Porosity in Castings: Causes, Effects, and Prevention

Have you ever wondered about the small holes or voids sometimes found in cast metal parts? This is a common defect known as porosity in castings. This type of defect refers to the presence of these pores within the solidified metal, and it can significantly affect the part’s strength and reliability. Two primary types of porosity are gas porosity, caused by trapped gases during solidification, and shrinkage porosity, resulting from the metal contracting as it cools. This article will explain the causes, effects, and prevention of porosity in castings.

What is Porosity in Castings?

Simply put, porosity in castings refers to the presence of voids, pores, or holes within the solidified metal. These imperfections can occur on the surface of the casting or be trapped inside. Porosity weakens the casting because these voids reduce the overall amount of solid material, making it more susceptible to cracking or breaking under stress. As mentioned in the introduction, there are two main types of porosity: gas porosity and shrinkage porosity. In the next section, I will explain what causes porosity to occur.

Causes of Porosity

Porosity in castings arises from two primary sources: gas porosity and shrinkage porosity. Let’s explore each of these in more detail.

Gas Porosity

Gas porosity in casting occurs when gases become trapped within the molten metal during the casting process and are unable to escape before the metal solidifies. These trapped gases create voids or pores within the casting. There are several ways gases can become trapped:

Dissolved gases in the molten metal: Just like water can dissolve air, molten metals can also dissolve gases from the atmosphere. The amount of gas a molten metal can hold depends on its temperature. As the metal cools and solidifies, its ability to hold these dissolved gases decreases. This causes the dissolved gases to come out of solution, forming tiny bubbles within the solidifying metal. If these bubbles don’t have time to escape, they become trapped, creating gas porosity.

Reactions with the mold material: The mold itself can sometimes contribute to gas porosity. When the molten metal comes into contact with the mold material (like sand or the binders used to hold the sand together), chemical reactions can occur. These reactions can generate gases that become trapped in the casting. For example, some binders used in sand casting can release gases when heated by the molten metal.

Turbulence during pouring: The way the molten metal is poured into the mold can also cause gas porosity. If the metal stream is turbulent or splashes around a lot during pouring, it can trap air within the mold cavity. This trapped air becomes incorporated into the casting as it solidifies, creating pores.

Gas evolution from core materials: In some casting processes, cores are used to create internal cavities or complex shapes within the casting. These cores are often made from sand and binders. If these core materials release gases during the casting process, these gases can become trapped, leading to porosity in castings.

Shrinkage Porosity

Shrinkage porosity is caused by the natural contraction of metals as they cool and solidify. When molten metal cools, it goes through several stages: it cools as a liquid, then it begins to solidify, and finally, the solid cools to room temperature. During the solidification process, the metal undergoes a significant decrease in volume.

Solidification shrinkage: As the molten metal cools and transforms from a liquid to a solid, it contracts or shrinks. This is a natural property of metals. If there isn’t enough molten metal available to fill the space created by this shrinkage, voids or pores form within the casting. This is called shrinkage porosity.

Lack of adequate feeding: In casting, risers are often used. Risers are reservoirs of molten metal designed to feed the casting as it solidifies, compensating for the shrinkage. If the risers are not properly designed or placed, they may not provide enough molten metal to fill the voids created by shrinkage, resulting in porosity. This “lack of adequate feeding” is a major cause of shrinkage porosity.

Influence of casting design: The design of the casting itself can also influence shrinkage porosity. Thicker sections of the casting cool more slowly than thinner sections. These thicker areas, often called “hot spots,” are the last to solidify. As these hot spots solidify, they shrink, and if they are not adequately fed by risers, they are prone to significant shrinkage porosity. Therefore, careful design is essential to minimize these hot spots and ensure proper feeding.

Types of Porosity

Porosity in castings can be categorized in a couple of ways: by the size of the pores and by their location within the casting.

Based on Size

When we talk about porosity based on the size of the pores, we generally distinguish between two types: microporosity and macroporosity.

Microporosity: Microporosity refers to very small, often microscopic pores that are distributed throughout the casting. These pores are so small that they are usually not visible to the naked eye. While they might not drastically reduce the overall strength of the casting in a single test, they can still have negative effects. For example, microporosity can reduce the casting’s resistance to fatigue, meaning it’s more likely to develop cracks and fail under repeated stress or vibration. It can also affect the surface finish, making it rougher.

Macroporosity: Macroporosity refers to larger, more visible voids or pores within the casting. These pores can be easily seen with the naked eye and can significantly weaken the casting. Macroporosity is a more serious defect than microporosity because these larger voids act as stress concentrators, meaning that stress is concentrated around these pores, making the casting much more likely to crack or break under load.

Based on Location

Porosity in castings can also be classified based on where it occurs within the casting: either on the surface or inside.

Surface Porosity: Surface porosity refers to pores that are open to the surface of the casting. These pores can affect the casting’s appearance, making it look rough or uneven. More importantly, surface porosity can also affect the casting’s performance. For example, it can reduce corrosion resistance because corrosive substances can get trapped in the pores. It can also cause leaks in applications where the casting needs to be pressure-tight.

Internal Porosity: Internal porosity refers to pores that are trapped within the interior of the casting, not open to the surface. These pores are harder to detect than surface porosity, as they cannot be seen with a simple visual inspection. However, internal porosity can still significantly weaken the casting and make it more prone to failure. Special non-destructive testing methods, like X-rays or ultrasound, are often needed to detect internal porosity.

Effects of Porosity on Casting Properties

Porosity in castings, whether it’s microporosity or macroporosity, surface or internal, can have several negative effects on castings’ properties and performance.

Effects on Mechanical Properties

The mechanical properties of a material describe how it behaves under different types of forces or stresses. Porosity directly impacts these properties:

• Reduced tensile strength: Tensile strength is a measure of how much pulling force a material can withstand before it breaks. Pores within a casting act as stress concentrators. This means that when a force is applied to the casting, the stress is concentrated around these pores, making them weak points. This significantly reduces the casting’s tensile strength, making it more likely to break under tension.
• Reduced fatigue strength: Fatigue strength is a measure of how much repeated or cyclic stress a material can withstand before it fails. Pores, especially those near the surface, can act as starting points for cracks. Under repeated stress, these cracks can grow and eventually lead to failure, even if the stress is much lower than the material’s tensile strength. Therefore, porosity in castings can reduce their fatigue strength.
• Reduced impact resistance: Impact resistance measures a material’s ability to withstand sudden impacts or shocks. Pores weaken the casting’s structure and make it less able to absorb impact energy. This means that a porous casting is more likely to fracture or break when subjected to a sudden blow.
• Reduced ductility and elongation: Ductility is a measure of a material’s ability to be deformed or stretched without breaking. Elongation is a measure of how much a material can be stretched before it fractures. Pores hinder the material’s ability to deform plastically. They act as obstacles to the movement of dislocations (defects in the crystal structure that allow metals to deform). This reduces both ductility and elongation, making the casting more brittle.

Effects on Other Properties

Porosity in castings also affects other important characteristics beyond mechanical properties.

• Leakage (pressure tightness): In applications where the casting needs to hold pressure or contain fluids (like pipes or valves), porosity can be a major problem. If the pores are interconnected, they can create leak paths through the casting, allowing fluids or gases to escape. This makes the casting unsuitable for pressure-tight applications.
• Surface finish: Surface porosity can result in a rough or uneven surface finish. This can be undesirable for both aesthetic reasons and functional reasons. A rough surface can increase friction, reduce corrosion resistance, and make it difficult to achieve a good seal with other components.
• Corrosion resistance: Pores, especially those open to the surface, can trap corrosive substances like moisture, salts, or chemicals. These trapped substances can accelerate corrosion, leading to premature failure of the casting. This is especially important in harsh environments or applications where the casting is exposed to corrosive media.

Prevention and Mitigation of Porosity

Preventing porosity is always the best approach, as it’s more cost-effective than trying to fix it after the casting is made. However, there are also methods to mitigate the effects of porosity if it does occur.

Mold Design and Gating Systems

The design of the mold and the gating system (the channels that guide the molten metal into the mold cavity) play a crucial role in preventing porosity.  

• Proper gating design: The gating system should be designed to ensure smooth, laminar flow of the molten metal into the mold cavity. This minimizes turbulence, which, as we discussed earlier, can trap air and cause gas porosity. The gating system should also be designed to fill the mold quickly and evenly.  
• Riser design and placement: Risers are reservoirs of molten metal that are connected to the casting. They are designed to feed the casting as it solidifies, compensating for the shrinkage that occurs during solidification. Proper riser design and placement are essential to prevent shrinkage porosity. The risers should be large enough and placed in strategic locations to provide enough molten metal to feed all parts of the casting as they solidify.  
• Chills: Chills are pieces of metal (often copper or cast iron) that are placed in the mold to accelerate cooling in specific areas. They are often used to promote directional solidification, meaning that the casting solidifies progressively from one end to the other. This helps to ensure that molten metal is always available to feed the solidifying areas, reducing shrinkage porosity.  

Melting and Pouring Practices

How the metal is melted and poured also has a big impact on porosity in castings.

• Controlling melting temperature: The melting temperature of the metal affects its ability to dissolve gases. Higher temperatures generally allow the metal to dissolve more gas. Therefore, controlling the melting temperature and avoiding excessively high temperatures can help reduce gas porosity.  
• Degassing techniques: Degassing is the process of removing dissolved gases from the molten metal before it is poured into the mold. Several degassing techniques are used, including fluxing (using chemical substances that react with the dissolved gases) and vacuum degassing (exposing the molten metal to a vacuum to remove dissolved gases).  
• Controlled pouring rate: Pouring the molten metal smoothly and at a controlled rate minimizes turbulence and reduces the chance of trapping air in the mold cavity. This helps prevent gas porosity.

Mold Materials and Binders

The materials used to make the mold can also contribute to porosity.

• Selecting appropriate mold materials: Certain mold materials can react with the molten metal and generate gases. Choosing mold materials that minimize these reactions can help reduce gas porosity.  
• Proper binder selection and control: Binders are used to hold the mold material (like sand) together. Some binders can release gases when heated by the molten metal. Choosing binders that generate minimal gas and carefully controlling the amount of binder used can help reduce gas porosity.  

Specialized Casting Techniques

Some specialized casting techniques are specifically designed to minimize porosity.

• Vacuum casting: In vacuum casting, the casting process is carried out under a vacuum. This significantly reduces the amount of gas present in the mold cavity, minimizing gas porosity.  
• Counter-pressure casting: In this process, a pressure is applied to the mold cavity during solidification. This pressure helps to prevent shrinkage porosity by forcing molten metal into the spaces created by shrinkage.

Post-Casting Treatments

Even with careful control of the casting process, some porosity may still occur. Post-casting treatments can be used to mitigate the effects of this porosity.  

• Hot isostatic pressing (HIP): HIP involves subjecting the casting to high temperature and high pressure in a sealed chamber. This process can effectively close internal pores, improving the casting’s density and mechanical properties.  
• Impregnation: Impregnation involves filling the pores in the casting with a sealant material, such as resin. This is often used to prevent leakage in pressure-tight applications.  

Detection of Porosity

Detecting porosity is crucial for guaranteeing the quality and integrity of castings. While some larger pores might be visible to the naked eye, many forms of porosity, especially microporosity and internal porosity, require specialized techniques for detection. Here are some common methods used to detect porosity in castings:

Visual Inspection

Visual inspection is the simplest and most basic method for detecting porosity. It involves carefully examining the surface of the casting for any visible pores or imperfections. While visual inspection can be effective for detecting larger surface pores (macroporosity), it has significant limitations. It cannot detect internal porosity or very small surface pores (microporosity). Therefore, visual inspection is often used as a preliminary screening method, but more advanced techniques are necessary for a complete assessment.

Non-Destructive Testing (NDT) Methods

Non-destructive testing (NDT) methods allow for the evaluation of a casting’s integrity without damaging or destroying the part. Several NDT methods are commonly used to detect porosity:

• Radiography (X-ray and gamma-ray): Radiography uses X-rays or gamma rays to penetrate the casting. The radiation passes through the casting and is captured on a detector (like film or a digital sensor). Pores within the casting absorb less radiation than the solid metal, creating darker areas on the image. This allows for the detection of both surface and internal porosity. Radiography is a powerful technique for detecting various types of porosity, but it requires specialized equipment and trained personnel.
• Ultrasonic testing: Ultrasonic testing uses high-frequency sound waves to probe the casting. A transducer emits sound waves into the casting, and these waves reflect back when they encounter a discontinuity, such as a pore. By analyzing the reflected waves, the size, location, and nature of the porosity can be determined. Ultrasonic testing is effective for detecting internal porosity and can be used on a wide range of materials.
• Dye penetrant inspection: Dye penetrant inspection is used to detect surface porosity. A liquid dye is applied to the surface of the casting and allowed to penetrate into any surface-connected pores. Excess dye is then removed, and a developer is applied. The developer draws the dye out of the pores, making them visible as colored indications on the surface. This method is relatively simple and inexpensive but is only effective for detecting surface-breaking defects.

Other NDT methods, such as eddy current testing and magnetic particle inspection, may also be used in specific cases, but radiography and ultrasonic testing are the most common for detecting porosity in castings. The selection of the appropriate detection method depends on factors such as the size and shape of the casting, the type of porosity being investigated, and the required level of sensitivity.

How We Guarantee Quality of Castings

At Taiyuan Simis Investment Casting Co., Ltd, we understand the critical importance of minimizing and eliminating porosity in castings. We employ a comprehensive approach, combining advanced techniques with strict quality control measures, to ensure that our castings meet the highest standards of quality and reliability.

• Advanced Casting Processes: We utilize a range of casting processes, including investment casting and shell mold casting, allowing us to select the optimal method for each specific project. This selection is based on factors like the part’s design, material, and required tolerances, helping to minimize the inherent risk of porosity from the outset.
• Optimized Mold and Gating Design: Our experienced engineers carefully design molds and gating systems to promote smooth metal flow and minimize turbulence during pouring. This helps to prevent gas entrapment, a major cause of gas porosity. We also utilize advanced simulation software to predict and optimize metal flow and solidification patterns, further reducing the risk of shrinkage porosity.
• Stringent Melting and Pouring Control: We maintain strict control over melting temperatures and employ degassing techniques to minimize the amount of dissolved gas in the molten metal. Our pouring procedures are carefully controlled to ensure a smooth, consistent flow, reducing the chance of trapping air during pouring.
• High-Quality Materials and Binders: We use only high-quality mold materials and binders that are carefully selected to minimize gas evolution during the casting process. This further reduces the risk of gas porosity caused by reactions with the mold.
• Rigorous Quality Control and Inspection: We have implemented a rigorous quality control system that includes inspections at various stages of the manufacturing process. This includes visual inspection, as well as advanced non-destructive testing methods like radiography and ultrasonic testing, to detect any potential porosity defects.
• Post-Casting Treatments (as needed): When necessary, we utilize post-casting treatments such as hot isostatic pressing (HIP) and impregnation to further mitigate the effects of any remaining porosity. HIP effectively closes internal pores, improving density and mechanical properties, while impregnation seals surface pores to prevent leakage.
• Commitment to Customer Satisfaction: Beyond technical expertise, we prioritize understanding our customers’ specific requirements and delivering tailored casting solutions that meet their needs and ensure reliable performance in their intended applications.

Through advanced techniques, rigorous controls, and a strong commitment to quality, Taiyuan Simis Investment Casting Co., Ltd. delivers high-quality castings with minimal porosity and ensures reliable performance. Contact us today for a quote and let us demonstrate our expertise.