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MCHE 365

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Casting uses the idea that a liquid metal can take the shape of any vessel ... drops below the liquidus TL and is complete when it reaches the solidus, TS. ... – PowerPoint PPT presentation

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Title: MCHE 365


1
Fundamentals of Metal Casting
  • CHAPTER 10

2
Topics
  • Introduction
  • Solidification of Metals
  • Fluid Flow
  • Fluidity of Metals
  • Heat Transfer
  • Defects

3
Overview of Casting
  • Casting uses the idea that a liquid metal can
    take the shape of any vessel containing it.
  • When the metal cools it has taken the shape of
    its container
  • Casting is one of the most versatile
    manufacturing processes

4
Advantages of Casting
  • Low cost and quick
  • Easier to manufacture
  • Can produce intricate shapes and internal
    openings
  • Can produce parts in one piece
  • Best suited for composite components

5
Introduction
  • Important Considerations
  • Flow of Molten Metal
  • Solidification and Cooling
  • Type of Mold Material

6
Solidification of Metals
  • Involves liquid metal turning back in to solid
    metal
  • The process is different for Pure metals and
    alloys
  • Can be divided into two steps
  • Formation of stable nuclei
  • Growth of crystals
  • Pure Metals
  • Have a clearly defined melting point
  • Temperature remains constant during freezing
  • Solidifies from the walls of the mold toward the
    center of the part

7
  • Grain Structure for Pure Metals
  • Two types of grains are formed for a pure metal
  • Fine equiaxed grains
  • Columnar
  • Rapid cooling at the walls produces fine equiaxed
    grains
  • Columnar grains grow opposite of the heat
    transfer throughout the mold following the chill
    zone
  • Equiaxed Grains
  • If crystals can grow approximately equally in all
    directions equiaxed grains will grow.
  • Large amounts of under cooling is needed near the
    wall of the mold.

8
Illustration of Cast Structures
9
Alloys
  • Solidification in alloys begins when the
    temperature drops below the liquidus TL and is
    complete when it reaches the solidus, TS.

10
Alloys
  • Within the TL and TS Temperature range, the alloy
    is like a slushy with columnar dendrites




11
Effects of Cooling Rates
  • Slow cool rates results in course grain
    structures (102 K/s)
  • Faster cooling rates produce finer grain
    structures (104 K/s)
  • For even faster cooling rates, the structures are
    amorphous (106 108 K/s)
  • Grain size influences strength of a material
  • Smaller grains have higher ductility and strength
  • Smaller grains help prevent hot tearing and/or
    cracks in the casting

12
Fluid Flow
  • Basic casting system
  • Fluid is pored though a pouring basin
  • Flows though the gating system into the mold
    cavity
  • Schematic of typical riser-gated casting.

Fig Schematic illustration of a typical
riser-gated casting. Risers serve as reservoirs,
supplying molten metal to the casting as it
shrinks during solidification.
13
Fluid Flow
  • Sprue is a vertical channel though which the
    molten metal flows downward in the mold
  • Runners channels that carry the molten metal
    from the sprue to the mold cavity
  • Gate is the portion of the runner though which
    the molten metal enters the mold cavity
  • Risers serve as reservoirs to supply the molten
    metal necessary to prevent shrinkage.
  • Principles of fluid flow
  • Bernoullis Theorem
  • Continuity
  • Flow Characteristics turbulence is an important
    consideration in gating systems.

14
  • Flow Characteristics
  • Reynolds Number is used to quantify this aspect
  • 0 lt Re lt 2000 gt laminar flow
  • 2000 lt Re lt 20 000 gtmixture of laminar and
    turbulent flow
  • Re gt 20 000 gt severe turbulence
  • Techniques for minimizing turbulence
  • Avoid sudden changes in flow direction
  • Dross or slag can be eliminated by vacuum casting
  • Use of filters eliminates turbulent flow in the
    runner system

15
Fluidity of Molten Metal
  • Fluidity of Molten Metal The capability of
    molten metal to fill mold cavities is called
    fluidity.
  • The following influence fluidity
  • Characteristics of molten metal
  • Viscosity
  • Surface tension
  • Inclusions
  • Solidification pattern of the alloy
  • Casting parameters
  • Mold design
  • Mold material and its surface characteristics
  • Degree of superheat
  • Rate of pouring
  • Heat transfer
  • Note Castability describes the case with which
    a metal can be cast to obtain a part with good
    quality.

16
Fluidity of Molten Metal
  • Fig A test method for fluidity using a spiral
    mold. The fluidity index is the length of the
    solidified metal in the spiral passage. The
    greater the length of the solidified metal, the
    greater the length of the solidified metal, the
    greater is its fluidity.

17
Heat Transfer
  • Important consideration in casting
  • Heat flow in the system
  • Complex
  • Depends of flow characteristics
  • Solidification Time
  • A function of the volume of a casting and its
    surface area
  • Solidification time C volume 2
  • surface area
  • Effects on solidification time
  • Mold Geometry
  • Skin thickness

18
Heat Transfer
  • Shrinkage causes dimensional changes and,
    sometimes cracking, is the result of the
    following
  • Contraction prior to solidification
  • Contraction during phase changes
  • Contraction as temperature drops to ambient
    temperature

Fig Solidified skin on a steel casting. The
remaining molten metal is poured out at times
indicated in the figure. Hollow ornamental and
decorative objects are made by a process called
slush casting, which based on this principle
19
Defects
  • Cavities
  • Internal or external
  • Blow holes
  • Pin holes
  • Shrinkage cavities
  • Discontinuities
  • Cracks
  • Cold or hot tearing
  • Cold shunts
  • Metallic projections
  • Fins
  • Flash
  • Massive projections
  • Swells
  • Rough surfaces

Fig Examples of hot tears in castings. These
defects occur because the casting cannot shrink
freely during cooling, owing to constraints in
various portions of the molds and cores.
Exothermic (heat-producing) compounds may be used
(as exothermic padding) to control cooling at
critical sections to avoid hot tearing.
20
Slags and other foreign material entrapped in the
metal can become inclusions too.
  • Incorrect Dimensions or Shape
  • Improper shrinkage allowance
  • Pattern mounting error
  • Irregular contraction
  • (iv) Deformed pattern
  • Warped casting
  • Inclusions form after melting, solidification
    and molding
  • Non-metallic
  • Harmful
  • Stress raisers
  • Reduce the strength of the casting
  • May react with
  • Environment
  • Crucible
  • Mold material
  • Defective surface
  • (i) Folds
  • Laps
  • Scars
  • (iv) Adhering sand layers
  • Oxide scale
  • Incomplete casting
  • (i) Misruns
  • (ii) Insufficient volume
  • (iii) Runout due to loss of metal from mold
  • (iv) Temperature too low when metal is poured
  • (v) Metal is poured to slow

21
Casting Defects
  • Fig Examples of common defects in castings.
    These defects can be minimized or eliminated by
    proper design and preparation of, olds and
    control of pouring procedures.

22
POROSITY Methods of removal of porosity by
(a)Internal and (b)External chills
  • Fig Various types of (a) internal and (b)
    external chills (dark areas at corners), used in
    castings to eliminate porosity caused by
    shrinkage. Chills are placed in regions where
    there is a larger volume of metal as shown in (c).

23
THE END
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