Chapter 17Powder MetalsPowder MetallurgyProduction of Metal PowdersParticle Size, Distribution, and shapePowder ParticlesMethods of Powder ProductionBlending PowdersCompaction of Metal PowdersEquipmentIsostatic PressingSlide 12Punch and Die MaterialsMetal Injection MoldingOther Shaping ProcessesSinteringSlide 17PowerPoint PresentationSecondary & Finishing OperationsSlide 20Design Considerations for P/MProcess CapabilitiesSlide 23Economics of Powder MetallurgyShaping CeramicsSLIP CASTINGProcessing steps involved in making ceramic partsChapter 17Processing of Powder Metals, Ceramics, Glass & SuperconductorsPowder Metals•Commonly used metals in P/M–Iron,Tin, Copper, Aluminum, and Nickel•It is a completive process with forging and machining•Parts can weigh as much as little as 2.5Kg or up to 50KgPowder MetallurgyabcFig: (a)Examples of typical parts made by powder-metallurgy processes. (b) Upper trip lever for a commercial irrigation sprinkler, made by P/M. This part is made of unleaded brass alloy; it replaces a die-cast part, with a 60% savings. (c) Main-bearing powder metal caps for 3.8 and 3.1 liter General Motors automotive engines.Production of Metal Powders•Powder production•Blending•Compaction•Sintering•Finishing operationsParticle Size, Distribution, and shape•Particle size is measured by screening•In addition to screen analysis one can use:–Sedimentation – measuring the rate that particles settle in a fluid–Microscopic analysis – using a scanning electron microscope–Light scattering–Optical – particles blocking a beam of light that is sensed by a photocell–Suspending particles in a liquid & detecting particle size and distribution Fig: Particle shapes in metal powders,and the processes by which they are produced.Iron powders are produced by many of these processesPowder ParticlesFig : (a) Scanning electron-microscopy photograph of iron-powder particles made by atomization. (b) Nickel-based superalloy powder particles made by the rotating electrode process.Methods of Powder Production Fig : Methods of metal-powder production by atomization;(a) melt atomization; (b) atomization with a rotating consumable electrode Fig: Methods of mechanical communication, to obtain fine particles: (a) roll crushing, (b) ball mill, & (c) hammer millingBlending Powders•Blending powders is the second step in the P/M process•Powders made by different processes have different sizes and shapes and must be well mixed•Powders of different metals can be mixed together•Lubricants can be mixed with the powders to improve their flow characteristics Fig: Some common equipment geometries for mixing or blending powders. (a) cylindrical, (b) rotating cube, (c) double cone, and (d) twin shell.Compaction of Metal Powders•Blended powders are pressed together•The powder must flow easily into the die•Size distribution is an important fact–They should not be all the same size–Should be a mixture of large and small particles•The higher the density the higher the strength Fig: Compaction of metal powder to form a bushing.The pressed powder part is called green compact. (b) Typical tool and die set for compacting a spur gearEquipment•Uses 100-300 ton press•Selection of the press depends on the part and the configuration of the partFig: A7.3 MN (825 ton) mechanical press for compacting metal powder.Isostatic Pressing•Cold isostatic Pressing (CIP)–Metal powder is placed in a flexible rubber mold–Pressurized hydrostatically–Uses pressures up to 150 KSI–Typical application is automotive cylinder liners Fig: Schematic diagram, of cold isostatic, as applied to forming a tube.The powder is enclosed in a flexible container around a solid core rod.Pressure is applied iso-statically to the assembly inside a high-pressure chamber.Isostatic Pressing•Hot Isostatic pressing–Container is made of high-melting-point sheet metal–Uses a inert gas as the pressurizing medium–Common conditions for HIP are 15KSI at 2000F–Mainly used for super alloy casting Fig: Schematic illustration of hot isostatic pressing.The pressure and temperature variation vs.time are shown in the diagramPunch and Die Materials•Depends on the abrasiveness of the powder metal•Tungsten-carbide dies are used•Punches are generally made of the similar materials•Dimensions are watched very closeMetal Injection Molding•MIM uses very fine metal powders blended with a polymer•The molded greens are then placed in a furnace to burn off the plastics•Advantages of injection molding–Produces complex shapes–Mechanical properties are nearly equal to those of wrought productsOther Shaping Processes•Rolling – powder is fed though the roll gap and is used to make coins and sheet metal•Extrusion – has improved properties and parts my be forged in a closed die to get final shape•Pressureless compaction – gravity filled die and used to make porous parts•Ceramic molds – molds are made by made by investment casting and the powder is compressed by hot isostatic pressing•Spray deposition – shape-generation process An example of powder rollingSintering•Sintering - Green compacts are heated in a furnace to a temperature below melting point•Improves the strength of the material•Proper furnace control is important for optimum properties Fig: Schematic illustration of two mechanism for sintering metal powders: (a) solid-state material transport; (b) liquid-phase material transport.R= particle radius, r=neck radius, and p=neck profile radiusSintering•Particles start forming a bond by diffusion•Vapor-phase transport – heated very close to melting temperature allows metal atoms to release to the vapor phaseMechanical PropertiesSecondary & Finishing Operations•To improve the properties of sintered P/M products several additional operations may be used:–Coining and sizing – compaction operations–Impact forging – cold or hot forging may be used•Parts may be impregnated with a fluid to reduce the porosity Fig: Examples of P/M parts,showing poor designs and good ones.Notes that sharp radii and re entry corners should be avoided and that threads and transverse holes have to be produced separately by additional machining operations.Secondary & Finishing Operations•Infiltration – metal infiltrates the pores of a sintered part to produce a stronger part and