Lecture-2.4.pdf
- 1. MANUFACTURING PROCESSES SUBJECT CODE: BEME301T 3RD SEMESTER UNIT-3 WELDING Sandhya M. Umare ASSISTANT PROFESSOR DEPARTMENT OF MECHANICAL ENGINEERING SVPCET NAGPUR
- 2. Resistance Welding PRINCIPLE: Resistance welding is a thermo-electric process where heat is generated at the interface of the parts to be joined by passing an electrical current through them or a precisely controlled time and under a controlled pressure (also called force) In resistance welding the parts to be joined are heated to a plastic state over a limited area by their resistance to flow of an electric current & mechanical pressure is used to complete the weld. A low voltage & very high current is passed through the joint for a very short time. This high current(amperage) heats the joint, due to contact resistance at the joint & melts it. The pressure on the joint is continuously maintained & the metal fuses under pressure. The heat generated in resistance welding can be expressed as H = I2 R t , Where, H = the total heat generated in the work,J I = electric current, A t = time for which the electric current is passing through the joint,s R = resistance of the electrodes, resistance of the work-piece plates, contact resistance between the two work-piece plates, contact resistance between the electrode &the work-piece.
- 3. The amount of heat released is directly proportional to the resistance. Resistance welding is the most commonly used method for joining steel sheets. No filler metal is needed. and the heat required for the weld pool is created by means of resistance to the flow of current by the work- pieces. to be welded, when a high welding current is directed through the work- pieces. An electro conductive contact surface is created between the work-pieces by pressing them together.
- 4. Water cooled electrodes made of alloyed copper are used in resistance welding. Electrodes convey a pressing force to the joint and direct the welding current to the joint in the appropriate manner. After welding, the electrodes rapidly cool down the welded joint. Work stages in resistance welding are very fast. The surfaces to be welded do not usually need to be cleaned before welding, in addition to which the weld does not usually require grinding or post heating. The resistance welding process can be easily automated. Resistance welding is a highly efficient production method that is particularly well-suited for automated production lines and mass production. Resistance welding is also suitable for small batch production, because the method is flexible, equipment simple and the welding process is easy to control. In addition, an important advantage of the method is that it can be used for joining a great number of metallic materials. Resistance welding is also suitable for the welding of the most common metal coated steel sheets. The maximum thickness of work-pieces that can be welded by means of resistance welding is approximately 6 mm for uncoated steels and 4 mm for coated steels. The major types of resistance welding are given as under: (1) Butt Welding (i) Flash butt welding (ii) Upset butt welding (2) Spot Welding (3)Seam Welding (4) Projection Welding (5) Percussion Welding
- 5. Butt Welding Their are two types of butt welding : upset & flash Upset Butt Welding: The parts to be welded are clamped edge to edge in copper jaws of the welding machine & are brought together in a solid contact so that their point of contact forms a locality of high electric resistance, while current flows to heat the joint. At this point pressure applied upsets or forges the parts together. Upset butt welding is used principally on non ferrous materials for joining bars, rods, wires, tubing, formed parts. The current densities range from 300-800 A/cm2 & the welding voltage is from 5 to 15 V. The welding pressure ranges from 17 to55 Mpa. Upset Butt Welding Diagram
- 6. Flash Butt Welding In flash butt welding process edges are brought together in light contact. A high voltage starts a flashing action between the two surfaces & continues as the parts advance slowly & forging temperature is reached. The upsetting action forces out the impurities caused by the flashing. The forced out metal is called the flash. The inner metal is then sound & free of oxides 7 cast metal. Many different materials & combinations can be flash welded; steels & ferrous alloys other than cast iron are probably the most easily welded. Those materials that cannot be flash butt welded are lead, tin, zinc,Antimony, bismuth, & their alloys. The copper alloys in which these metals are present in large percentage can not be flash butt welded. The flash butt welding is extensively used in in automobile construction – on the body, axle, wheels, frame & other parts. The current densities range from 300-800 A/cm2 & the welding voltage is from 5 to 15 V. The welding pressure ranges from 350 to 1400 Mpa Flash Butt Welding
- 7. Spot welding It is employed to join overlapping strips, sheets or plates at small areas. The pieces are assembled & placed between two electrodes. These electrodes must posses high electrical & thermal conductivity & retain the required strength at high temperatures. So they are made of pure copper for limited amount of service, & of alloys of copper & tungsten , or copper & chromium for continuous working. When the current is turned on, the pieces are heated at the area of contact to a welding temperature, & with the aid of mechanical pressure the electrodes are forced against the metal to be welded. The pressure may be developed by foot lever or by air pressure or by hydraulic cylinders. Practically all combinations of ductile metals & alloys can be spot welded. The spot welding method is used for fabricating all types of sheet metal structures where mechanical strength rather then water or air tightness is required. Spot Welding Diagram
- 8. Cont….. Before spot welding one must make sure that: (i) The job is clean, i.e., free from grease, dirt, paint, scale, oxide etc. (ii) Electrode tip surface is clean, since it has to conduct the current into the work with as little loss as possible. Very fine emery cloth may be used for routine cleaning. (iii) Water is running through the electrodes in order to (a)Avoid them from getting overheated and thus damaged, (b)Cool the weld. (iv) Proper welding current has been set on the current selector switch. (v) Proper time has been set on the weld-timer. Spot welding electrodes: Spot welding electrodes are made of materials which have (1) Higher electrical and thermal resistivities, and(2) Sufficient strength to withstand high pressure at elevated temperatures. So they are made of pure copper for limited amount of service, & of alloys of copper & tungsten , or copper & chromium for continuous working.For achieving the desired current density, It is important to have proper electrode shape for which three main types of spot welding electrodes are used which are pointed, domed and flat electrodes. Applications of Spot Welding: (i) It has applications in automobile and aircraft industries (ii) The attachment of braces, brackets, pads or clips to formed sheet-metal parts such as cases, covers or trays is another application of spot welding. (iii) Spot welding of two 12.5 mm thick steel plates has been done satisfactorily as a replacement for riveting. (iv) 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. (v) Containers and boxes frequently are spot welded.
- 9. Seam Welding It is a continuous type of spot welding wherein spot welds overlap each other to the desired extent. In this process coalescence at the overlapping surfaces is produced by the heat obtained from the resistance to the flow of electric current through the work pieces held together under pressure by rotating circular electrodes. The resulting weld is a series of overlapping resistance- spots welds made progressively along a joint by rotating the circular electrodes. 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 continuous spot welds seams are produced by the rotating electrodes and a regularly interrupted current. The material of the rotating circular electrodes is same as of spot welding electrodes. Their diameters may vary from 40 mm. to 350 mm. Welding currents range from 2000 to 5000A while the force applied to the electrodes may be as high as 5 to 6 kN. Welding speeds commonly range from 0.5 m/min to 3.5 m/min. Seam Welding Diagram Applications 1. It is used for making leak proof joints in fuel tanks of automobiles 2.Except for copper and high copper alloys, most other metals can be seam welded. 3.It is also used for making flange welds for use in water tight tanks.
- 10. Projection Welding It is modification of spot welding, This process is a resistance welding process in which two or more than two spot welds are made simultaneously on predetermined locations on the work-piece portions. The current & pressure are localized at weld section by use of embossed, machined ,or coined projections on one or both pieces of the work. These projections act to localize the heat of the welding circuit. The flattening out of these projections under pressure results in good welds at all points of contact. Fig shows the projection welding. The pieces to be welded are held in position under pressure being maintained by electrodes. The projected contact spot for welding should be approximately equal to the weld metal thickness. The welding of a nut on the automotive chassis is an example of projection welding. Projection Welding Diagram
- 11. Percussion Welding The operation is performed with one part held in a stationary holder & the other in a clamp mounted on the slide which is backed up against pressure from a heavy spring. In the welding operation, the movable clamp is released rapidly carrying the part forward. When two plates are about 1.5 mm. apart, sudden discharge of electrical energy is released, causing an intense arc between the two surfaces. To complete , it takes about 0.1 second. No upset or flash occurs at the weld. This area is limited to small areas of 144 mm2 maximum. Percussion welding is a fast method & it can handle dissimilar metals. This is highly suitable for welding small wires to electrical components. Percussion welding diagram
- 12. Thermit Welding It is primarily a fusion-welding process in which the weld is affected by pouring superheated liquid thermal steel around the parts to be welded. In the case of thermit pressure welding only the heat of the Thermit reaction is utilized to bring the surface of metal to be welded in a plastic state and mechanical pressure is then applied to complete the weld The thermit is a mixture of finely divided aluminum & iron oxide,the ratio by weight being approximately three parts of iron oxide to one part of aluminum. The mixture is placed in a refractory lined crucible,& it is ignited with aid of highly inflammable powder composed of barium peroxide. The reaction is non explosive & requires about a minute to go to completion. The temperature produced by the thermit reaction is approximately 3,000 0 C or about twice the temperature of the melting point of steel. The mixture burns according to following reaction There are two types of thermit welding processes: (i) plastic or pressure welding process &(ii) Fusion or non-pressure welding process
- 13. Cont…. Based on the chemical reaction between finely divided aluminum and iron oxide Weight is being 3parts of iron oxide to one of Al. 8Al + 3Fe3 O4 = 3Al2 O4 + 9Fe. During reaction oxygen leaves iron oxide and combines with aluminum producing Al. oxides or slag and superheated thermit steel. Temperature - 3000ﹾC twice the temperature of melting point of steel Working: wax pattern is shaped around the parts to be welded Sheet iron box is placed around the wax pattern and space between the pattern and box is filled and rammed with sand Pouring and heating gates and risers are cut and flame is directed into the heating opening. The wax pattern melts and drain but the heating continues to raise the temperature of the parts to be welded This preheating is done before the liquid metal pour into the mould in order to prevent chilling of the steel Then the burner or torch is removed and the preheating gate is plug with sand Superheated metal produced by the thermit reaction in a crucible is poured into the mould surrounding the surfaces to be welded. After the welding temperature is reached the mechanical presure is applied to complete the weldd
- 14. Diagram of Thermit welding
- 15. Solid State Welding Interatomic bonds may be established by bringing atoms of two surfaces in close enough proximity to assure adhesion. Relative movement of the surfaces under pressure and controlled roughness are helpful in breaking through surface films. While theoretically no pressure would be required for bonding Solid state welding includes 1) Friction 2) explosive 3) Ultrasonic 4) diffusion
- 16. Friction Welding In this process heat for welding obtained from mechanically induced sliding motion between rubbing surfaces of work piece One piece is fixed in chuck while other is rotating These parts are brought to rub against each other under pressure they get heated due to friction when the desired forging temperature is attained the rotation is stopped and the axial pressure increased to obtain forging action and joint is obtained This process is used for dissimilar metals such as Al. and titanium, copper and steel etc Friction welding diagram
- 17. Explosive Welding Strong metallurgical bonds can be produces between two metals It is carried out by bringing two metal surfaces with high velocity and high pressure and with proper orientation, so that large amount of plastic interaction takes place between two surfaces. Target plate – W/p held fixed Flyer plate – movable Least costly method Simple in working Large surface can be welded Explosive welding diagram
- 18. Ultrasonic Welding Can join similar or dissimilar metals Introduction of high frequency vibration energy (20000 to 60000 htz) into overlapping metals into the area to be joined No flux or filler material is used No electrical current passes through the weld No heat is applied Energy is in the form of mechanical energy Sonotrode (welding tool) coupled to the part to be welded and moves in the longitudinal direction the work piece remain static. The parts to be joined are clamped together between a welding tip and a supporting member under low static pressure. High frequency vibratory energy is then transmitted into the weld area for a brief interval. This process produces a sound bond without filler material and arc Thickness may vary from 0.38 to 2.5 mm depending on metal. Ultrasonic welding diagram
- 19. Diffusion Welding It is a process that does not necessarily need heat to produce fusion weld Rather it needs 2 kind of surfaces that comes in contact under pressure This pressure is applied for a period of hours In this process heating is not required if the temperature raised the diffusion rate will be cut sufficiently It might take many hours to perform certain bonding This process makes it possible to join metal to metal , metal to ceramic, Temperature approaches approx 900 ﹾC Diffusion welding diagram
- 20. Electron Beam Machine (New Methods) In EBW process, the heat is generated when the electron beam impinges on work piece. As the high velocity electron beam strikes the surfaces to be welded, their kinetic energy changes to thermal energy and hence causes the workpiece metal to melt and fuse. This process employs an electron gun in which the cathode in form of hot filament of tungsten is the source of a stream of electrons. The electrons emitted from filament by thermionic emission are accelerated to a high velocity to the anode because of the large potential difference that exists between them. The potential differences that are used are of the order of 30 kV to 175 kV. The higher the potential difference, higher would be the acceleration. The current levels are low ranging between 50 mA to 1000 mA. The electron beam is focused by a magnetic lens system on the workpieces to be welded. The depth of penetration of the weld depends on the electron speed which in turn is dependent upon the accelerating voltage. When the high velocity electron beam strikes the work-piece all the kinetic energy is converted to heat. EBM
- 21. Cont…. As these electrons penetrate the metal, the material that is directly in the path is melted which when solidifies form the joint. The penetration of the beam is high. The depth to width ratios lies between 10:1 to 30:1 can be easily realized with electron beam welding. It is also possible to closely control this penetration by controlling the accelerating voltage, beam current, and beam focus. The process can be used at higher welding speeds typically between 125 and 200 mm/sec. No filler metal or flux needs to be used in this process. It is possible to carry out the electron beam welding in open atmosphere. For welding in vacuum, the work-piece is enclosed in a box in which the vacuum is created. When electron beam moves in the normal atmosphere, the electrons would be impinging with the gas molecules in the atmosphere and would thus be scattered. This scattering increases the spot size of the electron beam and consequently there is lower penetration. As the vacuum increases, the scattering effect of the electron beam decreases and hence, penetration increases. The other advantage of using vacuum is that the weld metal is not contaminated. The EBW process is mainly used for welding of reactive metals (nuclear reactor components), titanium, zirconium, stainless steel, etc. for aero-space and automotive industries.
- 22. Laser Beam Machine (New Methods) Laser(light amplification by stimulated emission of radiation) is a concentrated is a concentrated beam of coherent beam of monochromatic radiation. Laser welding is performed by focusing coherent monochromatic light beam emitted by the laser source onto metal parts which are welded by surface heating & thermal conduction through the metal. The set up consists of radiation stimulated emission of radiation. The laser beam welding is mainly used for joining components that need to be joined with high welding speeds, thin and small weld seams and low thermal distortion. The high welding speeds, an excellent automatic operation and the possibility to control the quality online during the process make the laser welding a common joining method in the modern industrial production. LBM
- 23. Cont… The application range covers finest welding of non-porous seams in medical technology to precision spot welding in electronics or the jewelry industry, to deposit welding in tool and mold-making and welding complete car bodies in automobile construction. Electrons are atomic particles that exist at specific energy levels. These energy levels are unique and are different for every atom or molecule. Electrons in outer rings are at higher energy levels than those in the inner rings. A flash of light can bump electrons to higher energy levels by the injection of energy. When an electron drops from an outer ring to an inner ring or level, the excess of energy is given off as light. The wavelength or the color emitted is related to the amount of energy released. Laser beam welding is a fusion joining process that uses the energy from a laser beam to melt and subsequently crystallize a metal, resulting in a bond between parts. Laser beam welding can be successfully used to join many metals to themselves as well as to dissimilar metals. Main applications are related to welding steels, titanium, and nickel alloys.
- 24. Brazing …. Process Capabilities Brazing • Joining process in which a filler metal is melted and distributed by capillary action between faying surfaces of metal parts being joined • Filler metal Tm greater than 450°C (840°F) but less than Tm of base metal(s) to be joined • Dissimilar metals can be assembled with good joint strength – Typical products: carbide drill bits and carbide inserts on steel shank – Intricate lightweight shapes can be joined with little distortion
- 25. Soldering Soldering • Joining process in which a filler metal with Tm less than or equal to 4500C (8400F) is melted and distributed by capillary action between faying surfaces of metal parts being joined • No melting of base metals, but filler metal wets and combines with base metal to form metallurgical bond • Used extensively in the electronics industry • Because solders do not generally have enough strength, they are not used for load bearing (structural) applications • Can join various metals and thicknesses – Cu, Ag, Au are easy to solder – Al, SS are difficult to solder because of their, strong, thin oxide film.
- 26. Advantages of Brazing over welding Any metals can be joined, including dissimilar metals • Can be performed quickly and consistently, permitting high production rates • Multiple joints can be brazed simultaneously • In general, less heat and power required than FW • Problems with HAZ in base metal near joint are reduced • Joint areas that are inaccessible by many welding processes can be brazed, since capillary action draws molten filler metal into joint
- 27. Welding Defects Cracking Prevention: •Change the weld design to minimize stresses from shrinkage during cooling •Change welding process parameters, procedure and sequences. •Preheat components being welded •Avoid rapid cooling of the components after welding
- 28. Incomplete Fusion or Penetration Produces poor weld beads due to insufficient heat input. Prevention: •Raise the temperature of the base metal •Clean the weld area prior to welding •Adequate shielding gas •Change the design of joints and type of electrode Prof.
- 29. Porosity Caused by trapped gases during solidification of the weld area, chemical reactions during welding or contaminations. Prevention: •Proper selection of electrodes and filler material •Preheating the weld area •Proper cleaning and preventing contaminants from entering the weld zone.
- 30. Slag Inclusion Inclusions are compounds such as oxides, fluxes and electrode coating materials that are trapped in the weld zones. Slag Inclusion Prevention: •Clean the weld bead surface before the next layer is deposited. •Provide adequate shielding gas •Change the type of electrode
- 31. Due to under filling/overfilling and melting away of base metal. Prevention: •Improve welding technique. •Select proper electrode/filler material. Similar to slag inclusion
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