IE 337: Metal Casting - Spada UNS

Ceramics

2

This Time

 Ceramics

 Glass Processing

 Powder Processing: Ceramics and Metals

 Homework #5 on Thursday (2/25/10)

Ceramics

 General properties

 Hard

 High wear resistance

 Brittle

 High compressive strength

 High elastic modulus

 High temperature resistance

 Good creep resistance

 Low conductivity

 Low thermal expansion

 Good chemical inertness

Glasses Clay products

Refractories Abrasives Cements Advanced ceramics

-optical -composite reinforce -containers/

-household

-whiteware -bricks

-bricks for high T

(furnaces)

-sandpaper -cutting

-polishing

-composites

-structural engine -rotors -valves -bearings -sensors

Ceramics: Classification

Al₂O₃-SiO₂

Si₃N₄ ZrO₂ SiC

BN

Al₂O₃ AlN WC

Diamond ZrO₂

Al₂O₃

Common Ceramics

 Oxides: Al₂O₃, ZrO₂

 Nitrides: AlN, Si₃N₄, BN, TiN

 Carbides: WC, SiC, TiC, TaC

 Glasses: SiO₂ + others

 Carbon: Graphite, Diamond

Processed as powders

15m

sinter

Whiteware Ceramics

 Clay

 Quartz

 Feldspar

 Processing

 Water addition, mixing

 Air removal

 Shaping

 Drying

 Coating

 Firing

 Products

 Brick

 Structural Tile

 Drain / sewer pipe

 Decorative applications

 Bath / kitchen structures

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Refractory Material

 Retain properties at high temperature

 Mechanical

 Chemical

 Products

 Fire brick

 Insulating fibers

 Refractory linings

 Coatings

 Silica

 Alumina

 Magnesium Oxide

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Abrasives

 High hardness

 Examples

 Silicon carbide

 Aluminum oxide

 Cubic boron nitride

 Roughing Applications

 Grinding

 Cutting

 Water-jet

 Sawing

 Coatings

 Super-Finishing

 Honing

 Lapping

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Glasses

 Amorphous solid

 Vitreous (noncrystalline) structure

 Amorphous

 Cooled to semi-solid condition without crystallization

 Subject to creep

 Silica Glass

 Optical properties

 Thermal stability

 Products

 Window glass

 Fiber optics

 Chemical containers

 Lenses

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Glass Ceramics

 Crystalline solid

 0.1 to 1.0 micron grains

 Use of nucleating agents

 Glass Ceramic

 Efficient processing in glassy state

 Net shape process

 Good mechanical properties versus glass

 Low porosity

 Low thermal expansion

 Higher resistance to thermal shock

 Products

 Cookware

 Heat exchangers

 Missile radomes

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Cermets

 Combination of metals & ceramics

 “Cemented” carbides

 Bound with high temperature metal

 Properties

 High hardness

 High temperature resistance

 Improved toughness

 Improved strength

 Improved shock resistance

 Applications

 Crucibles

 Jet nozzles

 High temperature brakes

 Production

 Press powder in metal mold

 Sintering in controlled atmosphere

WC-Co

GLASS

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Shaping Methods for Glass

 Methods for shaping glass are different from those used for traditional and new ceramics

 Glassworking: principal starting material is silica

 Usually combined with other oxide ceramics that form glasses

 Heated to transform it from a hard solid into a viscous liquid; it is then shaped into the desired geometry while in this fluid condition

 When cooled and hard, the material remains in the amorphous state rather than crystallizing

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The typical process sequence in glassworking:

(1) preparation of raw materials and melting,

(2) shaping, and

(3) heat treatment

Glassworking Processes

 Piece Ware

 Flat and Tubular Glass

 Glass Fibers

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Piece Ware Shaping Processes

 Spinning – similar to centrifugal casting

 Pressing – for mass production of flat products such as dishes, bake ware, and TV faceplates

 Blow forming – for production of smaller-mouth containers such as beverage bottles and

incandescent light bulbs

 Casting – for large items such as large

astronomical lenses that must cool very slowly to avoid cracking

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Spinning of funnel shaped glass parts such as back sections of ‑ cathode ray tubes for TVs and computer monitors:

(1) gob of glass dropped into mold; and

(2) rotation of mold to spread molten glass on mold surface

Spinning

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Pressing of flat glass pieces: (1) glass gob is fed into mold from furnace; (2) pressing into shape by plunger; and (3) plunger is retracted and finished product is removed (symbols v and F indicate motion (velocity) and applied force)

Pressing

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Blow forming sequence: (1) gob is fed into inverted mold cavity; (2) mold is covered; (3) first blowing step; (4) partially formed piece is reoriented and transferred to second blow mold, and (5) blown to final shape

Blow Forming

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Casting

 A low viscosity glass can be poured into a mold

 Uses: massive objects, such as astronomical lenses and mirrors

 After cooling and solidifying, the piece must be finished by lapping and polishing

 Casting of glass is not often used except for special jobs

 Smaller lenses are usually made by pressing

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Starting glass from melting furnace is squeezed through opposing rolls whose gap determines sheet thickness, followed by grinding/ polishing

Rolling

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Molten glass flows onto the surface of a molten tin bath, where it spreads evenly, into a

uniform thickness and smoothness - no grinding or polishing is needed

Float Process

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Forming of Glass Fibers

Products can be divided into 2 categories:

1. Discontinuous fibrous glass for insulation and air filtration, in which the fibers are in a

random, wool like condition‑

 Produced by centrifugal spraying

2. Long continuous filaments suitable for fiber reinforced plastics, yarns, fabrics, and fiber optics

 Produced by drawing

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Continuous glass fibers of small diameter are produced by

pulling strands of molten glass

through small orifices in a heated plate made of a

platinum alloy

Drawing

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Heat Treatment

Annealing to eliminate stresses from temperature gradients

 Annealing temperatures are around 500C followed by slow cooling

Tempering to make the glass more resistant to scratching and breaking due to compressive stresses on its surfaces

 Heating to a temperature above annealing, followed by quenching of surfaces by air jets

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Finishing Operations

 Glass sheets often must be ground and

polished to remove surface defects and scratch marks and to make opposite sides parallel

 Decorative and surface processes performed on certain glassware products include:

 Mechanical cutting and polishing operations; and sandblasting

 Chemical etching (with hydrofluoric acid, often in combination with other chemicals)

 Coating (e.g., coating of plate glass with aluminum or silver to produce mirrors)

Figure 16.1 A collection of powder metallurgy parts (photo courtesy of Dorst America, Inc.).

Powder Processing Parts

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Powder Processing

1. The Characterization of Engineering Powders

2. Production of Metallic Powders

3. Conventional Pressing and Sintering

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Powder Metallurgy (PM)

Metal processing technology in which parts are produced from metallic powders

 Usual PM production sequence:

1. Pressing - powders are compressed into desired shape to produce green compact

 Accomplished in press using punch-and-die tooling designed for the part

2. Sintering – green compacts are heated to bond the particles into a hard, rigid mass

 Performed at temperatures below the melting point of the metal

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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 - ~ 97%

of starting powders are converted to product

 PM parts can be made with a specified level of porosity, to produce porous metal parts

 Examples: filters, oil impregnated bearings and ‑ gears

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More Reasons Why PM is Important

 Certain metals that are difficult to fabricate by other methods can be shaped by powder metallurgy

 Tungsten filaments for incandescent lamp bulbs are made by PM

 Certain alloy combinations and cermets made by PM cannot be produced in other ways

 Non-equilibrium microstructures possible

 PM compares favorably to most casting processes in dimensional control

 PM production methods can be automated for economical production

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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 distribution

 Particle shape and internal structure

 Surface area

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Measuring Particle Size

 Most common method uses screens of different mesh sizes

 Mesh count - refers to the number of openings per linear inch of screen

 A mesh count of 200 means there are 200 openings per linear inch

 Since the mesh is square, the count is equal in both directions, and the total number of openings per

square inch is 200² = 40,000

 Higher mesh count = smaller particle size

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Screen Mesh

Figure 16.2 Screen mesh for sorting particle sizes.

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Particle Shapes in PM

Figure 16.3 Several of the possible (ideal) particle shapes in powder metallurgy.

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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

 Easier flow of particles correlates with lower interparticle friction

 Lubricants are often added to powders to reduce

interparticle friction and facilitate flow during pressing 36

Particle Density Measures

 True density - density of the true volume of the material

 The density of the material if the powders were melted into a solid mass

 Bulk density - density of the powders in the loose state after pouring

 Because of pores between particles, bulk density is less than true density

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Packing Factor

Bulk density divided 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 spaces between larger ones, thus higher packing factor

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

 Pressure applied during compaction greatly increases packing of powders through rearrangement and

deformation of particles

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Porosity

Ratio of 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 possible existence of closed pores in some of the particles

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

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Chemistry and Surface Films

 Metallic powders are classified as either

 Elemental - consisting of a pure metal

 Pre-alloyed - each particle is an alloy

 Possible surface films include oxides, silica, adsorbed organic materials, and moisture

 As a general rule, these films must be removed prior to shape processing

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Production of Metallic Powders

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

 Any metal can be made into powder form

 Three principal methods by which metallic powders are commercially produced

1. Atomization 2. Chemical 3. Electrolytic

 In addition, mechanical methods are

occasionally used to reduce powder sizes

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Coventional PM Sequence

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.

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Blending and Mixing of Powders

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

 Blending - powders of same chemistry but possibly different particle sizes are

intermingled

 Different particle sizes are often blended to reduce porosity

 Mixing - powders of different elements/alloys are combined

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Compaction

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

 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

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Conventional Pressing in PM

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, (4) part

ejection.

Press for Conventional Pressing in PM

Figure 16.11 A 450 kN (50 ton) hydraulic press ‑ for compaction of PM parts (photo courtesy of Dorst America, Inc.).

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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

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Sintering Sequence

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.

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Sintering Cycle and Furnace

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

Limitations and Disadvantages

 High costs

 High tooling and equipment costs

 Metallic powders are expensive

 Typically requires a unique material or geometry to justify

 Problems in storing and handling metal powders

 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

 This is true for traditional punch and die

 Variations in density throughout part may lead to yield issues especially for complex geometries

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Interparticle Friction and Powder Flow

 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

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Angle of Repose

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.

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Powder Injection Molding

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shape

flow

dry/

debind

sinter (firing)

powder final

CERAMICS

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

Ceramics Processing

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Slip Casting

A suspension of ceramic powders in water,

called a slip, is poured into a porous plaster of paris mold where the water from the mix is absorbed to form a firm layer of clay

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, mold not drained

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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

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SLIP CASTING

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Tape Casting

Polyester Film Carrier

Slip Dried Tape

Doctor Blade

Polyester Film Roll

Fabrication process for thin ceramic sheets

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Miniaturization of Complex Circuits

 High Temperature Co-Fired Ceramic (HTCC)

 Low Temperature Co-Fired Ceramic (LTCC)

 Thick film metal traces are printed on several tape layers of ceramic and are co-fired

 Tape layers are electrically connected through vias

 Significant miniaturization of circuit form factor with this technology

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Extrusion

 Compression of clay through a die orifice to produce long sections of uniform

cross section‑

 Products: hollow bricks, shaped tiles, drain pipes, tubes, drill bit blanks, and insulators

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Extruder Sectional View

Components and features of a (single screw) extruder for ‑ plastics and elastomers

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Ceramic Extrusion: Examples

cordierite

catalytic converter

50 cells/cm²

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Powder Injection Molding (PIM)

 Ceramic particles are mixed with a

thermoplastic polymer, then heated and

injected into a mold cavity. Polymer provides flow characteristics for molding

Mold-Filling Interactions

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Air trap Weld-line

Short shot

Flashing Filler-

polymer separatio n

Jetting

Die Pressing

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Semi-Dry Pressing

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

 Joining

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Next Time

Chapter 30 & 31