CHAPTER 2
FUNDAMENTALS OF
METAL CASTING
Ass.Pr.Dr. Nguyen Ngoc Ha
1. FLUIDITY OF MOLTEN METAL
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
1. FLUIDITY OF MOLTEN METAL
• Fluidity: The capability of a molten metal to fill
mold cavities
• Viscosity: Higher viscosity decreases fluidity
• Surface tension: Decreases fluidity; ofte
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n caused
by oxide film
• Inclusions: Insoluble particles can increase
viscosity, reducing fluidity
• Solidification pattern: Fluidity is inversely
proportional to the freezing temperature range
1. FLUIDITY OF MOLTEN METAL
1. FLUIDITY OF MOLTEN METAL
• Mold design: The design and size of the sprue,
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
Fluidity Test
Fluidity Test
2. MOLD FILLING
• Bernouli’s equation:
• Reynold’s number:
Re= d/v
• Short filling time
• Potential turbulence
h + p/g + v2/2g = const
2. MOLD FILLING
• For this step to be successful, metal must flow
into all regions of the mold, most importantly the
main cavity, before solidifying
• Factors that determine success
– Pouring temperature
– Pouring rate
– Turbulence
• Pouring temperature should be sufficiently high
in order to prevent the molten metal to start
solidifying on its way to the cavity
2. MOLD FILLING
Pouring rate should neither be high (may stuck the
runner – should match viscosity of the metal) nor
very low that may start solidifying on its way to the
cavity
Turbulence should be kept to a minimum in order to
ensure smooth flow and to avoid mold damage and
entrapment of foreign materials. Also, turbulence
causes oxidation at the inner surface of cavity. This
results in cavity damage and poor surface quality of
casting.
Engineering Analysis of filling
1.v: velocity of
liquid metal at
base of sprue in
cm/sec; g:
981cm/sec.sec; h:
height of sprue in
cm
2.v1: velocity at
section of area
A1; v2: velocity at
section of area A2
3.V: volume of
mold cavity
COOLING EFFECTS ON MOLD CAVITIES
FILLED WITH MOLTEN METAL
Challenges of Molten Metal
• Hot metal readily forms oxides (dross or slag)
– Can be carried into the mold
– Can be controlled by pouring methods
– Control of temperature and atmosphere can slow
creation of slag
Challenges of Molten Metal
• Dissolved gases
– Porosity
– Can be controlled by:
• Vacuum degassing
• Gas flushing
• “Killing” - Reacting trapped gas with material that will
form buoyant compound that will float to surface
– Oxygen removed from copper by adding phosphorous
– Oxygen removed from steel by adding aluminum or silicon
Challenges of Molten Metal
Solubility of
hydrogen in
aluminum. Note the
sharp decrease in
solubility as the
molten metal begins
to solidify.
Challenges of Molten Metal
• Temperature Control
– Temp too Low
• Misruns
• Cold shuts
– Temp too High
• Excessive mold wear
• Higher reactivity of molten metal
• Penetration defects (excessive flash or entrapped sand
3. SOLIDIFICATION
3. SOLIDIFICATION
3.1.Classification of solidification processes
3.2.Solidification Defects
• Shrinkage - Contraction of a casting during
solidification.
• Microshrinkage - Small, frequently isolated pores
between the dendrite arms formed by the shrinkage
that accompanies solidification.
• Gas porosity - Bubbles of gas trapped within a casting
during solidification, caused by the lower solubility of
the gas in the solid compared with that in the liquid.
• Sievert’s law - The amount of a gas that dissolves in a
metal is proportional to the partial pressure of the gas
in the surroundings.
3.3. Solidification of Metals
• Transformation of molten metal back into
solid state
• Solidification differs depending on whether
the metal is
– A pure element or
– An alloy
– An eutectic alloy
3.3.1. Solidification of Pure Metals
(a) Temperature as a function of time for the solidification of
pure metals. Note that the freezing takes place at a constant
temperature. (b) Density as a function of time
3.3.2. Alloy Solidification
• 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
3.3.2. Alloy Solidification
Schematic illustration of alloy solidification and temperature distribution in
the solidifying metal. Note the formation of dendrites in the mushy zone.
3.3.3. Solidification of Eutectic Alloys
• Eutectic alloys solidify similar to pure metals.
• Eutectic point on phase diagram is a point at which
the liquid, on cooling, completely converts into solid
at one temp. No intermediate phase (L+S) exists.
• Al-Si (11.6% Si) and Cast Iron (4.3% C) are relevant
casting eutectic alloys.
3.4. Directional Solidification
• To minimize effects of shrinkage, it is desirable
for regions of the casting most distant from
the liquid metal supply to freeze first and for
solidification to progress from these regions
toward the riser(s)
– Thus, molten metal is continually available from risers to
prevent shrinkage voids
– The term directional solidification describes this aspect
of freezing and methods by which it is controlled
4.THERMODYNAMIC OF SOLIDIFICATION
4.1. Solidification of a pure element
4.2. Dendrite crystallization
Dendrite formatition
• In alloys, such as Fe-C, freezing and solidificaion occurs overa wide
range of temp. There is no fine line of demarcation exists between
the solid and liquid metal.
• Here, ‘start of freezing’ implies that grain formation while
progressing towards the center does not solidify the metal
completely but leaves behind the islands of liquid metals in
between grains which freeze later and there is multidirectional tree
like growth.
4.3. 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
Melt-Spinning
(a) Schematic illustration of melt-spinning to produce
thin strips of amorphous metal. (b) Photograph of
nickel-alloy production through melt-spinning. Source:
Siemens AG
5. KINETIC OF METAL SOLIDIFICATION
5.1. Solidification Time
• Once the material cools down to freezing
temperature, the solidification process for the
pure metals does not require a decrease in
temperature and a plateau is obtained in the
cooling curves, called thermal arrest. The
solidification time is total time required for the
liquid metal to solidify.
• Solidification time has been found to be directly
proportional to volume and inversely
proportional to surface area.
5.1. 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
where TTS = total solidification time; V = volume
of the casting; A = surface area of casting; n =
exponent with typical value = 2; and Cm is mold
constant.
n
TS m
V
T C
A
Mold Constant in Chvorinov's Rule
• Mold constant Cm depends on:
– Mold material
– Thermal properties of casting metal
– 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
What Chvorinov's Rule Tells Us
• Casting with a higher volume-to-surface area
ratio cools and solidifies more slowly than one
with a lower ratio
– To feed molten metal to the main cavity, TTS for riser
must be greater than TTS for main casting
• Since mold constants of riser and casting will
be equal, design the riser to have a larger
volume-to-area ratio so that the main casting
solidifies first
– This minimizes the effects of shrinkage
Solidification of Iron and Carbon Steels
(a) Solidification patterns for gray cast iron in a 180-mm (7-in.) square casting. Note
that after 11 minutes of cooling, dendrites reach each other, but the casting is still
mushy throughout. It takes about two hours for this casting to solidify completely. (b)
Solidification of carbon steels in sand and chill (metal) molds. Note the difference in
solidification patterns as the carbon content increases. Source: After H. F. Bishop and
W. S. Pellini
Solidified Skin on a Steel Casting
Solidified skin on a steel casting. The remaining molten metal
is poured out at the times indicated in the figure. Hollow
ornamental and decorative objects are made by a process
called slush casting, which is based on this principle. Source:
After H. F. Taylor, J. Wulff, and M. C. Flemings
5.2. Temperature Distribution during
Metal Solidification
Temperature
distribution at the
interface of the
mold wall and the
liquid metal
during the
solidification of
metals in casting
Comparison:
Sand Mold vs Metal Mold
Sand Mold
Sand casting Metal Mold
Die casting
5.3. Solidification of ingots and castings
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Development of the
ingot structure of a
casting during
solidification: (a)
Nucleation begins, (b)
the chill zone forms, (c)
preferred growth
produces the columnar
zone 3, and (d)
additional nucleation
creates the equiaxed
zone
5.3. Solidification of ingots and castings
- Because of the chilling
action of the mold wall, a
thin skin of solid metal is
initially formed at
interface immediately
after pouring.
- The skin formed initially
has equi-axed, fine
grained and randomly
oriented structure. This is
because of rapid cooling.
- As freezing proceeds, the
grains grow inwardly,
away from heat flow
direction, as needles or
spine of solid metal.
5.3. Solidification of ingots and castings
- On further growth of
spine, lateral branches
are formed, and as these
branches grow further
branches are formed at
right angle to the first
branches. This type of
growth is called dendritic
growth.
- The dendritic grains are
coarse, columnar and
aligned towards the
center of casting.
5.3. Solidification of ingots and castings
- The dendrites begin to form
with freezing. However, due to
large temperature spread
between solidus and liquidus,
the earlier portion of dendritic
grains extract higher % of
elements from liquid solution
than the portion of grain
formed later.
- As a result, the molten metal
in the center of mold cavity
depletes from the elements
and hence forms a different
structure (see Fig).
Pure metal
Fe-Ni Alloy
Grain Structure in Casting
Chill Zone – Rapid cooling near the
surface creates many nucleation sites and
many small, randomly oriented grains.
Columnar Zone – Directionally oriented
grains radiating inward from the surface
of the part.
Equiaxed Zone (not shown) – Randomly
oriented, spherical crystals. (isotropic
properties)
Three Cast Structures of Solidified Metals
•Schematic illustration of
three cast structures of
metals solidified in a square
mold:
(a) pure metals;
(b) solid-solution alloys;
and
(c) the structure obtained
by heterogeneous
nucleation of grains, using
nucleating agents.
5.4. Segregation
Composition change during solidification
5.4. Segregation
• The non-uniform distribution
of impurities or alloying
elements. The degree of
segregation depends not only
on the chemical composition
of the alloy, but also on the
rate of cooling, both of the
ingot as a whole, and of each
individual point within the
mass.
5.4. Segregation
• For example, near the surface, where the rate of
cooling is rapid, the segregated impurities are
trapped in the rapidly growing crystals. Fur- their
inside the ingot, where the cooling is slower, the
segregates will collect together and produce the
so-called ghosts, or they may tend to rise to the
surface and collect in the scrapped ingot head. In
normal segregation, the constituents with the
lowest melting points concentrate in the last
portions to solidify, but in inverse segregation this
is reversed. The segregation tends to form in
bands sloping inwards to the top of the ingot (A
segregate) and at the same time, due to
shrinkage, it takes a V shape (V segregate) along
the upper part of the ingot axis.
Microsegregation
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