Title: Manufacturing Processes for Engineering Materials 5th Edition in SI Units
1Manufacturing 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.
2Solidification 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.
3Solid 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.
4Phase 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
5Two-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.
7Phase 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
8The 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)
9The 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)
10The 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.
11Example 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.
12Cast 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
13Pure 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
14Alloys
- Pure metals are enhanced and modified by
alloying. - Solidification begins when temperature drops
below the liquidus, TL, and complete when
solidus, TS. - Freezing range
15Alloys
- 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!
16Structure-property relationships
- When alloy cooled slowly dendrite develops a
uniform composition. - For normal cooling cored dendrites are formed.
Columnar dendritic
Equiaxed dendritic
Equiaxed nondendritic
17Fluid 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.
18Fluid 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
19Fluid 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
20Fluid 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
21Fluid 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.
22Example 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,
23Fluidity 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
24Heat transfer
- Heat flow depends on casting material and the
mold and process parameters. - Temperature distribution in the mold-liquid metal
interface is shown below.
25Solidification 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
26Example 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
27Shrinkage
- 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!!
28Melting 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.
29Casting Alloys
30Ferrous 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
31Nonferrous casting alloys
- Types of alloys
- Aluminum-based alloys
- Magnesium-based alloys
- Copper-based alloys
- Zinc-based alloys
- High-temperature alloys
32Components 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
33Casting 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
34Expendable-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
35Expendable-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.
36Expendable-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.
37Expendable-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.
38Expendable-Mold, Permanent-Pattern Casting
Processes- Sand Casting
- The sand casting operation
39Shell-mold casting
- Can produce castings with close dimensional
tolerances, good surface finish and low cost.
40Plaster-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.
41Ceramic-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.
42Vacuum casting
- Alternative to investment, shell-mold and
green-sand casting. - Can be automated and production costs are similar
to green-sand casting.
43Expendable-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.
44Expendable-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
45Investment 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.
46Permanent-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.
47Die casting
- For non-ferrous metal casting such as Aluminum
- 2 types of process
- Hot-chamber process
- Cold-chamber process
48Die 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.
49Centrifugal 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
50Heat 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
51Heat 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.
52Heat treating ferrous alloys
- Martensite (BCT)
- Metastable phase by quenching
- Therefore, it is not shown
- in the phase diagram!
53Pearlite 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.
54Heat 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.
55Heat 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
56Heat 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.
57Heat 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.
58Heat 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.
59Heat 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.
60Case 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.
61Annealing
- 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
62Annealing
- 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.
63Tempering
- 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
64Design Considerations
- Advantages and limitations of casting processes
that impact design are given below.
65Defects in castings
- 7 categories of defects can develop in castings
- Metallic projections
- Cavities
- Discontinuities
- Defective surface
- Incomplete casting
- Incorrect dimensions or shape
- Inclusions
66Defects 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.
67General 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
68Economics 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.