CASTING NAME.docx

8/14/2019 CASTING NAME.docx

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DESIGN OF CASTINGS

FACTORS INVOLVED

Heat transfer between metal and mold

Design considerations in casting Designing for directional solidification and minimum stresses Principle of design of gating and risering

Heat transfer between metal and mold

Hot liquid metal takes time to lose its heat and solidify. The rate at which it can lose its heat is controlled by a number of resistances. The resistances to heat flow from the interior of the casting: The liquid, Solidified metal,

The metal and mould interface, The mould, The surrounding of the mould.

Thermal resistance and control the rate of loss of heat

Resistance 1: Casting

This type of freezing regime is not common for metal castings of high thermal conductivitysuch as the light alloys or Cu-based alloys.

The casting of Pb - Sb alloy into steel dies for the production of battery grids and terminals;the casting of steel into a copper mould; or the casting of hot wax into metal dies as in the

injection of wax patterns for investment casting.

Unidirectional flow of heat from a metal poured exactly at its melting point T, against amould wall initially at temperature To, The metal/mould interface


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Heat flow is controlled to a significant extent by the resistance at the metal mould interface.This occurs when both the metal and the mould have reasonably good rates of heat

conductance, leaving the boundary between the two the dominant resistance.

The interface becomes overriding in this way when an insulating mould coat is applied, orwhen the casting cools and shrinks away from the mould (and the mould heats up,

expanding away from the metal), leaving an air gap separating the two.

The metal/mould interface

As the casting cools and the mould heats up, the two remain in good thermal contact whilethe casting interface is still liquid. When the casting starts to solidify, it rapidly gains

strength, and can contract away from the mould. In turn, as the mould surface increases in

temperature it will expand and air gap is formed.

Inward and outward expansion

Variable air Gap


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Air Gap Vs Time

Metal / mould interface


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Resistance 3 : Mould

The rate of freezing of castings made in silica sand moulds is generally controlled by the rateat which heat can he absorbed by the mould.

sand mould acts as an excellent insulator, keeping the casting warm.

However, of course, ceramic investment and plaster moulds are even more insulating,avoiding premature cooling of the metal, and aiding fluidity to give the excellent ability to fill

thin sections for which these casting processes are renowned.

It is regrettable that the extremely slow cooling can contribute to rather poorer mechanicalproperties.

Increased Heat Transfer

The rate of heat extraction from a casting using a number of tricks. placement of chill blocks in the mould, adjacent to the casting, fins attached to the casting to increase the surface area through which heat can be

dissipated.

Convection

Convection is the bulk movement of the liquid under the driving force of density differencesin the liquid.

heavy solutes cause the liquid to sink, and the lighter solutes cause flotation. hot liquid will expand, becoming less dense, and will rise; cool liquid will contract, becoming

denser, and so will sink.

the hot metal is at the bottom and the cold metal at the top. As the casting starts to solidify,the cold liquid metal drifts downward, draining into the riser tube. Here it is replaced by hot

metal flowing up the heated riser tube and into the casting.

Convection Driven flow


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Thermal convection

Casting Design - Solidification

solidification (the freezing of molten metal inherent in all casting processes). The solidification of a casting can involve as many as three separate contractions as a result

of cooling:

o LiquidLiquid Contractiono LiquidSolid Contractiono SolidSolid Contraction

Liquid-liquid contraction occurs as a result of the liquid cooling from its pouring temperature(usually 110 to 165 C,or 200 to 300 F, above its melting point) down to the melting point or

solidification temperature.

Solidification

Solid-solid contraction occurs after a casting has solidified and as it cools from thesolidification temperature to room temperature.

The design engineer must be concerned with this contraction. To ensure that the dimensionsof the casting are correct, the pattern used to produce a given casting usually must be made

slightly larger than the casting dimensions at room temperature.

The pattern maker compensates for this pattern enlargement for a particular alloy by using ashrink rule specifically for the alloy involved.

Further, because the amount of solid contraction is a function of the particular metal to becast, problems with dimensions can often occur when changing alloys if the same pattern

equipment is used.


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Solidification

designers specifying the alloys should carefully consider any change in alloy to ensure thatthe cost of new equipment does not cancel out the benefits to be achieved by such changes.

Solid-solid contraction should also be considered in part design, because it is one of theprimary causes of warped and cracked castings.

A basic concept that governs the way castings cool is the casting modulus (the volume of aportion of a casting divided by the surface area of that portion of the casting).

This relationship of geometry to cooling is easily understood by considering the effects ofboth volume and surface area on cooling rates.

As volume increases, more hot metal will be contained within it, and the casting willtherefore take longer to cool.

Conversely, because all the heat within a casting must pass through a surface at themetal/mold interface, the greater the surface area, the faster the casting will cool.

Thus, as the volume-to-surface area ratio (casting modulus) increases, the time required forcooling and solidification is extended.

Solidification

Solidification

Thinner sections will have cooled and contracted by solid-solid contraction before thethicker section, and because of this the thicker section applies a compressive stress to the

thinner sections as it cools.

Such stresses have been measured to levels as high as 552 MPa (80 ksi), depending on thealloy and section size variations.

Therefore, a casting designed in this way will have a strong tendency toward warpage as aresult of the imposed stresses.

Although a casting may not be warped when taken from the mold, the internal stresses thatdevelop as a result of design can appear at later stages as cracks or warping, often after heat

treatment welding, and machining.


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If design features result in warpage, foundrymen often compensate for this by using tie barsthat are usually small cast-on bars attached to various parts of the casting to brace the

casting and therefore prevent warpage.

It can lead to the distortion of castings include improper heat-treating practices, difficult to-collapse molds or cores, shakeout procedures, rigging, and the way in which the casting

cools; these factors are under the control of the foundry engineer. Problems with warpage and cracks can often be eliminated through the cooperative effort of

casting designers and foundry engineers.

The importance of solid-solid contraction as applied to design means that an attempt shouldbe made to reduce dramatic variations in the section sizes of castings.

Liquidsolid contraction

Liquid-solid contraction is by far the greatest difficulty due to solidification that faces thefoundry engineer.

It is also one of the greatest opportunities for the design engineer to design for low cost,high quality, and timely delivery.

Most metals contract as they pass from the liquid to the solid state. Certain compositions ofgray and ductile iron are the exceptions to this rule in the major alloys produced by

foundries.

The entire founding process is possible only because volumetric contraction locates itself insolidifying castings in a systematic way.

Solidification sequence

The aspect of liquid-solid contraction that allows castings to be produced is that all of the

contraction is concentrated in the last portion(s) of the casting to solidify.

The foundry man uses this principle to produce sound castings by attaching a volume ofmetal to the last portion of the casting to solidify.

This technique is illustrated in Fig. 2. Such feed metal reservoirs are called risers. Properplacement of risers on castings changes the way in which both casting and riser(s) solidify

such that the riser is the last to solidify.

When used properly, this produces a casting free of shrinkage because all the shrinkage forthe entire mass of both casting and riser will be concentrated in the riser.

However, in many cases, the design of the casting restricts the proper placement of risers,making the production of sound castings difficult if not impossible.


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Riser Design

RISER DESIGN, or risering, deals with the development of suitable reservoirs of feed metal inaddition to the desired casting shape so that undesirable shrinkage cavities in the casting are

eliminated or moved to locations where they are acceptable for the intended application of

the casting. When metals solidify and cool to form a casting, they generally undergo three distinct stages

of volume contraction, or shrinkage.

Liquid shrinkage: The liquid metal loses volume as it gives up superheat and cools to itssolidification temperature.

Solidification shrinkage: The metal freezes, changing from a liquid to a higher-density solid.For pure metals, this contraction will occur at a single temperature, but for alloys it will take

place over some temperature range or freezing interval.

Solid shrinkage: The solid casting cools from its solidification temperature to roomtemperature.

Solid shrinkage (also called patternmaker's shrinkage), is accommodated by making thepattern (and therefore the mold cavity) somewhat larger than the desired dimensions of the

final casting.

Liquid shrinkage and solidification shrinkage are the concern of risering practice. In theabsence of risers, a casting would otherwise solidify.

To eliminate these undesirable defects in the casting, a riser will be added to accommodatethe liquid shrinkage and to supply liquid feed metal to compensate for the solidification

shrinkage within the casting (Fig. 3). Therefore, the shrinkage in the riser/casting system is

concentrated in the riser, which will then be removed from the finished casting.

The riser must often be larger than the casting it feeds, because it must supply feed metalfor as long as the casting is solidifying. Various methods are used to reduce the size of therequired riser, including chilling the casting (that is, reducing its solidification time) or

insulating the riser (that is, extending its solidification time).

Optimum Riser design

The riser/casting junction should be designed to minimize riser removal costs. The number and size of risers should be minimized to increase mold yield and to reduce

production costs.

Riser placement must be chosen so as not to exaggerate potential problems in a particularcasting design (for example, tendencies toward hot tearing or distortion)


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DEFECTS WITH POOR RISER DESIGN

Feed Metal Volume : The riser must be adequate to satisfy the liquid and solidificationshrinkage requirements of the casting. In addition, the riser itself will be solidifying, so the

total shrinkage requirement to be met will be for the riser/casting combination. The total

feeding requirement will depend on the specific alloy, the amount of superheat, the casting

geometry, and the molding medium.

Liquid shrinkage will depend on the alloy and the amount of superheat. As indicated in Fig.1, liquid shrinkage for carbon steels is generally considered to be in the range of 1.6 to

1.8%/100 C (0.9 to 1.0%/100 F) superheat. For graphitic cast irons, liquid shrinkage has

been variously reported in the range of 0.68 to 1.8%/100 C (0.38 to 1.0%/100 F).

Solidification Shrinkage.solidification shrinkage will vary considerably according to the alloymelted and that, within the graphitic cast irons, expansion may occur. This phenomenon is

often ascribed to the precipitation of the less dense graphite phase overcoming thecontraction associated with the solidification of austenite.

Mold Dilation. Mold wall movement after a mold cavity has been filled with liquid metal canenlarge the casting and thus increase the feed metal requirements. Such mold dilation is a

function of the molding medium, the mold filling temperature, and the alloy.

Casting Geometry. The shape of a casting will affect the size of the riser needed to meet itsfeed requirements for the obvious reason that the longer the casting takes to solidify, the

longer the riser must maintain a reservoir of liquid metal.

Riser Location

To determine the correct riser location, the methods engineer must make use of the conceptof directional solidification. If shrinkage cavities in the casting are to be avoided,

solidification should proceed directionally from those parts of the casting farthest from the

riser, through the intermediate portions of the casting, and finally into the riser itself, where

the final solidification will occur. Shrinkage at each step of solidification is thus fed by liquid

feed metal being drawn out of the riser.

The ability to achieve such directional solidification will depend on: The alloy and its mode of solidification The mold medium The casting design


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Directional Solidification

With the mold cavity filled, solidification will generally proceed from the mold wall, where askin of solid metal will form. As heat is lost to the mold, that skin will grow progressively

inward. Two conditions serve to change the rate of this growth. At the casting edge, where

the greater surface area allows more rapid transfer of heat to the mold, the solidification

rate will be faster. At the riser, where the mass of the riser provides more heat, and where

heat transfer to the mold is reduced at the internal angle of the riser/casting junction, the

rate of skin formation will be reduced. This combination of edge effect, or end effect, and

riser effect provides directional solidification.

Gate Design

A GATING SYSTEM is the conduit network through which liquid metal enters a mold andflows to fill the mold cavity, where the metal can then solidify to form the desired casting

shape.

The basic components of a simple gating system for a horizontally parted mold are shown inFig. 1.

A pouring cup or a pouring basin provides an opening for the introduction of metal from apouring device.

A sprue carries the liquid metal down to join one or more runners, which distribute themetal throughout the mold until it can enter the casting cavity through in gates.


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Gating design variables

Rapid mold filling Minimizing Turbulence. Avoiding Mold and Core Erosion. Removing Slag, Dross, and Inclusions. Promoting Favorable Thermal Gradients. Maximizing Yield. Economical Gating Removal. Avoiding casting distortion Compatibility With Existing Molding/Pouring Methods. Controlled Flow Conditions. Rapid mold filling can be important for several reasons. Especially with thin-section castings,

heat loss from the liquid metal during mold filling may result in premature freezing,

producing surface defects (for example, cold laps) or incompletely filled sections (misruns).

Superheating of the molten metal will increase fluidity and retard freezing, but excessive

superheat can increase problems of gas pickup by the molten metal and exaggerate the

thermal degradation of the mold medium. In addition, the mold filling time should be kept

shorter than the mold producing time of the molding equipment to maximize productivity.

Minimizing Turbulence. Turbulent filling and flow in the gating system and mold cavity canincrease mechanical and thermal attack on the mold. More important, turbulence may


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Controlled Flow Conditions. A steady flow rate of metal in the gating system should beestablished as soon as possible during mold filling, and the conditions of flow should be

predictably consistent from one mold to the next.

Directional Solidification

The process used to manufacture directionally solidified castings with a columnar structurerequires careful control to ensure that castings which are of acceptable quality are

produced. Specialized furnaces are used, and mold design is quite different from that used

for conventional investment castings.

directional solidification is in the manufacture of blades (rotating parts) for gas turbineengines.

These components are subjected to high stresses along their major axes, as well as hightemperatures. Because grain boundaries are weaker than grains at high temperatures, it is

logical to align them parallel to the axis of principal stress to minimize their effect on

properties.

The alloy that was originally used in directionally solidified turbine components was MAR M-200, a nickel base alloy containing 12.5% W.

The solidified structure consisted of tungsten-rich dendrites with high strength and creepresistance that grew to the length of the casting.

The grain-boundary material, which was parallel to the dendrites, was strong enough towithstand the transverse stresses on the components. The properties of the directionallysolidified alloy were far superior to those of the equi axed alloy.

the production of directionally solidified castings requires that both the thermal gradientand its rate of travel be controlled. For the case of nickel-base alloys, thermal gradients of 36

to 72 C/cm (165 to 330 F/in.) have been found to be effective (Ref 4), and rates of travel of

30 cm/h (12 in./h) can be used.

In columnar structures, the primary dendrites are aligned, as are the grain boundaries. Theprimary dendrites form around spines of the highest-melting constituent to freeze. As

freezing continues, the solid rejects solute into the residual liquid (segregation occurs) until

the final low-melting eutectic has frozen at the grain boundaries. Because segregation

products collect in the grain boundaries, it is important to consider the composition of these

grain boundaries in directional structures.

An ideal composition for directional solidification is one in which the primary dendrites formaround a strong spine, while the grain boundaries also retain their strength. A poor alloy is

one in which the segregation products form embrittling phases, especially adjacent to

secondary dendrite arms, which are normal to the primary stress axis.


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Defects unique to directional solidification

Equiaxed grains are most often freckles, which are caused by segregation of eutectic liquidthat is less dense than the bulk liquid in many alloys. This liquid forms jets within the mushy

zone, and as these jets freeze they form equiaxed grains. Freckles are usually cured by

increasing the thermal gradient and solidification rate in the casting.

Misoriented grains occur when the temperature ahead of the interface falls below theliquidus temperature and new grains nucleate. These grains will have a random orientation,

but because they are growing in gradient, they will be columnar. They can be eliminated by

increasing the gradient.

Shrinkageis sometimes encountered on the upper surfaces of directionally solidifiedcastings. There is no way to feed these surfaces; the addition of risers to these surfaces

usually interferes with radiation heat transfer from another part of the casting. The most

common solution is to invert the casting in order to minimize the surface area that is

susceptible to shrinkage.

Microporositymay occur in directionally solidified castings if the length of the mushy zone(length of the casting that is between the liquidus and solidus temperatures during

solidification) becomes too great for feed metal to reach into the areas where solidification

is taking place. Increasing the thermal gradient (which shortens the length of the mushy

zone) usually solves this problem.


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Mold or Core Distortion. A frequent cause of scrap in directionally solidified castings resultsfrom mold or core distortion. Because the mold and core are held at high temperatures for

long times while the casting solidifies, it is possible for the mold or core to sag or to undergo

local allotropic transformations of the refractory materials from which they are made. The

resulting changes in mold or core dimensions are reflected in the casting dimensions. Careful

control of the core and mold composition, their uniformity, and the firing conditions under

which they are made is required in order to avoid these dimensional problems.

Gating system

It refers to all the sections through which the molten metal passes while entering into themould cavity.

Elements of gating system:1.pouring cup

2.sprue

3.sprue well

4.runner

5.ingates

6.riser

Functions of gating system

Fill the mould cavity completely before freezing. Minimizing turbulence. Avoiding erosion Removing inclusions Regulate flow of molten metal. Consume least metalless scrap Trap contaminants. Establish directional solidifications. Comparison of gating system


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Design and location of ingates

Multiple ingates are preferable for the large castings A fillet should be used where an ingate meets a castingproduces less turbulence. The minimum ingate length should be three to five times the ingates width, depending on

the metal being cast.

Curved ingates should be avoided, as far as possible.

Design of riser * uses two methods CAINES method and Modulus Methode*

It acts as a reservoir of molten metal in the mould to compensate for shrinkage duringsolidification.

Guidelines for riser design and location: Riser must not solidify before casting. The volume of riser must be large enough to feed the entire shrinkage of the casting. The pressure head from the riser should enable complete cavity filling. Riser must be placed so that the liquid metal can be delivered to locations where it is most

needed.

Modulus method

Modulus of solidification of casting or riser is defined as the ratio of its volume and surfacearea.

Modulus method is based on chvorinovs rule.


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Canes methode

A casting with a higher modulus ( volume to surface area ratio) cools and solidifies moreslowly than the one with a lower modulus.

To feed molten metal to the casting. TST of the riser be greater than TST of the casting. Since mould constants of riser and casting will be equal, design the riser to have a larger

modulus, so that the main casting solidifies first,

Requirement of the riser to feed the casting: Modulus of the riser should be greater than modulus of casting, then only riser can succes

fully feed the casting.

Solidification of casting

Two steps:

Nucleation Growth

Nucleation:

It refers to the process in which tiny solid particles called nuclei are formed when liquidmetal cools below its liquidous temperature.

Two types of nucleation:


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Homogeneous nucleation : occurs without the help of foreign particles. Heterogeneous nucleation : occurs with the help of foreign particles ( such as the mould

material, impurities and added nucleating materials)

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