1 CHAPTER 13 POWDER METALLURGY. 8/1/2007ME 340 POWDER METALLURGY 2 I NTRODUCTION Powder Metallurgy is a manufacturing method to produce components by.

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CHAPTER 13

POWDER METALLURGY

8/1/2007 ME 340POWDER METALLURGY

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INTRODUCTION

Powder Metallurgyis 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


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PROCESSING STEPS IN PM

IF NECESSARY


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PROCESSING STEPS 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


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POWDER PRODUCTION

POWDER MANUFACTURINGPOWDER MANUFACTURING METHODSMETHODS

MECHANICALMECHANICALCOMMINUTIONCOMMINUTION

CHEMICALCHEMICALREACTIONSREACTIONS

ELECTROLYTIC ELECTROLYTIC DEPOSITIONDEPOSITION

METALMETALATOMIZATIONATOMIZATION

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


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MILLING TECHNIQUES

JAR MILLING ATTRITION MILLING


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ATOMIZATION TECHNIQUES

Disintegration of melt into droplets that freeze into particles

Production rates as high as 400Kg/min


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GAS ATOMIZATION WATER ATOMIZATION


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(a) 5-10 kg capacity water atomiser (a) 5-10 kg capacity water atomiser (b) 30Kg capacity inert gas atomiser(b) 30Kg capacity inert gas atomiser

(a)

(b)


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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)


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POWDERS CHARACTERIZATION (MORPHOLOGY)

Rounded and irregular, stainless steel, atomized

Sponge, palladiumelectrolytic

Porous & cubic Nickel, carbonyl decomposition

Crushed ribbon,Iron-based metallic glass

Irregular, titanium sodium reduced & milled

Angular, Niobium hydridemilled

Acicular,

tellurium, milled Spherical & agglomerated

Fines, Iron, atomized

polygonal Aggregates, Tungsten, Gas Reduced

Rounded & ligamental

Tin, Atomized

Spherical, Iron alloy,

centrifugally atomized Flake, tinSplat quenched


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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


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POWDERS PHYSICAL PROPERTIES

» Flow Properties given by flow rate and angle of repose

» Compressibility


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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


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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”


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COMPACTION


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COMPACTION


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COMPACTION


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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


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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)


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COLD COMPACTION


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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


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DIE PRESSING

)exp(0

0 A

kApp fr

l

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 areaAnd k is a factor representing radial to axial stress ratioFor an elastic solid:

1a

r

p

pk


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DIE PRESSING


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DIE PRESSING

Uniform fill density can be assured with the use of multipunch dies


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DIE PRESSING


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COLD ISOSTATIC PRESSING


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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)


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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


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POWDER INJECTION MOLDING

Taken from plastics technology

MIM Metal Injection Molding

CIM Ceramics Injection Molding

Typically 40% binder (70% paraffin wax + 30% polypropylene)


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POWDER INJECTION MOLDING


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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

3/1

shrinkageLinear

shrinkage Volumetric

))(())((constantMass

sintered

green

sintered

green

green

sintered

sinteredsinteredgreengreen

V

V

VV


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SINTERING


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SINTERING

3 important variables:

1. Atmosphere2. Temperature3. Time


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FINISHING OPERATIONS

»Repressing»Re-sintering

»Forging»Extrusion»Rolling»Machining»Heat Treatment

»Coining (Resizing)Increase density and improve dimensional tolerance»ImpregnationImmersion in heated oil; capillary action fills the pores.»InfiltrationImpregnation with a metal.


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DESIGN CONSIDERATIONS


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ADVANTAGES & DISADVANTAGES OF PM

Advantages Availability of a wide range of composites to obtain special mechanical and physical properties, such as stiffness, damping characteristics, hardness, density, toughness, and electrical and magnetic properties. Some of the highly alloyed new superalloys can be manufactures into parts by P/M processing. A technique for making parts from high-melting-point refractory metals, which would be difficult or uneconomical to make by other methods. High production rates on relatively complex parts, with automated equipment requiring little labor. Good dimensional control and, in many instances, elimination of machining and finishing operations, thus eliminating scrap and waste and saving energy. Capability for impregnation and infiltration for special applications.


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ADVANTAGES & DISADVANTAGES OF PM

Disadvantages

Size of parts, complexity of shape of parts, and press capacity. High cost of powder metals compared to other raw materials. High cost of tooling and equipment for small production runs Mechanical properties, such as strength and ductility, that are generally lower those obtained by forging. However, the properties of fully-dense P/M parts made by HIP or additional forging can be better than those made by other processes.


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TRENDS IN PM

1 CHAPTER 13 POWDER METALLURGY. 8/1/2007ME 340 POWDER METALLURGY 2 I NTRODUCTION Powder Metallurgy is a manufacturing method to produce components by.

Documents

density of powders

highest density

dietap density

diegreen density

vibration of powders

powders of different

pmparticles of desired

pressed powder