Have you ever faced order returns, soaring costs, or customer complaints due to issues such as voids on the surface or in the interior of castings? These problems likely stem from a single phenomenon—-casting porosity. It is one of the most common and challenging investment casting defects. This article will thoroughly explain the types of casting porosity, their causes, and preventive measures, helping you address porosity issues with greater confidence.

What is Casting Porosity?

Casting porosity refers to the presence of voids of varying sizes and shapes within the components. Some are visible on the surface, while others are hidden inside or even deeper within it; these internal voids can only be revealed after machining and grinding. If mishandled, porosity can significantly compromise the strength and performance of castings, leading to the scrapping of entire batches of components and even causing safety hazards. 

Different Types of Casting Porosity

Knowing the different types of porosity will help you identify the causes, characteristics, and preventive measures for each type, enabling you to improve the quality of the product effectively. Based on the causes of porosity, casting porosity can be divided into gas porosity and shrinkage porosity.

Gas porosity

These types of porosity are caused by gas and are typically round with smooth inner walls that are distributed randomly. Based on different causes, it can be categorized into three types.

Dissolved gas porosity

Causes

This type of porosity primarily occurs during the process of pouring. During the pouring of molten metal into the shell mold, the main causes include the impurities in the charge materials, excessive melting time, and excessively high temperatures, which cause the molten metal to dissolve an excessive amount of gas. Upon cooling and solidification, the solubility drops sharply, causing large amounts of gas to precipitate from the metal interior. Any gas that does not precipitate in time becomes “trapped” within the casting. This is what we call “dissolved gas porosity”.

Characteristics

Dissolved gas porosity commonly occurs during the investment casting of stainless steel, alloy steel, and high-temperature alloys. These three types of metals have a strong affinity for gases, causing them to dissolve large amounts of gas. Additionally, stainless steel has very high melting points, which typically range from 1580 to 1650°C. In terms of the current stainless steel materials, 304 stainless steel has better tolerance for high temperatures. A high melting point implies higher smelting temperatures, which leads to a significant increase in gas solubility.

These voids are typically fine in size, densely and uniformly distributed, and frequently appear throughout the casting or in thick sections where heat dissipation is slow.

Targeted preventive measures

To prevent this type of porosity, you should properly prepare the raw materials before starting the pouring. This includes keeping the charge materials dry, baking all tools and such In-furnace deoxidation operations to minimize the opportunity for gases to enter the molten metal. During smelting, keep the time as short as possible to reduce the degree of the molten metal, thereby lowering the solubility of gases in the molten metal and effectively preventing the formation of such porosity.

Mold gas porosity

Causes

The primary cause of this type of porosity is that the shell mold was not fired at a high enough temperature or for a sufficient duration during the process of shell building. This results in incomplete drying of volatile substances—such as moisture in the shell mold, wax parts, and coatings, which reduces the shell’s permeability. When undergoing thermal decomposition, these substances will generate a large amount of gas that enters the molten metal before it has solidified, thereby forming mold gas porosity.

Characteristics

These voids are characterized by their relatively large size and typically exhibit a round or pear-shaped morphology. They occur in limited quantities and tend to be locally concentrated, appearing predominantly on the upper surfaces, sharp corners, or thick-walled sections of the casting.

Targeted preventive measures

To prevent this type of porosity, you can heat the mold slowly, controlling the final baking temperature at around 1050–1100°C. In addition to maintaining the correct temperature, sufficient baking duration is essential. Standard castings should be held for 2–3 hours, while complex, thick-walled castings should be held for 3–5 hours. Correct heat treatment ensures the complete elimination of volatiles, thereby optimizing the shell’s permeability and reducing the generation of such porosity.

Entrained gas porosity

Causes

This type of porosity also occurs during the pouring process. However, unlike dissolved gas porosity, it results from an improperly designed gating system or an excessively fast pouring rate, which leads to turbulent flow, causing the molten metal to entrap air within the mold cavity.

Characteristics

These types of porosity frequently occur in thin-walled, complex castings. They are generally circular or elliptical, but their location is irregular. This type of porosity is the most common and the most difficult to prevent.

Targeted preventive measures

To prevent this type of porosity, you can improve the design of the gating system by enlarging the ingate diameter and reducing the velocity of the single metal stream, thereby minimizing turbulence as the molten metal enters the mold cavity.  You can also control the pouring speed by positioning the pouring cup of the mold shell close to the furnace nozzle and pouring at an angle to prevent turbulence in the molten metal, thereby reducing the formation of entrained gas porosity.

Shrinkage porosity

Shrinkage porosity is a type of porosity caused by casting shrinkage, which can be classified into two types. They are irregular in shape and have a rough internal surface, and are commonly found in thick-walled sections.

Shrinkage Cavity

Causes

All metals and alloys undergo solidification shrinkage as they transition from a liquid to a solid state. This is the most critical stage with the highest volume of contraction. If this volumetric shrinkage is not continuously compensated by a supply of molten metal, voids will form, which are known as a shrinkage cavity.

Characteristics

These types of shrinkage cavities appear as rough, angular voids or channels within the casting. They commonly occur in areas with thick walls or insufficient molten metal supply, as well as in alloys with a wide solidification range, such as copper and aluminum alloys.

Targeted Preventive Measures

The primary cause of shrinkage cavities is the volume contraction that occurs in metal solidification. Therefore, to prevent these cavities, you must not only provide feedings but also control the cooling speed and optimize the casting’s structural design.

• Core feeding method: riser designation

The riser serves as the core “molten metal reservoir” for feeding the casting. When designing it, you must adhere to two key principles.

First, the riser must be positioned near the thick-walled and hot-spot areas of the casting, that is, the areas where the molten metal solidifies last. This ensures a continuous supply of molten metal to these areas, which experience significant shrinkage.

Second, you need to ensure that the riser continues to supply molten metal even as the casting shrinks. By utilizing heating riser sleeves, you can ensure that the solidification time of the riser is later than that of the casting, thereby preventing the formation of internal voids.”

• Control the cooling speed

The key to this method is to ensure that the thicker sections of the casting cool first. You can place metal or graphite chill blocks in areas prone to hot spots, such as thick walls and hubs, to accelerate the cooling speed in these areas and prevent them from becoming isolated regions that solidify last. At the same time, to slow down the cooling speed of the remaining sections, you can place heat-retaining pads on the thinner sections, giving the thicker sections more time to solidify fully.

• Optimize the casting’s structural design

You can allow for a 1%–2% shrinkage allowance based on the casting wall thickness to compensate for volumetric shrinkage during solidification. To prevent sudden changes in thickness, you can use large-radius fillets to smooth transitions. This helps prevent localized overheating that could create isolated hot spots and avoids shrinkage defects exacerbated by stress concentration.

Micro porosity

Causes

These internal porosities are formed during the final stage of metal solidification. Because the rate of cooling is too fast, dendritic metal crystals to trap liquid metal within the crystals, which needs to feed the shrinkage, resulting in the formation of countless tiny voids.

Characteristics

Micro porosity resembles microscopic networks that are visible only under a microscope; they commonly occur in high-alloy steels and superalloys because they solidify over a wide temperature range. Rapid cooling reduces the time the molten metal flows between the dendrites, resulting in the formation of more microscopic voids.

Targeted Preventive Measures

The root cause of these micro porosities is insufficient cooling time. To prevent the formation of such porosity, you can address the issue through directional solidification. This involves slowing down the cooling process by lowering the temperature, thereby giving the molten metal more time to feed the shrinkage. However, it is important to note that the temperature should only be reduced by 50–100 degrees Fahrenheit to avoid causing other problems.

In addition to directional solidification, you can also perform hot isostatic pressing (HIP) after casting to collapse internal voids using high temperature and pressure, which is the most effective remediation method.

How does Porosity Affect Casting Quality?

Understanding the potential hazards caused by porosity will help you recognize the importance of minimizing porosity in your products. This will encourage you to place greater emphasis on preventing porosity during the production process, effectively reducing product scrap rates and lowering costs.

Affecting the strength of the product

The presence of porosity directly affects the strength of the product, primarily because it significantly reduces the effective bearing cross-sectional area, causing stress to concentrate near the voids. Over time, cracks will form near these voids, thereby shortening the product’s service life.

In the case of certain stainless steel or high-temperature alloy components, microcell effects can easily occur within the voids, accelerating localized pitting corrosion and reducing the component’s chemical stability.

Compromise the seal integrity

In the case of certain pump castings, the presence of porosity can easily lead to the formation of through-holes in the wall sections, which in turn can cause leaks when the parts are subjected to high-pressure fluids, compromising the overall integrity of the seal.

Degrade the surface quality

This is the primary reason for the high rate of returns for products containing porosity. Investment castings require high-precision surface finishes. During polishing, porosity hidden beneath the surface is exposed, leading to increased costs. During machining, the presence of porosity can also cause uneven cutting, which in turn affects surface tolerances.

Acceptance Criteria for Porosity in Castings

Understanding the acceptance criteria for porosity in castings and implementing strict foundry quality control will help you clearly define the permissible size and quantity of porosity. This not only enables you to quickly distinguish between acceptable and rejected parts but also standardizes acceptance standards, thereby ensuring consistent product quality and mitigating the risk of customer returns and complaints.

Since investment castings aim for high precision and surface quality, during acceptance, the casting is divided into Grade A, B, and C zones based on the stress distribution of the part. Void sizes are strictly limited in critical stress zones (Zone A), while a certain degree of micro-voids is permitted in non-stress zones (Zone C). During acceptance, X-ray inspection is typically used to assess internal porosity, and parts are judged for compliance based on ASTM E192 reference radiographs. Specific standards are shown in the table below.

This table applies only to investment castings with wall thicknesses of 1 inch (25.5 mm) or less and does not include isolated large porosity.

How to calculate the real cost of porosity in investment casting?

The later a porosity defect is detected, the greater the loss. Factories generally perform preliminary calculations using the formula: “Total scrap loss = Quantity × (Material Value + Process Cost).” Process costs encompass multiple aspects, including foundry costs, inspection costs, machining costs, and rejection costs.

The most severe loss is when subsurface porosity is discovered during machining, which requires remaking the entire part. While the cost of re-manufacturing is prohibitive, the loss of credibility with your partners is the most difficult to recover.

Final thoughts

The above provides a comprehensive overview of casting porosity. We hope you now have a clear understanding of what casting porosity is and how to prevent it. Although casting porosity can be challenging to address, a reliable foundry can help you effectively prevent it and minimize its impact.

If you have concerns about this issue, please contact us. Pingheng Machinery can assist you in resolving it. Through stringent quality control and comprehensive quality inspection, we deliver high-quality castings that exceed your expectations.

FAQ

Q1. Will “micro-porosity” inside a casting enlarge over time with extended use?

Generally speaking, this does not happen. However, if a part is subjected to high temperatures or heavy loads over an extended period, cracks may form around micron-sized voids within the casting as external pressure increases, and these cracks may gradually expand.

Q2. Is it still necessary to perform an X-ray inspection when the surface of the casting looks perfect?

Yes. Because voids are not limited to the surface, most of them are internal voids that are invisible to the naked eye. X-ray inspection can effectively prevent a large number of returns caused by excessive internal voids.

Q3. What is the difference between “gas porosity” and “shrinkage porosity”?

The most notable difference between gas porosity and shrinkage porosity is that they are caused by different factors and exhibit distinct shapes. Gas porosity is voids caused by gas, while shrinkage porosity is voids caused by casting shrinkage. Gas porosity is typically round with smooth inner walls, whereas shrinkage porosity is mostly irregular in shape and has rough interiors.

Q4. What is the difference between “shrinkage cavity” and “micro porosity”?

Macro cavity is caused by the contraction that occurs when a metal transitions from a liquid to a solid state, whereas micro porosity forms during the final stage of solidification. These two types differ in both their formation mechanisms and their appearance. Macro cavity is a large void visible to the naked eye, while micro porosity is tiny voids that are difficult to detect with the naked eye.

Q5. Does a higher shell firing temperature always guarantee better permeability?

Not necessarily. The higher the firing temperature, the more thoroughly the volatiles are removed; however, excessively high temperatures can cause the shell to over-sinter, which actually reduces its permeability. It is recommended to maintain the sintering temperature between 1050 and 1100 degrees.

Q6. Why does the scrap rate of investment castings increase during the rainy season?

Because the concentration of water molecules in the air increases during the rainy season, making it difficult to thoroughly dry the volatiles during shell-type firing. During casting, these water molecules release a large amount of gas into the molten metal, which creates numerous gas voids and consequently leads to a higher scrap rate.

Q7. Why are brass castings particularly susceptible to gas porosity?

Because the brass contains zinc, which has a boiling point of approximately 907 degrees Celsius. At temperatures of around 1,100 degrees Celsius, zinc evaporates rapidly, producing a large amount of gas. Furthermore, brass has a strong affinity for hydrogen. These two factors combined make brass castings highly prone to porosity during investment casting.

Q8. Can post-machining subsurface porosity be resolved by rework?

For standard civilian components, this issue can be resolved through repair and re-machining. However, in fields such as aerospace that require high-precision components, any part found to have subsurface porosity must be scrapped.