Casting defects can come in a variety of forms. Each has unique characteristics, root causes, and prevention methods. Knowing the different associated faults and reasons is essential to mastering casting. As a result, you may actively prevent these flaws and provide clients with high-quality items.
Casting defects are unwanted occurrences in a casting during the casting process. They may lead to bad shape, weak casting, and poor functionality. Casting defects are broken down into five main categories: gas porosity, shrinkage, mold material, pouring metal, and metallurgical defects. Some casting defects can be repaired, while others are tolerated. In this article, I’ll be discussing all types of casting defects, their causes, and remedies.
Contents
What are casting defects?
Casting defects are imperfections or inconsistencies that affect a component’s ability to meet quality standards. Imperfections in casting can occur due to several causes, including material failure, malfunctioning casting equipment, and inefficient casting methods. While some of these flaws may be overlooked or ignored, others can lead to weak casting, strange forms, and subpar functionality.
Types of Casting Defects
The following are types of casting defects, their causes, and remedies:
- Gas Porosity: Blowholes, open holes, pinholes
Shrinkage defects: shrinkage cavity
Mold material defects: Cut and washes, swell, drops, metal penetration, rat tail
Pouring metal defects: Cold shut, misrun, slag inclusion
Metallurgical defects: hot tears, hot spots.
Gas porosity casting defects
Gas porosity defects take place on cast metal when it traps gases like nitrogen, oxygen, or hydrogen during casting. These defects appear on metals as small holes and rounded, or circular cavities. The holes are formed when the casting cools and solidifies because the solid form of metal cannot store as much gas as the liquid form.
Gas porosity casting defects are caused by an escape route on mold and cores at which gases can pass and when mold and cores are not allowed to dry before use. Gas porosity casting defects are classified into three categories:
Pinholes:
Pinholes are very small holes of about 2 mm in size that appear on the surface of the casting. This defect happens because of the dissolution of the hydrogen gases in the molten metal. When the molten metal is poured into the mold cavity and as it starts to solidify, the solubility of the hydrogen gas decreases, and it starts escaping out of the molten metal, leaving behind a small number of holes known as pinholes.
Pinholes are caused by the use of high-moisture-content sand and the absorption of hydrogen or carbon monoxide gas by molten metal. The pouring of steel from wet ladles that is not sufficiently gasified is another cause of pinholes. The remedy is to reduce the moisture content of the molding sand. Good fluxing and melting practices should be used. Increasing the permeability of the sand and performing a rapid rate of solidification.
Blowhole:
Blowhole defects occur when gases are entrapped on the surface of the casting due to solidifying metal. This results in a rounded or oval cavity. They are often present in the cope part of the mold. Blowholes are larger cavities located inside the cast metal. They are difficult to locate before machining, and they require harmonic, ultrasonic, magnetic, or x-ray analysis.
These types of casting defects are caused by excessive moisture in the sand, low permeability of the sand, the sand grains being too fine, the sand being rammed hard, and not enough venting being provided.
To avoid blowholes, the moisture content in the sand must be controlled and kept at a desired level, and high-permeability sand should be used. Sand of appropriate grain size should be used, sufficient ramming should be done, and an adequate venting facility should be provided.
Read more: What are the types of casting processes?
Open holes:
These are the various kinds of blowholes that develop on the workpiece’s surface. They are developed as a result of air getting trapped when the metal is put into the mold. Open holes have the form of shallow scars.
On the surface of castings, these flaws are rounded or oval in shape. They frequently have bright yellow or brilliant white, smooth surfaces. As a result, following the casting process, you can detect gas porosity defects in the casting by visual inspection. On the other hand, to look for this flaw on non-machined surfaces, you need an X-ray machine.
Open holes are caused by loose ramming of the sand and improper cleaning of the mold cavity. Rapid pouring of the molten metal into the mold can result in the washing away of sand from the mold, and a hole is created. Ensure the proper ramming of the sand. The molten metal should be poured carefully into the mold, and proper cleaning of the molten cavity eliminates sand holes.
Shrinkage casting defects
These types of casting defects occur during the solidification of the casting. It appears with angular edges when compared with round surfaces of gas porosity. Shrinkage casting defects happen because metals are less dense as a liquid than a solid. Shrinkage with large cavities can cause the casting to eventually break under stress.
Shrinkage types of casting defects are classified into two:
Open Shrinkage defects:
These casting defects are located at the surface as pipes and burrow into the casting. It is open to the atmosphere and allows air to enter as the shrinkage cavity is formed.
Closed shrinkage defects:
these casting defects are also known as shrinkage porosity. It usually appears at the top of the hot spots or isolated pools of hot liquid. Closed shrinkage defects are formed within the casting. Some can be seen with the bare eye but some cannot.
The causes of shrinkage casting defects include high concentrations of metal in the mold areas and low injection pressure. High pouring temperature solidification of the metal in an uneven pattern, and poor design runner and gate could also cause the defect.
Shrinkage casting defect
remedies include cleaning the metal surfaces and using simple casting geometries with better runner and gating designs. To guarantee proper heat dissipation, add cooling coils, ribs, or internal chills. Increase the pressure of the metal injection to avoid the defect.
Mold material casting defects
Mold material casting defects occur on sands and can always be prevented by modifying the mold. These casting defects can affect the cast in several ways.
Cuts and washes:
Cuts and washes are areas of excess metals that occur when the molten metal washes away part of the molding sand. When this occurs, the sand does not have enough strength to resist erosion. These defects can be caused by molten metal flowing at an extremely high velocity, allowing the flow of too much metal through the gate.
Cuts and washes are prevented when the gating system is properly designed, improving the mold and core strength and adding more binders to the facing and core sand.
Read more: Difference between Forging and Casting
Fusion:
fusion is a thin crust with a brittle, glassy appearance firmly joined to the casting. This is caused when sand grains are fused with molten metal. It takes place when molten metal with extremely hot velocity is poured and the low refractoriness of clay or sand. Silica sand is mostly used during casting due to its high refractoriness.
The causes of fusion casting defects include low refractoriness of sand and clay, and sand grains mixing the metal. Too high a metal pouring temperature could also cause the defect.
Remember, refractoriness is the ability of molding material to resist the temperature of liquid metal to avoid fusion. To prevent fusion casting defects, reduce the temperature of the pouring metal, and improve the refractoriness of molding materials. Enhancing the refractoriness of molding material is also a remedy.
Run Out:
These casting defects occur when the molten metal escapes through the mold, which could lead to incomplete or missing casting. Run-outs are caused by the mold or flask, and they can be prevented by designing the casting mold with precision. One should also inspect and repair any defective molds before casting. Reducing the temperature of molten metal can stop the wear and tear of the mold. Using quality raw materials for mold work will help in resisting high temperatures.
Swell:
These casting defects occur on the vertical face of the castings and are caused by low-strength mold and improper or bad mold ramming. To prevent swell casting defects, molds should be built to withstand the pressure of liquid metal. Otherwise, the mold shape changes by moving a bit back and causing swells. These casting defects can also be prevented when the mold is properly rammed.
Drops:
These casting defects occur when the casting is still liquid. It is caused when pieces of metal drop into the liquid metal. It appears as an irregular-shaped projection on the surface of the casting. Using sand with lower strength, soft ramming, insufficient fluxing of molten metal, and the absence of reinforcement of sand in the cope can lead to this defect.
Rattail, Veins, and Buckles:
These casting defects occur in the casting as irregular lines or cracks. The rattail appears on casting when the hotness of the metal causes the sand to expand, which may be caused by poor expansion of the sand, the hot pouring temperature of the metal, or poor design.
Metal penetration:
Metal penetration occurs when molten metal penetrates gaps in the molding sand. These types of casting defects appear as a rough and uneven surface finish on the casting. Metal penetration is caused by various issues, which include: using sand with low strength and high permeability; using large or coarse sand grains; soft ramming of sand; and a lack of mold wash. All these can be prevented by high strength, small grain size, low permeability, and hard ramming of sand. Metal penetration can be removed from the surface of the casting by grinding down the affected parts.
Metallurgical defects
Metallurgical casting defects come in two types: hot tears and hand spots.
Hot tear/crack:
These casting defects take place in the casting pattern as narrow cracks. These cracks are obvious. These cracks are seen after the casting cools, or when it’s about to solidify. The hot tear occurs if the casted metal does not have sufficient strength to resist tensile forces during solidification, and it is mostly caused by poor mold design.
Hot or hard spots:
These casting defects are spots that are harder than the surrounding area. This spot cools more quickly than the surrounding material. Hard spot defects are the result of improper cooling practices and can be avoided by correcting cooling practices and considering changing the metal’s chemical composition. The casting defects can interfere during machining and increase tool wear.
Read more: Types of Casting patterns (Foundry Patterns)
Pouring defects
Pouring casting defects occur during the pouring process. They can be categorized into four categories:
Cold shut/lap:
These casting defects appear on the surface of the casting. It occurs as a line or crack with a round edge on the casting surface. The defect leads to full damage to the casting because it creates a weak spot. When molten metal enters the mold from two gates, the steam will meet at a point. Low temperatures can prevent fusion at the point, causing the steam to solidify before fusion and creating a cold shut. These types of casting defects are usually caused by a lack of fluidity in the molten metal or a poor design of the gating system, which is prevented by increasing the fluidity of the molten metal.
Misruns:
These casting defects are closely related to cold-shut. It occurs when the liquid metal is too cold to flow through the mold cavity before freezing and solidifying. The liquid metal does not fill the mold cavity. The misrun is an unfilled portion of the mold. This casting defect can be prevented by the molding design, gating system design, and molten metal fluidity.
Read more: Different Types of Die Casting and its Applications
Cold shots:
These casting defects are solid globules on the surface of the casting. It is caused by the splattering during the pouring of the liquid metal. These cold shuts are typically ball, drop, or pearl-shaped. To prevent splattering and cold shots, the pouring procedures should be modified, and the gating system should be designed to reduce the gate speed.
Slag inclusion (scab):
These types of casting defects are irregular metallic crusts located on the casting surface. They are typically thick but can be tough and seen with the naked eye. Scabs usually have sharp edges and irregular shapes and are firmly bonded to the casting. Removing scabs from the cast will expose a rat tail underneath. Slag inclusion is caused when molten metal containing slag particles is poured into the mold cavities and solidifies together. Slag can be removed in several ways, which include: melting the metal with flux in a vacuum or inert atmosphere; or adding a ceramic filter to the gating system.
Shift/ Mismatch:
These casting defects occur when the mold shifts due to misalignment of the cope (upper) and drag (lower) parts of the mold. Mold shift is usually reflected as a horizontal displacement. These casting defects occur when the box pins are lost, there are inaccurate pattern dowel pins, or the cope and drag have been carelessly placed, leading to misalignment. To avoid misalignment, these three causes must be examined.
Flash, Fin, and Burrs:
These casting defects are one of the most common. It is also called fins and burrs, which are unwanted or excess materials attached to the cast. Flash is a waste material that turns into dross after being melted. Flashes are caused by cracks and gaps on the core surface due to insufficient weight on the mold or improper clamping of the flask. To avoid fins, there should be enough weight on the top part of the mold so the two parts tightly fit together.
Read more: Understanding investment casting
Warping:
Warping is an unwanted casting deformity that occurs over time and may lead to a change in the dimensions of the product. These types of defects take place during solidification. They are caused by the different rates of solidification of different sections, which cause stress in the adjoining walls. Heat treatment can help remove the residual stress in iron casting.
Conclusion
The metal casting process can produce several surface defects. You will have greater chances for casting quality control if you have a deeper knowledge of these casting flaws. Your casting will be of higher quality if you are fully aware of casting defects and how to correct them.
While these defects may occasionally be expected, knowledgeable die-casting experts may help you avoid or correct them. As a result, you need to collaborate with a partner who supports quality control inspections. That is all for this article: types of casting defects their causes, and remedies. I hope it was helpful, if so, kindly share it with other students. Thanks for reading, see you next time!
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