21-05-2013, 11:53 AM
FUNDAMENTALS OF METAL CASTING
METAL CASTING.pdf (Size: 1.03 MB / Downloads: 191)
Solidification Processes
Starting work material is either a liquid or is in a highly
plastic condition, and a part is created through
solidification of the material
•Solidification processes can be classified according
to engineering material processed:
Metals
Ceramics, specifically glasses
Polymers and polymer matrix composites (PMCs)
Casting
Process in which molten metal flows by gravity or other
force into a mold where it solidifies in the shape of
the mold cavity
• The term casting also applies to the part made in the
process
• Steps in casting seem simple:
1. Melt the metal
2. Pour it into a mold
3. Let it freeze
Capabilities and Advantages of Casting
•Can create complex part geometries
•Can create both external and internal shapes
•Some casting processes are net shape; others are
near net shape
•Can produce very large parts
•Some casting methods are suited to mass production
Disadvantages of Casting
•Different disadvantages for different casting
processes:
Limitations on mechanical properties
Poor dimensional accuracy and surface finish for
some processes; e.g., sand casting
Safety hazards to workers due to hot molten
metals
problems
Parts Made by Casting
•Big parts: engine blocks and heads for automotive
vehicles, wood burning stoves, machine frames,
railway wheels, pipes, church bells, big statues, and
pump housings
•Small parts: dental crowns, jewelry, small statues,
and frying pans
•All varieties of metals can be cast, ferrous and
nonferrous
The Mold in Casting
•Contains cavity whose geometry determines part
shape
Actual size and shape of cavity must be slightly
oversized to allow for shrinkage of metal during
solidification and cooling
Molds are made of a variety of materials, including
sand, plaster, ceramic, and metal
Heating the Metal
• Heating furnaces are used to heat the metal to
molten temperature sufficient for casting
• The heat required is the sum of:
1. Heat to raise temperature to melting point
2. Heat of fusion to convert from solid to liquid
3. Heat to raise molten metal to desired
temperature for pouring
Solidification of Pure Metals
•Due to chilling action of mold wall, a thin skin of solid
metal is formed at the interface immediately after
pouring
•Skin thickness increases to form a shell around the
molten metal as solidification progresses
•Rate of freezing depends on heat transfer into mold,
as well as thermal properties of the metal
Mold Constant in Chvorinov's Rule
•Cm depends on mold material, thermal properties of
casting metal, and pouring temperature relative to
melting point
•Value of Cm for a given casting operation can be
based on experimental data from previous operations
carried out using same mold material, metal, and
pouring temperature, even though the shape of the
part may be quite different
Solidification Shrinkage
•Occurs in nearly all metals because the solid phase
has a higher density than the liquid phase
•Thus, solidification causes a reduction in volume per
unit weight of metal
•Exception: cast iron with high C content
Graphitization during final stages of freezing
causes expansion that counteracts volumetric
decrease associated with phase change
Achieving Directional Solidification
•Desired directional solidification is achieved using
Chvorinov's Rule to design the casting itself, its
orientation in the mold, and the riser system that
feeds it
•Locate sections of the casting with lower V/A ratios
away from riser, so freezing occurs first in these
regions, and the liquid metal supply for the rest of the
casting remains open
•Chills - internal or external heat sinks that cause
rapid freezing in certain regions of the casting