Metal Casting

1
Metal Casting

• A large sand casting weighing 680 kg for an air
  compressor frame

2
Basic Features

• 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.

3

• 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.

4

• 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.

5

• 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

6
Classification of casting processes
7
Open and Closed Mould
8
Sand Casting (Expandable-mould, Permanent-pattern
Casting)
9
Pattern geometry
10
Use of chaplets to avoid shifting of cores
Possible chaplet design and casting with core
11
Production steps in sand casting including
pattern making and mold making
12
Patterns

• Variety of patters are used in casting and the
  choice depends on the configuration of casting
  and number of casting required
• Single-piece pattern
• Split pattern
• Follow board pattern
• Cope and drag pattern
• Match plate pattern
• Loose-piece pattern
• Sweep pattern
• Skeleton pattern

13
(a)Split pattern (b) Follow-board (c) Match
Plate (d) Loose-piece (e) Sweep (f) Skeleton
pattern
14
Pattern allowances

• Shrinkage allowance
• Draft allowance
• Machining allowance
• Distortion allowance

15
Moulding Materials

• Major part of Moulding material in sand
  casting are
• 70-85 silica sand (SiO2)
• 10-12 bonding material e.g., clay cereal etc.
• 3-6 water
• Requirements of molding sand are
• Refractoriness
• Cohesiveness
• Permeability
• Collapsibility
• The performance of mould depends on following
  factors
• Permeability
• Green strength
• Dry strength

16
Effect of moisture, grain size and shape on mould
quality
17
Melting and Pouring

• 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
18

• 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.

19

• 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
20
The Gating System

• 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

21
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

22
Use of chills
23
Cooling and Solidification
Pure metal
Alloy
24
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.

25

• 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.

26
Crystal structure in Castings
27
Dendrite formation

• 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.

28
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.

29
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.

30
Why Riser?

• The shrinkage occurs in three stages,
• When temperature of liquid metal drops from
  pouring to zero temperature
• When the metal changes from liquid to solid
  state, and
• 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

31
Cleaning and Finishing

• 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.

32
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

33
Classification of casting defects
34
Surface Defects

• 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

35

• 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.

36
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.

37
Visible Defects
38

• 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.

39
Casting with expendable mould Investment Casting
40
Advantages and Limitations

• 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

41
Permanent mould casting Die casting
42
General Configuration of a Die Casting Machine
43

• 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

44
Cycle in Hot Chamber Casting
45
Cycle in Cold Chamber Casting
46
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.

47
Comparison of Casting Processes
48
(No Transcript)