Transcript Metal Casting A large sand casting weighing 680 kg for an air compressor frame.

Metal Casting
A large sand casting weighing 680 kg for an air
compressor frame
Basic Features
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Pattern and Mould
– A pattern is made of wood or metal, is a replica of the
final product and is used for preparing mould cavity
– Mould cavity which contains molten metal is
essentially a negative of the final product
– Mould material should posses refractory
characteristics and with stand the pouring
temperature
– When the mold is used for single casting, it made of
sand and known as expendable mold
– When the mold is used repeatedly for number of
castings and is made of metal or graphite are called
permanent mould
– For making holes or hollow cavities inside a casting,
cores made of either sand or metal are used.
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Melting and Pouring
– Several types of furnaces are available for
melting metals and their selection depends
on the type of metal, the maximum
temperature required and the rate and the
mode of molten metal delivery.
– Before pouring provisions are made for the
escape of dissolved gases. The gating system
should be designed to minimize the turbulent
flow and erosion of mould cavity.The other
important factors are the pouring
temperature and the pouring rate.
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Solidification and Cooling
– The properties of the casting significantly depends
on the solidification time cooing rate.
– Shrinkage of casting, during cooling of solidified
metal should not be restrained by the mould material,
otherwise internal stresses may develop and form
cracks in casting.
– Proper care should be taken at the design stage of
casting so that shrinkage can occur without casting
defects.
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Removal, Cleaning, Finishing and Inspection
– After the casting is removed from the mould it is
thoroughly cleaned and the excess material usually
along the parting line and the place where the
molten metal was poured, is removed using a potable
grinder.
– White light inspection, pressure test, magnetic
particle inspection, radiographic test, ultrasonic
inspection etc. are used
Classification of casting processes
Open and Closed Mould
Sand Casting (Expandable-mould,
Permanent-pattern Casting)
Pattern geometry
Use of chaplets to avoid shifting of cores
Possible chaplet design
and casting with core
Production steps in sand casting including
pattern making and mold making
Patterns
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Variety of patters are used in casting and the
choice depends on the configuration of casting
and number of casting required
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Single-piece pattern
Split pattern
Follow board pattern
Cope and drag pattern
Match plate pattern
Loose-piece pattern
Sweep pattern
Skeleton pattern
(a)Split pattern
(b) Follow-board
(c) Match Plate
(d) Loose-piece
(e) Sweep
(f) Skeleton
pattern
Pattern allowances
Shrinkage allowance
 Draft allowance
 Machining allowance
 Distortion allowance
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Moulding Materials
Major part of Moulding material in sand casting
are
1.
2.
3.
70-85% silica sand (SiO2)
10-12% bonding material e.g., clay cereal etc.
3-6% water
Requirements of molding sand are:
(a)
(b)
(c)
(d)
Refractoriness
Cohesiveness
Permeability
Collapsibility
The performance of mould depends on following
factors:
(a) Permeability
(b) Green strength
(c) Dry strength
Effect of moisture, grain size and shape
on mould quality
Melting and Pouring
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The quality of casting depends on the method of melting. The
melting technique should provide molten metal at required
temperature, but should also provide the material of good quality
and in the required quantity.
Pouring vessels
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Molten metal is prevented from oxidation by covering the molten metal with
fluxes or by carrying out melting and pouring in vacuum
Ladles which pour the molten metal from beneath the surface are used
The two main consideration during pouring are the temperature and pouring
rate
Fluidity of molten metal is more at higher temperature but it results into
more amount of dissolved gases and high temperature also damage the
mould walls and results into poor surface quality of the casting
To control the amount of dissolved gases low, the temperature should not
be in superheated range
In ferrous metals, the dissolved hydrogen and nitrogen are removed by
passing CO. In non-ferrous metals, Cl, He, or Ar gases are used.
Therefore, fluidity and gas solubility are two conflicting requirements. The
optimum pouring temp. is therefore decided on the basis of fluidity
requirements.The temp. should be able to fill the whole cavity at the same
time it should enter inside the voids between the sand particles.
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Cooling rate depends on casting material and configuration. It
also depends on volume and surface area of the casting also.
The pouring rate should be such that solidification does not
start and the cavity is completely filled without eroding mould
surface and undue turbulence.
On the basis of experience following empirical relations are
developed for pouring time
K: Fluidity factor
W: Weight In kg
Tp: Poring time in sec
The Gating System
1.
2.
3.
4.
Minimize turbulent flow so that absorption of
gases, oxidation of metal and erosion of mould
surfaces are less
Regulate the entry of molten metal into the
mould cavity
Ensure complete filling of mould cavity, and
Promote a temperature gradient within the
casting so that all sections irrespective of size
and shape could solidify properly
The Gating System
A: pouring basin
 B: Weir
 C: Sprue
 D: Sprue well
 E: Runner
 F: Ingates
 G: Runner break up
 H: Blind
 J: Riser
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Use of chills
Cooling and Solidification
Pure metal
Alloy
Mechanism of Solidification
Pure metals solidifies at a constant temp. equal to its
freezing point, which same as its melting point.
 The change form liquid to solid does not occur all at
once. The process of solidification starts with nucleation,
the formation of stable solid particles within the liquid
metal. Nuclei of solid phase, generally a few hundred
atom in size, start appearing at a temperature below the
freezing temperature. The temp. around this goes down
and is called supercooling or undercooling. In pure
metals supercooling is around 20% of the freezing temp.
 A nuclease, more than a certain critical size grows, and
causes solidification.
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By adding, certain foreign materials (nucleating agents) the
undercooling temp. is reduced which causes enhanced
nucleation.
 In case of pure metals fine equi-axed grains are formed near
the wall of the mold and columnar grain growth takes place
upto the centre of the ingot.
 In typical solid-solution alloy, the columnar grains do not extend
upto the center of casting but are interrupted by an inner zone
of equiaxed graines.
 My adding typical nucleating agents like sodium, magnesium or
bismuth the inner zone of equiaxed grained can be extended in
whole casting.
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Crystal structure in Castings
Dendrite formation
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In alloys, such as Fe-C, freezing and solidificaion occurs overa
wide range of temp. There is no fine line of demarcation exists
between the solid and liquid metal.
Here, ‘start of freezing’ implies that grain formation while
progressing towards the center does not solidify the metal
completely but leaves behind the islands of liquid metals in
between grains which freeze later and there is multidirectional
tree like growth.
Solidification Time
Once the material cools down to freezing
temperature, the solidification process for the
pure metals does not require a decrease in
temperature and a plateau is obtained in the
cooling curves, called thermal arrest. The
solidification time is total time required for the
liquid metal to solidify.
 Solidification time has been found to be directly
proportional to volume and inversely
proportional to surface area.
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Location of Risers and Open and
Closed Risers
•Top riser has the
advantage of
additional pressure
head and smaller
feeding distance over
the side riser.
•Blind risers are
generally bigger in
size because of
additional area of
heat conduction.
Why Riser?
 The
shrinkage occurs in three stages,
1.When temperature of liquid metal drops from pouring
to zero temperature
2.When the metal changes from liquid to solid state,
and
3.When the temperature of solid phase drops from
freezing to room temperature
 The shrinkage for stage 3 is compensated by providing
shrinkage allowance on pattern, while the shrinkage
during stages 1 and 2 are compensated by providing
risers.
 The riser should solidify in the last otherwise liquid
metal will start flowing from casting to riser. It should
promote directional solidification. The shape, size and
location of the risers are important considerations in
casting design
Cleaning and Finishing
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Casting is taken out of the mould by shaking and the
Moulding sand is recycled often with suitable
additions.
The remaining sand, some of which may be embedded
in the casting, is removed by means of Shot blasting.
The excess material in the form of sprue, runners,
gates etc., along with the flashes formed due to flow
of molten metal into the gaps is broken manuaaly in
case of brittle casting or removed by sawing and
grinding in case of ductile grinding.
The entire casting is then cleaned by either shot
blasting or chemical pickling.
Sometimes castings are heat treated to achieve better
mechanical properties.
Casting Defects
Defects
may occur due to one or more of
the following reasons:
– Fault in design of casting pattern
– Fault in design on mold and core
– Fault in design of gating system and riser
– Improper choice of moulding sand
– Improper metal composition
– Inadequate melting temperature and rate of
pouring
Classification of casting defects
Surface
Defect
Blow
Scar
Blister
Drop
Scab
Penetration
Buckle
Casting defects
Internal Defect
Blow holes
Porosity
Pin holes
Inclusions
Dross
Visible defects
Wash
Rat tail
Swell
Misrun
Cold shut
Hot tear
Shrinkage/Shift
Surface Defects
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These are due to poor design and quality of sand molds
and general cause is poor ramming
Blow is relatively large cavity produced by gases which
displace molten metal from convex surface. Scar is
shallow blow generally occurring on a flat surface. A
scar covered with a thin layer of metal is called blister.
These are due to improper permeability or venting.
Sometimes excessive gas forming constituents in
moulding sand
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Drop is an irregularly-shaped projection on the cope surface
caused by dropping of sand.
A scab when an up heaved sand gets separated from the mould
surface and the molten metal flows between the displaced sand
and the mold.
Penetration occurs when the molten metal flows between the
sand particles in the mould. These defects are due to
inadequate strength of the mold and high temperature of the
molten metal adds on it.
Buckle is a vee-shaped depression on the surface of a flat
casting caused by expansion of a thin layer of sand at the
mould face. A proper amount of volatile additives in moulding
material could eliminate this defect by providing room for
expansion.
Internal Defects
The internal defects found in the castings are mainly due to
trapped gases and dirty metal. Gases get trapped due to hard
ramming or improper venting. These defects also occur when
excessive moisture or excessive gas forming materials are
used for mould making.
 Blow holes are large spherical shaped gas bubbles, while
porosity indicates a large number of uniformly distributed tiny
holes. Pin holes are tiny blow holes appearing just below the
casting surface.
 Inclusions are the non-metallic particles in the metal matrix,
Lighter impurities appearing the casting surface are dross.
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Visible Defects
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Insufficient mould strength, insufficient metal, low pouring
temperature, and bad design of casting are some of the
common causes.
Wash is a low projection near the gate caused by erosion of
sand by the flowing metal. Rat tail is a long, shallow, angular
depression caused by expansion of the sand. Swell is the
deformation of vertical mould surface due to hydrostatic
pressure caused by moisture in the sand.
Misrun and cold shut are caused by insufficient superheat
provided to the liquid metal.
Hot tear is the crack in the casting caused by high residual
stresses.
Shrinkage is essentially solidification contraction and occurs due
to improper use of Riser.
Shift is due to misalignment of two parts of the mould or
incorrect core location.
Casting with expendable mould:
Investment Casting
Advantages and Limitations
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Parts of greater complexity and intricacy can be
cast
Close dimensional control 0.075mm
Good surface finish
The lost wax can be reused
Additional machining is not required in normal
course
Preferred for casting weight less than 5 kg,
maximum dimension less than 300 mm,
Thickness is usually restricted to 15mm
Al, Cu, Ni, Carbon and alloy steels, tool steels
etc. are the common materials
Permanent mould casting: Die casting
Graphite+oil
General Configuration of a Die Casting
Machine
In Die casting the molten metal is forced to
flow into a permanent metallic mold under
moderate to high pressures, and held under
pressure during solidification
 This high pressure forces the metal into
intricate details, produces smooth surface and
excellent dimensional accuracy
 High pressure causes turbulence and air
entrapment. In order to minimize this larger
ingates are used and in the beginning pressure
is kept low and is increased gradually
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Cycle in Hot Chamber Casting
Cycle in Cold Chamber Casting
Centrifugal Casting
•A permanent mold made of metal or ceramic is rotated at high
speed (300 to 3000 rpm). The molten metal is then poured into the
mold cavity and due to centrifugal action the molten metal conform
to the cavity provided in the mould.
•Castings are known for their higher densities in the outer most
regions.
•The process gives good surface finish
•Applications: pipes, bushings, gears, flywheels etc.
Comparison of Casting Processes