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FIGURE- SOLDERING PROCESS


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SOLDRING IRON AND SOLDER


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SOLDERING APPLICATION


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It is a process of joining metals with the help of hard solder. Brass is generally the main constituent of this solder. Brazing solder is commonly known as spelter which is an alloy of copper, zinc and tin. Hard solder provides much stronger joint as compared to soft soldering. However in this case the metal piece to be joined is heated to slightly above the melting point of the filler metal.

BRAZING


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BRAZING PROCESS


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APPLICATION OF BRAZING


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WELDING DEFECTS1. Heat affected zone(HAZ)


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Cracks may be of micro or macro size and may appear in the weld metal or base metal or base metal and weld metal boundary.

Different categories of cracks are longitudinal cracks, transverse cracks or radiating/star cracks and cracks in the weld crater.

Cracks occur when localized stresses exceed the ultimate tensile strength of material. These stresses are developed due to shrinkage during solidification of weld metal.

CRACKS


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Fig.- cracks


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Porosity results when the gases are entrapped in the solidifying weld metal.

These gases are generated from the flux or coating constituents of the electrode or shielding gases used during welding or from absorbed moisture in the coating. Rust, dust, oil and grease present on the surface of work pieces or on electrodes are also source of gases during welding.

Porosity may be easily prevented if work pieces are properly cleaned from rust, dust, oil and grease.

Futher, porosity can also be controlled if excessively high welding currents, faster welding speeds and long arc lengths are avoided flux and coated electrodes are properly baked.

Porosity


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Fig.- Porosity


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Slag inclusions may be in the form of slag or any other nonmetallic material entrapped in the weld metal as these may not able to float on the surface of the solidifying weld metal.

During arc welding flux either in the form of granules or coating after melting, reacts with the molten weld metal removing oxides and other impurities in the form of slag and it floats on the surface of weld metal due to its low density.

However, if the molten weld metal has high viscosity or too low temperature or cools rapidly then the slag may not be released from the weld pool and may cause inclusion.

Slag inclusion can be prevented if proper groove is selected, all the slag from the previously deposited bead is removed, too high or too low welding currents and long arcs are avoided.

Slag inclusions


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Fig.- slag inclusions


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Lack of fusion is the failure to fuse together either the base metal and weld metal or subsequent beads in multipass welding because of failure to raise the temperature of base metal or previously deposited weld layer to melting point during welding.

Lack of fusion can be avoided by properly cleaning of surfaces to be welded, selecting proper current, proper welding technique and correct size of electrode.

Lack of fusion


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Fig.- Incomplete fusion


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Excessive penetration or lack of penetration


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Imperfect shape means the variation from the desired shape and size of the weld bead.

During undercutting a notch is formed either on one side of the weld bead or both sides in which stresses tend to concentrate and it can result in the early failure of the joint.

Main reasons for undercutting are the excessive welding currents, long arc lengths and fast travel speeds.

Underfilling may be due to low currents, fast travel speeds and small size of electrodes. Overlap may occur due to low currents, longer arc lengths and slower welding speeds.

Imperfect shapes


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Imperfect shapes


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Fig.- imperfect shapes

welding ppt bmp unit 3

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