CASTING (RISER DESIGN)
Assist. Prof. Dr. İlkay KALAY Materials Science and Engineering Department
Çankaya University
MSE 324 CASTING and SOLIDIFICATION
Thermocouples
Casting
A metal casting may be defined as a metal object produced by pouring molten metal into mold containing a cavity which has the desired shape of casting, allowing the molten metal to solidify in the cavity, and then removing the casting.
Casting Advantages of casting processes
• Casting can produce complex shapes and can incorporate
internal cavities or hollow sections.
• Very large parts can be produced in one piece.
• Casting can utilize materials that are difficult or uneconomical to process by other means.
• The casting process can be economically competitive with other manufacturing processes.
Casting
The important factors in casting process are: 1. The flow of molten metal into mold cavity 2. Solidification of metal from its molten state 3. Heat transfer during solidification and cooling of the metal
in the mold 4. Influence of type of the mold
Casting Requirements:
- Mold cavity with desired shape and size - Melting process to provide molten metal - Pouring process to introduce the metal into the
mold - Solidification process controlled to prevent defects - Ability to remove the casting from the mold - Cleaning, finishing and inspection operations
Typical Casting Nomenclature
Typical Casting Nomenclature
Typical Casting Nomenclature Mold Cavity: The hollow mold area in which metal solidifies into the part Riser: An extra cavity to store additional metal to prevent shrinkage Gating System: Channels used to deliver metal into the mold cavity Pouring Cup: The part of the gating system that receives poured metal Downsprue: Vertical channel Runners: Horizontal channels
Typical Casting Nomenclature Flask: The box containing the mold Cope: The top half of any part of a 2-part mold Drag: The bottom half of any part of a 2-part mold Core: A shape inserted into the mold to form internal cavities Core Print: A region used to support the core
Fluid Flow Metal is poured through a pouring cup Risers hold and supply metal to prevent shrinking during solidification Gates are designed to prevent contaminants from reaching the mold cavity
Fluidity of Molten Metal
Fluidity: The capability of a molten metal to fill mold cavities Viscosity: Higher viscosity decreases fluidity Surface tension: Decreases fluidity; often caused by oxide film Inclusions: Insoluble particles can increase viscosity, reducing fluidity Solidification pattern: Fluidity is inversely proportional to the freezing
temperature range
Fluidity of Molten Metal Mold design: The design and size of the downsprue, runners, and risers
affect fluidity Mold material and surface: Thermal conductivity and roughness decrease
fluidity Superheating: The temperature increment above the melting point
increases fluidity Pouring: Lower pouring rates decrease fluidity because of faster cooling Heat transfer: Affects the viscosity of the metal
Casting Good casting requires a design effort to:
• Create a gating system (which consist from: pouring basin,
downsprue, and runner) to bring molten metal into the mold cavity
Casting Good casting requires a design effort to:
• Provide a riser (feeder) which is a reservoir to feed molten metal to
the casting as it solidifies to prevent internal and external shrinkage in the casting. The riser may have to provide up to 5-7% by volume for the casting as it solidifies.
The risers should be designed keeping the following in mind 1. the metal in the riser should solidify in the end of the process 2. the riser volume should be sufficient for compensating the
shrinkage in the casting.
Casting Good and sound casting requires a design effort to:
• Control heat flow , in order to make the last liquid to solidify is in
the riser
• Control the rate of heat flow so as to control the nature of the solidified product (microstructure , mechanical properties)
Fluidity of Molten Metal
Fluidity of molten metal helps in producing sound casting with fewer defects. • It fills not only the mold cavity completely and rapidly but
does not allow also any casting defect like “misrun” to occur in the cast object.
• Pouring of molten metal properly at correct temperature plays a significant role in producing sound castings.
• The gating system performs the function to introduce clean metal into mold cavity in a manner as free of turbulence as possible.
Fluidity of Molten Metal Prevent casting defect such as misruns without use of excessively high pouring temperatures is still largely a matter of experience. • To fill the complicated castings sections completely, flow
rates must be high but not so high as to cause turbulence.
• It is noted that metal temperature may affect the ability of molten alloy to fill the mold, this effect is metal fluidity.
• Often, it is desirable to check metal fluidity before pouring using fluidity test.
Fluidity of Molten Metal • Often, it is desirable to check metal fluidity before
pouring using fluidity test.
Fluidity tests, in which metal from the furnace is poured by controlled vacuum into a flow channel of suitable size, are very useful, since temperature (super-heat) is the most significant single variable influencing the ability of molten metal to fill mold. This test is an accurate indicator of temperature.
Heat Transfer • The metal that solidifies first is at the wall of the mold; this solid layer
thickens as time passes • Shrinkage during cooling can change the part dimensions and
sometimes cause cracking; it is caused by the metal’s thermal expansion properties and the phase change between liquid and solid.
Common Defects in Castings
Shrinkage
Solidification Time
Heat Transfer
Common Defects in Castings
Typical Casting Nomenclature
Riser Design- Feeding Distance
Two criteria determine whether or not a riser is adequate: 1. the solidification time of the riser relative to that of the
casting, and
2. the feeding distance of the riser.
Riser Design- Feeding Distance
Two criteria determine whether or not a riser is adequate: 1. the solidification time of the riser relative to that of the
casting.
To be effec(ve, a riser should con(nue to feed liquid metal to the cas(ng un(l the cas(ng has completely solidified. Thus, the riser must have a longer solidificaQon Qme than the cas(ng. For a riser of fixed volume, a minimum amount of heat loss will occur when the riser geometry has the smallest possible surface area.
Riser Design- Feeding Distance
Two criteria determine whether or not a riser is adequate: 2. the feeding distance of the riser.
• The feeding distance (FD) is the maximum distance over which a riser can supply feed metal such that the casting section remains relatively free of internal porosity.
• Hence, the feeding distance determines the number of risers
needed. • The feeding distance is always measured from the edge of the
riser to the furthest point in the casting section to be fed by that riser.
Centerline Feeding Resistance (CFR)
Caine’s Curve
Caine’s Equation
Naval Research Lab. Method
Naval Research Lab. Method
Naval Research Lab. Method
Naval Research Lab. Method
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