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.

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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.

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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.
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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.
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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
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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
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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.

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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.

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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
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(No Transcript)
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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
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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
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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.)
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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.
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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