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Material Joining

Material Joining. ISAT 430 Module 8. Joining Technologies. Joining is a many splendored thing. Welding Arc or melting Resistance or other Soldering & brazing Mechanical fastening (bolts & nuts). Seaming and crimping Adhesive bonding All are important for different reasons.

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Material Joining

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  1. Material Joining ISAT 430 Module 8

  2. Joining Technologies • Joining is a many splendored thing. • Welding • Arc or melting • Resistance or other • Soldering & brazing • Mechanical fastening (bolts & nuts). • Seaming and crimping • Adhesive bonding • All are important for different reasons. ISAT 430 Dr. Ken Lewis

  3. The Joined Vehicle ISAT 430 Dr. Ken Lewis

  4. Why Join? • The product may be impossible to make as a single piece. (See previous slide) • It may be more economical to make in pieces • A cooking pot (pot + handle) • Complicated products may be needed to taken apart for repair • Sewing machines, cars, hair dryers,…….. • Parts may have different functional purposes • Brake systems. • Rotors • pads ISAT 430 Dr. Ken Lewis

  5. Why Join2? • Making and assembling in one spot may be impossible • The space shuttle • Bridges • Pianos • … ISAT 430 Dr. Ken Lewis

  6. Welding • Fusion welding • Use heat to melt the base metals • In many operations a filler metal is added to the molten pool • Adds bulk and strength to the joint. • Solid state welding • Pressure or heat and pressure cause the coalescence • If heat is used, the temperature is below the melting point of the joined metals • No filler metal is used. ISAT 430 Dr. Ken Lewis

  7. Weld joints ISAT 430 Dr. Ken Lewis

  8. Fusion Welding • Oxyfuel gas welding • Uses a fuel gas and oxygen to produce the heat. • Arc welding • Heating is accomplished by an electric arc • Resistance welding • Heating is accomplished by the passage of an electric current • Others • Electron beam and laser welding ISAT 430 Dr. Ken Lewis

  9. Flux • One hears the word “FLUX” (Latin == ‘to flow’) bandied about. • All purpose word describing a material which • Protects the materials from unwanted oxidation • Helps dissolve and remove oxides • Provides a gaseous shield around the weld point ISAT 430 Dr. Ken Lewis

  10. Oxy Acetylene Welding

  11. Oxyfuel welding • Most common process uses acetylene, C2H2 as the fuel. • Heat is generated by two chemical reactions: • C2H2 + O2 2CO + H2 + Heat • CO + H2 + O2  CO2 + H2O • Temperatures in these flames can reach 3,300°C (6,000°F) ISAT 430 Dr. Ken Lewis

  12. Oxyacetylene Welding • The flame is directed by a welding torch. • A filler metal is sometimes added • Usually coated with a flux • Heat efficiency is low ~0.1 – 0.3 • Flame spreading, losses to the air, etc. ISAT 430 Dr. Ken Lewis

  13. Oxy acetylene flames • Neutral flame • Gas:Fuel = 1:1 • Useful • Oxidizing flame • Excess oxygen • Harmful to steel • Useful in copper and copper based alloys • Thin film of slag forms over the molten metal for protection. ISAT 430 Dr. Ken Lewis

  14. Oxy acetylene flames • Reducing flame • Excess fuel. • Temperature is lower • Used for brazing, soldering. • Other fuels, • Hydrogen, propane, butane, natural gas • Not useful, because the heat is low and the flames are reducing. ISAT 430 Dr. Ken Lewis

  15. Fuel Temperatures and Heats. ISAT 430 Dr. Ken Lewis

  16. Oxy-acetylene welding summary • Relatively cheap • Well suited to low volume and repair jobs. • Rarely used on stock thicker than ¼ inch • Rarely mechanized • Thus relies on the skill of the welder for quality. ISAT 430 Dr. Ken Lewis

  17. Arc Welding

  18. Shielded Welding -- overview ISAT 430 Dr. Ken Lewis

  19. Arc Welding • A fusion process wherein the coalescence of the metals is achieved from the heat of an electric arc formed between an electrode and the work. • An electric arc is a discharge of electric current across a gap I a circuit. • It is sustained by the presence of a thermally ionized column of gas (called a plasma). • Temperatures up to 30,000°C (54,000°F) a generated ISAT 430 Dr. Ken Lewis

  20. Arc Welding • The clamp completes the circuit • A filler metal may be used. • Two kinds of welding electrode • Consumable • Non -- consumable ISAT 430 Dr. Ken Lewis

  21. Consumable Electrode Welding • Shielded Metal Arc Welding (SMAW) • Consumable electrode coated with chemicals that provide flux and shielding • Sometimes called ‘stick’ welding • The filler metal (here the consumable electrode) is usually very close in composition to the metal being welded. • The coating contains • Cellulose, carbonates, a silicate binder • Sometime other metals to alloy the joint ISAT 430 Dr. Ken Lewis

  22. Shielded Metal Arc Welding ISAT 430 Dr. Ken Lewis

  23. Consumable Electrode Welding • Shielded Metal Arc Welding (SMAW) • The stick is about 12 – 18 inches long • Applications • Construction, pipelines, shipbuilding, fabrication job shops • Used for • Steels, stainless steels, cast irons • Not used for aluminum and its alloys, or copper and its alloys (energy density is too high) ISAT 430 Dr. Ken Lewis

  24. Consumable Electrode Welding2 • Gas Metal Arc Welding • Electrode is a consumable bare metal wire • Usually continuous and fed from a spool • The work area is protected by flooding it with a gas • Wire diameters range from 1/32 to ¼ inch in diameter ISAT 430 Dr. Ken Lewis

  25. Gas Metal Arc Welding ISAT 430 Dr. Ken Lewis

  26. Gas Metal Arc Welding • Shielding Gases • Inert – Ar (Argon), He (Helium) • Used for aluminum alloys and stainless steels • Active – CO2 • Used for low and medium carbon steels • The combination of bare electrode wire and shielding gas eliminates slag in the work area • Negates need for subsequent cleanup ISAT 430 Dr. Ken Lewis

  27. Gas Metal Arc Welding • Originally called “MIG” welding (for metal inert gas) • Used widely in factory fabrication • Better metal usage (no stubs) • Sticks or filler • High deposition rates • No slag ISAT 430 Dr. Ken Lewis

  28. Consumable Electrode Welding2 • Flux – cored Arc Welding • Developed in the 1950’ss • Electrode is a continuous consumable tubing • Interior of the electrode contains • Flux • De-oxidizers • Alloy elements ISAT 430 Dr. Ken Lewis

  29. Flux – Cored Arc Welding ISAT 430 Dr. Ken Lewis

  30. Flux – Cored Arc Welding • Two versions • Self shielded • Core contains flux and ingredients that generate a shielding gas • Gas shielded • Gas is applied externally • Used primarily for welding steels and stainless steels • Noted for producing high quality smooth joints ISAT 430 Dr. Ken Lewis

  31. Non-consumable Electrodes • Gas Tungsten Arc Welding • Known as “TIG” (tungsten inert gas) welding • The electrode is W (tungsten) • Tm = 6170°F (3410°C) • Actually it is slowly consumed • Shielding gases include Ar, He or a mixture ISAT 430 Dr. Ken Lewis

  32. Gas Tungsten Arc Welding ISAT 430 Dr. Ken Lewis

  33. Gas Tungsten Arc Welding (TIG) • Used for a wide variety of metals • can also be used to weld dissimilar metals (but not very well) • Most commonly used for aluminum and stainless steel • For steel • Slower and more costly than consumable welding • Except for thin sections or where very high quality is needed ISAT 430 Dr. Ken Lewis

  34. Resistance Welding

  35. Resistance Welding • This is a group of fusion welding processes that use heat and pressure to make the coalescence. • The heat comes from electrical resistance to current flow at the site of the weld. ISAT 430 Dr. Ken Lewis

  36. Resistance Welding • In order to obtain a strong bond in the weld nugget pressure is applied until the current is turned off. • Strength depends on the initial surface condition • Smoothness • Cleanliness • Presence of uniform thin oxides is not critical ISAT 430 Dr. Ken Lewis

  37. Resistance Welding • Currents range from 3,000 to 40,000A • Heat generated is given by: • Where: • I = current (amperes) • R = resistance (ohms) • T = time of current (seconds) • Q = heat in Joules ISAT 430 Dr. Ken Lewis

  38. Resistance Welding • The reason that the current is so high is because the R is usually so low ~~ 0.0001 ohm • Where: • I = current (amperes) • R = resistance (ohms) • T = time of current (seconds) • Q = heat in Joules ISAT 430 Dr. Ken Lewis

  39. A resistance spot welding operation is performed on two pieces of 0.0625 in thick sheet steel using 12,000 A for 0.23 seconds. The electrodes are 0.25 in. in diameter. Resistance is assumed to be 0.0001 ohm and the resulting weld nugget is 025 in. dia. And 0.1 in. thick. The melting energy for this metal is 155 BTU/in3. What portion of the heat generated was used to form the weld and what portion was merely siphoned off and dissipated into the surrounding metal? From the equation above: ISAT 430 Dr. Ken Lewis

  40. A resistance spot welding operation is performed on two pieces of 0.0625 in thick sheet steel using 12,000 A for 0.23 seconds. The electrodes are 0.25 in. in diameter. Resistance is assumed to be 0.0001 ohm and the resulting weld nugget is 025 in. dia. And 0.1 in. thick. The melting energy for this metal is 155 BTU/in3. What portion of the heat generated was used to form the weld and what portion was merely siphoned off and dissipated into the surrounding metal? The volume of the weld nugget is: Assuming a disc The heat necessary to melt this disc is ISAT 430 Dr. Ken Lewis

  41. A resistance spot welding operation is performed on two pieces of 0.0625 in thick sheet steel using 12,000 A for 0.23 seconds. The electrodes are 0.25 in. in diameter. Resistance is assumed to be 0.0001 ohm and the resulting weld nugget is 025 in. dia. And 0.1 in. thick. The melting energy for this metal is 155 BTU/in3. What portion of the heat generated was used to form the weld and what portion was merely siphoned off and dissipated into the surrounding metal? The wasted heat is just ISAT 430 Dr. Ken Lewis

  42. Resistance Spot Welding • Parts inserted • Electrodes close and force is applied • Weld time. Current switched on, force maintained • Current off, force maintained. Note: current sometimes left on at a reduced level for stress relief (micro anneal • Electrodes opened – repeat. ISAT 430 Dr. Ken Lewis

  43. Explosion Welding • First, some elimination of a preconceived notion • When something explodes, it does not just go KABLOOIE • There are different rates of explosion. • Deflagration • The reaction front proceeds at various rates, but at less than the speed of sound in the deflagrating material. • Black powder • Explosion • The reaction front proceeds at speeds greater than the speed of sound in the exploding material • Immense pressure build up. ISAT 430 Dr. Ken Lewis

  44. Explosion Welding2 • It just seems like it goes KABLOOIE • All are fast compared to normal burning • A slow explosion has a detonation rate of ~2,000 m/s • This is a heave • A fast explosion will have detonation rates approaching 8,000 m/s. • Provides brisance (a slap) ISAT 430 Dr. Ken Lewis

  45. Explosive welding • Contact pressures are extremely high • The explosive may be flexible plastic sheet. • Detonation speeds are usually low being 2400 – 3600 m/s • The impact ejects the interface effectively removing any oxides or surface impurities. • The impact mechanically interlocks the two surfaces ISAT 430 Dr. Ken Lewis

  46. Explosion Welding ISAT 430 Dr. Ken Lewis

  47. Explosive Welding • Pieces of up to 20ft x 7 ft have been joined • Very useful in cladding work • Cladding tantalum (Ta) to ordinary steel gives the inertness of Ta with the economy of steel. • Requires well trained personnel. ISAT 430 Dr. Ken Lewis

  48. Brazing and Soldering

  49. Faying surfaces = the surfaces to be joined. Brazing • A process which a filler metal is placed at or between the faying surfaces, the temperature is raised high enough to melt the filler metal but not the base metal. • The molten metal fills the spaces by capillary attraction. • Two types • Ordinary brazing (above) • Braze welding (similar to oxy-welding) ISAT 430 Dr. Ken Lewis

  50. Filler Metals for Brazing ISAT 430 Dr. Ken Lewis

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