Manufacturing Processes for Engineering Materials 5th Edition in SI Units

1
Manufacturing Processes for Engineering
Materials (5th Edition in SI Units)

• Chapter 5 Metal-Casting Processesand Equipment
  Heat Treatment
• Understanding phase diagram phase,
  solidification, solid-solution, eutectic, Fe-C
  diagram, micro-structure (pearlite)
• Principle of casting cavity, filling, shrinkage
• Casting processes how to make and fill the mold?
• Heat treatment process TTT CCT diagram,
  Martensite, tempering, Pearlite, annealing, etc.

2
Solidification of Metals

• Pure metals have defined melting points and
  solidification takes place at a constant
  temperature.
• When temperature reduced to the freezing point,
  latent heat of fusion is given off.
• Alloys solidify over a range of temperatures.

3
Solid solutions

• Solute is (minor element) added to the solvent
  (major element) to form a solution.
• Substitutional solid solutions - size of the
  solute atom is similar to that of the solvent
  atom.
• Interstitial solid solutions - size of the solute
  atom is smaller than that of the solvent atom.
• Intermetallic compounds - solute atoms are
  present among solvent atoms in certain specific
  proportions.

4
Phase diagrams (Equilibrium Phase diagram)

• A phase diagram or equilibrium diagram shows the
  relationships among temperature, composition and
  phases present in an alloy system.
• Equilibrium is where a system remains constant
  over an indefinite period of time.
• Phase Physically chemically uniform state
• Example is a binary phase diagram
• where two elements are in the system.

• Complete solid-solution (Isomorphous) diagram
• Solid-solution ??? (???)
• a is the solid-solution of Cu and Ni
• Should satisfy Hume-Rothery rule

5
Two-phase alloys

• Two or more elements soluble in a solid state and
  alloying elements are uniformly distributed.
• Example - where lead is added to copper
• one phase has small amount of lead in solid
  solution in copper
• another phase in which lead particles are
  dispersed throughout
• Effect of alloying Related to
• dislocation movement

6

• Eutectic diagram with limited solubility
• Eutectic means easily-melt.
• Eutectic point lowest temperature for L-gtS
• Eutectic composition 61.9wtSn-38.1wtPb
• (commercial eutectic solder 63Sn-37Pb)
• We will only discuss briefly about
  microstructures.

7
Phase diagrams

• Lever rule
• Composition of phases can be found by lever rule.
• Weight fraction of solid is proportional to the
  distance between C0 and CL
• Weight fraction of liquid is proportional to the
  distance between CS and C0
• Note Composition changes with temperature

8
The iron-carbon system

• Iron-carbon binary system is represented by the
  iron-iron carbide phase diagram.
• Ferrite (BCC) Austenite (FCC) Cementite
  (Fe3C) are the phases.
• Ferrite has low carbon solubility. With high C
  composition, additional C is transformed to Fe3C,
  and pearlite microstructure occurs.
• Austenite has high carbon solubility. When
  cooling, C is transformed to Fe3C.
• Pearlite It is not a phase but micro-structure.
• Laminating structure of ferrite
  Fe3C (soft hard -gt Good mech. property)

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The iron-carbon system

• Ferrite
• Solid solution of bcc iron
• Relatively soft and ductile
• Austenite
• Allotropic transformation from the bcc to fcc
• Denser and is ductile at elevated temperatures
• Cementite (Fe3C)
• Hard and brittle intermetallic compound
• Can be alloyed with elements
• Martensite (Not shown in Phase diagram, will
  discuss later)

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The iron-iron carbide phase diagram

• Eutectoid reaction - single solid phase
  (austenite) is transformed into 2 solid phases
  (ferrite and cementite).
• (Eutectoid means Eutectic-like)
• (Eutectic L-gtS, Eutectoid S-gtS)
• Effects of alloying elements in iron
• The effect is to shift eutectoid temperature and
  eutectoid composition ( of carbon in steel at
  the eutectoid point).
• Alloying elements will lower eutectoid
  composition.

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Example 5.1Determining the amount of phases in
carbon steel
Determine the amount of gamma and alpha phases in
a 10-kg, 1040 steel casting as it is being cooled
slowly to the following temperatures (a) 1173.15
K, (b) 1001 K, and (c) 999 K. Solution Find the
weigh percentages of each phase by the lever
rule (c) At 999 K, no gamma phase will be
present.
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Cast Structures

• Cast structure developed during solidification of
  metals and alloys depends on
• composition of the particular alloy
• the rate of heat transfer
• flow of the liquid metal during the casting
  process

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Pure Metals

• Pure metals, with preferred texture at the cool
  mold wall.
• Solid-solution alloys.
• Structure obtained by heterogeneous nucleation
  of grains.
• Homogeneous vs heterogeneous nucleation
• Epitaxial vs Equiaxed
• Competitive growth
• Grain Boundary

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Alloys

• Pure metals are enhanced and modified by
  alloying.
• Solidification begins when temperature drops
  below the liquidus, TL, and complete when
  solidus, TS.
• Freezing range

15
Alloys

• Effects of cooling rate
• For high cooling rates, structure becomes finer
  with smaller dendrite arm spacing.
• For higher cooling rates structures developed are
  amorphous.
• The developed structures and grain size influence
  the properties of the casting.
• Above phenomena are related to diffusion and
  expressed using CCT diagram!

16
Structure-property relationships

• When alloy cooled slowly dendrite develops a
  uniform composition.
• For normal cooling cored dendrites are formed.

Columnar dendritic
Equiaxed dendritic
Equiaxed nondendritic
17
Fluid Flow and Heat Transfer- Fluid flow

• The following is a gravity casting system.
• 2 principles of fluid flow are relevant to gating
  design Bernoullis theorem and the law of mass
  continuity.

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Fluid Flow and Heat Transfer- Fluid flow

• Bernoullis theorem
• Based on - principle of conservation of energy
  - frictional losses in a fluid system
• Conservation of energy requires that,

h elevation p pressure at elevation v
velocity of the liquid? density of the fluid
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Fluid Flow and Heat Transfer- Fluid flow

• Mass continuity
• States that for an incompressible liquid the rate
  of flow is constant.
• Subscripts 1 and 2 pertain to two different
  locations in the system.

Q volumetric rate of flow A cross-sectional
area of the liquid stream v velocity of the
liquid
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Fluid Flow and Heat Transfer- Fluid flow

• Sprue profile
• Relationship between height and cross-sectional
  area at any point in the sprue is given by
• Velocity of the molten metal leaving the gate is
• When liquid level reached height x, gate velocity
  is

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Fluid Flow and Heat Transfer- Fluid flow

• Flow characteristics
• Fluid flow in gating systems is turbulence, as
  opposed to laminar flow. (Which flow is
  preferred?)
• Reynolds number, Re, is used to characterize this
  aspect of fluid flow.
• Higher the Re, greater the tendency for turbulent
  flow.

v velocity of the liquid D diameter of the
channel ? density n viscosity of the liquid.
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Example 5.2Design and analysis of a sprue for
casting
The desired volume flow rate of the molten metal
into a mold is 0.01 m3/min. The top of the sprue
has a diameter of 20 mm and its length is 200 mm.
What diameter should be specified at the bottom
of the sprue in order to prevent aspiration? What
is the resultant velocity and Reynolds number at
the bottom of the sprue if the metal being cast
is aluminium and has a viscosity of 0.004
N-s/m2? Solution Since d1 0.02 m
? Therefore Assuming no frictional
losses, Thus,
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Fluidity of molten metal

• Characteristics of molten metal influence
  fluidity
• Viscosity- decreases as viscosity and the
  viscosity index increase
• Surface tension- reduce for high surface tension
  
• Inclusions- have adverse effects
• Solidification pattern of the alloy- inversely
  proportional to the freezing range

24
Heat transfer

• Heat flow depends on casting material and the
  mold and process parameters.
• Temperature distribution in the mold-liquid metal
  interface is shown below.

25
Solidification time

• Solidification time is a function of the volume
  of a casting and surface area (Chvorinovs rule).
  
• Effects of mold geometry and elapsed time on skin
  thickness and its shape are show.

C constant n 2
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Example 5.3Solidification times for various
solid shapes
Three pieces being cast have the same volume but
different shapes. One is a sphere, one a cube,
and the other a cylinder with a height equal to
its diameter. Which piece will solidify the
fastest and which one the slowest? Use n
2. Solution The volume is unity ? Respective
surface areas are Respective solidification
times t are
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Shrinkage

• Shrinkage in casting causes dimensional changes.
• Cracking is a result of
• Contraction of the molten metal
• Contraction of the metal during phase change
• Contraction of the solidified metal
• For L-gtS, always think of
• Solidification Shrinkage!!

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Melting Practice and Furnaces

• Melting has a direct bearing on the quality of
  castings.
• Fluxes are inorganic compounds that refine the
  molten metal by removing dissolved gases and
  various impurities.
• The metal charge may be composed of commercially
  pure primary metals, which can include remelted
  or recycled scrap.

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Casting Alloys
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Ferrous casting alloys

• Cast irons represent the largest amount of all
  metals cast and can cast into complex shapes.
• Types of irons
• Gray cast iron
• Ductile iron (nodular iron)
• White cast iron
• Malleable iron
• Compacted-graphite iron
• Cast steels
• Cast stainless steels

31
Nonferrous casting alloys

• Types of alloys
• Aluminum-based alloys
• Magnesium-based alloys
• Copper-based alloys
• Zinc-based alloys
• High-temperature alloys

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Components of Casting

• Path Sprue -gt Well -gt Runner -gt Mold cavity
• Riser Compensate volume loss due to shrinkage
• Location of riser?
• Core Make holes
• Core print
• Draft Prevent collapse
• of sand

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Casting Design

• Consider shrinkage during solidification
• Once you understand the effects of
  solidification, use common sense!!

• Hot spot location of
• slow cooling
• Reduce hot spots
• with uniform thickness
• Rounding to avoid
• stress concentration

34
Expendable-Mold, Permanent-Pattern Casting
Processes- Sand Casting

• The sand casting process consists of
• placing a pattern, having the shape of the
  desired casting, in sand to make an imprint
• incorporating a gating system
• filling the resulting cavity with molten metal
• allowing the metal to cool until it solidifies
• breaking away the sand mold
• removing the casting and finishing it

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Expendable-Mold, Permanent-Pattern Casting
Processes- Sand Casting

• Sands
• Sand is the product of the disintegration of
  rocks.
• Inexpensive and resistance to high temperatures.
• Most sand casting operations use silica sands
  (SiO2).
• Mold have good collapsibility to avoid defects in
  the casting.
• Clay is used as a cohesive agent to give sand
  better strength.

36
Expendable-Mold, Permanent-Pattern Casting
Processes- Sand Casting

• Types of sand molds
• 3 types green-sand, cold-box, and no-bake molds.
• Green molding sand is mixture of sand, clay, and
  water and is inexpensive.
• In skin-dried method, castings has high strength,
  better accuracy and surface finish.
• In no-bake mold process, a synthetic liquid resin
  is mixed with the sand and hardened in room
  temperature.

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Expendable-Mold, Permanent-Pattern Casting
Processes- Sand Casting

• Patterns
• Patterns are used to mold the sand mixture into
  the shape of the casting.
• Made from a combination of materials to reduce
  wear and tear.
• Material selection depends on size and shape of
  casting.
• Can be designed with features for applications
  and economic requirements.
• One-piece, Split and Match-plate patterns.

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Expendable-Mold, Permanent-Pattern Casting
Processes- Sand Casting

• The sand casting operation

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Shell-mold casting

• Can produce castings with close dimensional
  tolerances, good surface finish and low cost.

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Plaster-mold casting

• Also known as precision casting.
• Has high dimensional accuracy and good surface
  finish.
• Parts made are lock components, gears and valves.
• Patterns for plaster molding are generally made
  of aluminium alloys, thermosetting plastics,
  brass, or zinc alloys.

41
Ceramic-mold casting

• Also called cope-and-drag investment casting.
• Similar to the plaster-mold process, but uses
  refractory mold materials suitable for
  high-temperature applications.
• Used in casting ferrous, stainless steels and
  tool steels.

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Vacuum casting

• Alternative to investment, shell-mold and
  green-sand casting.
• Can be automated and production costs are similar
  to green-sand casting.

43
Expendable-Mold, Expendable-Pattern Casting
Processes- Expendable-pattern casting (lost foam)

• Uses a polystyrene pattern which evaporates with
  molten metal to form a cavity for the casting.

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Expendable-Mold, Expendable-Pattern Casting
Processes- Expendable-pattern casting (lost foam)

• Evaporative-pattern process has these
  characteristics
• Simple and has design flexibility
• Inexpensive flasks
• Polystyrene inexpensive and easily processed into
  patterns
• Casting requires minimum cleaning
• Operation automated and economical
• Cost to produce the die can be high

45
Investment casting (lost-wax process)

• Labor and materials is costly but little
  finishing is required.
• Suitable for casting high-melting-point alloys
  with good surface finish and close dimensional
  tolerances.

46
Permanent-Mold Casting Processes

• Labor and materials is costly but little
  finishing is required.
• Suitable for casting high-melting-point alloys
  with good surface finish and close dimensional
  tolerances.
• Can be automated for large production runs and is
  used for aluminium, magnesium and copper alloys.
• Castings have good surface finish, close
  dimensional tolerances and good mechanical
  properties.

47
Die casting

• For non-ferrous metal casting such as Aluminum
• 2 types of process
• Hot-chamber process
• Cold-chamber process

48
Die casting

• Process capabilities and machine selection
• Strength-to-weight ratio of die-cast parts
  increases with decreasing wall thickness.
• Good surface finish and dimensional accuracy.

49
Centrifugal casting

• 3 types of centrifugal casting
• 1) True centrifugal casting
• Good quality, dimensional
• accuracy and external surface detail casting
• Semi-centrifugal casting
• used to cast parts with rotational symmetry
• Centrifuging
• The properties within the castings vary by the
  distance from the axis of rotation

50
Heat Treatment

• Treatments induce phase transformations that
  influence mechanical properties.
• Depend on- alloy composition and
  microstructure- degree of prior cold work-
  rates of heating and cooling during heat treatment

51
Heat treating ferrous alloys

• Pearlite
• Fine pearlite ? ferrite and cementite lamellae
  thin and closely packed.
• Coarse pearlite ? lamellae are thick and widely
  spaced.
• The difference depends on the rate of cooling
  through the eutectoid temperature. (Diffusion)
• Transformation from austenite to pearlite is
  illustrated in isothermal transformation (IT)
  diagrams or time-temperature-transformation (TTT)
  diagrams.

52
Heat treating ferrous alloys

• Martensite (BCT)
• Metastable phase by quenching
• Therefore, it is not shown
• in the phase diagram!

53
Pearlite and Martensite

• Pearlite microstructure not phase
• Alternating layers of ferrite(soft, ductile)and
  cementite(hard, brittle)
• Improve mechanical property (strong and
  ductile)
• Why does such microstructure occur?
  Diffusion!!
• Martensite metastable phase (Quenching is
  needed to form martensite)
• Not shown in phase diagram because it does not
  occur under equilibrium condition
• Extremely brittle, cannot be used (carbon
  cannot be diffused by quenching)
• After heat treatment (tempering), mechanical
  properties are enhanced.

54
Heat treating ferrous alloys

• Martensite
• When austenite is cooled rapidly and fcc
  structure transformed to bct structure.
• Does not have many slip systems, thus extremely
  hard and brittle, low toughness and limited
  usage.
• Tempered martensite
• Tempering reduces martensites hardness and
  improves toughness.
• The bct is heated to an intermediate temperature
  which consists of bcc alpha ferrite and small
  cementite.

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Heat treating ferrous alloys

• Spheroidite
• Form when pearlite is heated below eutectoid
  temperature and held for a period of time.
• High toughness, low hardness and prevent
  propagation of cracks in metal during working.
• Bainite
• Very fine microstructure consists of ferrite and
  cementite.
• Stronger and more ductile

56
Heat treating nonferrous alloys and stainless
steels

• Nonferrous alloys do not undergo phase
  transformations.
• Alloys and stainless steels are hardened and
  strengthened by precipitation hardening.
• 3 stages are involved in the precipitation
  hardening process.

57
Heat treating nonferrous alloys and stainless
steels

• Nonferrous alloys do not undergo phase
  transformations.
• Alloys and stainless steels are hardened and
  strengthened by precipitation hardening.
• 3 stages are involved in the precipitation
  hardening process.

58
Heat treating nonferrous alloys and stainless
steels

• Solution treatment
• Alloy is heated to within the solid solution
  temperature and cooled rapidly.
• Has moderate strength and ductile.
• Precipitation hardening
• Alloy is reheated to intermediate temperature and
  held for some time.
• Cu atoms diffuse and combine with Al atoms which
  produces the theta phase.
• Stronger but less ductile.

59
Heat treating nonferrous alloys and stainless
steels

• Aging
• Above room temperature the process is called
  artificial aging.
• Over a period of time at room temperature is
  called natural aging.
• For extended period of time is known as
  overaging.
• There is an optimal time-temperature relationship
  to develop desired properties.
• Else it will overage and lose its strength and
  hardness.

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Case hardening

• Changing of the surface properties is essential
  for improving resistance to surface indentation,
  fatigue and wear.
• A small surface crack can propagate rapidly and
  cause total failure.

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Annealing

• Annealing is the restoration of a
  cold/heat-treated part to its original properties
• Increase ductility and reduce hardness and
  strength.
• Also applies to thermal treatment of glasses and
  weldments.
• Annealing process
• Heating
• Holding it at that temperature
• Cooling it slowly

62
Annealing

• Process annealing
• Restore workpiece ductility.
• If the temperature is high, grain growth may
  result adverse effects on the formability of
  annealed parts.
• Stress-relief annealing
• Residual stresses may have been induced during
  phase transformations.
• Stress relieving is allowing of slow cooling,
  such as in still air.

63
Tempering

• The steel is heated to a specific temperature and
  cooled at a prescribed rate.
• Use to reduce brittleness and residual stresses,
  increase ductility and toughness.
• In austempering, heated steel is quenched rapidly
  to avoid formation of ferrite/pearlite.
• Austempering is used to
• reduce cracking and distortion during quenching
• improve ductility and toughness

64
Design Considerations

• Advantages and limitations of casting processes
  that impact design are given below.

65
Defects in castings

• 7 categories of defects can develop in castings
• Metallic projections
• Cavities
• Discontinuities
• Defective surface
• Incomplete casting
• Incorrect dimensions or shape
• Inclusions

66
Defects in castings

• Porosity
• Porosity is detrimental to the ductility of a
  casting.
• Caused either by shrinkage or trapped gases.
• Porosity due to shrinkage can be reduced by
• Internal or external chills used in sand casting
• Making the temperature gradient steep
• Subjecting the casting to hot isostatic pressing
• Porosity due to gases where liquid metals have
  greater solubility.

67
General design considerations

• 2 types of design issues in casting
• Geometric features should be incorporated into
  the part
• Mold features needed to produce the desired
  casting
• Robust design of castings involve
• Designing the part so that the shape is easily
  cast
• Select a casting process and a suitable material
• Locate the parting line
• Locate and design the gating system
• Ensure that proper controls are used

68
Economics of Casting

• Cost of equipment per casting decreases as number
  of parts cast increases.
• When demand is small cost per casting increases,
  more economical to manufacture.
• Final decision depends on economic and technical
  considerations.