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CHAPTER 13

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CHAPTER 13

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  1. CHAPTER 13 POWDER METALLURGY

  2. INTRODUCTION • Powder Metallurgy • is a manufacturing method to produce components by bringing a powder of the starting material into desired end shape • The essential feature is that the bond between particles is produced without total melting ME 340 POWDER METALLURGY

  3. IF NECESSARY PROCESSING STEPS IN PM ME 340 POWDER METALLURGY

  4. PROCESSING STEPS IN PM • Particles of desired size are produced (production and characterization) • Blend particles to ensure even distribution (mixing) • Compact particles to impart desired shape (compaction) • Sinter parts to create strong, permanent bonds between particles (consolidation) • Finishing operations ME 340 POWDER METALLURGY

  5. POWDER PRODUCTION • Decomposition of solids by gas reduction, precipitation from gas or a liquid, or solid-solid reactive synthesis • High purity powder deposition at the cathode of electrolytic cells • Machining • Milling techniques Gas Atomization Liquid atomization Centrifugal Melt Explosion Plasma ME 340 POWDER METALLURGY

  6. MILLING TECHNIQUES JAR MILLING ATTRITION MILLING ME 340 POWDER METALLURGY

  7. ATOMIZATION TECHNIQUES • Disintegration of melt into droplets that freeze into particles • Production rates as • high as 400Kg/min ME 340 POWDER METALLURGY

  8. ATOMIZATION TECHNIQUES GAS ATOMIZATION WATER ATOMIZATION ME 340 POWDER METALLURGY

  9. ATOMIZATION TECHNIQUES (b) (a) (a) 5-10 kg capacity water atomiser (b) 30Kg capacity inert gas atomiser ME 340 POWDER METALLURGY

  10. POWDERS CHARACTERIZATION (MORPHOLOGY) • Particle Shape (Spheroidal, nodular, irregular, polygonal, ligaments, flakes) • Particle Size (too large may not display the desired structure and desired densities might not be obtained. Too small particles are difficult to handle and tend to agglomerate) • Particle Size Distribution (different processes are used to do the analysis such as sieve analysis, sedimentation, electron microscopy, and diffraction techniques) ME 340 POWDER METALLURGY

  11. Rounded and irregular, stainless steel, atomized Sponge, palladium electrolytic Acicular, tellurium, milled Spherical & agglomerated Fines, Iron, atomized Porous & cubic Nickel, carbonyl decomposition polygonal Aggregates, Tungsten, Gas Reduced Rounded & ligamental Tin, Atomized Crushed ribbon, Iron-based metallic glass Spherical, Iron alloy, centrifugally atomized Irregular, titanium sodium reduced & milled Flake, tin Splat quenched Angular, Niobium hydride milled POWDERS CHARACTERIZATION (MORPHOLOGY) ME 340 POWDER METALLURGY

  12. POWDERS PHYSICAL PROPERTIES • Specific Surface Area Indicates the surface available for bonding and also the area on which adsorbed contaminant may be present (cm2/gm) • Densities • Theoretical Density: Density when there is no porosity (actual reported density of material) • Apparent Density: Density when powder is in a loose state in die • Tap Density: Highest density achieved by vibration of powders in die • Green Density: Density of powders after compaction in die ME 340 POWDER METALLURGY

  13. POWDERS PHYSICAL PROPERTIES • Flow Properties given by flow rate and angle of repose • Compressibility ME 340 POWDER METALLURGY

  14. BLENDING OF POWDERS • To mix the powders in order to obtain uniformity • In order to impart special properties, powders of different materials may be mixed • To mix the powders with some type of lubricant to reduce die friction and aid ejection of the product from the compaction mold ME 340 POWDER METALLURGY

  15. COMPACTION • Purposes 1. To obtain the required shape, density, and particle-to particle contact 2. To impart sufficient strength for further handling of the part • The pressed powder is known as“green compact” ME 340 POWDER METALLURGY

  16. COMPACTION ME 340 POWDER METALLURGY

  17. COMPACTION ME 340 POWDER METALLURGY

  18. COMPACTION ME 340 POWDER METALLURGY

  19. COLD COMPACTION • Dry powders, which may be coated with lubricant or dry binder; are compacted by the application of pressure to form the so-called GREEN BODY • The density of the green body is function of: • The applied pressure • Powder shape (spherical powders compact to a higher density) • Powder size ME 340 POWDER METALLURGY

  20. COLD COMPACTION • SO WHAT ARE THE SOURCES OF GREEN STRENGTH? • Sliding combined with pressure promotes adhesion (sometimes cold welding) • Mechanical interlocking (especially with irregular shapes) • Bonding agents are used in the absence of previous mechanisms (ceramics) ME 340 POWDER METALLURGY

  21. COLD COMPACTION ME 340 POWDER METALLURGY

  22. DIE PRESSING • Widest application for net-shape (or near-net-shape) parts. • (a) Density is higher under the punch when compacting with a single punch in a fixed container; better uniformity is obtained with (b) a single punch and floating container or (c) with 2 counteracting punches ME 340 POWDER METALLURGY

  23. DIE PRESSING • For a single acting punch with applied pressure p0 the pressure at l depth in the body is: • Where: • Is wall friction • Afr is the frictional surface area. • A0 is the compacted area • And k is a factor representing radial to axial stress ratio • For an elastic solid: ME 340 POWDER METALLURGY

  24. DIE PRESSING ME 340 POWDER METALLURGY

  25. DIE PRESSING • Uniform fill density can be assured with the use of multipunch dies ME 340 POWDER METALLURGY

  26. DIE PRESSING ME 340 POWDER METALLURGY

  27. COLD ISOSTATIC PRESSING ME 340 POWDER METALLURGY

  28. COLD ISOSTATIC PRESSING • Powder is placed in deformable (reusable rubber) mold • Assembly is hydrostatically pressurized by means of a hydraulic fluid inside a pressure vessel (see figure 6.5) • No need to use lubricants or binders • Common pressure applied is between 300 MPa (45 kpsi) to 550 MPa (80 kpsi) ME 340 POWDER METALLURGY

  29. HOT ISOSTATIC PRESSING • Container made of high melting point sheet metal • Pressurizing medium is inert gas or vitreous (glasslike) fluids • Common conditions are 100 MPa and 1100oC ME 340 POWDER METALLURGY

  30. POWDER INJECTION MOLDING Taken from plastics technology MIMMetal Injection Molding CIMCeramics Injection Molding Typically 40% binder (70% paraffin wax + 30% polypropylene) ME 340 POWDER METALLURGY

  31. POWDER INJECTION MOLDING ME 340 POWDER METALLURGY

  32. SINTERING • The green compact is heated to attain the required final properties. In this course of heating several changes take place • Drying: liquid constituents are driven off at lower temperatures • Sintering: At higher temperatures (0.7 – 0.9 Tm) sintering takes place • Shrinkage: From the law of conservation of mass ME 340 POWDER METALLURGY

  33. SINTERING ME 340 POWDER METALLURGY

  34. SINTERING • 3 important variables: 1. Atmosphere 2. Temperature 3. Time ME 340 POWDER METALLURGY

  35. FINISHING OPERATIONS • Coining (Resizing) Increase density and improve dimensional tolerance • Impregnation Immersion in heated oil; capillary action fills the pores. • Infiltration Impregnation with a metal. • Repressing • Re-sintering • Forging • Extrusion • Rolling • Machining • Heat Treatment ME 340 POWDER METALLURGY

  36. DESIGN CONSIDERATIONS ME 340 POWDER METALLURGY

  37. ADVANTAGES & DISADVANTAGES OF PM ME 340 POWDER METALLURGY

  38. ADVANTAGES & DISADVANTAGES OF PM ME 340 POWDER METALLURGY

  39. TRENDS IN PM ME 340 POWDER METALLURGY