Materials Processing - PowerPoint PPT Presentation

Loading...

PPT – Materials Processing PowerPoint presentation | free to view - id: 21c343-OTg2M



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Materials Processing

Description:

The Fe-iron carbide phase diagram shows the eutectoid region. The horizontal line at the eutectoid temp., labeled A1, is the lower critical temperature (LCT) ... – PowerPoint PPT presentation

Number of Views:4687
Avg rating:5.0/5.0
Slides: 83
Provided by: DavidRe3
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Materials Processing


1
Materials Processing
Boeing 757 Landing Gear Door Beam Hinge
  • Chapter 14

2
Outline
  • Metal Forming Techniques
  • Casting Process
  • Miscellaneous Processes Powder Metallurgy and
    Welding
  • Thermal Processing Metals
  • Hardenability
  • Polymer Additives
  • Ceramic Fabrication Methods
  • Glass Forming
  • Particulate Forming

3
Metal Fabrication Techniques Overview
4
Forming
  • Forming operations (forging, rolling, drawing,
    extrusion) are where the shape of a metal is
    changed by plastic deformation.
  • Forming processes are commonly classified into
    hot-working and cold-working operations.

5
Hot Working
  • Hot-working refers to processes where metals are
    plastically deformed above their
    recrystallization temperature. This allows the
    material to recrystallize during deformation and
    prevents the materials from strain hardening the
    yield strength and hardness are not increased,
    while ductility is retained.
  • Hot-working processes rolling, extrusion or
    forging typically are used in the first step of
    converting a cast ingot into a wrought product.
  • Deformation energy requirements are less than for
    cold work.
  • The lower limit of the hot working temperature is
    determined by its recrystallization temperature.
    The upper limit for hot working is determined by
    excessive oxidation, grain growth, undesirable
    phase transformation.

6
Recrystallization
  • Recrystallization is the formation of a new set
    of strain-free and equiaxed grains that have low
    dislocation densities (pre-cold work state).
  • The driving force to produce the new grain
    structure is the internal energy difference
    between strained and unstrained material.
  • The new grains form as very small nuclei and grow
    until they consume the parent material.
  • Recrystallization temperature is 1/3 ltTm lt1/2.

7
Cold Working
  • When cold-working is excessive, the metal will
    fracture before reaching the final shape.
  • Cold-working operations are usually carried out
    in several steps with annealing used to soften
    the cold-worked metal and restore ductility.
  • A higher quality surface finish than hot working.
  • Closer dimensional control of the finished piece.
  • Cold-working of a metal results in an increase in
    strength or hardness and a decrease in ductility.

8
Forging
http//www.forging.it/
  • Forging is the process where metal (Fe, Ti, Al)
    is heated and shaped by plastic deformation
    (compressive forces). The compressive force
    typically comes from hammer blows or a press.
  • Forged articles have outstanding grain structures
    and the best combination of mechanical
    properties. Forging refines the grain structure
    and improves physical properties of the metal.
  • With proper design, the grain flow can be
    oriented in the direction of principal stresses
    encountered in actual use. Grain flow is the
    direction of the pattern that the crystals take
    during plastic deformation.

9
Forging
  • Physical properties (such as strength, ductility
    and toughness) are much better in a forging than
    in the base metal that has crystals randomly
    oriented.
  • Forgings are consistent from piece to piece,
    without any of the porosity, voids, inclusions
    and other defects. Also coating operations such
    as plating or painting are straightforward due to
    a good surface that needs very little
    preparation.
  • The forge or smithy is the workplace of a smith
    or a blacksmith. A basic smithy contains a forge
    for heating the metals to a temperature where
    work hardening ceases to accumulate, an anvil (to
    lay the metal pieces on while hammering), and a
    slack tub (to rapidly cool and harden forged
    metal pieces). Tools include tongs to hold the
    hot metal and hammers to strike the hot metal.

10
Die Forging
  • Most forging processes begin with open die
    forging.
  • Open die forging shapes heated metal parts
    between a top die attached to a ram and a bottom
    die attached to a hammer anvil or giant hydraulic
    press bed.
  • Metal parts are worked above their
    recrystallization temperatures (ranging from
    1900F to 2400F for steel) and gradually shaped
    into the desired configuration through hammering
    or pressing.
  • While impression or closed die forging confines
    the metal in dies, open die forging is never
    completely confined or restrained in the dies.
  • Wrenches, automotive crankshafts and piston
    connecting rods are typical objects formed by
    forging.
  • Some disadvantages of forging are the high cost
    (dies) and high residual stress produced.

Closed die forging - The shaping of hot metal
within the walls of two dies that come together
to completely enclose the work piece.
11
The Open Die Forging Process
  • Steps to produce a typical spindle-shaped part
  • Rough forging a heated billet between flat dies
    to the maximum diameter dimension.
  • A "fuller" tool marks the starting "step"
    locations on the fully rounded workpiece.
  • Forging or "drawing" down the first step to size.
  • The second step is drawn down to size. Note how
    the part elongates with each process step as the
    material is being displaced.
  • "Planishing" the rough forging for a smoother
    surface finish and to keep stock allowance to a
    minimum.

12
Rolling
  • The process of plastically deforming a metal by
    passing it between rollers a reduction in
    thickness results from compressive stresses
    exerted by the rolls.
  • This is the most widely used metalworking process
    because it lends itself to high production and
    close control of the final product.
  • After extraction processes, many molten metals
    are solidified by casting into large ingot molds.
    The ingots are normally subjected to hot rolling
    to produce a flat sheet or slab. These are more
    convenient shapes for subsequent metal forming
    operations (extrusion, forging, drawing).

13
Hot Rolling Cold Rolling
  • The principal rolling processes are hot rolling
    and cold rolling.
  • Hot rolling is the most common method of refining
    the cast structure of ingots and billets to make
    primary shapes.
  • Bars of circular or hexagonal cross section like
    I beams, channels, and railroad rails are
    produced in great quantity by hot rolling with
    grooved rolls.
  • Cold rolling is most often a secondary forming
    process that is used to make bar, sheet, strip
    and foil with superior surface finish and
    dimensional tolerances.

14
Extrusion
  • A bar of metal is forced through a die orifice by
    a compressive force that is applied to a ram
  • The extruded piece that emerges has the desired
    shape and a reduced cross-sectional area.
  • Extrusion products include rods and tubing, but
    shapes of irregular cross-sections may be
    produced form the more readily extrudable metals,
    like Al.
  • Extrusion is increasingly utilized in the working
    of metals difficult to form, like stainless
    steels, Ni-based alloys, and other
    high-temperature materials

15
Extrusion of Tubing
  • To produce tubing by extrusion, a mandrel must be
    fastened to the end of the extrusion ram
  • The mandrel extends to the entrance of the
    extrusion die, and the clearance between the
    mandrel and the die wall determines the wall
    thickness of the extruded tube
  • One method of extruding a tube is to use a hollow
    billet for the starting material

16
Drawing
  • Drawing is the pulling of a metal piece through a
    die having a tapered bore by means of a tensile
    force that is applied on the exit side
  • Rod, wire and tubing products are commonly
    fabricated in this way.
  • Wiredrawing usually starts with a coil of
    hot-rolled rod
  • Draw speeds vary from about 30 to 300 ft/min
  • In general, the term wire refers to small
    diameter products under 5 mm that may be drawn
    rapidly on multiple-die machines.

17
Casting Methods
18
Casting
  • Casting ? a fabrication process whereby a totally
    molten metal is poured into a mold cavity having
    the desired shape upon solidification, the metal
    assumes the shape of the mold but experiences
    some shrinkage.
  • Casting techniques are used when
  • The finished shape is so large or complicated
    that any other method would be impractical
  • A particular alloy is so low in ductility that
    forming by either hot or cold working would be
    difficult
  • In comparison to other fabrication processes,
    casting is the most economical.

19
Sand Casting
  • Sand can withstand T gt1600ºC
  • Sand is inexpensive and easy to mold.
  • A two-piece mold is formed by packing sand
    around a pattern that has the shape of the
    intended casting.
  • Often used for large parts, auto engine blocks

20
Die Casting
  • The liquid metal is forced into a mold (die)
    under pressure at a relatively high velocity,
    then allowed to solidify with the pressure
    maintained.
  • A two-piece permanent steel mold is used when
    clamped together, the two pieces form the desired
    shape.
  • When complete solidification has been achieved,
    the mold pieces are opened and the cast piece is
    ejected.
  • Rapid casting rates are possible, making this an
    inexpensive method a single set of molds may be
    used for thousands of castings.
  • This technique lends itself only to relatively
    small pieces and to alloys of low melting points
    such as Zn, Al and Mg

21
Investment Casting (lost-wax casting)
Investment Casting (low volume, complex
shapes like jewelry, turbine blades, jewelry and
dental crowns and inlays, and blades for gas
turbine and jet engine impellers)
  • Stage I Mold formed by pouring plaster
    of paris around wax pattern. Plaster
    allowed to harden.

Plaster die formed around wax prototype
  • Stage II Wax is melted and then
  • poured from moldhollow mold cavity
    remains.

Stage III Molten metal is poured into
mold and allowed to solidify.
22
Investment Casting (lost-wax casting)
23
Investment Casting (lost-wax casting)
24
Investment Casting (lost-wax casting)
25
Continuous Casting
  • Continuous casting (also called strand casting)
    is the process whereby molten steel is solidified
    into a "semi-finished" billet, bloom or slab for
    subsequent rolling in the finishing mills.
  • In the continuous casting process, molten metal
    is poured from the ladle into the tundish and
    then through a submerged entry nozzle into a mold
    cavity.
  • The mold is water-cooled so that enough heat is
    extracted to solidify a shell of sufficient
    thickness. The shell is withdrawn from the bottom
    of the mold at a "casting speed" that matches the
    inflow of metal, so that the process ideally
    operates at steady state. Below the mold, water
    is sprayed to further extract heat from the
    strand surface, and the strand eventually becomes
    fully solid when it reaches the ''metallurgical
    length''.

26
Casting Defects Cavities
  • Blowholes, pinholes, shrinkage cavities,
    porosity
  • Blowholes and pinholes are holes formed by gas
    entrapped during solidification.
  • Shrinkage cavities are cavities that have a
    rougher shape and sometimes penetrate deep into
    the casting.
  • Shrinkage cavities are caused by lack of proper
    feeding or non-progressive solidification.
  • Porosity is pockets of gas inside the metal
    caused by micro-shrinkage, e.g. dendritic
    shrinkage during solidification.

27
Dendrites of a shrinkage cavity in an aluminum
alloy
  • Discontinuities in castings that exhibit a size,
    shape, orientation, or location that makes them
    detrimental to the useful service life of the
    casting.
  • Some casting defects are remedied by minor repair
    or refurbishing techniques, such as welding.
  • Other casting defects are cause for rejection of
    the casting.

28
Casting Defects Discontinuities
  • Cracks in casting and are caused by hot tearing,
    hot cracking, and lack of fusion (cold shut)
  • A hot tear is a fracture formed during
    solidification because of hindered contraction.
  • A hot crack is a crack formed during cooling
    after solidification because of internal stresses
    developed in the casting.
  • Lack of fusion is a discontinuity caused when two
    streams of liquid in the solidifying casting meet
    but fail to unite.
  • Rounded edges indicate poor contact between
    various metal streams during filling of the mold.

29
Cast and Wrought Alloys
  • The distinctive metallurgical characteristics of
    castings are acquired during solidification,
    whereas with wrought materials, they are acquired
    during mechanical deformation.
  • The principal metallurgical difference between
    castings and wrought materials is that castings
    lack homogeneity.
  • They do not have the benefit of hot work to
    accelerate the diffusion of the chemical elements
    to achieve homogenization.
  • Cast alloys require significantly longer soaking
    times to achieve homogenization.
  • Cast alloys frequently contain more silicon to
    improve the fluidity of the molten metal.
  • Solidified castings contain high residual
    stresses from solid shrinkage, unless they are
    removed by a stress relief annealing process.

30
Metal Fabrication Methods
Nanophase Al-7.5Mg
31
Welding
  • In welding, two or more metal parts are joined to
    form a single piece when one-part fabrication is
    expensive or inconvenient.
  • Both similar and dissimilar metals may be welded.
  • The joining bond is metallurgical (involving some
    diffusion) rather than just mechanical, as with
    riveting and bolting.
  • A variety of welding methods exist, including arc
    and gas welding, as well as brazing and
    soldering.
  • Brazing is a joining process whereby a filler
    metal or alloy is heated to melting temperature
    above 450 C (840 F).
  • Soldering is a process where two or more metals
    are joined together by melting and flowing a
    filler metal into the joint, the melting point of
    the filler metal is below 400 C (752 F).
  • During arc and gas welding, the work pieces to be
    joined and the filler material are heated to a
    sufficiently high temperature to cause both to
    melt upon solidification, the filler material
    forms a fusion joint between the work pieces.

32
Heat-Affected Zone (HAF)
  • The heat-affected zone is the narrow region of
    the base metal adjacent to the weld bead, which
    is metallurgically altered by the heat of
    welding.
  • The heat-affected zone is usually the major
    source of metallurgical problems in welding.
  • The width of the heat-affected zone depends on
    the amount of heat input during welding and
    increases with the heat input. If the material
    was previously cold worked, the HAF may have
    experienced recrystallization and grain growth,
    and a diminishment of strength, hardness and
    toughness.

Generally, the heat-affected zone varies from 1.5
mm to 6.5 mm wide (0.06 in to 0.25 in)
33
Microstructural Changes Nearby HAF
  • For steels, the material in this zone may have
    been heated to temperatures sufficiently high so
    as to form austenite. Upon cooling to room
    temperature, the microstructural products that
    form depend on cooling rate and alloy
    composition.
  • For plain carbon steels, normally pearlite and a
    proeutectoid phase will be present.
  • For alloy steels, one microstructural product
    phase may be martensite, which is ordinarily
    undesirable because it is so brittle.
  • Upon cooling, residual stresses may form in this
    region that weaken the joint.
  • It can also lead to loss of corrosion resistance
    in stainless steels and nickel-base alloys.

34
Preheating and Post-Weld Heat Treatment
  • With carbon and low-alloy steels, the rapid
    cooling rate from the welding temperature is
    similar to quenching in heat treatment operations
  • The higher the carbon or alloy content, the more
    easily martensite is formed and the more brittle
    the martensite is
  • This situation may easily cause cracking as the
    steel cools down.
  • Steels that are susceptible to cracking must be
    preheated to cushion the effects of martensite
    formation.
  • They are also post-weld heat treated to temper
    (improve the toughness) any martensite that is
    formed and additionally stress relieve the joint.
  • Stress Relieving - Always done below the
    transformation temperature of the metal to
    minimize the welds residual stress. The
    temperature is held for roughly an hour until the
    residual stresses are minimized, then cooled very
    slowly to prevent new stresses from setting up in
    the metal.  

35
Carbon Equivalent
  • The carbon equivalent is a formula based on
    chemical composition that determines the need to
    preheat and post-weld carbon and low-alloy
    steels.
  • The higher the carbon equivalent, the greater the
    tendency toward cracking in the heat-affected
    zone.
  • Plain carbon steels with a carbon equivalent lt
    0.4 to 0.5 are considered readily weldable
    without the need for preheating or post-weld heat
    treatment.

Carbon Equivalent (CE) C Mn/6 Ni/20
Cr/10 Cu/40 Mo/50 V/10
36
Cracking in Welding
  • Cracking is rarely tolerated and must be removed
    by grinding
  • Crack formation is aggravated
  • by welding fixtures that do not permit
    contraction of the weld during cooling,
  • by narrow joints with large depth-to-width
    ratios,
  • by poor ductility of the deposited weld metal,
  • or by a high coefficient of thermal expansion
    coupled with low-heat conductivity in the parent
    metal

37
Hydrogen Cracking
  • Hydrogen cracking occurs in the heat-affected
    zone of some steels as hydrogen diffuses into
    this region when the weld cools
  • Hydrogen cracking is caused by atomic hydrogen.
  • The sources of atomic hydrogen are
  • organic material,
  • chemically bonded water in the electrode coating,
  • absorbed water in the electrode coating,
  • and moisture on the steel surface at the location
    of the weld

38
Methods of Avoiding Hydrogen Cracking
  • Using low-hydrogen electrodes, which includes
    baking and storing them in a low-temperature
    oven.
  • Preheating the surface of the steel before
    welding to remove moisture.
  • Post-weld heat treating immediately to force the
    hydrogen to escape.
  • Peening immediately after each pass is also
    beneficial because it induces compressive
    stresses and offsets the tendency toward cracking.

39
Powder Metallurgy
  • A fabrication technique involves the compaction
    of powdered metal, followed by a heat treatment
    to produce a more dense piece.
  • Powder metallurgy is especially suitable for
    metals
  • having low ductilities
  • having high melting temperatures
  • Production of P/M Parts
  • Preparation of Metal Powders
  • Compaction (pressing)
  • Sintering (densification) at elevated temperature

40
Thermal Processing of Metals
Common forms of heat treating processes.
41
Heat Treatment Temperature-Time Paths
A
  • Full Annealing
  • Quenching
  • Tempering
  • (Tempered Martensite)

P
B
A
100
50
0
b)
105 27.8 hrs
42
c14f04
Annealing
  • Annealing describes a heating, holding and
    cooling process to achieve specific metallurgical
    results.
  • The Fe-iron carbide phase diagram shows the
    eutectoid region.
  • The horizontal line at the eutectoid temp.,
    labeled A1, is the lower critical temperature
    (LCT). All austenite will have transformed into
    ferrite and cementite phases below the LCT.
  • The phase boundaries denoted A3 and Acm represent
    the upper critical temperature lines for
    hypoeutectoid and hypereutectoid steels. For
    temperatures above these boundaries, only
    austenite will exist.

43
Normalizing
  • An annealing treatment called normalizing is used
    to refine the grains (decrease the average grain
    size) and produce a more uniform size
    distribution fine grained pearlitic steels are
    tougher than coarse-grained ones.
  • To normalize, the temperature must be raised
    roughly 55 degrees above the upper critical
    temperature (above A3 or Acm depending on
    composition).

44
c14f05
Hardenability -- Steels
Hardenability measure of the ability to form
martensite Jominy end quench test used to
measure hardenability.
Plot hardness versus distance from the quenched
end.
45
c14f07
Hardness Changes with Distance
Correlation of hardenability and continuous
cooling information for and iron-iron carbon
alloy of eutectoid composition.
46
c14f08
Hardenability vs Alloy Composition
"Alloy Steels" (4140, 4340, 5140, 8640) --
contain Ni, Cr, Mo (0.2 to 2 wt) --
these elements shift the "nose" to longer times
(from A to B) -- martensite is easier to form
Hardenability curves for 5 alloys each with 0.4
wt C.
47
c14f09
  • Hardenability curves for 8600 series alloys where
    only carbon content is varied.
  • Hardness increases with carbon content.
  • Also, during production of steel, there is always
    a minor variation in composition and average
    grain size from one batch to another this
    results in some scatter of measured hardness
    values.
  • Hardenability band for an 8640 steel indicating
    maximum and minimum limits for hardness.

48
Influences of Quenching Medium Specimen Geometry
Effect of quenching medium
Medium air oil water
Hardness low moderate high
Severity of Quench low moderate high
49
Polymer Formation
  • Thermoplastic - can be reversibly cooled
    reheated, i.e. recycled
  • heat until soft, shape, then cool
  • ex polyethylene, polypropylene, polystyrene.
  • Thermoset - when heated, forms a molecular
    network (chemical reaction)
  • degrades (doesnt melt) when heated
  • ex urethane, epoxy
  • There are two types of polymerization
  • Addition polymerization
  • Condensation polymerization

50
Addition Polymerization
  • Initiation

50
51
Condensation Polymerization
51
52
Polymer Additives
  • Improve mechanical properties, processing,
    durability.
  • Fillers - Added to improve tensile strength
    abrasion resistance, toughness decrease cost.
    Examples carbon black, silica gel, wood flour,
    glass, limestone, talc.
  • Plasticizers - Added to reduce the glass
    transition temperature Tg below room temperature.
    Presence of plasticizer transforms brittle
    polymer to a ductile one. Commonly added to PVC.
  • Stabilizers Antioxidants, UV protection
  • Lubricants - Added to allow easier processing,
    polymer slides through dies easier (sodium
    stearate).
  • Colorants - Dyes and pigments
  • Flame Retardants - Substances containing
    chlorine, fluorine and boron.

53
Processing Plastics Compression Molding
  • Thermoplastics and thermosets
  • polymer and additives placed in mold cavity
  • mold heated and pressure applied
  • fluid polymer assumes shape of mold

53
54
Processing Plastics Injection Molding
  • Thermoplastics and some thermosets
  • when ram retracts, plastic pellets drop from
    hopper into barrel
  • ram forces plastic into the heating chamber
    (around the spreader) where the plastic melts as
    it moves forward
  • molten plastic is forced under pressure
    (injected) into the mold (die) cavity where it
    assumes the shape of the mold

Barrel
http//en.wikipedia.org/wiki/Injection_mold
54
55
Processing Plastics Extrusion
  • thermoplastics
  • plastic pellets drop from hopper onto the turning
    screw
  • plastic pellets melt as the turning screw pushes
    them forward by the heaters
  • molten polymer is forced under pressure through
    the shaping die to form the final product
    (extrudate)

55
56
Extrusion of Plastics
  • In the extrusion of plastics, raw thermoplastic
    material in the form of small beads (resin) is
    gravity fed from a top mounted hopper into the
    barrel of the extruder. Additives (colorants and
    UV inhibitors in either liquid or pellet form)
    are often used and can be mixed into the resin
    prior to arriving at the hopper.
  • The material enters through the feed throat (an
    opening near the rear of the barrel) and comes
    into contact with the screw. The rotating screw
    (normally turning at up to 120 rpm) forces the
    plastic beads forward into the barrel which is
    heated to the desired melt temperature of the
    molten plastic (which can range from 200C/400F
    to 275C/530F depending on the polymer).
  • In most processes, a heating profile is set for
    the barrel in which three or more independent PID
    controlled heater zones gradually increase the
    temperature of the barrel from the rear (where
    the plastic enters) to the front. This allows the
    plastic beads to melt gradually as they are
    pushed through the barrel and lowers the risk of
    overheating which may cause degradation in the
    polymer.

57
Processing Plastics Blown-Film Extrusion
  • The manufacture of plastic film for products like
    shopping bags is done using a blown film line.
  • This process is the same as a regular extrusion
    process up until the die. The die is an upright
    cylinder with a circular opening similar to a
    pipe die. The diameter can be a few cm to more
    than 3 m across. The molten plastic is pulled
    upward from the die by a pair of rollers high
    above the die.
  • Changing the speed of these rollers changes the
    gauge (wall thickness) of the film. Around the
    die sits an air-ring. The air-ring cools the film
    as it travels upward. In the center of the die is
    an air outlet where compressed air can be forced
    into the center of the extruded circular profile,
    creating a bubble.

57
58
c14f15
Ceramic Fabrication Methods
59
Glass Properties Viscosity
  • Glass or noncrystalline materials do not solidify
    in the same sense as crystalline materials. Upon
    cooling, a glass becomes more and more viscous
    with decreasing temperature.

Viscosity, h ,describes a fluid's internal
resistance to flow and may be thought of as a
measure of fluid friction. -- relates shear
stress (?) and velocity gradient (dv/dy)
h has units of (Pa-s)
60
Glass Properties
rdensity
Specific volume (1/r) vs Temperature (T)
Crystalline materials -- crystallize at
melting temp, Tm -- have abrupt change in
specific volume at Tm
Specific volume
Glasses -- do not crystallize --
change in slope in spec. vol. curve at
glass transition temperature, Tg --
transparent - no grain boundaries to
scatter light
solid
T
Tm
Tg
61
c14f17
Important in glass forming operations are the
viscosity-temperature characteristics of
glass. Temperatures Melting Point viscosity
10 Pa-s glass is fluid enough to be considered
liquid. Working Point viscosity 103 Pa-s
glass is easily deformed. Softening
Point viscosity 4x106 Pa-s max temp. glass
can be handled without altering
dimensions. Annealing Point viscosity 1012
Pa-s good atomic diffusion stress
relief. Strain Point viscosity 3 x 1013 Pa-s
below strain point, fracture will occur before
the onset of plastic deformation .
62
Log Glass Viscosity vs. Temperature
  • soda-lime glass 70 SiO2 balance Na2O
    (soda) CaO (lime)

Viscosity decreases with T
  • borosilicate (Pyrex) 13 B2O3, 3.5 Na2O,
    2.5 Al2O3
  • Vycor 96 SiO2, 4 B2O3
  • fused silica gt 99.5 wt SiO2

63
c14f18
Glass Blowing
  • Some glass blowing is done by hand.
  • The process is completely automated for the
    production of glass jars, bottles and light
    bulbs.
  • From a raw gob of glass, a parison (temporary
    shape) is formed by mechanical pressing in a
    mold.
  • This piece is inserted into a finishing or blow
    mold and forced to conform to the mold contours
    by the pressure created from a blast of air.
  • Drawing is used to form long glass parts (sheets,
    rods, tubing and fibers) that have a constant
    cross section.

64
Sheet Glass Forming
  • Sheet forming continuous casting
  • sheets are formed by floating the molten glass on
    a pool of molten tin

65
Heat Treating Glass
Annealing -- removes internal stresses
caused by uneven cooling. Tempering --
puts surface of glass part into compression --
suppresses growth of cracks from surface
scratches. -- sequence
66
Tempered Glass
  • Fully tempered glass is roughly 4 times stronger
    than annealed glass of the same thickness and
    configuration residual surface compression must
    be over 10,000 psi for 6mm thickness, according
    to ASTM C 1048.
  • Tempered glass is manufactured through a process
    of extreme heating and rapid cooling, making it
    harder than normal glass.
  • The typical process to produce tempered glass
    involves heating the glass to over 1,000 F, then
    rapidly cooling to lock the glass surfaces in a
    state of compression and the core in a state of
    tension.
  • When glass cools down to ambient temperature, the
    center plane of the glass contracts more than the
    surfaces. The contraction of the center plane
    pulls the surfaces into compression and the glass
    becomes very strong.
  • Tempered glass cannot be cut or drilled after
    tempering, and any alterations, such as
    edge-grinding, sandblasting or acid-etching, can
    cause premature failure.

67
Tempering Process
  • Fabrication occurs on electrically heated
    horizontal furnaces that heat the glass to a
    uniform temperature of roughly 1200F.
  • Ceramic rolls convey the glass through these
    furnaces at speeds regulated to ensure
    temperature uniformity and minimal optical
    distortions.
  • When the glass exits from the furnace, it is
    rapidly cooled by a series or air nozzles. This
    rapid cooling puts roughly 20 of the glass
    surface into a state of compression, with the
    center core in tension.

68
Shattered Tempered Glass
  • The brittle nature of tempered glass causes it to
    shatter into small oval-shaped pebbles when
    broken. This eliminates the danger of sharp
    edges. Due to this property, along with its
    strength, tempered glass is often referred to as
    safety glass.
  • Tempered glass breaks in a unique way. If any
    part of the glass fails, the entire panel
    shatters at once. This distinguishes it from
    normal glass, which might experience a small
    crack or localized breakage from an isolated
    impact.
  • Tempered glass might also fail long after the
    event that caused the failure.
  • Stresses continue to play until the defect
    erupts, triggering breakage of the entire panel.

69
Annealed Glass
  • Float glass (also called flat glass) has not
    been heat-strengthened or tempered.
  • Annealing float glass is the process of
    controlled cooling to prevent residual stress in
    the glass and is an inherent operation of the
    float glass manufacturing process.
  • Annealed glass can be cut, machined, drilled,
    edged and polished.
  • Annealing of glass is the process where the glass
    is heated and kept for a defined period of time
    to relive internal stresses.
  • Careful cooling under controlled conditions is
    essential to ensure that no stresses are
    reintroduced by chilling/cooling.

70
Different techniques for processing of advanced
ceramics.
The space shuttle makes use of 25,000 reusable,
lightweight, highly porous ceramic tiles that
protect the aluminum frame from the heat
generated during re-entry into the Earths
atmosphere.
71
Typical steps encountered in the processing of
ceramics.
Green ceramic - A ceramic that has been shaped
into a specific form but has not yet been
sintered.
72
Mechanical Properties of Advanced Ceramics
Typical Porcelain Composition (50) 1.
Clay (25) 2. Filler e.g. quartz (finely
ground) (25) 3. Fluxing agent (Feldspar)
-- aluminosilicates plus K, Na, Ca
-- upon firing - forms low-melting-temp.
glass
73
CEREC Technology
  • An optical 3D image is acquired with a small
    camera, directly in your mouth.
  • The computer and CEREC 3D software converts the
    digital picture to a three dimensional virtual
    model of your prepped tooth. Your dentist then
    designs your restoration right on screen using
    the software.
  • This software can handle single tooth
    restoration crowns, inlays (fillings), onlays
    (partial crowns), and veneers. After the design
    is complete, the data is transmitted via a
    wireless radio signal to the CEREC Milling Unit.
  • Diamond coated instruments mill a ceramic block
    to reproduce the design.
  • This is done during a single appointment using
    Computer Aided Design/Computer Aided Manufacture
    (CAD/CAM).

http//www.sirona.com/ecomaXL/index.php?siteSIRON
A_COM_cadcam_systems
74
Ceramic Materials
  • When creating CEREC restorations, you can choose
    from feldspar ceramics, glass ceramics and
    high-performance polymers.
  • They are biocompatible, clinically tested,
    durable and metal-free. Problems due to corrosion
    and incompatibility can be virtually ruled out.
  • The ceramic materials fulfill stringent standards
    in terms of fracture toughness, abrasion,
    aesthetics and machinability. Sirona has
    developed its own range of machinable ceramic
    blocks for the CEREC and inLab CAD/CAM systems.
  • Sirona inCoris materials consists of partially
    sintered framework ceramics they provides the
    basis for manufacturing high-precision
    all-ceramic crowns and bridge restorations made
    of aluminium and zirconium oxide.

75
Hydroplastic Forming
  • Hydroplastic forming - Processes where a moist
    ceramic clay body is formed into a useful shape.
  • Mill (grind) and screen constituents desired
    particle size.
  • Extrude the mass.
  • Dry and fire the formed piece.

76
Kaolinite
  • Clay is inexpensive.
  • Kaolinite is a clay mineral with the chemical
    composition Al2Si2O5(OH)4.
  • It is a layered silicate mineral, with one
    tetrahedral sheet linked through oxygen atoms to
    one octahedral sheet of alumina octahedra.
  • When water is added to clay, water molecules fit
    between layered sheets, reducing degree of van
    der Waals bonding (Can shear along vdW bonds
    more easily).
  • When external forces are applied, clay particles
    are free to move past one another, becoming
    hydroplastic.
  • Adding water to clay enables extrusion and slip
    casting.
  • Kaopectate, paper, pipes (smoking).

Structure of Kaolinite Clay
77
(No Transcript)
78
Drying and Firing
Drying as water is removed - interparticle
spacings decrease
shrinkage .
Drying too fast causes sample to warp or crack
due to non-uniform shrinkage
79
Slip Casting
  • A liquid clay body (a slip) is poured into a
    plaster mold and allowed to form a layer on the
    inside cavity of the mold.
  • In a solid cast mold, ceramic objects like
    handles and platters are surrounded by plaster on
    all sides with a reservoir for slip, and are
    removed when the solid piece is held within.
  • For a hollow cast mold, once the plaster has
    absorbed most of the liquid from the outside
    layer of clay the remaining slip is poured off
    for later use.
  • The cast piece is removed from the mold, trimmed
    and dried. This produces a green piece that is
    then fired, with or without decoration and glaze.
  • The technique is suited to the production of
    complex shapes, and is commonly used for toilets,
    basins, figurines and teapots. The technique can
    also be used for small scale production runs.

solid component
80
Powder Pressing used for both clay and non-clay
compositions. Powder (plus binder) compacted
by pressure in a mold -- Uniaxial compression
- compacted in single direction -- Isostatic
(hydrostatic) compression - pressure applied by
fluid - powder in rubber envelope -- Hot
pressing - pressure heat
Microstructure of a barium magnesium tantalate
(BMT) ceramic prepared using compaction and
sintering. (Courtesy Heather Shivey.)
81
Sintering
  • Sintering occurs during firing of a piece that
    has been powder pressed-- powder particles
    coalesce and of pore size is reduced.
  • Typically, ceramics with a small grain size are
    stronger than coarse-grained ceramics.
  • Finer grain sizes help reduce stresses that
    develop at grain boundaries due to anisotropic
    expansion and contraction.


Aluminum oxide powder -- sintered at
1700C for 6 minutes.
82
c14f28
Tape Casting
Tape casting - A process for making thin sheets
of ceramics using a ceramic slurry consisting of
binders, plasticizers, etc. The slurry is cast as
tape with the help of a blade onto a plastic
substrate. Used for integrated circuits and
capacitors Slip suspended ceramic particles
organic liquid
About PowerShow.com