1. 1 CHAPTER 13 POWDER METALLURGY

2. 8/1/2007ME 340 POWDER METALLURGY 2 I NTRODUCTION 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

3. 8/1/2007ME 340 POWDER METALLURGY 3 P ROCESSING S TEPS IN PM IF NECESSARY

4. 8/1/2007ME 340 POWDER METALLURGY 4 P ROCESSING S TEPS IN PM 1. Particles of desired size are produced (production and characterization) 2. Blend particles to ensure even distribution (mixing) 3. Compact particles to impart desired shape (compaction) 4. Sinter parts to create strong, permanent bonds between particles (consolidation) 5. Finishing operations

5. 8/1/2007ME 340 POWDER METALLURGY 5 P OWDER P RODUCTION POWDER MANUFACTURING METHODS METHODS MECHANICAL MECHANICALCOMMINUTIONCHEMICALREACTIONSELECTROLYTICDEPOSITIONMETALATOMIZATION Machining Milling techniques High purity powder deposition at the cathode of electrolytic cells Decomposition of solids by gas reduction, precipitation from gas or a liquid, or solid-solid reactive synthesis Gas Atomization Liquid atomization Centrifugal Melt Explosion Plasma

6. 8/1/2007ME 340 POWDER METALLURGY 6 M ILLING T ECHNIQUES JAR MILLINGATTRITION MILLING

7. 8/1/2007ME 340 POWDER METALLURGY 7 A TOMIZATION T ECHNIQUES Disintegration of melt into droplets that freeze into particles Production rates as high as 400Kg/min

8. 8/1/2007ME 340 POWDER METALLURGY 8 A TOMIZATION T ECHNIQUES GAS ATOMIZATIONWATER ATOMIZATION

9. 8/1/2007ME 340 POWDER METALLURGY 9 A TOMIZATION T ECHNIQUES (a) 5-10 kg capacity water atomiser (a) 5-10 kg capacity water atomiser (b) 30Kg capacity inert gas atomiser (a) (b)(b)

10. 8/1/2007ME 340 POWDER METALLURGY 10 P OWDERS C HARACTERIZATION (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)

11. 8/1/2007ME 340 POWDER METALLURGY 11 P OWDERS C HARACTERIZATION (MORPHOLOGY) Rounded and irregular, stainless steel, atomized Sponge, palladium electrolytic Porous & cubic Nickel, carbonyl decomposition Crushed ribbon, Iron-based metallic glass Irregular, titanium sodium reduced & milled Angular, Niobium hydride milled Acicular, tellurium, milled Spherical & agglomerated Fines, Iron, atomized polygonal Aggregates, Tungsten, Gas Reduced Rounded & ligamental Tin, Atomized Spherical, Iron alloy, centrifugally atomized Flake, tin Splat quenched

12. 8/1/2007ME 340 POWDER METALLURGY 12 P OWDERS P HYSICAL P ROPERTIES » Specific Surface Area Indicates the surface available for bonding and also the area on which adsorbed contaminant may be present (cm 2 /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

13. 8/1/2007ME 340 POWDER METALLURGY 13 P OWDERS P HYSICAL P ROPERTIES » Flow Properties given by flow rate and angle of repose » Compressibility

14. 8/1/2007ME 340 POWDER METALLURGY 14 B LENDING OF P OWDERS » 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

15. 8/1/2007ME 340 POWDER METALLURGY 15 C OMPACTION » 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”

16. 8/1/2007ME 340 POWDER METALLURGY 16 C OMPACTION

17. 8/1/2007ME 340 POWDER METALLURGY 17 C OMPACTION

18. 8/1/2007ME 340 POWDER METALLURGY 18 C OMPACTION

19. 8/1/2007ME 340 POWDER METALLURGY 19 C OLD C OMPACTION 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

20. 8/1/2007ME 340 POWDER METALLURGY 20 C OLD C OMPACTION 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)

21. 8/1/2007ME 340 POWDER METALLURGY 21 C OLD C OMPACTION

22. 8/1/2007ME 340 POWDER METALLURGY 22 D IE P RESSING 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

23. 8/1/2007ME 340 POWDER METALLURGY 23 D IE P RESSING For a single acting punch with applied pressure p 0 the pressure at l depth in the body is: Where:  Is wall friction A f r is the frictional surface area. A 0 is the compacted area And k is a factor representing radial to axial stress ratio For an elastic solid:

24. 8/1/2007ME 340 POWDER METALLURGY 24 D IE P RESSING

25. 8/1/2007ME 340 POWDER METALLURGY 25 D IE P RESSING Uniform fill density can be assured with the use of multipunch dies

26. 8/1/2007ME 340 POWDER METALLURGY 26 D IE P RESSING

27. 8/1/2007ME 340 POWDER METALLURGY 27 C OLD I SOSTATIC P RESSING

28. 8/1/2007ME 340 POWDER METALLURGY 28 C OLD I SOSTATIC P RESSING » 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)

29. 8/1/2007ME 340 POWDER METALLURGY 29 H OT I SOSTATIC P RESSING » Container made of high melting point sheet metal » Pressurizing medium is inert gas or vitreous (glasslike) fluids » Common conditions are 100 MPa and 1100 o C

30. 8/1/2007ME 340 POWDER METALLURGY 30 P OWDER I NJECTION M OLDING Taken from plastics technology MIM Metal Injection Molding CIM Ceramics Injection Molding Typically 40% binder (70% paraffin wax + 30% polypropylene)

31. 8/1/2007ME 340 POWDER METALLURGY 31 P OWDER I NJECTION M OLDING

32. 8/1/2007ME 340 POWDER METALLURGY 32 S INTERING 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

33. 8/1/2007ME 340 POWDER METALLURGY 33 S INTERING

34. 8/1/2007ME 340 POWDER METALLURGY 34 S INTERING 3 important variables: 1. Atmosphere 2. Temperature 3. Time

35. 8/1/2007ME 340 POWDER METALLURGY 35 F INISHING O PERATIONS » Repressing » Re-sintering » Forging » Extrusion » Rolling » Machining » Heat Treatment » Coining (Resizing) Increase density and improve dimensional tolerance » Impregnation Immersion in heated oil; capillary action fills the pores. » Infiltration Impregnation with a metal.

36. 8/1/2007ME 340 POWDER METALLURGY 36 D ESIGN C ONSIDERATIONS

37. 8/1/2007ME 340 POWDER METALLURGY 37 A DVANTAGES & D ISADVANTAGES OF PM

38. 8/1/2007ME 340 POWDER METALLURGY 38 A DVANTAGES & D ISADVANTAGES OF PM

39. 8/1/2007ME 340 POWDER METALLURGY 39 T RENDS IN PM