Shrinkage is a common problem in the casting industry, and it can lead to defects in the final cast metal product.
Are you seeking ways to reduce defects in cast metals?
In this article, we will unveil the secrets of shrinkage, and also provide you with appropriate solutions to minimize the defects in cast metals. Are you ready, let’s dive in!
Understanding the shrinkage in cast metals.
Shrinkage is a volume change that occurs during the metal transforms from
liquid to solid and it cools down to room temperature.
The shrinkage of stainless steel and nickel alloys might be 2-3% by volume typically, whereas it can be 5-6% for copper alloys.
In the cast metals product, shrinkage can cause defects like Pin Holes, Blowholes, and Sand Blow, which will weaken the strength of cast parts, sometimes, it can result in functional failures and safety issues of the final
Therefore, shrinkage during casting is an issue that needs to be carefully managed.
The impact of shrinkage on cast metal quality.
Shrinkage affects the quality of cast metal products negatively, it can lead to various types of defects.
Porosity is among the most common casting defects, which is caused by the metal shrinking, small voids or pockets are present in the metal, it is also called shrink porosity. It can weaken the part’s strength and reduce its resistance to fatigue.
Cracks often occur in the inner side of the casting sharp corner, the junction of the different thickness sections, and the hot spot area where the riser is connected with the casting.
Generally, the external cracks appear on the surface of the casting and can be detected visually or by similar non-destructive methods, while the internal cracks can only be found by X-ray inspection or destructive tests.
3. Dimensional accuracy
Shrinkage of the casting part occurs during the solidification process, which results in dimensional deviations from the desired specifications.
This can lead to improper fits of the assembly part, thus impacting the functionality of the final cast metal product.
What are the causes of the shrinkage defects?
There are a few causes leading to the shrinkage defects in the cast metal parts:
1). Cooling rate.
The cooling rate can impact the shrinkage behavior of the cast metal product.
A higher rate can increase shrinkage due to it doesn’t leave enough time for feeding the molten metal to compensate for the volume contraction.
While, a lower cooling rate can reduce shrinkage, due to it allows for more efficient feeding.
2). Chemical composition of the alloys.
The characteristics of melting and solidification are different for various casting alloys. Some kinds of alloys are more prone to shrink than others.
The shrinkage of low-carbon cast steel is bigger than that of gray cast iron because carbon is the element to form graphite, which can reduce the casting shrinkage.
However, Sulfur can prevent graphite forming, which can increase the potential for shrinkage.
3). Casting techniques.
Casting parameters such as pouring way, pouring speed and temperature can all impact the degree of shrinkage.
The higher of pouring temperature, the more overheating, which will increase the casting shrinkage.
How to minimize the shrinkage defects?
By understanding the above causes of shrinkage, we can apply some techniques to reduce the shrinkage of the casting part.
1). Adopt proper gating and riser systems.
A well-designed gating system can help to control the cooling rate and reduce shrinkage defects.
The gating system elements include a pouring cup, sprue, strut well, runner, and ingate. By choosing the appropriate dimensions and shape of elements, it can regulate the flow rate, minimize turbulence, and ensure smooth metal flow.
A riser, also known as a feeder, is a reservoir built into a metal casting mold to prevent cavities due to shrinkage. Selecting the proper size and location of the risers can supply the molten metal to cavities constantly and ensure uniform solidification.
2). Choose the suitable alloys.
Some elements can help to reduce the shrinkage during casting.
For example, carbon is the element to form the graphite, which will expand and compensate for the shrinkage.
Sulfur will increase the shrinkage, while manganese can react with Sulfur to synthesize MnS, so appropriate manganese can minimize the casting shrinkage.
3). Use simulation software to optimize casting designs.
Casting simulation software is a powerful tool in the model metal cast industry.
By simulating various casting parameters, it can identify the potential shrinkage defect and predict the cooling rate, then suggest modifications to optimize the mold design and casting parameters, to minimize the shrinkage defects.
shrinkage is a phenomenon that occurs commonly in the metal casting industry, it can lead to defects in the final cast part.
However, by understanding the causes and implementing proactive measures, such as optimizing the gating and riser system, simulating by software, and choosing the suitable alloys.
The shrinkage defects can be minimized to ensure good quality of the final cast metal product.
If you have any questions about shrinkage defects, feel free to reach out to us.