Metal Casting MCQ Quiz - Objective Question with Answer for Metal Casting - Download Free PDF

Explanation:

Centrifugal Casting

• Centrifugal casting is a manufacturing process in which molten metal is poured into a rotating mold and allowed to solidify while the mold is revolving at high speed. The rotation of the mold creates centrifugal force, which distributes the molten metal uniformly along the inner surface of the mold. This process is particularly useful for producing cylindrical or hollow components with high structural integrity.
• The centrifugal casting process relies on the principle of centrifugal force. When the mold rotates, the molten metal is forced outward and pressed firmly against the mold walls due to centrifugal force. This pressure ensures that the molten metal fills the mold completely and solidifies with minimal porosity or defects. The mold's rotation also helps in segregating impurities toward the center, which can be removed later.

Steps in the Process:

1. Preparation of Mold: The mold is cleaned and coated with a refractory material to prevent the molten metal from sticking to the mold surface.
2. Molten Metal Pouring: The molten metal is poured into the rotating mold. The mold is rotated at a controlled speed to generate the required centrifugal force.
3. Solidification: As the molten metal comes into contact with the cooler mold surface, it begins to solidify. The centrifugal force ensures uniform distribution of the metal along the mold walls.
4. Cooling and Removal: After the molten metal has completely solidified, the mold is stopped, and the casting is removed for further processing.

Advantages of Centrifugal Casting:

• Produces high-quality castings with minimal porosity and defects.
• Ideal for manufacturing cylindrical or hollow components like pipes, tubes, rings, and bushings.
• Requires no cores for hollow castings, simplifying the process.
• Impurities are pushed toward the center due to centrifugal force, making them easier to remove.
• Improved mechanical properties due to the directional solidification and grain structure formed during the process.

Applications:

• Production of pipes, tubes, and bushings in industries like automotive, aerospace, and construction.
• Manufacturing of rings, gear blanks, and other cylindrical components.
• Used in producing castings for chemical and petroleum industries due to its ability to create dense and defect-free components.

Explanation:

Cupola Furnace:

• The cupola furnace is one of the most common types of furnaces used in foundries for melting ferrous metals and alloys. It is a vertical, cylindrical structure with various zones that facilitate the melting process. The zone of the cupola furnace that starts above the melting zone and extends up to the bottom of the charging door is known as the preheating zone.

Working Principle:

• The preheating zone of the cupola furnace plays a critical role in preparing the charge materials for the melting process. As the charge materials (metal scrap, coke, flux) descend through the furnace, they pass through the preheating zone before entering the melting zone. In this zone, the heat generated in the lower zones (melting and combustion zones) rises upward, preheating the charge materials. This preheating helps in reducing the energy required for melting and improves the efficiency of the furnace.

Characteristics of the Preheating Zone:

• Located above the melting zone and below the charging door.
• Primarily serves to preheat the charge materials using the upward flow of hot gases.
• Enhances the thermal efficiency of the furnace by utilizing the residual heat from the combustion and melting zones.
• Ensures that the charge materials are sufficiently heated before entering the melting zone, reducing the time and energy required for melting.

Additional InformationStack zone:

The stack zone is the uppermost part of the cupola furnace, located above the preheating zone. This zone primarily serves as a passage for the escape of flue gases and does not play a direct role in preheating or melting.

Well:

• The well is the bottom-most part of the cupola furnace, where the molten metal collects after melting. It is located below the melting zone and does not extend to the charging door.

Combustion zone

• The combustion zone is the part of the furnace where the coke burns in the presence of air to generate the high temperatures required for melting. This zone is located directly below the melting zone and does not extend upward to the charging door.

Explanation:

Cold Shut

• A cold shut is a casting defect that occurs when two streams of molten metal flow together but fail to fuse completely. This results in a weak spot or seam in the casting, which can lead to reduced strength and potential failure of the part under stress. This defect is particularly common in components with complex geometries, such as automobile parts, where molten metal has to flow through intricate paths.

Formation of Cold Shut: A cold shut occurs due to improper fusion of molten metal streams. This can be attributed to factors such as:

• Low pouring temperature of the molten metal, which reduces its fluidity.
• Slow filling of the mold cavity, causing the molten metal to cool and solidify prematurely.
• Improper gating and riser design, leading to turbulence or uneven flow of molten metal.
• Inadequate venting of the mold, causing air pockets and preventing proper flow of metal.

Characteristics:

• Cold shuts appear as thin lines or seams on the surface of the casting.
• These seams are typically weak points where the metal is not fully bonded.
• The defect can often be detected visually or through non-destructive testing methods such as X-rays or ultrasonic testing.

Prevention:

To prevent cold shut defects, the following measures can be taken:

• Maintain proper pouring temperature to ensure adequate fluidity of the molten metal.
• Optimize gating and riser systems to ensure smooth and uniform flow of molten metal into the mold cavity.
• Use proper venting in the mold to eliminate air pockets and ensure even filling.
• Design the mold cavity to minimize abrupt changes in geometry, which can disrupt the flow of molten metal.
• Use materials with appropriate thermal properties for the mold to control cooling rates and prevent premature solidification.

Applications: Understanding and preventing cold shut defects is crucial in industries such as automotive manufacturing, where the strength and reliability of cast components are critical for safety and performance.

Explanation:

Directional solidification:

• In general metal casting processes, shrinkage is a real problem to overcome to obtain a successful casting. 
• To overcome this shrinkage problem, riser systems are used 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, the shrinkage cavity will not be filled with a liquid metal to compensate it.
• By the use of ‘chills’ inside the mold and liquid metal, you can start the solidification liquid metal that improves the directional solidification.
• Chills are small parts made of the same metal with liquid metal, that are placed inside the possible shrinkage cavity regions and most distant places from risers in the mold.

Concept:

Chvorinov's Rule relates solidification time of a casting to its volume and surface area as:

Given:

Small cube side = 2 cm, Large cube side = 4 cm

Time for small cube to solidify = 2 minutes

Calculation:

Volume of cube = L³ , Surface area = 6L²

So,

Now,

Explanation:

A casting defect is an irregularity in the metal casting process that is undesired.

Classification of casting defects is given as:

Explanation:

PROPERTIES OF MOULDING SAND:

Explanation:

Patterns

• A pattern is the replica of the casting to be formed with some modifications. These modifications are in the form of allowances.
• Patterns are of great variety because of their different designs, constructions, and methods of use. More commonly used types are
  • Solid/Single-piece pattern
  • Two-piece/Split pattern
  • Multi-piece pattern
  • Sweep pattern
  • Segmental pattern
  • Skeleton pattern
  • Shell pattern
  • Gated pattern
  • Loose piece pattern
  • Match plate pattern

Match plate pattern:

• Patterns are made in two pieces, one piece mounted on one side and the other on the other side of the plate called match plate
• The plate may carry one or group of patterns mounted on match plate
• Along with the pattern, gates and runners are also attached
• Produces accurate castings at faster rates
• One advantage of using a match plate is that several patterns can be moulded in the single flask, thus saving much time and labour.

Explanation:

Gating system:

The pouring basin is the first component where the molten metal first put. From where it slowly get into the vertical passage known as sprue. Sprue is connected with the gate and the gate is further connected with the pattern cavity. From the pattern cavity, one more vertical passage is provided called a riser. Mostly the shapes of these passes are cylindrical to avoid the erosion of the moulding sand.

From the diagram we can see that:

• The pouring basin acts as a reservoir for the molten metal. Where the molten metal is poured at the starting.
• The next component is sprue which feeds molted metal from the pouring basin to the gate.
• Gate regulates the molten metal into the mould cavity. When mould cavity gets filled with the molten metal the molten metal gets into the riser.
• The molten metal in the riser compensates for the liquid shrinkage by providing the molten metal back to the mould.

Explanation:

A casting defect is an irregularity in the metal casting process that is undesired.

Classification of casting defects is given as:

Explanation:

Casting defects:

Casting defects are those characteristics that generate imperfection in a casting exceeding the quality limits imposed by the design or service condition of the casting.

As per standard casting defects, there are of seven types:

1. Discontinuities:

Discontinuities in the form of cracks are caused due to hindrances in the contraction of the casting.

• Hot tears are caused because of the low strength of metals after solidification, causing the metal to fail in coping with the excessively high-stress setup by solid shrinkage of the metal. It can also happen due to a lack of collapsibility.

2. Metallic Projection:

It includes a small metallic projection on the casting surface and is caused due to wrong moulding practices or improper equipment.

• The mismatch is the displacement of cope half of the casting with respect to drag half and is caused due to loose connecting pins of moulding boxes.
• Lifts and shifts are caused because of misplacement of cope or core.
• Fins and flash are thin projections of metal occurring at the parting line on the casting surface.
• Swell is a localized enlargement of casting at its top side and is caused due to pressure of molten metal on soft rammed sand.

3. Cavities:

It may be internal or external surface depressions or hollowness.

• Blow-holes appear as internal voids having smooth round or oval holes as a result of entrapped gases, excessive moisture in sand, etc.
• Pin-holes or gas porosity is sub-surface porosity caused by gases absorbed by molten metal, particularly hydrogen.

4. Defective surface:

Defects under this category are caused due to wrong moulding, gate marking, and metal pouring.

• Fusion and metal penetration are caused in the mould by the very high temperature of the metal. Fusion appears as a rough surface on casting. Penetration is the passage of molten metal through sand grains and appears like an intimate mixture of metal and sand.
• Hard spots are developed on the casting surface due to the rapid cooling of that particular surface area.
• Cold shots are small spheres of metal appearing on the casting surface almost distinct from casting which is caused by the stream of metals that are too cold to fuse properly.
• Scabs or washes are due to erosion or breaking down of some sand from mould or core surface and the recess thus made is filled by metal and the same appears on casting as extra metal.
• A scar is a hollow cavity on a flat casting surface caused by the liquid metal pressure, which succeeds in pushing back the mould sand at certain places. A blister is a shallow blow on a flat casting surface covered by a thin layer of metal.

5. Incomplete castings:

When casting lacks metal supply during casting, it remains incomplete in its desired shape and size.

• Misrun and cold shut occur due to a lack of fluidity of molten metal. When metal fails to reach all sections of metal, the defects caused is misrun. Cold shut occurs when two-stream of molten metals approach each other in the mould from opposite directions.
• Pour short is the incomplete filling of mould due to less quantity of metal.
• Runout is the drainage of molten metal from the mould cavity, thus leaving the mould unfilled.

6. Incorrect dimension and shape:

It happens due to improper patterns with less shrinkage allowance and poor moulding practice.

• Warping is the deformation of the shape of the casting.
• The drop appears as an irregular deformation on casting due to falling of a portion of sand from the mould cavity and dropping into molten metal.

7. Inclusions:

Inclusions are the impurities (oxides, sand, nitrides, etc) in suspension in molten metal which, on solidification of casting, appear as inclusion defect that makes the casting poor in strength.

• Sponginess or honeycombing appears as small voids in large numbers and in close proximity on the casting surface and are caused due to dirt or swarf dissolved with metal.

Explanation:

• It is a process in which liquefied material, such as molten metal, is poured into the cavity of a specially designed mould and allowed to harden.
• After solidification, the workpiece is removed from the die to undergo various finishing treatments or for use as a final product.
• Casting is typically used to create intricate solid shapes of having many details, and cast products are found in a wide range of applications, including automotive components, aerospace parts, electronics, mechanical devices, and construction supplies.

Explanation:

Some of the defects encountered in casting are mentioned below.

Blow holes:

• Blow holes, gas holes, or gas cavities are well-rounded cavities having a clean and smooth surface. They appear either on the casting surface or in the body of a casting.

These defects are caused by: 

1. Excessive moisture content
2. Inadequate gas permeability of the moulding sand
3. Low pouring temperature

Shrinkage:

• Volumetric contraction both in a solid and liquid state. Shrinkage due to thermal contraction of a liquid convert to solid-state.

Porosity or Permeability:

• Permeability or porosity of the moulding sand is the measure of its ability to permit air to flow through it. Gases evolving from the molten metal and generated from the mould may have to go through the core to escape out of the mould. Hence cores are required to have higher permeability.

Inclusions:

• Particles of slag, refractory materials, sand, or deoxidation products are trapped in the casting during pouring upon solidification. The provision of choke in the gating system and the pouring basin at the top of the mould can prevent this defect.

Concept:

The total solidification time is the time required for the casting to solidify after pouring. This time is dependent on the size and shape of the casting by an empirical relationship known as Chvorinov’s rule.

For cube:

T₁ = 2 mins

T₂ = 4 T₁ = 8 T₁ = 8 min

Explanation:

Casting Defects:

External defects

• Surface defects → metal penetration, sand fusion, scabs, mould burst, flash, fins, incomplete filling, cold shuts, hot tear, un-fused chaplet, pipe formation, etc
• Dimensional defects → mismatch, distortion, core shift, dimensional inaccuracy, etc

Internal defects

• Internal cavities → blow holes, gas porosity, pinholes, shrinkage cavity, hot tears, internal cracks, etc
• Inclusions → sand inclusion, slag inclusion, un-fused internal chills, etc
• Metallurgical defects → improper composition, defective mechanical properties, internal stresses, etc

Fin (Casting defect)

• A thin, unintended projection on the surface of the casting is known as fin. Fins usually occurs at the parting line.

Fins are produced due to:

• Overflexible bottom boards
• Loose pattern plates
• In-adequately weighted sand
• Incorrectly assembled molds and cores.