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Introduction It is one of the oldest processes for shaping materials, which involves pouring molten metal into a mold cavity. Upon solidification, the molten metal takes the shape of the mold cavity. The figure below shows the outline of the metal-casting processes available to us:

Solidification of Metals After the molten metal is poured into the mold, the solidification takes place takes place at a clearly defined constant temperature, followed by cooling at the ambient temperature. These processes influence the size, shape, uniformity and chemical composition of grains formed throughout the casting. The factors affecting these events are:

Themal properties of both metal and mold Geometric relationship between volume and surface area of casting Shape of the mold

The diagram shown below is indicating the temperature as a function of time for solidification of pure metals. The freezing takes place at a constant temperature, known as the freezing point. When the metal solidifies and cools, it shrinks. Shrinkage can lead to;

Microcracking Porosity

These affects decrease the mechanical properties of the casting formed.

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At the mold walls, at ambient temperature the metal cools rapidly and produces a solidified skin or shell

The grains grow in a direction opposite to that of the heat transfer through the mold The grains that have favourable orientation in preference are called columnar grains As the driving force of the heat transfer is decreased away from the walls, the grains

become coarse The above described method of grain growth is called homogeneous nucleation,

meaning that grains grow themselves starting at the mold wall

Effects of cooling rates

Slow cooling rates (around 102 K/s) or long local solidification times give rise to coarse dendritic structures with large spacing between dendrite arms

For higher cooling rates (around 104 K/s) or short local solidification times, the structure of grains is much finer with smaller dendrite arm spacing

The structures and grain size influence the casting properties.

As grain size decreases, the strength and the ductility of the cast alloy increases Microporosity decreases The tendency for cracking of casting decreases

Lack of uniformity in grain size and grain distribution result in casting with

anisotropic properties

A major ratio which influences the rate of solidification is G/R ratio, where G is the thermal gradient and R is the rate at which in the liquid-solid interface moves.

Structure property relationships The relation between properties and structures developed during solidification are important for castings if they are to meet their design and service requirements.

When the alloy is cooled very slowly, each dendrite develops a uniform composition

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Under faster cooling rates, cored dendrites are formed. They have a surface composition different from that at their centres.

The darker shading at the inter-dendritic liquid near the dendrite roots indicate that these regions have a higher solute concentration.

The figure above illustrates three basic types of cast structures:

1. Columnar dendritic 2. Equiaxed dendritic 3. Equiaxed non dendritic

Because of the presence of thermal gradients in a solidifying mass of liquid metal, and due to gravity and the density differences, convection has a strong effect on the structures formed. Convection has the following effects:

Formation of an outer chill zone Grain size refinement Acceleration of transition from columnar to equiaxed grains

On reducing or eliminating convection gives rise to coarser and longer columnar

dendritic grains Fluid Flow Consider a basic gravity-casting system. The molten metal is poured through a pouring basin, making it flow through the gating system (sprue, runners and gates) into the mold cavity.

Sprue is a tapered vertical channel through which the molten metal flows downward in the mold

Runners are channels which carry the molten metal from sprue into the mold cavity Risers (also feeders) act as reservoirs of molten metal to supply any molten metal

required to prevent porosity because of shrinkage effects upon solidification

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Successful casting needs proper design and control of solidification process and to ensure proper fluid flow in the system. A properly designed gating system helps to avoid problems like, premature cooling, turbulence and gas entrapment.

Two basic principles having influence on gating system design: 1. Bernoulli’s Theorem 2. Law of mass continuity

Bernoulli’s theorem: It is based on the principle that the total energy related to pressure, velocity and the fluid level is constant. Mathematically it is given as;

Let 1 and 2 denote the two different locations in the system, the following relationship is satisfied:

Where ‘f’ represents the frictional loss in the liquid while travelling through the system. Mass Continuity: For incompressible flow of liquid, the discharge through any two given locations is constant

According to this, the fluid flow rate must be maintained throughout the system at all times.

The wall permeability is an important aspect otherwise some liquid will escape through the walls. This is very common in sand molds, so coatings are used to inhibit this behaviour in sand molds.

Sprue Design: Traditionally the sprue designs are tapered at the ends. Because if a sprue is designed with a constant cross section area throughout, then regions where liquid loses contact with the sprue walls will be developed. As a result, aspiration (a process due to which air is sucked in or entrapped in the liquid) takes place.

A tapered sprue is used to prevent molten metal separation from the sprue walls Straight sided sprues are supplied with choking mechanism at the bottom, which

slow the flow sufficiently to prevent aspiration effect in sprue.

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Assuming the pressure at the top and bottom of the sprue to be equal, with no frictional losses, the relationship between height and cross sectional area at any point in the sprue is given by a parabolic relationship:

Flow Characteristics:

An important consideration in fluid flow in gating system is that of turbulence In gating systems, Re (Reynolds Number) typically ranges from 2000 to 20,000 If Re exceeds 20,000 severe turbulence can harm the gating system, which results in

very high air entrainment and dross formation. Dross is the scum that forms on the surface of molten metal.

Techniques used to eliminate dross:

Skimming Using properly designed pouring basins and runner systems Using filters made of ceramics, mica or fibreglass

Fluidity of Molten Metal The ability of the molten metal to completely fill the mold cavity is called fluidity. The factors influencing fluidity are:

Viscosity: As viscosity and its temperature sensitivity increase, fluidity decreases Surface Tension: High value of surface tension reduces fluidity, because of formation of

an oxide film on the surface which increases the surface tension, e.g. an oxide film on the surface of pure molten Aluminium triples the surface tension

Inclusions: Due to their insolubility, they have a negative effect on the fluidity, e.g. effect of sand in oil to oil’s fluidity

Solidification pattern of the alloy: The fluidity is inversely proportional to freezing range. Metals have shorter freezing range, thus higher fluidity as compared to fluidity of alloys with higher freezing range

Mold design: Proper design of mold, sprue, runner etc have significant effect on the fluidity

Mold Material and its Surface Characteristics: A mold material with high thermal conductivity and rough surface will have low fluidity. Fluidity can be improved by heating the mold, it slows down the solidification of the metal. But, the casting develops coarse grains and has lower strength

Degree of superheat: It indicates the temperature of the liquid metal in excess of its melting point. Higher superheat improves fluidity by delaying solidification. Pouring temperature is much more appropriate to specify than superheat.

Rate of pouring: The slower the rate of pouring of molten metal in the mold, lower is the fluidity because of higher rate of cooling and solidification

The term castibility is used to describe the ease with which a metal can be cast to

produce a part with good quality. It not only includes fluidity, but the nature of casting practices as well.

Heat transfer It is a very important consideration in metal casting. It is a complex phenomenon, e.g. when casting thin sections, the metal flow rates must be high enough to avoid premature chilling and solidification. But, the flow rate must not be so high so as to cause excessive turbulence with negative effects.

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The figure shows a temperature distribution at the mold liquid-metal interface. Heat from the liquid metal is transferred through the mold wall to the surrounding air. The temperature drop at the air-mold and mold-metal interfaces is caused by the presence of boundary layers and imperfect contact at these interfaces. The shape of the curve depends upon the thermal properties of the molten metal and the mold.

Solidification time: A thin skin forms when the solidification begins, and as the time passes then skin gets thickened.

With flat mold walls thickness is proportional to the square root of time. Thus, doubling the time will make skin 41% thicker.

The solidification time can be found out by Chvorinov’s rule:

Where C = a constant that reflects: Mold material Metal properties (including latent heat) Temperature

Parameter ‘n’ has a value between .5 and 2. Shrinkage: The phenomenon of shrinkage, which results in dimensional changes like warping and cracking, is caused because of three sequential events:

1. Contraction of the molten metal as it cools prior to its solidification. 2. Contraction of the metal during phase change from liquid to solid (latent heat of fusion). 3. Contraction of the solidified metal (the casting) as its temperature drops to ambient

temperature.

The largest potential amount of shrinkage occurs during the cooling of the casting to ambient temperature.

Defects As per International Committee of Foundary Technical Associations, there are seven basic categories of casting defects, which are identified by bold capital letters:

A. Metallic Projections- fins, flash or projections like swells and rough surfaces

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B. Cavities- blowholes, pinholes and shrinkage cavities C. Discontinuities- cracks, cold or hot tearing and cold shuts D. Defective surface- surface folds, laps, scars, adhering sand layers and oxide scale E. Incomplete casting- misruns and runout F. Incorrect dimensions or shape G. Inclusions

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Summary of various casting processes Process Advantages Limitations Sand Almost any metal can be cast,

no limit to part size, low cost Finishing operations needed, coarse surface finish, wide tolerances

Shell Mold Good dimensional accuracy and surface finish; high production rate

Limit to part size, high pattern costs

Evaporative pattern Most metals can be cast, no limit to size; complex part shapes

Patterns have low strength and can be costly for low quantities

Plaster Mold Complex part shapes; good dimensional accuracy and surface finish; low porosity

Limited to non ferrous metals; limited part size and volume of production; mold making time is high

Ceramic mold Complex part shapes, close tolerance parts, good surface finish

Limited part size

Investment Intricate part shapes; excellent surface finish and accuracy; almost any metal can be cast

Part size limited, expensive patterns, molds and labor

Permanent Mold Good surface finish and dimensional accuracy; low porosity

High mold cost, limited part shape, not suitable for high melting point metals

Die Excellent dimensional accuracy and surface finish; high production rate

High die cost, limited part size, long lead time

Centrifugal Large cylindrical or tubular parts with good quality; high production rate

Expensive equipment, limited part shape

The figure below shows the Outline of production steps in a typical sand casting operation

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The major features of molds in sand casting are as follows: 1. The flask, which supports the mold itself. Two-piece molds consist of a cope on top and a

drag on the bottom; the seam between them is the parting line. When more than two pieces are used in a sand mold, the additional parts are called cheeks.

2. A pouring basin or pouring cup, into which the molten metal is poured. 3. A sprue, through which the molten metal flows downward. 4. The runner system, which has channels that carry the molten metal from the sprue to

the mold cavity. Gates are the inlets into the mold cavity. 5. Risers, which supply additional molten metal to the casting as it shrinks during

solidification. Two types of risers-a blind riser and an open riser. 6. Cores, which are inserts made from sand. They are placed in the mold to form hollow

regions or otherwise define the interior surface of the casting. Cores also are used on the outside of the casting to form features such as lettering on the surface or deep external pockets.

7. Vents, which are placed in molds to carry off gases produced when the molten metal comes into contact with the sand in the mold and the core. Vents also exhaust air from the mold cavity as the molten metal flows into the mold.

Inspection of Castings It is an essential step in maintaining casting quality. Castings can be inspected; visually, optically. Sub surface and internal defects are investigated through many NDTs. Design considerations for Castings The two design issues related to castings are:

1. Geometric features, tolerances etc., that should be incorporated in the part 2. Mold features that are needed to produce the desired casting

A good casting design involves the following steps: Part shape must be so, that it is easy to cast Select an appropriate casting process as per the part size, part shape, production volume

needed etc. Location of parting line in the mold Location and design of gates for adequate and uniform fluid flow Selection of an appropriate runner geometry Assuring good control measures and practices are in place

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Notes