2 POWDER METALLURGY Watch the sintering operation The Characterization of Engineering PowdersProduction of Metallic PowdersConventional Pressing and SinteringAlternative Pressing and Sintering TechniquesMaterials and Products for PMDesign Considerations in Powder MetallurgyWatch the sintering operation
3 Powder Metallurgy (PM) Metal processing technology in which parts are produced from metallic powdersPM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent machiningCertain metals that are difficult to fabricate by other methods can be shaped by PMTungsten filaments for lamp bulbs are made by PMPM process wastes very little material - ~ 97% of starting powders are converted to productPM parts can be made with a specified level of porosity, to produce porous metal partsExamples: filters, oil‑impregnated bearings and gearsModern powder metallurgy dates only back to the early 1800
4 Limitations and Disadvantages High tooling and equipment costsMetallic powders are expensiveProblems in storing and handling metal powdersDegradation over time, fire hazards with certain metalsLimitations on part geometry because metal powders do not readily flow laterally in the die during pressingVariations in density throughout part may be a problem, especially for complex geometries
5 Usual PM production sequence Blending and mixing (Rotating drums, blade and screw mixers)Pressing - powders are compressed into desired shape to produce green compactAccomplished in press using punch-and-die tooling designed for the partSintering – green compacts are heated to bond the particles into a hard, rigid massPerformed at temperatures below the melting point of the metal
6 PM Work MaterialsLargest tonnage of metals are alloys of iron, steel, and aluminumOther PM metals include copper, nickel, and refractory metals such as molybdenum and tungstenMetallic carbides such as tungsten carbide are often included within the scope of powder metallurgy
7 PM PartsA collection of powder metallurgy parts.
8 Engineering PowdersA powder can be defined as a finely divided particulate solidEngineering powders include metals and ceramicsParticle shapes
9 Measuring Particle Size Most common method uses screens of different mesh sizesMesh count - refers to the number of openings per linear inch of screenA mesh count of 200 means there are 200 openings per linear inchHigher mesh count = smaller particle sizeFigure Screen mesh for sorting particle sizes.
10 Interparticle Friction and Powder Flow Friction between particles affects ability of a powder to flow readily and pack tightlyA common test of interparticle friction is the angle of repose, which is the angle formed by a pile of powders as they are poured from a narrow funnel.Figure Interparticle friction as indicated by the angle of repose of a pile of powders poured from a narrow funnel. Larger angles indicate greater interparticle friction.
11 ObservationsEasier flow of particles correlates with lower interparticle frictionLubricants are often added to powders to reduce interparticle friction and facilitate flow during pressing
12 Let’s check!Smaller particle sizes show steeper angles or larger particle sizes?!Smaller particle sizes generally show greater friction and steeper angles!Finer particles
13 Let’s check! Which shape has the lowest interpartical friction? Spherical shapes have the lowest interpartical friction!Little friction between spherical particles!As shape deviates from spherical, friction between particles tends to increase
14 Particle Density Measures True density - density of the true volume of the materialThe density of the material if the powders were melted into a solid massBulk density - density of the powders in the loose state after pouringWhich one is smaller?!Because of pores between particles, bulk density is less than true density
15 Packing FactorTypical values for loose powders range between 0.5 and 0.7Bulk densityPacking factor =true densityHow can we increase the bulk density?If powders of various sizes are present, smaller powders will fit into spaces between larger ones, thus higher packing factorPacking can be increased by vibrating the powders, causing them to settle more tightlyPressure applied during compaction greatly increases packing of powders through rearrangement and deformation of particles
16 PorosityRatio of volume of the pores (empty spaces) in the powder to the bulk volumeIn principlePorosity + Packing factor = 1.0A highly porous ceramic material (http://www.glass-ceramics.uni-erlangen.de/)
17 PM Materials Metallic powders are classified as either Elemental - consisting of a pure metalPre-alloyed - each particle is an alloyFigure Iron powders produced by decomposition of iron pentacarbonyl (photo courtesy of GAF Chemical Corp); particle sizes range from about 0.25 ‑ 3.0 microns (10 to 125 -in).
18 PM Materials – Elemental Powders A pure metal in particulate formApplications where high purity is importantCommon elemental powders:IronAluminumCopperElemental powders can be mixed with other metal powders to produce alloys that are difficult to formulate by conventional methodsExample: tool steels
19 PM Materials – Pre-Alloyed Powders Each particle is an alloy comprised of the desired chemical compositionCommon pre-alloyed powders:Stainless steelsCertain copper alloysHigh speed steel
20 PM ProductsGears, bearings, sprockets, fasteners, electrical contacts, cutting tools, and various machinery partsAdvantage of PM: parts can be made to near net shape or net shapeWhen produced in large quantities, gears and bearings are ideal for PM because:The geometry is defined in two dimensions (cross section is uniform)There is a need for porosity in the part to serve as a reservoir for lubricant
22 Production of Metallic Powders In general, producers of metallic powders are not the same companies as those that make PM partsAny metal can be made into powder formThree principal methods by which metallic powders are commercially producedAtomization (by gas, water, also centrifugal one)ChemicalElectrolytic
23 Gas Atomization Method High velocity gas stream flows through expansion nozzle, siphoning molten metal from below and spraying it into containerFigure 16.5 (a) gas atomization methodCheck other figures as well (page 344)
24 Conventional Press and Sinter After metallic powders have been produced, the conventional PM sequence consists of:Blending and mixing of powdersCompaction - pressing into desired shapeSintering - heating to temperature below melting point to cause solid‑state bonding of particles and strengthening of partIn addition, secondary operations are sometimes performed to improve dimensional accuracy, increase density, and for other reasons
25 Blending and Mixing of Powders For successful results in compaction and sintering, the starting powders must be homogenized (powders should be blended and mixed)Blending - powders of same chemistry but possibly different particle sizes are intermingledDifferent particle sizes are often blended to reduce porosityMixing - powders of different chemistries are combined
26 CompactionApplication of high pressure to the powders to form them into the required shapeConventional compaction method is pressing, in which opposing punches squeeze the powders contained in a dieThe workpart after pressing is called a green compact, the word green meaning not yet fully processedThe green strength of the part when pressed is adequate for handling but far less than after sintering
27 Conventional Pressing in PM FillPressEjectFigure Pressing in PM: (1) filling die cavity with powder by automatic feeder; (2) initial and (3) final positions of upper and lower punches during pressing, (4) part ejection.Watch the single- and double-punch operations
28 SinteringHeat treatment to bond the metallic particles, thereby increasing strength and hardnessUsually carried out at between 70% and 90% of the metal's melting point (absolute scale)particle bonding is initiated at contact points
29 Densification and Sizing Secondary operations are performed to increase density, improve accuracy, or accomplish additional shaping of the sintered partRepressing - pressing sintered part in a closed die to increase density and improve propertiesSizing - pressing a sintered part to improve dimensional accuracyCoining - pressworking operation on a sintered part to press details into its surfaceMachining - creates geometric features that cannot be achieved by pressing, such as threads, side holes, and other details
30 Impregnation and Infiltration Porosity is a unique and inherent characteristic of PM technologyIt can be exploited to create special products by filling the available pore space with oils, polymers, or metalsTwo categories:ImpregnationInfiltration
31 ImpregnationThe term used when oil or other fluid is permeated into the pores of a sintered PM partCommon products are oil‑impregnated bearings, gears, and similar componentsAlternative application is when parts are impregnated with polymer resins that seep into the pore spaces in liquid form and then solidify to create a pressure tight part
32 InfiltrationOperation in which the pores of the PM part are filled with a molten metalThe melting point of the filler metal must be below that of the PM partResulting structure is relatively nonporous, and the infiltrated part has a more uniform density, as well as improved toughness and strengthTM (filler)<TM (Part)