Almost without exception, there is a common theme when discussing perceived issues in the use of metal castings: the concern that castings might contain some form of porosity that could make the component unsuitable for its intended application. Although technological advancements, such as computer simulations of the solidification of castings, have provided tools to minimize the porosity of castings, some porosity may still exist even in castings designed with the utmost care, whether they are microcastings, sand castings, or die-castings.
It is widely accepted that depending on the location of the porosity in the casting, certain levels of porosity are actually standard in many commercial applications, while in other applications, where the final strength of the casting is crucial, achieving zero porosity is required.
What is Porosity in Castings?
Porosity is often used to describe any void, hole, or discontinuity present in a casting. However, the morphology of the pores, in terms of size, shape, surface constituents, position, and frequency, is essential to define the specific defect, determine its cause, and analyze the extent to which it will negatively impact the properties or functionality of the casting.
In this article, the common types of porosity in castings will be discussed, how to detect them, how to minimize them, and how to manage them when zero porosity castings are required. For comprehensive information about our investment casting quality standards, explore our dedicated quality control processes.
Acceptance Criteria for Porosity in Castings
It is common for many foundries to produce castings with porosity. However, it is never acceptable to simply accept that porosity is permanently present, as the correct methodology for foundries is ALWAYS to apply the proper techniques to reduce or eliminate it. Certainly, we should never settle for product integrity that is below optimal; however, the most important point to highlight is that the presence of a certain amount of porosity does not necessarily mean that the cast product is unacceptable for its intended application. In fact, it is often the design of the casting itself that is a determining factor in the formation of porosity, and just as often, it will have little to no impact on the functionality of the product.
Naturally, there are also many situations where any porosity in a casting cannot be tolerated, and therefore it must be detected and removed, with the casting certified as 100% defect-free in terms of integrity.
Causes of Porosity in Castings
Molten metal generally presents voids due to shrinkage porosity, gas bubble porosity (or backpressure, as it is sometimes called), or inclusions of foreign bodies such as non-metallic slag, dust, or refractor fragments. Each of these causes is discussed in detail below.
Shrinkage
Since liquid metal (molten) has a greater volume than the same weight of solid metal, and since the casting process involves filling a mold with this molten metal, compensation must be provided so that the metal is fed into the casting while it solidifies and shrinks. This is done by inserting pouring risers around the casting, generally in the thicker sections of the casting, as these will be the last to solidify; thus, the molten metal is fed into the thinner sections first.
With improvements in computer simulations, the foundry is able to minimize most cases of shrinkage by working with the customer on the best design for the component, allowing the best flow of the casting metal.
However, there are also situations where a casting has so many adjacent thick sections that it is impossible to place risers in every heavy section. Many times, there will be small shrinkage pockets in a casting that cannot be eliminated, and therefore, there are various levels of acceptance criteria depending on the specific needs of the casting.
Coarse shrinkage discontinuities in a casting are almost always considered detrimental, but visible defects should be minimized and, in most cases, eliminated by design changes combined with the most effective feeding positions (or metal entry).
Gas Bubbles
Some alloys have the ability to dissolve large volumes of gases such as nitrogen, oxygen, or hydrogen when in the molten state. However, these alloys do not often retain these gases in their solid state. As a result, during the casting process, these gases come out of solution and form small bubbles or gas pockets while the casting solidifies, trapping them in the matrix.
These voids typically appear as small round holes and are almost always found in the thicker section of the casting. This condition is controlled through the use of degassing agents added to the metal. These agents react with the dissolved gases, forming solid slag, which can be removed or filtered from the molten metal before casting.
This technique is effective, but it does not always guarantee a 100% bubble-free casting, as the molten metal continues to react with the atmosphere even after degassing. Therefore, most specifications allow a certain amount of this type of void in the casting.
Inclusions
Inclusions are generally the most straightforward casting defect to understand. They are simply small fragments of non-metallic material that enter the casting. These can be small particles of refractory material from the furnace or the pouring cluster, ceramic shell fragments from the mold itself, or even small pieces of slag formed by the degassing agents. From the foundry's perspective, good practices, clean molds, and the use of ceramic filters minimize non-metallic inclusions, but there is no way to completely prevent small fragments from being carried into the metal flow and into the casting.
Although all three categories of porosity defects may appear harmful to the internal integrity of the casting, the fact is that typical small voids associated with castings can have varying impacts on functionality, ranging from no damage to being extremely detrimental if the voids are located in areas of high stress in the casting.
How to Avoid Porosity in Castings
Process controls to ensure a casting is as free as possible from these various forms of porosity are well understood by experienced foundries. The use of solidification prediction software, the use of appropriate degassing agents for the molten alloy, and the use of metallic filters are just a few of the available tools. Learn more about our advanced precision casting techniques and quality control measures.
With good foundry practices, porosity in a casting can be reduced to an acceptable level. However, it is always important to keep in mind that porosity in castings can almost never be completely eliminated due to the nature of the process, which involves molten metal flowing at high temperatures and being chemically active in the mold.
"Zero Porosity" Castings
The term "Zero Porosity" in castings is somewhat misleading, as there will inevitably always be some form of porosity that will find its way into the casting matrix due to the intrinsic nature of the casting process. However, when the possibility of porosity could lead to non-compliance of the cast component in its intended application, such as a component that retains pressure or an ultra-high strength component, there are post-casting techniques that can be used to maximize the density of the casting. These are the Hot Isostatic Processing (HIP) process and Impregnation.
HIP Process (Hot Isostatic Pressing)
Hot Isostatic Pressing (HIP) is a manufacturing process that uses high temperature and isostatic pressure to densify materials such as metals, ceramics, and composites. This process significantly improves the properties of materials by reducing porosity and increasing density. For more information about advanced metal processing techniques, visit the ASM International HIP processing guide.
Impregnation
Impregnation is another commonly used method to eliminate porosity in cast components. It is also called porosity sealing or sealing of porous metals. This technique involves filling the porous structure with a suitable material to achieve the desired sealing properties.
Contact Edr Fittings Srl for More Information
At Edr Fittings Srl, we are experts in the world of casting. We can collaborate with you to advise on the potential risks and the impact of porosity on your specific application, as well as the economic aspects of production if your design requires minimal or zero porosity. Contact us today to discuss your specific casting requirements and quality standards.
