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FUNDAMENTALS OF METAL CASTING
1.5 Solidification and Cooling of Metals
Metal casting is a process whereby molten metal is poured into a mold the shape of the desired finished product. Casting is one of the oldest metal shaping processes; according to Biblical records, casting technology reaches back almost 5500 years BCE, or 7500 years ago. Gold was the first metal to be discovered and shaped according to prehistoric people’s fancy. The Chinese made iron castings around 1000 BCE, and steel was fabricated in India about 500 BCE.
Four main elements are required in the process of casting: a pattern, a mold, cores, and the work-piece. The pattern, the original template from which the mold is prepared, creates a corresponding cavity in the casting material. Cores are used to produce tunnels or holes in the finished mold, and the workpiece is the final output of the process.
The cast metal remains in the mold until it has solidified, and it is then ejected or revealed to show the fabricated part or casting.
The casting process is divided into two broad types of casting:
•expendable-mold casting processes, and
•nonexpendable-mold casting processes.
Expendable-mold casting is a general classification that includes sand, plastic, shell, and investment (lost wax technique) moldings. All of these involve the use of temporary and nonreusable molds, and they all need gravity to help force the molten fluid into the casting cavities. In this process, the mold in which the molten metal solidifies must be destroyed in order to remove the casting. It is used only once.
Nonexpendable mold casting differs from expendable processes in that the mold need not be destroyed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting.
Steel cavities of molds are coated with some refractory wash layer like acetylene soot before processing to allow for easy removal of the workpiece and to promote longer tool life. The useful lifetime of permanent molds varies depending on maintenance: when the useful life is over, such molds require refinishing or replacement. Cast parts from a permanent mold generally show a 20% increase in tensile strength and a 30% increase in elongation as compared to the products of sand casting. Typically, permanent mold castings are used in forming iron, aluminum, magnesium, and copper-based alloys. The process is highly automated.
There are many different factors and variables that go into metal casting; each one can change the final product. That is why it is important that each one be considered in order to get a successful casting process.
Because different types of metal casting processes are available, it is one of the most used manufacturing processes. Casting technology is used to fabricate a large number of the metal components in designs we use every day. The reasons for this include the following advantages:
•Casting can produce very complex part geometries with internal and external shapes.
•It can be used to produce very small workpieces (a few hundred grams) to workpieces of very large size (over 100 tons).
•With some casting processes it is possible to manufacture final shapes that require no further manufacturing operations to achieve the required dimensions and tolerances of the parts.
•Casting is economical, with very little wastage. The extra metal in each casting is remelted and reused.
•Casting metal is isotropic: It has the same physical and mechanical properties in all directions.
•Some types of metal casting are very suitable for mass production.
There are also some disadvantages for different types of castings. These include the following:
•poor finish, wide tolerance (sand casting);
•limited workpiece size (shell molds and ceramic molds);
•patterns have low strength (expendable pattern casting);
•expensive, limited shapes (centrifugal casting);
•porosity (all types);
•environmental problems (all types).
To accomplish a casting process the worker must heat metal to the desired temperature for pouring. Heat is the energy that flows spontaneously from a higher temperature object to a lower temperature object through random interactions between their atoms. The heat energy required for heating metal to a pouring temperature is the sum of:
•the heat needed to raise the temperature to the melting point;
•the heat of fusion needed to convert it from a solid to a liquid;
•the heat needed to raise the molten metal to the desired temperature for pouring.
This energy can be expressed as a sum of phase energy by the following formula:
| Q | = | Qs + Qf + Ql | (1.1) | |
| Q | = | ρV[cs(Tm − T0) + Lf + c1(Tp − Tm)] | (1.1a) |
where
| Q | = | total heat energy, J (Btu) |
| Q s | = | heat energy for solid metal, J (Btu) |
| Q f | = | heat energy for fusion, J (Btu) |
| Q l | = | heat energy for liquid metal, J (Btu) |
| ρ | = | density, kg/m3 |
| V | = | volume of metal being heated, m3 (in3) |
| c s | = | specific heat for solid metal, J/kg°C (Btu/lbm°F) |
| c l | = | specific heat for liquid metal, J/kg°C (Btu/lbm°F) |
| T 0 | = | starting temperature, °C (°F) |
| T m | = | melting temperature of the metal, °C (°F) |
| T p | = | pouring temperature, °C (°F) |
| L f | = | heat of fusion of the metal, J/kg (Btu/lbm). |
The heat of fusion is the amount of heat required to convert a unit mass of a solid at its melting point into a liquid without an increase in temperature.
Equation (1.1a) can be used for the approximate calculation of the total heat energy because values cs, and c1 vary with the temperature; in addition, significant heat losses to the environment during heating are not implied by this equation.
Figure 1.1 shows a phase-change diagram of the process of heating for metal casting.
Fig. 1.1 Phase change diagram of the process of heating metal to a molten temperature sufficient for casting.
After the metal is heated to pouring temperature Tp the metal is ready for pouring. As the introduction of the molten metal into the mold includes the fluid flow in casting, we will describe a basic gravity casting system as shown in Fig. 1.2.
Fig. 1.2 Cross-section of a typical two-part sand mold.
Figure 1.2 presents a cross-section of a typical two-part sand mold and incorporates many features of the casting process. These are:
Drag. The drag is the bottom half of any of these features.
Core. A core is a sand shape that is inserted into the mold to produce internal features of a casting, such as holes or passages for water cooling.
Flask. The flask is the box that contains the molding aggregate.
Cope. In a two-part mold, the cope is the top half of the pattern, flask, mold, or core.
Core print. A core print is the region added to the pattern, core, or mold; the core print is used to place and support the core within the mold.
Riser. Risers serve as reservoirs of molten metal to supply any molten metal necessary to prevent shrinking during solidification.
Gating system. The gating system is the network of channels used to deliver the molten metal from outside the mold into the mold cavity.
Pouring basin. The pouring basin or cup is the portion of the gating system that initially receives the molten metal from the pouring vessel and controls its delivery to the rest of the mold. From the pouring basin the metal travels down the sprue (the vertical portion of the gating system), then along horizontal channels (called runners), and finally through controlled entrances, or gates, into the mold cavity.
One of the key elements to making a metal casting of high quality is the design of a good gating system. This is even more important if a casting is produced by a gravitational process.
The process of pouring the molten metal has to be performed carefully; if not, there will be various casting defects directly traceable to pouring the molten metal incorrectly in the stage of mold filling. For example, too rigorous a stream could cause mold erosion; highly turbulent flows could result in air and inclusion entrapments; and finally, relatively slow filling might generate cold shuts. Thus, the design of the gating and venting overflow systems has to take into consideration the proper control of the liquid metal as it fills the mold.
An optimum gating system design can reduce the turbulence of the molten material’s flow, minimizing gas, inclusions, and dross. If poor gating techniques are used, invariably, lower casting quality is the result because of damage to the molten metal received during its flow through the gating system. It could be even worse if the molten material is a material sensitive damage (dross and slag formation) during mold filling. Aluminum and its casting alloys are such materials.
Aluminum alloys are very reactive to oxygen, and they form an oxide, Al2O3. When flow is smooth, this oxide tends to form and remain on the surface of the steam. However, when flow is turbulent, the oxide goes into the molten metal and may carry gas or bubbles with it. Then, to avoid damage to the molten metal, the gating system must be designed to eliminate the air problems by avoiding conditions that permit aspiration due to the formation of lowpressure areas.
In order to achieve an adequate gating system design it is necessary to follow basic principles. Molten metal behaves according to fundamental hydraulic principles. Applying those fundamentals to the design of the gating system can be an advantage.
Several mathematical and theoretical factors affect the flow of molten metal through the gating system and into the mold, and it is a good idea to have an understanding of their effect on the process. They are Bernoulli’s theorem, law of continuity, the effects of the force of friction, and Raymond’s number.
a) Bernoulli’s Theorem
The Bernoulli effect is simply a result of the conservation of energy in the steady flow of an incompressible fluid; Bernoulli’s theorem states that the sum of the energies in a flowing liquid is constant at any two points. This can be written in the following form:
| (1.2) |
where
| h | = | elevation above a certain reference plane, m (in.) |
| p | = | pressure at that elevation, Pa (psi) |
| v | = | velocity of the liquid at that elevation, m/s (in./s) |
| ρ | = | density of the fluid, kg/m3 (lbm/in.3) |
| g | = | gravitation constant, m/s2 (in./s2) |
| f | = | frictional loss in the liquid as it travels downward through the system |
Subscripts 1 and 2 indicate two different elevations in the liquid flow.
Let us simplify equation (1.2). If we ignore friction losses we can assume that the system remains at atmospheric pressure throughout. Point 1 is defined as being at the top of the sprue and point 2 at its base; if point 2 is used as the reference plane (h2 = 0) and the metal is poured into the pouring basin, then the initial velocity at the top is zero (v1 = 0). Hence, equation (1.2) further simplifies to
| (1.3) |
The velocity of the liquid metal flow at the base of the sprue is
| (1.4) |
where
| h 1 | = | height (length) of the sprue, m (in.) |
| v 2 | = | velocity of the liquid metal at the base of the sprue, m/s (in./s) |
| g | = | gravitation constant, m/s2 (in./s2). |
b) Mass Continuity
The law of mass continuity states that for incompressible liquids and in a system with impermeable walls, the volume rate of flow remains constant throughout the liquid. Thus,
| Q = A1v1 = A2v2 | (1.5) |
where
| Q | = | volume rate of flow, m3/s (in.3/s) |
| A | = | cross-section area of the liquid stream, m2 (in.2) |
| v | = | velocity of the liquid in the system, m/s (in./s) |
Subscripts 1 and 2 indicate two different locations in the liquid flow.
As the liquid metal accelerates during its descent into the sprue opening, equation (1.5) indicates that the cross-sectional area of the channel must be reduced. Hence, the sprue should be tapered.
c) Flow Characteristics
A molten metal’s flow characteristics constitute very a important consideration in the gating system because of the possible consequences of turbulence. However, there are two different types of real fluid flow: laminar and turbulent. In laminar flow the fluid moves in layers called laminas. Laminar flow need not be in a straight line. For laminar flow, the flow follows the curved surface smoothly, in layers. Moreover, the fluid layers slide over one another without fluid being exchanged between the layers.
In turbulent flow, secondary random motions are superimposed on the principal flow and there is an exchange of fluid from one adjacent sector to another. More important, there is an exchange of momentum such that slow-moving fluid particles speed up and fast-moving particles give up their momentum to the slower moving particles and themselves slow down.
The factor that determines which type of flow is present is the ratio of the inertial forces to the viscous forces within the fluid. This ratio is expressed by the dimensionless Reynolds number as
| (1.6) |
where
| Re | = | Reynolds number |
| v | = | mean fluid velocity, m/s (in./s) |
| L | = | characteristic length (equal to diameter if a cross-section is circle), m (in.) |
| µ | = | dynamic fluid viscosity, Ns/m2 (lbm/in.2) |
| ρ | = | density of the fluid, kg/m3 (lbm/in.3) |
Fluid flows are laminar for Reynolds numbers up to 2100. A transition between laminar and turbulent flow occurs for Reynolds numbers between 2100 and 40,000, depending upon how smooth the tube junction is and how carefully the flow is introduced into the tube. Above Re = 40,000, the flow is always turbulent. Hence,
| Re | < | 2100 – laminar flow |
| Re | ≥ | 2100 < 40,000 – transition flow |
| Re | < | 40,000 – always turbulent |
The Reynolds number may be viewed another way:
The viscous forces arise because of the internal friction of the fluid. The inertia forces represent the fluid’s natural resistance to acceleration. In a flow with a low Reynolds number the inertia forces are negligible compared with the viscous forces, whereas in a flow that has a high Reynolds number the viscous forces are small relative to the inertia forces.
In foundry science fluidity is defined as the ability of the molten metal to flow easily before being stopped by solidification. Factors that affect the fluidity of molten metal are given in Table 1.1.
Table 1.1 Factors affecting molten metal fluidity
| FACTORS | DESCRIPTION |
| Viscosity | Viscosity is an internal property of a fluid that offers resistance to flow. If the temperature of liquid increases, the viscosity tends to decrease and the fluidity increases. |
| Surface tension | Surface tension is an effect within the surface layer of the molten metal that causes that layer to behave as an elastic sheet. Surface tension is caused by the attraction between the molecules of the liquid as a result of various intermolecular forces. A high surface tension of the molten metal decreases fluidity.Oxide films on the surface of the molten metal have a significant negative effect on fluidity. Fluxing processes in the heating of materials are used to reduce or eliminate oxidation and to improve the fluidity of surface metal layers. |
| Inclusions | Inclusions are slag or other foreign matter entrapped during molten metal casting. Metallic and nonmetallic inclusions have long been recognized as one of the most important quality issues for metal casting. The presence of inclusions is often the cause of decreased fluidity. |
| Composition | Composition is one of the main factors influencing fluidity. Small amounts of alloy additions to pure metals reduce fluidity. For example, among elements that decrease the casting fluidity of pure aluminum are Ti, Fe, Zr, and others. |
| Superheat | The difference between the melting temperature and the liquid temperature is also a very important factor influencing fluidity. Fluidity increases with increasing melt temperatures for given alloy compositions. The pouring temperature is often specified, rather than the superheat temperature, because it is easier to do so. |
| Rate of pouring | The slower the rate at which molten metal is poured, the lower the fluidity will be. |
| Mold material | Studies of the influence of the mold material have found that fluidity in fine grain sand is lower than in coarse sand. Significant metal penetration was observed in the coarse sand spiral, which resulted in an increase of the length of the spiral. |
| Heat transfer rate | If the heat transfer rate between the molting metal and the mold is reduced, fluidity increases. |
| Coating | An important function of mold coatings is to reduce the heating transfer rate between the flowing metal and the mold. |
The two tests commonly used to measure fluidity of the molten metal are the following:
•the spiral fluidity test, and
•the vacuum fluidity test.
Spiral test. The spiral test has been traditionally used in the foundry because it includes the effect of the mold material. The spiral test measures the length the metal flows inside a spiral-shaped mold.
Vacuum test. The vacuum test measures the length the metal flows inside a narrow channel when sucked from a crucible by a vacuum pump. Those tests are shown schematically in Fig. 1.3:
Fig. 1.3 A scheme of two fluidity tests: a) spiral test b) vacuum test.
The length of such casting, under standardized conditions, is taken as the fluidity index of that metal. The greater the length of the solidified metal, the greater is its fluidity.
1.5 SOLIDIFICATION AND COOLING OF METALS
After being poured into the mold, molten metal cools and solidifies. This section is primarily concerned about what happens after the metal actually is poured into a mold. A series of events and transitions takes place during the process of the solidification of the molten metal and its cooling to ambient temperature. These events greatly influence the size, shape, and chemical composition of the grains formed throughout the casting. The type of metal, the thermal properties of the molten metal and molds, the relationship between the volume and the surface area of the casting, and the shape of the mold are all significant factors that affect these transitions.
The cooling rate of a casting affects its microstructure, quality, and properties. The cooling curve illustrates the way in which molten metals solidify. There is a fundamental difference between the cooling curve observed during the solidification of a pure metal and that of an alloy.
a) Pure Metal
The temperature of metal at its pouring point is higher than its solidification temperature. The solidification temperature stays constant throught the period of time until all the liquid metal become solid.
The actual freezing of metal takes time, the local solidification time in casting during which the metal’s latent heat of fusion is released into the surrounding mold. The total solidification time is the time taken between pouring temperature and complete solidification. After complete solidification, the solidified metal, called the casting, is taken out of the mold and allowed to cool to ambient temperature. Figure 1.4 shows the cooling curve for a poured metal during casting.
Fig. 1.4 Cooling curve for a poured metal during casting.
The solidification process begins at the interface of mold and metal and over the entire outer skin of the casting. This rapidly cooling action causes the grains in the skin to become fine and randomly oriented. As cooling continues and the energy transfer continues through the solid thin layer of metal toward the mold material, energy travels in one direction while the energy of the solidification process travels in the opposite direction. Since the heat transfer is through the skin wall, the grains continually grow as dendritic growth until complete solidification has been achieved. The grains resulting from this dendritic growth are coarse and columnarly oriented toward the center of the casting. Schematic illustration of grain formation is shown in Fig. 1.5.
Fig. 1.5 Schematic illustration of grain structure in a casting of a pure metal.
b) Alloys
Solidification in the case of an alloy begins when the temperature of the molten metal drops below the liquidus and is completed when the solid form is reached. A phase diagram and cooling curve for alloys during casting is shown in Fig. 1.6.
Fig. 1.6 Phase diagram and cooling curve for alloy composition during casting.
While pure metals have a well-defined melting point temperature, for alloys there is a melting temperature range, over which liquid and solid co-exist. The melting range can be quite large, which is the case for cobalt–chromium alloys, resulting in a phenomenon known as coring. Coring means that individual grains do not have the same chemical composition from the center to the outer edge of the grain. The importance of coring is that it produces a microstructure that is in general more likely to be attacked by galvanic corrosion. However, in the case of cobalt–chromium, the metal is protected from corrosion by the formation of a very thin metal oxide on the surface, so signs of corrosion are rarely observed.
Solidification begins with the formation of solid nuclei and the growth of these nuclei into the liquid by the addition of atoms at the advancing interface. If the latent heat that evolves during solidification is not removed, the solid nuclei will heat back up to the melting temperature and solidification will stop. Consequently, the rate of solidification is determined primarily by the rate of the latent heat of melting.
On a macroscopic scale, the structure of casting depends on the rate of nucleation and the rate of heat removal from the casting, e.g., on the temperature of the molten liquid when it is poured into the mold and on the temperature of the mold walls. If the wall is cold and the casting is large, solidification will begin in the chill zone at the mold wall and proceed inward, parallel to the flow of heat out from the melt, shown in Fig. 1.7 as zone a.
Fig. 1.7 Grain structure in a casting: (a) chill zone, (b) columnar zone, (c) equiaxial grain structure.
The grains so formed are therefore elongated, or columnar, in shape. In Fig. 1.7, this is shown as zone b. If zone b can reach the center of the casting before the temperature there drops low enough for nucleation to occur, the entire casting structure will be composed of columnar grains. Usually this is not the case, since the center part of the casting begins to solidify before the columnar grains arrive there. Because the grains in the center of the casting are not forced to grow in any particular macroscopic direction, heat being removed isotropically, they are more equiaxial in shape. In Fig. 1.7 this is shown as zone c.
The relative amount of columnar versus equiaxial grains depends on such macroscopic factors as the rate of heat removal and the uniformity of cooling, as well as the presence of solid nuclei in the liquid region. A uniform but slow cooling rate, accompanied by a quiescent liquid pool, leads to a coarse grain structure in the interior of the casting, whereas a fast cooling rate and turbulent pool produce a fine grain structure, which is more desirable for good mechanical properties. In fact, a general rule about any solidification process is this: the faster the cooling rate, the finer the microstructure.
The total solidification time is the time required for the casting to solidify from molten metal after pouring. This time is a function of the volume of a casting and the surface area that is in contact with the mold.
According to Chvorinov’s rule, the mathematical relationship can be written as
| (1.7) |
where
| t | = | solidification time, s (min) |
| V | = | volume of the casting, m3(in.3) |
| A | = | surface area of the casting, m2(in.2) |
| c | = | mold constant |
| n | = | exponent (1.5 < n ≤ 2, but usually taken as 2). |
The mold constant c depends on the properties of the cast metal (heat of fusion, specific heat, and thermal conductivity), the mold material, and the pouring temperature. The value of c for a given casting operation can be based on experimental data from previous operations carried out using the same mold material, metal, and pouring temperature, even though the shape of the workpiece might be very complex.
The rule simply states that under the same conditions, a casting with a large surface area and small volume will cool more rapidly than a casting with a small surface area and large volume.
Like most materials, molten metals typically have a lower density than solid ones, so there is an expectation that the casting will be proportionally smaller (i.e., it will shrink) than the pattern from which it was cast. Shrinkage is result of the following factors:
•contraction of the liquid as it cools prior to its solidification
•contraction during phase change from a liquid to solid
•contraction of the solid as it continues to cool to ambient temperature.
These natural phenomena are manifest as either volumetric or linear shrinkage.
a) Volumetric Shrinkage
Volumetric, or liquid-to-solid shrinkage is the shrinkage of the metal as it goes from a state of disconnected atoms and molecules (liquid) to the formed crystals of atoms and chemical compounds, the building blocks of solid metal. The amount of solidification shrinkage varies a great deal from alloy to alloy. In the treatment of some alloys, disregard for this type of shrinkage may result in voids in the casting. Both the design engineer and the foundry engineer have the tools to combat this problem, but the designer has the most cost-effective tool, which is geometry. For some alloys finding that geometry can be very simple. For other alloys finding that geometry is the real essence of good casting design.
The shrinkage caused by solidification can leave cavities in a casting, weakening it. Risers provide additional material to the casting as it solidifies. The riser is designed to solidify later than the part of the casting to which it is attached. Thus, the liquid metal in the riser will flow into the solidifying casting and feed it until the casting is completely solid. In the riser itself, there will be a cavity showing where the metal was fed. Risers are often necessary to produce parts that are free of internal shrinkage voids.
Sometimes, to promote directional shrinking, chills must be used in the mold. A chill is any material that will conduct heat away from the casting more rapidly then the material used for molding. Thus, if silica sand is used for molding, a chill may be made of copper, aluminum, or graphite.
Table 1.2 presents some typical values of volumetric contraction during the solidification and cooling of various casting metals.
b) Linear Shrinkage
Shrinkage after solidification can be dealt with by using an oversized pattern designed for the relevant alloy. Pattern makers use special shrink rulers to make the patterns used by the foundry to make castings to the design size required. These rulers are 2–6% oversized, depending on the material to be cast. Using such a ruler during pattern-making will ensures an oversize pattern. Thus, the mold is larger also, so when the molten metal solidifies, it will shrink and the casting will be the size required by the design.
Table 1.2 Solidification and cooling/shrinking of various metals
Various defects can occur in manufacturing processes, depending on factors such as materials, part design, and processing techniques. While some defects affect only the appearance of parts, others can have major adverse effects on the structural integrity of the parts made. According to the International Committee of Foundry Technical Association, several basic categories of defects can develop in castings. For each basic category, only one typical defect is being presented here.
a) Metallic Projections
Consists of fins, flash, or massive projections.
Example: Joint flash or fins.
A flat projection of irregular thickness, often with lacy edges, perpendicular to one of the faces of the casting. It occurs along the joint or parting line of the mold, at a core print, or wherever two elements of the mold intersect.
Possible causes:
•Clearance between two elements of the mold or between the mold and the core
•Poorly fitting mold joint
•Flasks not being held at low burnout temperature long enough
•Speed being set too high on centrifugal casting machine.
Remedies:
•Care in pattern making, molding and core-making
•Control of dimensions
•Care in core setting and mold assembly
•Sealing of joints where possible.
b) Cavities
Cavities consist of rounded or rough internal or exposed cavities, including blowholes, pinholes, and shrinkage cavities.
Example: Blowholes, pinholes.
These formations are smooth-walled cavities, essentially spherical, often not contacting the external casting surface (blowholes). The largest cavities are most often isolated; the smallest, pinholes, appear in groups of varying dimensions. In specific cases the casting section can be strewn with blowholes or pinholes. The interior walls of blowholes and pinholes can be shiny, more or less oxidized, or, in the case of cast iron, covered with a thin layer of graphite. The defect can appear in all regions of the casting.
Possible causes:
•Excessive gas content in metal bath (charge materials, melting method, atmosphere, etc.). Dissolved gases are released during solidification.
•In the case of steel and cast irons, formation of carbon monoxide by the reaction of carbon and oxygen present as a gas or in oxide form. Blowholes from carbon monoxide may be increased in size by the diffusion of hydrogen or, less often, nitrogen.
•Excessive moisture in molds or cores.
•High level of aluminum or titanium in the base iron.
•Core binders that liberate large amounts of gas.
•Excessive amounts of additives containing hydrocarbons.
•Blacking and washes, which tend to liberate too much gas.
•Insufficient evacuation of air and gas from the mold cavity.
•Insufficient mold and core permeability.
•Entrainment of air due to turbulence in the runner system.
•Investment not mixed properly or long enough.
•Invested flasks not having been vibrated during vacuum cycle.
•Vacuum extended past working time.
Remedies:
•Making adequate provision for evacuation of air and gas from the mold cavity.
•Increasing permeability of mold and cores.
•Avoiding improper gating systems.
•Assuring adequate baking of dry sand molds.
•Controlling moisture levels in green sand molding.
•Reducing the aluminum content of the base iron metal.
•Increasing metal pouring temperature.
•Reducing amounts of binders and additives used or changing to other types.
•Using blackings and washes, which provide a reducing atmosphere.
•Keeping the sprue filled and reducing pouring height.
•Increasing static pressure by lengthening the runner height.
c) Discontinuities
Discontinuities include cracks, cold or hot tearing, and cold shuts. If the solidifying metal is constrained from shrinking freely, cracking and tearing can occur. Although many factors are involved in tearing, coarse grain size and the presence of low-melting segregates along the grain boundaries (inter-granular areas) increase the tendency for hot tearing. Incomplete castings result from the molten metal being at too low a temperature or from the metal being poured too slowly. Cold shut is an interface in a casting that lacks complete fusion because of the meeting of two streams of liquid metal from different gates.
Example: Hot cracking.
Hot cracking is a crack that is often scarcely visible because the casting in general has not separated into fragments. The fracture surfaces may be discolored because of oxidation. The design of the casting is such that the crack would not be expected to result from constraints during cooling.
Possible causes:
•Damage to the casting while hot, due to rough handling or excessive temperature at shakeout.
Remedies:
•Care in shakeout and in handling the casting while it is still hot.
•Sufficient cooling of the casting in the mold.
•For metallic molds; delay of knockout, assuring mold alignment, using ejector pins.
d) Defective Surface
Defective surfaces are ones that have folds, laps, scars, adhering sand layers, or oxide scale.
Example: Flow marks.
On the surfaces of otherwise sound castings, the defect appears as lines that trace the flow of the streams of liquid metal.
Possible causes:
•Oxide films that lodge at the surface, partially marking the paths of metal flow through the mold.
•Metal, flask or both being too hot.
Remedies:
•Increasing mold temperature.
•Lowering the pouring temperature.
•Modifying gate size and location (for permanent molding by gravity or low pressure).
•Tilting the mold during pouring.
•In die casting, vaporblast or sand blast mold surfaces that are perpendicular, or nearly perpendicular, to the mold parting line.
e) Incomplete Casting
Incomplete casting, such as misruns (due to premature solidification), insufficient volume of metal poured, and runout (due to loss of metal from mold after pouring).
Example: Poured shot.
The upper portion of the casting is missing. The edges adjacent to the missing section are slightly rounded, but all other contours conform to the pattern. The sprue, risers, and lateral vents are filled only to the same height above the parting line as is the casting (contrary to what is observed in the case of defect).
Possible causes:
•Insufficient quantity of liquid metal in the ladle.
•Premature interruption of pouring due to workman’s error.
•Metal too cold when cast.
•Mold too cold when cast.
Remedies:
•Having sufficient metal in the ladle to fill the mold.
•Checking the gating system.
•Instructing pouring crew and supervising pouring practice.
f) Incorrect Dimensions or Shape
Owing to factors such as improper shrinkage allowance, pattern mounting error, irregular contraction, deformed pattern, or warped casting.
Example: Distorted casting.
Inadequate thickness, extending over large areas of the cope or drag surfaces at the time the mold is rammed.
Possible causes:
•Rigidity of the pattern or pattern plate is not sufficient to withstand the ramming pressure applied to the sand.
Remedies:
•Assuring adequate rigidity of patterns and pattern plates, especially when squeeze pressures are being increased.
g) Inclusions
Inclusions form during melting, solidification, and molding. Generally nonmetallic, they are regarded as harmful because they act as stress raisers and reduce the strength of the casting. They can be filtered out during processing of the molten metal. Inclusions may form during melting because of reaction of the molten metal with the environment (usually oxygen) or the crucible material. Chemical reactions among components in the molten metal may produce inclusions; slags and other foreign material entrapped in the molten metal also become inclusions. Reactions between the metal and the mold material may produce inclusions. Spalling of the mold and core surfaces also produces inclusions, indicating the importance of the quality and maintenance of molds.
Example: Metallic inclusions.
Metallic or intermetallic inclusions can be of various sizes; they are distinctly different in structure and color from the base material and most especially different in properties. These defects most often appear after machining.
Possible causes:
•Combinations formed as intermetallics between the melt and metallic impurities (foreign impurities).
•Charge materials or alloy additions that have not completely dissolved in the melt.
•Molten metal containing excess flux or foreign oxides.
•During solidification, the formation and segregation of insoluble intermetallic compounds concentrating in the residual liquid.
•Contaminants in wax pattern.
Remedies:
•Assuring that charge materials are clean and eliminating foreign metals.
•Using small pieces of alloying material and master alloys in making up the charge.
•Being sure that the bath is hot enough when making the additions.
•Not making additions too near to the time of pouring.
•For nonferrous alloys, protecting cast iron crucibles with a suitable wash coating.
Examples of seven basic categories of defects are shown in Fig. 1.8.
h) Porosity
Porosity is the presence of holes, spaces, or gaps inside a solid. Two main sources of porosity during casting are shrinkage and gas porosity. The first of these occurs due to the volume contraction between solid and liquid during solidification; if additional liquid is not supplied to compensate, then porosity will appear in the casting.
Fig. 1.8 Schematic illustration of seven basic categories of casting defects: a) joint flash, b) blowholes, c) hot-cracks, d) flow marks, e) poured shot, f) distorted casting, g) metallic inclusion.
The most obvious porosity defects are caused by the entrapment of gases within the molten solution. Typically, hydrogen precipitates into melt by contact with the atmosphere or when there is too much moisture in the flux. Since hydrogen is highly soluble in molten metal, it is best to avoid superheating metals beyond their melting temperature and to avoid holding the material in a molten state any longer than is required. Gases can be scavenged from the molten metal by introducing an inert gas such as argon or nitrogen and bubbling it through the metal. To reduce the absorption of gases from the atmosphere, leaving any slag or dross, cover the molten metal until just prior to pouring it into the mold. Porosity is detrimental to the ductility of a casting and its surface finish, making it permeable and thus affecting the pressure tightness of a cast pressure vessel.
The loss in casting properties measured by a tensile test may reflect the amount of porosity in a casting. Because imperfections become areas of higher stress concentration, the percentage of property loss becomes greater when the strength requirement is higher. A metallographic examination can determine whether porosity exists in a casting. X-ray techniques are also used for nondestructive evaluations of porosity in castings.
REVIEW QUESTIONS
1.1Identify some of the important advantages of shape casting processes.
1.2What are some limitations and disadvantages of casting?
1.3Name the two basic mold types that distinguish casting processes.
1.4How can heat energy be expressed?
1.5What does “heat of fusion” mean in casting?
1.6Explain the phase diagram of heating metal to melting temperature.
1.7What is gravity sand casting?
1.8What is the difference between a pattern and a core in a sand casting?
1.9What is the law of mass continuity?
1.10Why should turbulent flow of molten metal into a mold be avoided?
1.11Identify factors that affect molten metal fluidity.
1.12How is solidification of pure metals different from solidification of alloys?
1.13What is “chill” in casting?
1.14What is Chvorinov’s rule in casting, and how can it be mathematically expressed?
1.15Why does shrinkage occur, and how can it be compensated for?
1.16Identify the most common defects in casting.
