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PART B — (2 × 16 = 32 Marks + 8 Marks = 40 Marks) 6. (a) (i) Explain Percusion welding with sketch and mention its advantages and disadvantages (Marks 8) Percussion welding (PEW) - It is a resistance welding process wherein coalescence* is produced simultaneously over the entire area of abutting surfaces by heat obtained from an arc produced by a rapid discharge of electrical energy, with pressure percussively (rapidly) applied during or immediately following the electrical discharge. Principle of Operation The following steps are involved in percussion welding. (i) The workpieces are cleaned by removing grease, dirt, paint, etc. (ii) The workpieces are clamped into machine or fixture. . (iii) Light force is applied and the ends or faces of two work-pieces are brought near. (iv) Arc between the faces of workpieces is struck by using any of the following methods: (a) Workpieces are brought into light contact to establish a flow of current. The workpieces are then retracted to draw the arc. (b) A nib of small cross sectional area is formed (by cutting) on one of the pieces. Current as it flows through the nib, explodes the nib and establishes an ionized path for the welding current to flow. (c) A DC voltage (high enough to jump the gap between the workpieces as they are moved toward each other) when applied, ionizes the air gap between the workpieces and starts the flow of current. (d) Another method involves superimposing an auxiliary high-frequency, high voltage AC on a low voltage current across the gap between the workpieces. The high frequency AC ionizes the air in the gap and the low voltage DC maintains the arc. The arc as established above heats the faces of the two work-pieces to be joined to the welding temperature. (v) At this stage, welding force is applied. It extinguishes the arc find holds parts together while weld cools. Welding force may be: Pneumatic, Electromagnetic, Spring force or Gravity (falling weights). (vi) Welding force is released. (vii) Workpieces are unclamped. Power Supplies for Percussion Welding (i) Low voltage (10 to 150 volts DC), / High voltage (1000 to 6000 volts DC), capacitive storage. (ii) Electromagnetic or inductive storage. (iii Low voltage (10 to 35 volts) AC that uses a transformer to furnish the welding voltage. Advantages of Percussion Welding (i) Because of the extreme brevity of the arc, fusion is confined to the surface of the parts being welded and there is almost complete absence of flash or upset. (ii) Heat treated or cold worked metals can be welded without annealing or destroying the heat treatment. Disadvantages of Percussion Welding (i) The process is limited to butt welded joints only. (ii) Since control of the path of an arc is difficult, the joint used is limited to about 1.5 to 3 sq. cm. 6.(a) (ii) Explain Gas metal arc welding with sketch and mention its advantages and disadvantages (Marks 8) MIG-Metal Inert Gas Welding (GMAW-Gas Metal Arc Welding) is a arc welding process , in which the weld is shielded by an external gas (Argon , helium, CO 2 , argon + Oxygen or other gas mixtures). Consumable electrode wire, having chemical composition similar to that of the parent material, is continuously fed from a spool to the arc zone. The arc heats and melts both the work pieces edges and the electrode wire. The fused electrode material is supplied to the surfaces of the work pieces, fills the weld pool and forms joint. Due to automatic feeding of the filling wire (electrode) the process is referred to as a semi- automatic. The operator controls only the torch positioning and speed. Advantages of Metal Inert Gas Welding (MIG, GMAW): Continuous weld may be produced (no interruptions); High level of operators skill is not required; Slag removal is not required (no slag); Disadvantages of Metal Inert Gas Welding (MIG, GMAW): Expensive and non-portable equipment is required; Outdoor application are limited because of effect of wind, dispersing the shielding gas. (Or)

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  • PART B (2 16 = 32 Marks + 8 Marks = 40 Marks) 6. (a) (i) Explain Percusion welding with sketch and mention its advantages and disadvantages (Marks 8) Percussion welding (PEW) - It is a resistance welding process wherein coalescence* is produced simultaneously over the entire area of abutting surfaces by heat obtained from an arc produced by a rapid discharge of electrical energy, with pressure percussively (rapidly) applied during or immediately following the electrical discharge.

    Principle of Operation The following steps are involved in percussion welding. (i) The workpieces are cleaned by removing grease, dirt, paint, etc. (ii) The workpieces are clamped into machine or fixture. . (iii) Light force is applied and the ends or faces of two work-pieces are brought near. (iv) Arc between the faces of workpieces is struck by using any of the following methods:

    (a) Workpieces are brought into light contact to establish a flow of current. The workpieces are then retracted to draw the arc. (b) A nib of small cross sectional area is formed (by cutting) on one of the pieces. Current as it flows through the nib, explodes the nib and establishes an ionized path for the welding current to flow. (c) A DC voltage (high enough to jump the gap between the workpieces as they are moved toward each other) when applied, ionizes the air gap between the workpieces and starts the flow of current. (d) Another method involves superimposing an auxiliary high-frequency, high voltage AC on a low voltage current across the gap between the workpieces. The high frequency AC ionizes the air in the gap and the low voltage DC maintains the arc. The arc as established above heats the faces of the two work-pieces to be joined to the welding temperature. (v) At this stage, welding force is applied. It extinguishes the arc find holds parts together while weld cools. Welding force may be: Pneumatic, Electromagnetic, Spring force or Gravity (falling weights). (vi) Welding force is released. (vii) Workpieces are unclamped. Power Supplies for Percussion Welding (i) Low voltage (10 to 150 volts DC), / High voltage (1000 to 6000 volts DC), capacitive storage. (ii) Electromagnetic or inductive storage. (iii Low voltage (10 to 35 volts) AC that uses a transformer to furnish the welding voltage. Advantages of Percussion Welding (i) Because of the extreme brevity of the arc, fusion is confined to the surface of the parts being welded and there is almost complete absence of flash or upset. (ii) Heat treated or cold worked metals can be welded without annealing or destroying the heat treatment. Disadvantages of Percussion Welding (i) The process is limited to butt welded joints only. (ii) Since control of the path of an arc is difficult, the joint used is limited to about 1.5 to 3 sq. cm.

    6.(a) (ii) Explain Gas metal arc welding with sketch and mention its advantages and disadvantages (Marks 8)

    MIG-Metal Inert Gas Welding (GMAW-Gas Metal Arc Welding) is a arc welding process, in which the weld is shielded by an external gas (Argon, helium, CO2, argon + Oxygen or other gas mixtures).

    Consumable electrode wire, having chemical composition similar to that of the parent material, is continuously fed from a spool to the arc zone. The arc heats and melts both the work pieces edges and the electrode wire. The fused electrode material is supplied to the surfaces of the work pieces, fills the weld pool and forms joint.

    Due to automatic feeding of the filling wire (electrode) the process is referred to as a semi-automatic. The operator controls only the torch positioning and speed.

    Advantages of Metal Inert Gas Welding (MIG, GMAW): Continuous weld may be produced (no interruptions); High level of operators skill is not required; Slag removal is not required (no slag);

    Disadvantages of Metal Inert Gas Welding (MIG, GMAW): Expensive and non-portable equipment is required; Outdoor application are limited because of effect of wind, dispersing the shielding gas. (Or)

  • 6. (b) (i) Explain Submerged arc welding with sketch and mention its advantages and disadvantages. (Marks 8)

    SAW - Submerged Arc Welding is a welding process, which utilizes a bare consumable metallic electrode producing an arc between itself and the work piece within a granular shielding flux applied around the weld.

    The arc heats and melts both the work pieces edges and the electrode wire. The molten electrode material is supplied to the surfaces of the welded pieces, fills the weld pool and joins the work pieces.

    Since the electrode is submerged into the flux, the arc is invisible. The flux is partially melts and forms a slag protecting the weld pool from oxidation and other atmospheric contaminations.

    Advantages of Submerged Arc Welding (SAW): Very high welding rate; The process is suitable for automation; High quality weld structure.

    Disadvantages of Submerged Arc Welding (SAW): Weld may contain slag inclusions; Limited applications of the process - mostly for welding horizontally located plates.

    6. (b) (ii) Explain Electro slag welding with sketch and mention its advantages and disadvantages. (Marks 8)

    ESW - Electroslag Welding is a welding process, in which the heat is generated by an electric current passing between the consumable electrode (filler metal) and the work piece through a molten slag covering the weld surface.

    Prior to welding the gap between the two work pieces is filled with a welding flux. Electroslag Welding is initiated by an arc between the electrode and the work piece (or starting plate). Heat, generated by the arc, melts the fluxing powder and forms molten slag. The slag, having low electric conductivity, is maintained in liquid state due to heat produced by the electric current.

    The slag reaches a temperature of about 3500F (1930C ). This temperature is sufficient for melting the consumable electrode and work piece edges. Metal droplets fall to the weld pool and join the work pieces.

    Electroslag Welding is used mainly for steels.

    Advantages of Electroslag Welding: High deposition rate - up to 45 lbs/h (20 kg/h); Low slag consumption (about 5% of the deposited metal weight); Low distortion; Unlimited thickness of work piece. Disadvantages of Electroslag welding: Coarse grain structure of the weld; Low toughness of the weld; Only vertical position is possible.

  • 7. (a) (i) Explain TIG welding with sketch and mention its advantages and disadvantages. (Marks 8) TIG - Tungsten Inert Gas Arc Welding (GTAW - Gas Tungsten Arc Welding) is a welding process, in which heat is generated by an electric arcstruck between a tungsten non-consumable electrode and the work piece.

    The weld pool is shielded by an inert gas (Argon, helium, Nitrogen) protecting the molten metal from atmospheric contamination. The heat produced by the arc melts the work pieces edges and joins them. Filler rod may be used, if required.

    Tungsten Inert Gas Arc Welding produces a high quality weld of most of metals. Flux is not used in the process.

    Advantages of Tungsten Inert Gas Arc Welding (TIG, GTAW): Weld composition is close to that of the parent metal; High quality weld structure Slag removal is not required (no slag); Thermal distortions of work pieces are minimal due to concentration of heat in small zone. Disadvantages of Tungsten Inert Gas Arc Welding (TIG, GTAW): Low welding rate; Relatively expensive; Requres high level of operators skill.

    7. (a) (ii) Explain Plasma arc welding with sketch and mention its advantages and disadvantages. (Marks 8)

    PAW - Plasma Arc Welding is the welding process utilizing heat generated by a constricted arc struck between a tungsten non-consumable electrode and either the work piece (transferred arc process) or water cooled constricting nozzle (non-transferred arc process). Plasma is a gaseous mixture of positive ions, electrons and neutral gas molecules. Transferred arc process produces plasma jet of high energy density and may be used for high speed welding and cutting ofCeramics, steels, Aluminum alloys, Copper alloys, Titanium alloys, Nickel alloys.

    Non-transferred arc process produces plasma of relatively low energy density. It is used for welding of various metals and forplasma spraying (coating). Since the work piece in non-transferred plasma arc welding is not a part of electric circuit, the plasma arc torch may move from one work piece to other without extinguishing the arc.

    Advantages of Plasma Arc Welding (PAW): Requires less operator skill due to good tolerance of arc to misalignments; High welding rate; High penetrating capability (keyhole effect);

    Disadvantages of Plasma Arc Welding (PAW): Expensive equipment; High distortions and wide welds as a result of high heat input.

    (Or)

  • 7. (b) (i) Explain Thermit welding with sketch and mention its advantages and disadvantages. (Marks 8)

    TW - Thermit Welding is a welding process utilizing heat generated by exothermic chemical reaction between the components of the thermit (a mixture of a metal oxide and aluminum powder). The molten metal, produced by the reaction, acts as a filler material joining the work pieces after Solidification.

    Thermit Welding is mainly used for joining steel parts, therefore common thermit is composed from iron oxide (78%) and aluminum powder (22%). The proportion 78-22 is determined by the chemical reaction of combustion of aluminum:

    8Al + Fe3O4 = 9Fe + 4Al2O3

    The combustion reaction products (iron and aluminum oxide) heat up to 4500F (2500C). Liquid iron fills the sand (or ceramic) moldbuilt around the welded parts, the slag (aluminum oxide), floating up , is then removed from the weld surface.

    Thermit Welding is used for repair of steel casings and forgings,for joining railroad rails, steel wires and steel pipes, for joining large cast and forged parts.

    (1) Thermit ignited; (2) crucible tapped, superheated metal flows into mold; (3) metal solidifies to produce weld joint.

    Advantages of Thermit Welding: No external power source is required (heat of chemical reaction is utilized); Very large heavy section parts may be joined.

    Disadvantages of Resistance Welding: Only ferrous (steel, chromium, nickel) parts may be welded; Slow welding rate; High temperature process may cause distortions and changes in Grain structure in the weld

    region. Weld may contain gas (Hydrogen) and slag contaminations.

    7. (b) (ii) Explain Electron beam welding with sketch and mention its advantages and disadvantages. (Marks 8) Electron Beam Welding is a welding process utilizing a heat generated by a beam of high energy electrons. The electrons strike the work piece and their kinetic energy converts into thermal energy heating the metal so that the edges of work piece are fused and joined together forming a weld after Solidification.

    The process is carried out in a vacuum chamber at a pressure of about 2*10-7 to 2*10-6 psi (0.00013 to 0.0013 Pa). Such high vacuum is required in order to prevent loss of the electrons energy in collisions with air molecules.

    The electrons are emitted by a cathode (electron gun). Due to a high voltage (about 150 kV) applied between the cathode and the anode the electrons are accelerated up to 30% - 60% of the speed of light. Kinetic energy of the electrons becomes sufficient for melting the targeted weld. Some of the electrons energy transforms into X-ray irradiation. Electrons accelerated by electric field are then focused into a thin beam in the focusing coil. Deflection coil moves the electron beam along the weld.

    Electron Beam is capable to weld work pieces with thickness from 0.0004 (0.01 mm) up to 6 (150 mm) of steel and up to 20 (500 mm) of aluminum. Electron Beam Welding may be used for joining any metals including metals, which are hardly weldable by otherwelding methods: refractory metals (tungsten, molybdenum, niobium) and chemically active metals (titanium, zirconium, beryllium). Electron Beam Welding is also able to join dissimilar metals.

    Advantages of Electron Beam Welding (EBW): Tight continuous weld; Low distortion; Narrow weld and narrow heat affected zone; Filler metal is not required. Disadvantages of Electron Beam Welding (EBW): Expensive equipment; High production expenses; X-ray irradiation.

  • 8. (a) Explain Friction welding with sketch and mention its advantages and disadvantages. (Marks 8) Friction Welding is a Solid State Welding process, in which two cylindrical parts are brought in contact by a friction pressure when one of them rotates. Friction between the parts results in heating their ends. Forge pressure is then applied to the pieces providing formation of the joint.

    Carbon steels, Alloy steels, Tool and die steels, Stainless steels, Aluminum alloys, Copper alloys, Magnesium alloys, Nickel alloys,Titanium alloys may be joined by Friction Welding. Advantages of Friction Welding: Simple operation and Low cost. Power Requirement is Low Time required is very less. No flux, Filler, Smoke and fumes are nil Heat dissipated is very quick. Disadvantages of Friction Welding: Heavy regid machine is required for work holding. Flat and angular joints are not possible. Continueous joints are not possible. (Or)

    8. (b) Explain Diffusion welding disadvantages. (Marks 8)DFW - Diffusion Welding is awork pieces with carefully cleaned surfaces and at an elevated temperature below the melting point of the metals. Bonding of the materials is a result of mutualatoms.

    A solid-state welding process that produces coalescence of the faying surfaces by the application of pressure at elevated temperature.

    The process does not involve macroscopic deformation, or relative motion of the workpieces.

    A solid filler metal may or may not be inserted between the faying surfaces

    In order to keep the bonded surfaces clean from oxides and other air contaminations, tprocess is often conducted in vacuum.Diffusion Welding. Diffusion Welding is often referred more commonly as(SSW). Diffusion Welding is able to bond dissimilar metals, which are difficult to weld by other welding processes: Steel to tungsten; Steel to niobium;

    Schematic representation of diffusion welding using electrical resistance for heating Diffusion Welding is used in aerospace and rocketry industries, electronics, nuclear applications, manufacturing composite materials 1

    Asperities come into contact. 1st stage deformation 2nd stage grain 3rd stage volumeAdvantages of Diffusion Welding: Dissimilar materials may be welded ( Welds of high quality are obtained (no No limitation in the work pieces thickness.Disadvantages of Diffusion Welding: Time consuming process with low productivity; Very thorough surface preparation is required prior to welding process; The mating surfaces must be precisely fitted to each other; Relatively high initial investments in equipment.

    ___________________

    Diffusion welding with sketch and mention its advantages and disadvantages. (Marks 8)

    is a Solid State Welding process, in which pressure applied to two work pieces with carefully cleaned surfaces and at an elevated temperature below the melting point of the metals. Bonding of the materials is a result of mutual diffusion

    state welding process that produces coalescence of the faying surfaces by the application of pressure at elevated temperature. The process does not involve macroscopic deformation, or relative motion of the

    A solid filler metal may or may not be inserted between the faying surfaces

    In order to keep the bonded surfaces clean from oxides and other air contaminations, tprocess is often conducted in vacuum. No appreciable deformation of the work pieces occurs in

    Diffusion Welding is often referred more commonly asDiffusion Welding is able to bond dissimilar metals, which are difficult to weld by other

    Steel to niobium; Stainless steel to titanium; Gold to copper alloys.

    Schematic representation of diffusion welding using electrical resistance for heating Diffusion Welding is used in aerospace and rocketry industries, electronics, nuclear

    composite materials. 1st Stage 2nd Stage 3

    1st stage deformation and interfacial boundary formation2nd stage grain boundary migration and pore elimination3rd stage volume diffusion pore elimination

    Advantages of Diffusion Welding: rials may be welded (Metals, Ceramics, Graphite, glass);

    Welds of high quality are obtained (no pores, inclusions, chemical No limitation in the work pieces thickness.

    Disadvantages of Diffusion Welding: rocess with low productivity;

    Very thorough surface preparation is required prior to welding process;The mating surfaces must be precisely fitted to each other;

    Relatively high initial investments in equipment. ___________________

    with sketch and mention its advantages and disadvantages. (Marks 8)

    process, in which pressure applied to two work pieces with carefully cleaned surfaces and at an elevated temperature below the melting

    diffusion of their interface

    state welding process that produces coalescence of the faying surfaces by the

    The process does not involve macroscopic deformation, or relative motion of the

    A solid filler metal may or may not be inserted between the faying surfaces

    In order to keep the bonded surfaces clean from oxides and other air contaminations, the No appreciable deformation of the work pieces occurs in

    Diffusion Welding is often referred more commonly as Solid State Welding Diffusion Welding is able to bond dissimilar metals, which are difficult to weld by other

    Gold to copper alloys.

    Schematic representation of diffusion welding using electrical resistance for heating Diffusion Welding is used in aerospace and rocketry industries, electronics, nuclear

    Stage 3rd Stage

    interfacial boundary formation and pore elimination

    , glass); segregation, distortions).

    Very thorough surface preparation is required prior to welding process;