Casting is a manufacturing process, in which hot molten metal is poured into a mold box, which contains a hollow cavity of the desired shape of the metal to be formed, and then allowed to solidify. That part solidified is known as a casting. Casting is a very versatile process and capable of being used in mass production of complex shapes that would be otherwise difficult or uneconomical to make by other methods.
Casting processes are naturally mind-boggling because of the stage change from fluid to a strong metal, with such a perplexing procedure, numerous potential deformities may result. There is always a chance that defect will arise and the occurrence of failure during all the process of production of the finished product. These defects formation in castings is one of the most problematic subjects in the foundry industries.
A defect in castings does not just happen. Casting defects
are unusually not by accidents, they occur because some step in manufacturing cycle does not get
properly controlled and somewhere along the line, something goes wrong.
A defect may be the result of a single clearly defined cause or a combination of factors. They are caused by wrong practice in one or more of the basic operations involved in the casting process as in the equipment used, the technique used in manufacturing or by the casting design.
A casting defect is an unwanted irregularity in a metal
casting process. Some defects can be tolerated while others can be repaired, otherwise, they must
be eliminated. A properly designed casting, a properly prepared mold, and
melted metal should result in a sound casting. However, if proper control is not exercised a variety of defects may result in a casting process.
There are different kinds of casting defect and various remedies to solve them. The remedy to one casting defect might be the cause of another type of defect.
Types of Casting Defects.
Casting defects are of diverse types, all of which are to be avoided when looking forward to preparing the perfect casting. Here is a list of the most frequently encountered ones, their causes, and remedial measures:
Misrun defect is a kind of incomplete casting defect, which occurs when the hot molten metal does not completely fill the mold cavity before solidifying. It can occur when the pouring temperature has been lowered thereby causing discontinuity of fluid streams. These form of defect is common in castings which have a large surface area to volume ratio. The unfilled portion of the mold cavity is known as misrun. Misrun defect can be easily identified by its round and smooth edges.
Causes of misrun defect
Decreased pouring temperature: as the temperature of decreases, the fluidity of the metal also decreases thereby causing the metal to solidify rapidly. An interruption in pouring, intermittent pouring due to inadequate metal supply, delayed pouring, back pressure during pouring and inadequate venting while pouring are factors that lower the pouring temperature.
The length to thickness ratio of the casting is too large (surface area to volume ratio is large): this causes an increase the cavity fill time (time for the molten metal to fill the mold box) in which the metal solidifies before the entire cavity distance is filled.
Improper design of the gating system.
Provide proper venting.
Modify the design of the gating system so as to decrease cavity fill time.
Increase the temperature of the metal to ensure fluidity and pour into the cavity without interruption.
Ensuring uniform thickness throughout the section.
COLD SHUT OR COLD LAP OR KNIT LINES
Cold lap, also called as Cold shut is formed when two flows of liquid metal that flow together do not fuse properly together in the mold cavity due to the premature freezing of one of such metal leaving behind a weak spot. When the molten metal is transferred through the gates, multiple liquid fronts will have to flow together and become one solid. If the flowing metal fronts are too cool, they may not flow together but will leave a seam in the part. Such a seam is called a cold shot or cold lap. Cold shut is the most common types of casting defect. When a surface with such defects is subjected to certain types of loading, it begins to crack, the crack is most visible in surfaces with round edges.
Causes of Cold Shut
Low pouring temperature of metal: pouring metal into the mold cavity while they are yet cool will cause their fluidity to reduce sporadically thereby making them solidify prematurely.
Inadequate gating system: gating systems will go a long way to determine the quality of your final product, the better your gating system, the better your finished product.
Thin section of casting.
Ensure you superheat your metal before pouring into the mold cavity.
Confirm your cavity walls are thick enough while designing the entire process by using minimum permissible cavity thickness for the process.
Make certain that your gating system is appropriately placed.
Improve the fluidity of the metal by changing its chemical composition and altering the microstructure of the metal.
Hot tears are another common type of casting defect; they are cracks which originate during solidification. It arises due to the formation of irrevocable failures of semisolid casting. Hot tears occur during the last stage of solidification when the volume fraction of solid is above 85 to 95 percent and the solid phase is organized in a continuous network of grains because of the imbalance in temperature that occurs during this stage. Hot tears occur in the form of sharp and broken rugged lines at the edges of castings. Alloys with large freezing range are more susceptible to this defect.
Causes of Hot Tears
Impeding the contraction forces that occur during material solidification.
It can arise as a result of the thermal contraction in the solidifying metal: Pouring the molten metal while its pouring temperature is yet to reach is a common cause of hot tear
Application of Incorrect solidification methods
Low amount of eutectic cells at the grain boundary
An excess amount of Sulphur/Phosphorus present
Designing sound solidification system: solidification should be made to begin furthermost from the feed gate or riser and moves progressively toward it so that the final shrinkage void will probably be located external to the desired casting, in either the riser or the gating system.
Improve casting mold design
The pouring temperature should be accurate
Proper placement of a gate
Ensure uniform thickness throughout the section
THERMAL (SOLIDIFICATION) SHRINKAGE
Thermal shrinkage is a noticeable volumetric reduction that begins to
occur once the molten metal enters the mold
cavity and begin to cool.
There are three basic stages of thermal shrinkage:
(1) shrinkage of the liquid metal as it cools to the temperature where solidification begins,
(2) solidification contraction as the liquid turns into solid, and
(3) shrinkage of solid metal as the solidified material cools to room temperature.
The extent of liquid metal shrinkage depends on the coefficient of thermal contraction (it is a constant which specifies the percentage reduction in the length, area or volume of the material per degree of temperature change) and the amount of superheat.
Liquid contraction is rarely a problem, however, because the metal in the gating system continues to flow into the mold cavity as the liquid already in the cavity cools and contracts.
As the metal changes state from liquid to crystalline solid, the new atomic arrangement is usually more efficient, and significant amounts of shrinkage can occur. The actual amount of shrinkage varies from alloy to alloy, its noteworthy that not all metals contract upon solidification. Some actually expand, such as gray cast iron, where low-density graphite flakes form as part of the solid structure.
Causes of Thermal Shrinkage
The density of a casting alloy in the molten state is not as much as its density in the solid state: when an alloy changes stage from the molten state to the solid state, it generally shrivels. This shrinkage happens when the casting begins solidifying.
A complication in casting geometry.
The pressure of metal is excessively low.
Poor gating and runner design.
Increased metal fixation at an explicit location.
Contamination of metal.
Reduced metal volume in course of hardening
Increase the metal pouring pressure: The general strategy for elimination of shrinkage porosity is to guarantee that fluid metal under pressure keeps on streaming into the voids as they develop.
Design the casting with additional tolerance.
Use simple geometry in castings and avoid every form of complexity.
Increased metal pressure.
Improve the gating and runner design.
Remove the contaminants, clean the metal surface.
Provide core hole in the center and offset the ribs
If you detect thermal shrinkage porosity in your castings, lowering the pouring temperature wouldn’t be the correct solution because it can lead to another defect, namely cold shut.
Porosity is any void or hole in a casting. It can be attributed to two main sources:
- solidification porosity (caused by solidification shrinkage)
- gas porosity (caused by gas entrapment)
Most alloys have a higher density in their
solid state as compared to their density in the liquid state. As a result,
shrinkage porosity forms during solidification. Due to the turbulent manner in which metal enters and fills the cavity, gas often becomes entrapped
in the metal, resulting in gas porosity. If a casting needs to be
pressure tight, then the porosity can allow gas and fluids to seep from the part.
In addition, the porosity can weaken the casting.
Gas porosity is the formation of bubbles within the casting after it has cooled. This occurs because most liquid materials can hold a large amount of dissolved gas, but the solid form of the same material cannot, so the gas forms bubbles within the material as it cools.
The gas can be from trapped air, Nitrogen, oxygen, and hydrogen are the most encountered gases in cases of gas porosity, moisture from water-based die lubricants or steam from cracked cooling lines. Usually internal, caused by trapped gases of various kinds in the die. Gas porosity comes from three main sources in die-casting, namely trapped air, steam and burned lubricant.
Air is present in the cavity before the shot. It can easily be trapped as the metal starts to fill the cavity. The air is then compressed as more and more metal streams into the cavity and the pressure rises. When the cavity is full it becomes dispersed as small spheres of high-pressure air. The swirling flow can cause them to become elongated. Small gas holes either at the surface or just below the surface or fairly uniformly dispersed over the surface
Gas porosity may present itself on the surface of the casting as porosity or the pore may be trapped inside the metal, which reduces strength in that vicinity.
shrinkage porosity, are defects that form within the casting. It can be describedas internal cracks in the casting which come from several sources. At the midpoint of thick areas of casting, shrinkage can result in a number of little voids known as shrinkage porosity. shrinkage porosity, are defects that form within the casting.
Isolated pools of liquid can also form inside solidified metal; these are called hot spots. The shrinkage defect usually forms at the top of the hot spots, in the event that the shrinkage porosity is little in diameter and restricted to the midpoint of thick sections it will more often than cause no issues. Be that as it may, on the off chance that it is bigger in size, or consolidated, it can extremely debilitate a casting. It is additionally a specific setback for castings which should be gas-tight or watertight.
Causes of Porosity
- Turbulence from pouring the molten metal into the mold cavity.
- The complexity of casting geometry.
- Poor gating and runner design.
- Low metal pouring temperature and pressure.
- The high gas pressure in the mold arising from molding material having high moisture and/or volatile content and/or low permeability
- Presence of slag on the metal surface.
- Metal section is too thin.
- Insufficient fluidity of molten metal.
- Mold cavities should be streamlined to reduce turbulence.
- Clean the metal to remove slag from the metal surface.
- Modify metal composition to increase fluidity.
- Reduce gas pressure in the mold by an appropriate adjustment to molding material properties.
- Proper venting of molds and cores and modify runner and gating system.
- If possible, modify casting design to avoid thin sections.
Gas porosity can also be prevented by melting the metal in a vacuum, in an environment of low-solubility gases, such as carbon dioxide or argon, or under a flux that prevents contact with the air. Other methods include Vacuum Degassing, Gas Flushing, or Precipitation
A mismatch is a defect in which the top or bottom part of the casting shifts above or below the center line causing the casting to be defective. It occurs when the cope and drag relatively displace from their normal positions. Segments dislocated above the parting line leads to this serious defect.
Causes of Mismatch
The major cause of mismatch is the absence of cope and drag parts of the mold, in their proper positions. This can occur as a result of either of the following:
- The careless placing of cope on drag.
- Loose box pins
- Imprecise or worn out pattern dowel pins
- Faulty pattern designs of the top and bottom parts
- Remedial Measures
· Use of proper gating system.
· Heavyweight to be kept on top of casting. ensuring no dislocation of cope part.
· Ensure the top and bottom parts are aligned properly.
· Replace worn out dowel pins with new ones.
· Use locators to match top & bottom parts.
· Use C-Clamps to clamp the mold box
Inclusions are due to the presence of forging, nonmetallic particles in cast metal. These inclusions can occur in the form of oxides, slag, dirt, sand or nails. These inclusions can limit the mechanical properties and fatigue performance as well as lead to cosmetic defects in the casting. The most common form of inclusion is sand inclusion. Sand inclusions stuck under the casting surface and are most visible during the treatment.
Causes of Inclusions
- Sand, dirt reis not properly cleaned from molds.
- Improper gating system
- Alloy additions which have not completely dissolved in the melt.
- Uneven compaction of molds
- Tilting of metal stream directly to cores causing erosion.
- Mold breakage during assembly.
- Improper pouring practices leading to mold disturbances.
- Uneven sand mixing (in case of sand inclusion)
- Use of properly dressed cores.
- Ensure proper pouring time.
- Maintain optimum pouring height.
- Improve the gating system.
clean and pure molten metal.
In the case of sand inclusion,
- Ensure proper ramming of sand for uniform compaction.
- Use of high bentonite content.
- Frequently clean of the mold boxes.
- A proper mixing ratio of reclaimed sand and binder should be enforced.
There are many other types of casting defects though they are not as prevalent as the aforementioned ones. These includes: