PARTICULATE PROCESSING OF METALS AND CERAMICS

Presented ByD.Manesi

Contributed By : ©2010 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 4/e”

THE SECTION Of POWDER METALLURGY

1. The Characterization of Engineering Powders2. Production of Metallic Powders3. Conventional Pressing and Sintering4. Alternative Pressing and Sintering Techniques5. Materials and Products for PM6. Design Considerations in Powder Metallurgy

POWDER METALLURGY (PM)

• Metal processing technology in which parts are produced from metallic powders

• In the usual PM production sequence, the powders are compressed (pressed) into the desired shape and then heated (sintered) to bond the particles into a hard, rigid mass1. Pressing is accomplished in a press-type machine using

punch-and-die tooling designed specifically for the part to be manufactured

2. Sintering is performed at a temperature below themelting point of the metal

WHY POWDER METALLURGY IS IMPORTANT

• PM parts can be mass produced to net shape or near net shape, eliminating or reducing the need for subsequent machining

• PM process wastes very little material - about 97% of the starting powders are converted to product

• PM parts can be made with a specified level of porosity, to produce porous metal partsExamples: filters, oil-impregnated bearings and

gears

MORE REASONS WHY PM IS IMPORTANT

• Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgyExample: Tungsten filaments for incandescent lamp

bulbs are made by PM• Certain alloy combinations and cermets made by

PM cannot be produced in other ways• PM compares favorably to most casting processes in

dimensional control• PM production methods can be automated

foreconomical production

LIMITATIONS AND DISADVANTAGESWITH PM PROCESSING

• High tooling and equipment costs• Metallic powders are expensive• Problems in storing and handling metal powders

Examples: degradation over time, fire hazards with certain metals

• Limitations on part geometry because metal powders do not readily flow laterally in the die during pressing

• Variations in density throughout part may be a problem, especially for complex geometries

PM WORK MATERIALS

• Largest tonnage of metals are alloys of iron, steel, and aluminum

• Other PM metals include copper, nickel, and refractory metals such as molybdenum and tungsten

• Metallic carbides such as tungsten carbide are often included within the scope of powder metallurgy

Figure 16.1 - A collection of powder metallurgy parts (courtesy ofDorst America, Inc.)

ENGINEERING POWDERS

A powder can be defined as a finely divided particulate solid

• Engineering powders include metals and ceramics

• Geometric features of engineering powders:Particle size and distributionParticle shape and internal structureSurface area

MEASURING PARTICLE SIZE

• Most common method uses screens of different mesh sizes

• Mesh count - refers to the number of openings per linear inch of screenA mesh count of 200 means there are 200 openings

per linear inchSince the mesh is square, the count is the same in

both directions, and the total number of openings per square inch is 2002 = 40,000

Higher mesh count means smaller particle size

Figure 16.2 - Screen mesh for sorting particle sizes

Figure 16.3 - Several of the possible (ideal) particle shapes inpowder metallurgy

INTERPARTICLE FRICTION ANDFLOW CHARACTERISTICS

• Friction between particles affects ability of a powder to flow readily and pack tightly

• A 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 16.4 - 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

OBSERVATIONS

• Smaller particle sizes generally show greater friction and steeper angles

• Spherical shapes have the lowest interpartical friction

• As shape deviates from spherical, friction between particles tends to increase

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 mass• Bulk density - density of the powders in the

loose state after pouringBecause of pores between particles, bulk density

is less than true density

PACKING FACTOR = BULK DENSITYDIVIDED BY TRUE DENSITY

• Typical values for loose powders range between 0.5 and 0.7

• If powders of various sizes are present, smaller powders will fit into the interstices of larger ones that would otherwise be taken up by air, thus higher packing factor

• Packing can be increased by vibrating the powders, causing them to settle more tightly

• Pressure applied during compaction greatlyincreases packing of powders through rearrangement and deformation of particles

POROSITY

Ratio of the volume of the pores (empty spaces) in the powder to the bulk volume

• In principle, Porosity + Packing factor = 1.0• The issue is complicated by the possible

existence of closed pores in some of the particles

• If internal pore volumes are included in above porosity, then equation is exact

CHEMISTRY AND SURFACE FILMS

• Metallic powders are classified as eitherElemental - consisting of a pure metalPre-alloyed - each particle is an alloy

• Possible surface films include oxides, silica, adsorbed organic materials, and moistureAs a general rule, these films must be removed

prior to shape processing

PRODUCTION OF METALLIC POWDERS

• In general, producers of metallic powders are not the same companies as those that make PM parts

• Virtually any metal can be made into powder form• Three principal methods by which metallic powdersare

commercially produced1. Atomization2. Chemical3. Electrolytic

• In addition, mechanical methods are occasionally used to reduce powder sizes

GAS ATOMIZATION METHOD

• High velocity gas stream flows through an expansion nozzle, siphoning molten metal from below and spraying it into a container

• Droplets solidify into powder form

Figure 16.5 (a) gas atomization method

Figure 16.6 - Iron powders produced by decomposition of iron pentacarbonyl; particle sizes range from about 0.25 - 3.0 microns (10 to 125 -in) (photo

courtesy of GAF Chemicals Corporation, Advanced Materials Division)

VIDEO GAS ATOMIZATION METHOD

CONVENTIONAL PRESS AND SINTER

• After the metallic powders have been produced, the conventional PM sequence consists of three steps:1. Blending and mixing of the powders2. Compaction - pressing into desired part shape3. Sintering - heating to a temperature below the melting point to cause solid-state bonding of particles and strengthening of part

• In addition, secondary operations are sometimes performed to improve dimensional accuracy, increase density, and for other reasons

Figure 16.7 - Conventional powder metallurgy production sequence: (1) blending, (2) compacting, and (3) sintering; (a) shows the condition of the particles while (b) shows the

operation and/or workpart during the sequence

BLENDING AND MIXING OF POWDERS

• For successful results in compaction and sintering, the starting powders must be homogenized

• Blending - powders of the same chemistry but possibly different particle sizes are intermingledDifferent particle sizes are often blended to reduce

porosity• Mixing - powders of different chemistries are

combinedPM technology allows mixing various metals into alloys

that would be difficult or impossible to produce by other means

Video Blending

COMPACTION

Application of high pressure to the powders to form them into the required shape

• The conventional compaction method is pressing, in which opposing punches squeeze the powders contained in a die

• The workpart after pressing is called a green compact, the word green meaning not yet fully processed

• The green strength of the part when pressed is adequate for handling but far less than after sintering

Figure 16.9 - 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, and (4)

ejection of part

Figure 16.11 - A 450 kN (50-ton) hydraulic press for compaction of powder metallurgycomponents. This press has the capability to actuate multiple levels to produce

complex PM part geometries (photo courtesy Dorst America, Inc.).

SINTERING

Heat treatment to bond the metallic particles, thereby increasing strength and hardness

• Usually carried out at between 70% and 90% of the metal's melting point (absolute scale)

• Generally agreed among researchers that the primary driving force for sintering is reduction of surface energy

• Part shrinkage occurs during sintering due to pore size reduction

Figure 16.12 - Sintering on a microscopic scale: (1) particle bonding is initiated at contact points; (2) contact points grow into "necks"; (3) the pores between particles are reduced in

size; and (4) grain boundaries develop between particles in place of the necked regions

Figure 16.13 - (a) Typical heat treatment cycle in sintering; and (b) schematic cross-section of a continuous sintering furnace

DENSIFICATION AND SIZING

Secondary operations are performed to increasedensity, improve accuracy, or accomplish additional shaping of the sintered part

• Repressing - pressing the sintered part in a closed die to increase density and improve properties

• Sizing - pressing a sintered part to improve dimensional accuracy

• Coining - pressworking operation on a sintered part to press details into its surface

• Machining - creates geometric features that cannot be achieved by pressing, such as threads, side holes, and other details

IMPREGNATION AND INFILTRATION

• Porosity is a unique and inherent characteristic of PM technology

• It can be exploited to create special products by filling the available pore space with oils, polymers, or metals

• Two categories:1. Impregnation2. Infiltration

IMPREGNATION

The term used when oil or other fluid is permeated into the pores of a sintered PM part

• Common products are oil-impregnated bearings, gears, and similar components

• An alternative 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

INFILTRATION

An operation in which the pores of the PM part are filled with a molten metal

• The melting point of the filler metal must be below that of the PM part

• Involves heating the filler metal in contact with the sintered component so capillary action draws the filler into the pores

• The resulting structure is relatively nonporous, and the infiltrated part has a more uniform density, as well as improved toughness and strength

ALTERNATIVE PRESSING AND SINTERINGTECHNIQUES

• The conventional press and sinter sequence is the most widely used shaping technology in powder metallurgy

• Additional methods for processing PM parts include:Isostatic pressingHot pressing - combined pressing and sintering

MATERIALS AND PRODUCTS FOR PM

• Raw materials for PM are more expensive than for other metalworking because of the additional energy required to reduce the metal to powder form

• Accordingly, PM is competitive only in a certain range of applications

• What are the materials and products that seem most suited to powder metallurgy?

PM MATERIALS –ELEMENTAL POWDERS

A pure metal in particulate form• Used in applications where high purity is important• Common elemental powders:

IronAluminumCopper

• Elemental powders are also mixed with other metal powders to produce special alloys that are difficult to formulate by conventional methods Example: tool steels

PM MATERIALS –PRE-ALLOYED POWDERS

Each particle is an alloy comprised of the desired chemical composition

• Used for alloys that cannot be formulated by mixing elemental powders

• Common pre-alloyed powders:Stainless steelsCertain copper alloysHigh speed steel

PM PRODUCTS

• Gears, bearings, sprockets, fasteners, electrical contacts, cutting tools, and various machinery parts

• Advantage of PM: parts can be made to near net shape or net shapeThey require little or no additional shaping after PM

processing• When produced in large quantities, gears and bearings

are ideal for PM because:The geometry is defined in two dimensionsThere is a need for porosity in the part to serve as a

reservoir for lubricant

PM PARTS CLASSIFICATION SYSTEM

• The Metal Powder Industries Federation (MPIF) defines four classes of powder metallurgy part designs, by level of difficulty in conventional pressing

• Useful because it indicates some of the limitations on shape that can be achieved with conventional PM processing

Figure 16.16 - Four classes of PM parts (side view shown; cross-section is circular): (a) Class I - simple thin shapes, pressed from one direction; (b) Class II - simple but thicker

shapes require pressing from two directions; (c) Class III – two levels of thickness, pressed from two directions; and (d) Class IV - multiple levels of thickness, pressed

from two directions, with separate controls for each level

DESIGN GUIDELINES FOR PM PARTS - I

• Economics usually require large quantities to justify cost of equipment and special toolingMinimum quantities of 10,000 units are suggested

• PM is unique in its capability to fabricate parts with a controlled level of porosityPorosities up to 50% are possible

• PM can be used to make parts out of unusual metals and alloys - materials that would be difficult if not impossible to produce by other means

DESIGN GUIDELINES FOR PM PARTS - II

• The part geometry must permit ejection from die after pressingThis generally means that part must have verticalor

near-vertical sides, although steps are allowedDesign features such as undercuts and holes on the

part sides must be avoidedVertical undercuts and holes are permissible because

they do not interfere with ejectionVertical holes can be of cross-sectional shapes other

than round without significant difficulty

Figure 16.17 - Part features to be avoided in PM: side holes and (b) side undercuts since part ejection is impossible

DESIGN GUIDELINES FOR PM PARTS - III

• Screw threads cannot be fabricated by PM; if required, they must be machined into the part

• Chamfers and corner radii are possible by PM pressing, but problems arise in punch rigidity when angles are too acute

• Wall thickness should be a minimum of 1.5 mm (0.060 in) between holes or a hole and outside wall

• Minimum recommended hole diameter is 1.5 mm (0.060 in)

Figure 16.19 - Chamfers and corner radii are accomplished but certain rules should be observed: (a) avoid acute angles; (b) larger angles preferred for punch rigidity; (c)

inside radius is desirable; (d) avoid full outside corner radius because punch is fragile at edge; (e) problem solved by combining radius and chamfer

THE SECTION Of PROCESSING OF CERAMICS AND CERMETS

• Processing of Traditional Ceramics• Processing of New Ceramics• Processing of Cermets• Product Design Considerations

TYPES OF CERAMICS AND THEIR PROCESSING

• Ceramic materials divide into three categories: 1. Traditional ceramics – particulate processing2. New ceramics – particulate processing3. Glasses – solidification processing

• The solidification processes for glass are covered in a different slide set

• The particulate processes for traditional and new ceramics as well as certain composite materials are covered in this slide set

OVERVIEW OF CERAMICS PARTICULATE PROCESSING

• Traditional ceramics are made from minerals occurring in nature – Products include pottery, porcelain, bricks, and

cement • New ceramics are made from synthetically

produced raw materials– Products include cutting tools, artificial bones, nuclear

fuels, and substrates for electronic circuits • The starting material for all of these items is

powder

OVERVIEW OF CERAMICS PARTICULATE PROCESSING - CONTINUED

• For traditional ceramics, the powders are usually mixed with water to temporarily bind the particles together and achieve the proper consistency for shaping

• For new ceramics, substances other than water are used as binders during shaping

• After shaping, the green parts are fired (sintered), whose function is the same as in powder metallurgy: – To effect a solid state reaction which bonds the material

into a hard solid mass

Figure 17.1 Usual steps in traditional ceramics processing: (1) preparation of ‑raw materials, (2) shaping, (3) drying, and (4) firing

Part (a) shows the workpart during the sequence, while (b) shows the condition of the powders

PREPARATION OF THE RAW MATERIAL FOR TRADITIONAL CERAMICS

• Shaping processes for traditional ceramics require the starting material to be a plastic paste – This paste is comprised of fine ceramic powders

mixed with water • The raw ceramic material usually occurs in

nature as rocky lumps, and reduction to powder is the purpose of the preparation step in ceramics processing

COMMINUTION

Reducing particle size in ceramics processing by use of mechanical energy in various forms such as impact, compression, and attrition

• Comminution techniques are most effective on brittle materials such as cement, metallic ores, and brittle metals

• Two general types of comminution operations: 1. Crushing 2. Grinding

CRUSHING

Reduction of large lumps from the mine to smaller sizes for subsequent further reduction

• Several stages may be required (e.g., primary crushing, secondary crushing), the reduction ratio in each stage being in the range 3 to 6

• Crushing of minerals is accomplished by compression against rigid surfaces or by impact against surfaces in a rigid constrained motion

Jaw CrusherLarge jaw toggles back and forth to crush lumps

against a hard, rigid surface

Figure 17.2 ‑

Crushing operations: (a) jaw crusher

Roll CrusherCeramic lumps are squeezed between rotating rolls

Figure 17.2 ‑ Crushing operations: (c) roll crusher

GRINDING

In the context of comminution, grinding refers to the operation of reducing the small pieces after crushing to a fine powder

• Accomplished by abrasion, impact, and compaction by hard media such as balls or rolls

• Examples of grinding include: – Ball mill– Roller mill– Impact grinding

Ball MillHard spheres mixed with stock are rotated inside a

large cylindrical container; the mixture is carried up the container wall as it rotates, and then pulled back down by gravity for grinding action

Figure 17.3 ‑ Mechanical methods of producing ceramic powders: (a) ball mill

Roller MillStock is compressed against a flat horizontal grinding

table by rollers riding over the table surface

Figure 17.3 ‑

Mechanical methods of producing ceramic powders: (b) roller mill

INGREDIENTS OF CERAMIC PASTE FOR SHAPING

1. Clay (hydrous aluminum silicates) - usually the main ingredient because of ideal forming characteristics when mixed with water

2. Water – creates clay-water mixture with suitable plasticity for shaping

3. Non plastic raw materials, such as ‑ alumina and silica - reduce shrinkage in drying and firing but also reduce plasticity of the mixture during forming

4. Other ingredients, such as fluxes that melt (vitrify) during firing and promote sintering, and wetting agents to improve mixing of ingredients

SHAPING PROCESSES

• Slip casting – The clay-water mixture is a slurry

• Plastic forming methods – The clay is plastic

• Semi dry pressing ‑– The clay is moist but has low plasticity

• Dry pressing – The clay is basically dry (less than 5% water) and

has no plasticity

Figure 17.4 Four categories of shaping processes used for traditional‑ ceramics,

compared to water content and pressure required to form the clay

SLIP CASTING

A suspension of ceramic powders in water, called a slip, is poured into a porous plaster of paris mold so that water from the mix is absorbed into the plaster to form a firm layer of clay at the mold surface

• The slip composition is 25% to 40% water • Two principal variations:

– Drain casting - the mold is inverted to drain excess slip after a semi solid layer has been formed, thus ‑producing a hollow product

– Solid casting - to produce solid products, adequate time is allowed for entire body to become firm

Figure 17.5 Sequence of steps in drain casting, a form of slip casting: (1) ‑slip is poured into mold cavity, (2) water is absorbed into plaster mold

to form a firm layer, (3) excess slip is poured out, and (4) part is removed from mold and trimmed

OVERVIEW OF PLASTIC FORMING

• The starting mixture must have a plastic consistency, with 15% to 25% water

• Variety of manual and mechanized methods – Manual methods use clay with more water

because it is more easily formed• More water means greater shrinkage in drying

– Mechanized methods generally use a mixture with less water so starting clay is stiffer

PLASTIC FORMING METHODS

• Hand modeling (manual method)• Jiggering (mechanized method)• Plastic pressing (mechanized method)• Extrusion (mechanized method)

HAND MODELING

Creation of the ceramic product by manipulating the mass of plastic clay into the desired geometry

• Hand molding - similar only a mold or form is used to define portions of the part geometry

• Hand throwing on a potter's wheel is another refinement of handcraft methods – Potter's wheel = a round table that rotates on a vertical

spindle, powered either by motor or foot operated ‑treadle

– Products of circular cross section can be formed by ‑throwing and shaping the clay, sometimes using a mold to provide the internal shape

Jiggering Similar to potter's wheel methods, but hand

throwing is replaced by mechanized techniques

Figure 17.6 ‑ Sequence in jiggering: (1) wet clay slug is placed on a convex mold; (2) batting; and (3) a jigger tool imparts the final product shape

PLASTIC PRESSING

Forming process in which a plastic clay slug is pressed between upper and lower molds contained in metal rings

• Molds are made of porous material such as gypsum, so when a vacuum is drawn on the backs of the mold halves, moisture is removed from the clay

• The mold sections are then opened, using positive air pressure to prevent sticking of the part in the mold

• Advantages: higher production rate than jiggering and not limited to radially symmetric parts

EXTRUSION Compression of clay through a die orifice to produce

long sections of uniform cross section, which are then ‑cut to required piece length

• Equipment utilizes a screw type action to assist in ‑mixing the clay and pushing it through die opening

• Products: hollow bricks, shaped tiles, drain pipes, tubes, and insulators

• Also used to make the starting clay slugs for other ceramics processing methods such as jiggering and plastic pressing

Semi-dry Pressing Uses high pressure to overcome the clay’s low

plasticity and force it into a die cavity

Figure 17.7 ‑ Semi‑dry pressing: (1) depositing moist powder into die cavity, (2) pressing, and (3) opening the die sections and ejection

DRY PRESSING

Process sequence is similar to semi dry pressing - the ‑main distinction is that the water content of the starting mix is typically below 5%

• Dies must be made of hardened tool steel or cemented carbide to reduce wear since dry clay is very abrasive

• No drying shrinkage occurs, so drying time is eliminated and good dimensional accuracy is achieved in the final product

• Typical products: bathroom tile, electrical insulators, refractory brick, and other simple geometries

CLAY VOLUME VS. WATER CONTENT

• Water plays an important role in most of the traditional ceramics shaping processes

• Thereafter, it has no purpose and must be removed from the clay piece before firing

• Shrinkage is a problem during drying because water contributes volume to the piece, and the volume is reduced when it is removed

Figure 17.8 Volume of clay ‑as a function of water content

Relationship shown here is typical; it varies for different clay compositions

DRYING

The drying process occurs in two stages: • Stage 1 - drying rate is rapid and constant as water

evaporates from the surface into the surrounding air and water from the interior migrates by capillary action to the surface to replace it – This is when shrinkage occurs, with the risk of warping

and cracking• Stage 2 - the moisture content has been reduced

to where the ceramic grains are in contact– Little or no further shrinkage occurs

Figure 17.9 Typical drying rate curve and associated volume reduction ‑(drying shrinkage) for a ceramic body in drying Drying rate in the

second stage of drying is depicted here as a straight line; the function is sometimes concave or convex

FIRING OF TRADITIONAL CERAMICS

Heat treatment process that sinters the ceramic material

• Performed in a furnace called a kiln • Bonds are developed between the ceramic grains,

and this is accompanied by densification and reduction of porosity

• Therefore, additional shrinkage occurs in the polycrystalline material in addition to that which has already occurred in drying

• In the firing of traditional ceramics, a glassy phase forms among the crystals which acts as a binder

GLAZING

Application of a ceramic surface coating to make the piece more impervious to water and enhance its appearance

• The usual processing sequence with glazed ware is: 1. Fire the piece once before glazing to harden the

body of the piece2. Apply the glaze3. Fire the piece a second time to harden the glaze

PROCESSING OF NEW CERAMICS

• The manufacturing sequence for the new ceramics can be summarized in the following steps: 1. Preparation of starting materials2. Shaping3. Sintering4. Finishing

• While the sequence is nearly the same as for the traditional ceramics, the details are often quite different

PREPARATION OF STARTING MATERIALS

• Strength requirements are usually much greater for new ceramics than for traditional ceramics

• Therefore, the starting powders must be smaller and more uniform in size and composition, since the strength of the resulting ceramic product is inversely related to grain size

• Greater control of the starting powders is required • Powder preparation includes mechanical and

chemical methods

SHAPING OF NEW CERAMICS

• Many of the shaping processes for new ceramics are borrowed from powder metallurgy (PM) and traditional ceramics – PM press and sinter methods have been adapted to the

new ceramic materials • And some of the traditional ceramics forming

techniques are used to shape the new ceramics, such as: slip casting, extrusion, and dry pressing

• The processes described here are not normally associated with the forming of traditional ceramics, although several are associated with PM

HOT PRESSING

Similar to dry pressing except it is carried out at elevated temperatures so sintering of the product is accomplished simultaneously with pressing

• This eliminates the need for a separate firing step

• Higher densities and finer grain size are obtained, but die life is reduced by the hot abrasive particles against the die surfaces

©2002 John Wiley & Sons, Inc. M P Groover, “Fundamentals of Modern

Manufacturing 2/e”

ISOSTATIC PRESSING

Uses hydrostatic pressure to compact the ceramic powders from all directions

• Avoids the problem of nonuniform density in the final product that is often observed in conventional uniaxial pressing

• Same process used in powder metallurgy

POWDER INJECTION MOLDING (PIM)

Ceramic particles are mixed with a thermoplastic polymer, then heated and injected into a mold cavity

• The polymer acts as a carrier and provides flow characteristics for molding

• Upon cooling which hardens the polymer, the mold is opened and the part is removed

• Because temperatures needed to plasticize the carrier are much lower than those required for sintering the ceramic, the piece is green after molding

• The plastic binder is removed and the remaining ceramic part is sintered

SINTERING OF NEW CERAMICS

• Since the plasticity needed to shape the new ceramics is not normally based on water, the drying step required for traditional green ceramics can be omitted for most new ceramic products

• The sintering step is still very much required • Functions of sintering are the same as before:

1. Bond individual grains into a solid mass2. Increase density3. Reduce or eliminate porosity

FINISHING OPERATIONS FOR NEW CERAMICS

• Parts made of new ceramics sometimes require finishing, which has one or more of the following purposes: 1. Increase dimensional accuracy 2. Improve surface finish3. Make minor changes in part geometry

• Finishing usually involves abrasive processes – Diamond abrasives must be used to cut the

hardened ceramic materials

CEMENTED CARBIDES

A family of composite materials consisting of carbide ceramic particles imbedded in a metallic binder

• Classified as metal matrix composites because the metallic binder is the matrix which holds the bulk material together

• However, the carbide particles constitute the largest proportion of the composite material, normally between 80% and 95% by volume

A BINDER IS NEEDED FOR CEMENTED CARBIDES

• The carbide powders must be sintered with a metal binder to provide a strong and pore free part ‑– Cobalt works best with WC, while nickel is better with TiC

and Cr3C2 • Usual proportion of binder metal is 4% up to 20%• Powders of carbide and binder metal are thoroughly

mixed wet in a ball mill to form a homogeneous sludge

• The sludge is then dried in a vacuum or controlled atmosphere to prevent oxidation in preparation for compaction

COMPACTION

• Most common process is cold pressing, used for high production of cemented carbide parts such as cutting tool inserts – Dies must be oversized to account for shrinkage

during sintering (shrinkage can be 20% or more)– For high production, the dies are made with

WC Co liners to reduce wear‑– For smaller quantities, large flat sections may be

pressed and then cut into smaller pieces– Other methods: isostatic pressing and hot pressing

SINTERING OF WC-CO

• It is possible to sinter WC (and TiC) without a metal binder, but the resulting material is less than 100% of true density – Using a binder yields a structure virtually free of

porosity • Sintering of WC Co involves liquid phase sintering ‑

– The usual sintering temperatures for WC Co are ‑1370 1425‑ C (2500 2600‑ F), which is below cobalt's melting point of 1495C (2716F)

– Thus, the pure binder metal does not melt at the sintering temperature

Figure 17.11 WC Co phase diagram‑ ‑

SINTERING OF WC-CO - CONTINUED

• However, WC dissolves in Co in the solid state so WC is gradually dissolved during the heat treatment, and its melting point is reduced so melting occurs – As the liquid phase forms, it flows and wets the WC

particles, further dissolving the solid – Presence of molten metal also serves to remove gases

from the internal regions of the compact • These mechanisms cause a rearrangement of the

remaining WC particles into a closer packing, which results in significant densification and shrinkage of the WC Co mass ‑

SECONDARY OPERATIONS

• Subsequent processing is usually required after sintering to achieve adequate dimensional control of the cemented carbide parts

• Grinding with a diamond or other very hard abrasive wheel is the most common secondary operation performed for this purpose

• Other secondary operations include – Electric discharge machining– Ultrasonic machining

The End

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