Metal Casting Processes

Metal Casting Processes

Metals are one of the most important engineering materials that we use in todays technologies ranging from customer products to aerospace… everywhere. Important thing is, giving the reqired shape and properties to metals that they will use in such engineering applications. One of these metal shaping processes is casting. Casting is very prominent and very important process to obtain required shape of materials, not only metals. But in this extent, we will assess the topics about metal casting;

  • Properties of metal casting processes,
  • Advantages and disadvantages of metal casting processes,
  • Classification of metal casting processes according to different aspects,
  • General aspects of metal casting processes.
Metal casting process.
Metal casting.

What is Metal Casting?

As we stated just above, casting is the process of giving a shape to metal, by liquefying them with heating. Then pour them into prepared molds that have the intended shape in the cavity. So, the design of a mold for casting metals is a very important thing. The casting of metals generally has two categories; ingot casting and shape casting.

We can explain the ingot casting of materials like this: They cast metals in the form of bulk or simple shape. Before casting these metals, we make required adjustments in the way of obtaining ingots. Such as the addition of alloying elements to obtain required material characteristics or mechanical characteristics. Metallurgy techniques are very important in this step of casting the metals. They use these bulk metal ingots in further shaping operations such as forging, rolling, etc.

Metal cast ingots.
Metal cast ingots.

As you understand from its name shape casting is the casting process of metals to obtain the required shape metal. In this process, we use metallurgy techniques, but the general design of the required mold cavity is more prominent. We can use ingots or other scrap metals as liquid metals to pour them.

Some Terminology and Classifications of Metal Casting Processes

If there is a constant metal casting process in a place, we call this place foundry and the workforce called in the foundry is foundrymen

In shape casting processes of metals, mold design and mold cavity design is the very important thing. We must adopt an engineering-level approach. In mold design, the most important parameter is shrinkage. Every material and metal shrinks upon solidification. We consider this shrinkage, in the phase of mold design.

Foundry.
A typical foundry place.

We design the cavity of the mold oversize, according to the produced product’s shape. We make this oversized design of mold according to calculations of shrinkage.

Types of molds are also another important parameter in the phase of metal casting mold design. There are generally two types of molds; open mold and closed mold.

Open molds are the type of mold where we are casting the metal directly from the open side of the mold that is open to the atmosphere. Designing this type of mold is much easier if we compare it with a closed mold. There are no complex ventilating systems and gating systems to achieve a good molding process of metals. We can cast ingots in open molds.

Also…

Closed molds are generally the most used to obtain more complex metal parts. We make the casting of liquid metal by gating system that has a pouring cup. We pour the metal into this pouring cup of closed mold, and downspout and runner systems transmit the poured metal into the closed mold cavity. Risers, pouring cups and runners, mold cavity, core, and the placement of molding cavity according to the parting line are very important parameters in terms of the design of the mold.

So, closed mold design is very complex and has engineering-level design characteristics. An engineer must design a closed mold in detail to obtain flawless geometry from it.

We use molds in metal casting we also classified according to their use. We use some of the molds only one time and we use some of the others in lots of processes. Because of this difference, the types of molds are; permanent molds and expendable molds.

Permanent molds are the kinds of mold that we can use lots of time without crashing them. We can produce lots of products from permanent molds. So, we use the material in the production of permanent molds must be a refractory material, and must withstand cycling heat loads. In general, permanent molds have a collapsible system to take the molded material from it after solidification.

We use expendable molds generally only for one time. Because they demolish generally to extract the molded material after solidification. We use various kinds of refractory and cheap materials for it such as ceramics. With expendable molds, we can produce more intricate metal parts.

What are the Advantages and Disadvantages of Metal Casting?

  • We can produce very large metal parts up to 100 tons with metal casting processes.
Cast metal part.
Very big cast metal part.
  • If the material can be liquid upon melting such as metals, the casting of this material is generally possible.
  • We can produce very complex internal and external shapes with metal casting methods.
  • Mass production with casting methods can sound impossible slightly but the situation is not like that. Certain metal casting methods are available for mass production.
  • We can establish net-shaped final metal products with only one casting process. But sometimes, the surface characteristics can not be sufficient. In these situations, we can apply some polishing and machining processes. In this case, a product that we produce with metal casting methods is called the near-net-shaped product.

Disadvantages of metal casting methods;

  • Environmental problems are the most serious downside of metal casting methods. Because of the very high temperatures, they consume lots of energy to melt tons of metals. Also, they release very hazardous gases into the atmosphere because of molten metals. These conditions are generally harmful to the workforce.
  • In some metal casting processes, we achieve poor surface characteristics. After that, we can require some machining operations to obtain optimum surface characteristics.
  • Mechanical properties of produced products from metal casting methods can be affected in a bad way because of possible porosity formation during the solidification.

What are the Types of Metal Casting?

All the metal casting processes are classified into two groups; Expandable mold metal casting processes and permanent mold metal casting processes.

Expandable Mold Metal Casting Processes

In expandable mold metal casting processes, we destroy the obtained mold for casting operation to extract the solidified metal part from it.

Sand Casting Processes

The sand metal casting method is one of the most used methods to produce metal parts. We can cast a wide variety of metal parts and a wide variety of materials in sand casting processes. In sand casting, we make mold cavities and mold from sand which we mix with special resins to obtain a solid mold cavity. We place sprues, gates, and cores in a specific orientation. Risers are also very important structures in sand casting processes.

Vacuum Molding Processes

In this kind of metal casting process, we obtain the mold cavity with sand again. We give the shape to sand with a special application of vacuum on the sand with the use of films, and these two parts of sand molds are combined. Then, we pour the molten metal inside the mold cavity.

Vacuum molding processes.
Vacuum molding proces illustration.

Shell Molding Processes

Shell molding method is another expandable mold casting method in that we make the mold from the sand. In this process, we heat the metal pattern, then we spill the sand that we mix with special resins on these heated metal patterns. With the application of heat, resin inside the sand cures, and a solid half of mold take place. Two halves of sand shells combine and we pour metal inside it. After the solidification, we extract the metal part by destroying the sand shell mold.

Shell molding.
Shell molding steps.

Plaster/Ceramic Mold Casting

In plaster and ceramic mold metal casting methods, there is no use of sand to obtain mold cavities. Here, as you understand from the name, we use plaster and ceramic materials to obtain mold cavities. In the slurry form of ceramic and plaster, we give the required mold cavity shape, after the solidification of plaster or ceramic, molten metal is poured inside the mold. Obtained metal parts are very good in terms of surface characteristics.

Clay metal casting.
Example of ceramic mold(Source Of Image: ClayKing).

Investment Casting

In the investment casting method, we combine a pattern tree in which lots of patterns to each other with sprues and gates, to produce by using a special wax. Then this wax is dipped into a slurry of refractory material, and coated with it. When the refractory slurry coating solidifies, we apply heat to melt the wax. So, we obtain mold to pour molten metal inside it.

Investment casting in metal casting.
Image Source: Ppcpinc

Expanded Foam(Polystyrene) Process

They make patterns from polystyrene foams in an expanded polystyrene process, and they use sand to obtain mold cavities by using these foam patterns. They pour the molten metal directly on these foam patterns. And we turn this foam pattern into a gas state with the extreme heat of the molten metal. Molten metal immediately takes the place of polystyrene foam pattern and we obtain the metal part.

Expanded foam process.
Expanded foam molding process.

Permanent-Mold Metal Casting Processes

Permanent-mold metal casting methods are the metal casting methods in which we do not destroy the created mold. And we can use it in lots of casting processes. The permanent means that there is no need for making a new mold for each cycle of metal casting. In general, we make molds from metals.

Squeeze Metal Casting Method

As you understand from its name that in squeeze casting, we pour the molten metal inside half of the metal mold. The other half of the metal mold presses on the first half. With this pressure application, molten metal fills all the cavities inside the metal mold.

Squeeze metal casting.
Explanation of squeeze casting.

Centrifugal Metal Casting Process

In general, they use a gravitational force for filling the mold cavities with molten metal. But in centrifugal casting methods, we obtain centrifugal force by rotating the mold. This centrifugal force presses the molten metal inside cavities. There are three types of centrifugal metal casting processes available in the industry; the First one is the true centrifugal metal casting prosses in which they pour the molten metal inside a hollow tube mold to obtain hollow tube shapes such as bushings, rings, etc. The second one is the semi centrifugal metal casting process in which we cast the shapes like pulleys and rims that has radial symmetries. The last one is centrifuge casting which has similar characteristics to semi centrifugal metal casting. We produce just a bunch of parts that do have not radial symmetry.

Vertical true centrifugal casting method.
Vertical true centrifugal casting process(Image Source: Rudradefence).

Semisolid Metal Casting Process

They say semi-solid metal is generally for metal alloys that are available on market. As you know, metal alloys have no exact completely melting temperature like pure metals. There is a transition from solid to liquid phase for metal alloys. In this transition phase, we call their situation semi-solid metal. Take a look at the liquidus and solidus curves for metal alloys to understand the logic

Thixomolding machine.
Thixomolding machine(SRC:Alfa machinery)

They heat the metal alloys to temperatures between solidus and liquidus temperatures to obtain semi-solid metal. To obtain semi-solid metal, a small ratio of metal must be solidified. In this solidification mechanism, generally, dendritic growth occurs. This is a microstructural mechanism of solidification. The dendritic structure is not a desirable thing in the semi-solid metal casting method. We desire a solid globule granule inside the liquid phase instead of a dendritic microstructure in semi-solid metal casting.

To obtain the globule microstructure, we use some shaking and stirring mechanisms to shake and stir the semi-solid metal.

After this process, we inject semi-solid metal inside the mold via a screw mechanism. It is just like metal injection molding.

Which Metals do They Use in Semi-Solid Metal Casting?

As in the squeeze casting method, both ferrous and non-ferrous metals are cast in semi-solid metal casting processes. But because of their low melting temperature, we desire non-ferrous metals.

Magnesium alloys are the common materials in semi-solid metal casting processes. They push magnesium granules inside the mold cavity via a rotating screw mechanism. During this pushing activity of magnesium, they heat the granules to obtain a semi-solid phase. The stirring effect that originated from the rotating screw also lead to the needed globular solidification microstructure inside cast magnesium material. This process has a different name called ‘thixomolding‘.

In the casting of aluminum alloys, they use another different method in semi-solid casting. In the casting of aluminum alloys, the starting form of them is solid billets that do have not dendritic microstructure inside them. They heat these billets to a semi-solid temperature to obtain slurry to fill the mold cavities with that. They use die-casting equipment to fill the mold cavities. And we call this special process ‘rheocasting‘.

What are the Advantages of the Semi-Solid Metal Casting Process?

The most important advantage of the semi-solid metal casting process is the zero or very low porosity inside the cast part, which leads to very good mechanical properties compared with the parts produced by other casting processes.

Because the of use of equipment to cast metal, we can obtain very complex part geometries and details with a very good tolerance control over dimensions in the semi-solid casting method.

In some parts such as engine casings, we can obtain thin part sections with the desired characteristics.

These are the general characteristics of the semi-solid metal casting process. Do not forget to leave your comments and questions about the semi-solid metal casting process below.

Die Casting

The die casting process is one of the most used permanent-mold metal casting methods. The word ‘die’ refers to the use of metal mold halves in the die casting process. They force the molten metal inside metal molds with the force of plunger mechanisms. These plunger mechanisms are generally hydraulic. There are two types of die casting processes; The first one is the cold-chamber die casting process in which they supply the molten metal to the cold chamber, and that plunger pushes this molten metal inside the mold. The second die casting process is the hot-chamber die casting process in which they supply solid metal inside the hot chamber, and the chamber of the die casting machine is constantly hot. The plunger pushes the part of the molten metal inside this hot chamber. When the chamber is running out of molten metal, they adds solid metal.

Die metal casting process.
Die casting.

What are the Metal Casting Defects?

You can find information about 14 casting defects that we encounter mostly in metal casting processes generally.

Shrinkage Cavity Problem

Shrinkage is a phenomenon that occurs in metal casting processes because of the difference in volume between the liquid state and the solid state of metals. In the liquid state, the volume of metal will be higher than in the solid state. When metal transforms into the solid phase in solidification in casting, internal voids can occur because of this volume reduction.

Shrikage problem.
Shrinkage problem illustration.

Solution of Shrinkage:  A proper riser design for the casting process will generally solve this problem.

Misruns in Casting

Misruns are general problems in casting processes that are generally occurring because of the unsuitable gating and mold design, insufficient fluidity of molten metal, very thin sections in mold, and low pouring temperature of molten metal which that we pour.

Misrun.
Illustration of misrun.

Solution of Misruns: You might understand the solution actually above information about misrun reasons. Proper design of mold sections, good calculation of required pouring temperature of molten metal, and suitable design of gatings will solve the problem of misruns occurring in metal casting processes.

Microporosity Problem

Local shrinkage causes the microporosity by local shrinkage which leads to contraction of unsolidified sections near the solidified sections dendritically. Because of these localized contractions, little porosities inside the volume may occur.

Microporosity.
Microporosity on cast metals.

Solution of Microporosity: In casting methods, you need to design the solidification manner of metals also. You need to use chillers for critical sections inside the mold cavity.

Cold Shots

This is a casting phenomenon that generally occurs. Splattering of metal during the pouring of metal created metal globules that entrap inside the mold.

Cold shots.
Illustration of cold shots(Image Source: Tfgusa).

Solution of Cold Shots Problem: We must accomplish a proper gating system design to prevent the formation of metal globules via splattering.

Cold Shuts

In metal alloys, generally, we use two types of metals. When there is insufficient mixing of liquid metals, fluidity of metal can prevent the fluxing of other metal that has low fluidity.

Cold shuts.
Cold shut illustration(Image Source: Mechanical Booster).

Solution of Cold Shuts: Actually, you can find the answer from the information about cold shuts above. You need to mix portions of different metals at sufficient pouring temperature.

Hot Tearing

The hot tearing problem takes place in specific shapes of molds. When we pour and solidify the metalinside the mold, shrinkage of metal inside mold can cause some crackings because of the tensile stresses that occur on part because of the mold shape. We also call the hot tearing ‘hat cracking’.

Hot tear problem in metal casting.
Various versions of hot tear problems.

Solution of Hot Tearing Problem: In expandable-mold casting processes, mold is collapses after the full solidification completes. In permanent-mold casting processes, we remove the part immediately after solidification. Because hot tearing takes place in the early stages of cooling after solidification.

Sand Wash Problem

We encounter sand wash problem generally in sand casting operations. The form of sand mold surfaces can erode because of the hot fluxing metal. And we can see this erosion on the surface of metal after solidification.

Solution of Sand Wash Problem: Obtaining required mold strength and providing non-turbulence flux of molten metal inside the mold is important for preventing sand wash problems.

Mold Crack

This is also another problem that occurs because of the insufficient strength of mold. Generally, molten metal infiltrates into crackings of mold.

Mold crack.
Mold crack problem(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217).

Solution of Mold Crack Problem: The solution is straightforward. We must establish a sufficient strength for mold structure.

Pinholes

This problem is also related to sand casting operations. Because of the entrapped gases inside molten metal and mold cavity which we fill with molten metal, we can see pores beneath the surface of metal parts.

Solution of Pinholes: The permeability of mold sand must be sufficient for the evacuation of gases. Also, we can drill small holes for venting inside the mold cavity on sand molds.

Pinholes.
Place of pinholes in mold(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217)

Penetration Problem In-Mold

When the fluidity of molten metal is high, it can penetrate the inside surface of the sand mold. We can see the defect of penetration at the surface of the metal part as a mixture of sand and cast metal.

Penetration in metal casting.
Penetration of molten metal inside sand(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217)

Solution of Penetration Problem In-Mold: Strick packing of sand mold is important to alleviate the penetration problem. This will increase the strength of sand mold.

Sand Blow

This is the most critical sand casting defect. In sand blow defects, there can be big voids just beneath the surface of solidified metal parts. This is generally because of the mold gasses entrapped inside the mold.

Sand blow.
Sand blow illustration(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217).

Solution of Sand Blow Problem: We must optimize the moisture content of mold sand must correctly. Low permeability is also the reason.

Core Shift

Because of the buoyancy of core material inside high-density molten metal, the core can displace vertically inside the sand mold.

Core shift in metal casting.
Lateral view of core shift(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217).

Solution of Core Shift Problem: Required consolidation of core placement regions must be done to prevent core shift problems.

Scabs

At the inner surfaces of mold, mold and can take a shape of crust which can penetrate inside the molten metal.

Scab.
Scab(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217).

Solution of Scabs: More rigid design of sand mold could prevent this problem.

Mold Shift

Mold shifting is an external problem because of the displacement or misplacement of cope or drag according to each other on parting line.

Mold shift.
Cope and drag shift illustration(Image Source: https://www.wiley.com/en-us/Fundamentals+of+Modern+Manufacturing:+Materials,+Processes,+and+Systems,+7th+Edition-p-9781119475217).

Solution Of Mold Shift Problem: We must doouble check the placement of cope and drag before the pouring of molten metal.

Solidification in Metal Casting

Directional solidification in metal casting.
Directional solidification expression in casting.

In general metal casting processes, shrinkage is a real problem to overcome to obtain a succesful cast part. To overcome this shrinkage thing, they use riser systems to compensate for shrinkage cavities. In risers, there is liquid metal to fill the shrinkage cavities that occur during the solidification of cast metal.

In the solidification phase of the main part inside the mold, the connection between the shrinkage cavity region and the riser is very important to fill it. If the liquid connection between the riser and shrinkage cavity region is lost by solidification of this region, liquid metal will not fill the shrinkage cavity to compensate for it. This is a very big problem in terms of the mechanical properties of the obtained geometry after solidification.

So, neat solidification we must overcome this situation. So again, you need to design the solidification direction of the liquid metal inside the mold. This is done by ‘directional solidification’ methods.

How the Directional Solidification Take Place?

External chills in metal casting.
External chills illustration.

By the use of ‘chills’ inside the mold and liquid metal, you can start the solidification of liquid metal whenever you want. These chills are small parts that are made of the same metal as liquid metal, which they place inside the possible shrinkage cavity regions and most distant places from risers in the mold.

So, the solidification will start from the most distant places and, the liquid connection between possible shrinkage cavity regions and risers will not be interrupted with solidification in this region.

Also, we can start the possible shrinkage cavity regions solidification first with the addition of chillers. This can be very helpful to compensate for the shrinkage of cavities. We call these kinds of chillers ‘internal chillers’.

They use external chillers also for starting the solidification from the intended places of interface with mold walls. They place these external chillers at the surface of mold walls.

Temperature and Time in Sand Casting

Just like other elements in nature, pure metals have a constant melting temperature. This melting temperature of metals is also equal to the freezing temperature. But if we take a look at the solidification inside a sand casting, it has a quite original solidification mechanism.

When the liquid metal pours inside a sand mold, it has a liquid form and the temperature of this liquid metal is higher than the melting temperature to prevent the solidification inside gatings. When we fill the mold cavity inside the sand casting with liquid metal, the solidification process starts.

T-T-T diagrams in metal casting.
Temperature-time curve of pure metals(Src: Fundamentals Of Modern Manufacturing).

We can express the solidification of liquid metal with the above diagram showing the temperature and time relation of solidifying pure metals. As you from the diagram, the pouring temperature is the highest temperature and the beginning of the casting. When the liquid pure metal fills the mold cavity, cooling will begin. When we react to the freezing temperature after a time passed, the solidification of metal starts from the outer surface of the mold.

From the interface of mold wall and liquid metal, the solidification of liquid metal starts. After the formation of a thin solid film, this film thickens with time and gives up the ‘latent heat of fusion of metal through the mold walls.

What is ‘Dendritic Growth’ In Metal Casting?

Sections of pure metal near the mold wall will have fine grains in the structure because of the rapid cooling of these sections. These grains are randomly oriented and they grow with the passing of time.

Dendritic growth.
Dendritic growth of metals inside molds(Src: Fundamentals Of Modern Manufacturing).

When the solidification goes into the inner sections of the mold, these grains will turn into spikes that orient inside the mold. Up to the full solidification of pure metal, these spike grains will turn into long branches that orient inside the mold. These long branches of molecular structures unite in the innermost section of the mold cavity. We illustrated this molecular structure above. We call this solidification phenomenon of metals dendritic growth.

The time that passed between the starting of freezing and the ending of it, we call ‘local solidification time.’ In this time section, solidification of pure metal takes place but the temperature is constant. When the pure metal is completely solidified, solid cooling starts up to room temperature.

‘Total solidification time’ refers to the time between pouring and complete cooling of pure metal in sand casting operations.

This can be the general expression of the solidification of pure metals inside sand casting molds. We design metal parts and molds according to these solidification and grain growth mechanisms.

Metal Casting Furnaces

We must establish this procurement of molten metal inside the casting sites right in time. So, a manufacturing site that makes casting operations must have special furnaces to supply molten metal for casting operations. There are various types of melting furnaces that are available in the casting market. A proper selection of the melting furnace is very important.

In the selection phase of proper melting furnace equipment for a foundry, the important parameters are;

  • Type of metal or metal alloy that we cast,
  • Melting temperature of metal or metal alloy in the foundry. This can vary according to the types of cast metals that we use in that foundry.
  • Environmental restrictions are very important. We must make comprehensive research on whether the selected melting furnace equipment is proper for the environmental regulations in that country or region.
  • The production capacity of the foundry is a very important parameter.
  • Investment costs of selected melting furnace systems and maintenance and operational costs such as workforce required are very important parameters.

Here you can find summary information about the types of melting furnace types for casting operations;

Crucible Furnaces

We use crucible furnaces generally for melting non-ferrous metal alloys because of their low melting temperatures. In the mechanism of crucible melting furnaces, there is a melting pot that we make from refractory materials. This melting pot is placed inside the crucible furnace which is heated with the application of burning fuel. These crucible furnaces generally have enclosed structures that keep the heat inside. The general fuel types that are used in crucible furnaces are coal and natural gas.

Tilted crucible furnaces.
Tilted type crucible furnaces(Source: REMIX).

There are three types of crucible furnaces available;

  • Stationary pot crucible furnaces: The pot is the stationary inside furnace that molten metal is taken from it with the use of a tap at the bottom of the furnace.
  • Lift-out pot crucible furnace: When the metal inside the pot is fully melted, the pot is lifted with the help of cranes inside the manufacturing site to serve the molten metal to the molds.
  • Tilting pot furnace: The pot is stationary inside the crucible furnace, and the whole furnace system is tilted to pour the molten metal inside it.

For one service, several hundred kilograms of molten metal can be served by using crucible furnaces.

Induction Furnaces

Electric arc furnace in metal casting.
A small type of electric arc furnace(Image Source: lyshennai).

Induction furnaces are the most successful technique to obtain high-quality and pure metals and metal alloys. In induction furnaces, an electric charge is used to produce induction which produces an electromagnetic field. This electromagnetic field vibrates the atoms of metal, then its vibration creates heat. Eventually, the metal inside the induction furnace is completely molten.

Because there is no direct connection between metal and the melting equipment, the state of metal can be controlled in a very good way. Thus, very accurate compositions can be obtained in high quality.

All types of metals can be melted inside induction furnaces such as cast irons, ferrous and non-ferrous metals, and metal alloys, etc.

Cupola Furnaces

Cupola furnaces.
Cupola furnaces.

They are big vertical and round structures that have a ‘charge’ inside them to produce the required molten metal. Also, they are the biggest type of melting furnaces that produce the highest tonnage of molten metal. In general, cupolas are used for melting cast irons. Charge is placed inside the big cylinder which is coated with refractory liner inside. The constituents of charge;

  • Coke: Coke is used as fuel to melt the material inside the cupola furnace.
  • Limestone: With the addition of a proper amount of limestone which is also called ‘flux’, ash and other impurities reacted with it and form ‘slag’. This slag-on molten material forms a protective layer to prevent oxidizing environment inside the cupola furnace.
  • Metal parts and other alloying elements: Required metal parts and other alloying elements are provided into the cupola furnace from the charging hole. Metal parts can be leftovers or scraps produced from other casting processes, such as in the form of risers, sprues, etc.

At the bottom of cupola furnaces, there is a tapping hole that the molten metal is taken from the cupola furnace.

Cupola furnaces are the general application for cast iron casting operations for very high amounts.

Direct Fuel-Fired Melting Furnaces

Unlike crucible furnaces that we stated above, there is a direct interaction of fire that is produced with the combustion of fuel which is generally natural gas, with the metal. This process takes place inside a structure called ‘hearth’ which has a closed structure to melt the material inside it. When the metal is melted inside the direct fuel-fired furnace, metal is tapped from the bottom of the hearth. Melting processes of non-ferrous metals are generally made with direct-fuel-fired melting furnaces.

Electric Arc Furnaces

Big electric arc furnace in metal casting operations.
The very big tonnage of the electric arc furnace is tilted(Image Source: Compudas).

Electric arc furnaces are one of the biggest furnaces that can produce molten metal up to 25-45 tonnes per hour. Because of that, they are used for very high production rates in foundries. The working principle of electric arc furnaces is very easy. An electric arc is produced with an electric current to produce heat. But the energy consumption is very high compared with other technologies.

Fluidity in Metal Casting

Fluidity test of molten metals.
Fluidity test for cast metals.

What is the Fluidity of Cast Metals?

Fluidity is a very important parameter and this needed to be evaluated in a very serious approach. The fluidity of liquids is generally defined as the inverse of viscosity. As you know, the internal shear forces of fluids lead to the situation of viscosity to withstand the fluxing. The desire to flux in a fluid is defined as fluidity.

Some factors affect the fluidity of cast metals;

  • The viscosity of liquid metal: As we stated above when the viscosity of a liquid increase, fluidity decreases. There is an inverse relation between the viscosity and fluidity of a liquid metal.
  • Pouring temperature and melting temperature of metal: There is always a difference between the pouring temperature of the metal and the melting temperature of liquid metal. This difference guarantees the solidification of metal before reaching into the mold, inside gates. So melting temperature and the pouring temperature are important parameters in terms of fluidity. Because, with the increasing temperature, fluidity also increases.
  • Heat transfer: If the metal is in a molten state, the temperature of this metal is very high. So, there is a great heat transfer between the molten metal and its surroundings. When the molten metal advances in gates and channels to reach the mold cavity, heat transfer inside these gates is a very important parameter to adjust the difference between the pouring temperature and melting temperature. When the heat energy is given to the surrounding by the metal fluid, the temperature of the metal fluid decreases. So, fluidity decreases.

What are the Effects of Higher Temperatures on Metal Casting?

You may think that we need to increase the pouring temperature of cast metal to the highest possible point, to increase the fluidity. But there are also another problems in the casting periods with very high pouring temperatures, these problems are;

  • Penetration of metal liquid into the sand: In sand casting operations, very hot liquid metal may penetrate sand casting barriers, which will lead to the problem of surface defects and uneven cast metal geometries.
  • Oxidizing of metal: Metal tends to oxidize, and increases exponentially with the increasing temperature. This means that very high pouring temperatures will lead to the oxidizing of metal which is not a desirable thing in metallurgy.
  • Gas porosities: Boiling of metal liquid will lead to trapped gas porosities inside the molten metal. After solidification, these voids that originated from these gas porosities will lead the very serious mechanical property problems.

But freezing mechanisms of metal alloys and metals are quite different. Metal fluids freeze at a constant temperature, which means much more easy manipulation of casting temperature. But in metal alloys, there is no exact freezing temperature. In a temperature range, two phases of fluid and solid occur. So, temperature manipulation of temperature is not easy. The solid phase can interfere with the flux of fluid sides.

Conclusion

As you see above, metal casting processes are very important processes in engineering. Lots of kinds of metal production processes depend on the metal casting processes in general. So, the market cap of the metal casting processes is maybe the biggest in the industry.

There are various kinds of metal casting processes available. Different sizes and different parts are produced with different casting processes. But the most common ones, are sand casting processes. In general, we classify these processes as permanent mold and expendable mold casting processes. So, the situation of the mold is very important in these processes.

Also, there are various kinds of casting defects that we need to deal with. We are dealing with these defects in general kinds of applications. So, it is very important to use to deal with them. And also, if we know the general solidification mechanisms in the metal casting processes, we will probably not have any problem with them. So it is very important to have these processes.

To create a metal casting process, we need to melt the metal in different furnaces. There are different sizes and capacities of furnaces available in the market in general. So, metal casting is a very important thing that we need to consider in general.

Also, they are very high-temperature processes that we need. In high temperatures, the behavior of the materials changes. So, it is very important to have very good casting performances from these systems.

Finally, do not forget to leave your comments and questions below about the metal casting processes.

Your precious feedbacks are very important to us.

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FAQs

What is the metal casting process?

Metal casting is a manufacturing method in that we are giving the intended shapes to metals by melting and pouring them into the molds. There are various types of these processes which generally vary according to the material and part size. But the most important one is the sand casting process. In these processes, we create a mold that has a cavity inside them from the sand. And we pour the molten metal inside this mold. After the solidification, we obtain the general shape of the metal.

Can I cast metal at home?

It depends on the type of metal. For example, you can melt and cast metals that have low melting temperatures such as aluminum. But also, you can use materials that have very low melting temperatures. Also, it is very hard to melt the metals such as steel with home furnaces. So, you need to have industry-type furnaces to melt these materials. And this is a very hard thing to do.

What can I use to cast metal?

You can use very basic sand mold and sand mold halves. And you can print a model by using 3D printers and give the sand this shape. And then, you can pour the molten metal inside the cavity to obtain parts. So, it is very easy like this to obtain cast metals. But you need to take some instructions and applying education about it.

What are the types of metal casting?

There are different types of these processes. We can classify them as expandable mold casting methods and permanent mold casting methods. In expandable casting, the mold is destructed after the solidification of the metal part. And in permanent molds, we use mold for lots of cycles without destructing it.

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