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Welding Processes
- 2. WELDING ▫ 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, boilers, general repair work and ship building.
- 3. WELDING ▫ 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, boilers, general repair work and ship building.
- 6. II-Arc Welding (AW) A fusion welding process in which coalescence of the metals is achieved by the heat from an electric arc between an electrode and the work. • Electric energy from the arc produces temperatures ~ 10,000 F (5500 C), hot enough to melt any metal • Most AW processes add filler metal to increase volume and strength of weld joint
- 7. What is an Electric Arc? An electric arc is a discharge of electric current across a gap in a circuit . • It is sustained by an ionized column of gas (plasma) through which the current flows. • To initiate the arc in AW, electrode is brought into contact with work and then quickly separated from it by a short distance
- 8. • A pool of molten metal is formed near electrode tip, and as electrode is moved along joint, molten weld pool solidifies in its wake. Arc Welding
- 9. Two Basic Types of AW Electrodes • Consumable – consumed during welding process ▫ Source of filler metal in arc welding • Nonconsumable – not consumed during welding process ▫ Filler metal must be added separately if it is added
- 10. Consumable Electrodes • Forms of consumable electrodes ▫ Welding rods (a.k.a. sticks) are 9 to 18 inches and 3/8 inch or less in diameter and must be changed frequently ▫ Weld wire can be continuously fed from spools with long lengths of wire, avoiding frequent interruptions • In both rod and wire forms, electrode is consumed by the arc and added to weld joint as filler metal
- 11. Nonconsumable Electrodes • Made of tungsten which resists melting • Gradually depleted during welding (vaporization is principal mechanism) • Any filler metal must be supplied by a separate wire fed into weld pool
- 12. Arc Shielding • At high temperatures in AW, metals are chemically reactive to oxygen, nitrogen, and hydrogen in air. ▫ Mechanical properties of joint can be degraded by these reactions ▫ To protect operation, arc must be shielded from surrounding air in AW processes • Arc shielding is accomplished by: ▫ Shielding gases, e.g., argon, helium, CO2 ▫ Flux
- 13. Flux A substance that prevents formation of oxides and other contaminants in welding, or dissolves them and facilitates removal. • Provides protective atmosphere for welding • Stabilizes arc • Reduces spattering
- 14. Power Source in Arc Welding • Direct current (DC) vs. Alternating current (AC) ▫ AC machines less expensive to purchase and operate, but generally restricted to ferrous metals ▫ DC equipment can be used on all metals and is generally noted for better arc control
- 15. Consumable Electrode AW Processes • Shielded Metal Arc Welding • Gas Metal Arc Welding • Flux Cored Arc Welding‑ • Electrogas Welding • Submerged Arc Welding
- 16. Shielded Metal Arc Welding (SMAW) Uses a consumable electrode consisting of a filler metal rod coated with chemicals that provide flux and shielding • Sometimes called "stick welding"
- 18. Welding Stick in SMAW • Composition of filler metal usually close to base metal • Coating: powdered cellulose mixed with oxides and carbonates, and held together by a silicate binder • Welding stick is clamped in electrode holder connected to power source • Disadvantages of stick welding: ▫ Sticks must be periodically changed ▫ High current levels may melt coating prematurely SMAW Applications • Used for steels, stainless steels, cast irons, and certain nonferrous alloys • Not used or rarely used for aluminum and its alloys, copper alloys, and titanium
- 19. Gas Metal Arc Welding (GMAW) Uses a consumable bare metal wire as electrode with shielding by flooding arc with a gas • Wire is fed continuously and automatically from a spool through the welding gun • Shielding gases include argon and helium for aluminum welding, and CO2 for steel welding • Bare electrode wire plus shielding gases eliminate slag on weld bead ▫ No need for manual grinding and cleaning of slag
- 20. Gas Metal Arc Welding
- 22. GMAW Advantages over SMAW • Better arc time because of continuous wire electrode ▫ Sticks must be periodically changed in SMAW • Better use of electrode filler metal than SMAW ▫ End of stick cannot be used in SMAW • Higher deposition rates • Eliminates problem of slag removal • Can be readily automated
- 23. Flux Cored Arc Welding (FCAW)‑ Adaptation of shielded metal arc welding, to overcome limitations of stick electrodes - two versions ▫ Self shielded FCAW - core includes compounds that‑ produce shielding gases ▫ Gas shielded FCAW - uses externally applied shielding‑ gases • Electrode is a continuous consumable tubing (in coils) containing flux and other ingredients (e.g., alloying elements) in its core
- 25. Electrogas Welding (EGW) Uses a continuous consumable electrode, flux cored wire‑ or bare wire with externally supplied shielding gases, and molding shoes to contain molten metal • When flux cored electrode wire is used and no external‑ gases are supplied, then special case of self shielded‑ FCAW • When a bare electrode wire used with shielding gases from external source, then special case of GMAW
- 27. Submerged Arc Welding (SAW) Uses a continuous, consumable bare wire electrode, with arc shielding by a cover of granular flux • Electrode wire is fed automatically from a coil • Flux introduced into joint slightly ahead of arc by gravity from a hopper ▫ Completely submerges operation, preventing sparks, spatter, and radiation
- 30. SAW Applications and Products • Steel fabrication of structural shapes (e.g., I beams)‑ • Seams for large diameter pipes, tanks, and pressure vessels • Welded components for heavy machinery • Most steels (except hi C steel) • Not good for nonferrous metals
- 31. Nonconsumable Electrode Processes • Gas Tungsten Arc Welding • Plasma Arc Welding • Carbon Arc Welding
- 32. Gas Tungsten Arc Welding (GTAW) Uses a nonconsumable tungsten electrode and an inert gas for arc shielding ▫ In Europe, called "WIG welding" • Used with or without a filler metal ▫ When filler metal used, it is added to weld pool from separate rod or wire • Applications: aluminum and stainless steel mostly
- 33. Gas Tungsten Arc Welding
- 35. Advantages and Disadvantages of GTAW Advantages: • High quality welds for suitable applications • No spatter because no filler metal through arc • Little or no post-weld cleaning because no flux Disadvantages: • Generally slower and more costly than consumable electrode AW processes
- 36. Plasma Arc Welding (PAW) Special form of GTAW in which a constricted plasma arc is directed at weld area • Tungsten electrode is contained in a nozzle that focuses a high velocity stream of inert gas (argon) into arc region to form a high velocity, intensely hot plasma arc stream • Temperatures in PAW reach 28,000°C (50,000°F), due to constriction of arc, producing a plasma jet of small diameter and very high energy density
- 38. Advantages and Disadvantages of PAW Advantages: • Good arc stability and excellent weld quality • Better penetration control than other AW processes • High travel speeds • Can be used to weld almost any metals Disadvantages: • High equipment cost • Larger torch size than other AW processes ▫ Tends to restrict access in some joints
- 39. III-Resistance Welding (RW) A group of fusion welding processes that use a combination of heat and pressure to accomplish coalescence • Heat generated by electrical resistance to current flow at junction to be welded • Principal RW process is resistance spot welding (RSW)
- 42. Components in Resistance Spot Welding • Parts to be welded (usually sheet metal) • Two opposing electrodes • Means of applying pressure to squeeze parts between electrodes • Power supply from which a controlled current can be applied for a specified time duration
- 43. Advantages and Drawbacks of Resistance Welding Advantages: • No filler metal required • High production rates possible • Lends itself to mechanization and automation • Lower operator skill level than for arc welding • Good repeatability and reliability Disadvantages: • High initial equipment cost • Limited to lap joints for most RW processes
- 44. Resistance Spot Welding (RSW) Resistance welding process in which fusion of faying surfaces of a lap joint is achieved at one location by opposing electrodes • Used to join sheet metal parts • Widely used in mass production of automobiles, metal furniture, appliances, and other sheet metal products ▫ Typical car body has ~ 10,000 spot welds ▫ Annual production of automobiles in the world is measured in tens of millions of units
- 45. (a) Spot welding cycle (b) Plot of force and current •Cycle: (1) parts inserted between electrodes, (2) electrodes close, (3) current on, (4) current off, (5) electrodes opened Spot Welding Cycle
- 46. Resistance Seam Welding (RSEW) Uses rotating wheel electrodes to produce a series of overlapping spot welds along lap joint • Can produce air tight joints‑ • Applications: ▫ Gasoline tanks ▫ Automobile mufflers ▫ Various sheet metal containers
- 49. Resistance Projection Welding (RPW) A resistance welding process in which coalescence occurs at one or more small contact points on the parts • Contact points determined by design of parts to be joined ▫ May consist of projections, embossments, or localized intersections of parts
- 51. (1) Start of operation, contact between parts is at projections; (2) when current is applied, weld nuggets similar to spot welding are formed at the projections Resistance Projection Welding
- 52. IV-Other Fusion Welding Processes FW processes that cannot be classified as arc, resistance, or oxyfuel welding . • Use unique technologies to develop heat for melting • Applications are typically unique • Processes include: ▫ Electron beam welding ▫ Laser beam welding ▫ Thermit welding
- 53. Electron Beam Welding (EBW) Fusion welding process in which heat for welding is provided by a highly focused, high intensity stream of‑ ‑ electrons striking work surface. • Electron beam gun operates at: ▫ High voltage (e.g., 10 to 150 kV typical) to accelerate electrons
- 55. EBW Vacuum Chamber When first developed, EBW had to be carried out in a vacuum chamber to minimize disruption of electron beam by air molecules. Three Vacuum Levels in EBW: 1.High vacuum welding – welding in same vacuum chamber as beam generation to produce highest quality weld 2.Medium vacuum welding – welding in separate chamber but partial vacuum reduces pump 3.Non-vacuum welding – welding done at or near atmospheric pressure, with work positioned close to electron beam generator - requires vacuum divider to separate work from beam generator
- 56. EBW Advantages and Disadvantages of EBW Advantages: • High quality welds, deep and narrow profiles‑ • Limited heat affected zone, low thermal distortion • No flux or shielding gases needed Disadvantages: • High equipment cost • Precise joint preparation & alignment required • Vacuum chamber required • Safety concern: EBW generates x rays‑
- 57. Laser Beam Welding (LBW) Fusion welding process in which coalescence is achieved by energy of a highly concentrated, coherent light beam focused on joint. • LBW normally performed with shielding gases to prevent oxidation • Filler metal not usually added • High power density in small area ▫ So LBW often used for small parts
- 58. VIDEO
- 59. Comparison: LBW vs. EBW • No vacuum chamber required for LBW • No x rays emitted in LBW‑ • Laser beams can be focused and directed by optical lenses and mirrors • LBW not capable of the deep welds and high depth to width ratios of EBW‑ ‑ ▫ Maximum LBW depth = ~ 19 mm (3/4 in), whereas EBW depths = 50 mm (2 in)
- 60. Thermit Welding (TW) FW process in which heat for coalescence is produced by superheated molten metal from the chemical reaction of thermite • Thermite = mixture of Al and Fe3O4 fine powders that produce an exothermic reaction when ignited • Also used for incendiary bombs • Filler metal obtained from liquid metal • Process used for joining, but has more in common with casting than welding
- 61. (1) Thermit ignited; (2) crucible tapped, superheated metal flows into mold; (3) metal solidifies to produce weld joint Thermit Welding
- 62. TW Applications • Joining of railroad rails • Repair of cracks in large steel castings and forgings • Weld surface is often smooth enough that no finishing is required
- 63. 5-Solid State Welding (SSW) • Coalescence of part surfaces is achieved by: ▫ Pressure alone, or ▫ Heat and pressure If both heat and pressure are used, heat is not enough to melt work surfaces ▫ For some SSW processes, time is also a factor • No filler metal is added • Each SSW process has its own way of creating a bond at the faying surfaces • Explosion welding • Friction welding • Friction stir welding • Ultrasonic welding
- 64. Explosion Welding (EXW) SSW process in which rapid coalescence of two metallic surfaces is caused by the energy of a detonated explosive • No filler metal used • No external heat applied • No diffusion occurs - time is too short • Bonding is metallurgical, combined with mechanical interlocking that results from a rippled or wavy interface between the metals
- 65. Explosive Welding Commonly used to bond two dissimilar metals, e.g., to clad one metal on top of a base metal over large areas (1) Setup in parallel configuration (2) during detonation of the explosive charge
- 66. Friction Welding (FRW) SSW process in which coalescence is achieved by frictional heat combined with pressure • When properly carried out, no melting occurs at faying surfaces • No filler metal, flux, or shielding gases normally used • Process yields a narrow HAZ • Can be used to join dissimilar metals • Widely used commercial process, amenable to automation and mass production
- 67. (1) Rotating part, no contact; (2) parts brought into contact to generate friction heat; (3) rotation stopped and axial pressure applied; (4) weld created Friction Welding
- 68. Friction Welding Applications: • Shafts and tubular parts • Industries: automotive, aircraft, farm equipment, petroleum and natural gas Limitations: • At least one of the parts must be rotational • Flash must usually be removed (extra operation)
- 69. Friction Stir Welding (FSW) SSW process in which a rotating tool is fed along a joint line between two workpieces, generating friction heat and mechanically stirring the metal to form the weld seam • Distinguished from FRW because heat is generated by a separate wear-resistant tool rather than the parts • Applications: butt joints in large aluminum parts in aerospace, automotive, and ship building
- 70. Friction Stir Welding (1) Rotating tool just before entering work (2) partially completed weld seam
- 71. Friction Stir Welding • Advantages ▫ Good mechanical properties of weld joint ▫ Avoids toxic fumes, warping, and shielding issues ▫ Little distortion or shrinkage ▫ Good weld appearance • Disadvantages ▫ An exit hole is produce when tool is withdrawn ▫ Heavy duty clamping of parts is required
- 72. Ultrasonic Welding (USW) Two components are held together, and oscillatory shear stresses of ultrasonic frequency are applied to interface to cause coalescence . • Oscillatory motion breaks down any surface films to allow intimate contact and strong metallurgical bonding between surfaces • Temperatures are well below Tm • No filler metals, fluxes, or shielding gases • Generally limited to lap joints on soft materials
- 73. (a) General setup for a lap joint (b) close up of weld area‑ Ultrasonic Welding
- 74. USW Applications • Assembly of aluminum sheet metal panels • Welding of tubes to sheets in solar panels • Assembly of small parts in automotive industry