Temperature & Density for Castings - JUfilesjufiles.com/wp-content/uploads/2016/05/Casting-Slides.pdf · Manufacturing Processes for Engineering Materials, ... Manufacturing Processes

Manufacturing Processes for Engineering Materials, 5th ed.

Kalpakjian • Schmid

© 2008, Pearson Education

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Metal Casting




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Important factors in casting

• Solidification of the metal from its molten

state and accompanying shrinkage

• Flow of the molten metal into the mold cavity

• Heat transfer during solidification and

cooling of the metal in the mold

• Mold material and its influence on the

casting operation




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Solidification of Metals

• Pure metals have clearly defined melting or

freezing points and solidification takes place

at a constant temperature.

• The latent heat of fusion is released during

the freezing leading to this fixed temperature

of melting/freezing

• After solidification the solid metal contracts a

it cools down to room temperature




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• Alloys, on the other hand, solidify over a

range of temperatures (freezing range)

between the Liquidus and the Solidus

• In this range the alloy is in a mushy or pasty

state




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Temperature & Density for Castings

FIGURE 5.1 (a) Temperature as a function of time for the solidification of pure metals. Note that

freezing takes place at a constant temperature. (b) Density as a function of time.




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Cast Structures – Pure Mretals

• A typical grain structure of a solidified pure

metal consists of the following zones:

• A chill zone (skin) as the shell in contact with

the mold walls solidify. This is characterized

by fine grained shell around the casting due

to the initially high cooling rates.




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• Columnar grains which have a preferential

growth direction due to the fact that

solidification proceeds in opposite direction

to heat transfer

• Grains having substantially different direction

of growth are blocked from further growth




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Texture in Castings

FIGURE 5.5 Schematic illustration of three cast structures of metals solidified in a square mold: (a) pure

metals, with preferred texture at the cool mold wall. Note in the middle of the figure that only favorable

oriented grains grow away from the mold surface; (b) solid-solution alloys; and (c) structure obtained by

heterogeneous nucleation of grains.




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Cast Structures / Alloys

• As the alloy enter the mushy state within its

freezing range, it develops columnar

dendritic grains

• The alloys tend to develop a third zone of

equi-axed grains in the middle of the casting

• If heterogeneous nucleation is used then a

fully equi-axed grained structure may be

obtained




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Alloy Solidification & Temperature

FIGURE 5.6 Schematic illustration of alloy solidification and temperature distribution in the solidifying metal.

Note the formation of dendrites in the semi-solid (mushy) zone.




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Solidification Patterns for Gray Cast

Iron

FIGURE 5.7 Schematic illustration of three basic types of cast structures: (a) columnar dendritic; (b)

equiaxed dendritic; and (c) equiaxed nondendritic. Source: After D. Apelian.




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Effect of Cooling Rate

• Slow cooling rates (100 °K/s) or long

solidification times result in coarse dendritic

structures with large spacing between the

dendrite arms

• Higher cooling rates (10000 °K/s) or short

solidification times result in coarse dendritic

structures with smaller spacing between the

dendrite arms




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• For even higher cooling rates (106 - 108

°K/s) solidification proceeds with the

formation of amorphous structures (without

any ordered crystalline structure)




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Cast Structures

FIGURE 5.9 Schematic illustration of cast structures in (a) plane

front, single phase, and (b) plane front, two phase. Source: After D.

Apelian.

FIGURE 5.8 Schematic illustration of three

basic types of cast structures: (a) columnar

dendritic; (b) equiaxed dendritic; and (c)

equiaxed nondendritic. Source: After D.

Apelian.




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Structure Property relationships

• As the structure becomes more fine-grained:

• The strength and ductility increase

• Microporosity (intedendritic shrinkage voids)

decrease)

• Tendency for cracking (hot tearing)

decreases




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• A lack of uniformity in grain size and

distribution results in anisotropic properties.

• Slow cooling is more likely, however, to

result in uniform composition of the

dendrites, while fast cooling is likely to result

in cored dendrites




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• The surface of the dendrites having

compositions different to their core due to

solute rejection in the liquid is known as

“microsegregation”




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Macrosegregation

• Differences in composition throughout the

casting itself




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Normal segregation

• when the solidification front moves as a

plane front (see figure 5.9) , lower melting

point constituents move towards the center.




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Inverse Segregation

• When the center of the casting has lower

concentration of alloying elements as in the

case of solid-solution alloys




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Gravity Segregation

• Higher density inclusions and compounds

sink while lighter ones float to the surface




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Innoculants

• Nucleating agents added to the casting to

enhance heterogeneous nucleation and

extend the equi-axed grains zone (see figure

5.5b)

• An example is TiB2 in Aluminum alloys to

refine grain patterns and improve

mechanical properties




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Rheocasting

• Breaking dendrites by mechanical or

electromagnetic action to promote

heterogeneous nucleation (Fig. 5.8-b)




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Fluid Flow

• Consider a basic gravity casting system

• The molten metal is poured through a

pouring basin

• it then flows through the sprue to the well

and into the runners to the mold cavity

• Risers (also known as feeders) serve as

reservoirs of molten metal to supply the

metal necessary to substitute for shrinkage




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• Successful casting requires proper design

and control of the solidification process to

ensure adequate fluid flow during casting




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Requirements of gating system

• Contaminants trapping; causing the

contaminants (oxides and other inclusions)

to adhere to the gating system walls and

prevent their reach to the mold cavity

• Avoid or minimize problems such as

premature cooling, turbulence and gas

entrapment and introduction of impurities

into the molten metal




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Fluidity of the molten metal

• The ability of the molten metal to fill mold

cavities

• Fluidity depends on:

• Molten metal characteristics and

• Casting parameters




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Molten metal characteristics

• Viscosity. Fluidity decrease as viscosity

Increase

• Surface tension. A higher surface tension of

the liquid metal (such as the presence of

oxide film on the metal surface) significantly

affects fluidity in an adverse manner




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• Inclusions. The presence of inclusions

significantly affects fluidity in an adverse

manner (observe the flow of a liquid through

sand particles)

• Solidification pattern of the alloy. Fluidity is

inversely proportional to the freezing range




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Casting Parameters

• Mold design. The design and dimensions of

the gating system, risers, etc.,

• Mold material and its surface characteristics.

The higher the thermal conductivity of the

mold material the lower the fluidity

• Higher degree of super heat improves fluidity

• Heat transfer directly affects the molten

metal viscosity and, hence, fluidity




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Test for Fluidity

• A common test is to let molten metal flow

along a channel that is at room temperature

• The distance the metal travels before it

solidifies and stops running is an indication

of its fluidity




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Mold Features

FIGURE 5.10 Schematic illustration of a typical sand mold showing various features.




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Temperature Distribution

FIGURE 5.11 Temperature distribution at the mold wall and liquid-metal interface during solidification of metals in casting.

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Temperature & Density for Castings - JUfilesjufiles.com/wp-content/uploads/2016/05/Casting-Slides.pdf · Manufacturing Processes for Engineering Materials, ... Manufacturing Processes