1.0 Fundamentals of Casting

Chapter 52005 Pearson Education South Asia Pte Ltd
CASTING
2005 Pearson Education South Asia Pte Ltd
Chapter Outline
What is Metal Casting ?
It is defined as the process of producing a metallic component by pouring and allowing the liquid (metal or alloy) to solidify in side a mold cavity of pre-designed shape.
1.0 Fundamentals of Metal Casting
Terminology !!
1.0 Fundamentals of Metal Casting
Casting is one of the most versatile manufacturing processes
An ancient art with primitive skills
First metals cast were probably gold and copper.
Cu melts at 1084° C
Gold melts at 1064° C
Egyptians were expert at investing or “lost wax” casting
Allowed small intricate shapes to be cast.
Bronze casting
Made when people realized that copper tin alloys cast much easier than copper alone.
Bronze casting of bells became important
Introduction
What is casting??
Melt metal/ alloy
Solidification (by cooling the melt in the mold)
remove the cast component (also called as casting) from the mold
Finishing
Advantages of Casting
Best suited for composite components
Can produce intricate shapes and internal openings
Flow of molten metal
Solidification of Metals
After molten metal is poured into a mold, a series of events takes place during the solidification of the metal and its cooling to ambient temperature.
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Pure Metals
Density as a function of time
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Pure Metals
After the temperature of the molten metal drops to its freezing point, its temperature remains constant while the latent heat of fusion is given off.
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Pure Metals
Pure metals
Solid-solution alloys
Nucleating agents
Alloys
Solidification in alloys begins when the temperature drops below the liquidus, TL , and is complete when it reaches the solidus, TS
Fig 10.4 shows the Schematic illustration of alloy solidification and temperature distribution in the solidifying metal.
Note the formation of dendrites in the mushy zone.
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Alloys
Alloys
Within this temperature range, the alloy is in a mushy or pasty state consisting of columnar dendrites.
The width of the mushy zone (where both liquid and solid phases are present) is an important factor during solidification.
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Alloys
For alloys, a short freezing range generally involves a temperature difference of less than 50°C, and a long freezing range greater than 110°C.
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Alloys
Alloys
Effects of Cooling Rates
Slow cooling rates course grain structures, large spacing between dendrite arms.
Faster cooling rates finer grain structures.
Grain size influence the properties of the casting.
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Fluid Flow
Fig 10.8 shows the schematic illustration of a typical riser-gate casting.
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Basic casting system:
Flows though the gating system into the mold cavit
Sprue – is a vertical channel though which the molten metal flows downward in the mold
Runners – channels that carry the molten metal from the sprue to the mold cavity
Gate – is the portion of the runner which the molten metal enters the mold cavity
Risers – serve as reservoirs to supply the molten metal necessary to prevent shrinkage.
Fluid Flow
Fluid Flow
Bernoulli’s theorem
This theorem is based on the principle of the conservation of energy and relates pressure, velocity, the elevation of the fluid at any location in the system, and the frictional losses in a system that is full of liquid.
Mass Continuity
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Fluid Flow
Flow Characteristics
0 < Re < 2000 laminar flow
2000<Re<20,000 mixture of laminar & turbulent flow
Re > 20 000 => severe turbulence
Techniques for minimizing turbulence
Dross or slag can be eliminated by vacuum casting
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Fluidity of a Molten Metal
The capability of the molten metal to fill mold cavities is called fluidity, which consists of two basic factors: (1) characteristics of the molten metal and (2) casting parameters.
Molten Metal
1. Viscosity
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2. Surface Tension
A high surface tension of the liquid metal reduces fluidity.
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3. Inclusions
Because they are insoluble, inclusions can have a significant adverse effect on fluidity.
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4. Solidifications pattern of alloy
The manner in which solidification takes place can influence fluidity.
Moreover, fluidity is inversely proportional to the freezing range.
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Casting parameters
1. Mold Design
The design and dimensions of the spruce, runners, and risers all influence fluidity.
Mold material and its surface characteristics
Higher the thermal conductivity of the mold and the rougher its surfaces, the lower the fluidity of the molten metal.
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3. Degree of superheat
Superheat (defined as the increment of temperature of an alloy above its melting point) improves fluidity by delaying solidification.
The pouring temperature often is specified instead of the degree of superheat, because it is specified more easily.
4. Rate of pouring
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5. Heat Transfer
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Test for fluidity
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Heat Transfer
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Solidification time
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Solidification time
The solidification time is a function of the volume of a casting and its surface area (Chvorinov’s rule):
where C is a constant that reflects (a) the mold material, (b) the metal properties (including latent heat), and (c) the temperature.
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Shrinkage
Shrinkage, which causes dimensional changes and (sometimes) cracking, is the result of the following three sequential events:
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Shrinkage
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Defects
hot tears in castings:
These defects occur because the casting cannot shrink freely during cooling, owing to constraints in various portions of the molds and cores.
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Defects
Common Defects in Casting
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Defects
The International Committee of Foundry Technical Associations has developed a standardized nomenclature, consisting of seven basic categories of casting defects identified with boldface capital letters:
A—Metallic projections
G—Inclusions
Porosity
Porosity in a casting may be caused by shrinkage, or gases, or both.
Porous regions can develop in castings because of shrinkage of the solidified metal.
Microporosity also can develop when the liquid metal solidifies and shrinks between dendrites and between dendrite branches.
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Porosity
Porosity caused by shrinkage can be reduced or eliminated by various means such as:
Adequate liquid metal should be provided to avoid cavities caused by shrinkage.
Internal or external chills, as those used in sand casting also are an effective means of reducing shrinkage porosity.
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Porosity
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Casting is a solidification process in which molten metal is poured into a mold and allowed to cool.
Solidification of pure metals takes place at a constant temperature, whereas solidification of alloys occurs over a range of temperatures.
Composition and cooling rates of the molten metal affect the size and shape of the grains and the dendrites.
Melting practices have a direct effect on the quality of castings, as do foundry operations.
Concept Summary
Class Activity
Explain the reasons why heat transfer and uid ow are important in metal casting?
We know that pouring metal at a high rate into a mold has certain disadvantages. Are there any disadvantages to pouring it very slowly?
Why Reynolds number is important in casting?
Explain the reason of hot tearing in casting?
State the different between the short and long freezing ranges?
What is different between the solidification of pure metals and metal alloys?
How to test for fluidity?
Why it is important to remove dross/slag during pouring? How?
What are some of the limitations of casting?
What does heat of fusion mean in casting?
(10.1)
range
Freezing
S
L
T
T

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1.0 Fundamentals of Casting