2. Course Content
• Sand molding process, properties of molding sand, Pattern and its
allowances , Solidification and cooling, Riser design, cleaning,
finishing and heat treatments of casting, permanent mold die casting,
hot and cold chamber die casting, centrifugal casting, investment
casting, Defects in casting and inspection, Product design
considerations.
6. Casting is the process
whereby a part is produced by
solidification (of a molten
metal) to take the shape of a
mold.
Why casting?
• Versatile to many types of metals
• Potential for rapid and cost-effective production
• Wide range of length scales (mm to m!)
• Complex part geometries (including internal cavities)
• Capable of producing parts to net shape
This Photo by Unknown Author is licensed under CC BY-SA-NC
7. Capabilities
• Dimensions
sand casting - as large as you like
small - 1 mm or so
• Tolerances
0.005 in to 0.1 in
• Surface finish
die casting 8-16 micro-inches (1-3 μm)
sand casting - 500 micro-inches (2.5-25 μm)
8. Casting and history
• 9000 BC : Earliest metal objects of wrought native copper: Near East
• 6500 BC : Earliest life-size statues : Jorden
• 3000–1500 B.C. : Bronze age: arsenical copper and tin bronze alloys
• 3000–2500 B.C : Lost wax casting of small objects
• 2000 B.C. : Bronze age
• 600 BC: Cast Iron
• 16th Century: Sand introduced as mold material
• 1818 A.D.: Production of cast steel by crucible process
• 1837 A.D. : Development of first molding machine
• 1919 A.D. – First electric arc furnace used in the U.S.
• 1996 A.D. : Cast Metal Matrix Composites in automobile applications
• 2006 A.D.: Casting simulations coupled to mechanical performance
simulations
http://link.springer.com/10.1007/978-3-319-46633-0
9. Casting and history
The earliest known
casting in existence
“A Copper Frog”,
cast in 3200 B.C
Mold prepared by
smooth textured stone
with an axe
The Iron pillar of Delhi
Constructed by Chandragupta II (reigned c.
375–415 CE)
http://link.springer.com/10.1007/978-3-319-46633-0
10. Casting and history
Coin of Samudragupta (c. 350—375) with Garuda
pillar. British Museum.
Bronze Chola Statue of Nataraja at the Metropolitan
Museum of Art, New York City.
https://en.wikipedia.org/wiki/History_of_metallurgy_in_the_Indian_subcontinent
11. Volume of the global casting production in 2019, by
country(in million metric tons)
https://www.statista.com/statistics/237526/casting-production-worldwide-by-country/
14. General sequence (all casting processes):
• Pattern/mold making
• Melt preparation
• Mold filling
• Cooling and solidification
• Removal (‘breakout’) of the parts
16. Important criteria for casting materials
• Melting point and latent heat
• Density versus temperature
• Solubility with other elements
• Diffusion rates
• Reactivity (especially to oxygen)
• Outgassing (vapor pressure)
18. Sand Casting: Basic Steps
• Basic steps in casting are:
– Preparation of pattern(s), core(s) and mold(s)
– Melting and pouring of liquefied metal
– Solidification and cooling to room temperature
– Removal of casting - shakeout
– Inspection (for possible defects)
19. Pattern Making
• Pattern is a replica of the exterior surface of part to be cast – used to create the
mold cavity
• Pattern materials – wood, metal, plaster, plastic
20. Pattern Making
• Advancement in pattern making
• Rapid Prototyping
Pattern design Consideration
• Metal Shrinkage
• Easily Removable
21. Pattern Making
• Pattern usually larger than cast part Allowances made for
• Shrinkage: to compensate for metal shrinkage during cooling from
freezing to room temp
• Shrinkage allowance = αL(Tf – T0)
expressed as per unit length for a given material
• α = coeff. of thermal expansion, Tf = freezing temp
T0 = room temp
e.g. Cast iron allowance = 1/96 in./ft
aluminum allowance = 3/192 in./ft
22. Pattern Making
• Pattern allowances made for:
– Machining: excess dimension that is removed by machining; depends
on part dimension and material to be cast
e.g. cast iron, dimension 0-30 cm, allowance = 2.5 mm; aluminum, allowance =
1.5 mm
– Draft: taper on side of pattern parallel to direction of extraction from
mold; for ease of pattern extraction; typically 0.5~2 degrees
24. Heating and Pouring
• The heat energy required is the sum of
(1) the heat to raise the temperature to the melting point, Tm
(2) the heat of fusion to convert it from solid to liquid, Hf
(3) the heat to raise the molten metal to the desired temperature for
pouring Tp
25. Pouring
• An important step in casting since it impacts mold filling ability and
casting defects
26. Pouring
• Key aspects of pouring
• Pouring rate
• Too slow → metal freezes before complete mold filling
• Too fast → inclusion of slag, aspiration of gas, etc.
• Reynolds number: Laminar versus turbulent flow
27. Pouring
• Flow velocity of molten metal at base of sprue
𝑣 = 2𝑔ℎ
• Volume flow rate at any location
• 𝑄 = 𝑣1 𝐴1= 𝑣2 𝐴2
where v = the velocity of the liquid metal at the base of the sprue,
cm/s (in/sec); g =981 cm/s/s (386 in/sec/sec); and h = the height of
the sprue, cm (in
where Q = volumetric fl ow rate, cm3/s (in3/sec); v =
velocity as before; A = crosssectional=area of the
liquid, cm2 (in2); and the subscripts refer to any two
points in the flow system.
28. Pouring Analysis (Sprue/Gating Design)
• As metal is poured into mold, the
effective “head” decreases
• Velocity of metal at point 3:
𝑉3 = 2𝑔(ℎ𝑡 − ℎ)
• Mold filling time
29. Solidification of pure metals
• Metal releases latent heat as it freezes;; this accounts for up to ~50%
of the energy transfer.
• As a result, solid and liquid co-exist in the mold for a significant time.
30. Formation of cast microstructure
• Grain size is
inversely
proportional to
cooling rate.
• Smaller grains
give higher
strength
• The grains
generally grow in
a direction
opposite to that
of the heat
transfer out
through the mold.
31. Solidification of Alloys
• Alloys
• Solidify over a temperature range
• – Composition and microstructure determined by
phase diagram of alloy
32. Shrinkage
• Shrinkage: most metals shrink when cooled from the liquid state
• – Liquid shrinkage
• – Solidification shrinkage
• – Solid shrinkage
33. Heat Transfer During Solidification
• The solidification time is
a function of the volume
of a casting and its
surface area (Chvorinov’s
rule):
40. Die casting
• Pressure: ~1-1000 Mpa
• Cycle time: ~10’s of seconds for average components (tools/toys)
• Dies are endangered by heat- induced cracking and corrosion
(accelerated at high temperature) hence need tool-grade steel or
other special materials
47. Investment casting: key points
• Use of wax template enables excellent surface finish with little/no
post-processing
• Ceramic shell enables casting of high melting point metals/alloys.
• Metal typically poured in vacuum oven (reduces defects).
• Very labor intensive : robots!
• Why investment casting?
• Jewelry: complex geometries, high tolerances and fine features
• Jet engine parts: smooth surface finish, compatibility with high
temperature alloys
48. Investment casting of turbine blades
• Careful control of solidification can give single crystal blades (= very
high strength and fatigue life under cyclic load at high temperature)
49. Investment casting of turbine blades
• Careful control of solidification can give single crystal blades (= very
high strength and fatigue life under cyclic load at high temperature)
Split patterns consist of two pieces, dividing the part along a plane coinciding
with the parting line of the mold. Split patterns are appropriate for complex part
geometries and moderate production quantities. The parting line of the mold is predetermined
by the two pattern halves, rather than by operator judgment.
For higher production quantities, match-plate patterns or cope-and-drag patterns
are used. In match-plate patterns, the two pieces of the split pattern are attached
to opposite sides of a wood or metal plate. Holes in the plate allow the top and
bottom (cope and drag) sections of the mold to be aligned accurately. Cope-and drag
patterns are similar to match-plate patterns except that split pattern halves are attached to separate plates, so that the cope and drag sections of the mold can be
fabricated independently, instead of using the same tooling for both
Split patterns consist of two pieces, dividing the part along a plane coinciding
with the parting line of the mold. Split patterns are appropriate for complex part
geometries and moderate production quantities. The parting line of the mold is predetermined
by the two pattern halves, rather than by operator judgment.
For higher production quantities, match-plate patterns or cope-and-drag patterns
are used. In match-plate patterns, the two pieces of the split pattern are attached
to opposite sides of a wood or metal plate. Holes in the plate allow the top and
bottom (cope and drag) sections of the mold to be aligned accurately. Cope-and drag
patterns are similar to match-plate patterns except that split pattern halves are attached to separate plates, so that the cope and drag sections of the mold can be
fabricated independently, instead of using the same tooling for both
Metals shrink while cooling and, generally, also shrink when they solidify shrinkage can lead to microcracking and the associated porosity, which can adversely affect the mechanical
properties of the casting.
At the mold walls, which are at ambient temperature at first, or typically are much cooler than the molten metal, the metal cools rapidly, producing a solidified skin, or shell, of fine equiaxed grains.
Those grains that have favorable orientation
grow preferentially, and are called columnar
grains
