Manufacturing Engineering Technology in SI Units, 6th Edition Chapter 17: Powder-Metal Processing and Equipment Presentation slide for courses, classes, lectures et al. Copyright © 2010 Pearson Education South Asia Pte Ltd

Chapter Outline Introduction Production of Metal Powders Compaction of Metal Powders Sintering Secondary and Finishing Operations Design Considerations Process Capabilities Economics of Powder Metallurgy Copyright © 2010 Pearson Education South Asia Pte Ltd

Introduction Raw materials for metals and alloys are in a molten state (casting) or in solid form (metalworking) Powder metallurgy (PM) process involves metal powders compacted into desired shapes and sintered to form a solid piece Copyright © 2010 Pearson Education South Asia Pte Ltd

Production of Metal Powders Powder-metallurgy process consists of: Powder production Blending Compaction Sintering Finishing operations Copyright © 2010 Pearson Education South Asia Pte Ltd

Production of Metal Powders: Methods of Powder Production The choice of producing metal powders depends on the requirements of the end product The microstructure, bulk and surface properties, chemical purity, porosity, shape, and size distribution of the particles depend on the particular process used Copyright © 2010 Pearson Education South Asia Pte Ltd

Production of Metal Powders: Methods of Powder Production Atomization Involves a liquid-metal stream produced by injecting molten metal through a small orifice Stream is broken up by jets of inert gas or air or water known as gas or water atomization Copyright © 2010 Pearson Education South Asia Pte Ltd

Production of Metal Powders: Methods of Powder Production Atomization Size and shape of the particles formed depend on the temperature of the molten metal, rate of flow, nozzle size, and jet characteristics In centrifugal atomization, the centrifugal forces break up the stream and generate particles Copyright © 2010 Pearson Education South Asia Pte Ltd

Production of Metal Powders: Methods of Powder Production Reduction The reduction of metal oxides uses gases, such as hydrogen and carbon monoxide, as reducing agents Electrolytic Deposition Used either aqueous solutions or fused salts Carbonyls Metal carbonyls are formed by letting iron or nickel react with carbon monoxide Reaction products are decomposed to iron and nickel Copyright © 2010 Pearson Education South Asia Pte Ltd

Production of Metal Powders: Methods of Powder Production Comminution Involves crushing milling in a ball mill, or grinding of brittle or less ductile metals into small particles A ball mill is a machine with a rotating hollow cylinder partly filled with steel or white cast-iron balls Copyright © 2010 Pearson Education South Asia Pte Ltd

Production of Metal Powders: Methods of Powder Production Mechanical Alloying Powders of two or more pure metals are mixed in a ball mill Under the impact of the hard balls, the powders fracture and bond together by diffusion Copyright © 2010 Pearson Education South Asia Pte Ltd

Production of Metal Powders: Particle Size, Shape, and Distribution Particle size is controlled by passing the metal powder through screens of various mesh sizes, called screening Other methods for particle-size analysis are: Sedimentation Microscopic analysis Light scattering Optical methods Suspending particles Copyright © 2010 Pearson Education South Asia Pte Ltd

Production of Metal Powders: Particle Size, Shape, and Distribution Particle Shape Particle shape is described in terms of aspect ratio or shape factor Aspect ratio is the ratio of the largest dimension to the smallest dimension of the particle Shape Factor A measure of the ratio of the surface area of the particle to its volume Copyright © 2010 Pearson Education South Asia Pte Ltd

Production of Metal Powders: Particle Size, Shape, and Distribution Size Distribution Affects the processing characteristics of the powder Distribution of particle size is given in terms of a frequency-distribution plot Properties of metal powders that affect their behavior in processing are: Flow properties Compressibility Density Copyright © 2010 Pearson Education South Asia Pte Ltd

Production of Metal Powders: Blending Metal Powders Blending (mixing) powders is the next step in powder- metallurgy processing Powder mixing must be carried out under controlled conditions in order to avoid contamination or deterioration Copyright © 2010 Pearson Education South Asia Pte Ltd

Production of Metal Powders: Blending Metal Powders Hazards Metal powders can be explosive due to high surface area–to-volume ratio Precautions include: Grounding equipment Preventing sparks and avoiding friction as a source of heat Avoiding dust clouds, open flames, and chemical reactions Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders Compaction is the step where the blended powders are pressed into various shapes in dies Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders Purposes of compaction are to obtain the required shape, density and particle-to-particle contact Pressed powder is known as green compact Density depends on the pressure applied Higher the density of the compacted part, the higher are its strength and elastic modulus Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders May be necessary to use multiple punches to ensure that the density is more uniform throughout the part Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Equipment Compacting pressure required depends on the characteristics and shape of the particles, method of blending and lubricant Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Equipment Hydraulic presses with capacities as high as 45 MN, can be used for large parts Press selection depends on part size and the configuration, density requirements, and production rate Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Isostatic Pressing Green compacts is subjected to hydrostatic pressure to achieve more uniform compaction and density In cold isostatic pressing (CIP), the metal powder is placed in a flexible rubber mold Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Isostatic Pressing Green compacts is subjected to hydrostatic pressure to achieve more uniform compaction and density In cold isostatic pressing (CIP) the metal powder is placed in a flexible rubber mold Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Isostatic Pressing In hot isostatic pressing (HIP), the container is made of a high-melting-point sheet metal and the pressurizing medium is a high-temperature inert gas Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Isostatic Pressing The HIP process is used to produce superalloy components for the aircraft and aerospace industries It also is used: To close internal porosity To improve properties in superalloy and titanium-alloy castings for the aerospace industry As a final densification Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Isostatic Pressing Advantages of hot isostatic pressing are: Produces fully dense compacts of uniform grain structure and density Handling larger parts Limitations of HIP: Wider dimensional tolerances Higher equipment cost and production Small production quantities Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Isostatic Pressing EXAMPLE 17.1 Hot Isostatic Pressing of a Valve Lifter Figure shows a valve lifter for heavy-duty diesel engines Produced from a hot-isostatic-pressed carbide cap on a steel shaft Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Miscellaneous Compacting and Shaping Processes Powder-injection Molding Also called metal-injection molding Very fine metal powders are blended with a 25 to 45% polymer or a wax-based binder Mixture undergoes a process similar to die casting Advantages of powder-injection molding are: Complex shapes Good dimensional tolerances High production rates Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Miscellaneous Compacting and Shaping Processes EXAMPLE 17.2 Mobile Phone Components Produced through Metal Injection Molding Powder-metal components for mobile phones to achieve a flip-open feature Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Miscellaneous Compacting and Shaping Processes Forging Products from powder forging (PF) are fully dense, good surface finish, good dimensional tolerances, and a uniform and fine grain size Rolling Also called roll compaction Metal powder is fed into the roll gap in a two-high rolling mill and compacted into a continuous strip Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Miscellaneous Compacting and Shaping Processes Extrusion Powders is compacted by extrusion whereby the powder is encased in a metal container and hot extruded Pressureless Compaction The die is gravity filled with metal powder and the powder is sintered directly in the die Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Miscellaneous Compacting and Shaping Processes Spray Deposition It is a shape-generation process die Basic components are An atomizer A spray chamber with an inert atmosphere A mold for producing preforms Copyright © 2010 Pearson Education South Asia Pte Ltd

Compaction of Metal Powders: Punch and Die Materials Selection depends on the abrasiveness of the powder metal and the number of parts to be produced Close control of die and punch dimensions is essential for proper compaction and die life Large clearance will allow the metal powder to enter the gap Die and punch surfaces must be polished for improved die life and overall performance Copyright © 2010 Pearson Education South Asia Pte Ltd

Sintering Sintering is the process whereby green compacts are heated in a furnace to below the melting point but high enough to allow bonding (fusion) of the individual particles Copyright © 2010 Pearson Education South Asia Pte Ltd

Sintering Strength of the bond between the particles depends on the complex mechanisms of diffusion of: Plastic flow Evaporation of volatile materials in the compact Recrystallization Grain growth Pore shrinkage Copyright © 2010 Pearson Education South Asia Pte Ltd

Sintering Continuous-sintering furnaces have 3 chambers: Burn-off chamber High-temperature chamber Cooling chamber The sintering mechanisms are diffusion, vapor-phase transport, and liquid-phase sintering Copyright © 2010 Pearson Education South Asia Pte Ltd

Sintering Mechanical Properties Affecting mechanical properties are temperature, time, and processing history Porosity cannot be avoided completely due to voids remaining after compaction and gases evolve during sintering Copyright © 2010 Pearson Education South Asia Pte Ltd

Sintering Mechanical Properties Copyright © 2010 Pearson Education South Asia Pte Ltd

Sintering Mechanical Properties Copyright © 2010 Pearson Education South Asia Pte Ltd

Sintering Mechanical Properties Copyright © 2010 Pearson Education South Asia Pte Ltd

Secondary and Finishing Operations Further improvement for the properties of sintered PM products are: Coining and sizing Preformed and sintered alloy-powder Subject to finishing operations Reduce porosity by impregnating PM components with a fluid Infiltration Electroplating Copyright © 2010 Pearson Education South Asia Pte Ltd

Design Considerations Certain design principles that should be followed: Simple and uniform shape of the compact Provision must be made for ejection of the green compact PM parts made with acceptable dimensional tolerances Part walls should not be less than 1.5 mm thick Copyright © 2010 Pearson Education South Asia Pte Ltd

Design Considerations Copyright © 2010 Pearson Education South Asia Pte Ltd

Design Considerations Copyright © 2010 Pearson Education South Asia Pte Ltd

Process Capabilities Process capabilities of powder metallurgy are: A technique for making parts from high-melting-point refractory metals High production rates Good dimensional control Availability of a wide range of compositions Capability of impregnation and infiltration Copyright © 2010 Pearson Education South Asia Pte Ltd

Process Capabilities Limitations of PM are: High cost of metal powder High cost of tooling and equipment Limitations on part size and shape complexity Mechanical properties are lower Copyright © 2010 Pearson Education South Asia Pte Ltd

Economics of Powder Metallurgy Powder metallurgy can produce parts at net or near- net shape Eliminate many secondary manufacturing and assembly operations High initial cost of punches, dies, and equipment Economical for quantities over 10,000 pieces and labor costs would not be high Reduces scrap Copyright © 2010 Pearson Education South Asia Pte Ltd

Economics of Powder Metallurgy CASE STUDY 17.1 Powder Metallurgy Parts in a Snowblower Collection of PM parts in a commercial snowblower Copyright © 2010 Pearson Education South Asia Pte Ltd