What are Casting Defects?

Casting is a major manufacturing process in the engineering industry and produces components for a range of industries using many different methods. The components will have a range of functions and roles including structural strength. A casting defect is a defect in a cast component that compromises its function. Depending on the function of the cast component, some defects will be benign. Below, we discuss 5 common casting defects and how to prevent them.

5 Common Casting Defects

1) Gas Porosity Casting Defects

Gas porosity is one of the most common casting defects that occurs during the cooling process of casting and results in bubbles or air voids within the casting structure. It is similar to the weld defect, porosity. The volume of gas that can be held within a liquid and solid is different, with a liquid capable of holding a greater volume of gas. As the casting structure cools and solidifies, it no longer holds as much gas as it moves from a liquid to a solid. As a result, the gas forms bubbles in the structure.

The location of the gas porosity can be on the surface of the material or within the structure. The porosity defect harms the structure as it reduces the strength of the metal in the defect.

Types of Gas Porosity Casting Defects

Pinholes

Pinholes, as the name suggests, have the appearance of very small holes and tend to be around 2 mm in size. Generally, pinholes are found in the cope (upper part of the mould) in areas that are poorly ventilated. They tend to form in clusters at the surface of the metal meaning, it is achievable to identify them without the use of specialised equipment.

Blowholes

Blowholes, also known as ‘blows’, are a larger type of gas porosity defect compared to pinholes. Unfortunately, these larger defects can occur within the structure of the metal, making them difficult to detect and sometimes only detected as a result of the following machining operations. On the other hand, blowholes in the centre of the casting structure will require non-destructive testing such as ultrasonic testing to detect them.

Open Holes

Open holes are gas porosity defects that are on the surface and ‘open’ to the surface. Due to being on the surface, they are very easy to detect.

How to Prevent Gas Porosity Casting Defects?

• – Melt the metal in a vacuum or an environment of low solubility gases such as carbon dioxide or argon.
• – Melt the metal under a flux to prevent the metal from reacting with the atmosphere.
• – Use a mould design that minimises the turbulent flow of the molten metal because turbulent flow introduces gases.  

Hot Tears

Hot tears, also known as hot cracking, are a metallurgical defect and is a failure that occurs during the cooling portion of the casting process. The casting defect hot tear occurs because the metal is weaker when it is hot and is unable to withstand the residual stresses in the material as it cools and solidifies. Studies have been undertaken and established that hot tears tend to occur in the late stages of solidification when the volume fraction of the solid is above 85%.

During solidification, the liquid flows through the mush zone (a term used to describe the state of the casting material during solidification – between solid and liquid states). It is the mutual existence of two phases (solid and liquid). During solidification, the amount of liquid flowing through the mushy zone decreases and can no longer compensate for shrinkage resulting in pores. Hot tears then form from these pores.

A hot tear can result in a casting becoming redundant and unsuitable for its intended purpose. Even if the hot tear is small, there is a chance it can propagate in service and result in a complete fracture of the casting.

Hot tear casting defects have the visual appearance of irregular crevices that are branch-like. As a result, some hot tears are easily seen, however, others are more difficult and will require a process such as magnetic particle inspection to identify them.

How to Prevent Hot Tears?

The primary reason for hot tears is the contraction forces when the metal undergoes solidification. In order to prevent hot tears, it is best to practice designing a mould to minimise the contraction forces that the metal will undergo.

Additional methods of minimising hot tears are not overheating the casting material and increasing the temperature of the mould. Both these methods reduced the temperature delta and hence the residual stress during solidification in the metal.

Inclusion Casting Defects

Similarly to welding, casting material can contain inclusion defects. The inclusions in casting material are commonly slag inclusions and sand inclusions.

Slag Inclusions in Castings

Slag inclusions consist of non-metallic materials trapped within the metal structure. Although it is detrimental to a casting structure, slag is a necessary compound during the creation of castings. Slag has a positive impact on castings as silica and calcium-based slags alter the viscosity of the metal.

Cold Shut

A cold shut casting defect is a type of pouring defect. A cold shut occurs when a part of the mould is filled from two separate flows, and they are too cold to fuse. Cold shut defects have the appearance of an open seam or a crack with a round edge.

The part that has not formed together is separated by an oxide meaning it is not fully homogeneous and potentially has a structural impact.

What Causes Cold Shut Casting Defects?

A cold shut is normally caused as a result of pouring issues. As a result, they form due to poor gate design or a lack of molten metal fluidity.

How to Prevent Cold Shuts in Castings 

To prevent cold shuts, it is necessary to optimise the gating system used to transfer the molten metal into the mould. Improving the gating system reduces the time it takes for the molten metal to fill the mould and will lose less temperature as a result. An additional preventative measure is to increase the pouring temperature to overcome premature solidification and ensure the two flows are warm enough to fuse when they meet in the mould.

Shrinkage Casting Defects

Shrinkage occurs in castings because metals are less dense in the liquid state than in the solid-state. It is a process that will occur, however, a shrinkage defect is when the shrinkage is uneven and results in a distorted shape. When calculating the mould size and molten of molten metal required, shrinkage is integrated into the calculations.

Some shrinkage defects can result in a similar appearance as porosity (small cavities), however, tend to appear on angular edges, unlike porosity.

Shrinkage defects can be split into two: open shrinkage defects and closed shrinkage defects.  

Open Shrinkage Casting Defects

Open shrinkage defects appear on the surface of the cast material and are exposed to the atmosphere. As the shrinkage cavity starts to form, air compensates and results in the defect. When the metal shrinks unevenly, the air inside the mould into that area.

Closed Shrinkage Defects

Conversely, closed shrinkage defects within the casting structure tend to form because one part of the liquid is hotter than the remaining material. They are closed shrinkage defects that tend to be at the top of hot spots or isolated pools of liquid.

Due to the nature of closed shrinkage defects, they tend to be undetectable by eye however they can sometimes be visually seen.

How to Prevent Shrinkage Casting Defects

There are several ways to prevent shrinkage casting defects, both open and closed. The primary methods of preventing them are:

• – Designing a gate system containing risers to ensure a continuous flow of molten metal.
• – Lowering the casting temperature to minimise the volume deficit.
• – Optimising local heat dissipation by the use of internal chills, cooling coils and cooling ribs.

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