MSE 440/540: Processing of Metallic Materials
Instructors: Yuntian Zhu
Office: 308 RBII
Ph: 513-0559
ytzhu@ncsu.edu
Lecture 3: Casting I,
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Casting process
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USS New York LPD 21
Built from 24 tons of scrap steel from the World Trade Center
The steel was melted down in a foundry in Amite, LA to cast the ship’s bow
section. When it was poured into the molds on Sept 9, 2003, “those big rough
steelworks treated it with total reverence”, recalled Navy Capt. Kevin Wensing.
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Our focus here is relatively small parts
Advantages:
• Can create complex part geometries
• Can produce net-shape parts
• Cheap
Disadvantages :
– 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
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Open Molds and Closed Molds
Two Categories of Casting Processes
1. Expendable mold processes – one time use
–
Mold materials: sand, plaster, and similar materials, plus binders
2. Permanent mold processes – multiple use
–
Made of metal (or, less commonly, a ceramic refractory material)
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Sand Casting Mold
Definition: Cope, Drag, parting line, riser, runner, cavity, core,
downsprue
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Cooling Curve for a Pure Metal
• A pure metal
solidifies at a
constant
temperature equal
to its freezing
point (slightly
below melting
point)
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Riser
Reservoir in the mold which is a source of liquid metal to
compensate for shrinkage of the part during solidification
• The riser must be designed to freeze after the main casting in
order to satisfy its function
• To minimize waste in the unit operation, it is desirable for the
volume of metal in the riser to be a minimum
• Since the shape of the riser is normally designed to maximize
the V/A ratio, this allows riser volume to be reduced to the
minimum possible value
Quiz: why do we want to
maximize the V/A ratio?
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Solidification of Pure Metals
• Due to chilling action of mold
wall, a thin skin of solid metal is
formed at the wall surface
immediately after pouring
• Grain structure in a casting of a
pure metal, showing randomly
oriented grains of small size
near the mold wall, and large
columnar grains oriented
toward the center of the casting
http://www.youtube.com/watch?v=M95bhPrDwA0
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Solidification of Alloys
• Most alloys freeze over a temperature range
• Phase diagram for a copper-nickel alloy system and
cooling curve for a 50%Ni-50%Cu composition
Quiz: any problem with
this cooling curve?
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Solidification of Alloys
• Characteristic grain
structure in an alloy
casting, showing
segregation of alloying
components in center of
casting
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Cellular and Dendritic Solidification
Constitutional Supercooling:
T
k=
T1
Cell Structure:
T2
XS
XL
XL
T3
XS
TE
XE
Dendritic structure:
X0
Condition for non-constitutional cooling (Planar):
T1 - T3
T >
Dv
'
L
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No Diffusion in Solid, Diffusional Mixing in Liquid
ìï 1 - k - x üï
X L = X 0 í1+
e Dvý
ïî
ïþ
k
D/v = Characteristic width of concentration profile
Fig. 4.22,
T
k=
T1
T2
T3
XS
XL
XL
XS
TE
XE
X0
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Solidification Time
• Total solidification time tTS = time required for
casting to solidify after pouring
• tTS depends on size and shape of casting by
relationship known as Chvorinov's Rule
n
tTS
æV ö
= Cm ç ÷
è Aø
where V = volume of the casting; A = surface
area of casting; n = exponent with typical
value = 2; and Cm is mold constant.
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Shrinkage during Solidification and Cooling
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HW Assignment
• Read Chapter 6
• HW: Example 5.2 on page 109
• Problems 5.8, 5.9, 5.11, 5.12, 5.13,5.16
• Quiz for online students: why the columnar structure
grow perpendicular to the mold wall? (submit it
directly to Dr. Zhu by email: ytzhu@ncsu.edu)
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